Chapter 3Structure and Stereochemistry

of Alkanes

Organic Chemistry, 6th EditionL. G. Wade, Jr.

Jo BlackburnRichland College, Dallas, TX

Dallas County Community College District2006,Prentice Hall

Chapter 3 2

Classification Review

Chapter 3 3

Alkane Formulas• All C-C single bonds

• Saturated with hydrogens

• Ratio: CnH2n+2

• Alkane homologs: CH3(CH2)nCH3

• Same ratio for branched alkanes

=>

C

H

C

H

H

H C H

H

HC H

H

H

Isobutane, C4H10

C

H

C

H

H

H C C

H

HH H

H

H

Butane, C4H10

Chapter 3 4

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 for decane! =>

Chapter 3 5

Alkane Examples

=>

Chapter 3 6

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 7

Longest Chain• The number of carbons in the longest

chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 82.)

• 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 CH3

H3C

H3C

=>

Chapter 3 8

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 9

Name Alkyl Groups

• CH3-, methyl

• CH3CH2-, ethyl

• CH3CH2CH2-, n-propyl

• CH3CH2CH2CH2-, n-butylCH3 CH CH2 CH3

sec-butyl

CH3 CH

CH3

CH2

isobutyl

CH3 CH CH3

isopropyl

CH3C

CH3

CH3

tert-butyl

=>

Chapter 3 10

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 11

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

=>

Chapter 3 12

Isobutyl Groups

CH

H

H

C

CH H

C

HH

H H

CH

H

H

C

CH H

C H

HH

H

H

H

A primary carbon A tertiary carbon

=>

isobutyl tert-butyl

Chapter 3 13

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 14

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.

12

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

Chapter 3 15

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).

•

Chapter 3 16

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

=>

Chapter 3 17

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

=>

Chapter 3 18

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

CH3 bp 58°Cmp -135°C

=>

bp 50°Cmp -98°C

CH3 C

C 3

CH3

CH2 CH3

Chapter 3 19

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 =>

Chapter 3 20

Reactions of Alkanes

• CombustionCH3CH2CH2CH3 + O2 CO2 + H2O

heat8 10132

long-chain alkanes catalyst

shorter-chain alkanes

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

=>

• Cracking and hydrocracking (industrial)

• Halogenation

Chapter 3 21

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 22

Ethane Conformers

• Staggered conformer has lowest energy.

• Dihedral angle = 60 degrees

H

H

HH

H H

Newmanprojection

sawhorse

=>

model

Chapter 3 23

Ethane Conformers (2)• Eclipsed conformer has highest energy

• Dihedral angle = 0 degrees

=>

Chapter 3 24

Conformational Analysis• Torsional strain: resistance to rotation.

• For ethane, only 12.6 kJ/mol

=>

Chapter 3 25

Propane ConformersNote slight increase in torsional strain

due to the more bulky methyl group.

=>

Chapter 3 26

Butane Conformers C2-C3

• Highest energy has methyl groups eclipsed.

• Steric hindrance

• Dihedral angle = 0 degrees

=>totally eclipsed

Chapter 3 27

Butane Conformers (2)

• Lowest energy has methyl groups anti.

• Dihedral angle = 180 degrees

=>

anti

Chapter 3 28

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

conformer• Dihedral angle = 120 degrees

=>eclipsed

Chapter 3 29

Butane Conformers (4)

• Gauche, staggered conformer

• Methyls closer than in anti conformer

• Dihedral angle = 60 degrees

=>gauche

Chapter 3 30

Conformational Analysis

=>

Chapter 3 31

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 =>

Chapter 3 32

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 33

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 34

Cis-Trans Isomerism

• Cis: like groups on same side of ring

• Trans: like groups on opposite sides of ring =>

Chapter 3 35

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 - =>

Chapter 3 36

Heats of Combustion/CH2 Alkane + O2 CO2 + H2O

Long-chain

658.6 kJ 658.6

697.1 686.1664.0 663.6 kJ/mol

=>

662.4

Chapter 3 37

Cyclopropane• Large ring strain due to angle compression

• Very reactive, weak bonds

=>

Chapter 3 38

Cyclopropane (2)

Torsional strain because of eclipsed hydrogens

=>

Chapter 3 39

Cyclobutane• Angle strain due to compression

• Torsional strain partially relieved by ring-puckering

=>

Chapter 3 40

Utilizando los datos de la tabla dada a continuación. Demuestre cuantitativamente que la tensión total en ciclobutano es aproximadamente 26.4 kcal/mol. Describa los factores que contribuyen a esta tensión en ciclobutano. • cicloalcano Ho

comb (kcal/mol)

Tensión Total (kcal/mol)

ciclopropano 499.8 27.6

ciclobutano 655.9 26.4

ciclopentano 793.5 6.5

ciclohexano 944.5 0

cicloheptano 1108.3 6.3

ciclooctano 1268.9 9.6

Chapter 3 41

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

hydrogens would be eclipsed.

• Puckered conformer reduces torsional strain.

=>

Chapter 3 42

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 43

Chair Conformer

=>

Chapter 3 44

Boat Conformer

=>

Chapter 3 45

Conformational Energy

=>

Chapter 3 46

Axial and Equatorial Positions

=>

Chapter 3 47

Monosubstituted Cyclohexanes

=>

Chapter 3 48

1,3-Diaxial Interactions

=>

Chapter 3 49

Disubstituted Cyclohexanes

=>

Chapter 3 50

Cis-Trans Isomers

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

=>

CH3

CH3

One axial, one equatorial

Chapter 3 51

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 52

• 3) Considere el siguiente ciclohexano sustituído:

• a) Dibuje la conformación silla correspondiente.

• (b) Haga la representación Newman correspondiente. (Recuerde especificar la perspectiva que representa)

• c) Dibuje ambas sillas (interconversión de los

confórmeros). Señale la que considere más estable.

• d) Utilizando los valores de la tabla de Go de sustituyentes, calcule el Go para el proceso de interconversión.

• e) Calcule el por ciento de cada confórmero

Sust Go kcal/mol

CH3 1.74

OCH3 0.75

CH(CH3)2 2.61

CH(CH3

)2

H3

C

OCH3

Chapter 3 53

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

Chapter 3 54

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-decalin

Chapter 3 55

Bicyclo[4.4.0]decane

=>

Chapter 3 56

End of Chapter 3