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William H. Brown Christopher S. Foote Brent L. Iverson

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1 William H. Brown Christopher S. Foote Brent L. Iverson
Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson

2 Alkanes and Cycloalkanes Chapter 2

3 Structure Hydrocarbon: a compound composed only of carbon and hydrogen
Saturated hydrocarbon: a hydrocarbon containing only single bonds Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain Aliphatic hydrocarbon: another name for an alkane

4 Hydrocarbons

5 Structure Shape tetrahedral about carbon
all bond angles are approximately 109.5°

6 Drawing Alkanes Line-angle formulas
an abbreviated way to draw structural formulas each vertex and line ending represents a carbon

7 Constitutional Isomerism
Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms example: C4H10

8 Constitutional Isomerism
do these formulas represent constitutional isomers? find the longest carbon chain number each chain from the end nearest the first branch compare chain lengths as well the identity and location of branches

9 Constitutional Isomerism
World population is about 6,000,000,000

10 Nomenclature - IUPAC Suffix -ane specifies an alkane
Prefix tells the number of carbon atoms

11 Nomenclature - IUPAC Parent name: the longest carbon chain
Substituent: a group bonded to the parent chain alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R-

12 Nomenclature - IUPAC 1.The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix 2. The parent chain is the longest chain of carbon atoms 3. Each substituent is given a name and a number 4. If there is one substituent, number the chain from the end that gives it the lower number

13 Nomenclature - IUPAC 5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc. use commas to separate position numbers

14 Nomenclature - IUPAC 6. If there are two or more different substituents, list them in alphabetical order number from the end of the chain that gives the substituent encountered first the lower number

15 Nomenclature - IUPAC 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization alphabetize the names of substituents first and then insert these prefixes

16 Nomenclature - IUPAC Alkyl groups

17 Nomenclature - Common The number of carbons in the alkane determines the name all alkanes with four carbons are butanes, those with five carbons are pentanes, etc. iso- indicates the chain terminates in -CH(CH3)2; neo- that it terminates in -C(CH3)3

18 Classification of C & H Primary (1°) C: a carbon bonded to one other carbon 1° H: a hydrogen bonded to a 1° carbon Secondary (2°) C: a carbon bonded to two other carbons 2° H: a hydrogen bonded to a 2° carbon Tertiary (3°) C: a carbon bonded to three other carbons 3° H: a hydrogen bonded to a 3° carbon Quaternary (4°) C: a carbon bonded to four other carbons

19 Cycloalkanes General formula CnH2n Structure and nomenclature
five- and six-membered rings are the most common Structure and nomenclature to name, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring if only one substituent, no need to give it a number if two substituents, number from the substituent of lower alphabetical order if three or more substituents, number to give them the lowest set of numbers and then list substituents in alphabetical order

20 Cycloalkanes Line-angle drawings each line represents a C-C bond
each vertex and line ending represents a C

21 Cycloalkanes Example: name these cycloalkanes

22 Bicycloalkanes Bicycloalkane: an alkane that contains two rings that share two carbons Bicyclo[4.4.0]decane (Decalin) Bicyclo[4.3.0]nonane (Hydrindane) Bicyclo[2.2.1]heptane (Norbornane)

23 Bicycloalkanes Nomenclature
parent is the alkane of the same number of carbons as are in the rings number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc. show the lengths of bridges in brackets, from longest to shortest

24 IUPAC - General prefix-infix-suffix
prefix tells the number of carbon atoms in the parent infix tells the nature of the carbon-carbon bonds suffix tells the class of compound Nature of Carbon-Carbon Bonds in the Parent Chain Suffix Class Infix -e hydrocarbon -an- all single bonds -ol alcohol -en- one or more double bonds -al aldehyde -yn- one or more triple bonds -amine amine -one ketone -oic acid carboxylic acid

25 IUPAC - General 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

26 Conformations Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond Newman projection: a way to view a molecule by looking along a carbon-carbon single bond

27 Conformations Staggered conformation: a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon

28 Conformations Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon

29 Conformations Torsional strain also called eclipsed interaction strain
strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol

30 Conformations Dihedral angle (Q): the angle created by two intersecting planes

31 Conformations Ethane as a function of dihedral angle

32 Conformations The origin of torsional strain in ethane
originally thought to be caused by repulsion between eclipsed hydrogen nuclei alternatively, caused by repulsion between electron clouds of eclipsed C-H bonds theoretical molecular orbital calculations suggest that the energy difference is not caused by destabilization of the eclipsed conformation but rather by stabilization of the staggered conformation this stabilization arises from the small donor-acceptor interaction between a C-H bonding MO of one carbon and the C-H antibonding MO on an adjacent carbon; this stabilization is lost when a staggered conformation is converted to an eclipsed conformation

33 Conformations anti conformation
a conformation about a single bond in which the groups lie at a dihedral angle of 180°

34 Conformations Steric strain (nonbonded interaction strain):
the strain that arises when atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow Angle strain: strain that arises when a bond angle is either compressed or expanded compared to its optimal value The total of all types of strain can be calculated by molecular mechanics programs such calculations can determine the lowest energy arrangement of atoms in a given conformation, a process called energy minimization

35 Conformations conformations of butane as a function of dihedral angle

36 Anti Butane Energy-minimized anti conformation
the C-C-C bond angle is 111.9° and all H-C-H bond angles are between and 107.9° the calculated strain is 9.2 kJ (2.2 kcal)/mol

37 Eclipsed Butane calculated energy difference between (a) the non-energy-minimized and (b) the energy-minimized eclipsed conformations is 5.6 kJ (0.86 kcal)/mol

38 Cyclopropane angle strain: the C-C-C bond angles are compressed from 109.5° to 60° torsional strain: there are 6 sets of eclipsed hydrogen interactions strain energy is about 116 kJ (27.7 kcal)/mol

39 Cyclobutane puckering from planar cyclobutane reduces torsional strain but increases angle strain the conformation of minimum energy is a puckered “butterfly” conformation strain energy is about 110 kJ (26.3 kcal)/mol

40 Cyclopentane puckering from planar cyclopentane reduces torsional strain, but increases angle stain the conformation of minimum energy is a puckered “envelope” conformation strain energy is about 42 kJ (6.5 kcal)/mol

41 Cyclohexane Chair conformation: the most stable puckered conformation of a cyclohexane ring all bond C-C-C bond angles are 110.9° all bonds on adjacent carbons are staggered

42 Cyclohexane In a chair conformation, six H are equatorial and six are axial

43 Cyclohexane For cyclohexane, there are two equivalent chair conformations all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa

44 Cyclohexane Boat conformation: a puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other there are four sets of eclipsed C-H interactions and one flagpole interaction a boat conformation is less stable than a chair conformation by 27 kJ (6.5 kcal)/mol

45 Cyclohexane 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

46 Cyclohexane

47 Methylcyclohexane Equatorial and axial methyl conformations

48 G0 axial ---> equatorial
given the difference in strain energy between axial and equatorial conformations, it is possible to calculate the ratio of conformations using the following relationship

49 Cis,Trans Isomerism Stereoisomers: compounds that have
the same molecular formula the same connectivity a different orientation of their atoms in space Cis,trans isomers stereoisomers that are the result of the presence of either a ring (this chapter) or a carbon-carbon double bond (Chapter 5)

50 Isomers relationships among isomers

51 Cis,Trans Isomers 1,2-Dimethylcyclopentane

52 Cis,Trans Isomerism 1,4-Dimethylcyclohexane

53 Cis,Trans Isomerism trans-1,4-Dimethylcyclohexane
the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = kJ/mol

54 Cis,Trans Isomerism cis-1,4-Dimethylcyclohexane

55 Cis,Trans Isomerism The decalins

56 Steroids The steroid nucleus Cholestanol

57 Bicycloalkanes Norbornane drawn from three different perspectives

58 Bicycloalkanes Adamantane

59 Physical Properties Intermolecular forces of attraction (example)
ion-ion (Na+ and Cl- in NaCl) ion-dipole (Na+ and Cl- solvated in aqueous solution) dipole-dipole and hydrogen bonding dispersion forces (very weak electrostatic attraction between temporary dipoles)

60 Physical Properties Low-molecular-weight alkanes (methane....butane) are gases at room temperature Higher molecular-weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature

61 Physical Properties Constitutional isomers have different physical properties

62 Oxidation of Alkanes Oxidation is the basis for their use as energy sources for heat and power heat of combustion: heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water

63 Heat of Combustion Heat of combustion for constitutional isomers

64 Heats of Combustion For constitutional isomers [kJ (kcal)/mol]
( ) ( ) (1304.6) (1303.0) 8 C O 2 + 9 H 2 O

65 Heat of Combustion strain in cycloalkane rings as determined by heats of combustion

66 Sources of Alkanes Natural gas Petroleum Coal 90-95% methane
gases (bp below 20°C) naphthas, including gasoline (bp °C) kerosene (bp °C) fuel oil (bp °C) lubricating oils (bp above 350°C) asphalt (residue after distillation) Coal

67 Gasoline Octane rating: the percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties

68 Synthesis Gas A mixture of carbon monoxide and hydrogen in varying proportions which depend on the means by which it is produced

69 Synthesis Gas Synthesis gas is a feedstock for the industrial production of methanol and acetic acid it is likely that industrial routes to other organic chemicals from coal via methanol will also be developed

70 Alkanes and Cycloalkanes
End Chapter 2

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