Organic Chemistry – Some Basic Principles and Techniques

Organic chemistry is the chemistry of carbon compounds, and almost everything in it flows from two properties of carbon: tetravalency (carbon forms four covalent bonds) and catenation (carbon atoms link to one another in chains and rings). Together these let carbon build the enormous variety of molecules that make up living things, fuels, medicines and plastics — context NEET uses to root the subject in biology.

The shapes of organic molecules are explained by hybridisation. When carbon forms four single bonds it is $sp^3$ hybridised, giving a tetrahedral geometry with bond angles of about $109.5^{\circ}$ (as in methane). A carbon with one double bond is $sp^2$ hybridised, planar, with angles near $120^{\circ}$ (as in ethene). A carbon with a triple bond (or two double bonds) is $sp$ hybridised and linear, with $180^{\circ}$ angles (as in ethyne). NEET regularly asks you to read off hybridisation and shape from a structure.

A single bond is a sigma ($\sigma$) bond formed by head-on overlap; a double bond is one sigma plus one pi ($\pi$) bond, and a triple bond is one sigma plus two pi bonds. Sigma bonds allow free rotation, while pi bonds restrict rotation — the reason geometrical isomerism exists. As the bond order increases, bonds get shorter and stronger, so a $\text{C}\equiv\text{C}$ triple bond is shorter and stronger than a $\text{C=C}$ double bond, which is shorter than a $\text{C–C}$ single bond.

Organic structures are drawn in several shorthand ways: complete structural formulae, condensed formulae (e.g. $\text{CH}_3\text{CH}_2\text{OH}$), and bond-line (skeletal) formulae where carbons sit at the ends and bends of lines and hydrogens on carbon are implied. Being fluent in converting between these and counting hydrogens correctly is a basic NEET skill that underlies the whole chapter.

To name the millions of organic compounds unambiguously, chemists use the IUPAC system. An IUPAC name is built from three parts: a root (the longest continuous carbon chain — meth, eth, prop, but, pent...), a suffix for the principal functional group (-ane, -ene, -yne, -ol, -al, -one, -oic acid, etc.), and prefixes for substituents (methyl, chloro, etc.). The chain is numbered so that the principal functional group, then double/triple bonds, then substituents, get the lowest locants. Substituent prefixes are listed alphabetically. Mastering this stepwise procedure is one of the highest-yield NEET skills in the chapter.

There is a priority order for choosing the principal functional group when several are present (carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine, and so on); the highest-priority group takes the suffix and the rest become prefixes. Getting this priority right is a frequent NEET trap, especially in compounds with two functional groups.

Isomerism — different compounds with the same molecular formula — is the other pillar of this topic. Structural (constitutional) isomers differ in connectivity and include chain isomerism (different skeletons), position isomerism (same group at different positions), functional isomerism (different functional groups, e.g. an alcohol vs an ether), metamerism (different alkyl groups around a functional group), and tautomerism (a dynamic equilibrium, classically keto–enol).

Stereoisomers have the same connectivity but differ in spatial arrangement. Geometrical (cis–trans) isomerism arises from restricted rotation about a C=C double bond or in rings. Optical isomerism arises when a molecule has a chiral centre (a carbon with four different groups) and is non-superimposable on its mirror image; such enantiomers rotate plane-polarised light in opposite directions. NEET often asks you to count the number of isomers or identify which type a given pair shows, so recognising each category quickly is essential.

How an organic molecule reacts is governed by the distribution of electrons within it, described by a set of electronic effects. The inductive effect (I) is the permanent polarisation of sigma bonds caused by a nearby electronegative or electropositive group. Electron-withdrawing groups (like $\text{-NO}_2$, halogens) show a $-I$ effect; electron-donating groups (like alkyl groups) show a $+I$ effect. The inductive effect is permanent but weakens rapidly with distance along the chain — a point NEET tests when comparing acid strengths.

The resonance (mesomeric) effect (M or R) involves the delocalisation of pi electrons or lone pairs through a conjugated system. Groups that push electron density into the system show $+M$ (e.g. $\text{-OH}, \text{-NH}_2, \text{-OR}$); groups that pull it out show $-M$ (e.g. $\text{-NO}_2, \text{-C=O}$). Resonance gives extra stability to molecules and ions: the real structure is a hybrid of the contributing resonance forms, and greater delocalisation means greater stability. This explains, for example, the special stability and reactivity of benzene and the acidity of carboxylic acids and phenols.

A third effect, hyperconjugation, is the stabilising delocalisation of the electrons of adjacent C–H sigma bonds into an empty or partially filled p-orbital (often called the 'no-bond resonance'). The more alpha (adjacent) C–H bonds available, the greater the hyperconjugative stabilisation. This is the main reason a tertiary carbocation is more stable than a secondary or primary one, and why more-substituted alkenes are more stable — a recurring NEET theme.

The electromeric effect is a temporary effect that appears only in the presence of an attacking reagent, when a pi bond shifts completely to one atom. Together, these electronic effects let you predict the stability of intermediates, the strength of acids and bases, the orientation of substitution on benzene, and the direction of addition reactions — making them the analytical toolkit for the rest of organic chemistry.

Organic reactions proceed through short-lived reaction intermediates formed when bonds break. A covalent bond can break in two ways. In homolytic fission the bond splits evenly, each atom keeping one electron, producing free radicals (species with an unpaired electron). In heterolytic fission the bond splits unevenly, one atom taking both electrons, producing ions — a carbocation (carbon with a positive charge, electron-deficient) and an anion, or a carbanion (carbon with a negative charge and a lone pair).

The stability order of these intermediates governs reaction pathways. Carbocations follow $3^\circ > 2^\circ > 1^\circ > \text{methyl}$, stabilised by $+I$ and hyperconjugation (and by resonance, e.g. allyl and benzyl cations). Carbanions show the reverse alkyl order and are stabilised by electron-withdrawing groups. Free radicals follow a stability order similar to carbocations. Reagents are classified as electrophiles (electron-loving, electron-deficient, e.g. $\text{H}^+, \text{NO}_2^+$) and nucleophiles (nucleus-loving, electron-rich, e.g. $\text{OH}^-, \text{NH}_3$); recognising these is essential for predicting mechanisms, a core NEET skill.

Because organic compounds are rarely pure when first made, a set of purification techniques is used. Crystallisation purifies solids using differential solubility. Simple distillation separates liquids with well-separated boiling points; fractional distillation separates liquids with close boiling points; steam distillation purifies steam-volatile, water-immiscible substances; and distillation under reduced pressure handles liquids that decompose near their boiling point. Sublimation purifies solids that vaporise directly (camphor, naphthalene), and chromatography separates mixtures by differential adsorption or partition.

Once pure, a compound's composition is found by qualitative analysis (detecting elements: nitrogen, sulphur and halogens by the sodium fusion / Lassaigne's test; carbon and hydrogen by combustion) and quantitative estimation (e.g. Dumas and Kjeldahl methods for nitrogen, Carius method for halogens). NEET commonly asks which technique suits a given separation or which test detects a particular element, so matching the method to the property is the practical takeaway.