Which of the following is a straight-chain alkyl carboxylic acid with the chemical formula CH 3 CH 2 CH 2 CO 2 H?

Correct — D. Count the carbons in CH3CH2CH2CO2H: three in the chain, plus the one inside the -CO2H group. That is a four-carbon straight-chain acid.

Four carbons means butanoic acid, whose common name is butyric acid — the smell of rancid butter, and the source of the name, from the Latin butyrum.

The options climb the series one carbon at a time, so the count alone settles it on option (d).

Why the others are wrong

• (a)
  Methanoic acid — Methanoic acid, better known as formic acid, is HCOOH with a single carbon. It is the acid of ant and nettle stings and cannot match a four-carbon formula.
• (b)
  Ethanoic acid — Ethanoic acid, or acetic acid, is CH3COOH with two carbons — the acid in vinegar. The formula given has two more CH2 groups than that.
• (c)
  Propionic acid — Propionic acid, or propanoic acid, is CH3CH2COOH with three carbons. It is the close decoy, exactly one CH2 short of the formula in the question.

Carboxylic acids are named from the number of carbons in the chain, and the acid carbon itself is counted.

The first four are methanoic (one carbon), ethanoic (two), propanoic (three) and butanoic (four). Each keeps an older common name that SSC still uses: formic, acetic, propionic and butyric.

With a condensed formula like CH3CH2CH2CO2H the method is to read left to right and add the carbons up before looking at the options. Do that and the near-miss propionic acid stops being tempting.

Key facts

• CH3CH2CH2CO2H is butanoic acid, commonly called butyric acid, a straight-chain four-carbon acid.
• Methanoic acid is formic acid, HCOOH, with one carbon.
• Ethanoic acid is acetic acid, CH3COOH, the acid in vinegar.
• Butyric acid gives rancid butter its smell, and its name comes from the Latin butyrum, butter.

Common traps

• Forgetting that the carbon in -CO2H counts, and naming the compound propionic acid.
• Reading the flattened subscripts as separate atoms and losing the carbon count.

No directly related past PYQ was found.