Halogenoalkanes: reaction mechanisms

Question 1

Halogenoalkane’s with aqueous sodium or potassium hydroxide :OH

Conditions needed

Answer 1

Nucleophilic substitution

Warm aqueous solution with ethanol as solvent

Alcohol ROH is formed

Question 2

Halogenoalkane’s with cyanide ions
:CN

Conditions needed

Answer 2

Nucleophilic substitution

Warm aqueous alcoholic solution

Nitriles are formed

Question 3

Halogenoalkane’s with ammonia :NH3

Conditions needed

Answer 3

Nucleophilic substitution

Excess concentrated solution of ammonia in ethanol, carried out under pressure

Produces an amine RNH2

Question 4

OH- ion as a base

Conditions needed

Answer 4

Elimination reaction

The sodium (or potassium) hydroxide dissolved in ethanol and mixed with the halogenoalkane.

No water present, mixture is heated

Double bond is formed

Question 5

Elimination mechanism eg hydrogen bromide

Answer 5

The OH- ion uses its lone pair to form a bond with one of the hydrogen atoms on the carbon next to the C-Br bond. These hydrogen atoms are slightly partially positive

The electron pair from the C-H bond now becomes part of a carbon-carbon double bond

The bromine takes the pair of electrons in the C-Br bond and leaves as a bromide ion (the leaving group)

Question 6

Electrophilic addition reaction mechanism

Answer 6

The electrophile is attracted to the C=C double bond as it has a high electron density

Electrophiles are positively charged and accept a pair of electrons from the double bond. The electrophile may be a positively charged ion or have a positively charged area.

A positive ion (carbocation) is formed

A negatively charged ion forms a bond with the carbocation