Question

42) Which of the following is an example of polar, aprotic solvent? a. hexane C. carbon tetrachloride b. acetone d. ethanol 43) Which of the following is the best leaving group? OH b. OCH d. H0

Answer 1

42) Acetone is an example of polar aprotic solvent. ​​​​​​

Generally solvents with dielectric constant greater than about 5 are considered polar whereas those with less than 5 are considered non polar.

In polar protic and aprotic solvents, the distinguishing feature is the presence of a - OH group, and that's most common characteristics feature of a protic solvent.

However there are exceptions like nitro methane CH3NO2 which is also considered a protic solvent. This suggest that Bronsted acidity is the most important feature because nitro methane is most acidic ,with pka of about 10.

But Acetone is still considered polar aprotic solvent, despite the fact that it is relatively acidic, and not significantly less acidic than alcohols. Also Acetone (and other carbonyl containing solvents) are indeed poor solvents, when using strong bases due to their relatively high acidity.

Whereas Hexane and CCl4 are non polar solvents. And ethanol is polar protic solvent.

43) (d) H2O is the best leaving group.

Good leaving groups are weak bases as they are stable on their own like H2O, I-, Br-, Cl- etc. The weaker the base, the better the leaving group.

On the other hand, strong bases are bad leaving groups. This is why alcohols don't take part in SN2 reaction as - OH is a terrible leaving group.

A pKa table is a handy guide to leaving groups Functional group/Example pka Conjugate base Hydroiodic acid HI -10 Hydrobromic acid HBr -9 Br Hydrochloric acid HCI -6 CI Excellent leaving groups (extremely weak bases) Sulfuric acid H2SO4 -3 HSO4 SO Sulfonic acids OH -3 Me Me (tosic acid) Hydronium H30 -1.7 H2O ion Hydrofluoric acid H-F 3.2 F Exception: F is typically an extremely poor leaving group (forms strong bonds) 4 Carboxylic acids H2C OH H3C Moderate leaving groups (weak bases) Protonated amines NH, CI 9-11 NH3 Water HO-H 16 HO Poor leaving groups (strong bases) Alcohols CH3O-H 16-18 сное Amine NH ~35 NH2 Hydrogen H-H 42 Extremely poor leaving groups (very strong bases) CH, Alkane CH3 ~50 H2C1 нус