Alkyl Halides Part 4 PDF

Alkyl halides Organic chemistry E2 reaction The most common mechanism for dehydrohalogenation is the E2 mechanism. It exhibits second-order kinetics, and both the alkyl halide and the base appear in the rate equation, i.e., rate = k[(CH3)3CBr][¯OH] The reaction is concerted—all bonds are broken and...

Alkyl halides Organic chemistry E2 reaction The most common mechanism for dehydrohalogenation is the E2 mechanism. It exhibits second-order kinetics, and both the alkyl halide and the base appear in the rate equation, i.e., rate = k[(CH3)3CBr][¯OH] The reaction is concerted—all bonds are broken and formed in a single step. E2 reaction E2 reaction Factors affect on the rate of E2 reactions E2 reaction Factors affect on the rate of E2 reactions 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, negatively charged bases like¯OH and ¯OR. Polar aportic solvents increase the rate of E2 reactions. E2 reaction Factors affect on the rate of E2 reactions E2 reaction Increasing the number of R groups on the carbon with the leaving group forms more highly substituted, more stable alkenes in E2 reactions. In the reactions below, since the disubstituted alkene is more stable, the 3° alkyl halide reacts faster than the 1o alkyl halide. E2 reaction E2 reaction E2 reaction The Zaitsev (Saytzeff) Rule Recall that when alkyl halides have two or more different β carbons, more than one alkene product is formed. When this happens, one of the products usually predominates. The major product is the more stable product—the one with the more substituted double bond. This phenomenon is called the Zaitsev rule. E2 reaction The Zaitsev Rule The Zaitsev rule: the major product in β elimination has the more substituted double bond. A reaction is regioselective when it yields predominantly or exclusively one constitutional isomer when more than one is possible. Thus, the E2 reaction is regioselective. E1 reaction E2 reaction Mechanisms of Elimination—E1 The dehydrohalogenation of (CH3)3CCI with H2O to form (CH3)2C=CH2 can be used to illustrate the second general mechanism of elimination, the E1 mechanism. An E1 reaction exhibits first-order kinetics: rate = k[(CH3)3CCI] E1 reaction Mechanisms of Elimination—E1 The E1 reaction proceeds via a two-step mechanism: the bond to the leaving group breaks first before the π bond is formed. The slow step is unimolecular, involving only the alkyl halide. The E1 and E2 mechanisms both involve the same number of bonds broken and formed. The only difference is timing. In an E1, the leaving group comes off before the β proton is removed, and the reaction occurs in two steps. In an E2 reaction, the leaving group comes off as the β proton is removed, and the reaction occurs in one step. E1 reaction E1 reaction Factors in E1 reactions The rate of an E1 reaction increases as the number of R groups on the carbon with the leaving group increases. The strength of the base usually determines whether a reaction follows the E1 or E2 mechanism. Strong bases like ¯OH and ¯OR favor E2 reactions, whereas weaker bases like H2O and ROH favor E1 reactions. E1 reaction Factors in E1 reactions Example E1 reactions are regioselective, favoring formation of the more substituted, more stable alkene. Zaitsev’s rule applies to E1 reactions also. E1 reaction E1 reaction SN1 and E1 Reactions SN1 and E1 reactions have exactly the same first step—formation of a carbocation. They differ in what happens to the carbocation. When is the Mechanism E1 or E2 The strength of the base is the most important factor in determining the mechanism for elimination. Strong bases favor the E2 mechanism. Weak bases favor the E1 mechanism. E2 reaction E2 Reactions and aklene, Alkyne Synthesis A single elimination reaction produces a π bond of an alkene. Two consecutive elimination reactions produce two π bonds of an alkyne. E2 reaction E2 Reactions and Alkyne Synthesis Two elimination reactions are needed to remove two moles of HX from a dihalide substrate. Two different starting materials can be used—a vicinal dihalide or a geminal dihalide. E2 reaction E2 Reactions and Alkyne Synthesis The reason that stronger bases are needed for this dehydrohalogenation is that the transition state for the second elimination reaction includes partial cleavage of the C—H bond. In this case however, the carbon atom is sp2 hybridized and sp2 hybridized C—H bonds are stronger than sp3 hybridized C—H bonds. As a result, a stronger base is needed to cleave this bond. E2 reaction E2 reaction E2 reaction Predicting the Mechanism from the Reactants—SN1, SN2, E1 or E2 Good nucleophiles that are weak bases favor substitution over elimination—Certain anions always give products of substitution because they are good nucleophiles but weak bases. These include I¯, Br¯, HS¯, and CH3COO¯. E2 reaction Predicting the Mechanism from the Reactants—SN1, SN2, E1 or E2 Bulky nonnucleophilic bases favor elimination over substitution—KOC(CH3)3, DBU, and DBN are too sterically hindered to attack tetravalent carbon, but are able to remove a small proton, favoring elimination over substitution. E2 reaction Predicting the Mechanism from the Reactants—SN1, SN2, E1 or E2 E2 reaction Predicting the Mechanism from the Reactants—SN1, SN2, E1 or E2 E2 reaction Predicting the Mechanism from the Reactants—SN1, SN2, E1 or E2