Question

Compound A, \(\mathrm{C}_{10} \mathrm{H}_{18} \mathrm{O},\) undergoes reaction with dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\) at \(50^{\circ} \mathrm{C}\) to yield a mixture of two alkenes, \(\mathrm{C}_{10} \mathrm{H}_{16}\). The major alkene product, \(\mathrm{B}\). gives only cyclopentanone after ozone treatment followed by reduction with zinc in acetic acid. Identify \(\mathbf{A}\) and \(\mathbf{B}\), and write the reactions.

Short answer

Compound A is 1-methylcyclopentanol; Compound B is 1-methyl-1-cyclopentene.

Step-by-step solution

Analyzing the Compound A

Compound A has the formula \( \mathrm{C}_{10} \mathrm{H}_{18} \mathrm{O} \), which is consistent with an alcohol because it contains an oxygen atom likely bonded as a hydroxyl group (\(-\mathrm{OH}\)). Since Compound A reacts with dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\), this suggests a dehydration reaction to form alkenes.

Dehydration and Formation of Alkenes

In the presence of sulfuric acid, an alcohol undergoes dehydration, losing a molecule of water to form an alkene. Therefore, Compound A \((\mathrm{C}_{10} \mathrm{H}_{18} \mathrm{O})\) loses water \((\mathrm{H}_2\mathrm{O})\) to become \(\mathrm{C}_{10} \mathrm{H}_{16}\), forming a mixture of alkenes. The major alkene must be highly substituted due to the stability conferred by substituents.

Identifying the Major Alkene B

The major product, Alkene B, undergoes ozonolysis producing cyclopentanone. This indicates that B has a cyclopentane ring with a double bond. Given that it leads to cyclopentanone, B is likely 1-methyl-1-cyclopentene.

Identifying Compound A

Since Compound B, 1-methyl-1-cyclopentene, forms from a dehydration reaction, Compound A must be the corresponding alcohol. Therefore, Compound A is likely 1-methylcyclopentanol, which, when dehydrated, would primarily form 1-methyl-1-cyclopentene as the major product.

Writing the Reactions

1. Dehydration of 1-methylcyclopentanol: \[ \text{1-methylcyclopentanol}\, \underset{\text{dilute } \mathrm{H_2SO_4}}{\longrightarrow} \text{1-methyl-1-cyclopentene (B) + H_2O} \] 2. Ozonolysis of 1-methyl-1-cyclopentene: \[ \text{1-methyl-1-cyclopentene (B)} \xrightarrow{\mathrm{O_3, Zn}} \text{cyclopentanone} \]

Key concepts

Alkene Stability

Understanding alkene stability is crucial when predicting the products of reactions. Alkenes are hydrocarbons with at least one carbon-to-carbon double bond, \((C=C)\). Their stability is greatly influenced by substituents that are bonded to the double-bonded carbons.
Substituents can stabilize alkenes due to hyperconjugation and the electron-donating effects of alkyl groups.
The more highly substituted an alkene is, the more stable it becomes.

• Tetrasubstituted alkenes are more stable than trisubstituted alkenes.
• Trisubstituted alkenes are more stable than disubstituted alkenes.
• Disubstituted alkenes are more stable than monosubstituted alkenes.

This is why in the dehydration reaction of an alcohol, the major product tends to be the more stable alkene, often the one with the greatest substitution degree.
Understanding this concept helps us predict which alkene will form as the major product in a reaction like the dehydration of 1-methylcyclopentanol.

Ozonolysis Reaction

Ozonolysis is a critical reaction in organic chemistry, particularly for breaking down alkenes.
This process uses ozone \((O_3)\) to cleave the double bonds of an alkene, yielding carbonyl-containing compounds, such as aldehydes, ketones, or carboxylic acids, depending on the substrate.
During the reaction, ozone initially adds across the double bond, forming a molozonide intermediate. This then rearranges to create an ozonide which is subsequently reduced to yield the final carbonyl products.

• For example, an alkene with a terminal double bond may produce an aldehyde and a ketone.
• An internal double bond typically results in two ketone fragments.

In the case of our example, the ozonolysis of 1-methyl-1-cyclopentene results in the formation of cyclopentanone, indicating that the alkene originally had a double bond within a five-membered ring structure. This provides a useful fingerprint for identifying unknown compounds and their structures.

Cyclopentanone Formation

The formation of cyclopentanone during ozonolysis is an excellent clue to the structure of the initial alkene.
In simple terms, if the treatment of an alkene results in cyclopentanone, it indicates the presence of a cyclopentane ring with a double bond.
In our specific exercise, this insight helps us identify alkene B as 1-methyl-1-cyclopentene. Here's why:

• When 1-methyl-1-cyclopentene undergoes ozonolysis, the double bond within the five-membered ring is cleaved.
• The cyclic structure ensures that the break-up leads to the formation of a single ketone, cyclopentanone, since the ring itself dictates the resulting carbonyl group.

This specificity is useful because cyclopentanone is a simple straightforward product that provides direct insight into the original carbon structure, making it easier to deduce the precursor alkene's make-up and confirm mechanisms like the dehydration reaction involved.