Advices For Studying Organic Chemistry

(DMF) (DMSO) (DMA) (HMPA)

1) Polar aprotic solvents dissolve ionic compounds, and they solvate cations very

well.

Na

OH 2

H 2 O OH 2

H 2 O

OH 2

OH 2

+

Na

O

O O

O

O

O

+ S(CH^3 )^2

(H 3 C) 2 S S(CH 3 ) 2

(H 3 C) 2 S

S

S

H 3 C CH 3

H 3 C CH 3

A sodium ion solvated by molecules A sodium ion solvated by molecules
of the protic solvent water of the aprotic solvent dimethyl sulfoxide

2) Polar aprotic solvents do not solvate anions to any appreciable extent

because they cannot form hydrogen bonds and because their positive centers

are well shielded from any interaction with anions.

i) “Naked” anions are highly reactive both as bases and nucleophiles.

ii) The relative order of reactivity of halide ions is the same as their relative

basicity in DMSO:

F–> Cl– > Br– > I–

iii) The relative order of reactivity of halide ions in alcohols or water:

I– > Br– > Cl– > F–

3) The rates of SN2 reactions generally are vastly increased when they are

carried out in polar aprotic solvents.

4) Solvent effects on the SN2 reaction of azide ion with 1-bromobutane:

N 3 − + CH 3 CH 2 CH 2 CH 2 Br Solvent CH 3 CH 2 CH 2 CH 2 N 3 + Br−

Solvent HMPA CH 3 CN DMF DMSO H 2 O CH 3 OH