Elimination

Question 1

What is elimination?

Answer 1

Breaking of a sigma bond and formation of a new pi bond

Question 2

What reagent is required to form a alkene from alkyl halide?

Answer 2

Strong base - source of OH- ions

Question 3

What reagent is required to form an alkene from alcohol?

Answer 3

Conc. H2SO4 - strong acid

The acid here has to be concentrated as the reverse reaction from the alkene to the alcohol also involves the acid catalyst. Making it concentrated removes source of H2O.

Question 4

What is the name of the rule associated with elimination?

Answer 4

Zaitzev’s rule

Question 5

What is Zaitzev’s rule?

Answer 5

Elimination reactions with unsymmetrical molecules can form multiple, constitutional isomers giving the more substituted alkene as the major product

Question 6

What are the mechanisms of elimination?

Answer 6

E1 unimolecular

E2 bimolecular

Question 7

Explain the E1 mechanism

Answer 7

Analogous to SN1

The rds involves only the alkyl halide and occurs via a two step process with formation of an intermediate carbocation

Hence, E1 is favoured for 3 > 2 > 1

Question 8

Explain the E2 mechanism

Answer 8

Analogous to SN2

The rds involves two species - the alkyl halide and the nucleophile. A single step process.

NOT NECESSARILY FAVOURED BY PRIMARY ALKYL HALIDES

Question 9

What are the factors that affect competition between SN1 and E1 pathways?

Answer 9

Substitution reactions usually have lower energy than elimination reactions

However, with bulky nucleophiles more steric hindrance would occur with substitution hence elimination can be favoured.

Question 10

What are the factors that affect competition between SN2 and E2 pathways?

Answer 10

Substitution reactions usually have lower energy then elimination reactions

High temp and high basicity nucleophiles favour elimination

Very poor nucleophiles will also favour elimination