Hydrocarbons formulas for JEE Main & Advanced

22 formula entries for Hydrocarbons, in the order the chapter builds them. Each shows the formula, when to use it and the first trap to avoid.

Alkanes: General Formula & Nomenclature

C_{n} H_{2 n + 2}

When to use. When naming or writing formulas for saturated hydrocarbons.

Trap. Forgetting to include all carbon atoms in the longest chain when naming branched alkanes.

Alkanes: Combustion Enthalpy

Δ H_{c} = n Δ H_{f} (C O_{2}) + (n + 1) Δ H_{f} (H_{2} O) - Δ H_{f} (alkane)

(negative for combustion)

When to use. When calculating heat released or balancing combustion reactions.

Trap. Forgetting to balance oxygen correctly; coefficient depends on n.

Alkanes: Free Radical Halogenation

R - H + X_{2} \overset{h ν}{\to} R - X + H X

When to use. When predicting products of halogenation or relative rates.

Trap. Assuming all H atoms are equally reactive; tertiary H is more reactive due to stability of radical.

Alkenes: General Formula & Isomerism

C_{n} H_{2 n}

When to use. When analyzing alkene structure or predicting isomers.

Trap. Forgetting that alkenes with same substituents on each carbon show cis-trans isomerism only if each carbon has two different groups.

Alkenes: Addition Reactions (Markovnikov's Rule)

R C H = C H_{2} + H X \to R C H X - C H_{3}

(Markovnikov)

When to use. When predicting major product of HX addition to alkenes.

Trap. Applying Markovnikov's rule to symmetrical alkenes where both carbons have same number of H.

Alkenes: Ozonolysis

R_{2} C = C R_{2} + O_{3} \to R_{2} C = O + R_{2} C = O

(after reduction)

When to use. When determining alkene structure from ozonolysis products.

Trap. Confusing reductive vs oxidative workup products; reductive gives aldehydes, oxidative gives acids.

Alkynes: General Formula & Acidity

R - C \equiv C - H + N a N H_{2} \to R - C \equiv C^{-} N a^{+} + N H_{3}

When to use. When identifying acidic hydrogens or preparing alkynides.

Trap. Assuming all alkynes are acidic; only terminal alkynes (with H on sp carbon) are acidic.

Alkynes: Addition Reactions (Hydration)

R - C \equiv C H + H_{2} O \overset{H g^{2 +} / H^{+}}{\to} R - C O - C H_{3}

When to use. When predicting product of alkyne hydration.

Trap. Forgetting that hydration of terminal alkyne gives a methyl ketone, not an aldehyde.

Alkynes: Reduction to Alkenes/Alkanes

R - C \equiv C - R + H_{2} \overset{L i n d l a r}{\to} c i s - R C H = C H R

When to use. When selectively reducing alkynes to alkenes or alkanes.

Trap. Assuming Lindlar's catalyst gives trans-alkene; it gives cis-alkene.

Aromatic Hydrocarbons (Arenes)

C_{6} H_{6}

(benzene)

When to use. When determining aromaticity of a compound.

Trap. Assuming all cyclic compounds with alternating double bonds are aromatic; must satisfy Hückel's rule.

Benzene: Structure & Stability

Resonance structures:

\leftrightarrow

When to use. When explaining benzene's reactivity or stability.

Trap. Assuming benzene has alternating single and double bonds (Kekulé structure); it is a resonance hybrid.

Electrophilic Aromatic Substitution (EAS) Mechanism

C_{6} H_{6} + E^{+} \to C_{6} H_{5} E + H^{+}

When to use. When predicting products of EAS reactions (nitration, halogenation, sulfonation, Friedel-Crafts).

Trap. Forgetting that the σ-complex is non-aromatic and less stable; rate-determining step is usually formation of σ-complex.

Directing Effects in EAS

Ortho/para directors:

+ I

+ M

groups; Meta directors:

- I

- M

groups

When to use. When predicting major product in disubstituted benzene reactions.

Trap. Assuming all deactivating groups are meta-directors; halogens are deactivating but ortho/para-directing due to +M effect.

Friedel-Crafts Reactions

C_{6} H_{6} + R C l \overset{A l C l_{3}}{\to} C_{6} H_{5} R + H C l

When to use. When synthesizing alkylbenzenes or aryl ketones.

Trap. Forgetting that Friedel-Crafts alkylation can lead to polyalkylation and carbocation rearrangements.

Alkanes: Preparation from Alkyl Halides (Wurtz Reaction)

2 R - X + 2 N a \overset{d r y e t h e r}{\to} R - R + 2 N a X

When to use. When preparing symmetrical alkanes from alkyl halides.

Trap. Assuming Wurtz reaction works well with different alkyl halides; gives mixture of three alkanes.

Alkenes: Preparation by Dehydrohalogenation

R - C H_{2} - C H_{2} X + K O H (a l c .) \to R - C H = C H_{2} + K X + H_{2} O

When to use. When preparing alkenes from alkyl halides.

Trap. Forgetting that alcoholic KOH favors elimination, while aqueous KOH favors substitution.

Alkynes: Preparation from Vicinal Dihalides

R - C H X - C H_{2} X \overset{2 K O H (a l c .)}{\to} R - C \equiv C H + 2 K X + 2 H_{2} O

When to use. When synthesizing alkynes from dihalides.

Trap. Forgetting that two moles of base are needed for complete elimination.

Benzene: Nitration

C_{6} H_{6} + H N O_{3} \overset{H_{2} S O_{4}}{\to} C_{6} H_{5} N O_{2} + H_{2} O

When to use. When introducing nitro group on benzene.

Trap. Forgetting that the electrophile is $N O_{2}^{+}$ , not $N O_{3}^{-}$ or $H N O_{3}$ .

Benzene: Sulfonation

C_{6} H_{6} + H_{2} S O_{4} \to C_{6} H_{5} S O_{3} H + H_{2} O

When to use. When introducing sulfonic acid group or as a blocking group in synthesis.

Trap. Forgetting that sulfonation is reversible; used to block positions temporarily.

Alkanes: Conformations (Newman Projections)

Staggered:

Δ H \approx 12 kJ/mol

more stable than eclipsed for ethane

When to use. When analyzing stability or energy of alkane conformations.

Trap. Assuming all staggered conformations are equally stable; butane's anti is more stable than gauche due to steric hindrance.

Alkenes: Stability (Heat of Hydrogenation)

Stability

\propto 1 / ∣ Δ H_{h y d r o g e n a t i o n} ∣

(more stable alkene has the smaller magnitude of

Δ H_{h y d r o g e n a t i o n}

, i.e. less heat released)

When to use. When comparing alkene stability.

Trap. Assuming that trans-alkenes are always more stable than cis; true for most, but steric effects can reverse.

Alkynes: Ozonolysis

R - C \equiv C - R^{'} + O_{3} \to R C O O H + R^{'} C O O H

When to use. When determining alkyne structure from ozonolysis products.

Trap. Forgetting that terminal alkynes give CO₂ and carboxylic acid (RCOOH).

Values to remember

$1.39 \overset{˚}{A}$ Benzene: Structure & Stability
$152 kJ/mol$ Benzene: Structure & Stability
$12 kJ/mol$ Alkanes: Conformations (Newman Projections)

Read the full chapter: Hydrocarbons for JEE, with what JEE asks and the common traps.