Question

What factors would favor a reaction to be SN1 rather than SN 2?

Why it is critical to keep the temperature of the water bath within (+ or -) ½ degree celcius?

In which of these solvents would the rate constant for an SN 1 reaction be greatest: 50% water?

Answer 1

The factors favoring S_N¹ reaction over S_N² are listed below:

    1. Since the first step of the S_N¹ 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. So, Benzylic and allylic carbocation will give better reaction.

2. The S_N¹ tends to proceed with weak nucleophiles – generally neutral compounds such as solvents like CH₃OH, H₂O, CH₃CH₂OH etc. As these nucleophiles can not knock out the leaving group according to S_N² route the S_N¹ pathway is preffered.

3. The S_N¹ 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.

Water bath is in equilibrium with the surrounding can easily transfer a small amount of heat. Hence, it is critical to keep the temperature of the water bath within (+ or -) ½ degree celcius.