Question

Predict the major product for each of the following reactions:

Predict the major product for each of the following reactions:

Answer 1

Concepts and reason

Alkyne chlorination is an addition reaction between an alkyne and a chlorine molecule. In general, for alkynes double addition reactions are possible. This means that two equivalents of chlorine molecules can be added successfully.

Fundamentals

Chlorine addition to the alkyne is Trans phase addition to the triple bond through a chlorinum ion as an intermediate. In this addition reaction, alkyne acts as HOMO while chlorine molecule acts as LUMO. The chlorine molecule is partially polarized; an alkene will attack on positive charge side by pushing the electron cloud towards the other chlorine atom.

Here, the alkyne is in an electron rich space, which acts as a nucleophile while the chlorine molecule acts as an electrophile.

If we add one more equivalent chlorine molecule to the obtained Trans product, we get a tetra-chlorinated product.

Formation of chloroinium ion intermediate:

Here, the nucleophile is chloride ion, which is produced from the chlorine molecule in step 1.

Since chlorine is larger atom, a nucleophile attack is from behind the chloronium ion cycle present in the intermediate of the reaction.

The obtained Trans product in step 1 reacts with another equivalent of chlorine and gives a tetra-chlorinated alkane as the product by following the same mechanistic pathway of alkyne chlorination.

Ans:

The product on bromination to the given alkyne is