Which functional group is present in alkanals?

Step 1: Analyze the question The question asks for the functional group in alkanals. The name gives a clue: "alkanal" = alkane + "-al", and the suffix -al marks the aldehyde family.

Step 2: Explain the concept A functional group is the atom or group of atoms that gives an organic compound its characteristic chemical properties. Alkanals (aldehydes) have the general formula RCHO (or CₙH₂ₙO). The functional group is a carbonyl group (C=O) in which the carbonyl carbon is bonded to at least one hydrogen atom, written as -CHO, at the end of the carbon chain.

Step 3: Check each option

• A. Carbonyl group (C=O): Correct. The -CHO group of alkanals contains the C=O carbonyl. (Ketones, the alkanones, also contain C=O, but there the carbon is bonded to two alkyl groups.)
• B. Hydroxyl group (-OH): This is the functional group of alkanols (alcohols), such as ethanol.
• C. Alkyl group (-R): This is just a hydrocarbon branch, not a functional group that defines a family.
• D. Carboxyl group (-COOH): This is the functional group of alkanoic acids (carboxylic acids), such as ethanoic acid.

Step 4: Answer Examples of alkanals:

• Methanal: HCHO
• Ethanal: CH₃CHO
• Propanal: CH₃CH₂CHO

Each contains the C=O group, so the answer is A. Carbonyl group (C=O).

Exam tip: Remember the suffixes:

• -ol → alcohol (-OH)
• -al → aldehyde (-CHO, a carbonyl)
• -one → ketone (C=O in the middle of the chain)
• -oic acid → carboxylic acid (-COOH)

A simple way to remember: "AL" for "Aldehyde, At the end of the chain."