Question

1. How would the reaction be affected if bromoethane were used instead of iodoethane?

2. How would the reaction be affected if 2-iodopropane were used instead of iodoethane?

experiment: http://legacy.earlham.edu/~seuka/Biochemistry/Ex.%2007%20Saccharin%20Alkylation.pdf

Answer 1

The effect of substituent is discussed below.

1. The effect on the reaction if bromoethane were used instead of iodoethane : Iodo is a better leaving group than bromo. As the size increases, the ability of the leaving group to leave increases. If we move down the periodic table, size increases. With an increase in size, basicity decreases, and the ability of the leaving group to leave increases. Therefore, the ability of I to leave would be higher then Br. Hence, the rate of the reaction would be slower when bromoethane were to be used instead of iodoethane in the reaction.

2. The effect on the reaction if 2-iodopropane were used instead of iodoethane : Here we have to consider say the reaction to go by an SN2/SN1/E2/E1 pathways. Consider the reaction undergoes an SN2 reaction pathway, In primary alkyl halide such as iodoethane, the steric crowding in the transition state (higher energy) with five bonds would be less as compared to the secondary alkyl halide 2-iodopropane. Thus an SN2 reaction would be less favoured in when 2-iodopropane is used as the substrate for the reaction as when compared to iodoethane. On the other hand, secondary alkyl halide are also prone to undergo an substitution reaction by an SN1 pathway. Here, the secondary carbocation formed after the Br leaves in the slow step would be more stable then when Br leaves from the iodoethane substrate wherein we get a primary carbocation. Hence in this case, the reaction would be more favorable when 2-iodopropane is used as the substrate when compared to iodoethane. In case of elimination reaction again, keeping the same points in mind we considered for the substitution reaction, in case of E2 pathway, the reaction would be slower for 2-iodopropane than compared to iodoethane due to steric crowding in the transition state (higher energy). However, in case of the E1 pathway, the reaction would be more favorable when 2-iodopropane is the substrate then when iodoethane is the substrate in the reaction due to more stable secondary carbocation formation as when compared to the primary carbocation in case of iodoethane.