1559T_ch07-113-13110/22/0520:20Pa9e115 EQA Keys to the Chopler·115 Note that on)is ac-bond:大Hr+: C-H bond:- its kineti the nucleophile attaches to the cationi For the pra If,however,elimination is desired,it may be achieved by addition of strong base to tertiary halides.At high concentrations.in fact.a second elimination mechanism with second-order kinetic behavior occurs (E2 rea with the loss of the leaving group.Examine Figure 7-8 closely.Note the electron motion toward the chlorine quite simi electron motion of an Sx2 proc 山- As t any haloalkane with aBC -H bond can und text an 7-8.Substitution vs.Elimination:General Guidelines for Prediction 1.Is the nucleophile a strong base? 3.Is the subst If the answer to at least two more for practice. CH;Na+-NH2? Note that only the carbon–hydrogen bond (the one next to the positive carbon) is susceptible to cleavage in this manner. Other COH bonds would not give such stable products if they were to be broken: Because the E1 process involves the same rate-determining step as the SN1 reaction, its kinetics are the same: first order. E1 elimination almost always accompanies SN1 substitution. The difference is simple: In SN1, the nucleophile attaches to the cationic carbon; in E1, it attaches to and removes a proton. For the prac￾tical purposes of synthesis, the presence of the E1 “side reaction” can limit the usefulness of SN1 substitution. If, however, elimination is desired, it may be achieved by addition of strong base to tertiary halides. At high concentrations, in fact, a second elimination mechanism with second-order kinetic behavior occurs (E2 reac￾tion). Section 7-7 describes its details pictorially. As in the E1 process, the electrons in a COH bond move toward the electrophilic carbon; in the E2 reaction, however, this electron movement occurs simultaneously with the loss of the leaving group. Examine Figure 7-8 closely. Note the electron motion toward the chlorine￾bearing carbon: It is actually quite similar to the electron motion of an SN2 process! The tertiary halide can￾not undergo SN2 displacement, but the E2 mechanism is a way for it to move electrons in a similar manner, getting them from a COH bond instead of from an external nucleophile. As the rest of this section shows, any haloalkane with a COH bond can undergo E2 elimination. In the case of 1 and 2 halides, E2 and SN2 reactions compete. However, as the following sections in the text and below will show, it is very easy to predict the favored products in these cases. 7-8. Substitution vs. Elimination: General Guidelines for Prediction The key consideration for synthetic purposes is elimination versus substitution, and, as the text shows, the pref￾erence in most cases can be determined by answering the following three questions: 1. Is the nucleophile a strong base? 2. Is the nucleophile sterically very bulky? 3. Is the substrate sterically hindered (i.e., 3, 2, or 1 with branching)? If the answer to at least two out of these three questions is “yes,” elimination will be favored. Otherwise sub￾stitution will predominate. Check the reactions in the chapter to see how these guidelines work. Here is one more for practice. Na  NH2 ? Liquid NH3 CH3CH2CH2CH2I H C  C H bond: C A cyclopropane (Strained) C C H C C H C  C H bond: H C A carbene (Very unstable) Keys to the Chapter • 115 Both of these are very uncommon processes. Their occurrence is limited to situations where normal elimination cannot take place. 1559T_ch07_113-131 10/22/05 20:20 Page 115