matriculation chemistry ( hydrocarbon ) part 2 alkane

Physical properties of Physical properties of alkanes & cycloalkanesalkanes & cycloalkanes

Physical Physical statestate

Boiling Point

Solubility

Physical statePhysical state At room temperature (25At room temperature (25ooC) and atmospheric C) and atmospheric

pressure (1 atm), for unbranched alkanes,pressure (1 atm), for unbranched alkanes,

CC11 – C – C4 4 : gases: gases

CC55 – C – C1717 : liquids : liquids

CC18 18 - more : solids- more : solids

Boiling pointsBoiling points

The boiling points of the straight alkanes The boiling points of the straight alkanes show a show a regular increase with increasing regular increase with increasing molecular weightmolecular weight. .

BranchingBranching of the alkanes chain, of the alkanes chain, lower the lower the boiling point.boiling point.

Mr ↑ boiling point ↑

C-H is non polar bond C-H is non polar bond Intermolecular forces exist – Intermolecular forces exist – London London

dispersiondispersionThe The London dispersionLondon dispersion forces increase : forces increase : As molecular weight increases,As molecular weight increases, Molecular size increasesMolecular size increases Molecular surface area increaseMolecular surface area increase

Therefore, more energy is required to Therefore, more energy is required to separate molecules from one anotherseparate molecules from one another

Result - a higher boiling point.Result - a higher boiling point.

Mr ↑ boiling point ↑

Chain branching Chain branching :: makes a molecule more compactmakes a molecule more compact Surface area reducesSurface area reduces The strength of the London dispersion The strength of the London dispersion

forces reduceforces reduce Lower boiling points.Lower boiling points.

Isomeric Alkanes

Have different boiling point due to branching

PentanePentane

CHCH33CHCH22CHCH22CHCH22CHCH33

3737ooCC

2-methylbutane2-methylbutane

CHCH33CHCHCHCH22CHCH

CHCH33

28.528.5ooCC

2,2-2,2-dimethylpropanedimethylpropane

CHCH33

CHCH33CCHCCH33

CHCH33

99ooCC

CH3CH2CH2CH3

CH3CH2CH2CH3

CH3CHCH3

CH3

CH3CHCH3

CH3

2-methylpropane

butane

For example, butane and 2-methylpropane both have a molecular formula C4H10, but the atoms are arranged differently

In butane the C atoms are arranged in a single chain, but 2-methylpropane is a shorter chain with a branch

Name Molecular structureBoiling point

Butane has a higher boiling point because the dispersion forces are greater.

The molecules are longer (and so set up bigger temporary dipoles) and can lie closer together than the shorter, fatter 2-methylpropane molecules.

Cycloalkanes

The boiling points of cycloalkanes are 10oC to 15oC higher than the corresponding straight chain alkanes.

CycloalkaneCycloalkane Boiling pointBoiling point alkanealkane Boiling Boiling pointpoint

CyclobutaneCyclobutane 1313ooCC ButaneButane -0.5-0.5ooCC

CyclopentaneCyclopentane 4949ooCC PentanePentane 36..336..3ooCC

SolubilitySolubility

Alkanes – less dense than waterAlkanes – less dense than water

Alkanes and cycloalkanes are almost Alkanes and cycloalkanes are almost totally insoluble in watertotally insoluble in water

They are They are non-polar non-polar moleculemolecule Unable to form hydrogen bond with Unable to form hydrogen bond with

HH22OO

Liquid alkanes and cycloalkanes are Liquid alkanes and cycloalkanes are soluble in one another, and they generally soluble in one another, and they generally dissolve in non-polar solvents.dissolve in non-polar solvents.

Good solvents for alkanes are benzene, Good solvents for alkanes are benzene, carbon tetrachloride, chloroform, and other carbon tetrachloride, chloroform, and other hydrocarbons.hydrocarbons.

Nature sources of alkanes;

Natural gas

oil

Natural gas contains primarily methane and ethane, with some propane and butane

Oil is a mixture of liquid alkanes and other hydrocarbons

CHEMICAL REACTION OF CHEMICAL REACTION OF ALKANESALKANES

Non-reactivity of alkanes

Alkanes are generally inert towards many Alkanes are generally inert towards many chemical reagents chemical reagents (bases, acids, dehydrating (bases, acids, dehydrating agents and aqueous oxidizing agents)agents and aqueous oxidizing agents)

C and H has nearly the same electronegativity, C and H has nearly the same electronegativity, the C-H bonds of alkanes are only slightly the C-H bonds of alkanes are only slightly polarised (non polar)polarised (non polar)

Reaction of alkanes

Combustion Halogenation

Combustion of alkanes

Alkanes burnt in air ( oxygen ) to give carbon dioxide gas, water and heat.

CxHy+ O2 → x CO2 + H2O

+ Heat 4

yx

2

y

Examples:

C4H8+ 3O2 → 4CO2 + 4H2O

+ Heat

Combustion of alkane in excess oxygen

Examples:

C4H8+ 4O2 → 4CO + 4H2O

+ Heat

Combustion of alkane in limited oxygen

Alkanes are unreactive towards polar or ionic reagents but can react with non-polar reagents such as oxygen and bromine.

The low reactivity of alkanes toward The low reactivity of alkanes toward many reagents explain why alkanes many reagents explain why alkanes were originally called were originally called paraffinsparaffins. .

(Latin : parum affinis= low affinity)(Latin : parum affinis= low affinity)..

Halogenation: a free radical Halogenation: a free radical substitution reactionsubstitution reaction

Alkanes react with Alkanes react with halogenhalogen (chlorine & bromine) (chlorine & bromine) to produce to produce haloalkanes haloalkanes in the presence of in the presence of light light or temperature >> 100 or temperature >> 100 ooC. C.

R–H + XR–H + X22 R–X + HX R–X + HX

With methane, the reaction produces a mixture of With methane, the reaction produces a mixture of halomethane and a hydrogen halide.halomethane and a hydrogen halide.

hv

Examples:Examples:

i. CH4 + Cl2 hv CH3Cl + CH2Cl2

+ CHCl3 + CCl4

+ HCl

ii. CH3CH3 + Cl2 hv CH3CH2Cl + HCl

CH3 CCH2Cl + HCl

| | CH3 CH3

CH3 CH3

| |iii. CH3 CCH3 + Cl2 hv

(ii) & (iii): 1 product only because all the hydrogen atoms are identical

iv. CH3CH2CH3 + Br2 hv CH3CH2CH2Br

( minor)+ CH3CHCH3

| Br ( major)+ HBr

CH4 + Cl2 CH3Cl + HCl

i. Chain initiation step

Reaction mechanism

hv

hvCl – Cl Cl – Cl 2Cl•2Cl•

ii. Chain propagation steps

3HC• + Cl Cl_ CH3Cl + Cl•

HH-C H + •Cl •CH3 + HCl

H

iii. Chain termination step.

•CH3 + •CH3 CH3CH3

•Cl + •CH3 CH3Cl

•Cl + •Cl Cl2

Example Example

The bromination of 2-methylbutane yields a The bromination of 2-methylbutane yields a mixture of isomers.mixture of isomers.

CH3 C CH2 CH3

CH2Br

H

CH3 C CH3

H

CH3

CHBr CH3 C CH3

CH3

Br

CH2

Increasing % yield

ExerciseExercise

Chlorination reaction of certain Chlorination reaction of certain alkanes can be used for laboratory alkanes can be used for laboratory preparations, for example in the preparations, for example in the preparation of chlorocyclopentane preparation of chlorocyclopentane from cyclopentane. Give the from cyclopentane. Give the mechanism for the reaction.mechanism for the reaction.