Question

In an organic chemistry lab we were looking at the substrate and nucleophile effects in Sn2 reactions. Our experimental data showed that 1-bromobutane with 1.0mL of 0.25M NaI in 2-Butanone reacted faster than 1-bromobutane with 1.0mL 0.50M NaI in 2-butanone. Also, our experimental data showed that 1-bromobutane with 1.0mL of 0.50M NaI reacted faster than 1-bromobutane with 2.0mL of 0.05M NaI.

These result are the opposite of the theoritcal rate aren't they? From my understanding, the Sn2 reaction rate should should double when the nucleophile or substarte is doubled.

Answer 1

The rate of a SN2 substitution reaction depends both on the concentration of the substrate and the nucleophile as

Rate = k*[Substrate]*[Nucleophile] where k is the second order rate constant.

The first instance is a bit doubtful; the reaction should procced faster with 0.50 M NaI faster than with 0.25 M NaI. It is not clear if you started out with the same concentration of the starting material. A possible error can creep in if you didn’t have the same amount of the substrate in both the cases or there was some glitch with the recording of the time. Moreover, if you used a higher volume of 0.25 M NaI, then the effective number of moles of NaI may be higher in the first case and hence, the reaction can proceed faster.

The second experiment is in line with theory. The concentration of NaI in the first case is higher than the concentration of NaI in the second case; therefore, the first reaction proceeds faster as demanded by theory.