Alkyl Halides - Elimination Reactions | Organic Chemistry 1

An alkyl halide typically undergoes a nucleophilic substitution reaction or an elimination reaction. In both cases, the alkyl halide acts as an electrophile, reacting with an electron-rich reagent. In a substitution, the nucleophile attacks the carbon atom bearing the good leaving group, while in an elimination, the base removes a proton to form a π bond, and 2 carbons are involved in the reaction

The degree of substitution of an alkene corresponds to the number of alkyl groups attached to the C=C double bond. Thus, a monosubstituted alkene has one carbon atom bonded to the carbons of the double bond. A disubstituted alkene has two carbon atoms bonded to the carbons of the double bond, and so on
 

Alkenes more highly alkylated are more stable, thus the order is tetra > tri > di > mono-substituted. Indeed, alkyl groups are electron-donating via hyperconjugation: they can donate electron density to the neighboring sp²-hybridized carbon atoms of a π bond. The resulting electron density delocalization has a stabilizing effect

The major product of a β elimination reaction is the alkene with the most substituted double bond

The Zaitsev product is the product of the β elimination reaction with the most substituted C=C double bond

An E2 reaction is an elimination reaction which proceeds via a concerted mechanism: E2 reactions are bimolecular with simultaneous bond-making and bond-breaking steps. The kinetic rate involves 2 components: the base and the electrophile. Therefore, the E2 reaction is favored by strong bases

E2 reactions are bimolecular with simultaneous bond-making and bond-breaking steps: the carbon-hydrogen and carbon-halogen bonds break to form a new double bond. E2 reactions occur when the geometry is anti-periplanar
 

E2 reactions occur when the hydrogen and halogen atoms are oriented on opposite sides of the molecule. This geometry is called anti-periplanar and is preferred to syn-periplanar geometry

An E1 reaction is an elimination reaction that takes place via an intermediate carbocation. E1 reactions are unimolecular with a bond-breaking step followed by a bond-making step. The kinetic rate only involves the starting material. Since the base does not appear in the rate equation, weak bases favor E1 reactions

The mechanism of an E1 reaction has two steps. In the first step, the leaving group comes off to form a planar carbocation. In the second step, a β proton is removed by the base to give the alkene. Because of this two-step mechanism, E1 reactions do not require an anti periplanar geometry
 

The rate of an E1 reaction is rate = k [R-X]. Indeed, the slowest step of an E1 reaction is the formation of the carbocation (first step). Therefore, it determines the rate and is called the rate-determining step

The main difference between the E1 and E2 reactions is their number of steps. The E1 reaction takes place in two steps and has a carbocation intermediate. In contrast, the E2 reaction takes place in one step and has no intermediate. Thus, E1 is a first-order reaction, while E2 is a second-order reaction requiring an anti periplanar geometry

The strength of the base is the most important factor in determining the mechanism of an elimination reaction:

• Strong bases (negatively charged bases such as HO^- and RO^-) favor E2 reactions
• Weak bases (neutral bases such as H₂O and ROH) favor E1 reactions

Polar protic solvents (H₂O, ROH, RCOOH) favor E1 reactions because the carbocation intermediates are stabilized by solvation while polar aprotic solvents (CH₃CN, ROR, RCOR, DMSO) favor E2 reactions because the negatively charged bases do not interact with the solvent and are therefore stronger

Substitution reactions compete with β elimination reactions. The structure of the alkyl halides and/or nucleophiles determines the type of reaction:

• Strong nucleophiles that are weak bases favor substitution over elimination
• Bulky, non-nucleophilic bases (^tBuOK, LDA, DBU) favor elimination over substitution

1. Determine the nature of the alkyl halide (primary, secondary or tertiary)
2. Determine the nature of the base or nucleophile (strong, weak, non-nucleophilic, bulky)