Alkanes and Cycloalkanes

Name: Alkanes and Cycloalkanes
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Description: Alkanes and Cycloalkanes

Alkanes and Cycloalkanes
Chapter 3 Organic Compounds: Alkanes and Cycloalkanes

Introduction The structural theory is the basis of organic chemistry:
Organic compounds can be grouped into families by their common structural features The members of a given family often have similar chemical and physical behavior

Hydrocarbons - are organic molecules. consisting only of carbon (C)
Hydrocarbons - are organic molecules consisting only of carbon (C) and hydrogen (H) atoms.

I. Functional Groups Definition Types of Functional Groups

A. Definition Functional Group - is a group of atoms within a larger molecule that has a characteristic chemical reactivity It reacts in a typical way, generally independent of the rest of the molecule

The chemistry of every organic molecule, regardless of size and complexity, is determined by its functional groups.

B. Types of Functional Groups
Table 3.1 lists a wide variety of functional groups. The functional groups of a compound affect: its reactions its structure its physical properties

Functional Groups with Carbon-Carbon Multiple Bonds

Functional Group with Carbon-Carbon Multiple Bonds: Alkene
Alkenes have a C-C double bond

Functional Group with Carbon-Carbon Multiple Bonds: Alkyne
Alkynes have a C-C triple bond

Functional Group with Carbon-Carbon Multiple Bonds: Arene
Arenes have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring

Functional Groups with Carbon Singly Bonded to an Electronegative Atom

Functional Group with Carbon Singly Bonded to an Electronegative Atom: Alkyl halide
Alkyl halides have a C bonded to a halogen (C-X)

Functional Group with Carbon Singly Bonded to an Electronegative Atom: Alcohol
Alcohols have a C bonded to O of a hydroxyl group (C-OH)

Functional Group with Carbon Singly Bonded to an Electronegative Atom: Ether
Ethers have two C’s bonded to the same O (C-O-C)

Functional Group with Carbon Singly Bonded to an Electronegative Atom: Amine
Amines have a C bonded to N (C-N)

Functional Group with Carbon Singly Bonded to an Electronegative Atom: Thiol
Thiols have a C bonded to SH group (C-SH)

Functional Group with Carbon Singly Bonded to an Electronegative Atom: Sulfide
Sulfides have two C’s bonded to same S (C-S-C)

Bonds are polar, with partial positive charge on C (+) and partial negative charge (-) on electronegative atom

Functional Groups with Carbon-Oxygen Double Bond (Carbonyl Groups)
These compounds behave similarly but differ depending on the identity of the atoms attached on the carbonyl-group carbon.

Aldehydes: have one hydrogen bonded to C=O Ketones: have two C’s bonded to the C=O

Carboxylic Acids: have -OH bonded to the C=O Esters: have one ether-like C-O bonded to the C=O

Amides: have one amine-like N bonded to the C=O Acid chlorides: have a Cl bonded to the C=O

Carbonyl C has partial positive charge (+) Carbonyl O has partial negative charge (-).

Practice Problem: Identify the functional groups in each of the
Practice Problem: Identify the functional groups in each of the following molecules:

Practice Problem: Propose structures for simple molecules that
Practice Problem: Propose structures for simple molecules that contain the following functional groups: Alcohol Aromatic ring Carboxylic acid Amine Both ketone and amine Two double bonds

Practice Problem: Identify the functional groups in the following
Practice Problem: Identify the functional groups in the following model of arecoline, a veterinary drug used to control worms in animals. Convert the drawing into a line-bond structure and a molecular formula (red = O, blue = N)

II. Alkanes Alkane Isomers Alkyl Groups Naming Alkanes
Properties of Alkanes

Overview Alkanes - are saturated aliphatic hydrocarbons.
They are compounds that contain only carbons and hydrogens (hydrocarbons), all connected exclusively by single bonds (saturated) They are a large family of molecules

Alkanes - have the general formula
CnH2n+2 where n is an integer

A. Alkane Isomers Isomers - are different compounds with the same molecular formula

For an alkane with less than three carbons, there is only one possible structure.

The molecular formula of an alkane with more than three carbons can give more than one structure

Branched-chain alkanes - are compounds whose carbon chains branch.
Straight-chain alkanes or normal alkanes - are compounds whose carbons are connected in a row. The C’s are connected to no more than 2 other C’s Branched-chain alkanes - are compounds whose carbon chains branch. There are one or more C’s connected to 3 or 4 C’s

Constitutional Isomers - are isomers that differ in how their atoms are arranged in chains
They are isomers with different connectivity, i.e different order of attachment of their atoms

Constitutional isomers may have:

Condensed Structures of Alkanes
An alkane can be represented in a brief form or in many types of extended form A condensed structure does not show bonds but lists atoms, such as CH3CH2CH3 (propane) CH3C(CH3)2CH3 (2,2-dimethylpropane)

Practice Problem: Draw structures of the five isomers C6H14

Practice Problem: Propose structures that meet the following
Practice Problem: Propose structures that meet the following descriptions Two isomeric esters with the formula C5H10O2 Two isomeric nitriles with the formula C4H7N

Practice Problem: How many isomers are there with the
Practice Problem: How many isomers are there with the following descriptions? Alcohols with the formula C3H8O Bromoalkanes with the formula C4H9Br

B. Alkyl Groups AlkyI group - is the partial structure that remains when a hydrogen atom is removed from an alkane - has the general abbreviation “R” (for Radical, an incomplete species or the “rest” of the molecule)

Replace -ane ending of alkane with -yl ending
Naming Alkyl groups Replace -ane ending of alkane with -yl ending -CH3 is “methyl” from methane -CH2CH3 is “ethyl” from ethane

Combining an alkyl group with any of the functional groups gives the name of the compound

Degree of a Carbon atom Degree (o) is applied to sp3 carbons only !
Degree (o) of a carbon is equal to the number of carbons attached directly to it. 1o primary o secondary 3o tertiary o quaternary

Alkyl groups are classified by the connection site
Types of Alkyl groups Alkyl groups are classified by the connection site primary alkyl group (attached to 1oC) secondary alkyl group (attached to 2oC) tertiary alkyl group (attached to 3oC) quaternary alkyl group (attached to 4oC)

Naming Alkyl groups: Common System
Free valence indicates carbon # 1 Prefixes are necessary to indicate arrangement of carbons in structure: n – unbranched chain and free valence is on 1o C iso – one -CH3 on next to last carbon sec – free valence on 2o C and used only for butyl tert – free valence is on 3o C neo – two -CH3’s next to the last carbon

iso – one -CH3 on next to last carbon

sec – free valence on 2o C and used only for butyl tert – free valence is on 3o C

tert – free valence is on 3o C neo – two -CH3’s next to the last carbon

Degree of a Hydrogen atom
Degree (o) of a hydrogen is the same as the o of carbon to which it is attached No such thing as a quaternary H !

Practice Problem: Draw the eight 5-carbon alkyl groups (pentyl
Practice Problem: Draw the eight 5-carbon alkyl groups (pentyl isomers)

Practice Problem: Identify the carbon atoms in the following
Practice Problem: Identify the carbon atoms in the following molecules as primary, secondary, tertiary or quaternary

Practice Problem: Identify the hydrogen atoms on the
Practice Problem: Identify the hydrogen atoms on the compounds shown as primary, secondary, or tertiary

Practice Problem: Draw structures of alkanes that meet the
Practice Problem: Draw structures of alkanes that meet the following descriptions An alkane with two tertiary carbons An alkane that contains an isopropyl group An alkane that has one quaternary and one secondary carbon

C. Naming Alkanes The International Union of Pure and Applied Chemistry (IUPAC) devised a system of nomenclature that uses:

Steps to naming alkanes1
Find the parent hydrocarbon Find the longest continuous chain of carbon atoms IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA

Find the parent hydrocarbon Find the longest continuous chain of carbon atoms If two chains have equal length, choose the main chain with more substituents IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA

Number the atoms in the main chain The correct sequence is when the substituents have the lowest possible number IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA

Identify and number the substituents Name and locate the substituents IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA

Write the name as a single word Use hyphens to separate the different prefixes, and use commas to separate numbers. Use multiplicity prefixes di-, tri-, tetra-, … Put the substituents in alphabetical order (Do not consider multiplier prefixes) IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA

Name a complex substituent as though it were itself a compound Name the complex substituent using the point of attachment as C#1 Put it in alphabetical order (numerical prefix is included) Set it off in parentheses IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA

Practice Problem: Give IUPAC names for the following compounds

Practice Problem: Draw structures corresponding to the
Practice Problem: Draw structures corresponding to the following IUPAC names 3,4-Dimethylnonane 3-Ethyl-4,4-dimethylheptane 2,2-Dimethyl-4-propyloctane 2,2,4-Trimethylpentane

Practice Problem: Name the eight 5-carbon alkyl groups

Practice Problem: Give the IUPAC name for the following
Practice Problem: Give the IUPAC name for the following hydrocarbon, and convert the drawing into skeletal structure.

D. Properties of Alkanes
Alkanes are called paraffins (low affinity compounds) They do not react as most chemicals, i.e. they are chemically inert to most lab reagents However, they do react with O2, halogens and a few other substances

Oxidation: Reaction with O2
Alkanes will burn in a flame, producing carbon dioxide, water, and heat O2 D Alkane CO2 + H2O DH = - The larger the value of DH, the less stable the alkane If hydrocarbons do not contain the same amount of carbons, one must compare DH/CH2 rather than DH

Halogenation Alkanes react with Cl2 in the presence of light to replace H’s with Cl’s (not controlled) X2 hn Alkane Alkyl halides + HX X2 = Br2 or Cl2

Physical Properties Intermolecular Forces Solubility

Intermolecular Forces
To a first approx. C-H and C-C bonds are nonpolar: Dipole moments of alkanes are ~0 Alkanes are nonpolar The IMF’s for alkanes are van der Waals interactions (i.e induced-dipole-induced-dipole forces, London or dispersion forces) For compounds of similar M.W., alkanes have lower b.p’s and m.p’s.

Boiling points and melting points increase as size of alkane increases.
This is due the presence of dispersion forces

Dispersion forces are weak intermolecular forces that arise due to transient nonuniform electron distribution (temporary molecular dipoles).

Increased branching lowers an alkane’s boiling point
Branched-chain alkanes are more nearly spherical than straight-chain alkanes, thus have smaller surface areas, and consequently have smaller dispersion forces. Compounds Boiling Point Pentane 36.1 oC Isopentane 27.85 oC Neopentane 9.5oC Octane 125.7oC Isooctane 99.3oC

Solubility “Like dissolves like”
Alkanes are nonpolar; they dissolve best in solvents which are either nonpolar or weakly polar # Carbons 4 ~ # Hydrophilic sites 1

III. Cycloalkanes Naming Cycloalkanes Cis-Trans Isomerism

Overview Cycloalkanes – are alicyclic (aliphatic cyclic) compounds
They are alkanes that have carbon atoms that form a ring Their structure is shown as a regular polygon with the number of vertices equal to the number of C’s

Cycloalkanes – are rings of -CH2- units. – have the general formula
Simple Cycloalkanes Cycloalkanes – are rings of -CH2- units. – have the general formula CnH2n where n is an integer

Complex Cycloalkanes Naturally occurring materials contain cycloalkane structures Examples: chrysanthemic acid, prostaglandins, steroids (cyclopropane)

(cyclopentane) (cyclohexane and cyclopentane)

Properties of Cycloalkanes
Boiling points increase as ring size increases. Melting points are affected by the shapes and the way that crystals pack so they do not change uniformly

A. Naming Cycloalkanes Cycloalkanes are also named by the rules devised by the International Union of Pure and Applied Chemistry (IUPAC).

Steps to naming cycloalkanes1
Find the parent hydrocarbon Count the number of carbon atoms in the ring and the number in the largest substituent chain If # Cring = or > # Csubstituent, then it is named as an alkyl-substituted cycloalkane If # Cring < # Csubstituent, then it is named as an cycloalkyl-substituted alkane IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA

Number the substituents and write the name The correct sequence is when the substituents have the lowest possible number For an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible. When two or more substituents could potentially be assigned the same numbers, number them by alphabetical order. IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA

When two or more substituents could potentially be assigned the same numbers, number them by alphabetical order.

Practice Problem: Give IUPAC names for the following cycloalkanes

Practice Problem: Draw structures corresponding to the
Practice Problem: Draw structures corresponding to the following IUPAC names 1,1-Dimethylcyclooctane 3-Cyclobutylhexane 1,2-Dichlorocyclopentane 1,3-Dibromo-5-methylcyclohexane

Practice Problem: Name the following cycloalkane

B. Cis-Trans Isomerism In open-chain alkanes, free rotation is possible around C-C bonds because s bonds are cylindrically symmetrical:

In cycloalkanes, free rotation is limited by the ring structure
The common ring sizes (C3, C4, C5, C6, C7) are severely restricted in their molecular motions Larger cycloalkanes have increasingly more rotational freedom

Rings have two “faces” and substituents are labeled as to their relative facial positions
There are two different 1,2-dimethyl-cyclopropane isomers: one with the two methyls on the same side (cis) of the ring one with the methyls on opposite sides (trans)

Stereoisomers Stereoisomers – are compounds with atoms connected in the same order but which differ in three-dimensional orientation The terms “cis” and “trans” should be used to specify stereoisomeric ring structures

Constitutional isomers – are compounds with atoms connected in different order

Practice Problem: Name the following substances, including the
Practice Problem: Name the following substances, including the cis- or trans- prefix

Practice Problem: Draw the structures of the following molecules
trans-1-Bromo-3-methylcyclohexane cis-1,2-Dimethylcyclobutane trans-1-tert-Butyl-2-ethylcyclohexane

Practice Problem: Name the following substances, including the
Practice Problem: Name the following substances, including the cis- or trans- prefix (red-brown = Br)

IV. Bicyclic Alkane Systems
Naming Bicyclic Systems

Overview Bicycloalkanes – are alkanes containing two rings that share two carbon atoms Bridgehead atoms – are the shared carbon atoms Bridges – are the carbon chains connecting the bridgeheads

Bicycloalkanes – have the general formula
CnH2n-2 where n is an integer

Spiroalkanes – are cycloalkanes in which two rings share only one carbon atom

A. Naming Bicyclic Alkane Systems
The International Union of Pure and Applied Chemistry (IUPAC) has devised rules to name bicyclic alkane systems.

Steps to naming bicycloalkanes1
Find the parent name The parent name of a bicycloalkane is that of the unbranched alkane of the same number of carbon atoms as are in the bicyclic ring system Numbering begins at one bridgehead carbon atom IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA #1 #1

Number the bridges in order of decreasing size Longest bridge is written first Proceed along the longest bridge to the second bridgehead carbon, then along the next longest bridge back to the original bridgehead carbon, and so on until all carbon atoms are numbered. If there are two bridges of the same length, the correct sequence is when the substituents have the lowest possible number If there are more than one substituents, give an alphabetical preference. IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA

Show the bridge lengths in brackets Count the number of carbons linking the bridgeheads and place them in decreasing order in brackets between the prefix bicyclo and the parent name and with periods separating each number. IE – is the energy required to remove an electron from an atom or ion in the gas phase Atom in ground state (g) + energy  Atom+ (g) + e- Change in energy = ionization energy EA – is the energy involved when a gaseous atom absorbs an electron to form a negative ion of the element. Atom (g) + e-  Atom- Change in Energy = EA Bicyclo [3.2.0] heptane 7-methylbicyclo [4.2.0]octane

Chapter 3 The End

Alkanes and Cycloalkanes

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