If a primary alkyl halide and a chiral tertiary halide both undergo substitution reactions, the reaction with the tertiary halide will have more products because:

Question

Quizplus · Accepted Answer

11ecba2d_12bb_6c22_a3bf_1f180f4f7c86_MD0008_00 Substitution reactions can occur in one of two ways: an S_N1 or an S_N2 mechanism. These substitution reactions differ in number of steps and stereospecificity. An S_N1 reaction occurs in two steps and is not stereospecific, whereas an S_N2 reaction occurs in a single step and is stereospecific. Stereospecific reactions occur when stereoisomers that undergo the same reaction give different stereoisomers of the product. The stereochemistry of the starting material determines the stereochemistry of the product. The first step of an S_N1 reaction involves breaking the bond between the leaving group and the adjoining carbon, forming a planar carbocation intermediate. In the second step, the nucleophile can add to the top or bottom of the planar carbocation, so two products form. In contrast, S_N2 reactions occur in a single step and the nucleophile always attacks on the side opposite the leaving group (backside attack), causing an inversion at the electrophilic carbon. An S_N2 reaction forms only one product. Tertiary alkyl halides form more stable carbocations than less sterically hindered alkyl halides and readily undergo S_N1 reactions, whereas primary alkyl halides undergo S_N2 reactions only. Chiral tertiary alkyl halides form two products (stereoisomers) in S_N1 reactions because S_N1 reactions are not stereospecific. (Choices A and C) S_N2 reactions occur in one step, and primary alkyl halides undergo this reaction more readily than the more sterically hindered tertiary halides. However, S_N2 reactions are stereospecific and only one stereoisomer is formed. (Choice D) The leaving group on a tertiary halide is sterically hindered by the bulk of the other substituents. Educational objective: Stereospecific reactions occur when stereoisomers undergo the same reaction and give different stereoisomers of the product. Tertiary alkyl halides readily undergo S_N1 reactions, which are not stereospecific because the nucleophile can attack the planar carbocation intermediate from the top or the bottom of the carbocation, forming two stereoisomers. Primary alkyl halides readily undergo S_N2 reactions, which are stereospecific because the nucleophile attacks from the opposite side, where the leaving group is eliminated (backside attack).

If a Primary Alkyl Halide and a Chiral Tertiary Halide