MIND MAP 5.1 ALKANE

MIND MAP
ALKANE

Prepared by: NFAG, KMPh

CHEMICAL PROPERTIES OF ALKANE

Equation Combustion of methane, CH4: Equation Combustion of propane, CH3CH2CH3:

Excess oxygen CH4 + 2O2 CO2 + 2H2O Excess oxygen CH3CH2CH3 + 5O2 3CO2 + 4H2O

Limited/ Very limited oxygen CH4 + 3/2 O2 CO + 2H2O Limited/ Very limited oxygen CH3 CH2CH3 + 7/2 O2 3CO + 4H2O
CH4 + O2 C + 2H2O CH3 CH2CH3 + 2O2 3C + 4H2O

CH4 + Br2 UV CH3Br + HBr CH3CH2CH3 + Br2 UV CH3CH(Br)CH3 + Br
CH2Cl2 CH2Cl2

Bromine test: Br2/ CH2Cl2 (UV) Bromine test: Br2/ CH2Cl2 (UV)
The reddish brown colour of bromine is decolourised The reddish brown colour of bromine is decolourised

Chain Initiation Step Chain Initiation Step

Br Br uv 2 Br Br Br uv 2 Br

Chain Propagation step CH3 + HBr Chain Propagation step CH3CHCH3 + HBr
Br
H H
CH3CHCH3+ Br CH3CHCH3 + Br
CH3 + Br
Br Br + CH3CHCH3
Br Br + CH3 CH3Br + Br
Br2 Chain termination step
Chain termination step

Br + Br Br + Br Br2

CH3 + CH3 CH3CH3 CH3CHCH3 + CH3CHCH3 CH3CHCH3
CH3CHCH3
CH3 + + Br CH3Br CH3CHCH3 + Br
Prepared by: NFAG, KMPh Br
CH3CHCH3

CHEMICAL PROPERTIES OF CYCLIC ALKANE

+ Br2 UV Br CH3 + Br2 UV Br
CH2Cl2
CH2Cl2 + HBr CH3 + HBr

Bromine test: Br2/ CH2Cl2 (UV) Bromine test: Br2/ CH2Cl2 (UV)
The reddish brown colour of bromine is decolourised The reddish brown colour of bromine is decolourised

Chain Initiation Step Chain Initiation Step

Br Br uv 2 Br Br Br uv 2 Br

Chain Propagation step Chain Propagation step

H + HBr H + HBr
Br + Br Br
+ Br
Br Br + Br Br Br +
+ Br

Chain termination step Chain termination step

Br + Br Br2 Br + Br Br2

+

+

Br Br

+ Br + Br

Prepared by: NFAG, KMPh

PHYSICAL PROPERTIES OF ALKANES

(BOILING POINT OF ALKANES)

Boiling points based on molecular weight Boiling points of isomeric alkanes
(similar molecular weight)

No. of C ↑ (Molar mass ↑), Molecular size ↑, Surface area ↑, No. of branches ↑, Surface area ↓, London Forces ↓, Boiling point ↓
London forces ↑, Boiling point ↑

Example: Example:

Pentane Propane Pentane 2-methylbutane 2,2-

(b.p: 37oC) (b.p: -42.1oC) dimethylpropane

H3C CH2 CH2 CH2 CH3 H3C CH2 CH3 (b.p: 37oC) (b.p: 28.5oC) (b.p: 9oC)

The boiling points of alkanes show a regular increase with increasing H3C CH2 CH2 CH2 CH3 H3C CH CH2 CH3 CH3
molecular weight because the number of carbon increase. CH3 H3C C CH3

C-H is nonpolar bond. CH3
-Type of intermolecular forces exist is London forces/ dispersion
forces. Branching affect the boiling point :
-the more branches, molecule becomes more compact,
The London forces/ dispersion forces stronger: -surface area reduces,
-as molecular weight higher/ molecular size bigger, -this causes the branched alkanes to have a weaker Van Der Waals/London forces,
-molecular surface area increase, -lower boiling points.
-more energy is required to separate molecules,
-higher boiling point. Pentane has the higher boiling point than 2-methylbutane and 2,2-dimethylpropane
because it is linear/ no branch.
Pentane has higher boiling point than propane because it has the
highest molecular weight. 2,2-dimethylpropane has the lowest boiling point because it has the most branch
molecules.

Prepared by: NFAG, KMPh

Boiling points of alkanes and cycloalkanes

Cyclic alkane, molecule closer together, Intermolecular forces ↑, energy needed to break the bond ↑, Boiling point ↑

Example:

Alkane A Alkane B Alkane C

CH2 CH3
H3C C CH3
H2C CH2 H3C CH2 CH2 CH2 CH3
CH3
CH2 CH2

CH3 CH2
H3C C CH3
H2C CH2
CH3
, H3C CH2 CH2 CH2 CH3 , CH2 CH2
B C
A

Increasing boiling point
Alkane A, B and C have similar molecular weights (5 carbon atoms).

Boiling point of A is higher than C because alkane A is in cyclic structure which is closer molecule, therefore has stronger
London forces.

The presence of branches in alkane B causes the molecule becomes more compact and has smaller contact surface area.
While alkane C is linear chain and has bigger surface area. London forces of alkane B is weaker than alkane C.

Prepared by: NFAG, KMPh

PHYSICAL PROPERTIES OF ALKANES

(SOUBILITY OF ALKANES)

Water Organic Solvent

• Alkanes is less dense than water. • Liquid alkanes and cycloalkanes are

• Alkanes and cycloalkanes are almost soluble in one another, and they generally

totally insoluble in water. dissolve in non-polar solvents.

• They are non-polar molecule and • Good solvents for alkanes are benzene,

unable to form hydrogen bond with carbon tetrachloride, chloroform, and

polar solvent like H2O. other hydrocarbons.

Prepared by: NFAG, KMPh

Prepared by: NFAG, KMPh