Alkanes

Physical and Chemical Properties

Attractive Forces Acting Between Ions & Molecules“Secondary Forces”

Pure Electrostatic Attractions - chiefly ionic compounds

Dipole-dipole attractions - chiefly between molecules

δ−δ+ δ−δ+ δ−δ+

δ−δ+ δ−δ+ δ−δ+ δ−δ+

δ−δ+

Dispersion Forces - Very weak, due to a temporary shift in electron distribution dependent on the size of the molecule

Attractive Forces Acting Between Ions & Molecules

Physical Properties of Alkanes

Alkanes are Nonpolar Only dispersion forces act between molecules.

Most alkanes are insoluble in water.

Boiling points, melting points, and densities generally increase with the size of the alkane:

Decane > Heptane > Butane

Physical Properties of Alkanes

Compound B.P. ºC

CH3CH3 -89

CH3(CH2)3CH3 36

CH3(CH2)7CH3 151

CH3(CH2)11CH3 235

CH3(CH2)16CH3 316

Physical Properties of Alkanes

Boiling points generally decrease with increased branching as a result of decreased surface area. Consider C9H20

CH3CH2CH2CH2CH2CH2CH2CH2CH3

H3C CHCH2CH2CH2CH

CH3

CH3

CH3

H3C C

CH3

CH3

CH2 C

CH3

CH3

CH3

Physical Properties of Alkanes

3-D Structures of Isomeric Nonanes

n-nonane, bp 151ºC

2,6-dimethylheptane, bp 135ºC

2,2,4,4-tetramethylpentane, bp 122ºC

Physical Properties of AlkanesThe Basis for Fractional Distillation

Hot crude oil is fed in at t h e f o o t o f t h e fractionating column and mixed with steam. Various constituents, or fractions, of the crude oil separate at various points of the column.

# C’s1-4

7

13

16

19

22

25+

Physical Properties of AlkanesThe Basis for Fractional Distillation

Physical Properties of Alkanes

Comparison of Boiling Points for Alkanes and Alcohols

Compound B.P. ºC Compound B.P. ºC

CH3CH3 -89 CH3-OH 65

CH3(CH2)3CH3 36 CH3(CH2)3-OH 118

CH3(CH2)7CH3 151 CH3(CH2)7-OH 194

CH3(CH2)11CH3 235 CH3(CH2)11-OH 255

CH3(CH2)16CH3 316 CH3(CH2)16-OH 308

Note: All organic molecules may have “Alkane-like” portions of their structures which confer “hydrocarbon-like” properties on the molecules

Boiling involves breakdown of

dispersion forces + polar forces.

Boiling involves breakdown of

dispersion forces

Physical Properties of AlkanesComparison of the Boiling of Ethane (a hydrocarbon) and Methanol (an alcohol)

methanol, bp +65ºC

ethane, bp -89ºC

Boiling involves breakdown of

dispersion forcesBoiling involves

breakdown of dispersion forces +

polar forces.

Physical Properties of Alkanes

Comparison of the Boiling of a Hydrocarbon and an Alcohol

Liquid Gas

Liquid Gas

However, the boiling points are very

similar....Why ??

Physical Properties of Alkanes

Comparison of Boiling Points for Alkanes and Alcohols

Compound B.P. ºC Compound B.P. ºC

CH3CH3 -89 CH3-OH 65

CH3(CH2)3CH3 36 CH3(CH2)3-OH 118

CH3(CH2)7CH3 151 CH3(CH2)7-OH 194

CH3(CH2)11CH3 235 CH3(CH2)11-OH 255

CH3(CH2)16CH3 316 CH3(CH2)16-OH 308

Note: All organic molecules may have “Alkane-like” portions of their structures which confer “hydrocarbon-like” properties on the molecules

Chemical Reactions of

Alkanes

Chemical Reactions of Alkanes

Combustion - Burning in Oxygen

“Propane burns in oxygen to yield carbon dioxide and water.”

C3H8 + O2 ---------------> CO2 + H2O

“Propane burns in oxygen to yield carbon dioxide and water.”“Propane burns in oxygen to yield carbon dioxide and water.”

C3H8 + O2 ---------------> CO2 + H2O

C3H8 + ? O2 ---------------> 3 CO2 + 4 H2O

C3H8 + 5 O2 ---------------> 3 CO2 + 4 H2O

Chemical Reactions of Alkanes

“Butane burns in oxygen to yield carbon dioxide and water.”

C4H10 + O2 ---------------> CO2 + H2O

“Butane burns in oxygen to yield carbon dioxide and water.”“Butane burns in oxygen to yield carbon dioxide and water.”

C4H10 + O2 ---------------> CO2 + H2O

C4H10 + ? O2 ---------------> 4 CO2 + 5 H2O

2 C4H10 + ? O2 ---------------> 8 CO2 + 10 H2O

2 C4H10 + 13 O2 ---------------> 8 CO2 + 10 H2O

Combustion - Burning in Oxygen

Chemical Reactions of AlkanesHalogenation - Reaction with Halogen + UV light

“Methane reacts with chlorine gas in the presence of ultraviolet light to yield chloromethane and hydrogen chloride gas.”

H C H

H

H

Cl H C

H

H

H+ +Cl Cl Cl

uv light

“Methane reacts with chlorine gas in the presence of ultraviolet light to yield chloromethane and hydrogen chloride gas.”

CH4 + Cl2 CH3Cl + HCluv light

CH4 CH3ClCl2

uv light

Chemical Reactions of AlkanesHalogenation - Reaction with Halogen + UV light

CH4 CH3ClCl2

uv light

H C H

H

H

Cl2

uv light H C

H

H

Cl

A SUBSTITUTION REACTION

Chemical Reactions of Alkanes

Halogenation

“Ethane reacts with chlorine gas in the presence of ultraviolet light to yield chloroethane and hydrogen chloride gas.”

H C C

H

H

Cl2

uv light C C

H

H

ClH

H

H

H

H

H

C2H6 C2H5ClCl2

uv light

A SUBSTITUTION REACTIONDoes it make any difference which hydrogen is exchanged in

the substutution reaction ?

Chemical Reactions of Alkanes

Note: It makes no difference which of the hydrogens in methane or ethane is replaced because all of the hydrogens within each compound

are identical.

Methane Ethane

Chemical Reactions of Alkanes

Structures of Halogenated Alkanes

1-chloropropane

chloromethane

2-chloropropane

chloroethane

Chemical Reactions of Alkanes

“Propane reacts with chlorine gas in the presence of ultraviolet light to yield two different compounds with the formula C3H7Cl.”

Halogenation

Cl2

uv lightH C C

H

H

C

H

H

H

H

H

H C C

H

H

C

H

H

H

Cl

Hor

H C C

H

H

C

H

Cl

H

H

H

1-chloropropane

2-chloropropane

bp 46ºC

bp 36ºC

45%

55%

Chemical Reactions of Alkanes

Structures of Halogenated Alkanes

1-chloropropane

chloromethane

2-chloropropane

chloroethane

Halogenation - Chlorination

Chemical Reactions of Alkanes

Halogenation - Chlorination vs Bromination

Chemical Reactions of Alkanes

1-chloropropane

2-chloropropane

45%

55%

Cl2

uv lightH C C

H

H

C

H

H

H

H

H

H C C

H

H

C

H

H

H

Hor

H C C

H

H

C

H H

H

H

Cl

Cl

Br2

uv lightH C C

H

H

C

H

H

H

H

H

H C C

H

H

C

H

H

H

Hor

H C C

H

H

C

H H

H

H

Br

Br

1-bromopropane

2-bromopropane

3%

97%Bromine is more selective !!

Halogenation - Bromination

Chemical Reactions of Alkanes

Substitution takes place at the carbon which has the fewest hydrogens in the

starting material !!