Aldehyde: Definition, Reactions, Formula & Structure

Instructor: Felicia Fullilove
There are numerous organic functional groups. In this lesson, we will learn about the volatile, and often sweet-smelling, aldehyde. Read on to learn more about aldehyde, including its formula and structure, as well as some common reactions that it can undergo.

Definition

What gives almonds their sweet aroma? Or cinnamon its distinct odor? What if I told you similar molecules contribute to their aromas? This molecule is called an aldehyde. According to Chemwiki, the aldehyde found in almonds is known as benzaldehyde, while the aldehyde found in cinnamon bark is cinnamaldehyde.

Cinnamon
Cinnamon

An aldehyde is a common functional group in organic chemistry. Britannica states that aldehydes can be found in perfume fragrances as well as natural and synthetic hormones. In an aldehyde, a carbonyl group is single-bonded to a hydrogen atom. A carbonyl is a carbon that is double bonded to an oxygen atom. The carbonyl carbon is also bonded to another hydrogen atom or a carbon/hydrogen chain, typically known as an R group.

Aldehyde
aldehyde

Reactions

Aldehydes can be subjected to a number of reactions. In fact, the Virtual Organic Chemistry Book lists over 10 common reactions of aldehydes! A key similarity in the reactions of aldehydes is that the carbonyl is the site of reactivity. The carbonyl carbon of aldehyde is slightly positive because it is double bonded to an electronegative oxygen. Due to the positive nature of the carbonyl carbon, aldehydes are considered electrophiles. Electrophiles, or electron 'loving' molecules, attract the electrons from molecules called nucleophiles. A nucleophile is a molecule capable of donating a pair of electrons to form a new covalent bond. You can think of electrophiles and nucleophiles like magnets. The positive magnet (electrophile) attracts the negative magnet (nucleophile).

Aldehydes are electrophiles
electrophile

When aldehydes react with nucleophiles, the nucleophile forms a new bond with the carbonyl carbon. This action is called a nucleophilic attack. Molecules that act as nucleophiles typically have oxygens, such as H2O, or nitrogen's, as in NH3, with lone pairs of electron free to 'attack' the carbonyl carbon. In organic chemistry, the nucleophilic attack of aldehydes is classified as an addition reaction. Often in reactions of aldehydes, nucleophilic attack is followed by loss of water. This process is called condensation.

Nucleophilic attacked upon an aldehyde followed by condensation.
NucCon

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