HydrocarbonsNEET Chemistry · Class 11 · NCERT Chapter 9

Markovnikov's rule applies to the electrophilic addition of unsymmetrical reagents (like HX or H₂O) across a double bond. It states that the hydrogen atom (or the positive part of the reagent) adds to the carbon of the double bond that already has more hydrogen atoms. The underlying reason is that the more substituted (more stable) carbocation intermediate is formed preferentially. Example: propene + HBr gives 2-bromopropane (Markovnikov product), not 1-bromopropane.

When HBr adds to an alkene in the presence of peroxides (e.g., H₂O₂ or ROOR), the reaction proceeds via a free radical mechanism instead of ionic. The bromine atom (not HBr as an ionic species) adds first to give the more stable radical intermediate. This produces the anti-Markovnikov product (Br goes to the less substituted carbon). This effect is specific to HBr; HCl and HI do not show this effect with peroxides.

A hypothetical cyclohexatriene (three alternating double bonds, like Kekulé's structure) would have a heat of hydrogenation of about −360 kJ/mol (3 × −120 kJ). But the actual benzene has a heat of hydrogenation of only −208 kJ/mol. The difference (about 152 kJ/mol) is the resonance energy. This extra stability comes from the complete delocalisation of the 6 pi electrons over all 6 carbon atoms in a ring, following Hückel's rule (4n + 2 π electrons with n = 1).

Groups attached to a benzene ring influence where the incoming electrophile attacks. Ortho/para directors (also called ring-activating groups) have a lone pair that can donate electron density to the ring via resonance (+M effect). They activate the ring and direct the electrophile to the ortho and para positions. Examples: −OH, −NH₂, −OCH₃, −alkyl. Meta directors (ring-deactivating groups) withdraw electron density from the ring by −M or −I effects, leaving the meta positions relatively electron-rich. Examples: −NO₂, −COOH, −SO₃H, −CHO, −CN.

Acidity depends on how well the conjugate base (the carbanion) is stabilised. The sp carbon in alkynes has more s-character (50%) than sp² (33%) or sp³ (25%) carbon. More s-character means electrons are held closer to the nucleus, stabilising the negative charge on carbon. So the sp carbanion (from an alkyne) is more stable → alkyne is more acidic than alkene or alkane. Acidic order: alkyne (pKa ≈ 25) >> alkene (pKa ≈ 44) > alkane (pKa ≈ 50).

Ozonolysis cleaves the double bond completely. The double bond is replaced by two C=O groups. Reductive ozonolysis (with Zn/H₂O or Me₂S) gives aldehydes from terminal carbons (=CH₂ → HCHO) and ketones from internal carbons (=CR₂ → R₂C=O). Oxidative ozonolysis (with H₂O₂ or KMnO₄) converts aldehydes further to carboxylic acids. This reaction is used to determine the position of a double bond in an unknown alkene.