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PROBLEM 7.50

Expert-verified
Organic Chemistry
Found in: Page 291

Short Answer

Question: Rank the species in each group in order of increasing nucleophilicity.

a. CH3CH2S- ,CH3CH2O- ,CH3CO2- in CH3OH

b. CH3NH2 ,CH3SH ,CH3OH in acetone

c. -OH ,F- ,Cl- in acetone

d. HS- ,F- ,I- in CH3OH

Answer

a. CH3CO2- < CH3CH2O- < CH3CH2S- IN CH3OH

b. CH3SH < CH3NH2 < CH3OH in acetone

c. Cl- <OH- < F- , in acetone

d. F- <Cl- < HS- IN CH3OH

See the step by step solution

Step by Step Solution

Nucleophilicity

A nucleophile is one that can donate its electron (e-) pair with ease. This capacity of donating is termed nucleophilicity.

The greater the capacity to donate the electron pair, the greater will be the nucleophilicity.

The factors affecting nucleophilicity

The conjugate base of an acid is a better nucleophile than its acid. The more the electronegativity of an atom, the less is its nucleophilicity.

Polar protic solvents form hydrogen bonds around the conjugate base of acid, and strong conjugate bases become bulky in polar protic solvents and become less nucleophilic.

For example, HF is less nucleophilic in polar protic solvents like water.

The reverse trend is observed in polar aprotic solvents like acetone.

If the molecule is bulkier, its nucleophilicity will be lesser.

The nucleophilicity of the given species

a. The compound methanol is a polar protic solvent.

So, it can form a hydrogen bond around the given species. The one with the highest electronegative atom will be able to form a bulky cloud and cannot donate its electron pair, thus becoming a weak nucleophile.

The order among the given species is shown below:

CH3CO2- < CH3CH2O- < CH3CH2S- IN CH3OH

b. The compound CH3COCH3 is an aprotic solvent. The compound with the highest electronegative atom will become the best nucleophile; the order is shown below:

CH3SH < CH3NH2 < CH3OH in acetone

c. The compound CH3COCH3 (acetone) is an aprotic solvent. Among the given species, fluorine has the highest electronegativity and will become the best nucleophile; the order is shown below:

Cl- <OH- < F- , in acetone

d. The given solvent, methanol, is a polar protic solvent. The compound that consists of a less electronegative atom will act as the best nucleophile. The order is shown below:

F- <Cl- < HS- IN CH3OH

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