Organic Chemistry : Identifying Nucleophiles

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #11 : Identifying Nucleophiles

Which of the following is the worst nucleophile?

Possible Answers:

Correct answer:

Explanation:

 is least nucleophilic because its oxygen has no negative charge, and oxygen is more electronegative than sulfur, thus the lone pair of electrons on oxygen are more stable than they would be on sulfur or nitrogen.

Example Question #12 : Nucleophiles And Electrophiles

Which of the following sentences correctly explains why iodide () is a better nucleophile than fluoride () in a polar protic solvent?

Possible Answers:

is more electronegative than  which causes  to interact more with the solvent (the more abundant substance) than with the target molecule.

 is more electronegative than  which causes  to interact more with the target molecule than with the solvent (the more abundant molecule).

 is less electronegative than  which causes  to interact more with the solvent (the more abundant substance) than with the target molecule.

 is less electronegative than  which causes  to interact more with the target molecule than with the solvent (the more abundant molecule).

None of the other answers.

Correct answer:

is more electronegative than  which causes  to interact more with the solvent (the more abundant substance) than with the target molecule.

Explanation:

 forms strong hydrogen bonds with polar protic solvents, which makes it less able to interact with any target molecules (electrophiles). Note: in a polar aprotic solvent, hydrogen bonds cannot form, and  is more nucleophilic than .

Example Question #13 : Nucleophiles And Electrophiles

Which of the following compounds can be considered a Lewis base?

Possible Answers:

Correct answer:

Explanation:

A Lewis base is a compound that donates a pair of electrons. The only compound shown that possesses an electron pair that can be donated is ammonia.  is the Lewis base.  and  are Lewis acids and are therefore wrong. Neither carbon dioxide nor methane are Lewis bases.

Example Question #12 : Identifying Nucleophiles

Which of these qualities applies to a good nucleophile?

Possible Answers:

Wants to accept electrons

Less bulky

Is a conjugate acid

More bulky

Correct answer:

Less bulky

Explanation:

A good nucleophile should not be bulky. If the nucleophile is bulky, the compound will not be able to reach the carbonyl carbon, where the reaction occurs. With increased steric hindrance due to a bulkier nucleophile, the reaction will run slowly, and the compound is not a good nucleophile. Good nucleophiles also want to give away electrons so that the reaction can occur. This means that conjugate bases make better nucleophiles than acids.

Example Question #13 : Identifying Nucleophiles

Which of the following is least likely to result in substitution when added to a secondary halide.

Possible Answers:

Correct answer:

Explanation:

To promote elimination rather than substitution, a molecule must be a strong base and weakly nucleophilic. Typically weak nucleophiles are large and bulky, which therefore have difficulty undergoing backside attack. Only tert-butoxide is a weak nucleophile, due to its branching, meaning it is least likely to participate in substitution. 

Example Question #14 : Identifying Nucleophiles

Which of the following is the best leaving group?

Possible Answers:

Correct answer:

Explanation:

 is the weakest base as its negative charge is spread out more evenly over the large atom. Thus, it is the best leaving group of these four halogens. It is the best leaving group due to its largest size and distribution of negative charge. 

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