Propose mechanisms for the following reactions. In most cases, more products are formed than are shown here. You only need to explain the formation of the products shown, however.

The chemical reaction in which one or more carbon atoms add to a cyclic ring that alters the ring size (expands) is called ring expansion. The ring expansion reactions are significant in organic synthesis because it is very difficult to carry out the single cyclization reactions.

In this reaction, the lone pairs of electrons present in the oxygen of the OH group attack ZnCl₂ that forms a positive charge on the oxygen atom and a negative charge on ZnCl₂ . There will be the cleavage of the C-O bond and there will not be any primary carbocation intermediate. The chlorine atom attacks the carbocation leading to the formation of the desired product. The mechanism is shown below.

Mechanism of the reaction

In this reaction, the lone pairs of electrons present in the oxygen of the OH group attack hydrogen that forms a positive charge on the oxygen atom. On heating, there will be the formation of a carbocation. The obtained tertiary carbocation removes its adjacent protons and leads to the formation of the desired products. The mechanism is shown below.

Mechanism of the reaction

The proton abstraction takes place in the same way like previous steps. The removal of water by heating and rearrangements takes place like the previous steps leading to the formation of the desired products. The mechanism is shown below.