Hydrocarbons

1. Alkenes: Electrophilic Addition

The most characteristic reaction of alkenes. Markovnikov's Rule: The negative part of the addendum goes to the carbon with fewer hydrogen atoms. Peroxide Effect: Only HBr shows Anti-Markovnikov addition in the presence of peroxides.

2. Aromaticity and Hückel's Rule

A compound is aromatic if it is Cyclic, Planar, Conjugated, and has $(4n+2)\pi$ electrons. Non-aromatic compounds fail one of the first three criteria. Anti-aromatic compounds have $4n\pi$ electrons.

Interactive diagram: open this page in a browser to use it.

3. Alkanes

• Wurtz reaction: $2RX + 2Na \xrightarrow{dry\ ether} R{-}R$. Good for symmetric alkanes; mixed halides give a mixture.
• Kolbe's electrolysis of sodium carboxylates gives alkanes at the anode.
• Free-radical halogenation: chlorination is fast and unselective; bromination is slower and highly selective for 3° H.

4. Alkynes

Terminal alkynes are weakly acidic (sp carbon). They react with $NaNH_2$ or ammoniacal $AgNO_3$ (white precipitate) and ammoniacal $Cu_2Cl_2$ (red precipitate); internal alkynes do not. Hydration with $HgSO_4$/dil. $H_2SO_4$ gives a carbonyl compound: ethyne gives ethanal, other alkynes give ketones.

5. Reduction

• Lindlar's catalyst (Pd/$BaSO_4$ or $CaCO_3$, poisoned) reduces alkynes to cis-alkenes.
• Na in liquid $NH_3$ (Birch-type) reduces alkynes to trans-alkenes.

6. Electrophilic Aromatic Substitution

Benzene keeps its aromatic ring by substituting rather than adding. Common reactions: nitration ($NO_2^+$ from conc. $HNO_3$ + conc. $H_2SO_4$), halogenation ($FeCl_3$), sulphonation, and Friedel–Crafts alkylation and acylation ($AlCl_3$).

• Ortho/para directors (activating): $-OH$, $-NH_2$, $-OCH_3$, $-CH_3$.
• Ortho/para directors (deactivating): halogens.
• Meta directors (deactivating): $-NO_2$, $-CN$, $-CHO$, $-COOH$, $-SO_3H$.

Practice questions

1. Addition of HBr to propene in the presence of peroxide gives:

   1. 2-Bromopropane
   2. 1-Bromopropane
   3. 1,2-Dibromopropane
   4. Propane
   Answer

   B. Peroxides trigger a free-radical (anti-Markovnikov) addition, so Br goes to the terminal carbon.

2. Which reagent reduces but-2-yne to cis-but-2-ene?

   1. Na/liquid NH₃
   2. H₂, Lindlar's catalyst
   3. LiAlH₄
   4. H₂/Ni excess
   Answer

   B. Lindlar's poisoned catalyst stops at the alkene and adds H₂ from one face (syn), giving the cis isomer.

3. Which of the following is aromatic?

   1. Cyclobutadiene
   2. Cyclooctatetraene
   3. Cyclopentadienyl anion
   4. Cycloheptatriene
   Answer

   C. The cyclopentadienyl anion is cyclic, planar, fully conjugated and has 6 π electrons (4n+2, n=1).

4. -NO₂ group on benzene is:

   1. Ortho/para directing and activating
   2. Meta directing and deactivating
   3. Ortho/para directing and deactivating
   4. Meta directing and activating
   Answer

   B. Its strong −I and −M effects remove electron density, most of all from the ortho and para positions.

5. Which compound gives a white precipitate with ammoniacal AgNO₃?

   1. Ethene
   2. But-2-yne
   3. Propyne
   4. Benzene
   Answer

   C. Only terminal alkynes have an acidic H that forms a silver acetylide.