Question

Give structures of compounds \(\mathrm{A}, \mathrm{B}\) and \(\mathrm{C}\) : ethyl oxalate \(+\) ethyl acetate + sodium ethoxide, then \(\mathrm{H}^{+} \rightarrow \mathrm{A}\left(\mathrm{C}_{8} \mathrm{H}_{20} \mathrm{O}_{5}\right)\) \(\mathrm{A}+\) ethy 1 bromoacetate \(+\mathrm{Zn}\), then \(\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{B}\left(\mathrm{C}_{12} \mathrm{H}_{20} \mathrm{O}_{7}\right)\) \(\mathrm{B}+\mathrm{OH}^{-}+\) heat, then \(\mathrm{H}^{+} \rightarrow \mathrm{C}\left(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{7}\right)\), citric acid.

Short answer

In this series of reactions: 1) Sodium ethoxide deprotonates ethyl acetate, which then attacks ethyl oxalate, resulting in Compound A (\(C_{8}H_{20}O_{5}\)). 2) Ethyl bromoacetate reacts with zinc forming a carbanion that attacks the carbonyl carbon of compound A, giving Compound B (\(C_{12}H_{20}O_{7}\)). 3) Heating compound B with hydroxide, followed by acidification, results in the formation of citric acid (Compound C, \(C_{6}H_{8}O_{7}\)).

Step-by-step solution

React ethyl oxalate, ethyl acetate, and sodium ethoxide

Sodium ethoxide would act as a base and remove the acidic hydrogen from ethyl acetate, generating a resonance-stabilized enolate, which then attacks the carbonyl of ethyl oxalate. The result of this reaction is Compound A with a molecular formula of \(C_{8}H_{20}O_{5}\).

React Compound A, ethyl bromoacetate and Zn

Ethyl bromoacetate can be considered as a carbonyl compound with a good leaving group (bromo group). When it reacts with zinc, it forms a carbanion. This carbanion is a strong nucleophile. It can react with compound A as carbonyl compound by attacking its carbonyl carbon to eliminate ethoxide anion. This gives us compound B, with a molecular formula of \(C_{12}H_{20}O_{7}\).

React Compound B with an Hydroxide Ion and heat, then add an acid

Heating compound B with hydroxide would form a carboxylate ion by losing a molecule of ethanol. Then adding \(H^+\) to the resulting solution will give citric acid (Compound C), with a molecular formula of \(C_{6}H_{8}O_{7}\). It's worth noting that citric acid is a well-known organic compound, and its structure is widely accepted. This can help in reassuring that the reactions conducted in steps 1, 2 and 3 are feasible as they lead to a known compound with the specified molecular formula.

Key concepts

Ethyl Oxalate

Ethyl oxalate is an organic compound with the formula \((C_2H_5)_2C_2O_4\). It is commonly used in organic synthesis as a reagent. This ester derivative of oxalic acid has a structure where two ethyl groups are attached to the oxalate ion. In chemical reactions, ethyl oxalate acts primarily as an electrophile, due to the presence of the electron-withdrawing carbonyl groups. This makes it reactive with nucleophiles, such as enolate ions, which are formed in the reaction between ethyl acetate and sodium ethoxide. Understanding the properties of ethyl oxalate is important for predicting the outcomes of reactions where it might be used. Some key properties include:

• Colorless liquid at room temperature.
• Boiling point around 183°C.
• Sweet, pungent odor.

Sodium Ethoxide

Sodium ethoxide \((NaOCH_2CH_3)\) is a strong base made by dissolving sodium metal in ethanol. In organic chemistry, it is commonly used as a base to generate enolate ions from esters or ketones. The base strength of sodium ethoxide makes it effective in deprotonating acidic hydrogen atoms in molecules like ethyl acetate, promoting further chemical reactions. Sodium ethoxide’s role as a base in the original solution highlights its ability to:

• Form enolate ions by removing acidic protons.
• Promote nucleophilic attacks on electrophiles like carbonyl groups.
• Facilitate various elimination and condensation reactions.

Its reactivity must be carefully controlled to ensure desired outcomes in organic synthesis.

Ethyl Bromoacetate

Ethyl bromoacetate is an organic compound that serves as a versatile building block in synthetic chemistry. The molecular structure of ethyl bromoacetate \(BrCH_2CO_2C_2H_5\) includes an ester group and a bromine atom, making it a suitable carbonyl compound with a good leaving group. This configuration allows ethyl bromoacetate to be effectively used in various chemical reactions, particularly those forming carbanions:

• Acts as an electrophile due to the bromine, facilitating reactions with nucleophiles.
• Upon treatment with zinc, forms carbanions that are reactive nucleophiles.
• Participates in formation of complex structures by nucleophilic displacement.

The reaction with zinc in the step-by-step solution shows ethyl bromoacetate's ability to generate carbanions, which then react with compound A to form compound B.

Citric Acid

Citric acid is a well-known organic compound with a molecular formula of \(C_6H_8O_7\). It is widely found in citrus fruits and is used in various industries, from food to pharmaceuticals, due to its weak acid properties and pleasant sour taste. In the original problem, citric acid represents the final product (compound C) and confirms the feasibility of the reactions described. Understanding citric acid involves knowing its:

• Role as a tricarboxylic acid with three -COOH groups.
• Use as a buffering agent in chemical solutions.
• Ability to chelate metals, which is useful in stabilizing and preserving foods.

The formation of citric acid in the reaction demonstrates the predictable and controlled outcomes possible in organic synthesis, where intermediate compounds eventually lead to well-characterized end products.

Molecular Formula

Molecular formulas in chemistry represent the types and numbers of atoms in a molecule. They serve as vital tools in identifying substances and predicting the outcomes of chemical reactions. In the given reaction sequence, understanding the molecular formula of each compound is essential:

• Compound A: \(C_8H_{20}O_5\) shows how the transformation from ethyl oxalate and ethyl acetate occurs.
• Compound B: \(C_{12}H_{20}O_7\) is derived from further reactions with ethyl bromoacetate.
• Compound C (Citric Acid): \(C_6H_8O_7\) indicates the successful conversion into a known product.

A deep understanding of molecular formulas aids in synthesizing new compounds and verifying the presence of the correct reaction products through their chemical equations.