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 type of reactions: elimination reactions (E reactions), competition between E1 and E2, competition between SN and E 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. The kinetic rate only involves the starting material
 

Mechanism:

In the first step, the leaving group leaves, forming a carbocation. In the second step, the nucleophile attacks the carbocation to give the product. Due to planar carbocation, SN1 reactions result in racemization of stereochemistry at the reaction centre (nucleophiles can attack from both sides)
The first step is slower and therefore determines the rate: it is the rate-determining step

 



​​​​​​​Neighbouring group participation in SN1 is important: the more stable the carbocation intermediate (hyperconjugation), the faster the SN1 reaction:
​​​​​​​


​​​​​​​The nature of the nucleophile has no importance

SN2 versus SN1

SN2

SN1

 

1 step

2 steps

rate = k [RX] [Nu-]

rate = k [RX]

inversion of configuration

racemisation

inhibited by steric hindrance
⇒ SN2 faster with primary haloalkanes

the more stable the carbocation, the faster the reaction
⇒ SN1 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 elimination reactions, the major product is the most substituted alkene. 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
 


Mechanism:

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. The kinetic rate only involves the starting material
 

Mechanism:

In the first step, the leaving group leaves, forming a carbocation. In the second step, a proton is removed by the base to give the alkene. Due to planar carbocation, E1 reactions do not always occur with an anti periplanar geometry. The major product will be the Zaitsev product
The first step is slower and therefore determines the rate: it is the rate-determining step

 

E2 versus E1

E2

E1

 

1 step

2 steps

rate = k [RX] [Base]

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: