Hydrocarbons — Practice

Question 1

Predict the product of CH3-CH=CH2 + H2O with an acid catalyst.

Accepted Answer

By Markovnikov's rule, H+ adds to the terminal CH2 (more hydrogens), forming the more stable secondary carbocation CH3-C+H-CH3. Water then adds to give propan-2-ol (CH3-CHOH-CH3).

Question 2

Why does benzene undergo electrophilic substitution rather than addition?

Accepted Answer

Addition would break the delocalised aromatic pi-system and lose about 152 kJ/mol of resonance (aromatic) stabilisation energy. Substitution replaces a hydrogen while preserving the aromatic sextet, so it is strongly favoured.

Question 3

Identify A and B: CH3COONa + NaOH(soda lime) -> A; A + Cl2/UV -> B.

Accepted Answer

A = CH4 (decarboxylation of sodium acetate). B = CH3Cl (free-radical chlorination of methane).

Question 4

What is the peroxide effect? Give an example.

Accepted Answer

In the presence of organic peroxides, HBr adds to an alkene against Markovnikov's rule (anti-Markovnikov), via a free-radical mechanism. Example: CH3-CH=CH2 + HBr (peroxide) -> CH3-CH2-CH2Br (1-bromopropane).

Question 5

How would you distinguish ethane, ethene, and ethyne?

Accepted Answer

Baeyer's test (cold dilute KMnO4): ethane gives no reaction (purple persists); ethene and ethyne both decolourise it. To separate the latter two, ethyne (a terminal alkyne) gives a white precipitate with ammoniacal AgNO3, while ethene does not.

Practice — Hydrocarbons

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