Animated Solution for Chemistry - Organic Chemistry: Identify A in the given chemical reaction.

Imagine you are looking at a long, flexible chain of carbon atoms. The molecule given to us is an open-chain alkane. Before we can predict what will happen to it, we must first identify exactly what it is.

By finding the longest continuous carbon chain, we can determine its IUPAC name. If we start numbering from the right side of the molecule as drawn, we trace a main chain consisting of exactly $6$ carbon atoms. Attached to the second carbon of this chain is a single methyl group (${\text{CH}}_3$). Therefore, the chemical identity of our starting material is 2-methylhexane.

Now, let's shift our focus to the reaction arrow. It is loaded with specific, powerful conditions: ${\text{Mo}}_2{\text{O}}_3$ (Molybdenum(III) oxide) acting as a catalyst, a scorching temperature of $773\text{ K}$, and a high pressure of $10-20\text{ atm}$.

These conditions are the classic signature of an aromatization reaction. Aromatization is a fascinating chemical transformation where aliphatic, open-chain alkanes containing six or more carbon atoms are forced to fold onto themselves. The catalyst strips away hydrogen atoms, forcing the carbon chain to close into a ring and form alternating double bonds. This process is formally known as dehydrocyclization.

So, how does 2-methylhexane behave under these extreme conditions? The $6$-carbon main chain is the perfect length to form a highly stable, six-membered aromatic ring—benzene.

As the main chain cyclizes and loses hydrogen to form the aromatic $\pi$-electron cloud, the methyl group attached to the second carbon doesn't just vanish. It remains firmly bonded to the newly formed benzene ring. A benzene ring with a single methyl group attached to it is universally known as Toluene.

Let's look at the stoichiometry of this transformation. Our reactant, 2-methylhexane, has the molecular formula ${\text{C}}_7{\text{H}}_{16}$. The product, toluene, has the formula ${\text{C}}_7{\text{H}}_8$.

To balance the equation, the reaction must release the excess hydrogen atoms as diatomic hydrogen gas.

Exactly $4$ molecules of ${\text{H}}_2$ are liberated. The major product 'A' is undeniably toluene.

This reaction is a high-yield concept for competitive exams. It is crucial to remember the patterns: - If you start with n-hexane ($6$ carbons), the product is Benzene. - If you start with n-heptane ($7$ carbons), the product is Toluene. - If you start with 2-methylhexane (also $7$ carbons), the product is also Toluene.

Understanding the mechanism of dehydrocyclization allows you to predict the aromatic products of even larger alkanes, such as n-octane forming various isomers of xylene or ethylbenzene. Always count your carbons carefully!