1

Chapter 3Structure and Stereochemistry

of Alkanes

Organic Chemistry, 5th EditionL. G. Wade, Jr.

Chapter 3 2

ALKANE FORMULAS•Four carbon/or hydogen atoms bonded toeach carbon atom

• All C-C single bonds•Ratio: CnH2n+2

NOTE: always an even number ofhydrogen atoms in a hydrocarbon

•Alkane homologs : each member in aalkane series different from the nextmember by a CH2 group

Chapter 3 3

CH3CH2CH2CH2CH2CH3CH3CH2CH2CH2CH3

C6H14

CH2

C5H12

Chapter 3 4

Lower Membered Alkanestrivial (common) names

• METHANE

• ETHANE

• PROPANE

CH4

Condensed Line-angle

CH3CH3

CH3CH2 CH3

2

Chapter 3 5

BUTANES

CH3CH2 CH2 CH3CH3CH CH3

CH3n-BUTANE iso-BUTANE

Note: 1 H hereChapter 3 6

Pentanes

CH

H

C

H

H

C

H

H

C

H

C

HH

H

H

H

n-pentane, C5H12

CHH

C CHH

C H

H

H

HH

CH

HH

isopentane, C5H12

=>

C

CH3

CH3H3C

CH3

neopentane, C5H12

Chapter 3 7

Writing Isomers -Heptanes

1. Start with longest straight chain possible

Obviously you do not add the extra carbon to end carbon!!!

2. Shorten chain by one carbon and add the "extra" at all possible posi

2-Methylhexane

2-Methylhexane

3-Methylhexane

3-Methylhexane Chapter 3 8

3. Shorten chain by one more carbon. This gives a 5-carbon chain (pentane). Two carbons are to be added as two methyl groups or a single two-carbon group (ethyl group).

THIS is 3-methylhexane!!

3

Chapter 3 9

Shorten chain by yet one more carbon. This gives a 4-carbon chain(butane). Three carbons are to be added as three methyl group.

NOTE: Can’t derive another butane chain by adding an ethyl group.

2,2,3-trimethylbutane3,3-dimethylpentane

Chapter 3 10

Common Names

• Isobutane, “isomer of butane”• Isopentane, isohexane, etc., methyl

branch on next-to-last carbon in chain.• Neopentane, most highly branched• Five possible isomers of hexane,

18 isomers of octane and 75 fordecane!

=>

Chapter 3 11

IUPAC Names• Find the longest continuous carbon

chain.• Number the carbons, starting closest to

the first branch.• Name the groups attached to the chain,

using the carbon number as the locator.• Alphabetize substituents.• Use di-, tri-, etc., for multiples of same

substituent. =>

Chapter 3 12

Longest Chain• The number of carbons in the longest

chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 81.)

• If there are two possible chains with the same number of carbons, use the chain with the most substituents.

C

CH3

CH2

CH3

CH CH2 CH2 CH3

CH CH2 CH3H3C

H3C

=>

4

Chapter 3 13

Number the Carbons• Start at the end closest to the first

attached group.• If two substituents are equidistant, look

for the next closest group.

1

2

3 4 5

6 7CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3

=>

Chapter 3 14

Name Alkyl Groups

• CH3-, methyl• CH3CH2-, ethyl• CH3CH2CH2-, n-propyl• CH3CH2CH2CH2-, n-butyl

CH3 CH CH2 CH3

sec-butyl

CH3 CH

CH3

CH2

isobutyl

CH3 CH CH3

isopropyl

CH3C

CH3

CH3tert-butyl

=>

Chapter 3 15

Propyl Groups

C

H

H

H

C

H

H

C

H

H

H

n-propyl

C

H

H

H

C

H

C

H

H

H

isopropyl

H

A primary carbon A secondary carbon

=>

Chapter 3 16

Butyl Groups

C

H

H

H

C

H

C

H

H

C

H

H

H

C

H

H

H

C

H

C

H

HH

C

H

H

n-butyl sec-butyl

H

H

A primary carbon A secondary carbon

=>

5

Chapter 3 17

Isobutyl Groups

CH

H

H

C

CH H

C

HH

H H

CH

H

H

C

CH H

C H

HH

HH

H

A primary carbon A tertiary carbon

=>

isobutyl tert-butyl

Chapter 3 18

Alphabetize

• Alphabetize substituents by name.• Ignore di-, tri-, etc. for alphabetizing.

CHH3C

CH3

CH

CH2CH3

CH2 CH2 CH

CH3

CH3

3-ethyl-2,6-dimethylheptane=>

Chapter 3 19

Complex Substituents• If the branch has a branch, number the

carbons from the point of attachment.• Name the branch off the branch using a

locator number.• Parentheses are used around the

complex branch name.1

2

31-methyl-3-(1,2-dimethylpropyl)cyclohexane =>

Chapter 3 20

Physical Properties• Solubility: hydrophobic• Density: less than 1 g/mL• Boiling points increase with

increasing carbons (little less for branched chains).

Melting points increase with increasing carbons (less for odd-number of carbons).

•

6

Chapter 3 21

Boiling Points of AlkanesBranched alkanes have less surface area contact,so weaker intermolecular forces.

=>

Chapter 3 22

Melting Points of AlkanesBranched alkanes pack more efficiently intoa crystalline structure, so have higher m.p.

=>

Chapter 3 23

Branched Alkanes

• Lower b.p. with increased branching• Higher m.p. with increased branching• Examples:

H

CH3CH

CH3

CH2 CH2 CH3

bp 60°Cmp -154°C

CH3CH

CH3

CHCH3

CH3bp 58°C

mp -135°C

=>

bp 50°Cmp -98°C

CH3 C

C 3

CH3

CH2 CH3

Chapter 3 24

Major Uses of Alkanes

• C1-C2: gases (natural gas)• C3-C4: liquified petroleum (LPG)• C5-C8: gasoline• C9-C16: diesel, kerosene, jet fuel• C17-up: lubricating oils, heating oil• Origin: petroleum refining

=>

7

Chapter 3 25

Reactions of Alkanes

• CombustionCH3CH2CH2CH3 + O2 CO2 + H2Oheat

8 10132

long-chain alkanes catalystshorter-chain alkanes

CH4 + Cl2 CH3Cl + CH2Cl2 CHCl3 CCl4+ +heat or light

=>

• Cracking and hydrocracking (industrial)

• Halogenation

Chapter 3 26

Conformers of Alkanes

• Structures resulting from the free rotation of a C-C single bond

• May differ in energy. The lowest-energy conformer is most prevalent.

• Molecules constantly rotate through all the possible conformations.

=>

Chapter 3 27

Ethane Conformers

• Staggered conformer has lowest energy.• Dihedral angle = 60 degrees

H

H

HH

H H

Newmanprojection

sawhorse

=>

modelChapter 3 28

Ethane Conformers (2)• Eclipsed conformer has highest energy• Dihedral angle = 0 degrees

=>

8

Chapter 3 29

Conformational Analysis• Torsional strain: resistance to rotation.• For ethane, only 3.0 kcal/mol

=>Chapter 3 30

Propane ConformersNote slight increase in torsional straindue to the more bulky methyl group.

=>

Chapter 3 31

Butane Conformers C2-C3• Highest energy has methyl groups eclipsed.• Steric hindrance• Dihedral angle = 0 degrees

=>totally eclipsed Chapter 3 32

Butane Conformers (2)

• Lowest energy has methyl groups anti.• Dihedral angle = 180 degrees

=>

anti

9

Chapter 3 33

Butane Conformers (3)• Methyl groups eclipsed with hydrogens• Higher energy than staggered

conformer• Dihedral angle = 120 degrees

=>eclipsedChapter 3 34

Butane Conformers (4)• Gauche, staggered conformer• Methyls closer than in anti conformer• Dihedral angle = 60 degrees

=>gauche

Chapter 3 35

Conformational Analysis

=>Chapter 3 36

Higher Alkanes• Anti conformation is lowest in energy.• “Straight chain” actually is zigzag.

CH3CH2CH2CH2CH3

CH C

CC

CH H H H

H H

H H

HH H =>

10

Chapter 3 37

Cycloalkanes• Rings of carbon atoms (CH2 groups)• Formula: CnH2n

• Nonpolar, insoluble in water• Compact shape• Melting and boiling points similar to

branched alkanes with same number of carbons =>

Chapter 3 38

Naming Cycloalkanes

• Cycloalkane usually base compound• Number carbons in ring if >1 substituent.• First in alphabet gets lowest number.• May be cycloalkyl attachment to chain.

CH2CH3

CH2CH3

CH3 =>

Chapter 3 39

Cis-Trans Isomerism

• Cis: like groups on same side of ring• Trans: like groups on opposite sides of ring

=>

Chapter 3 40

Cycloalkane Stability• 5- and 6-membered rings most stable• Bond angle closest to 109.5°• Angle (Baeyer) strain• Measured by heats of combustion

per -CH2 -=>

11

Chapter 3 41

Heats of Combustion Alkane + O2 → CO2 + H2O

Long-chain

157.4 157.4166.6 164.0

158.7 158.6

=>

158.3

Chapter 3 42

Cyclopropane• Large ring strain due to angle compression• Very reactive, weak bonds

=>

Chapter 3 43

Cyclopropane (2)

Torsional strain because of eclipsed hydrogens

=>Chapter 3 44

Cyclobutane• Angle strain due to compression• Torsional strain partially relieved by

ring-puckering

=>

12

Chapter 3 45

Cyclopentane• If planar, angles would be 108°, but all

hydrogens would be eclipsed.• Puckered conformer reduces torsional strain.

=>Chapter 3 46

Cyclohexane

• Combustion data shows it’s unstrained.• Angles would be 120°, if planar.• The chair conformer has 109.5° bond

angles and all hydrogens are staggered.• No angle strain and no torsional strain.

=>

Chapter 3 47

Chair Conformer

=>

Chapter 3 48

Boat Conformer

=>

13

Chapter 3 49

Conformational Energy

=>Chapter 3 50

Axial and Equatorial Positions

=>

Chapter 3 51

Monosubstituted Cyclohexanes

=>Chapter 3 52

1,3-Diaxial Interactions

=>

14

Chapter 3 53

Disubstituted Cyclohexanes

=>Chapter 3 54

Cis-Trans Isomers

Bonds that are cis, alternate axial-equatorial around the ring.

CH3

CH3

=>

Chapter 3 55

Bulky Groups• Groups like t-butyl cause a large energy

difference between the axial and equatorial conformer.

• Most stable conformer puts t-butyl equatorial regardless of other substituents.

=> Chapter 3 56

Bicyclic Alkanes

• Fused rings share two adjacent carbons.• Bridged rings share two nonadjacent C’s.

bicyclo[3.1.0]hexane

=>

bicyclo[2.2.1]heptane

15

Chapter 3 57

Cis- and Trans-Decalin

• Fused cyclohexane chair conformers• Bridgehead H’s cis, structure more flexible• Bridgehead H’s trans, no ring flip possible.

H

H

cis-decalin

H

H=>

trans-decalinChapter 3 58

End of Chapter 3