Question

If one hydrogen in a hydrocarbon is replaced by a halogen atom, the number of isomers that exist for the substituted compound depends on the number of types of hydrogen in the original hydrocarbon. Thus there is only one form of chloroethane (all hydrogens in ethane are equivalent), but there are two isomers of propane that arise from the substitution of a methyl hydrogen or a methylene hydrogen. How many isomers can be obtained when one hydrogen in each of the compounds named below is replaced by a chlorine atom?

(a) n-pentane

(b) 2-methylbutane

(c) 2,4-dimethylpentane

(d) methylcyclobutane

Short answer

a. The number of isomers that can be obtained when one hydrogen inn-pentane is replaced by a chlorine atom is three.

b. The number of isomers that can be obtained when one hydrogen in2-methylbutane is replaced by a chlorine atom is four.

c. The number of isomers that can be obtained when one hydrogen in 2,4-dimethylpentane is replaced by a chlorine atom is three.

d. The number of isomers that can be obtained when one hydrogen in methylcyclobutane is replaced by a chlorine atom is four.

Step-by-step solution

Subpart (a) The number of isomers that can be obtained when one hydrogen in n-pentane is replaced by a chlorine atom.

In the given structure, there are three different types of hydrogens; each one will produce one product, resulting in three separate chlorination products:

Thus, the three different products are;

In the first case, the chlorine is substituted with the hydrogen at the terminal carbon.

In the second, the chlorine is substituted with the hydrogen at the second carbon.

In the third case, chlorine is substituted with hydrogen in the central carbon.

Subpart (b) The number of isomers that can be obtained when one hydrogen in   2-methylbutane is replaced by a chlorine atom.

There will be four separate chlorinated products as isomers since there are four different hydrogens.

The four possible products are:

In the first case, the chlorine is substituted with the hydrogen at the terminal carbon.

In the second, the chlorine is substituted with the hydrogen at the second carbon (more substituted).

In the third, the chlorine is substituted with the hydrogen at the second carbon that is less substituted.

In the fourth case, chlorine is substituted with hydrogen at the least hindered terminal carbon.

Subpart (c) The number of isomers that can be obtained when one hydrogen in  2, 4-dimethylpentane is replaced by a chlorine atom.

There will be three sorts of products from the sum of three types of hydrocarbogens, as illustrated below:

The three possible products are:

In the first case, the chlorine is substituted with the hydrogen at the terminal carbon.

In the second, the chlorine is substituted with the hydrogen at the second carbon (more substituted).

In the third, the chlorine is substituted with the hydrogen at the central carbon.

Here, all four goods are identical.

Both products are same.

Subpart (d) The number of isomers can be obtained when one hydrogen in methylcyclobutane is replaced by a chlorine atom.

When one hydrocarbon of the given compound methyl-cyclobutane is replaced with one chlorine atom of the given compound, four isomers are obtainedby changing where the chlorine atom is bonded to the given compound.

The four possible products are: