Aldehyde Functional Group

Core Concepts

In this tutorial, you will learn about the aldehyde functional group. You will also learn about some the differences between aldehydes vs ketones, reactions aldehydes can undergo, and some examples of compounds containing an aldehyde group.

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What is an Aldehyde?

An aldehyde is a common functional group in organic chemistry, and a class of organic compounds. In addition, they are primarily derivatives of alcohols. Aldehydes are responsible for natural and synthetic hormones. The structure of an aldehyde consists of a carbonyl group single bonded to a hydrogen atom.

Aldehyde Structure

The carbonyl carbon is double bonded to an oxygen atom, and it is bonded to another hydrogen atom or a carbon-hydrogen chain (shown as the R group in the molecule). An aldehydes position is at the end of a compound due to it requiring a hydrogen bond.

Aldehydes vs. Ketone

Aldehydes and ketones are very similar in structure. They both contain a carbonyl group and a double bond to oxygen. However, the main difference between aldehydes and ketones is the position of the carbonyl group in the molecule. Aldehydes have to have hydrogen attached to the carbon instead of another carbon (as found in ketones).

In aldehydes, the carbonyl group is located at the end of the carbon chain, while in ketones, the carbonyl group is located in the middle of the carbon chain. Here is the general structure of an aldehyde:


And here is the general structure of a ketone:


where R and R’ are alkyl groups (carbon chains).

Aldehydes are typically more reactive than ketones because the carbonyl group is more accessible in aldehydes due to its location at the end of the carbon chain. This makes aldehydes more susceptible to reduction and oxidation reactions. Ketones are less reactive than aldehydes because the carbonyl group is shielded by the alkyl groups on either side, making it less accessible.

Both of these functional groups have resonance through their double bond to oxygen, this resonance allows for the carbonyl group to be polar.

Resonance of Ketones and Aldehydes

Aldehydes also tend to be more reactive than ketones due to the presence of the hydrogen atom. Due to the hydrogen atom, there is less steric hindrance in an aldehyde than a ketone due to there being only a single large substituent (the R group) attached to the carbonyl group instead of two. In addition, an aldehyde group is more electrophilic than a ketone because aldehydes do not have two alkyl groups (these reduce the groups electrophilicity).

Naming Aldehydes

For naming aldehydes, the IUPAC nomenclature is more precise than using common naming. In IUPAC naming, the “-e” suffix is removed from the parent alkane chains and replaced with “-al”. However, if the aldehyde attaches to a ring, the “-carbaldehyde” suffix adds to the parent alkane name instead. The functional group of an aldehyde is always located at the lowest possible position number, so it is not included in the name.

Some examples of aldehydes:


Reactions of Aldehydes

Aldehydes are a fairly reactive functional group. This is due to the carbon of the aldehyde which contains a partial positive charge. Overall, aldehydes can undergo many addition reactions such as reacting with Grignard reagents. Some reactions that aldehydes can undergo are the following:

Nucleophilic addition:

aldehyde nucleophilic addition
Aldehydes reacting with hydrogen cyanide

Reacting with Grignard reagents:

aldehyde reaction
Aldehyde reacting with example Grignard reagent

Reduction with NaBH4 or LiAlH4:

aldehyde reduction
Aldehyde reacting with LiAlH4

Hydration reaction:

aldehyde hydration
Aldehyde undergoing a hydration reaction

Real Life Examples of Aldehydes

Aldehydes are in our everyday lives. Down below are two examples of compounds with aldehyde groups and their functions.

Vanillin – responsible for taste and smell of vanilla
Formaldehyde – one chemical used to preserve dead bodies

. Some common examples of aldehydes include:

  1. Formaldehyde: Formaldehyde is a colorless, flammable gas with a strong, pungent smell. It is commonly used as a disinfectant and preservative in laboratories and mortuaries, and it is also used in the production of plastics, resins, and textiles.
  2. Acetaldehyde: Acetaldehyde is a colorless liquid with a sweet, fruity smell. It is a natural byproduct of alcoholic fermentation and is also produced industrially from ethylene. It is used in the production of acetic acid (vinegar), perfumes, and flavors.
  3. Propionaldehyde: Propionaldehyde is a colorless liquid with a pungent, unpleasant smell. It is produced industrially from propylene and is used as a solvent and in the production of plastics and resins.
  4. Butyraldehyde: Butyraldehyde is a colorless liquid with a pungent, fruity smell. It is produced industrially from butane and is used as a solvent and in the production of plastics, resins, and fragrances.
  5. Valeraldehyde: Valeraldehyde is a colorless liquid with a strong, unpleasant smell. It is produced industrially from valeric acid and is used as a solvent and in the production of plastics and resins.

Further Reading