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alcohol

Primary amine

Halogenoalkanes

nitrileHeat under reflux NaOH(aq) in ethanol or water in ethanol

BOX 7a-A: REACTIONS OF HALOGENOALKANES

BOX 7a-B: HETEROLYTIC NUCLEOPHILIC SUBSTITUTION

HANDOUT 7a REACTIONS OF HALOGENOALKANES

Objectives:

recall the chemistry of halogenoalkanes as exemplified by the following nucleophilic substitution reactions of

bromoethane: hydrolysis, formation of nitriles, formation of primary amines by reaction with ammonia and by the elimination of hydrogen bromide from 2-bromopropane

describe the mechanism of nucleophilic substitution (by both SN1 and SN2 mechanisms) in halogenoalkanes

NH3(aq) in ethanol

Heat

KCN(aq) in ethanolHeat under reflux

Nucleophilic

substitutionNucleophilic

substitution

Nucleophilic

substitution

Heat under reflux

Pure NaOH in ethanol

alkene

This mechanism is associated with polar covalent bonds (e.g. carbon-halogen).

C B r

The positive charge on the carbon

allows attack by nucleophiles suchas :OH or :CN which have lone pairs to

donate.

SN2 MechanismThe mechanism involves the substitution of a nucleophile which is second order (two molecules in rate determining step).

C Br

H3C

HH

HO C Br

H3C

HH

HO C Br

CH3

HH

HO

transition state

The nucleophile attacks the C+ and

starts to form a covalent bond with it. At

the same time the C+-Hal bond starts

to break heterolytically.This is one continuous process with the

carbon-nucleophile bond gettingstronger and the carbon-halogen getting

weaker.

There are two mechanisms for

nucleophilic substitution, (1) SN1

and (2) SN2.

SN1 Mechanism

This mechanism involves the substitution of a

nucleophile which is first order (one molecule in rate

determining step). This mechanism occurs in two stages:

C

CH3

H3C Br

CH3

C

CH3

H3C Br

CH3

BOX 7a-B contd.

C

CH3

H3C

CH3

C

CH3

H3C

CH3

OH OH

Stage 1:

Stage 2:

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HANDOUT 7b RELATIVE STRENGTH OF C-HAL BONDS

Objectives:

Interpret the different reactivities of halogenoalkanes e.g. CFCs, anaesthetics, flame retardants, and plastics with particular reference to hydrolysis and to the relative strengths of the

C-Hal bonds)

explain the uses of fluoroalkanes and fluorohalogenoalkanes in terms of their relative chemical inertness

recognise the concern about the effect of chlorofluoroalkanes on the ozone layernes

BOX 7b-A: REACTIVITIES OF HALOGENOALKANES

The relative strength of C-Hal bond is C-Cl > C-F > C-Br > C-I.

In general the greater the bond strength the lower the reactivity so that

fluorocarbons are less reactive than alkanes and the other haloalkanesincreasingly more reactive and less stable from Cl to I.

BOX 7b-B: THE USES OF HALOGEN COMPOUNDS

Due to their inertness and stability, many halogenoalkanes are used in

several purposes.

Plastics

Poly(chloroethene) or PVC and poly(tetrafluoroethene)/PTFE, also

known as Teflon are polymers which are synthesised from

halogenoalkanes.

Anaesthetics

Trichloromethane (chloroform), CHCl3, was once used as anaesthetics.

It was a better anaesthetics than ethoxyethane (ether) which was also

used an Anaesthetics Since ether is highly flammable. However,

although chloroform is not flammable, it causes liver damage. Therefore,

it is seldom in use now.

Flame retardants

Bromochlorodifluoromethane (BCF) is very effective at extinguishing fire.

However, it is not in beneral use because the breakdown products arepoisonous.

BOX 7b-B contd.

CFCs

Chlorofluorocarbons, for example dichlorodifluoromethane or

trichlorofluoromethane, have been used as refrigerant, aerosolpropellants or blowing agents.

The inertness and the high stability of CFCs are the reasons for

choosing these.

However, due to the stability, these agents accumulate in the

atmosphere and when they reach stratosphere, they undergo

photodissociation by ultraviolet radiation which produces chlorine free

radicals.The radicals decompose ozone into oxygen, thus depleting ozone layer

in the atmosphere.

Therefore, some governments have decided to restrict the uses of CFCs

and change to alternatives such as 1,1,1,2-tetrafluoroethane (CF3CH2F).

The presence of hydrogen atom increases its reactivity so it is brokendown more easily in atmosphere. Even if it reaches stratosphere, it does

not produce the damaging chlorine free radicals.