The following questions are based on the reactions of benzaldehyde (6) shown here. (Sections 23.

The following questions are based on the reactions of...

Key Concepts

Aldehyde Reactivity

Aromatic aldehydes feature a strongly electrophilic carbonyl carbon that is susceptible to attack by nucleophiles. This reactivity underpins a variety of transformations including nucleophilic addition, the formation of intermediates like cyanohydrins, and subsequent condensation or oxidation reactions. Understanding this concept is fundamental to manipulating these compounds in synthesis.

Cyanohydrin Formation

When a cyanide ion attacks the carbonyl carbon of an aldehyde, a cyanohydrin intermediate is formed. This reversible addition is not only a prototypical example of nucleophilic addition to a carbonyl group but also serves as a key step in catalytic processes such as the benzoin condensation, where cyanide acts as a catalyst to couple two aldehyde molecules.

Benzoin Condensation

The benzoin condensation is a reaction in which two molecules of an aromatic aldehyde, in the presence of a nucleophilic catalyst (often cyanide), couple to form an ??hydroxy ketone known as benzoin. This process involves the generation of an acyl anion equivalent through cyanide addition and serves as a classic example of umpolung reactivity, where the normal electrophilic character of the aldehyde is temporarily reversed.

Oxidation Reactions

Oxidation of secondary alcohols is a key transformation in organic synthesis, exemplified by the conversion of benzoin to a diketone (benzil). Such oxidations are crucial for altering the oxidation state of a molecule, thereby enabling further synthetic manipulation or rearrangement reactions.

Rearrangement Mechanisms

Rearrangement reactions, such as the benzilic acid rearrangement, involve the migration of substituents (often aryl groups) under basic conditions, leading to a new carbon framework. These mechanisms typically proceed through nucleophilic addition, followed by a shift of a group and reorganization of the molecular structure, highlighting the importance of understanding intermediate formation and transition state stabilization in reaction pathways.

Transcript

00:01 So we're asked to answer the following questions, and we're starting with benzaldehyde.

00:13 So in the first one, we're reacting benzaldehyde with ethyl magnesium bromide.

00:20 So if a green yard reagent, this carbon is nucleophilic.

00:22 It's going to attack the carbonyl carbon, curate the ketone, eventually to an alcohol.

00:34 So we're going to get, it's also going to add on whatever was attached to magnesium bromide onto the carbon that was attacked.

00:44 So we have the addition of an ethyl group.

00:54 So reacting with chrymic acid, which is a strong oxidizing agent, it's going to convert it into a ketone.

01:02 If we react the churmic acid with a primary alcohol, it would convert it to a carboxyl acid.

01:08 But here we just have a secondary alcohol, so it doesn't matter what we reacted with.

01:16 And finally, with methyl magnesium bromide, attacking the carbonyl carbon, and we get the addition of a methyl.

01:43 So for b, we have to create the start.

01:48 Structure from benzaldehod.

02:05 So whenever you react to ketone with either ethane -1 -2 -thial or ethane -one -2 -dial, so the dial meaning two alcohols, like ethylene glycol, it's going to replace the ketone with these two groups, the two alcohols, and drop the hydrogens.

02:28 But in the case of ethane -1 -2 -thyel, it's going to replace the ketone with the two sulfur atoms and drop the hydrogen.

02:39 We're just going to keep the two carbon chain between them.

02:45 So reacting with ethane 1 -2thyle, we'll give us that.

02:56 And then reacting with rainy nickel, we'll get rid of it.

03:08 So we still have a methyl reader.

03:11 I will go back to it.

03:15 So it's a good way to get rid of ketone, i mean, protecting groups, like ethyl and glycol for ketones.

03:22 By reacting with rainy nickel.

03:27 And then lastly, we have to create, converts structure 2 into 12.

03:35 And we're starting with benz -aldehyde again.

03:48 So we want to create this.

04:01 So whenever you have this rearrangement of, or this arrangement of a ketone next to an al -keen, it's pretty characteristic of an aldol reaction.

04:12 So the al, implying with an aldehyde, or reacting with an aldehyde, and the ol is with an alcohol or an alcohol.

04:25 So we're starting with benz -aldehyde, we want to get this structure...

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