Further Reactions of Haloalkanes | Organic Chemistry 1

Further reactions of haloalkanes are studied in this chapter: In-depth study of SN reactions, competition between SN1 and SN2, introduction to another important types of reactions: elimination (E reactions), competition between E1 and E2, competition between SN and elimination reactions.

SN1 Reactions

SN1 reactions = nucleophilic substitutions which proceed through an intermediate carbocation: SN1 reactions are unimolecular with a bond-breaking step following by a bond-making step.
Because of the planar carbocation, SN1 reactions give racemization of stereochemistry at the reaction centre (nucleophiles can attack from both sides).
The first step is slower and therefore determines the rate. The kinetic rate only involves the starting material.

 

 

The more stable the carbocation intermediate (hyperconjugation), the faster the SN1 reaction.
The nature of the nucleophile has no importance.

Neighbouring group participation in SN1 is important:

SN2 vs. SN1

SN2

SN1

1 step

2 step

rate = k [RX][Nu-]

rate = k[RX]

inversion of configuration

racemisation

inhibited by steric hindrance
⇒ faster with primary haloalkanes

the more stable the carbocation intermediate, the faster the SN1 reaction
⇒ faster with tertiary haloalkanes

favored when strong nucleophiles are used

the nature of the nucleophile has no importance

 

Alkyl Halides Reactions

- Substitution reactions (SN Reactions):


 

- Elimination reactions (E Reactions):


 

In the elimination reactions, the more highly substituted alkene product predominates.
This is the Zaitsev's rule and the major product is called 'Zaitsev Product'.

 

E2 Reactions

E2 reactions = nucleophilic eliminations which do not proceed via an intermediate: E2 reactions are bimolecular with simultaneous bond-making and bond-breaking steps.
The kinetic rate involves 2 components: the nucleophile (generally a base) and the electrophile reagents.
E2 reactions always occur with an anti periplanar geometry (preferred over syn periplanar geometry).
 


 

 

 

 

 


 

E1 Reactions

E1 reactions = nucleophilic eliminations which proceed through an intermediate carbocation: E1 reactions are unimolecular with a bond-breaking step following by a bond-making step.
Because of the planar carbocation, SN1 reactions do not always occur with an anti periplanar geometry.
As we previously saw, the Zaitsev product predominates. 
The first step is slower and therefore determines the rate. The kinetic rate only involves the starting material.

 

E2 vs. E1

E2

E1

1 step

2 steps

rate = k [RX][Nu-]

rate = k[RX]

Anti periplanar geometry is preferred over syn periplanar geometry

No geometric requirement

 

Competition between SN1, SN2, E1, E2

Substitution reactions can compete with elimination reactions. 
The structure of the haloalkanes and/or nucleophiles determines the reaction:
 

 

good nucleophiles 
(RS-, NC-, I-, Br-):

strong sterically hindered bases
(tBuOK, LDA):

 

Primary alkyl halides:

SN2

E2

Secondary alkyl halides:

SN2 (+ SN1)

E2 (+ E1)

Tertiary alkyl halides:

SN1 + E1

E1 (+ SN1)


 

SN1 and E1 reactions always compete.
SN1 predominates over E1, when the reaction is carried out under neutral conditions.
E1 occurs with strong bases as well as HO- or CH3O-.

 

Common strong sterically hindered bases: