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The Zaitsev (Saytseff) Rule, When alkyl halides have two or more different β carbons, more than one alkene product, is formed., In such cases, the major product is the more stable product—the one with the more, substituted double bond. This phenomenon is called the Zaitsev rule., , The Zaitsev product or the more substituted alkene product is more stable than the less, substituted product. The stability of the more substituted alkene is a result of number of, different contributing factors, including hyperconjugation., Each alkyl group that can involve in hyperconjugation with the double bond stabilizes, it by approximately 6 kcal/mol

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The E2 Mechanism, , The most common mechanism for dehydrohalogenation is the E2 mechanism., y halide and the base appear, pp, in the, It exhibits second-order kinetics,, and both the alkyl, rate equation, rate = k[(CH3)3CBr][HO−], g step., p, The reaction is concerted—all bonds are broken and formed in a single, E2 reactions are regioselective and favor the formation of Zaitsev products.

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Factors Affectingg the Rate of an E2 Reaction, There are close parallels between E2 and SN2 mechanisms in how the identity of the, base, the leaving group and the solvent affect the rate., The base appears in the rate equation, so the rate of the E2 reaction increases, as the strength of the base increases., E2 reactions are generally run with strong,, strong negatively charged bases like OH−, and OR−., Polar aprotic solvents increase the rate of E2 reactions, There is a partial breaking of the bond to the leaving group in the transition, state. So, the better the leaving group the faster the E2 reaction., Rate of reaction follows the order,, order, R−I > R−Br > R−Cl > R−F

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Factors Affectingg the Rate of an E2 Reaction, The SN2 and E2 mechanisms differ in how the R group affects the reaction rate., As the number of R groups on the carbon with the leaving group increases, the rate of, the E2 reaction increases., , The increase in E2 reaction rate with increasing alkyl substitution can be rationalized in, terms of transition state stability., In the transition state, the double bond is partially formed. Thus, the transition state for, a more substituted alkene is lower in energy, reducing the activation energy for the, reaction and making the reaction faster.

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Characteristics of an E2 Reaction, Kinetics, , –, , Second order, , Mechanism, , –, , Single step, , Identity of R group –, , More substituted halides react faster, Rate: R3CX > R2CHX > RCH2X, , Strength of the base –, , Stronger bases favor the reaction, , Leaving group, , –, , Better leaving group leads to faster, reaction rates, , Type of solvent, , –, , Favored by polar aprotic solvents, , E2 reactions are stereoselective, resulting in the formation of trans-double bonds, preferably.

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The E1 Mechanism, , The E1 reaction proceeds via a two-step mechanism: the bond to the leaving group, b k first, breaks, fi t before, b f, th π bond, the, b d is, i formed., f, d The, Th slow, l, step, t is, i unimolecular,, i l l involving, i l i only, l, the alkyl halide., It exhibits first-order kinetics,, rate, t = k[(CH3)3CCl], , E1 reactions also are regioselective and follow Zaitsev rule

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Characteristics of an E1 Reaction, Kinetics, , –, , First order, , Mechanism, , –, , Two steps, , Identity of R group –, , More substituted halides react faster, Rate: R3CX > R2CHX > RCH2X, , Strength of the base –, , Favored by weaker bases such as H2O and ROH, , Leaving group, , –, , Better leaving group leads to faster reaction rates., Just as in SN1 reactions,, reactions the rate determining step, involves the C—X bond cleavage, , Type of solvent, , –, , Favored by polar protic solvents, which can, stabilize the ionic intermediates

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Stereochemistry of the E2 Reaction, The transition state of an E2 reaction consists of four atoms from the substrate (one, hydrogen atom, two carbon atoms, and the leaving group, X) aligned in a plane. There, are two ways for the C—H and C—X bonds to be coplanar., , E2 elimination occurs most often in the anti periplanar geometry. This arrangement, allows the molecule to react in the lower energy staggered conformation,, conformation and allows, the incoming base and leaving group to be further away from each other., The anti periplanar geometry also allows direct interaction between the bonding, electrons of C—H bond and the anti-bonding orbital of the C—X bond.

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E2 Reactions in 6-Membered Rings, The stereochemical, Th, t, h i l requirement, i, t off an anti, ti periplanar, i l, geometry, t in, i an E2 reaction, ti has, h, important consequences for compounds containing six-membered rings., Chlorocyclohexane, y, , For E2 elimination, the C, C—Cl, Cl bond must be anti periplanar to the C, C—H, H bond on a β, carbon, and this occurs only when the H and Cl atoms are both in the axial position., The requirement for trans-diaxial geometry means that elimination must occur from the, less stable conformer.

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Dehydrohalogenation of trans-1-Chloro-2-methylcyclohexane, , The conformer with Cl in an axial orientation has just one β-H atom. Only one product, is formed, which is not what is predicted by the Zaitsev rule., , In conclusion,, conclusion with substituted cyclohexanes,, cyclohexanes E2 elimination should occur with a trans, diaxial arrangement of the leaving group and the β-H, and as a result of this, requirement, the more substituted alkene is not necessarily the major product.

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E1cB Reaction, An elimination, A, li i ti reaction, ti that, th t happens, h, when, h a compoundd bearing, b i a poor leaving, l i group, and an acidic hydrogen is treated with a base., , E1cB stands for Elimination Unimolecular conjugate Base., Base The reaction is, unimolecular from the conjugate base of the starting compound, which in turn is, formed by deprotonation of the starting compound by a suitable base., The electron withdrawing group (EWG) can be a carbonyl group (keto, aldehyde,, ester), a nitro group, an electron deficient aromatic group etc. Dehydration of aldol is, the most common E1cB reaction, H, HO, , O, , OH, H2O, , OH, HO, , O, , HO, , O, , O