Aldehyde: Structure, Properties & Naming

An aldehyde is an organic compound whose carbonyl carbon carries at least one hydrogen atom. That single hydrogen is what separates an aldehyde from a ketone, and it controls almost everything about how aldehydes behave: their naming, their reactivity, and how you tell them apart in the lab.

If you can recognize the $\text{-CHO}$ group sitting at the end of a carbon chain, you already know the most important structural fact about this family.

The Carbonyl Group at the Heart of an Aldehyde

Every aldehyde contains a carbonyl group, a carbon double-bonded to oxygen, written $\text{C=O}$. In an aldehyde this carbonyl carbon is bonded to one hydrogen and to one other group (a carbon chain or, in the special case of formaldehyde, a second hydrogen).

The carbonyl carbon is $sp^2$ hybridized, so the three atoms attached to it sit in a plane at roughly $120^\circ$ angles. Oxygen is far more electronegative than carbon, so the double bond is strongly polarized:

$$\overset{\delta+}{\text{C}}=\overset{\delta-}{\text{O}}$$

That polarity is the key to aldehyde chemistry. The carbon is electron-poor and acts as an electrophile, while the oxygen is electron-rich. Because the carbonyl carbon also carries a hydrogen, it sits relatively exposed, which makes aldehydes more reactive than ketones toward attack at that carbon.

Naming Aldehydes with IUPAC Rules

The systematic name of an aldehyde replaces the final -e of the parent alkane with -al.

• Find the longest carbon chain that includes the carbonyl carbon.
• The $\text{-CHO}$ carbon is always carbon number $1$, so it never needs a locant.
• Add substituent names and numbers as prefixes.

Formula	IUPAC name	Common name
$\text{HCHO}$	methanal	formaldehyde
$\text{CH}_3\text{CHO}$	ethanal	acetaldehyde
$\text{CH}_3\text{CH}_2\text{CHO}$	propanal	propionaldehyde
$\text{CH}_3(\text{CH}_2)_2\text{CHO}$	butanal	butyraldehyde

When the $\text{-CHO}$ group is attached to a ring, the suffix -carbaldehyde is used, as in cyclohexanecarbaldehyde. Benzaldehyde ($\text{C}_6\text{H}_5\text{CHO}$) keeps its retained common name.

Physical Properties

The polar carbonyl gives aldehydes a permanent dipole, so they have higher boiling points than alkanes of similar mass. However, aldehyde molecules cannot donate a hydrogen bond to each other (there is no $\text{O-H}$ or $\text{N-H}$), so their boiling points fall below those of comparable alcohols.

• Solubility: small aldehydes such as methanal and ethanal mix freely with water because the carbonyl oxygen accepts hydrogen bonds from water. Above about four or five carbons, water solubility drops sharply.
• Odor: short-chain aldehydes are sharp and pungent, while many larger aromatic aldehydes are pleasant; benzaldehyde smells of almonds and cinnamaldehyde of cinnamon.
• State: methanal is a gas at room temperature; most other common aldehydes are liquids.

Key Reactions of Aldehydes

Nucleophilic Addition

The defining reaction of the carbonyl group is nucleophilic addition. A nucleophile attacks the electrophilic carbon, the $\pi$ bond breaks, and the oxygen picks up the negative charge before being protonated. A classic example is the addition of hydrogen cyanide to form a cyanohydrin:

$$\text{RCHO} + \text{HCN} \rightarrow \text{RCH(OH)CN}$$

Aldehydes react faster than ketones here because the lone hydrogen leaves the carbon less crowded and less shielded by electron-donating alkyl groups.

Oxidation

Aldehydes are easily oxidized to carboxylic acids, because the carbonyl carbon still holds a hydrogen that can be removed:

$$\text{RCHO} + [\text{O}] \rightarrow \text{RCOOH}$$

This easy oxidation is exactly why aldehydes give positive results with mild oxidizing tests, and it is the basis for telling them apart from ketones.

Reduction

Reducing agents such as $\text{NaBH}_4$ or $\text{LiAlH}_4$ add hydrogen across the carbonyl and convert an aldehyde into a primary alcohol:

$$\text{RCHO} + 2[\text{H}] \rightarrow \text{RCH}_2\text{OH}$$

Telling Aldehydes from Ketones

Because aldehydes oxidize easily and ketones do not, mild oxidizing reagents give a clean test.

Test	Reagent	Aldehyde	Ketone
Tollens'	ammoniacal $\text{Ag(NH}_3)_2^+$	silver mirror	no change
Fehling's	blue $\text{Cu}^{2+}$ complex	brick-red ppt	no change
Benedict's	blue $\text{Cu}^{2+}$ complex	brick-red ppt	no change

In Tollens' test the aldehyde reduces silver ions to metallic silver, depositing a shiny mirror on the glass:

$$\text{RCHO} + 2\text{Ag}^+ + 3\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag} + 2\text{H}_2\text{O}$$

In Fehling's and Benedict's tests the aldehyde reduces the deep-blue copper(II) complex to a brick-red precipitate of copper(I) oxide, $\text{Cu}_2\text{O}$. Ketones leave all three reagents unchanged.

Worked Example 1: Naming and Predicting a Product

Name $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$ and predict the product of mild oxidation.

The longest chain has four carbons and ends in $\text{-CHO}$, so the parent is butane with the -al suffix: butanal. Numbering starts at the carbonyl carbon. Because it is an aldehyde, mild oxidation removes the carbonyl hydrogen and produces the carboxylic acid:

$$\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO} + [\text{O}] \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$$

The product is butanoic acid.

Worked Example 2: Identifying an Unknown

An unknown liquid gives a silver mirror with Tollens' reagent and a brick-red precipitate with Fehling's solution. Reduction with $\text{NaBH}_4$ gives a primary alcohol. What functional group is present?

Both Tollens' and Fehling's are positive, which rules out a ketone, since ketones do not respond to either reagent. Reduction to a primary alcohol confirms the carbon was a terminal carbonyl. Together these point to an aldehyde.

Aldehydes in Everyday Life

Aldehydes are everywhere once you know the smell. Formaldehyde is used to make resins, plastics, and preservatives. Cinnamaldehyde gives cinnamon its aroma, benzaldehyde supplies almond flavor, and vanillin is the aldehyde behind vanilla. In the body, the partial oxidation of ethanol to acetaldehyde is responsible for much of the discomfort of a hangover, which is one reason this small molecule matters well beyond the chemistry classroom.