Question

All of the following reactions may result in a ketone EXCEPT: A. ozonolysis of an alkene. B. aldol condensation. C. oxidation of a primary alcohol D. Friedel-Crafts acylation

Short answer

C. oxidation of a primary alcohol

Step-by-step solution

Analyze each reaction type

The first step is to understand what kind of products each reaction type typically yields.

Ozonolysis of an alkene

Ozonolysis of an alkene breaks the double bond and typically forms aldehydes or ketones, depending on the structure of the starting alkene. This reaction can indeed form a ketone.

Aldol condensation

Aldol condensation typically involves the combination of two aldehyde or ketone molecules to form a β-hydroxy ketone and then dehydrating it to form an α,β-unsaturated ketone. This reaction results in a ketone.

Oxidation of a primary alcohol

Oxidation of a primary alcohol first forms an aldehyde and then further oxidation can turn it into a carboxylic acid. It does not form a ketone.

Friedel-Crafts acylation

Friedel-Crafts acylation is a reaction where an acyl group is introduced into an aromatic ring. This process typically results in the formation of a ketone as the main product.

Identify the exception

By analyzing the products, it can be seen that all reactions except the oxidation of a primary alcohol result in the formation of a ketone. Thus, the oxidation of a primary alcohol is the exception.

Key concepts

Ozonolysis of an Alkene

Ozonolysis is a reaction used in organic chemistry to break alkenes (compounds with a carbon-carbon double bond) into smaller pieces. It involves the reaction of ozone (O3) with the alkene.
The process can produce both aldehydes and ketones, depending on the structure of the starting alkene. If the carbon atoms attached to the double bond are bonded to hydrogen atoms, an aldehyde might form. However, if both carbons are bonded to other carbons, a ketone is the likely product.
This breakdown process is incredibly useful for determining the positions of double bonds within unknown alkenes.

• The basic steps are as follows:
• React the alkene with ozone.
• Allow the intermediate ozonide to form.
• Treat the ozonide with a reducing agent, often zinc/water or dimethyl sulfide.

Consequently, ozonolysis can indeed result in ketone formation.

Aldol Condensation

Aldol condensation is a powerful reaction used to create larger molecules by combining smaller molecules through the formation of new carbon-carbon bonds. This reaction typically involves aldehydes and ketones.
In an aldol condensation reaction, two carbonyl compounds (typically aldehydes or ketones) react to form a β-hydroxy ketone (aldol) which can further dehydrate to form an α,β-unsaturated ketone.
The reaction follows these steps:

• An enolate ion is generated from a carbonyl compound (usually by using a base).
• The enolate ion then attacks another carbonyl compound, forming a β-hydroxy ketone (aldol).
• Subsequent dehydration of the aldol produces an α,β-unsaturated ketone.

Since one of the key products includes the formation of a ketone, the aldol condensation is indeed a reaction that can form ketones.

Oxidation of Primary Alcohol

Oxidation of a primary alcohol is a two-step process that typically does not result in ketone formation. Instead, it proceeds as follows:
1. The primary alcohol (R-CH2OH) is first oxidized to an aldehyde (R-CHO).
2. The aldehyde then undergoes further oxidation to form a carboxylic acid (R-COOH).
The specific steps include:

• Oxidizing the primary alcohol using an oxidizing agent like PCC (pyridinium chlorochromate) or an equivalent to get the aldehyde.
• Further oxidation using stronger oxidizing agents such as KMnO4 or H2CrO4 to convert the aldehyde into a carboxylic acid.

As primary alcohols oxidize via this pathway, they do not form ketones. Secondary alcohols, on the other hand, oxidize to ketones.

Friedel-Crafts Acylation

The Friedel-Crafts acylation is an essential reaction in organic chemistry for introducing an acyl group into an aromatic ring. This reaction, involving an aromatic substrate, an acyl chloride, and a Lewis acid catalyst like AlCl3, predominantly results in the formation of a ketone.
Here's how it functions:

• The acyl group (RCO-) from the acyl chloride reacts with the aromatic ring using a catalyst.
• The catalyst helps to generate a positively charged acylium ion (RCO+), which is highly reactive.
• This acylium ion then reacts with the aromatic ring forming a substituted aromatic ketone.

The reaction is very useful in the synthesis of various aromatic ketones because of its ability to precisely introduce the acyl group into the aromatic system.
Thus, Friedel-Crafts acylation is a key reaction for ketone formation.