The general formula for the Alkanals is?

Analyze the question: You're asked for the general formula of the alkanals, which are the aldehydes.

Explain the concept: Organic compounds are grouped into homologous series by their functional group. The name tells you the family:

• Alkanals (the suffix -al) are aldehydes. Their functional group is –CHO, with the carbonyl carbon bonded to at least one hydrogen.
• Alkanones (the suffix -one) are ketones, with the group >C=O between two alkyl groups.
• Alkanols (the suffix -ol) are alcohols, with the group –OH.
• Alkanoates (the suffix -oate) are esters, with the group –COO–.

Step-by-step solution:

1. Alkanal means aldehyde, so the functional group must be –CHO.
2. Attach an alkyl group (R) to the –CHO group, which gives R–CHO.
3. Examples: methanal (HCHO), ethanal (CH₃CHO), propanal (CH₃CH₂CHO).
4. The general molecular formula is CₙH₂ₙO.

Checking the options:

• A. R₂CO is the formula for alkanones (ketones), so it is wrong.
• B. RCHO is the formula for alkanals (aldehydes), so it is correct.
• C. ROH is the formula for alkanols (alcohols), so it is wrong.
• D. RCOOR is the formula for alkanoates (esters), so it is wrong.

Answer: B. RCHO

Tip: Link the suffix to the family: -al = aldehyde = RCHO, -one = ketone = R₂CO, -ol = alcohol = ROH, -oate = ester = RCOOR. Also remember that an aldehyde always has an H on the carbonyl carbon, and that is why it is written as CHO.