Question

Look up the structure of each of the following solvents and determine whether they will speed up or slow down the reaction (t-butyl chloride Sn 1 reaction with NaOH) as compared to ethanol (they all dissolve in water). Tetrahydrofuran, Dimethyl sulfoxide, Isopropanol, Glycerol

Answer 1

SN1 reaction is unimolecular nucleophillic substitution reaction which takes place in two steps. First step is slow step which is rate determining step involves the formation of carbocation intermediate.

Polar protic solvents speed up the SN1 reaction because the large dipole moment of solvent stabilize the intermediate carbocation.

Examples of polar protic solvents are ethanol, isopropanol ,glycerol(trihydric alcohol) contain O-H bond can form H bond with themselves and can also solvate the carbocation thus stabilizes it.

DMSO ( CH₃SOCH₃) and THF(C₄H₈O five membered ring contain 4 C and one O) are polar aprotic solvent .they does not contain N-H or O-H bond thus unable to form H bond with themselves hence move freely in solution and decreases the reactivity of nucleophile but in SN1 reaction nucleophile is not a part of rate determining step.

As polar aprotic solvent does not contain acidic H and does not form H bond thus cannot stabilize carbocation thus slow down the reaction as compared to polar protic solvent which can solvate the carbocation thus stabilizes it , lowers its energy and increases the rate of reaction.