Pharmchem
Since we’ve gone through the different factors that impact the SN1 and SN2 reactions, it’s worthwhile to review and summarize the different factors behind each of these two reactions. But first – have you ever heard of the Hobo on the bench?
You’re in a park on a lovely summer day and you want to sit on a bench. Trouble is, a hobo is sleeping on it. So what do you do?
There are two options. 1. You can kick the hobo off and sit on the bench. 2. You can wait for the hobo to leave, and then sit on the bench.
Think about that for a second. In the meantime, let’s compare the SN1 and the SN2.
The Mechanism * The SN2 reaction is concerted. That is, the SN2 occurs in one step, and both the nucleophile and substrate are involved in the rate determining step. Therefore the rate is dependent on both the concentration of substrate and that of the nucleophile. * The SN1 reaction proceeds stepwise. The leaving group first leaves, whereupon a carbocation forms that is attacked by the nucleophile.
The Big Barrier - this is the most important thing to understand about each reaction. What’s the one key factor that can prevent this reaction from occurring? * In the SN2 reaction, the big barrier is steric hindrance. Since the SN2 proceeds through a backside attack, the reaction will only proceed if the empty orbital is accessible. The more groups that are present around the vicinity of the leaving group, the slower the reaction will be. That’s why the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest) * In the SN1 reaction, the big barrier is carbocation stability. Since the first step of the SN1 reaction is loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation. Carbocation stability increases with increasing substitution of the carbon (tertiary > secondary >> primary) as well as with resonance.
The dependence of rate upon the substrate *
You’re in a park on a lovely summer day and you want to sit on a bench. Trouble is, a hobo is sleeping on it. So what do you do?
There are two options. 1. You can kick the hobo off and sit on the bench. 2. You can wait for the hobo to leave, and then sit on the bench.
Think about that for a second. In the meantime, let’s compare the SN1 and the SN2.
The Mechanism * The SN2 reaction is concerted. That is, the SN2 occurs in one step, and both the nucleophile and substrate are involved in the rate determining step. Therefore the rate is dependent on both the concentration of substrate and that of the nucleophile. * The SN1 reaction proceeds stepwise. The leaving group first leaves, whereupon a carbocation forms that is attacked by the nucleophile.
The Big Barrier - this is the most important thing to understand about each reaction. What’s the one key factor that can prevent this reaction from occurring? * In the SN2 reaction, the big barrier is steric hindrance. Since the SN2 proceeds through a backside attack, the reaction will only proceed if the empty orbital is accessible. The more groups that are present around the vicinity of the leaving group, the slower the reaction will be. That’s why the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest) * In the SN1 reaction, the big barrier is carbocation stability. Since the first step of the SN1 reaction is loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation. Carbocation stability increases with increasing substitution of the carbon (tertiary > secondary >> primary) as well as with resonance.
The dependence of rate upon the substrate *