Ch3 ch3 ome h3c meoh ch ch3 3 нас 3 br alkyl halides undergo nucleophilic substitution and elimination reactions. when the kinetics of the reaction are measured, if the rate of the reaction is found to be dependent only upon the concentration of the alkyl halide the reaction is first order. the substitution reaction is thus termed sn1, and the elimination reaction is termed el. these reactions are unimolecular and occur in two steps. the first step is rate-limiting and involves the loss of the leaving group to form a carbocation. in the second, fast, step the nucleophile adds to the carbocation in the sn reaction or elimination occurs to give an alkene in the el reaction. because the carbocation is planar, the nucleophile can add to either face and therefore racemization is usually observed although solvent effects can influence this somewhat. el elimination follows zaitsev's rule and typically yields the most substituted alkene as the major product. conditions which favor the sni/e1 pathway include the use of a weak nucleophile and a polar protic solvent. the same reaction conditions apply for both sn1 and el, therefore these reactions will compete. it is usually difficult to predict which pathway, substitution or elimination, will predominate and so a mixture of products is frequently observed. in contrast to the el reaction which involves a carbocation intermediate, the elcb reaction takes place through a carbanion intermediate. base-induced abstraction of a proton in a slow, rate-limiting step gives an anion which then expels the leaving group from the adjacent carbon. the reaction is particularly suited to the elimination of substrates which contain poor leaving groups two carbons removed from a carbonyl group. the poor leaving group disfavors the alternative el and e2 reactions, and the carbonyl group to stabilize the anion via resonance. draw curved arrows to show the movement of electrons in this step of the mechanism. arrow-pushing instructions ch ch3 ch3 ch br h3c h3c