Question

A chemist added water and acid to an unknown ester of formula \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{2}\) to produce an acid \(\mathrm{X}\) and an alcohol \(\mathrm{Y}\). Oxidation of \(\mathrm{Y}\) with chromic acid produced \(\mathrm{X}\). Determine the structure of the original ester. Write equations for all reactions that occur.

Short answer

The original ester has the structure \(\mathrm{C}_{3}\mathrm{H}_{7}\mathrm{COOC}_{2}\mathrm{H}_{5}\). The hydrolysis reaction is \(\mathrm{C}_{3}\mathrm{H}_{7}\mathrm{COOC}_{2}\mathrm{H}_{5} + \mathrm{H}_{2}\mathrm{O} \xrightarrow[]{\text{acid}} \mathrm{C}_{3}\mathrm{H}_{7}\mathrm{COOH} + \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\), and the oxidation of alcohol with chromic acid is \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH} \xrightarrow[]{\text{chromic acid}} \mathrm{C}_{3}\mathrm{H}_{7}\mathrm{COOH}\).

Step-by-step solution

- Recall the general structure of an ester and find the possible structure of \(\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{2}\)

An ester has the general formula \(\mathrm{R}_{1}\mathrm{COOR}_{2}\), where R1 and R2 are organic groups, possibly alkyl or aryl groups. In this case, the given formula is \(\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{2}\). Therefore, we need to find possible structures with a combination of R1 and R2 groups that add up to the 6 carbons. As there are 6 carbons in total, we can have the following combinations of R1 and R2: 1. R1 = \(\mathrm{C}_{5}\mathrm{H}_{11}\) and R2 = \(\mathrm{H}\) (5 carbons in R1 and 1 carbon in R2) 2. R1 = \(\mathrm{C}_{4}\mathrm{H}_{9}\) and R2 = \(\mathrm{CH}_{3}\) (4 carbons in R1 and 2 carbons in R2) 3. R1 = \(\mathrm{C}_{3}\mathrm{H}_{7}\) and R2 = \(\mathrm{C}_{2}\mathrm{H}_{5}\) (3 carbons in R1 and 3 carbons in R2)

- Write the reaction of the ester with water and acid (ester hydrolysis)

Ester hydrolysis is the reaction of an ester with water and acid, resulting in the breaking of the ester bond and formation of an acid (X) and an alcohol (Y). The general reaction is: \(\mathrm{R}_{1}\mathrm{COOR}_{2} + \mathrm{H}_{2}\mathrm{O} \xrightarrow[]{\text{acid}} \mathrm{R}_{1}\mathrm{COOH} + \mathrm{R}_{2}\mathrm{OH}\)

- Consider the oxidation of Y with chromic acid to produce X

The oxidation reaction of alcohol (Y) with chromic acid produces acid (X). This means that the oxidation converts the alcohol into the same acid with the same carbon chain. Also, this implies that R1 and R2 should have the same number of carbons since both contribute to the acid formation. This observation eliminates possibilities 1 and 2 from our previous list of combinations, leaving us with the following possibility: R1 = \(\mathrm{C}_{3}\mathrm{H}_{7}\), R2 = \(\mathrm{C}_{2}\mathrm{H}_{5}\)

- Determine the structure of the original ester

Since R1 and R2 are \(\mathrm{C}_{3}\mathrm{H}_{7}\) and \(\mathrm{C}_{2}\mathrm{H}_{5}\) respectively, the original ester has the following structure: \(\mathrm{C}_{3}\mathrm{H}_{7}\mathrm{COOC}_{2}\mathrm{H}_{5}\)

- Write equations for all the reactions that occur

Hydrolysis of the ester: \(\mathrm{C}_{3}\mathrm{H}_{7}\mathrm{COOC}_{2}\mathrm{H}_{5} + \mathrm{H}_{2}\mathrm{O} \xrightarrow[]{\text{acid}} \mathrm{C}_{3}\mathrm{H}_{7}\mathrm{COOH} + \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\) Oxidation of Y with chromic acid: \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH} \xrightarrow[]{\text{chromic acid}} \mathrm{C}_{3}\mathrm{H}_{7}\mathrm{COOH}\) Thus, the structure of the original ester is \(\mathrm{C}_{3}\mathrm{H}_{7}\mathrm{COOC}_{2}\mathrm{H}_{5}\), and the reactions involved are ester hydrolysis and the oxidation of alcohol in the presence of chromic acid.

Key concepts

Ester Hydrolysis

Ester hydrolysis is a fundamental reaction in organic chemistry. In simple terms, it involves breaking down an ester into an acid and an alcohol.
This process requires the presence of water and an acid catalyst, such as hydrochloric acid. When added, the acid helps in breaking the ester bond between the carbonyl carbon and the oxygen atom.

During the hydrolysis of an ester, the molecules rearrange like so:

• The ester usually with the formula \( \mathrm{R}_1\mathrm{COOR}_2 \), reacts with water.
• An acid catalyst facilitates this reaction breaking it into \( \mathrm{R}_1\mathrm{COOH} \) (an acid) and \( \mathrm{R}_2\mathrm{OH} \) (an alcohol).

The key here is that the original ester breaks down while retaining the carbon skeletons of the R groups. It is "hydrolysis" because water, \( \mathrm{H_2O} \), plays a critical role in adding the required hydrogen and oxygen atoms necessary for the transformation. This process can easily take apart esters and is widely used in the breakdown or transformation of organic molecules.

Organic Chemistry Reactions

Organic chemistry reactions are the backbone of creating new compounds and materials from organic molecules. One crucial example is ester reactions, which showcase the versatility of organic transformations.

Reactions can be facilitated through various mechanisms, such as:

• **Addition Reactions**: Where atoms are added to a double or triple bond.
• **Substitution Reactions**: Involving the exchange of an atom or group in the molecule for another.
• **Elimination Reactions**: Where atoms are removed, generally forming double or triple bonds in the process.
• **Rearrangement Reactions**: These involve the reorganization of the atoms within the molecule without adding or removing atoms.

In context, the breaking of an ester bond through hydrolysis represents a simple yet powerful substitution reaction.
Understanding organic chemistry reactions allows for predicting reaction outcomes and creating efficient pathways for synthesizing new compounds. Clarity in exploring how reactants transform into products is beneficial not only for fundamental science but also for applications like drug development, materials science, and more.

Chromic Acid Oxidation

Chromic acid oxidation is an important method used to oxidize alcohols in organic chemistry. It provides a powerful means of transforming primary alcohols into carboxylic acids.

Here's how the reaction typically unfolds:

• When an alcohol like ethanol (\( \mathrm{C_2H_5OH} \)) is treated with chromic acid (\( \mathrm{H_2CrO_4} \)), it is oxidized.
• The process converts the hydroxyl group (\( \mathrm{-OH} \)) into a carbonyl (\( \mathrm{=O} \)), forming a carboxylic acid.
• Primary alcohols usually convert to carboxylic acids, whereas secondary alcohols become ketones.

This specific oxidation is a great way to increase the oxidation state of carbon in organic substances.It involves the transfer of electrons from the alcohol to the oxidizing agent, chromic acid.
The transformation from an alcohol to an acid via this oxidation manifests not merely a change in structure but greatly influences the chemical properties, making it a well-utilized approach in synthetic organic chemistry and biochemical applications.