Question

Formulate a reasonable mechanism for the reduction of acetaldehyde by triethylborane to give ethoxydiethylborane.

Short answer

The mechanism for the reduction of acetaldehyde by triethylborane involves two main steps: 1. Nucleophilic attack of a boron ethyl group on the carbonyl carbon of acetaldehyde, forming a tetrahedral intermediate with a positive charge on the boron atom. 2. Transfer of a hydride from an adjacent boron ethyl group to the carbonyl oxygen, resulting in the formation of ethoxydiethylborane (CH3CH2OB(C2H5)2) and diethylborane as byproduct.

Step-by-step solution

Identifying the reactants

First, let's identify the reactants in the reaction we need to analyze. We have acetaldehyde (CH3CHO) and triethylborane (B(C2H5)3).

Identifying the products

Now, let's identify the product of the reaction: ethoxydiethylborane (CH3CH2OB(C2H5)2).

Proposing the mechanism

Now that we know the reactants and products, we can propose a mechanism for the reduction of acetaldehyde by triethylborane. The reaction mechanism will likely involve the following steps: 1. Nucleophilic attack of a boron ethyl group on the carbonyl carbon of acetaldehyde. 2. Transfer of a hydride from an adjacent boron ethyl group to the carbonyl oxygen, forming the product and the byproducts. Let's describe each step in more detail.

Nucleophilic attack of boron ethyl group on the carbonyl carbon

In this step, one of the ethyl groups (C2H5) attached to the boron atom in triethylborane acts as a nucleophile and attacks the carbonyl carbon in acetaldehyde. This results in the formation of a tetrahedral intermediate with four groups attached to the carbon, and a positive charge on the boron atom.

Transfer of a hydride from an adjacent boron ethyl group to the carbonyl oxygen

Next, one of the remaining ethyl groups attached to the boron atom donates a hydride (H-) to the carbonyl oxygen in the tetrahedral intermediate. This results in the formation of ethoxydiethylborane and diethylborane. Since two ethyl groups are still attached to the boron after the reaction, the product is consistent with the formation of ethoxydiethylborane (CH3CH2OB(C2H5)2). In conclusion, the mechanism for the reduction of acetaldehyde by triethylborane consists of two main steps: the nucleophilic attack of a boron ethyl group on the carbonyl carbon of acetaldehyde, followed by the transfer of a hydride from an adjacent boron ethyl group to the carbonyl oxygen leading to the formation of ethoxydiethylborane.

Key concepts

Organic Reaction Mechanisms

Understanding organic reaction mechanisms is critical for studying organic chemistry. Reaction mechanisms refer to the step-by-step sequence of elementary reactions by which an overall chemical change occurs. Each step involves a reconfiguration of atoms' electrons and understanding these steps is key to predicting the products of a reaction.

When analyzing a reaction mechanism, chemists visualize the breaking and forming of bonds and the intermediate species formed along the way, such as carbocations, carbanions, free radicals, and others. In the reaction mechanism for the reduction of acetaldehyde by triethylborane, the formation of intermediates and the rearrangement of electrons between the reactants lead to the final product. Visualizing the mechanism can also reveal information about the reaction's rate, whether it's stepwise or concerted, and the involvement of catalysts or external agents.

For students struggling with reaction mechanisms, it's essential to practice by writing out each elementary step, understanding the role of each reactant, and the nature of the transition states. It's also useful to know common reaction patterns, such as nucleophilic substitution, elimination, and addition reactions, as these provide a basis for understanding new reactions.

Nucleophilic Attack

A nucleophilic attack is a fundamental process in organic chemistry in which a nucleophile, a species rich in electrons, targets a positive or partially positive charge on an electrophile, an electron-deficient atom or group. This concept is illustrated in the reaction between acetaldehyde and triethylborane.

In the proposed mechanism, an ethyl group from the triethylborane acts as the nucleophile. Nucleophiles can be neutral molecules with free pairs of electrons or anions. The boron atom, before the attack, is slightly electron-deficient due to its three alkyl groups, so the electrophilic carbon atom of the acetaldehyde’s carbonyl group becomes a prime target for the nucleophile.

Key Points in Nucleophilic Attack:

• Identification of electrophilic and nucleophilic centers is vital.
• The attacking nucleophile must have a lone pair or a negative charge.
• The electrophilic center typically features a positive or partial positive charge.
• Formation of a tetrahedral intermediate is common when a carbonyl compound is involved.

This step is crucial as it sets the stage for subsequent rearrangements that lead to the final product.

Hydride Transfer

Hydride transfer is an essential mechanism in organic chemistry where a hydride ion (H⁻), which is a hydrogen atom with an additional electron, is transferred from one molecule to another. This can be part of reduction processes where the gaining of a hydride ion signifies a reduction in the receiving molecule.

In the reduction of acetaldehyde by triethylborane, following the nucleophilic attack, a hydride transfer occurs from an adjacent ethyl group attached to the boron. The transfer of this negatively charged hydrogen species effectively reduces the carbonyl group to an alcohol, exemplifying a common method of reducing carbonyl compounds.

Understanding Hydride Transfer:

• It involves the movement of a hydride ion, not a proton.
• This is often a key step in reduction reactions.
• The source must have a hydride available for donation.
• Accurate identification of the hydride donor and acceptor is crucial.

For students, recognizing the patterns of hydride transfer can greatly help in understanding and predicting the outcomes of organic reactions.