Which Factors Determine the Rate of SN1 and SN2 Reactions?

[Soaring Crane]

1) Predict the relative reaction times from fastest to slowest for the following compounds with NaI in acetone: 1-chlorobutane, 1-bromobutane, 2-chlorobutane.

I am assuming that this is under SN2 reaction conditions with the solvent and compound given.


2-chlorobutane = secondary substrate = slowest
1-bromobutane = primary substrate = fastest (Br is larger than Cl)
1-chlorobutane = primary substrate = moderate rate

2) Indicate whether the following statements refer to SN1 or SN2 reactions.

tertiary alcohols - SN1??
sp2 planar like transition state - SN1?? I am unsure of this one.
reaction rate independent of the concentration and nature of nucleophile = SN1??
inversion and retention of stereochemistry - SN2??
primary and secondary alcohols - SN2?
carbocation intermediate - SN1 ??

Thanks.

 

[sicjeff]

You are correct in your assumptions. I performed a similar experiment just yesterday for my organic lab.

 

[Blueshift5]

You are correct in your predictions for the relative reaction times. The reaction rates for SN2 reactions generally follow the trend of 1° > 2° > 3° substrates, with 1-bromobutane being the fastest due to the larger and more polarizable bromine atom. 2-chlorobutane is the slowest due to steric hindrance from the two methyl groups.

As for the statements, you are mostly correct. Tertiary alcohols tend to undergo SN1 reactions due to the stability of the carbocation intermediate. Sp2 planar transition states are typically associated with SN1 reactions, but can also occur in SN2 reactions with certain substrates. The reaction rate for SN1 reactions is indeed independent of the concentration and nature of the nucleophile, as it is the leaving group that determines the rate. Inversion and retention of stereochemistry is a characteristic of SN2 reactions, as the nucleophile attacks from the backside of the substrate, resulting in inversion of configuration. Primary and secondary alcohols tend to undergo SN2 reactions due to their less hindered nature. And lastly, the formation of a carbocation intermediate is a characteristic of SN1 reactions, as the leaving group leaves before the nucleophile attacks, leaving behind a positively charged carbocation.

Overall, nucleophilic substitution refers to a type of reaction where a nucleophile replaces a leaving group in a molecule, resulting in a new product. The reaction mechanism can be either SN1 or SN2, depending on the nature of the substrate and reaction conditions. It is an important concept in organic chemistry, and understanding the factors that influence the reaction rate and mechanism is crucial for predicting and controlling reactions in the laboratory.