Basic Organic Chemistry and Hydrocarbons — NEET Chemistry

Organic chemistry looks like endless reactions to memorise — until you learn the logic underneath. Almost everything follows from a few electronic effects and the stability of a handful of intermediates. Master why electrons flow where they do, and reactions become predictable rather than memorised. This foundation, plus the hydrocarbons built on it, is NEET's single largest organic block — a reliable 4–5 questions on nomenclature, isomerism, stability orders and reaction mechanisms. This chapter builds that logic from the ground up and applies it to alkanes, alkenes, alkynes and aromatics, reasoning through every mechanism.

Part A — General Organic Chemistry (the logic)

1. IUPAC nomenclature

A systematic name has three parts: substituent prefixes + parent chain (root + saturation) + principal functional-group suffix.

• Root = longest carbon chain containing the principal functional group (meth-, eth-, prop-, but-, pent-…).
• Suffix for saturation: -ane (single), -ene (double), -yne (triple).
• Principal functional group gets the lowest locant and the characteristic suffix. Priority order (highest first): carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine; others (halo, nitro, alkyl) are named as prefixes.
• Number the chain to give the principal group the lowest locant.

Worked example 1.1. Name CH₃–CH(OH)–CH₂–CHO. The principal group is the aldehyde (higher priority than OH), so it is C-1: a 4-carbon chain → butanal, with OH as a "hydroxy" prefix at C-3 → 3-hydroxybutanal.

2. Isomerism

Structural (constitutional) isomers — same formula, different connectivity:

• Chain — different carbon skeleton (n-butane vs isobutane).
• Position — same skeleton, group in a different place (1-propanol vs 2-propanol).
• Functional — different functional group (ethanol C₂H₆O vs dimethyl ether).
• Metamerism — different alkyl groups either side of a functional group (diethyl ether vs methyl propyl ether).
• Tautomerism — a dynamic equilibrium by proton shift (keto ⇌ enol).

Stereoisomers — same connectivity, different spatial arrangement:

• Geometrical (cis–trans / E–Z) — restricted rotation about a C=C double bond (needs two different groups on each carbon).
• Optical — non-superimposable mirror images (enantiomers) around a chiral centre (a carbon with four different groups); they rotate plane-polarised light oppositely. A 50:50 mix is a racemic mixture (optically inactive).

Worked example 2.1. Does 2-butene show geometrical isomerism? Does butan-2-ol show optical isomerism? 2-butene: yes — the C=C carbons each carry an H and a CH₃, so cis and trans forms exist. Butan-2-ol: yes — C-2 bears four different groups (H, OH, CH₃, C₂H₅), so it is chiral and optically active.

3. Electronic effects — why electrons flow

Inductive effect (I) — permanent polarisation of σ bonds by an electronegative atom, transmitted along the chain and weakening with distance.

• −I (electron-withdrawing): –NO₂, –CN, –COOH, halogens.
• +I (electron-donating): alkyl groups (–CH₃).

Resonance / mesomeric effect (M) — delocalisation of π/lone-pair electrons through conjugation; stronger and further-reaching than induction. +M groups (–OH, –NH₂, –OR, halogens) donate electron density into the ring; −M groups (–NO₂, –C=O, –CN) withdraw it.

Hyperconjugation — delocalisation of σ(C–H) electrons into an adjacent empty p orbital or π system ("no-bond resonance"). More α-hydrogens → more hyperconjugation → more stability. It explains alkene and carbocation stability trends.

Electromeric effect — a temporary, complete shift of a π electron pair in the presence of an attacking reagent.

Worked example 3.1. Why is a tertiary carbocation more stable than a primary one? A tertiary carbocation has three alkyl groups that donate electron density by +I and, crucially, provide many α C–H bonds for hyperconjugation, spreading and stabilising the positive charge. A primary carbocation has few such stabilising groups.

4. Reaction intermediates, reagents and reaction types

Bond cleavage: homolytic (each atom keeps one electron → free radicals) or heterolytic (one atom keeps both → ions).

Three intermediates and their stability:

• Carbocation (positive C): stability 3° > 2° > 1° > methyl (stabilised by +I and hyperconjugation; benzyl/allyl extra-stable by resonance).
• Carbanion (negative C): stability methyl > 1° > 2° > 3° (the reverse — alkyl groups destabilise the negative charge).
• Free radical (unpaired electron): stability 3° > 2° > 1° (like carbocations, but less pronounced).

Reagents: electrophiles (electron-pair seekers, e.g. H⁺, NO₂⁺, carbocations) and nucleophiles (electron-pair donors, e.g. OH⁻, CN⁻, NH₃).

Reaction types: substitution, addition, elimination, and rearrangement.

Worked example 4.1. Arrange these carbocations in order of stability: CH₃⁺, (CH₃)₂CH⁺, (CH₃)₃C⁺, C₆H₅CH₂⁺. Benzyl (resonance-stabilised) ≈ most stable, then tertiary, then secondary, then methyl: C₆H₅CH₂⁺ > (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃⁺.

Part B — Hydrocarbons

5. Alkanes

Alkanes ($C_n{H}_{2n+2}$) are saturated, largely unreactive ("paraffins"), with tetrahedral $s{p}^3$ carbons.

• Preparation: hydrogenation of alkenes (H₂/Ni), Wurtz reaction (2R–X + 2Na → R–R), decarboxylation of sodium salts (soda-lime).
• Halogenation proceeds by a free-radical chain mechanism (initiation → propagation → termination), needing UV light:

$C{l}_2\stackrel{h\nu }{\to }2C{l}^{\bullet }(\text{initiation});C{H}_4+C{l}^{\bullet }\to C{H}_3^{\bullet }+HCl,C{H}_3^{\bullet }+C{l}_2\to C{H}_{3}Cl+C{l}^{\bullet }(\text{propagation})$

• Conformations (from C–C rotation): staggered (lowest energy, anti) vs eclipsed (highest) in ethane; NEET tests which is more stable.

Worked example 5.1. Why does methane react with chlorine only in sunlight (or UV)? Halogenation is a free-radical reaction; the initiation step needs energy (UV) to homolytically split Cl₂ into chlorine radicals. In the dark, no radicals form and no reaction occurs.

6. Alkenes

Alkenes ($C_n{H}_{2n}$) have a C=C (one σ + one π); the π bond is the reactive site for electrophilic addition.

Markovnikov's rule: in adding HX to an unsymmetrical alkene, H adds to the carbon with more hydrogens (so the halogen adds to the more substituted carbon) — because the reaction goes through the more stable carbocation.

Anti-Markovnikov (peroxide/Kharasch effect): with HBr in the presence of peroxides, addition reverses (free-radical mechanism), putting Br on the less substituted carbon. This works only for HBr, not HCl or HI.

Other key additions:

• H₂ / Ni → alkane; X₂ (Br₂ water) → vicinal dihalide (decolourises bromine water — a test for unsaturation).
• Hydration (H₂O/H⁺) → alcohol (Markovnikov).
• Ozonolysis (O₃ then Zn/H₂O) cleaves C=C into two carbonyls — used to locate the double bond.

Worked example 6.1. Product of HBr addition to propene (CH₃–CH=CH₂), with and without peroxide? Without peroxide (Markovnikov): H to the terminal CH₂ (more H's), Br to the middle carbon → 2-bromopropane (via the more stable secondary carbocation). With peroxide (anti-Markovnikov): Br to the terminal carbon → 1-bromopropane.

7. Alkynes

Alkynes ($C_n{H}_{2n-2}$) have a C≡C (one σ + two π), with $sp$ carbons.

• Terminal alkynes are weakly acidic — the $sp$ C–H (50% s-character) holds its electrons tightly, so the H is acidic enough to be removed by strong bases and to form metal acetylides (test: white ppt with ammoniacal AgNO₃, red with Cu₂Cl₂). Acidity: HC≡CH > H₂C=CH₂ > H₃C–CH₃ (s-character order $sp>s{p}^2>s{p}^3$).
• Addition of H₂O (dilute H₂SO₄/HgSO₄) gives, via an enol, an aldehyde (from ethyne) or ketone (Markovnikov).

Worked example 7.1. Why is ethyne (acetylene) acidic but ethane is not? In ethyne the C–H bond is on an $sp$ carbon (50% s-character), which holds the bonding electrons closer to carbon, making the H more easily lost as H⁺. Ethane's $s{p}^3$ C–H (25% s-character) holds its H far more tightly — hence non-acidic.

8. Aromatic hydrocarbons

Aromaticity (Hückel's rule): a ring is aromatic if it is cyclic, planar, fully conjugated, and has $(4n+2)$ π electrons ($n=0,1,2,\dots$). Benzene (6 π electrons, $n=1$) is the archetype — unusually stable, with all C–C bonds equal (resonance hybrid).

Electrophilic aromatic substitution (EAS) is benzene's signature reaction — it substitutes rather than adds, to preserve aromaticity. Five standard reactions, all via an electrophile attacking the π cloud:

Directing effects — a group already on the ring steers the next one:

• Ortho/para directors (activating): –OH, –NH₂, –OR, –CH₃, halogens (halogens are o/p-directing but deactivating). They donate electron density (+M/+I).
• Meta directors (deactivating): –NO₂, –COOH, –CN, –C=O, –SO₃H. They withdraw electron density (−M/−I).

Worked example 8.1. Where does nitration of nitrobenzene occur, and why? The –NO₂ group is a meta director (electron-withdrawing, deactivating), so a second –NO₂ enters the meta position, giving 1,3-dinitrobenzene. The ring also reacts more slowly (deactivated).

Worked example 8.2. Is phenol more or less reactive than benzene toward EAS, and where does substitution occur? More reactive — the –OH group donates electron density (+M), activating the ring — and it is an ortho/para director, so substitution occurs at those positions.

9. Common traps NEET sets here

• Carbocation stability 3° > 2° > 1°; carbanion stability is the reverse (methyl > 1° > 2° > 3°).
• Markovnikov (more stable carbocation) vs peroxide/anti-Markovnikov (HBr only).
• Halogens are o/p-directing but deactivating — the exception to the "activating = o/p" pattern.
• Aromaticity needs 4n+2 π electrons, planar and fully conjugated — count carefully.
• Alkyne acidity from $sp$ character — HC≡CH is acidic; alkanes are not.
• Bromine-water decolourisation = test for unsaturation (alkene/alkyne).
• Benzene substitutes, not adds, to keep aromaticity.
• IUPAC priority: –COOH > –CHO > >C=O > –OH > –NH₂ for the principal suffix.

10. Memory aids

• "3° cation wins, 3° anion loses" — opposite stability orders.
• "Markovnikov: rich get richer" — H to the H-rich carbon; peroxide flips it (HBr only).
• "Halogens: o/p but deactivating" — the directing exception.
• "4n+2 makes it aromatic" — Hückel's rule.
• "More s-character, more acidic" — $sp$ > $s{p}^2$ > $s{p}^3$ C–H.
• "Activators send ortho/para, deactivators send meta" — EAS directing.
• "Benzene swaps, it never adds" — electrophilic substitution.

11. Exam protocol

1. Name by IUPAC: longest chain with the principal group, lowest locants, priority suffix.
2. Classify isomerism (chain/position/functional/tautomer; geometrical/optical) — spot chiral centres and C=C restriction.
3. Predict reactivity from electronic effects (I, M, hyperconjugation) and intermediate stability (3° cation, methyl anion).
4. Alkanes: free-radical halogenation (needs UV); staggered > eclipsed.
5. Alkenes: electrophilic addition by Markovnikov (stable carbocation), peroxide reverses HBr; bromine-water test.
6. Alkynes: terminal-H acidity from $sp$ character; hydration to carbonyls.
7. Aromatics: check 4n+2; EAS reactions and o/p vs meta directing by the substituent already present.