CHP 8 Questions , 33, 35, Nucleophilic Substitution Reactions
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1 CHP 8 Questions , 33, 35, Nucleophilic Substitution Reactions 8.1 The General Reaction In the reaction of chloromethane with hydroxide ion the hydroxide ion is substituted for the chlorine group. The hydroxide ion is donating a pair of electrons to the carbon and is termed a Nucleophile. The nucleophile is electron rich species that seeks an electron deficient site. Nucleophiles are Lewis bases. The carbon of the chloromethane is electron deficient and termed an Electrophile. Electrophiles are Lewis acids. The carbon atom of chloromethane, electrophile, accepts a pair of electrons from the hydroxide ion, nucleophile. To avoid exceeding the proper valence of carbon, the chlorine atom departs with the electron pair of the C-Cl bond. The chloride is termed the Leaving Group. Example The concept of nucleophiles and electrophiles is one of the most important concepts in organic chemistry. This reaction is termed a nucleophilic substitution reaction since a nucleophile replaces the leaving group. The general reaction can be depicted as Nu: - + R-L Nu-R + L: Reaction Mechanisms A reaction mechanism shows the individual steps of a reaction the order in which the bonds are broken and formed. In a nucleophilic substitution reaction there are three possible mechanisms: 1) the bond to the leaving group is broken first, followed by formation of the bond with the nucleophile, 2) the bond to the nucleophile is formed first, then the bond to the leaving group is broken, 3) the bond breaking and formation occur simultaneously. Pathways 1 and 3 occur while pathway 2 does not since it would require a pentavalent carbon. Examples:
2 8.3 Bimolecular Nucleophilic Substitution Considering the reaction of hydroxide ion with chloroethane: OH - + CH 3 CH 2 Cl CH 3 CH 2 OH + Cl - The rate of this reaction is dependent upon the concentration of hydroxide ion and chloroethane. The reaction is second order reaction where both EtCl and OH - are involved with the rate determining step of the reaction. Rate = k[etcl][oh - ] This reaction is consistent with mechanism 3 above and is termed a second order nucleophilic substitution reaction (S N 2). The reaction coordinate diagram for this reaction involves a single transition state. 8.4 Stereochemistry of the S N 2 Reaction Substitution reactions that occur on a chiral carbon can have three possible outcomes. The sterocenter can be retained, inverted or racemized. Figure 8.2 Reactions that occur via an S N 2 reaction occur with inversion of the stereocenter. The incoming nucleophile approaches the carbon from the opposite side of the leaving group. At the transition state the carbon has a trigonal planar geometry and is sp 2 hybridized. The leaving group departs as the nucleophile carbon bond is fully formed. Figure 8.3
3 8.5 Effect of Substituents on the Rate of the S N 2 Reaction The rate of the S N 2 reaction decreases moving from methyl chloride to ethyl chloride (a 1 o alkyl halide) to isopropyl chloride (2 o ) to tert-butyl chloride (3 o ). The relative rates are listed in Table 8.1: MeCl 30 EtCl 1 iprcl t-bu 0 This is explained by increases in the energy of the transition state due to steric interactions between the alkyl substituents and the incoming nucleophile. Fig 8.4 One can observe the increased steric constraints of the different alkyl chlorides via space filling models. Fig 8.5 While the most important differences in the rate of S N 2 reactions depend upon the structure of the electrophilic carbon (methyl, 1 o, 2 o, or 3 o ). There are exceptions to this rule. Neopentyl chloride, a 1 o alkyl chloride, reacts 2500 times slower than isopropyl chloride. This is due to the large steric bulk of the t-butyl substituent of neopentyl chloride. Both allyl chloride and benzyl chloride react much faster than ethyl chloride. This is due to the resonance stabilization of the transition state. Example 8.6 Unimolecular Nucelophilic Substitution If we consider the reaction of acetate ion with tert-butyl chloride: CH 3 CO (CH 3 ) 3 CCl (CH 3 ) 3 CO 2 CCH 3 + Cl - The reaction looks similar to that of hydroxide ion with methyl chloride. However, the rate of this reaction is only dependent upon the concentration of the tert-butyl chloride. Rate = k[t-bucl] This indicates that only t-bucl is present at the rate limiting transition state. The reaction is termed a unimolecular nucleophilic substitution or (S N 1) reaction. The reaction
4 mechanism has two steps. The first step is the unimolecular dissociation of the t-bucl to form a reactive intermediate (the t-bu cation) followed by rapid reaction with the nucelophile. This can be indicated by the reaction coordinate diagram in Figure 8.6. Upon dissociation of the chloride ion a reactive intermediate is formed. This reactive intermediate is a high energy, reactive species. But a local energy minimum. In the S N 1 reaction the intermediate is a carbocation. The cationic carbon has only 6 electrons and is sp 2 hybridized. The transition state for conversion of the t-bucl to t-bu + and Cl - has structural characteristics similar to both the starting material and the intermediate. (EX) Hammond s Postulate states that the structure of the transition state is closer to that of the species to which it is closer in energy. Fig Effect of substituents on the rate of the S N 1 reactions. Methyl and primary alkyl chlorides do not undergo S N 1 reactions. They only undergo S N 2. The formation of the carbocation is the rate limiting step of the reaction and the stability of the carbocations follows the following order: 3 o >2 o >1 o >Me. The substitution of an alkyl group for a hydrogen results in significant stabilization of the carbocation. This is due to overlap of the adjacent sigma bond which provides electronic delocalization via hyperconjugation. Example. Resonance stabilization of adjacent π-bonds provides significant increases in the rates of S N 1 reactions. Example
5 //INSERT 8.8 to 8.12// 8.12 Competition between S N 1 and S N 2 Reactions S N 1 Reactions are favored by: 1) Stabilized carbocations (3 o or resonance stabilized are best, 2 o is ok) 2) Polar solvent 3) Poor nucleophiles are preferred. S N 2 Reactions are favored by: 1) Unhindered electrophilic carbon (Me and 1 o excellent, 2 o is ok) 2) Polar aprotic solvent. 3) Strong Nucleophiles are preferred Intramolecular Reactions When a molecule contains both a nucleophile and an electrophile one can observe intramolecular reactions. For reactions which generate 5 and 6 membered rings the reaction can occur much more rapidly than the intermolecular reaction (> 10 3 ). This is primarily due to the close proximity of the nucleophile and electrophile lower entropic cost, and lack of ring strain no unfavorable enthalpy. Example 8.14 Competing Reactions In addition to S N 1 and S N 2 reactions, other reactions can occur in competition. Elimination reactions (CHP 9) occur when the leaving group and a hydrogen are lost from adjacent carbons, resulting in the formation of a double bond. Figure 8.11 Another side reaction that can occur in S N 1 reactions is the 1,2-hydride (or alkyl) shift. This usually occurs when a carbocation is generated adjacent to a potentially more stable carbocation. Typically this is a change from a secondary carbocation to a tertiary carbocation. Example
6 It is also important to understand that allylic carbocations provide more than one site for nucleophilic attack. Example
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