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.
- You can kick the hobo off and sit on the bench.
- You can wait for the hobo to leave, and then sit on the bench.
- 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.
- For the SN2, since steric hindrance increases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest).
- For the SN1, since carbocation stability increases as we go from primary to secondary to tertiary, the rate of reaction for the SN1 goes from primary (slowest) << secondary < tertiary (fastest)
- The SN2 tends to proceed with strong nucleophiles; by this, generally means negatively charged nucleophiles such as CH3O(-), CN(-), RS(-), N3(-), HO(-), and others.
- The SN1 tends to proceed with weak nucleophiles – generally neutral compounds such as solvents like CH3OH, H2O, CH3CH2OH, and so on.
- The SN2 reaction is favored by polar aprotic solvents – these are solvents such as acetone, DMSO, acetonitrile, or DMF that are polar enough to dissolve the substrate and nucleophile but do not participate in hydrogen bonding with the nucleophile.
- The SN1 reaction tends to proceed in polar protic solvents such as water, alcohols, and carboxylic acids. These also tend to be the nucleophiles for these reactions as well.
- Since the SN2 proceeds through a backside attack, if a stereocenter is present the SN2 reaction will give inversion of stereochemistry.
- By contrast, if the SN1 leads to the formation of a stereocenter, there will be a mixture of retention and inversion since the nucleophile can attack from either face of the flat carbocation.
- In the SN2, the nucleophile (you) forms a bond to the substrate (bench) at the same time the leaving group (hobo) leaves.
- In the SN1, the leaving group (hobo) leaves the substrate (bench), and then the nucleophile (you) forms a bond.