How are SN1 and SN2 reactions carried out?

Question

Elias Aguila · Accepted Answer

Add reagent A to reagent B in some suitable solvent....

You could write whole chapters in an organic text to answer this question. And of course what follows is merely a guide to further reading. We know that #S_(N)1# reactions...#"substitution nucleophilic unimolecular"#, are in essence #"BOND-BREAKING"# reactions, that occur thru an intermediate, whereas #S_(N)2# reactions...#"substitution nucleophilic bimolecular"#, are in essence #"BOND-MAKING"# reactions. Furthermore, in the former case, bond-breaking determines the rate; that is, for a hydrocarbyl halide... #R-Xstackrel("rate determining step")rarrunderbrace(R^(+) + Cl^(-))_"slow"# Because this mechanism is favoured by solvents that tend to solvate the individual ions, such as alcohols and water, the resulting carbocation has a long enough lifetime to react with an electron-rich nucleophile, such as water or hydroxide ion. #R^(+) +HO^(-)stackrel("fast")rarrROH# And, by contrast, in an #S_(N)2# reaction, BOND-MAKING between hydroxide, and the hydrocarbyl halide is the rate determining step... #R-X +HO^(-)stackrel("slow")rarrROH+X^(-)# The rate of reaction depends both on the concentration of hydroxide ion, and of the alkyl halide, (hence bimolecular kinetics), and importantly the reaction PROCEEDS with retention of any stereochemistry around the carbon centre...an optically active substrate will give rise to an optically active product given an #S_(N)2# mechanism... And in solvents such as ethers, the individual ions are not so strongly solvated, and as a consequence their nucleophilicity, and reactivity TEND to be enhanced.

Santiago Alfonso · Answer

undefined SN1 reactions proceed in two steps: ionization of the substrate to form a carbocation, followed by nucleophilic attack. SN2 reactions occur in a single concerted step, with simultaneous nucleophilic attack and displacement of the leaving group.