Which of the following is an open chain organic compound?

The correct answer is Acetaldehyde.

To understand why, we must first distinguish between open chain (acyclic) and closed chain (cyclic) organic compounds.

Open Chain Compounds (Aliphatic Compounds):
These are organic compounds in which carbon atoms are linked together in a straight or branched chain, with no ring structure formed. The chain has a definite start and end point.

Closed Chain / Cyclic Compounds:
These are compounds where the carbon atoms (or other atoms) form a closed ring or loop. They can be carbocyclic (only carbon in the ring) or heterocyclic (other atoms like O, N, S in the ring).

Now, let us analyze each option one by one:

Option 1 — Benzene (C₆H₆):
Benzene is an aromatic cyclic compound. Its six carbon atoms are arranged in a closed hexagonal ring with alternating double bonds. It is a closed chain compound — NOT an open chain compound.

Option 2 — Tetrahydrofuran (THF):
Tetrahydrofuran is a heterocyclic compound. It contains a five-membered ring with four carbon atoms and one oxygen atom in the ring. Because a ring is present, it is a closed chain compound — NOT an open chain compound.

Option 3 — Toluene (C₇H₈):
Toluene is a benzene derivative — it is a benzene ring with a methyl group (–CH₃) attached to it. Since it still contains the closed benzene ring, it is a cyclic (closed chain) compound — NOT an open chain compound.

Option 4 — Acetaldehyde (CH₃CHO):
Acetaldehyde is an aliphatic aldehyde. Its structure is a simple open chain:

{\mathrm{CH}}_3-\mathrm{CHO}

It consists of two carbon atoms linked in a straight chain — a methyl group (CH₃) attached to a formyl group (CHO). There is no ring formation whatsoever. The carbon chain has a clear beginning and end. Therefore, Acetaldehyde is a perfect example of an open chain (acyclic) organic compound.

Summary Table:

Conclusion: Acetaldehyde (CH₃CHO) is the only open chain organic compound among the given options, as it contains no ring structure and its carbon atoms are simply arranged in a straight, unbranched chain.