Question

The most likely protonation site in the following molecule is :
A. $C-1$
B. $C-2$
C. $C-3$
D. $C-6$

Answer 1

To solve this question, we should first know what is protonation, its mechanism, and what the protonation sites are. Protonation is the addition of a ($H_{{}}^{+}$) to an atom or a molecule to form a conjugate acid. It is also known as hydronation.

Complete step-by-step answer:
Protonation is a chemical reaction that has usage in stoichiometric and catalytic processes. The protonation process can happen more than once in some biological macromolecules, they are called polybasic. In acid-base reactions, protonation and deprotonation both occur. The theory is known as “Bronsted – Lowry acid” in which the chemical substance helps in the protonation of another substance.
The mobility of protons in many solvents is very high which makes the process of protonation rapid. The rate of the protonation depends on the acidity of the protonation of species which means strong acids show a high rate of protonation as compared to weak acids.
When we add a proton at the position $C_{{}}^{1}$, it forms the tropylium carbocation ( The delocalization of $\pi$ electrons results in the formation of a stable aromatic species ) therefore it is considered to be the most reactive site towards protonation.

When we add a proton at any other carbon atom except for $C_{{}}^{1}$, it causes hindrance in the way of delocalization of the$\pi$electrons by interrupting the planarity of molecules and makes the molecule unstable.

So, Option (A) $C-1$ is correct.

Note: A carbon atom having a positive charge and three bonds are called carbocation, They are carbon cations that have a positive charge on an atom of carbon. A carbon atom has a negative charge and its trivalent is called carbanion.