Classification of Organic Compounds

Two Broad Classes

All organic compounds may be divided into two broad classes based upon the pattern of the chain of carbon atoms. The first class is the open-chain or aliphatic compounds, whose carbon atoms are joined in chains that do not close back on themselves. The second is the closed-chain or cyclic compounds, in which at least one ring of atoms is present. Every other distinction in organic chemistry — saturated against unsaturated, carbocyclic against heterocyclic, alicyclic against aromatic — is layered on top of this single skeletal criterion.

The skeleton tells you the shape; it does not by itself tell you the chemistry. A second, independent classification rests on the functional group — the atom or group of atoms responsible for a compound's characteristic reactions. The two systems run in parallel: ethanol, $\ce{CH3CH2OH}$, is simultaneously an acyclic compound (by skeleton) and an alcohol (by functional group). Holding both lenses at once is the habit this topic builds.

The Classification Tree

The cleanest way to fix the hierarchy is to read it as a branching tree. Organic compounds split first into acyclic and cyclic; cyclic compounds split into homocyclic (carbocyclic) and heterocyclic by ring composition; and homocyclic compounds split once more into alicyclic and aromatic by their properties. The schematic below traces every branch in the order NIOS Chapter 23 presents them.

Acyclic (Aliphatic) Compounds

Open-chain compounds include all hydrocarbons, saturated and unsaturated, together with their derivatives, in which the carbon atoms form an open chain. Saturated hydrocarbons contain only single bonds between carbon atoms, as in ethane $\ce{CH3-CH3}$ and propane $\ce{CH3-CH2-CH3}$. Unsaturated compounds contain at least one carbon–carbon double bond ($\ce{-C=C-}$) or triple bond ($\ce{-C#C-}$).

Propene, $\ce{CH3-CH=CH2}$, and propyne, $\ce{CH3-C#CH}$, are the standard unsaturated illustrations; isobutylene, $\ce{(CH3)2C=CH2}$, shows that a branch does not change the classification — a branched chain is still an open chain. The word "aliphatic" is used interchangeably with "open-chain" throughout NIOS, and the two terms should be treated as synonyms for NEET purposes.

Cyclic Compounds

Cyclic compounds possess at least one ring system. They are first divided into two sub-classes — homocyclic and heterocyclic — purely on the basis of the atoms that make up the ring. When the ring is built from carbon atoms only, the compound is homocyclic, also called carbocyclic. Homocyclic compounds are then divided again into alicyclic and aromatic groups by their properties.

Alicyclic vs Aromatic

Alicyclic compounds are saturated or unsaturated cyclic hydrocarbons that resemble the aliphatic hydrocarbons in their properties. Cyclopropane (a three-membered ring), cyclobutane, cyclopentane and cyclohexane are the textbook members; each corner of the condensed polygon structure stands for a $\ce{-CH2-}$ group. Because they behave much like open-chain alkanes, the prefix "ali-" (from aliphatic) is attached to "cyclic".

Aromatic compounds form a special group of homocyclic compounds with a distinctive set of properties and, historically, a characteristic smell or aroma — which is the origin of the name. This group is built around the benzene ring and includes benzene together with its derivatives. The deeper account of why aromatic rings behave so differently — the delocalised electron system and electrophilic substitution — belongs to the Hydrocarbons chapter; here the point is simply that aromatic is a property-defined sub-class of the carbocyclic branch.

Heterocyclic Compounds

Heterocyclic compounds contain one or more atoms other than carbon — usually oxygen, nitrogen or sulphur — within the ring. The non-carbon atom is called a heteroatom, and its presence is what removes the compound from the homocyclic (carbocyclic) branch. NIOS gives furan (an oxygen heterocycle), thiophene (sulphur), pyrrole and pyridine (nitrogen) as the standard examples.

A note on overlap that NEET sometimes probes: a ring can be both heterocyclic and aromatic. Pyridine and furan are heteroaromatic — their rings carry both a heteroatom and aromatic character. The strict NIOS tree introduces aromatic compounds under the all-carbon (benzenoid) homocyclic branch, so the cleanest reading at this level is that aromaticity is a property some heterocyclic rings also share, while the formal "aromatic compounds" sub-class in the tree refers to benzene-ring systems. Treat non-benzenoid and heteroaromatic aromaticity as material for the Hydrocarbons chapter rather than the introductory tree.

Homologous Series

Classifying skeletons is only half the organising idea. The second is the homologous series: a series of compounds in which the molecular formula of each member differs from those of its neighbours by a $\ce{CH2}$ group. Each series carries a general name and a general formula, so a single formula and a single set of reaction rules describe an entire family. The open-chain saturated hydrocarbons form the series of alkanes; the open-chain unsaturated hydrocarbons form two series, the alkenes (one C=C) and the alkynes (one C≡C).

The defining characteristics of a homologous series, drawn directly from the NIOS treatment, are worth memorising as a checklist, because NEET assertion–reason items often hinge on one of them.

Functional Groups

A functional group is an atom or group of atoms that is responsible for the characteristic properties of a compound — for example $\ce{-Cl}$, $\ce{-Br}$, $\ce{-I}$, $\ce{-COOH}$, $\ce{-OH}$ and $\ce{-NH2}$. The functional group, far more than the carbon skeleton, dictates how a molecule reacts. Compounds that carry the same functional group constitute a family, which is exactly why the homologous series and the functional group are two faces of the same organising principle: a homologous series is, in effect, a single functional group threaded onto a growing carbon chain.

A compound bearing only one functional group is a monofunctional derivative; one bearing several is polyfunctional, and in naming such compounds a priority order decides which group is treated as principal. The table below lists the common aliphatic functional groups exactly as NIOS Table 23.5 presents them, with the family name and a worked example for each.

When more than one functional group is present, one group is given preference over the others in deciding the parent. The NIOS priority sequence runs $\ce{-COOH}$, $\ce{-COOR}$, $\ce{-SO3H}$, $\ce{-COX}$, $\ce{-CONH2}$, $\ce{-CHO}$, $\ce{-CO-}$, $\ce{-CN}$, $\ce{-OH}$, $\ce{-SH}$, $\ce{-O-}$, $\ce{-NH2}$, $\ce{-X}$, $\ce{-NO2}$, then $\ce{-C=C-}$ and $\ce{-C#C-}$. Thus in $\ce{CH3-CH(Br)-CH2-CH(OH)-COOH}$ the carboxyl group outranks both hydroxyl and bromo, fixing the compound as a pentanoic acid derivative.