Question

When cis-2-decalone is dissolved in ether containing a trace of \(\mathrm{HCl}\), an equilibrium is established with trans-2-decalone. The latter ketone predominates in the equilibrium mixture.

Short answer

Question: Explain the process of equilibrium and why trans-2-decalone predominates in a mixture of cis-2-decalone dissolved in ether containing a trace of HCl. Answer: When cis-2-decalone is dissolved in ether containing a trace of HCl, an equilibrium is established between the cis- and trans-2-decalone isomers. The HCl acts as a catalyst, promoting the isomerization without being consumed in the reaction. The equilibrium favors the more stable trans-2-decalone due to its lower energy and less strained molecular structure, resulting in a mixture where trans-2-decalone predominates.

Step-by-step solution

Understanding cis- and trans-2-decalone

Before diving into the main topic, it's important to know what cis-2-decalone and trans-2-decalone are. These are isomers of each other, meaning they share the same molecular formula, but their structures are different. The cis- and trans- prefixes indicate that the isomers have different arrangements of substituents around the C2 position of the decalone ring, which is a 10-membered cyclic ketone.

Discussing the role of HCl

When a trace amount of HCl is added to the ether solution containing cis-2-decalone, it acts as a catalyst in the process of reaching the equilibrium between cis- and trans-2-decalone. A catalyst is a substance that increases the rate of reaction without being consumed in the reaction. In this case, HCl promotes the isomerization of cis-2-decalone to trans-2-decalone without being used up in the reaction.

Establishing equilibrium

The equilibrium state refers to the point at which the rate of the forward reaction (conversion of cis-2-decalone to trans-2-decalone) equals the rate of the reverse reaction (conversion of trans-2-decalone to cis-2-decalone). This means that the concentrations of both reactants and products remain constant, as long as no external factors affecting the equilibrium are altered. When the cis-2-decalone is dissolved in ether containing a trace of HCl, it induces a reaction that forms trans-2-decalone as a product and establishes an equilibrium between the reactants and products.

Reasoning behind the predominance of trans-2-decalone

In this particular equilibrium, trans-2-decalone predominates over cis-2-decalone. This can be explained by considering the stability of two isomers: trans-2-Decalone is more stable than cis-2-decalone due to the less strained and more evenly distributed molecular structure, which results in overall lower energy for the trans isomer. As a result, the equilibrium lies towards the more stable form (trans-2-decalone) in order to minimize the system's energy. In summary, when cis-2-decalone is dissolved in ether containing a trace of HCl, an equilibrium is established between the cis- and trans-2-decalone isomers. The HCl acts as a catalyst, promoting the isomerization, and the equilibrium favors the more stable trans-2-decalone due to its lower energy and less strained molecular structure.