Question

A compound named Y-terpinene, an isomer of \(\alpha\) -terpinene, is isolated along with the a isomer from the oils of a number of plant products. Upon ozonolysis, followed by an oxidative workup, \(\Upsilon\) -terpinene yields two compounds, one of which is 4 -methyl-3-oxopentanoic acid. The second compound is an acid which decarboxylates upon heating and gives acetone. Two structures consistent with the data are possible for \(\Upsilon\) -terpinene. What are they? On the basis of the isoprene rule, one of them can be discarded as highly improbable. What is the structure of \(\Upsilon\) -terpinene?

Short answer

The two possible structures for Y-terpinene based on ozonolysis products are: Structure 1: \(CH_3C(CH_3)=CCH_2C\text{(COOH)}=CHCH_2\text{(COOH)}\) Structure 2: \(CH_3C(CH_3)=CCH_2\text{(COOH)}=CHCH_2C\text{(COOH)}=CH_2\) Applying the isoprene rule, Structure 2 is more likely to be found in nature as it has more isoprene subunits connected to each other. Hence, the most probable structure of Y-terpinene is: \[CH_3C(CH_3)=CCH_2\text{(COOH)}=CHCH_2C\text{(COOH)}=CH_2\]

Step-by-step solution

Analyze the products of ozonolysis

One of the products obtained after ozonolysis is 4-methyl-3-oxopentanoic acid. The second product is an acid that, upon decarboxylation, produces acetone. This implies that the second compound contains a carboxylic acid adjacent to the acetone's carbonyl group.

Reverse engineer the terpinene structure from its products

We have identified two products after ozonolysis: 1. 4-methyl-3-oxopentanoic acid 2. A carboxylic acid that, upon decarboxylation, produces acetone (a ketone) Now, let's connect these parts to create a possible terpinene structure. Considering the formation of the 4-methyl-3-oxopentanoic acid, there is a linkage between the carbonyl carbon and the 4-methyl-3-oxopentanoic acid's alpha carbon. The other product suggests a connection between the carbonyl carbon and the carboxylic acid's alpha carbon. The resulting structure can be represented as: Structure 1: \[CH_3C(CH_3)=CCH_2C\text{(COOH)}=CHCH_2\text{(COOH)}\] However, we can also come up with another possible structure considering the same products: Structure 2: \[CH_3C(CH_3)=CCH_2\text{(COOH)}=CHCH_2C\text{(COOH)}=CH_2\]

Apply the isoprene rule

Now, we will apply the isoprene rule to evaluate which structure is more probable. The isoprene rule states that terpenes are formed by connecting isoprene units head-to-tail, meaning that all terpene carbon atoms can be grouped into one or more isoprene subunits. Comparing both structures: - Structure 1 has only one C=C bond connected to an isoprene subunit (CH₂C(CH₃)=C). - Structure 2 has two C=C bonds connected to isoprene subunits (CH₂C(CH₃)=C and CH₂=C(CH₂COOH)CH). Based on the isoprene rule, Structure 2 is more likely to be found in nature as it has more isoprene subunits connected to each other. Therefore, Y-terpinene's most probable structure is: Structure 2: \[CH_3C(CH_3)=CCH_2\text{(COOH)}=CHCH_2C\text{(COOH)}=CH_2\]

Key concepts

Ozonolysis in Organic Chemistry

Ozonolysis is a method used in organic chemistry to fragment unsaturated hydrocarbons—compounds with double or triple carbon-carbon bonds. This powerful reaction involves the use of ozone (O₃) to cleave these bonds, creating smaller molecules often in the form of carbonyl compounds, such as aldehydes, ketones, and acids.

In practice, the reaction comprises two stages: the addition of ozone to the double bond to form a molozonide, which is unstable, and its subsequent rearrangement to a more stable ozonide. Upon reductive or oxidative workup, the ozonide is broken down into carbonyl fragments. The oxidative workup may also introduce oxygen-containing groups, such as carboxylic acids.

Understanding the outcomes of ozonolysis enables chemists to reverse engineer the starting material's structure, based on the fragmentation products, which is particularly useful in structure determination of complex molecules such as terpenes.

Isoprene Rule

The isoprene rule, also known as the biogenetic isoprene rule, is a guiding principle in the structural biology of terpenes. It is based on the observation that natural terpenes are constructed from 5-carbon units, similar to isoprene (C5H8).

Terpenes can be classified according to the number of isoprene units they contain. Monoterpenes, for example, have two isoprene units, while sesquiterpenes have three. According to the rule, isoprene units are joined 'head-to-tail' to form the larger structures.

This rule allows for the prediction of terpene structures and assists in determining which of possible structural isomers is more likely to occur in nature. The isoprene units in natural terpenes usually form regular patterns which often simplifies the deduction of complex structures.

Terpenes Structure

Terpenes are a diverse class of organic compounds known for their roles in creating the fragrances and flavors of plants. Structurally, terpenes are derived from hydrocarbons consisting of multiple isoprene (C5H8) units.

Variability in Terpenes

Despite sharing a basic isoprene-based structure, terpenes are highly variable owing to the many ways these units can combine and be modified. Structural intricacies such as ring formation and the addition of functional groups give rise to an extensive range of compounds with unique biochemical properties and activities.

Importance of Terpene Structure

The structure of terpenes not only determines their scent and taste but can also affect their biological functions. Their configuration impacts their interaction with biological systems, influencing their therapeutic and ecological roles.

Organic Compound Decarboxylation

Decarboxylation is a chemical reaction where a carboxyl group (-COOH) is removed from an organic molecule, releasing a molecule of carbon dioxide (CO2). This reaction is significant in both biochemical processes and synthetic organic chemistry.

In a laboratory setting, decarboxylation typically requires heat. The reaction plays a crucial role in the fragmentation analysis of organic compounds, providing insight into their structure. When paired with techniques such as ozonolysis, decarboxylation products (like acetone from Y-terpinene) can corroborate the presence of specific structures and pathways in the compound subjected to analysis.

Structural Isomerism

Structural isomerism, or constitutional isomerism, occurs when molecules with the same molecular formula have different bonding patterns and atomic organization, resulting in distinct properties. There are several types of structural isomerism:

• Chain isomerism: Isomers differ in the arrangement of the carbon skeleton.
• Positional isomerism: Isomers have functional groups in different positions on the same carbon skeleton.
• Functional isomerism: Isomers have different functional groups.

Understanding isomerism is crucial in organic chemistry because isomers can exhibit dramatically different physical and chemical behaviors. Identifying the correct isomer requires consideration of synthesis pathways, stability, and often, rules like the isoprene rule for terpenes.