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Alkanes: Preparation and

Chemical Reactions

General methods for the

Preparation of Alkanes

1. By hydrogenation of alkenes and alkynes :

 Alkenes and alkynes on catalytic hydrogenation give alkanes

H 2

C

CH 2

H 2

Ni , 200

o

CH 3

CH 3

H 3

C C

H

CH 2

H 2

Pt

H 3

C

H 2

C CH 3

R C C R +^

H 2

Ni

R C

H

C

H

R (^) +

H 2

Ni

RCH 2

CH 2

R

HC CH +

2H 2

Ni, 200

o

CH 3

CH 3

H 2

Pt

3. By Hydrolysis of Grignard reagents



Organic compounds in which a metal

atom is directly linked to carbon atom

are known as organometallic

compound.



Alkyl or aryl magnesium halide (R-

MgX) are also called Grignard

reagents or organometallic

compounds.

Grignard reagent on double decomposition with water or

with other compounds having active H (the hydrogen

attached to O, N, F or triple bonded carbon atom are

known as active hydrogen) give alkane.

e.g. RX+ Mg RMgX

Mg

R

X

HOH

Mg

OH

X

R H

Mg

R

X

HNH

2

Mg

NH 2

X

R H

Limitations of wurtz reaction :

• (^) a) Methane cannot be obtained by this method
• (^) b)The reaction fails in case of tertiary halides
• (^) Mechanism :

The mechanism of Wurtz reaction is although not clear

however two mechanisms are proposed for this reaction.

Mechanism

• Involving formation of reactive organometallic intermediate

compound:

RX + 2 Na → [RNa] + NaX

Intermediate

RX + [RNa] → R-R + NaX

5. Kolbe's electrolysis method:

2 RCOONa aq

electrolysis

R R + 2CO 2

• 2NaOH + H 2

Anode

Cathode

Alkanes are formed, on electrolysis of

concentrated aqueous solution of sodium or

potassium salt of saturated mono

carboxylic acid.

At cathode, Metal atom is liberated, which reacts to

form H

2

and metal hydroxide.

At anode, RCOO- decompose to form CO

2

and

free radical R. which unite to form alkane R-R.

© Prentice Hall 2001 Chapter 8 10

CHEMICAL PROPERTIES OF ALKANES

Reactivity of Alkanes:

• Stable to acids,alkalis and oxidizing agents

(due to no difference in electronegativity

of carbon and hydrogen).

• Alkanes have strong, nonpolar  bonds.
• (^) No reaction with nucleophiles or

electrophiles

• Not much reactivity - paraffins (little

affinity)

CHEMICAL REACTIONS OF ALKANES

• There are 2 major types of reactions of

alkanes :

• Substitution Reaction
• Thermal and Catalytic Reactions

1. Halogenation : involves substitution of hydrogen

atoms of alkanes with halogen atoms.

Involves the formation of free radical

R-H + X

2

, heat or hv  R-X + HX

a) heat or light required for reaction.

b) Reactivity of Halogens: X

2

: F

2

Cl

2

Br

2

 I

2

c) Relative rate of H abstraction : 3

o

2

o

1

o

CH

3

d) bromine is more selective in abstracting

hydrogen from alkane (less reactive than chlorine

in free radical halogenation.

CHEMICAL REACTIONS OF ALKANES

:Example

16

Reaction Mechanism: Free Radical Substitution Reaction

Mechanism of reaction :

1. Chain initiation

• homolytic fission of chlorine molecules by heat or light into two chlorine free

radicals.

• Atom or group of atom posessing an odd/unpaired electron is free radical.

Step 1:

Chlorination

2. Chain propagation

Step 1:

Step 2:

• steps 1 and 2 repeat hundreds or thousands of time due to formation of

the reactive intermediate in each step

 chain reaction