Organic Chemistry
The key players in the reactions of aldehydes and ketones are ketones and aldehydes themselves, containing a carbon-oxygen double bond, known as a carbonyl. Carbonyls have a partial positive charge on carbon, which attracts nucleophiles to attack the carbonyl carbon. This results in a tetrahedral product being formed. Some important nucleophiles include water and alcohols. When water or alcohol attacks aldehydes or ketones, the products formed can be hydrates, hemi-acetals, or acetals.
These products are formed reversibly, allowing the original carbonyl to be reverted back to the ketone or aldehyde under certain conditions. Cyclic acetals can protect aldehydes and ketones during synthesis, and carbohydrates spend most of their time as cyclic hemi-acetals. Reactions with primary amines form imines, while reactions with secondary amines form enamines. Lastly, the reaction with hydrocyanic acid (HCN) results in the formation of cyanohydrins.
Lesson Outline
<ul> <li>Carbonyl group importance in ketones and aldehydes: <ul> <li>Contains carbon-oxygen double bonds</li> <li>Carbon has large partial positive charge</li> </ul> </li> <li>Nucleophile attacks carbonyl carbon: <ul> <li>Formation of tetrahedral shape</li> </ul> </li> <li>Oxygen-based nucleophiles (water and alcohols): <ul> <li>Hydrate formation from water</li> <li>Acetal formation from two equivalents of alcohol</li> <li>Hemi-acetal formation from one equivalent of alcohol</li> <li>Products formed reversibly</li> </ul> </li> <li>Cyclic acetal protective properties in synthesis</li> <li>Carbohydrates and cyclic hemi-acetals</li> <li>Amine nucleophiles (nitrogen-based): <ul> <li>Primary amines form imines</li> <li>Secondary amines form enamines</li> </ul> </li> <li>Hydrocyanic acid (HCN) reaction: <ul> <li>Formation of cyanohydrins</li> </ul> </li> </ul>
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FAQs
Aldehydes and ketones are both carbonyl compounds containing a carbon-oxygen double bond (C=O). The primary difference between them is that aldehydes have a hydrogen atom bonded to the carbonyl carbon, while ketones have two alkyl groups or an aryl group bonded to the carbonyl carbon. This difference in structure results in aldehydes being more reactive than ketones due to the electron-donating alkyl groups in ketones that make the carbonyl group less electrophilic, as well as the reduced steric hindrance in aldehydes.
Nucleophiles react with carbonyl compounds by attacking the electrophilic carbonyl carbon, which is partially positively charged because of the polar carbon-oxygen double bond. This nucleophilic attack generates a tetrahedral intermediate in which the carbonyl oxygen becomes negatively charged. Subsequent reaction steps depend on the type of nucleophile and reaction conditions, giving rise to different products such as hydrates, hemi-acetals, acetals, cyclic acetals, imines, enamines, and cyanohydrins.
In the presence of an alcohol, aldehydes and ketones can undergo nucleophilic addition reactions, leading to the formation of different products depending on the reaction conditions. The main reaction pathways are the formation of hydrates, hemi-acetals, and acetals. Hydrates are formed when an alcohol is added and removed, resulting in a geminal diol. Hemi-acetals are formed when a single equivalent of alcohol reacts with the carbonyl group, and acetals are formed when two equivalents of alcohol react with the carbonyl group under acidic conditions.
When carbonyl compounds, such as aldehydes and ketones, react with primary amines, imines are formed through a nucleophilic addition-elimination mechanism. This involves the nucleophilic attack of the amine nitrogen on the carbonyl carbon, followed by the elimination of a water molecule and subsequent double bond formation between the nitrogen and carbon atoms. The reaction with secondary amines proceeds similarly, but it results in the formation of enamines, which contain a nitrogen atom bonded to a carbon-carbon double bond.
Cyanohydrins are formed through the reaction of aldehydes or ketones with hydrogen cyanide (HCN), in which HCN acts as a nucleophile attacking the electrophilic carbonyl carbon. The resultant cyanohydrins are valuable synthetic intermediates due to the presence of both alcohol and nitrile functional groups. The nitrile group can be further hydrolyzed to a carboxylic acid or reduced to an amine, rendering cyanohydrins useful in the synthesis of a variety of complex organic molecules. Additionally, cyanohydrin formation can serve as a key step in protecting the carbonyl group during multistep synthetic sequences.
