Every formula, rule and trap from the chapter on one page. Keep it open while you solve PYQs, or download it and stick it above your desk.

Hydrocarbons: one-page cheat sheet

NEET Chemistry · Unit 15 · Organic Chemistry

ScoreChemby CSJ Sir
Weightage: 3–4 questions · 12–16 marks

1 · Alkanes: preparation

Hydrogenationalkene + H

_{2}

over Ni, Pt or Pd

Wurtz2RX + 2Na (dry ether)

\to

R–R; symmetrical, even carbons; no CH

_{4}

Soda-limeRCOONa + NaOH (CaO,

Δ

)

\to

RH + Na

_{2}

_{3}

; one carbon less

Kolbeelectrolysis of RCOONa(aq): 2RCOO

^{-}

\to

R–R + 2CO

_{2}

(anode)

SteamCH

_{4}

+ H

_{2}

O (Ni, 1273 K)

\to

CO + 3H

_{2}

2 · Conformations & b.p.

Staggereddihedral

6 0^{\circ}

: most stable

Eclipseddihedral

0^{\circ}

: least stable; barrier 12.5 kJ mol

^{- 1}

Pentanesb.p.: n-pentane (309 K)

>

isopentane (301 K)

>

neopentane (282.5 K)

Branching makes the molecule more spherical, so the surface area and b.p. fall.

3 · Free-radical halogenation

ReactivityF

_{2}

>

_{2}

>

_{2}

>

_{2}

; Br

_{2}

is the most selective

H replaced

3^{\circ} > 2^{\circ} > 1^{\circ}

Chaininitiation

Cl_{2} h ν 2 Cl \cdot

; propagation; termination (radicals pair)

OxidationCH

_{4}

: Cu, 523 K, 100 atm gives CH

_{3}

OH; Mo

_{2}

_{3}

gives HCHO

4 · Alkenes

Prep.alc. KOH on RX (Saytzeff); Zn on vicinal dihalide; alcohol + conc. H

_{2}

_{4}

From alkyneLindlar's catalyst gives cis; Na / liq. NH

_{3}

gives trans

C=C134 pm: one

σ

+ one

π

cis / transcis: higher dipole moment and b.p.; trans: higher m.p.

Needs two different groups on each carbon. 1,2-Dimethylcyclohexane shows it; pent-1-ene does not.

5 · Addition & oxidation

MarkovnikovH goes to the carbon with more H; via the more stable cation

PeroxideHBr only: anti-Markovnikov, free-radical

OzonolysisO

_{3}

, then Zn/H

_{2}

O: C=C splits into two C=O

Baeyercold dilute alkaline KMnO

_{4}

is decolourised; glycol forms

_{2}

/ CCl

_{4}

red-orange colour discharged

6 · Alkynes

Shapelinear, 180°, C≡C 120 pm

HC \equiv CH > CH_{3} C \equiv CH ≫ CH_{2} = CH_{2} > CH_{3} CH_{3} (acidity)

TerminalNaNH

_{2}

gives acetylide; Tollens' gives a precipitate

KucherovHgSO

_{4}

/ dil. H

_{2}

_{4}

: ethyne gives ethanal, others give ketones

Trimerisation3 HC≡CH (red-hot Fe tube, 873 K)

\to

_{6}

_{6}

7 · Benzene & EAS

Aromaticplanar, cyclic,

(4 n + 2) π

electrons; C–C 139 pm

Benzene fromethyne, sodium benzoate + soda-lime, n-hexane reforming

Not benzenebenzenediazonium chloride + H

_{2}

O gives phenol

Reaction	Electrophile
nitration	NO $_{2}^{+}$ (HNO $_{3}$ + H $_{2}$ SO $_{4}$ )
halogenation	Cl $^{+}$ (Cl $_{2}$ + FeCl $_{3}$ )
sulphonation	SO $_{3}$ (fuming H $_{2}$ SO $_{4}$ )
Friedel–Crafts	R $^{+}$ or RCO $^{+}$ (AlCl $_{3}$ )

Steps: attack by E $^{+}$ , arenium ion, loss of H $^{+}$ .

8 · Directing effects

Group	Effect
OH, NH $_{2}$ , OCH $_{3}$ , CH $_{3}$	activating, o/p
F, Cl, Br, I	deactivating, o/p
NO $_{2}$ , CN, CHO, COOH, SO $_{3}$ H	deactivating, meta

Friedel–Crafts fails on strongly deactivated rings and on aniline.

9 · Tests & stability

Alkene stabilitymore alkyl groups on C=C, more stable (hyperconjugation)

Terminal alkyneammoniacal AgNO

_{3}

gives a white precipitate; Cu

_{2}

_{2}

gives red

UnsaturationBaeyer's and bromine tests, both negative for alkanes

Seen in NEET 2021

3-Methylbut-1-ene + HBr / peroxide:

(CH_{3})_{2} CHCH = CH_{2} \to (CH_{3})_{2} CHCH_{2} CH_{2} Br

1-Bromo-3-methylbutane (anti-Markovnikov).

Seen in NEET 2024

An alkene gives HCHO and 2-methylpropanal on reductive ozonolysis.

The alkene is 3-methylbut-1-ene.

Seen in NEET 2020, 2022, 2023

Wurtz fails in high yield for unsymmetrical alkanes such as n-heptane.

Least stable ethane conformer: dihedral angle $0^{\circ}$ .

2-Bromopentane + alc. KOH gives pent-2-ene (Saytzeff).

Six traps to remember

Wurtz coupling gives high yields only for symmetrical, even-carbon alkanes.
Soda-lime decarboxylation shortens the chain by one carbon.
The peroxide effect works only with HBr, not HCl or HI.
Lindlar's catalyst gives cis-alkenes; Na / liq. NH $_{3}$ gives trans.
Halogens are deactivating but ortho/para-directing.
Benzenediazonium chloride with water gives phenol, not benzene.

Hydrocarbons: free one-page cheat sheet