Question

The number of benzylic hydrogen atoms in the ethylbenzene is:
A) 3
B) 5
C) 2
D) 7

Answer 1

Hint: The benzylic hydrogen atoms are referred to as the total number of hydrogen atoms that are attached to the carbon atom which is next to the benzene ring. The benzylic positions are the position that is neighbouring to the ring and can take part in conjugation.

Complete Solution :
- Ethylbenzene is an organic molecule. The general formula of ethylbenzene is as $\text{ }{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}\text{C}{{\text{H}}_{\text{2}}}\text{C}{{\text{H}}_{\text{3}}}\text{ }$. Here, an ethyl group is attached to one of the carbon atoms of the aromatic ring. It is also known as the ethylbenzol or phenyl ethane. It belongs to the benzene and its substituent derivative class.
- Here, the compound contains the one benzene ring, thus it is monocyclic.
The structure of ethylbenzene is as follows:

We are interested to determine the benzylic hydrogens on the ethyl benzene.
- Let's first understand what a benzylic position in organic molecules is. The position which is adjacent to the aromatic ring that can conjugate with the aromatic system is termed as the benzylic position.
- This position has special features.
- The benzylic hydrogen atoms are referred to as the total number of hydrogen atoms that are attached to the carbon atom which is next to the benzene ring.
- The ethylbenzene has the ethyl bonded to the benzene ring. The carbon atom of the ethyl groups which is forming a bond with the ring has two hydrogen atoms on it. These hydrogens atoms are called the benzylic hydrogen atoms.
Thus, ethylbenzene has 2 benzylic hydrogen atoms.
So, the correct answer is “Option C”.

Note: Note that, the benzylic positions are very reactive and are useful as the synthetic route. The reason is, the benzylic carbon is conjugated with the pi aromatic system. Thus it does take part in the extended conjugation. The resonance stabilizes the benzylic carbon atom, thus it is susceptible to the substitution or elimination reactions such as: \[\text{ }{{\text{S}}_{\text{N}}}\text{1 }\] ,\[\text{ }{{\text{S}}_{\text{N}}}\text{2 }\], E, etc.