Question

Ethanal is allowed to react with ethanol (excess) in presence of dry HCl gas. The product formed is (A) ethoxyethane (B) 1,2 -diethoxyethane (C) 1,1 -diethoxyethane (D) 1 -ethoxyethanol

Short answer

The product formed when ethanal (CH3CHO) reacts with ethanol (CH3CH2OH) in the presence of dry HCl gas is 1-ethoxyethanol. This reaction occurs via a nucleophilic addition mechanism, where the protonated ethanol attacks the electrophilic carbonyl group of ethanal. The final product can be represented as CH3CH(OCH2CH3)OH. Therefore, the correct answer is (D) 1-ethoxyethanol.

Step-by-step solution

Identify the reactants and the catalyst

The reactants in the given problem are ethanal and ethanol, while dry HCl gas is acting as a catalyst. Let's write down their chemical structures: - Ethanal (CH3CHO) - Ethanol (CH3CH2OH) - Dry HCl gas (HCl)

Identify the nucleophile and electrophile

In this reaction, ethanol, containing the hydroxyl group (OH), acts as a nucleophile due to the lone pair of electrons on the oxygen atom. Ethanal, having a carbonyl group (C=O), serves as the electrophile because of the electrophilic carbon atom.

Determine the reaction mechanism

In the presence of dry HCl gas, the nucleophile (ethanol) is protonated, transforming the hydroxyl group into a better leaving group (-OH2+). This reaction proceeds via a nucleophilic addition reaction, where the nucleophile attacks the electrophilic carbonyl group of ethanal, leading to the formation of a new product.

Predict the product

When the protonated ethanol attacks the electrophilic carbonyl group in ethanal, the product formed will be 1-ethoxyethanol. The reaction can be represented as follows: CH3CHO + H3CCH2OH (in the presence of HCl) → CH3CH(OCH2CH3)OH The correct answer is (D) 1-ethoxyethanol.

Key concepts

Nucleophilic Addition Reaction

A nucleophilic addition reaction is a type of organic reaction where a nucleophile forms a bond with an electrophile, typically at a carbonyl group. This type of reaction is common in aldehydes and ketones because of the polarized nature of the carbonyl (C=O) bond.

In our example of ethanal reacting with ethanol, the nucleophile is ethanol. This is because it has a pair of non-bonded electrons on the oxygen atom of its hydroxyl group (OH). These electrons are ready to be donated, making ethanol an excellent nucleophile.

Dry HCl gas serves as a catalyst, protonating the nucleophile to form a better attacking species. The protonation of ethanol makes the oxygen more positively charged, turning the hydroxyl group into a better leaving group. This step is crucial because it enhances the reactivity of the nucleophile.

The nucleophile then attacks the electrophilic carbon atom of ethanal's carbonyl group, resulting in the carbon-oxygen double bond becoming a single bond, while the carbon forms a new bond with the oxygen of ethanol. This entire process culminates in the formation of an acetal, reinforcing the concept of nucleophilic addition.

Electrophile and Nucleophile

Electrophiles and nucleophiles play essential roles in organic reactions. An electrophile is a chemical species that seeks electrons, while a nucleophile is one that donates electrons.

In the reaction between ethanal and ethanol, the electrophile is ethanal. This is due to the carbon atom in its carbonyl group, which is electron deficient and thus highly attractive to electron-rich species. The partially positive charge of the carbon makes it an ideal target for nucleophilic attack.

Ethanol acts as the nucleophile in this case. The oxygen in its hydroxyl group has a pair of free electrons, which can be readily shared, making ethanol highly nucleophilic. The initial step of protonating ethanol with dry HCl enhances this nucleophilicity, making it even more reactive towards the electrophile.

Understanding these roles highlights how electrophiles and nucleophiles interact in organic reactions, allowing you to predict the outcome and the sequence of steps within a given mechanism.

Organic Reaction Mechanisms

Organic reaction mechanisms provide a detailed step-by-step account of how reactants transform into products. These mechanisms reveal the sequence and nature of the chemical bonds that break and form during a reaction.

In the discussed acetal formation process, the mechanism commences with the protonation of the nucleophile (ethanol) using dry HCl. This step alters the reactivity of the ethanol by making it capable of easily donating electrons.

Next, the nucleophile carries out a nucleophilic attack on the electrophilic center of ethanal. This is the core transformation step in the mechanism, where the nucleophile’s electrons form a new bond with the carbon atom of the aldehyde, leading to the development of a new structure - in this case, 1-ethoxyethanol.

The intricacies of such mechanisms underscore the dynamic nature of chemical reactions in organic chemistry, explaining how reactants convert into products via distinct pathways. Understanding organic reaction mechanisms is crucial for mastering advanced organic synthesis and predicting the outcomes of various chemical reactions.