Chapter 2: Alkanes and Cycloalkanes

Chapter 2: Alkanes and Cycloalkanes

Chapter Outline (2.1) The structure of alkanes
(2.2) Constitutional isomerism in alkanes (2.3) Nomenclature of alkanes and the IUPAC system (2.4) Cycloalkanes (2.5) Conformations of alkanes and cycloalkanes (2.6) Cis,trans isomerism in cycloalkanes and bicycloalkanes (2.7) Physical properties of alkanes and cycloalkanes (2.8) Reactions of alkanes (2.9) Sources and importance of alkanes

Alkanes Members of a larger group of organic compounds called hydrocarbons Hydrocarbon: Compound composed of only carbon and hydrogen atoms Are saturated hydrocarbons and contain only carbon-carbon single bonds Referred to as aliphatic hydrocarbons Physical properties of the higher members of this class resemble those of the long carbon-chain molecules found in animal fats and plant oils General molecular formula - CnH2n+2 (2-1) The structure of alkanes

Figure 2.1 - Four Classes of Hydrocarbons
(2-1) The structure of alkanes

Figure 2.2 - Lewis Structures and Ball-and-Stick Models of Methane and Ethane
Four bonds about each carbon are arranged in a tetrahedral manner All bond angles are approximately 109.5° (2-1) The structure of alkanes

Drawing Alkanes Line-angle formulas
Abbreviated way to draw structural formulas Each vertex and line ending represents a carbon atom (2-1) The structure of alkanes

Table Names, Molecular Formulas, and Condensed Structural Formulas for the First 20 Alkanes with Unbranched Chains (2-1) The structure of alkanes

Join In (1) Which of the following choices is not an alkane? C12H26
(2-1) The structure of alkanes Correct Answer 2. C10H20

Constitutional Isomers
Compounds with the same molecular formula but a different connectivity of their atoms Example - For C4H10, the following connectivities are possible: Butane and 2-methylpropane are constitutional isomers Are different compounds Differ in physical and chemical properties (2-2) Constitutional isomerism in alkanes

Constitutional Isomerism
To determine whether two or more structural formulas represent constitutional isomers, write and compare the molecular formulas of compounds All compounds that have the same molecular formula but different structural formulas are constitutional isomers Ability of carbon to form strong bonds with other carbon atoms results in a staggering number of constitutional isomers (2-2) Constitutional isomerism in alkanes

Example 2.1 - Constitutional Isomers
Do the condensed formulas in each pair represent the same compound or constitutional isomers? (2-2) Constitutional isomerism in alkanes

Example Solution (a) The molecules are drawn here as both condensed structural formulas and line-angle formulas Each formula has an unbranched chain of six carbons The two are identical and represent the same compound (2-2) Constitutional isomerism in alkanes

Example Solution (b) Each formula has a chain of five carbons with two —CH3 branches Although the branches are identical, they are at different locations on the chains These formulas represent constitutional isomers (2-2) Constitutional isomerism in alkanes

Example 2.2 - Line-Angle Formulas
Write line-angle formulas for the five constitutional isomers with the molecular formula C6H14 Solution In solving problems of this type, you should devise a strategy and then follow it First, draw a line-angle formula for the constitutional isomer with all six carbons in an unbranched chain Then draw line-angle formulas for all constitutional isomers with five carbons in a chain and one carbon as a branch on the chain Finally, draw line-angle formulas for all constitutional isomers with four carbons in a chain and two carbons as branches (2-2) Constitutional isomerism in alkanes

Example 2.2 - Line-Angle Formulas (continued)
No constitutional isomers with only three carbons in the longest chain are possible for C6H14 (2-2) Constitutional isomerism in alkanes

Join In (2) What functional group is not present in the naturally occurring compound shown below? Carboxylic acid Amine Primary alcohol Secondary alcohol Methyl group (2-2) Constitutional isomerism in alkanes Correct Answer 3. Primary alcohol

Join In (3) A brilliant chemist determined the number of secondary carbons and secondary hydrogens for an unknown alkane Well, actually, the chemist is having trouble deciding which numbers are correct Which of these answers is possible for an alkane? 2° hydrogens 2° carbons (2-2) Constitutional isomerism in alkanes Correct Answer 5. 2° hydrogens - 8; 2° carbons - 4

Join In (4) Which of the following is not an isomer of hexane?
2,2-Dimethylbutane 2,2-Dimethylpentane 3-Methylpentane 2-Methylpentane 2,3-Dimethylbutane (2-2) Constitutional isomerism in alkanes Correct Answer 2. 2,2-Dimethylpentane

Join In (5) How many constitutional isomers of C4H10O are alcohols? 1
2 3 4 5 (2-2) Constitutional isomerism in alkanes Correct Answer 4. 4

Nomenclature of Alkanes - The IUPAC System
IUPAC - International Union of Pure and Applied Chemistry IUPAC name of an alkane with an unbranched carbon chain consists of: Prefix that indicates the number of carbon atoms in the chain Suffix -ane to show that the compound is a saturated hydrocarbon IUPAC name of an alkane with a branched chain consists of: A parent name that indicates the longest chain of carbon atoms in the compound Substituent names that indicate the groups bonded to the parent chain (2-3) Nomenclature of alkanes and the IUPAC system

Table Prefixes Used in the IUPAC System to Show the Presence of 1 to 20 Carbon Atoms in an Unbranched Chain (2-3) Nomenclature of alkanes and the IUPAC system

Rules of the IUPAC System for Naming Alkanes
Name for an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms in the chain and the ending -ane The longest chain of carbon atoms should be selected as the parent chain for branched-chain alkanes Name of the parent chain becomes the root name Each substituent is given a name and a number Number shows the carbon atom of the parent chain to which the substituent is bonded Hyphen should be used to connect the number to the name (2-3) Nomenclature of alkanes and the IUPAC system

Rules of the IUPAC System for Naming Alkanes (continued 1)
If there is one substituent, number the parent chain from the end that gives the substituent the lower number In case of two or more identical substituents: Number the parent chain from the end that gives the lower number to the substituent encountered first Indicate the number of times the substituent appears by the prefix di-, tri-, tetra-, and so on Use commas to separate position numbers (2-3) Nomenclature of alkanes and the IUPAC system

In case of two or more different substituents: List the substituents in alphabetical order Number the chain from the end that gives the substituent encountered first the lower number If there are different substituents in equivalent positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number (2-3) Nomenclature of alkanes and the IUPAC system

Prefixes di-, tri-, tetra-, and so on are not included in alphabetizing Alphabetize the names of substituents first and then insert the prefixes In case of two or more parent chains of identical length, choose the parent chain with the greater number of substituents (2-3) Nomenclature of alkanes and the IUPAC system

Substituents with unbranched chains are named by dropping -ane from the name of the parent alkane and replacing it with -yl Unbranched alkyl substituents are named methyl, ethyl, propyl, butyl, pentyl, and so forth Substituents with branched chains are named according to rules 2 and 3 (2-3) Nomenclature of alkanes and the IUPAC system

Alkyl Group Substituent group derived by removal of a hydrogen atom from an alkane Represented by the symbol R— Named by dropping the -ane from the name of the parent alkane and adding the suffix -yl (2-3) Nomenclature of alkanes and the IUPAC system

Table 2.3 - Names for Alkyl Groups with One to Five Carbons
(2-3) Nomenclature of alkanes and the IUPAC system

Example 2.3 - IUPAC Nomenclature I
Write the IUPAC and common names for these alkanes (2-3) Nomenclature of alkanes and the IUPAC system

Example Solution Number the longest chain in each compound from the end of the chain toward the substituent that is encountered first For (a), the longest chain is four carbons (a butane) with a methyl group on carbon 2 For (b), the longest chain is seven carbons (a heptane), with substituents on carbons 2 and 4 (2-3) Nomenclature of alkanes and the IUPAC system

Nomenclature of Alkanes - Common Nomenclature
Total number of carbon atoms in an alkane, regardless of their arrangement, determines the name Example - All alkanes with the molecular formula C4H10 are called butanes, those with the molecular formula C5H12 are called pentanes, and so on Prefix n- is used to indicate that an alkane chain is unbranched Iso- indicates that one end of an unbranched chain terminates in a (CH3)2CH— group Neo- indicates that the chain terminates in —C(CH3)3 (2-3) Nomenclature of alkanes and the IUPAC system

The IUPAC System - A General System of Nomenclature
Name given to any compound with a chain of carbon atoms consists of a prefix, an infix, and a suffix Prefix indicates the number of carbon atoms in the parent chain Infix - Modifying element inserted into a word Indicates the nature of the carbon-carbon bonds in the parent chain (2-3) Nomenclature of alkanes and the IUPAC system

The IUPAC System - A General System of Nomenclature (continued 1)
Suffix indicates the class of compound to which the substance belongs (2-3) Nomenclature of alkanes and the IUPAC system

The IUPAC System - A General System of Nomenclature (continued 2)
prop-en-e = propene eth-an-ol = ethanol but-an-one = butanone but-an-al = butanal pent-an-oic acid = pentanoic acid cyclohex-an-ol = cyclohexanol eth-yn-e = ethyne eth-an-amine = ethanamine (2-3) Nomenclature of alkanes and the IUPAC system

Join In (6) What is the best IUPAC name for the following compound?
2-Ethyl-3-isopropylhexane 1-Isopropyl-3-ethylhexane 4-Isopropyl-5-ethylhexane 3-Methyl-4-isopropylheptane 4-Isopropyl-3-methylheptane (2-3) Nomenclature of alkanes and the IUPAC system Correct Answer 5. 4-Isopropyl-3-methylheptane

Join In (7) What is the systematic name of the alkane shown below?
3,7-dimethyl-4-propyloctane 2-methyl-5-(1-methylpropyl)octane 2-ethyl-6-methyl-3-propylheptane 3-methyl-4-(3-methylbutyl)heptane 2,6-dimethyl-5-propyloctane (2-3) Nomenclature of alkanes and the IUPAC system Correct Answer 5. 2,6-dimethyl-5-propyloctane

Classification of Carbon and Hydrogen Atoms
Classification of carbon atoms Primary (1°) - Carbon bonded to one carbon atom Secondary (2°) - Carbon bonded to two other carbon atoms Tertiary (3°) - Carbon bonded to three other carbon atoms Quaternary (4°) - Carbon bonded to four other carbon atoms Hydrogen atoms are classified as primary, secondary, or tertiary depending on the type of carbon to which each is bonded Primary hydrogen - Hydrogen bonded to a 1° carbon Secondary hydrogen - Hydrogen bonded to a 2° carbon Tertiary hydrogen - Hydrogen bonded to a 3° carbon (2-3) Nomenclature of alkanes and the IUPAC system

Classification of Carbon and Hydrogen Atoms - Example
(2-3) Nomenclature of alkanes and the IUPAC system

Join In (8) What kind of carbon is not present in the molecule below?
Primary Secondary Tertiary Quaternary All 4 kinds of carbon are present (2-3) Nomenclature of alkanes and the IUPAC system Correct Answer 4. Quaternary

Join In (9) How many secondary hydrogens are there in the following molecule? 2 3 4 5 6 (2-3) Nomenclature of alkanes and the IUPAC system Correct Answer 5. 6

Join In (10) C6H14 has 5 isomers
How many of these isomers have at least one tertiary carbon? 1 2 3 4 (2-3) Nomenclature of alkanes and the IUPAC system Correct Answer 4. 3

Cycloalkanes Saturated hydrocarbons that contain carbons joined to form a ring General formula - CnH2n Five- and six-membered rings are the most common Rings are represented using line-angle formulas Each ring is represented by a regular polygon that has the same number of sides as there are carbon atoms in the ring (2-4) Cycloalkanes

Cycloalkanes - Nomenclature
Add the prefix cyclo- to the name of the corresponding open-chain alkane Name each substituent on the ring There is no need to give a number if there is only one substituent Number the ring beginning with the substituent of lower alphabetical order if there are two substituents Number the ring to give the substituents the lowest set of numbers and list them in alphabetical order in case of three substituents (2-4) Cycloalkanes

Example 2.5 - IUPAC Nomenclature III
Write the molecular formula and the IUPAC name for each cycloalkane (2-4) Cycloalkanes

Example Solution (a) Replace each vertex and line terminus with a carbon and add hydrogens as necessary to give each carbon four bonds Molecular formula of this compound is C8H16 Because there is only one substituent on the ring, there is no need to number the atoms of the ring This compound’s name is (1-methylethyl)cyclopentane The substituent also could be named isopropyl, giving the alternative IUPAC name isopropylcyclopentane (2-4) Cycloalkanes

Example Solution (b) Two substituents - Ethyl and 1,1-dimethylethyl The IUPAC name of the cycloalkane is 1-ethyl-4-(1,1-dimethylethyl)cyclohexane Substituents also could be named ethyl and tert-butyl, giving the cycloalkane the alternative IUPAC name 1-tert-butyl-4-ethylcyclohexane Molecular formula is C12H24 (2-4) Cycloalkanes

Example Solution (c) Number the ring to give the three substituents the lowest set of numbers and then list them in alphabetical order Name of this compound is 2-ethyl-1,4-dimethylcyclohexane, and its molecular formula is C10H20 (2-4) Cycloalkanes

Bicycloalkanes Alkanes containing two rings that share two carbons
General formula - CnH2n−2 Examples (2-4) Cycloalkanes

Example 2.6 - General Formulas
Write the general formula for an alkane, a cycloalkane, and a bicycloalkane How do these general formulas differ? Solution General formulas are CnH2n+2 for an alkane, CnH2n for a cycloalkane, and CnH2n−2 for a bicycloalkane Each general formula in this series has two fewer hydrogens than the previous member of the series (2-4) Cycloalkanes

Example 2.7 - Molecular Formulas
Following are line-angle formulas and common names for three bicyclic compounds Write the molecular formula of each compound (2-4) Cycloalkanes

Example 2.7 - Solution The molecular formula of α-pinene is C10H16
α-Pinene is a major component, often as high as 65% by volume, of pine oil and turpentine The molecular formula of camphor is C10H16O Camphor, obtained from the camphor tree, Cinnamonium camphora, is used in the manufacture of certain plastics, lacquers, and varnishes The molecular formula of caryophyllene is C15H24 Caryophyllene is one of the fragrant components of oil of cloves (2-4) Cycloalkanes

Join In (11) What is the IUPAC name for the following molecule?
1-(2-Methylbutyl)-cyclopentane 2-(3-Methylbutyl)-cyclopentane 2-(2-Methylbutyl)-cyclopentane (1,2-Dimethylpropyl)-cyclopentane (2,2-Dimethylpropyl)-cyclopentane (2-4) Cycloalkanes Correct Answer 4. (1,2-Dimethylpropyl)-cyclopentane

Join In (12) How many secondary hydrogens are there in the cycloalkane shown below? 2 3 4 5 6 (2-4) Cycloalkanes Correct Answer 3. 4

Intramolecular Strain
Chemicals are physical entities that possess optimal structures similar to any macroscopic object One is said to put a strain on the system if one disturbs the optimal structure Strain: Measure of the energy stored in a compound due to a structural distortion Types - Torsional strain, steric strain, and angle strain Interconversion between the strained and relaxed structures results from collisions and thermal motions (2-5) Conformations of alkanes and cycloalkanes

Conformations of Alkanes
Conformation: Any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond Different conformations are often called conformational isomers or conformers Staggered conformation: Conformation about a C—C bond in which the atoms or groups on one carbon are as far apart as possible from atoms or groups on an adjacent carbon Newman projection: A way to view a molecule by looking along a C—C bond to help evaluate the relative orientations of attached groups (2-5) Conformations of alkanes and cycloalkanes

Conformations of Alkanes (continued 1)
Example - Ball-and-stick model and Newman projection of a staggered conformation of ethane Eclipsed conformation Conformation about a C—C bond in which the atoms or groups on one carbon are as close as possible to the atoms or groups on an adjacent carbon (2-5) Conformations of alkanes and cycloalkanes

Conformations of Alkanes (continued 2)
Example - Ball-and-stick model and Newman projection of an eclipsed conformation of ethane Energy relationships among conformations Dihedral angle (θ): Angle created by two intersecting planes Example - Dihedral angles in ethane (2-5) Conformations of alkanes and cycloalkanes

Conformations of Alkanes - Ethane
Has infinite number of conformations that differ only in the degree of rotation about the C—C bond Rotation is not completely free as there is a small energy barrier between conformations Staggered conformation is the lowest energy (most stable) conformation Eclipsed conformation is the highest energy (least stable) conformation At room temperature, molecules undergo collisions with sufficient energy to cross the energy barrier between extreme conformations Rotation about the C—C from one conformation to another occurs rapidly (2-5) Conformations of alkanes and cycloalkanes

Conformations of Alkanes - Ethane (continued)
Difference in energy between an eclipsed conformation and a staggered conformation is approximately 12.6 kJ (3.0 kcal)/mol and is referred to as torsional strain Torsional strain (eclipsed-interaction strain): Arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation Occurs when pairs of hydrogens H(4)-H(6), H(5)-H(8), and H(3)-H(7) on adjacent carbons are forced into eclipsed positions (2-5) Conformations of alkanes and cycloalkanes

Figure 2.8 - Energy of Ethane as a Function of Dihedral Angle
(2-5) Conformations of alkanes and cycloalkanes

Theoretical molecular orbital calculations
Conformations of Alkanes - Origin of the Torsional Strain in the Eclipsed Conformations of Ethane Original thought - Strain is caused by repulsion between eclipsed hydrogen nuclei Alternate thought - Strain is caused by repulsion between electron clouds of the adjacent C—H bonds Theoretical molecular orbital calculations Suggest that energy difference is not caused by destabilization of the eclipsed conformation, but rather by stabilization of the staggered conformation Stabilization arises from a small donor-acceptor interaction between a filled C—H bonding MO of one carbon and the empty or unfilled C—H antibonding MO on an adjacent carbon Donor-acceptor stabilization is lost when a staggered conformation is converted to an eclipsed conformation (2-5) Conformations of alkanes and cycloalkanes

Conformations of Alkanes - Butane
Has two types of staggered conformations and two types of eclipsed conformations Staggered confirmations Anti conformation: Conformation about a single bond in which two groups on adjacent carbons lie at a dihedral angle of 180° Gauche conformation: Conformation about a single bond of an alkane in which two groups on adjacent carbons lie at a dihedral angle of 60° Methyl is eclipsed by methyl in one eclipsed conformation (θ = 0°) Methyl is eclipsed by hydrogen in the other eclipsed conformation (θ = 120°) (2-5) Conformations of alkanes and cycloalkanes

Conformations of Alkanes - Butane (continued)
While dealing with the relative stabilities of the conformations, one encounters angle strain and steric strain along with torsional strain Angle strain: Arises when a bond angle is either compressed or expanded compared to its optimal value Steric strain (nonbonded interaction or van der Waals strain) Arises when nonbonded atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii allow (2-5) Conformations of alkanes and cycloalkanes

Figure Energy of Butane as a Function of the Dihedral Angle about the Bond between Carbons 2 and 3 (2-5) Conformations of alkanes and cycloalkanes

Energy-minimized conformation of anti butane (θ = 180°)
Conformations of Alkanes - Comparison of Anti and Eclipsed Conformations of Butane Energy-minimized conformation of anti butane (θ = 180°) Each C—C—C bond angle is 111.9° All H—C—H bond angles are between 107.4° and 107.9° Calculated strain is 9.2 kJ (2.2 kcal)/mol (2-5) Conformations of alkanes and cycloalkanes

Energy-minimized eclipsed conformation
Conformations of Alkanes - Comparison of Anti and Eclipsed Conformations of Butane (continued) Energy-minimized eclipsed conformation Calculated energy difference between the (a) non-energy-minimized and (b) energy-minimized eclipsed conformations is 3.6 kJ (0.86 kcal)/mol Represents a balance between a decrease in steric strain and an increase in angle strain Calculated strain is 21 kJ (5.0 kcal)/mol (2-5) Conformations of alkanes and cycloalkanes

Conformations of Alkanes - Energy-Minimized Gauche Conformation of Butane
Approximately 3.8 kJ (0.90 kcal)/mol higher in energy than the anti, staggered conformation Difference in energy is caused by the steric strain (nonbonded interaction strain) between the two methyl groups Two gauche conformations (θ = 60° and 300°) have equal energies but are not identical Related by reflection (2-5) Conformations of alkanes and cycloalkanes One of two equivalent energy-minimized gauche conformations of butane

Example 2.8 - Newman Projections
Following is the structural formula of 1,2- dichloroethane: Draw Newman projections for all staggered conformations formed by rotation from 0° to 360° about the carbon- carbon single bond Which staggered conformation(s) has (have) the lowest energy? Which has (have) the highest energy? Which, if any, of these staggered conformations are related by reflection? (2-5) Conformations of alkanes and cycloalkanes

Example Solution If we take the dihedral angle when the chlorines are eclipsed as a reference point, staggered conformations occur at dihedral angles 60°, 180°, and 300° (2-5) Conformations of alkanes and cycloalkanes

Example 2.8 - Solution (continued)
We predict that the anti conformation (dihedral angle θ = 180°) has the lowest energy Two gauche conformations (dihedral angle θ = 60° and 300°) are of higher but equal energy We are not given data in the problem to calculate the actual energy differences Two gauche conformations are related by reflection (2-5) Conformations of alkanes and cycloalkanes

Join In (13) What is the correct order of stability, starting with the most strained, for the conformations shown below? A B, C A, C, B B, A, C C, A, B C, B, A (2-5) Conformations of alkanes and cycloalkanes Correct Answer 5. C, B, A

Join In (14) Which statement about bond rotations around C—C bonds in ethane and ethylene is correct? Rotation around σ bond is easy because the orbital overlap does not change with rotation Rotation around π bond is easy because there are p orbitals available in all directions of space Rotation around σ bond is very hard because the electron pairs from the adjacent bonds must pass by each other Rotation around π bond is hard because the electrons are delocalized Rotation around σ bond is hindered because hydrogens on the adjacent carbons bump into each other (2-5) Conformations of alkanes and cycloalkanes Correct Answer 1. Rotation around σ bond is easy because the orbital overlap does not change with rotation

Join In (15) Which of the following represents the Newman projection of 2-bromo-3-chlorobutane? a b c d e (2-5) Conformations of alkanes and cycloalkanes Correct Answer 3. c

Join In (16) The lowest energy conformation of decane resembles a straight rod It is a fully stretched “zig-zag” conformation with all bonds staggered and all large substituents anti to each other Imagine we bend this chain at carbon 5 What is the minimum strain that such a once-bent conformation (bent rod) must have? 3.8 kJ/mol 4.0 kJ/mol 6.0 kJ/mol 0.0 kJ/mol 11.0 kJ/mol (2-5) Conformations of alkanes and cycloalkanes Correct Answer kJ/mol

Join In (17) What is the energy difference between the two lowest-energy conformations of 2,3-dimethylbutane? Energy of one gauche interaction Energy of two gauche interactions Energy of two H ↔ CH3 eclipsing interactions Energy of three H ↔ CH3 eclipsing interactions Energy of two CH3 ↔ CH3 eclipsing interactions (2-5) Conformations of alkanes and cycloalkanes Correct Answer 1. Energy of one gauche interaction

Join In (18) The highest energy conformation of ethane has 12 kJ/mol of strain The two highest conformations of butane have 16 and 19 kJ/mol of strain, respectively How much strain does the highest energy conformation of 2,3-dimethylbutane have? 16 kJ/mol 20 kJ/mol 26 kJ/mol 31 kJ/mol 37 kJ/mol (2-5) Conformations of alkanes and cycloalkanes Correct Answer kJ/mol

Conformations of Cycloalkanes
Structures and energies are highly dependent on the size of the rings Small ring strain: Associated with ring sizes below six that arises from nonoptimal bond angles Interconversions of cyclic alkanes of four carbons or more involve rotations along C—C single bonds of the rings Rotations are limited to certain angles (2-5) Conformations of alkanes and cycloalkanes

Conformations of Cycloalkanes - Cyclopropane
Angle strain is introduced by the compression in the optimal bond angle Observed C—C—C bond angles are 60° Bond angle predicted for sp3 hybridized carbon atoms ° Torsional strain Introduced by six pairs of eclipsed C—H bonds Combined angle and torsional strain energy is about 116 kJ (27.7 kcal)/mol Intramolecular strain causes cyclopropane and its derivatives to undergo several ring-opening reactions not seen with larger cycloalkanes (2-5) Conformations of alkanes and cycloalkanes

Conformations of Cycloalkanes - Cyclobutane
If cyclobutane were planar: All C—C—C bond angles would be 90° Torsional strain would be maximized with eight pairs of eclipsed hydrogen interactions Puckering of the ring: Reduces torsional strain associated with eclipsed interactions Increases angle strain caused by the compression of C—C—C bond angles Energy is a minimum in the puckered (butterfly) conformation Measured C—C—C bond angles are 88° Strain energy is about 110 kJ (26.3 kcal)/mol (2-5) Conformations of alkanes and cycloalkanes

Figure Cyclobutane (2-5) Conformations of alkanes and cycloalkanes

Conformations of Cycloalkanes - Cyclopentane
Puckering from planar cyclopentane reduces torsional strain but increases angle strain The ring twists by rotations along the C—C bonds into the envelope conformation Average C—C—C bond angle is reduced from 108° to 105° Number of eclipsed C—H interactions is also reduced Exists as a dynamic equilibrium of five equivalent envelope conformations Each carbon atom alternates as the out-of-plane atom Total strain energy in puckered cyclopentane is about 27 kJ (6.5 kcal)/mol (2-5) Conformations of alkanes and cycloalkanes

Figure Cyclopentane (2-5) Conformations of alkanes and cycloalkanes

Conformations of Cycloalkanes - Cyclohexane
Chair conformation: Most stable puckered conformation of a cyclohexane ring All bond C—C—C bond angles are 110.9° All hydrogens on adjacent carbons are staggered No two atoms are close enough to each other for nonbonded interaction strain to exist (2-5) Conformations of alkanes and cycloalkanes

Conformations of Cycloalkanes - Cyclohexane (continued)
C—H bonds in a chair conformation are arranged in the following orientations: Axial bonds Equatorial bonds (2-5) Conformations of alkanes and cycloalkanes

Conformations of Cycloalkanes - Nonplanar Conformations of Cyclohexane
Boat conformation: Carbons 1 and 4 of the rings are bent toward each other Less stable than a chair conformation because of: Torsional strain associated with four pairs of eclipsed C— H interactions Steric strain between the two flagpole hydrogens Difference in energy between chair and boat conformations - Approximately 27 kJ (6.5 kcal)/mol Twist-boat conformation Approximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation Approximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation (2-5) Conformations of alkanes and cycloalkanes

Figure 2.17 - Boat and Twist-Boat Conformations of Cyclohexane

Figure 2.18 - Interconversion of (a) a Chair Conformation to (b) a Boat Conformation

Figure Energy Diagram for the Interconversion of Chair, Twist-Boat, and Boat Conformations of Cyclohexane (2-5) Conformations of alkanes and cycloalkanes

Large difference in energy between the conformations
Conformations of Cycloalkanes - Interconversion between Chair, Twist-Boat, and Boat Conformations of Cyclohexane Large difference in energy between the conformations At room temperature, molecules in the chair conformation make up more than 99.99% of the equilibrium mixture (2-5) Conformations of alkanes and cycloalkanes Interconversion of two equivalent chair conformations

Figure 2.20 - Interconversion of Alternate Chair Conformations of Cyclohexanes
Change occurs in the relative orientations in space of the hydrogen atoms bonded to each carbon when one chair is converted to the other All hydrogen atoms axial in one chair become equatorial in the other and vice versa Interconversion occurs rapidly at room temperature (2-5) Conformations of alkanes and cycloalkanes

Example 2.9 - Axial versus Equatorial Groups I
Following is a chair conformation of cyclohexane showing one methyl group and one hydrogen Indicate using a label whether each group is equatorial or axial Draw the alternative chair conformation and again label each group as axial or equatorial (2-5) Conformations of alkanes and cycloalkanes

Example Solution (2-5) Conformations of alkanes and cycloalkanes

Join In (19) What kind of strain is relieved the most by ring puckering in cyclopentane? Torsional Steric Angle Steric and angle Torsional and steric (2-5) Conformations of alkanes and cycloalkanes Correct Answer 1. Torsional

Join In (20) Which of the above molecules represents the most stable conformation of trans-1,3-dimethylcyclohexane? a b c d e (2-5) Conformations of alkanes and cycloalkanes Correct Answer 4. d

Join In (21) Which of the following does not contribute to the overall energy of a cycloalkane? Torsional strain Angle strain Hydrogen bonding Steric strain All of these contribute to the overall energy of a cycloalkane (2-5) Conformations of alkanes and cycloalkanes Correct Answer 3. Hydrogen bonding

Join In (22) Which of the following statements is true concerning the isomers cis-1,2-dimethylcyclohexane and cis-1,3-dimethylcyclohexane? They are not constitutional isomers They represent different conformations of the same molecule The favored conformer of the 1,3-isomer is more stable than that of the 1,2-isomer The favored conformer of the 1,3-isomer and that of the 1,2-isomer are equal in energy The relative stability of the two molecules cannot be determined (2-5) Conformations of alkanes and cycloalkanes Correct Answer 3. The favored conformer of the 1,3-isomer is more stable than that of the 1,2-isomer

Join In (23) What isomer of dichlorocyclohexane is represented by the Newman projection shown below? 1-axial; 3-axial 1-equatorial; 4-equatorial 1-equatorial; 3-equatorial 1-equatorial; 3-axial 1-equatorial; 4-axial (2-5) Conformations of alkanes and cycloalkanes Correct Answer 5. 1-equatorial; 4-axial

Join In (24) Which of the above structures represents the most stable conformation of 1-tert-butyl-3,5-dimethylcyclohexane? a b c d e (2-5) Conformations of alkanes and cycloalkanes Correct Answer 4. d

Join In (25) What is the energy difference between the two chair conformations of trans-1,2-dimethylcyclohexane? Zero One gauche interaction Two gauche interactions Three gauche interactions Four gauche interactions (2-5) Conformations of alkanes and cycloalkanes Correct Answer 4. Three gauche interactions

Join In (26) Imagine a molecule of cyclopropane with all σ C—C bonds made of pure p orbitals What would the H—C—H bond angles be in such a molecule? 60° 90° 109.5° 120° 180° (2-5) Conformations of alkanes and cycloalkanes Correct Answer °

Diaxial (Axial-Axial) Interaction
Steric strain arising from interaction between an axial substituent and an axial hydrogen (or another group) on the same side of a chair conformation of a cyclohexane ring Called 1,3-diaxial interaction as it originates between groups on carbons 1 and 3 of a cyclohexane ring Helps describe the relative stabilities of chair conformations with equatorial and axial substituents (2-5) Conformations of alkanes and cycloalkanes

Figure 2.21 - Two Chair Conformations of Methylcyclohexane
Steric strain introduced by two diaxial interactions makes the axial methyl conformation less stable by approximately 7.28 kJ (1.74 kcal)/mol (2-5) Conformations of alkanes and cycloalkanes

Calculation of the Ratio of the Two Conformations at Equilibrium
Uses the equation ΔG0 = −RT ln Keq ΔG0 = Change in Gibbs free energy Keq = Equilibrium constant T = Temperature in kelvins R = Universal gas constant that has the value J ( cal)·K−1·mol−1 For methylcyclohexane At any given instant at room temperature, percentage of equatorial is (18.9/19.9) ×100% = about 95% (2-5) Conformations of alkanes and cycloalkanes

Table 2.4 - ΔG0 (Axial-Equatorial) for Monosubstituted Cyclohexanes at 25°C

Drawing Alternative Chair Conformations of Cyclohexane
Step 1 - Draw two sets of parallel lines, each set at a slight angle Step 2 - Complete each chair by drawing the ends connected to the parallel lines, in each case making one end tip up and the other end tip down Step 3 - Draw axial bonds as vertical lines drawn on the side of the larger angle (greater than 180°) at each ring atom Step 4 - Draw the equatorial bonds using the bonds of the ring as guides for the angles (2-5) Conformations of alkanes and cycloalkanes

Example 2.10 - Axial versus Equatorial Groups II
Label all methyl-hydrogen (CH3/H) diaxial interactions in the following chair conformation of 1,2,4-trimethylcyclohexane (2-5) Conformations of alkanes and cycloalkanes

Example Solution There are four methyl-hydrogen 1,3-diaxial interactions in this example Each axial methyl group has two sets of 1,3-diaxial interactions with parallel hydrogen atoms on the same side of the ring Equatorial methyl group has no diaxial interactions (2-5) Conformations of alkanes and cycloalkanes

Example 2.11 - Equilibrium Populations of Conformations
Calculate the ratio of the diequatorial to diaxial conformation of this disubstituted cyclohexane at 25°C (2-5) Conformations of alkanes and cycloalkanes

Example Solution For these two chair conformations, ΔG0 (2 axial CH3 → 2 equatorial CH3) = 2 × (−7.28 kJ/mol) = −14.56 kJ (3.5 kcal)/mol Substituting this value in the equation ΔG0 = −RT ln Keq gives a ratio of 357:1 Thus, at any given instant at room temperature, approximately 357/358 ×100% = 99.7% of the molecules of this compound are in the diequatorial chair conformation (2-5) Conformations of alkanes and cycloalkanes

Join In (27) Which of the following statements is true?
An axial group encounters more steric strain than an equatorial group All substituted cycloalkanes possess axial and equatorial bonds Only cyclopentane rings possess axial and equatorial bonds Cyclohexane derivatives are more stable with axial substituents than with equatorial substituents A cyclic compound cannot contain a double bond (2-5) Conformations of alkanes and cycloalkanes Correct Answer 1. An axial group encounters more steric strain than an equatorial group

Join In (28) Which of the above will have the highest energy after the chair flip? a b c d e (2-5) Conformations of alkanes and cycloalkanes Correct Answer 2. b

Figure 2.22 - Relationship between Stereoisomers and Constitutional Isomers
(2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Cis,Trans Isomers Stereoisomers that have the same connectivity but a different arrangement of their atoms in space Result of the presence of either a ring or a carbon-carbon double bond Without breaking and reforming one or more bonds, cis isomer cannot be converted to the trans isomer and vice versa (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Cis,Trans Isomerism in Cycloalkanes
Example - 1,2-Dimethylcyclopentane Carbons 1 and 2 are stereocenters Stereocenter: Atom about which exchange of two groups produces a different stereoisomer Stereoisomers are referred to as having either a cis or a trans configuration Configuration: Arrangement of atoms about a stereocenter (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Example 2.12 - Cis versus Trans Isomerism
Which cycloalkanes show cis,trans isomerism? For each that does, draw the cis and trans isomers Methylcyclopentane 1,1-Dimethylcyclopentane 1,3-Dimethylcyclobutane (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Example Solution Methylcyclopentane does not show cis,trans isomerism Has only one substituent on the ring 1,1-Dimethylcyclopentane does not show cis,trans isomerism Because both methyl groups are bonded to the same carbon, only one arrangement is possible for them (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Example 2.12 - Solution (continued)
1,3-Dimethylcyclobutane shows cis,trans isomerism In the following structural formulas, cyclobutane is drawn as a planar ring viewed first from the side and then from above (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Cis,Trans Isomerism in 1,4-Dimethylcyclohexane
Chair conformations of trans-1,4-dimethylcyclohexane In one chair, the two methyl groups are axial, and in the alternative chair, they are equatorial Diequatorial-methyl chair conformation is more stable by approximately 14.6 kJ (3.5 kcal)/mol Chair conformations of cis-1,4-dimethylcyclohexane In one chair, one methyl group is equatorial and the other is axial, and in the alternative chair, the orientations in space of the methyl groups are reversed (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Trans-1,4,Dimethylcyclohexane

Cis-1,4-Dimethylcyclohexane

Example 2.14 - Substituted Cyclohexane Rings
Here is one cis,trans isomer of 2,4-dimethylcyclohexanol Complete the alternative chair conformations on the right (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Example Solution For (a), the CH3 group on carbon 2 must be below the plane of the ring, which on this carbon is axial CH3 group on carbon 4 must be above the plane of the ring, which on this carbon is equatorial For (b), methyl group on carbon 2 is equatorial Methyl group on carbon 4 is axial (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Cis,Trans Isomerism in Bicycloalkanes
Stereoisomers of decalin Trans-decalin is the more common stereoisomer of decalin Each ring is locked into one chair conformation (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Cis,Trans Isomerism in Bicycloalkanes - Steroids
Present in human metabolism as cholesterol, steroid hormones, and bile acids Carbon skeleton consists of six-membered rings and five-membered ring Ring system is present in both animal and plant steroids Stereorepresentations of cholestanol (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Cis,Trans Isomerism in Bicycloalkanes - Steroids (continued 1)
Drawing a six-membered ring in which an added CH2 group forms a bridge between carbons 1 and 4 When viewed from the side (c), one can notice that the one-carbon bridge locks the six-membered ring into a boat conformation Locked boat conformation embedded in the molecule is not obvious in images (a) and (b) (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Cis,Trans Isomerism in Bicycloalkanes - Steroids (continued 2)
Drawing adamantane (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes

Join In (29) Glucosamine (R=H), a naturally occurring substance found in the exoskeleta of marine invertebrates, is believed by some to be helpful in relieving arthritic symptoms in humans What are, respectively, the relative positions of the amino group to the CH2OH substituent and OR group? Cis, cis Trans, trans Cis, trans Trans, cis Cannot be determined from this kind of perspective drawing (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes Correct Answer 2. Trans, trans

Join In (30) What happens to a substituent in a chair conformation of cyclohexane when all bonds rotate in a synchronized fashion? Nothing, it stays where it was It “flips” to an alternative position on the same carbon It moves to the next carbon The carbon to which the substituent is attached changes hybridization It moves from a cis to a trans position (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes Correct Answer 2. It “flips” to an alternative position on the same carbon

Join In (31) What are the relative geometries of ring fusions (starting from the left) for the steroid shown below? Cis, cis, cis Cis, cis, trans Cis, trans, trans Trans, cis, cis Trans, trans, cis (2-6) Cis,trans isomerism in cycloalkanes and bicycloalkanes Correct Answer 3. Cis, trans, trans

Physical Properties of Alkanes - Intermolecular Forces of Attraction
Strongest attractive forces occur between ions Example - Between Na+ and Cl− in NaCl Weaker attractive forces Dipole-dipole interactions and hydrogen bonding Dispersion forces Weakest intermolecular attractive forces resulting from the interaction between temporary induced dipoles Account for the ability to liquefy low-molecular-weight, nonpolar substances (2-7) Physical properties of alkanes and cycloalkanes

Figure 2.23 - Dispersion Forces in a Neon Atom
Distribution of electron density averaged over time is symmetrical, and there is no net polarity Temporary polarization of one atom induces temporary polarization in adjacent atoms Electrostatic attractions between temporary dipoles are called dispersion forces (2-7) Physical properties of alkanes and cycloalkanes

At room temperature, alkanes with:
Physical Properties of Alkanes - Boiling Points, Melting Points, and Density Boiling points of alkanes are lower than other types of compounds of the same molecular weight Boiling and melting points of alkanes increase as molecular weight increases At room temperature, alkanes with: 1 to 4 carbons are gases 5 to 17 carbons are colorless liquids 18 or more carbons are white, waxy solids All liquid and solid alkanes are less dense than water (1.0 g/mL) (2-7) Physical properties of alkanes and cycloalkanes

Table 2.5 - Physical Properties of Some Unbranched Alkanes
(2-7) Physical properties of alkanes and cycloalkanes

Constitutional isomers have different physical properties
Table Physical Properties of the Isomeric Alkanes of Molecular Formula C6H14 Constitutional isomers have different physical properties (2-7) Physical properties of alkanes and cycloalkanes

Example 2.15 - Alkane Boiling Points
Arrange the alkanes in each set in order of increasing boiling point Butane, decane, and hexane 2-Methylheptane, octane, and 2,2,4-trimethylpentane (2-7) Physical properties of alkanes and cycloalkanes

Example Solution (a) All of these compounds are unbranched alkanes As the number of carbon atoms in the chain increases, dispersion forces between molecules increase, as does the amount of energy required to put the molecules into motion Therefore, the larger the unbranched alkane, the higher the boiling point Decane has the highest boiling point, whereas butane has the lowest (2-7) Physical properties of alkanes and cycloalkanes

Example Solution (b) These three alkanes are constitutional isomers with molecular formula C8H18 Their relative boiling points depend on the degree of branching 2,2,4-Trimethylpentane, the most highly branched isomer, has the smallest surface area and the lowest boiling point Octane, the unbranched isomer, has the largest surface area and the highest boiling point (2-7) Physical properties of alkanes and cycloalkanes

Join In (32) Dipole moments usually are measured on collections of molecules and represent average values for all conformations present Which of the following collections of molecules will not show a dipole moment? 1,2-dibromoethane trans-1,2-dibromocyclohexane cis-1,2-dibromocyclohexane cis-1,4-dibromocyclohexane trans-1,4-dibromocyclohexane (2-7) Physical properties of alkanes and cycloalkanes Correct Answer 4. trans-1,4-dibromocyclohexane

Reactions of Alkanes - Oxidation
Basis for the use of alkanes as energy sources for heat and power Balanced equations for the complete oxidation of methane Energy of the products is less than that of the reactants Difference in energy is given off as the heat of combustion (ΔH0) (2.8) Reactions of alkanes

Table 2.7 - Heats of Combustion of Four Constitutional Isomers of C8H18
(2.8) Reactions of alkanes

Figure Heats of Combustion in kJ (kcal)/mol for Four Isomeric Alkanes with the Molecular Formula C8H18 (2.8) Reactions of alkanes

Figure 2.25 - Strain Energy of Cycloalkanes as a Function of Ring Size
(2.8) Reactions of alkanes

Sources of Alkanes Natural gas Petroleum Coal 90%–95% methane
Mixture of other relatively low-boiling alkanes, chiefly propane, butane, and 2-methylpropane Petroleum Fractional distillation - Separation process in refining petroleum Coal (2-9) Sources and importance of alkanes

Figure 2.26 - Fractional Distillation of Petroleum
(2-9) Sources and importance of alkanes

Octane Rating Used to express the quality of gasoline as a fuel for internal combustion engines Percent of isooctane in a mixture of isooctane and heptane that has antiknock properties equivalent to the test gasoline Reference fuels (2-9) Sources and importance of alkanes

Synthesis Gas Mixture of carbon monoxide and hydrogen in varying proportions depending on the means by which it is manufactured Preparation (2-9) Sources and importance of alkanes

Synthesis Gas (continued)
Methanol and acetic acid are produced from carbon monoxide and hydrogen Routes to develop other organic chemicals from coal via methanol is likely to be seen in the future (2-9) Sources and importance of alkanes

Problem 2.63 This is a stereorepresentation of glucose
Convert this stereorepresentation to a planar hexagon representation Convert this stereorepresentation to a chair conformation Which substituent groups in the chair conformation are equatorial? Which are axial? (2-9) Sources and importance of alkanes

Problem Solution All substituents in chair conformation are equatorial, making it a particularly stable chair (2-9) Sources and importance of alkanes

Chapter 2: Alkanes and Cycloalkanes

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Chapter 2: Alkanes and Cycloalkanes

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