Chemistry · General Principles Of Organic Chemistry · NEET
In NCERT, acyclic (open chain) compounds are also called aliphatic compounds. But be careful: the word aliphatic is broader. Straight or branched open chains are aliphatic, and alicyclic rings (like cyclohexane) behave like aliphatic compounds too. So all acyclic compounds are aliphatic, but not every aliphatic compound is acyclic. Cyclohexane is aliphatic in behaviour yet it is cyclic.
No. A ring only means the carbon chain closes on itself. Rings are of two kinds: alicyclic rings (plain rings like cyclopropane, cyclohexane) that behave like aliphatic compounds, and aromatic rings (benzene and benzene-like) that have special stability from delocalised pi electrons. So a ring is not automatically aromatic. Cyclohexane is cyclic but NOT aromatic.
NCERT defines alicyclic (aliphatic cyclic) as carbon atoms joined in the form of a ring. When the ring is made of only carbon atoms it is called homocyclic. Examples are cyclopropane, cyclobutane and cyclohexane. They show some properties similar to open chain aliphatic compounds, which is why the name is ALI-cyclic.
A heterocyclic compound is a ring that contains at least one atom other than carbon (a hetero atom such as nitrogen, oxygen or sulphur) inside the ring. Furan (O in ring), pyridine and pyrrole (N in ring) and thiophene (S in ring) are examples. A ring made of only carbon is homocyclic; a ring with a hetero atom is heterocyclic. Heterocyclic rings can be aromatic too (heteroaromatic).
Benzene is aromatic, not alicyclic. Even though benzene is a six-membered ring like cyclohexane, its six pi electrons are delocalised over the ring, giving it special aromatic stability. Cyclohexane has only single bonds and is alicyclic. Same ring size, different class.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Acyclic (open chain, also called aliphatic) and cyclic (closed chain or ring). Cyclic compounds are then divided into alicyclic and aromatic.
Homocyclic rings are made of carbon atoms only. Heterocyclic rings contain at least one hetero atom (N, O or S) inside the ring, for example pyridine, furan and thiophene.
It is the base of the whole organic unit. Naming, functional groups, aromaticity and reaction type all depend on knowing whether a compound is open chain, alicyclic, aromatic or heterocyclic. NTA often hides the trap in words like cyclohexane (alicyclic) versus benzene (aromatic).
Yes. Rings that are aromatic and also contain a hetero atom are called heteroaromatic (or heterocyclic aromatic) compounds, such as pyridine, furan, pyrrole and thiophene.
Cyclohexane is alicyclic, and NCERT notes that alicyclic compounds show some properties similar to aliphatic compounds. So it behaves in an aliphatic way even though it is a ring.
