JEE Chemistry Haloalkanes and Haloarenes Notes Set 06

Elimination Reactions

In an elimination two atoms or groups are removed from the substrate, usually leaving a pi bond. Alpha (1,1) elimination gives a carbene, beta (1,2) elimination gives an alkene, and gamma (1,3) elimination gives a three-membered ring. The beta-elimination can follow three mechanisms: E1, E2 and E1cB.

E1 Reaction

Two steps: slow ionisation to a carbocation, then loss of a beta-hydrogen to a base. Rate = k[RX], first order. It shares the first step with SN1, so the two compete and substitution products accompany the alkene; rearrangement of the cation is possible.
Rate follows cation stability: 3 degree > 2 degree > 1 degree. Favoured by tertiary and secondary substrates, polar protic solvents, weak bases and heat.
Dehydration of alcohols with H3PO4 or H2SO4 is E1 for secondary and tertiary alcohols (and for primary alcohols branched at the beta carbon, through rearrangement). Reactivity of alcohols: 3 degree > 2 degree > 1 degree. Acid with a non-nucleophilic anion (H3PO4, H2SO4) is used; HCl and HBr give halides instead.

E2 Reaction

One concerted step: the base removes a beta-hydrogen while the leaving group departs. Rate = k[RX][base]; second order. The H and the leaving group must be anti-periplanar, which makes the reaction stereospecific; in cyclohexanes both must be axial.
Favoured by strong bases (alcoholic KOH, NaOEt, tert-BuOK), heat and a good leaving group. Rate of dehydrohalogenation: 3 degree > 2 degree > 1 degree, and R-I > R-Br > R-Cl. A carbon with no beta-hydrogen cannot undergo E2. A kinetic isotope effect (C-H faster than C-D) is observed because the C-H bond breaks in the slow step.

Saytzeff and Hofmann Orientation

Saytzeff rule: the more substituted (more stable) alkene is the major product, for example with small bases such as ethoxide and with alcohol dehydration. Conjugated alkenes are favoured over non-conjugated ones and trans over cis.
Hofmann rule: the least substituted alkene is the major product when the base or leaving group is bulky (tert-BuOK; quaternary ammonium hydroxides; fluorides), because the most accessible hydrogens are removed.

E1cB Reaction

Two steps: a base removes the beta-hydrogen first, giving a carbanion, and then the leaving group is expelled. It needs an acidic beta-hydrogen (next to C=O, SO2R, NO2 or CN) and a poor leaving group (OH, OR).
Examples: dehydration of beta-hydroxy aldehydes and ketones to enones in base or acid (4-hydroxypentan-2-one gives pent-3-en-2-one), and the elimination of PhSO2CH2CH2OR. Evidence: recovered starting material shows hydrogen-deuterium exchange when the solvent is EtOD.

Substitution Versus Elimination

Aqueous KOH gives mainly substitution (alcohols); alcoholic KOH on heating gives mainly elimination (alkenes). A tertiary halide with a strong base in alcohol gives both ether and alkenes.