What is an aldehyde? (with photo)

Vanillin is an aromatic aldehyde obtained from vanilla beans.

Aldehydes are organic molecules, which means they are made of carbon atoms. They can be distinguished from other organic molecules because they contain an aldehyde, or formyl, group. This group is formed by a carbon atom bonded to a hydrogen atom and to an oxygen atom by a single and double bond, respectively. Aldehydes are just one of many different types of organic molecules and compounds. They differ greatly in their physical properties and are usually formed by the oxidation of an alcohol.

Ketones and aldehydes are very similar in that they both have carbon atoms doubly bonded to oxygen – they are called carbonyl groups. Aldehydes can be distinguished from ketones based on the location of the carbonyl group. It is found at the end of the molecule in an aldehyde, while inside the ketone molecule the carbonyl group is found between two other carbon atoms.

The chemical formula of aldehydes is R-CHO, where R represents the rest of the molecule that is linked to the carbon of the aldehyde group through a single bond. Aldehydes are very common compounds and the simplest is formaldehyde. The chemical structure of formaldehyde is a single hydrogen atom that is bonded once to the carbon of the aldehyde group. It has the chemical formula CH 2 O.

The physical properties of aldehydes differ greatly due to the wide range of R groups attached to the -CHO group. It is the rest of the molecule that determines how the aldehyde will appear and act. Many different fragrances are aldehydes, and these molecules are also found in many essential oils. Examples include vanillan from the vanilla bean and cinnamaldehyde, which provides the smell and taste of cinnamon.

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Aldehydes can be formed by the oxidation of an alcohol. During this chemical reaction, the alcohol loses a hydrogen atom to another molecule – in other words, it has been dehydrogenated. The resulting molecule now has the -CHO group of an aldehyde at the end of it, instead of the -CH 2 OH group of an alcohol molecule. Through the addition of water, an aldehyde can form an aldehyde hydrate, which is then oxidized to a carboxylic acid. These are common organic acids, like the acetic acid found in vinegar and the formic acid that is produced by ants.

The type of alcohol that undergoes oxidation determines the type of resulting molecule. To form aldehydes from alcohol oxidation, a primary alcohol is oxidized or loses a hydrogen atom. Primary alcohols have only one R group attached to the -CH 2 OH group; secondary and tertiary alcohols also exist, with two and three R groups, respectively. When secondary alcohols are oxidized, ketones form. In contrast, tertiary alcohols cannot be oxidized.

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