Question

Terpinene (1) and \(\gamma\)-terpinene (2) are isomeric compounds \(\left(\mathrm{C}_{10} \mathrm{H}_{16}\right)\) that are constituents of many plants. Upon catalytic hydrogenation, they both afford 1isopropyl-4-methylcyclohexane. However, on ozonolysis followed by oxidative workup, each compound yields different products. Provide structures for \(\mathbf{1}\) and \(\mathbf{2}\) and explain your reasoning.

Short answer

Terpinene (1) has two double bonds at positions 1 and 3. \(γ-terpinene\) (2) has three double bonds at positions 1, 3, and 5.

Step-by-step solution

Analyze the hydrogenation product

When both terpinene (1) and \(\text{γ-terpinene (2)}\) undergo catalytic hydrogenation, they both convert into 1-isopropyl-4-methylcyclohexane. This suggests that both compounds share a common cyclohexane skeleton with similar substituents.

Identify the possible double bond arrangements

Since terpinene and γ-terpinene are isomers with molecular formula \(\text{C}_{10} \text{H}_{16}\), they are likely to have different arrangements of double bonds. Terpinene commonly has two double bonds, while \(\text{γ-terpinene}\) typically has three double bonds.

Structural implications of ozonolysis

Upon ozonolysis followed by oxidative workup, different products will form depending on the position of the double bonds. For \(\text{γ-terpinene}\) (2), which has three double bonds, breaking these will yield different structures and combinations of carbonyl groups compared to terpinene (1) with two double bonds.

Determine the structures of 1 and 2

Given that both 1 and 2 share a common cyclohexane skeleton and their different responses to ozonolysis, we can deduce the possible structures: \(1: \text{Terpinene}\) has two double bonds at positions 1 and 3 in the cyclohexane ring, while \(2: \text{γ-terpinene}\) has three double bonds at positions 1, 3, and 5 in the cyclohexane ring.

Confirm reasoning

The structures of terpinene and \(\text{γ-terpinene}\) are consistent with their different ozonolysis products and the resultant 1-isopropyl-4-methylcyclohexane after hydrogenation.

Key concepts

Catalytic Hydrogenation

Catalytic hydrogenation is a chemical reaction where hydrogen is added to a molecule in the presence of a catalyst, typically a metal such as palladium, platinum, or nickel. This reaction is especially important for converting unsaturated compounds (those with double or triple bonds) into saturated ones (single bonds only).

For terpinene (1) and \(\text{γ-terpinene}\) (2), this process means adding hydrogen atoms to the carbon-carbon double bonds. The result for both compounds is the same product: 1-isopropyl-4-methylcyclohexane. This indicates that the differences in the initial structures of these isomeric compounds lie in the positioning and number of double bonds, not in the overall carbon skeleton of the molecule. The addition of hydrogen effectively 'saturates' these double bonds, leaving behind a cycle of six carbon atoms with an isopropyl group and a methyl group attached.

Ozonolysis

Ozonolysis is a reaction where ozone (O\textsubscript{3}) is used to cleave the carbon-carbon double bonds in a molecule. This reaction typically forms carbonyl compounds such as aldehydes, ketones, and carboxylic acids depending on the presence of hydrogen atoms on the carbon atoms involved.

For compounds like terpinene (1) and \(\text{γ-terpinene}\) (2), which differ in their number of double bonds, ozonolysis will yield different products, revealing the nature of these double bonds. Specifically, terpinene has two double bonds, leading to fewer cleavage sites and fewer carbonyl-containing products, while \(\text{γ-terpinene}\) has three double bonds, resulting in more cleavage sites and potentially more types of carbonyl compounds.

This difference in products upon ozonolysis helps us deduce the specific structure of each isomer, confirming the arrangement and number of their double bonds.

Double Bond Arrangements

Double bond arrangements are crucial in determining the chemical properties and reactivity of isomeric compounds. For terpinene (1) and \(\text{γ-terpinene}\), the arrangement and number of double bonds define their unique characteristics, even though both share the same molecular formula, \(\text{C}_{10}\text{H}_{16}\).

In terpinene, two double bonds might typically be placed at positions 1 and 3 of the cyclohexane ring. This sets up a structure where there are less strained or non-conjugated double bonds. On the other hand, \(\text{γ-terpinene}\) is structured with three double bonds at positions 1, 3, and 5. This arrangement implies a highly conjugated system that can dictate different reactivity patterns compared to terpinene.

Understanding these arrangements not only helps in chemical reactions like hydrogenation and ozonolysis but also in predicting the physical properties and potential uses of these compounds in various applications, such as in pharmaceuticals, fragrances, and more.