Alkanes

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AromaticAromaticAromaticAromaticAliphaticAliphaticAliphaticAliphatic

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AromaticAromaticAromaticAromaticAliphaticAliphaticAliphaticAliphatic

AlkanesAlkanesAlkanesAlkanes AlkynesAlkynesAlkynesAlkynesAlkenesAlkenesAlkenesAlkenes

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AliphaticAliphaticAliphaticAliphatic

AlkanesAlkanesAlkanesAlkanes

Alkanes are hydrocarbons in which all of the bonds are single bonds.

CC CCHH HH

HH HH

HH HH

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AliphaticAliphaticAliphaticAliphatic

AlkenesAlkenesAlkenesAlkenes

Alkenes are hydrocarbons that contain a carbon-carbon double bond.

CC CC

HH HH

HH HH

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AliphaticAliphaticAliphaticAliphatic

AlkynesAlkynesAlkynesAlkynes

Alkynes are hydrocarbons that contain a carbon-carbon triple bond.

HCHC CHCH

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AromaticAromaticAromaticAromatic

The most common aromatic hydrocarbons are those that contain a benzene ring.

HH

HH

HH

HH

HH

HH

CCnnHH22nn+2+2CCnnHH22nn+2+2

Introduction to Alkanes:Methane, Ethane, and Propane

Methane (CH4) CH4 Ethane (C2H6) CH3CH3

Propane (C3H8) CH3CH2CH3

bp -160°Cbp -160°C bp -89°Cbp -89°C bp -42°Cbp -42°C

The Simplest Alkanes

tetrahedralbond angles = 109.5°bond distances = 110 pmbut structure seems inconsistent withelectron configuration of carbon

Structure of Methane

Structure of EthaneStructure of Ethane

CHCH33CHCH33

CC22HH66

tetrahedral geometry at each carbon

C—H bond distance = 110 pm

C—C bond distance = 153 pm

n-Butane CH3CH2CH2CH3

Isobutane (CH3)3CH

bp -0.4°Cbp -0.4°C bp -10.2°Cbp -10.2°C

nn-Pentane-Pentane

CHCH33CHCH22CHCH22CHCH22CHCH33

IsopentaneIsopentane

(CH(CH33))22CHCHCHCH22CHCH33

NeopentaneNeopentane

(CH(CH33))44CC

CC55HH1212CC55HH1212

Number of Constitutionally Isomeric Alkanes

CH4 1 C8H18 18

C2H6 1 C9H20 35

C3H8 1 C10H22 75

C4H10 2 C15H32 4,347

C5H12 3 C20H42 366,319

C6H14 5 C40H82 62,491,178,805,831

C7H16 9

Nomenclature of Alkanes Rules for naming compounds are given by the

International Union for Pure and Applied Chemistry (IUPAC).

To name alkanes: Find the longest chain and use it as the name of the

compound. Number the carbon atoms starting with the end

closest to the substituent. Name and give the location of each substituent.

When two or more substituents are present list them in alphabetical order.

Nomenclature of Alkanes

1. Determine the number of carbons in the parent hydrocarbon

CH3CH2CH2CH2CHCH2CH2CH3

CH3

12345678

CH3CH2CH2CH2CHCH2CH3

CH2CH2CH3

45678

123

CH3CH2CH2CHCH2CH2CH3

CH2CH2CH2CH3

1234

5 6 7 8

2. Number the chain so that the substituent gets the lowest possible number

CH3CHCH2CH2CH3

CH3

1 2 3 4 5

2-methylpentane

CH3CH2CH2CHCH2CH2CH2CH3

CHCH3

CH3

1 2 3 4 5 6 7 8

4-isopropyloctane

CH3CHCH2CH2CH3

CH3

common name: isohexanesystematic name: 2-methylpentane

3. Number the substituents to yield the lowest possible number in the number of the compound

CH3CH2CHCH2CHCH2CH2CH3

CH3 CH2CH3 5-ehtyl-3-methyloctanenot

4-ethyl-6-methyloctanebecause 3<4

(substituents are listed in alphabetical order)

4. Assign the lowest possible numbers to all of the substituents

CH3CH2CHCH2CHCH3

CH3CH3

2,4-dimethylhexane

CH3CH2CH2C

CH3

CH3

CCH2CH3

CH3

CH3

3,3,4,4-tetramethylheptane

CH3CH2CHCH2CH2CHCHCH2CH2CH3

CH2CH3

CH2CH3 CH2CH3

CH3

3,3,6-triethyl-7-methyldecane

5. When both directions lead to the same lowest number for oneof the substituents, the direction is chosen that gives the lowest possible number to one of the remaining substituents

CH3CHCH2CHCH3

CH3

CH3 CH3

2,2,4-trimethylpentanenot

2,4,4-trimethylpentanebecause 2<4

CH3CH2CHCHCH2CHCH2CH3

CH3

CH3 CH2CH3

6-ethyl-3,4-dimethyloctanenot

3-ethyl-5,6-dimethyloctanebecause 4<5

6. If the same number is obtained in both directions, the firstgroup receives the lowest number

CH3CH2CHCH2CHCH2CH3

CH3

CH2CH3

3-ethyl-5-methylheptanenot

5-ethyl-3-methylheptane

CH3CH2CHCH3

Cl

Br

2-bromo-3-chlorobutanenot

3-bromo-2-chlorobutane

7. In the case of two hydrocarbon chains with the same number ofcarbons, choose the one with the most substituents

CH3CH2CHCH2CH2CH3

CHCH3

CH31

2

3 4 5 6

3-ethyl-2-methylhexane (two substituents)

CH3CH2CHCH2CH2CH3

CHCH3

CH3

1 2 3 4 5 6

3-isopropylhexane (one substituent)

8. Certain common nomenclatures are used in the IUPAC system

CH3CH2CH2CH2CHCH2CH2CH3

CHCH3

CH3

4-isopropyloctaneor

4-(1-methylethyl)octane

CH3CH2CH2CH2CHCH2CH2CH2CH2CH3

CH2CHCH3

CH3

5-isobutyldecaneor

5-(2-methylpropyl)decane

Nomenclature of AlkanesNomenclature of Alkanes

Types of Carbons

primary (1o) - bonded to only 1 other carbon

secondary (2o) - bonded to 2 other carbons

tertiary (3o) - bonded to 3 other carbons quaternary (4o) - bonded to 4 other

carbons

CCH3

CH3

CH3

CH2 CH CH3

CH3

Conformations are different spatial Conformations are different spatial arrangements of a molecule that are arrangements of a molecule that are

generated by rotation about single generated by rotation about single bonds.bonds.

Conformations are different spatial Conformations are different spatial arrangements of a molecule that are arrangements of a molecule that are

generated by rotation about single generated by rotation about single bonds.bonds.

Eclipsed conformationEclipsed conformationEclipsed conformationEclipsed conformation

EthaEthanene

EthaEthanene

Staggered conformationStaggered conformationStaggered conformationStaggered conformation

EthaEthanene

EthaEthanene

Projection formulas of the staggeredconformation of ethane

NewmanNewman SawhorseSawhorse

HH

HH

HH HH

HH HH

HH

HH HHHH

HH

HH

Anti relationships

HH

HH

HH HH

HH HH

HH

HH HHHH

HH

HH

Two bonds are anti when the angle between them is 180°. Two bonds are anti when the angle between them is 180°.

180°180°

Gauche relationshipsGauche relationships

HH

HH

HH HH

HH HH

HH

HH HHHH

HH

HH

Two bonds are gauche when the angle between them is 60°. Two bonds are gauche when the angle between them is 60°.

60°60°

An important point:An important point:

The terms anti and gauche applyThe terms anti and gauche apply

only to bonds (or groups) on only to bonds (or groups) on adjacentadjacent

carbonscarbons, and only to , and only to staggeredstaggered

conformations.conformations.

0°0° 60° 60° 120° 120° 180° 240° 180° 240° 300°300° 360°360°

12 kJ/mol12 kJ/mol12 kJ/mol12 kJ/mol

Conformational Analysis of Butane: C2-C3 Rotation

The most stable conformation of unbranchedalkanes has anti relationships between carbons

HexaneHexane

Reactions with AlkanesReactions with Alkanes

The C-C and C-H bonds are very strong. Therefore, alkanes are very unreactive.

At room temperature alkanes do not react with acids, bases, or strong oxidizing agents.

Alkanes do combust in air (making them good fuels):

2C2H6(g) + 7O2(g) 4CO2(g) + 6H2O(l) H = -2855 kJ

Alkanes are very unreactive compounds because they have only strong s bonds and atoms with no partial charges

However, alkanes do react with Cl2 and Br2

Reaction of Alkane with Cl2 or Br2

Petroleum is a complex mixture of alkanes and cycloalkanes that can be separated by distillation