Alkenes

Question 1

Preparation reactions

Answer 1

• dehydration of alcohols (elimination reaction)
• base induced elimination of hydrogen halide from haloalkanes

Question 2

Dehydration of alcohols process

Answer 2

Either by passing alcohol vapour over a hot aluminium oxide catalyst or reaction alcohol with concentrated sulphuric acid or concentrated phosphoric acid. Phosphoric acid is better to use as there are less side reactions and less charring occurs.

Question 3

Base induced elimination of hydrogen halides from haloalkanes process

Answer 3

Heating the haloalkane under reflux with ethanolic potassium hydroxide. In this reaction OH- is a base as it is a proton acceptor.

Question 4

Reactions alkenes take part in (all are electrophilic addition)

Answer 4

• addition of hydrogens to form alkanes
• addition of halogens to form dihaloalkanes
• addition of hydrogen halides to form monohaloalkanes
• addition of water, using an acid catalyst, to form alcohols

Question 5

Markovnikov’s rule

Answer 5

States that when hydrogen halide (H-H) or water (H-OH) is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogen atoms already attached to it.

Question 6

The 3 different mechanisms for electrophilic addition reactions that are considered at AH

Answer 6

• addition of hydrogen halides (carbocation intermediate)
• addition of water using an acid catalyst (carbocation intermediate)
• addition of halogens (cyclic ion intermediate)