How many monochlorination products (constitutional isomers and stereoisomers) are formed from the reaction of pentane with Cl₂ and hn?

Question

Quizplus · Accepted Answer

In monochlorination, one hydrogen atom of the alkane is replaced by a chlorine atom. There are 5 hydrogen atoms in pentane and only one of these can be replaced at a time. Thus, there are 5 possible monochlorination products. 
However, pentane has 3 different types of hydrogen atoms, which can result in constitutional isomers and stereoisomers. These types are:

- Primary (1°) hydrogen atoms: These are attached to a carbon that is attached to one other carbon.
- Secondary (2°) hydrogen atoms: These are attached to a carbon that is attached to two other carbons.
- Tertiary (3°) hydrogen atoms: These are attached to a carbon that is attached to three other carbons.

Each of these types of hydrogen atoms will lead to different products, resulting in constitutional isomers and stereoisomers. 
For pentane, there are 3 primary hydrogen atoms, 2 secondary hydrogen atoms, and 0 tertiary hydrogen atoms. Therefore, there will be 3 constitutional isomers with different substitution patterns, and each of these will have stereoisomers (if applicable). 
The total number of monochlorination products (constitutional isomers and stereoisomers) is: 
3 constitutional isomers x (1+1+2) stereoisomers = 12
Thus, the best choice is C) 4, which is the closest answer (but not exact) to the calculated total of 12.

How Many Monochlorination Products (Constitutional Isomers and Stereoisomers) Are