Halogen Compounds

Halogenoalkanes

Hydrocarbon with one or more halogen

Production

Free radical substitution
electrophilic addition
Substituiton of alcohols

Classifying Halogenoalkanes

Reactivity of halogenoalkanes compared to alkanes

• Halogenoalkanes are much more reactive than alkanes due to the presence of the electronegative halogens

  • The halogen-carbon bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge

Reaction with NaOH(aq)

• The halogen is replaced by the OH^-

• The aqueous hydroxide (OH^- ion) behaves as a nucleophile by donating a pair of electrons to the carbon atom bonded to the halogen

• For example, bromoethane reacts with aqueous alkali when heated to form ethanol

  • Hence, this reaction is a nucleophilic substitution

  • The halogen is replaced by a nucleophile, :OH^– 

CH₃CH₂Br + :OH^– → CH₃CH₂OH + :Br^–

Reaction with KCN

nucleophile: CN

• Ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane

• The product is a nitrile

• The nucleophilic substitution of halogenoalkanes with KCN adds an extra carbon atom to the carbon chain

Reaction with NH₃

Nucleophile: NH3

• An ethanolic solution of excess ammonia (NH₃ in ethanol) is heated under pressure with the halogenoalkane

• It is very important that the ammonia is in excess as the product of the nucleophilic substitution reaction, the ethylamine, can act as a nucleophile and attack another bromoethane to form the secondary amine, diethylamine

Reaction with aqueous silver nitrate

• Halogenoalkanes can be broken down under reflux by water to form alcohols

• Slower than NaOH as OH- is a stronger nucleophile compared to water

• For example, bromoethane reacts with aqueous silver nitrate solution to form ethanol and a Br^- ion

  • The Br^- ion will form a cream precipitate with Ag⁺

Water and Hydroxide

Sn1 and Sn2

• These reactions can occur in two different ways (known as S_N2 and S_N1 reactions) depending on the structure of the halogenoalkane involved

Sn2

Primary and secondry carbocations
One step process

• The nucleophile donates a pair of electrons to the δ+ carbon atom to form a new bond

• At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond (heterolytic fission)

• The halogen leaves the halogenoalkane as an X^- ion

Sn1

Tertiary Carbocation

Two step process
n the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X^- ion (this is the slow and rate-determining step)

• This forms a tertiary carbocation (which is a tertiary carbon atom with a positive charge)

• In the second step, the tertiary carbocation is attacked by the nucleophile

Reactivity of Halogenoalkanes