JEE Chemistry Hydrocarbons Notes Set 10

Free Radical Substitution in Alkanes

Alkanes react with halogens in light or at 250-400 degrees C by a chain reaction in three stages.
Initiation: \( \ce{X2 ->[h\nu] 2X.} \). Propagation: \( \ce{X. + R-H -> R. + HX} \) (rate determining) and \( \ce{R. + X2 -> R-X + X.} \). Termination: two radicals combine, for example \( \ce{X. + X. -> X2} \), \( \ce{R. + R. -> R-R} \) or \( \ce{R. + X. -> R-X} \).
Reactivity of halogens: F2 > Cl2 > Br2 > I2. Fluorine reacts explosively even in the dark and must be diluted with inert gas. Iodination is reversible because the HI formed reduces R-I back to R-H; it works only in the presence of an oxidising agent such as HIO3, HNO3 or HgO.
Reactivity of C-H bonds: 3 degree > 2 degree > 1 degree, following the stability of the alkyl radical. Chlorine is not very selective (propane gives about 45 percent 1-chloro and 55 percent 2-chloro products), whereas bromine is highly selective (2-methylpropane gives over 99 percent tert-butyl bromide).
Initiators such as peroxides start chains; inhibitors such as oxygen stop them by trapping alkyl radicals as peroxy radicals.
Allylic and benzylic substitution: Cl2 or Br2 at high temperature or in light, or N-bromosuccinimide (NBS) or N-chlorosuccinimide, substitute the hydrogen on the carbon next to a C=C bond or a benzene ring, because the allyl and benzyl radicals are resonance stabilised. NBS keeps the Br2 concentration low so that addition to the double bond does not compete.

Free Radical Addition of HBr (Peroxide Effect)

In the presence of peroxides or light, HBr adds to an unsymmetrical alkene in the anti-Markovnikov sense: \( \ce{R-CH=CH2 + HBr ->[R2O2] R-CH2-CH2-Br} \). This is the Kharasch (peroxide) effect and it works only for HBr (not for HCl or HI).
Mechanism: the peroxide generates Br. radicals; Br. adds to the terminal carbon of the alkene so as to give the more stable alkyl radical (secondary radical from a terminal alkene); this radical takes H from HBr, giving the product and a new Br. radical, which continues the chain.
The peroxide effect is a free-radical chain reaction, so the bromine, not the hydrogen, ends up on the carbon with more hydrogens.

Electrophilic Addition to Alkenes

The pi electrons of C=C are loosely held, so alkenes act as nucleophiles towards electrophiles. One weak pi bond (about 251 kJ per mole) is broken and two strong sigma bonds (about 694 kJ per mole) form, so the reaction is exothermic by roughly 443 kJ per mole.
Mechanism: the electrophile attacks first (slow step) to give either an open carbocation or a cyclic bridged ion; the nucleophile then attacks in a fast step. The rate depends on the stability of the intermediate cation, so alkenes with electron-releasing groups react faster: tetra-substituted > tri- > di- > mono-substituted > ethene, and ERG-CH=CH2 > CH2=CH2 > EWG-CH=CH2.
Markovnikov rule: the electrophile (H+) goes to the carbon that gives the more stable carbocation, so the nucleophile ends up on the more substituted carbon.

Halogenation. Cl2 and Br2 (not F2 or I2) add through a cyclic halonium ion, so the addition is anti and there is no rearrangement. Bromine adds to cyclopentene to give trans-1,2-dibromocyclopentane.
Halohydrin formation. X2 with water (HOX) adds X to one carbon and OH to the other, anti, through the halonium ion, with OH on the more substituted carbon.
Addition of HX. Markovnikov addition through a carbocation; rearrangement (hydride or methyl shift) can occur.
Acid-catalysed hydration (dilute H2SO4): Markovnikov addition of water through a carbocation, rearrangement possible.
Oxymercuration-demercuration (Hg(OAc)2 in water or THF, then NaBH4): Markovnikov alcohol with no rearrangement because a bridged mercurinium ion is formed; with an alcohol in place of water it gives an ether.
Hydroboration-oxidation (BH3 in THF, then H2O2 and OH-): anti-Markovnikov, syn addition of H and OH; no rearrangement.

Electrophilic Addition to Alkynes and Dienes

Alkynes react like alkenes but can add two molecules of reagent. Halogens: one equivalent of Br2 gives the trans-dihalide (anti addition), and a second equivalent gives the tetrahalide. HX adds in the Markovnikov sense twice to give a gem-dihalide, and a single addition gives a vinyl halide. The vinyl cation is less stable than an alkyl cation, so alkynes are somewhat less reactive than alkenes towards electrophiles.
HOX adds to give an enol and finally an alpha,alpha-dihaloketone (major) with some alpha-haloketone.
Hydration: with dilute H2SO4 and Hg2+ the enol formed tautomerises to a ketone (Markovnikov), so propyne gives acetone and other terminal alkynes give methyl ketones; ethyne gives acetaldehyde. Hydroboration-oxidation of a terminal alkyne is anti-Markovnikov and gives an aldehyde: propyne gives propanal.
Partial reduction: H2 over Lindlar catalyst (Pd/CaCO3, quinoline) gives the cis-alkene (syn addition); Na or Li in liquid NH3 gives the trans-alkene.
Conjugated dienes with HBr or Br2 give both 1,2- and 1,4-addition products. At low temperature (-80 degrees C) the 1,2-product forms faster and predominates (kinetic control); at higher temperature (about 40 degrees C) the more stable 1,4-product predominates (thermodynamic control).