3. Compare Sn1 and Sn2 reactivities of alkyl halides with regard to

a)stereochemical outcomes
b)an increase in the nucleophilic concetration
c)an increase in alkly halide concentration
d)an increase in the polarity of the solvent
e)rearrangements

To compare the Sn1 and Sn2 reactivities of alkyl halides, we need to consider several factors. Let's examine each of the given scenarios:

a) Stereochemical outcomes:
- Sn1 (Substitution Nucleophilic Unimolecular): In Sn1 reactions, a carbocation intermediate is formed before the nucleophile attacks. As a result, Sn1 reactions often lead to racemization (formation of both R and S stereoisomers) because the nucleophile can attack from either face of the planar carbocation.
- Sn2 (Substitution Nucleophilic Bimolecular): In Sn2 reactions, the nucleophile attacks the alkyl halide simultaneously while the leaving group is leaving. This concerted mechanism leads to inversion of stereochemistry, meaning that the configuration of the stereocenter is inverted during the reaction.

b) An increase in nucleophilic concentration:
- Sn1: Since Sn1 reactions involve a carbocation intermediate, the rate of the reaction is largely dependent on the concentration of the nucleophile. Therefore, an increase in nucleophilic concentration will increase the rate of the Sn1 reaction.
- Sn2: Sn2 reactions are bimolecular and involve a direct attack by the nucleophile. An increase in nucleophilic concentration will also increase the rate of the Sn2 reaction because a higher concentration of nucleophile increases the chances of a successful collision with the alkyl halide.

c) An increase in alkyl halide concentration:
- Sn1: In Sn1 reactions, the rate-determining step is the formation of the carbocation intermediate, which is independent of the concentration of the alkyl halide. Therefore, an increase in alkyl halide concentration does not significantly affect the rate of Sn1 reactions.
- Sn2: On the other hand, Sn2 reactions involve a direct collision between the nucleophile and the alkyl halide. Therefore, an increase in alkyl halide concentration will increase the rate of the Sn2 reaction because a higher concentration increases the chances of collision between the nucleophile and the alkyl halide.

d) An increase in the polarity of the solvent:
- Sn1: Sn1 reactions are facilitated by polar solvents. A polar solvent stabilizes the carbocation intermediate through solvation, making the reaction faster. Therefore, an increase in solvent polarity will increase the rate of Sn1 reactions.
- Sn2: In contrast, Sn2 reactions are hindered by polar solvents. A polar solvent can solvate the nucleophile, making it less reactive and reducing its ability to attack the alkyl halide. Therefore, an increase in solvent polarity will decrease the rate of Sn2 reactions.

e) Rearrangements:
- Sn1: Sn1 reactions can often lead to rearrangements due to the presence of a carbocation intermediate. Rearrangement occurs when the alkyl group shifts to form a more stable carbocation, followed by attack of the nucleophile on the rearranged carbocation.
- Sn2: Sn2 reactions do not involve carbocation intermediates, so rearrangements do not occur.

In summary, Sn1 reactions lead to racemization, are dependent on nucleophilic concentration, are independent of alkyl halide concentration, are facilitated by polar solvents, and can involve rearrangements. Sn2 reactions lead to inversion of stereochemistry, are dependent on both nucleophilic and alkyl halide concentrations, are hindered by polar solvents, and do not involve rearrangements.