chemistry

Carbocation rearrangements may give products where there is change in ring size. An example is shown below:   Fig. 1: A carbocation rearrangement with ring change is shown. The rearrangement gives a more stable tertiary carbocation Another example of carbocation rearrangement and ring change (ring expansion) is shown in Fig. 2.   The reaction is a pinacol rearrangement and in this case leads to ring expansion.  Fig. 2: A carbocation rearrangement with ring change is shown. The rearrangement gives a more stable tertiary carbocation since it is stabilized by the lone pairs of the oxygen atom. This rearrangement is known as the pinacol – pinacolone rearrangement.

Carbocation rearrangements Carbocation Rearrangements: 1,2-H and 1,2-alkyl Shifts In a rearrangement a group moves from one atom to another in the same molecule. Most are migrations between adjacent atoms and are called 1,2-shifts . Carbocation rearrangements occur more frequently on secondary carbocations to form tertiary which are more stable and energetically more favorable . In general, the bonding electrons of a carbocation may shift between adjacent atoms to form a more stable carbocation . What types of carbocation rearrangements are possible? When does a carbocation rearrangement occur? Two types of carbocation rearrangements are possible: 1,2-H shift and 1,2-alkyl shift . 1,2-H shift (called 1,2 hydride shift , hydride ion = H: - ) : If a carbocation is vicinal to a tertiary carbon bearing a H, a 1,2-H shift should occur (Fig. 1). Fig. 1: Two possible types of rearrangement. The carbocation desires electron pair to complete th...

Any species that has an unshared pair of electrons (any Lewis base) can act as a nucleophile whether it is neutral or has a negative charge. A nucleophile (Nu: - ) is a species that attacks an electrophile (E + ) (electron pair acceptor) by making a pair of electrons available to it; it is an electron pair donor: Nu: -    +     E +         →       Nu-E    +     by-products S N 2 reactions and nucleophiles The rates of S N 2 reactions are dependent of the identity of the nucleophile since it does appear in the rate determining step: R =  [Nu: - ] * [E + ] This may be illustrated by the effect of changing the nucleophile from H 2 O to OH - for CH 3 Br which reacts by an S N 2 mechanism (Fig. 1). The rate of reaction is multiplied by 5000: Fig. 1: Effect of changing the nucleophile from H 2 O to OH - in the S N 2 reaction of CH 3 Br. Th...