Review page introducing the SN1 mechanism

For 
(CH3)3CBr + OH–  (CH3)3COH + Br–  

The rate depends only on substrate:  rate = k[(CH3)3CBr].

This nucleophilic substitution is unimolecular (SN1).
The halide must react in the rate-determining step to produce an intermediate that reacts with nucleophile as shown below.
 
Step 1:
(CH3)3CBr  (CH3)3C+ + Br–    slow
(C-leaving group bond is broken)
Step 2:
(CH3)3C+ + OH– (CH3)3COH  fast
(C-Nu bond is formed)

Experimental evidence consistent with this mechanism includes:
  • Substrates with one or more chiral centres react to give a mixture of stereoisomers (approximately an equal mixture of R and S at the reacting centre).

    Therefore, reaction of compounds with a single stereogenic centre (non-racemic) produce a racemic mixture (approximately 1:1 of both enantiomers), while reaction of compounds with multiple stereogenic centres give diastereoisomers.
     
    •  Roll your mouse over the image to see that the geometry about the positive carbon is planar, and that there is at this carbon a vacant p-orbital with lobes on both sides of the positive carbon.
    • The nonbonding pair on the nucleophile may form a bond to the positive carbon using either lobe of this orbital.

     
  • >
    (CH3)3CBr - 3°>(CH3)2CHBr - 2° >CH3CH2Br - 1°
    The carbon with the green dot is positive
    in the intermediate.
    The rate is faster for substrates with more substitution at the carbon bearing leaving group

    Therefore carbocations with more bonds to saturated carbon at the positive carbon are of lower energy.
    Thus the activation energy for their formation is smaller, and the rate of their formation is faster.
     

Differences to the SN2 mechanism include:
  • rate of the reaction is independent of the concentration of nucleophile
  • charge builds up in the transition state of the rate-determining step
  • different stereochemical outcome
Dr Sheila Woodgate
© The University of Auckland 2002