Unimolecular Nucleophilic Substitution
We already know the SN2 mechanism that accounts for (S)-2-octanol as the major 'inverted' product of our experiment. But, how can we explain the 6% formation of (R)-2-octanol? We have to draw on an alternative mechanism: unimolecular nucleophilic substitution (SN1).
C-Br bonds are highly polarized towards the more electronegative halogen atom. The presence of the polar solvent may prompt the spontaneous heterolytic cleavage of the C-Br bond, the resulting ions being more or less separated (solvated) by the solvent.
The so-formed carbocation is planar, the charged carbon being sp2 hybridized. The empty p orbital, orthogonal to the plane, can be attacked equally through either of its two lobes.
The attack of the nucleophile by either of the two sides gives rise to the two possible enantiomers. Since the probability is the same, this mode of attack renders the two enantiomers in the same ratio, i.e. it produces a racemic mixture.
The SN1 reactions - a good example is the solvolysis of tert-butyl chloride - follow first-order kinetics.
Rate = k[(CH3)3CCl] mol/L·s
This contrasts with the second-order kinetics of the SN2 mechanism.
This first-order kinetics, which only depends on the haloalkane, means that, in the SN1 mechanism, only the haloalkane takes part in the rate-limiting step.
Look at the differences in motion:
The carbocation is attacked by the nucleophile.
In the solvolysis of tert-butyl chloride the nucleophile is the solvent itself, which collapses with the carbocation. Another solvent molecule removes the extra proton.
The respective energy profiles of the SN1 and SN2 processes are as follows:
The SN1 reaction lacks stereospecificity because it takes place through the formation of a planar carbocation that can be attacked equally by either of its two sides.
In the substitution reaction the SN1 and SN2 processes compete with each other.
