Chapter 2Alkanes

Hydrocarbon: a compound composed of only carbon and hydrogen.

Alkanes: hydrocarbons containing only carbon-carbon single bonds.

H-C C-HH-C-C-HH

H

H

H HC C

H

H H

Hydrocarbons

Alkanes(Chapter 11)

Alkenes(Chapter 12)

Alkynes(Chapter 12)

Arenes(Chapter 13)

Only carbon-carbon single

bonds

One or more carbon-carbondouble bonds

One or morecarbon-carbontriple bonds

One or morebenzene-like

rings

Ethane Ethene(Ethylene)

Ethyne(Acetylene)

Benzene

The first two alkanes are methane and ethane.

H-C-HH

HH-C-C-HH

H

H

H

Methane Ethane

• Table 2.1 The first 10 alkanes with unbranched chains

CH4 CH4C2H6 CH3CH3C3H8 CH3CH2CH3C4H10 CH3(CH2)2CH3C5H12 CH3(CH2)3CH3

C6H14 CH3(CH2)4CH3C7H16 CH3(CH2)5CH3C8H18 CH3(CH2)6CH3C9H20 CH3(CH2)7CH3C10H22 CH3(CH2)8CH3

CH4 CH4C2H6 CH3CH3C3H8 CH3CH2CH3C4H10 CH3(CH2)2CH3C5H12 CH3(CH2)3CH3

C6H14 CH3(CH2)4CH3C7H16 CH3(CH2)5CH3C8H18C9H20C10H22

CondensedStructural Formula

MolecularFormulaName

decane

nonane

octane

heptane

hexane

pentane

butane

propane

ethanemethane

CondensedStructural Formula

MolecularFormulaName

CondensedStructural Formula

MolecularFormulaName

decane

nonane

octane

heptane

hexane

pentane

butane

propane

ethanemethane

CondensedStructural FormulaName

The IUPAC name of an alkane with an unbranched chain of carbon atoms consists of two parts: (1) a prefix: the number of carbon atoms in

the chain.(2) the suffix -ane: shows that the compound

is a saturated hydrocarbon.

Prefixmeth-eth-prop-but-pent-

hex-

oct-non-dec-

12345

67hept-8910

Number ofCarbon Atoms

Number ofCarbon AtomsPrefix

The name of an alkane with a branched chain of carbon atom consists of:a parent name: the longest chain of carbon

atomssubstituent names: the groups bonded to the

parent chain

CH3

CH3CH2CH2CHCH2CH2CH2CH38

substituent

4-Methyloctane

1 2 3 4 5 6 7 8

parent chain

12

34

5

6

7

Alkyl group: a substituent group derived from an alkane by removal of a hydrogen atom.commonly represented by the symbol R-.named by dropping the -ane from the name of

the parent alkane and adding the suffix -yl.

-CH2CH3

-CH3

-CH2CH2CH3

-CHCH3CH3

-CH2CH2CH2CH3

-CH2CHCH3CH3

-CHCH2CH3CH3

-CCH3

CH3

CH3

tert-butyl

sec-butyl

isobutyl

butyl

isopropyl

propyl

ethyl

methylName

CondensedStructural Formula

CondensedStructural FormulaName

If there is one substituent, number the parent chain from the end that gives the substituent the lower number.

CH3

CH3CH2CH2CHCH35

2-Methylpentane(not 4-methylpentane)

123

4

If the same substituent occurs more than once:Number the parent chain from the end that

gives the lower number to the substituent encountered first.

Indicate the number of times the substituent occurs by a prefix di-, tri-, tetra-, penta-, hexa-and so on.

Use a comma to separate position numbers.

CH3CH2CHCH2CHCH3

CH3 CH3

2,4-Dimethylhexane(not 3,5-dimethylhexane)

12

34

56

If there are two or more different substituents: list them in alphabetical order.number the chain from the end that gives the

lower number to the substituent encountered first.

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.

CH3CH2CHCH2CHCH2CH3

CH3

CH2CH3

12

34

56

7

3-Ethyl-5-methylheptane(not 3-methyl-5-ethylheptane)

Do not include the prefixes di-, tri-, tetra-, and so on, or the hyphenated prefixes sec- and tert- in alphabetizing; alphabetize the names of substituents first,

and then insert these prefixes

CH2CH3

CH3CCH2CHCH2CH3

CH3

CH34-Ethyl-2,2-dimethylhexane

(not 2,2-dimethyl-4-ethylhexane)

23

45

61

Halogens as substituent groupsSame priority as alkyl groupsFluoro, chloro, bromo, iodo

Common names; in this older system, • The number of carbon atoms determines the

name.• The first three alkanes are methane, ethane,

and propane.

• All alkanes of formula C4H10 are called butanes, all those of formula C5H12 are called pentanes, etc.

• For alkanes beyond propane, iso shows that one end of an otherwise unbranched chain terminates in (CH3)2CH-

• For more complex alkanes, use the IUPAC system.

Cycloalkanes

Cyclic hydrocarbon: a hydrocarbon that contains carbon atoms joined to form a ring.

Cycloalkane: a cyclic hydrocarbon in which all carbons of the ring are saturated. Cycloalkanes of ring sizes ranging from 3 to

over 30 carbon atoms are found in nature.

• Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant in nature.

Cyclopentane Cyclohexane

NomenclatureTo name a cycloalkane, prefix the name of the

corresponding open-chain alkane with cyclo-, and name each substituent on the ring.

• If there is only one substituent on the ring, there is no need to give it a location number.

• If there are two substituents, number the ring beginning with the substituent of lower alphabetical order.

Isopropylcyclopentane1-tert-Butyl-4-methylcyclohexane

1 4

Conformation: any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond.

• following are three conformations for a butane molecule.

Most crowdedconformation

rotate by 120°

rotate by 60°

Least crowdedconformation

Intermediatecrowding

Cyclopentane The most stable conformation of a

cyclopentane ring is an envelope conformation.

Cyclohexane

The most stable conformation of a cyclohexane ring is the chair conformation.all bond angles are approximately 109.5°.

In a chair conformation,six C-H bonds are equatorial.six C-H bonds are axial.

HHH

HHH

(a) Ball-and-stick modelshowing all 12 hydrogens

axis through thecenter of the ring

H H

H

H

H

H

(b) The six equatorialC-H bonds

(c) The six axial C-H bonds

• The more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial.

CH3

Equatorial methylcyclohexane

CH3

Axial methylcyclohexane

Cis-trans isomers

Cis: on the same side. Trans: across from each other.

Same molecular formula, same connectivity—different orientation of atoms in space. Stereoisomers.

Viewing a cyclopentane ring edge-on:

trans-1,2-Dimethyl-cyclopentane

cis-1,2-Dimethyl-cyclopentane

CH3

H

CH3

H

H

HH

H

HH

H

H

CH3

H3C

H

HH

HH

H

View from above:

CH3trans-1,2-Dimethyl-

cyclopentanecis-1,2-Dimethyl-

cyclopentane

H3C CH3 H3C

View of the cyclohexane ring as a planar hexagon:

trans-1,4-Dimethylcyclohexane cis-1,4-Dimethylcyclohexane

H

H3C

CH3

H

H

H3C

H

CH3

or or

CH3

CH3

CH3

CH3

Physical Properties

The most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity.

• The electronegativity difference between carbon and hydrogen is 2.5 - 2.1 = 0.4 on the Pauling scale.

• Given this small difference, we classify a C-H bond as nonpolar covalent.

• Alkanes are nonpolar compounds and the only interaction between their molecules are the very weak London dispersion forces.

Melting and boiling pointsBoiling points of alkanes are lower than those

of almost any other type of compound of the same molecular weight.

In general, both boiling and melting points of alkanes increase with increasing molecular weight.

CH4CH3CH3CH3CH2CH3CH3(CH2)2CH3

CH3(CH2)3CH3CH3(CH2)4CH3CH3(CH2)5CH3CH3(CH2)6CH3CH3(CH2)7CH3CH3(CH2)8CH3

methane

ethanepropane

butane

pentane

hexane

heptane

octanenonane

decane

Name

CondensedStructrualFormula

mp(°C)

bp(°C)

-182

-183

-190-138

-130

-95

-90

-57-51

-30

-164

-88

-420

36

69

98

126151

174

(a gas)

(a gas)

(a gas)(a gas)

0.626

0.659

0.684

0.7030.718

0.730

*For comparison, the density of H2O is 1 g/mL at 4°C.

Mol wt(amu)

16.0

30.144.1

58.1

72.2

86.2

100.2

114.2128.3142.3

Density of Liquid

(g/mL at 0° C)*

• Alkanes that are constitutional isomers are different compounds and have different physical and chemical properties.

bp (°C)Namehexane

2-methylpentane3-methylpentane

2,3-dimethylbutane

2,2-dimethylbutane

68.7

60.363.3

58.0

49.7

Hexane

2,2-Dimethylbutane

Solubility: a case of “like dissolves like”.Alkanes are not soluble in water; they are

unable to form hydrogen bonds with water.Alkanes are soluble in each other.Alkanes are also soluble in other nonpolar

organic compounds, such as toluene and diethyl ether.

Reactions

Oxidation (combustion)Oxidation of hydrocarbons, including alkanes

and cycloalkanes, is the basis for their use as energy sources for heat [natural gas, liquefied petroleum gas (LPG), and fuel oil] and power (gasoline, diesel fuel, and aviation fuel).

CH3CH2CH3 5O2 3CO2 4H2O++Propane

530 kcal/mol+

CH4 2O2 CO2 2H2OMethane

++ 212 kcal/mol+

Reaction with halogens (halogenation)Halogenation of an alkane is a substitution

reaction.

CH4 Cl2 CH3Cl HCl+ heator light

+Methane Chloromethane

(Methyl chloride)

CH3Cl Cl2 CH2Cl2 HCl+heat

+Dichloromethane

(Methylene chloride)

CH2Cl2Cl2

CHCl3Cl2

CCl4heat heatTrichloromethane

(Chloroform)Tetrachloromethane

(Carbon tetrachloride)

Summary of Topics: Chapter 2

NomenclatureCis-trans isomers

Conformations—particularly cyclohexane Properties (mp/bp; solubility) Reactions:

CombustionRadical halogenation