Organic Chemistry : Identifying Nucleophiles

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #1 : Identifying Nucleophiles

Which of the following compounds would be the best nucleophile?

Possible Answers:

Correct answer:

Explanation:

A nucleophile acts by donating a pair of electrons to another atom's nucleus. In general, a negatively charged compound is going to be a stronger nucleophile than a neutral compound. In addition, as one proceeds down a given column of the periodic table, the nucleophilicity increases because the electrons are not held as tightly to the nucleus (electronegativity decreases).

is the best nucleophile, because it has a negative charge (more electron density), and its electrons are held less tightly than those of because sulfur is less electronegative than oxygen.

Example Question #2 : Identifying Nucleophiles

The given molecule is known as voacamine. Multi-cyclic molecules with a high nitrogen content such as this one are known as alkaloids, and tend to be highly toxic. A chemist is attempting to react a sample of voacamine with an electrophilic reagent known as Boc anhydride, which is typically used to "protect" (or react with to chemically mask) nitrogen functionality in order to lessen the toxicity of the molecule. What nucleophilic moiety in voacamine will react first with Boc anhydride?

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Possible Answers:

Green

Blue

Gold

Purple

Red

Correct answer:

Blue

Explanation:

There are two major types of nitrogen-containing moieties in this molecule.

First, there are the aromatic nitrogenated groups, such as the purple, green, and gold. All three of these nitrogens, when reacted with an electrophile such as Boc anhydride, would produce positively charged species. This alone would be unfavorable, however, as these nitrogens each donate a lone pair to their aromatic systems, donating this lone pair to an electrophile would break the aromaticity of the system. Breaking aromaticity is always highly unfavorable, and hence, none of these three would readily react with Boc anhydride.

Second, there are the aliphatic nitrogenated groups, such as the red and blue. Of these two, the red is tertiary and the blue is secondary. This means the red would produce a positively charged, tetrasubstituted product when reacting with Boc anhydride, whereas the blue would not form a charged product. The blue amine is also more sterically available, and is the correct answer, as it has the best ability to act as a nucleophile. 

Example Question #3 : Nucleophiles And Electrophiles

Which of the following is the strongest nucleophile?

Possible Answers:

Correct answer:

Explanation:

The molecules are almost exactly the same, except that each molecule contains a different group 6 atom. Size increases as we move down the group 6 column, and therefore nucleophelicity increases. Larger and less electronegative atoms hold onto their electrons more loosely, and are stronger nucleophiles.

Example Question #3 : Identifying Nucleophiles

Rank the following compounds in order of increasing nucleophilicity.

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Possible Answers:

IV, III, I, II

I, II, IV, III

II, I, III, IV

I, II, III, IV

Correct answer:

II, I, III, IV

Explanation:

The periodic trends of electronegativity and charge stability are useful tools for predicting nucleophilic strength. First, it is important to recognize that the two charged species,  and  are the two strongest nucleophiles. This is because the destabilizing negative charge present in these species may be neutralized by donating a lone pair to the formation of a chemical bond. As we know, opposite charges attract, so species bearing a full negative charge are drawn to electron-poor regions. Uncharged species such as water and ammonia carry a lone pair capable of bonding, but are less energetically drawn towards positive charges. 

Ammonia is a stronger nucleophile than water because nitrogen is less electronegative than oxygen. What this means is that the nitrogen-bound lone pair of ammonia is more loosely contained than the oxygen-bound lone pairs of water. As a result, they are more easily donated to form a bond at an electron-poor carbon.

From this trend, one might expect that fluoride ions would be less nucleophilic than chloride ions since fluorine is more electronegative. However, moving down a group of the periodic table, atomic radius increases. Anions are stabilized by spreading electron density across an electron cloud of greater volume, such as that of  compared to the smaller . As such, the correct ordering of species is II, I, III, IV.

Example Question #4 : Identifying Nucleophiles

Select the strongest nucleophile in an aprotic solvent.

Possible Answers:

Correct answer:

Explanation:

In aprotic solvents, nucleophilicity increases with electronegativity when dealing with atoms in the same group (column on the periodic table).

Example Question #6 : Nucleophiles And Electrophiles

Which is the strongest nucleophile in an aprotic solvent?

Possible Answers:

Correct answer:

Explanation:

When discussing nucleophilic strength, we can begin to see trends. However, it is important to note that the question asked for the strongest nucleophile in aprotic solvent. The correct answer is .

In polar protic solvents, electronegative nucleophiles tend to hydrogen bond with the solvent, inhibiting the nucleophile's nucleophilicity. However, in aprotic solvents, this does not occur and so basicity correlates to nucleophilicity.

Example Question #5 : Identifying Nucleophiles

What is not true about a nucleophile?

Possible Answers:

A strong nucleophile is a strong base

They donate electrons to form a bond

Strong nucleophile are weak acids

A chlorine ion is an example of a nucleophile

None of these

Correct answer:

None of these

Explanation:

Nucleophile are electron-rich species that form bonds with electron-poor species. When thinking in terms of acids and bases, bases tend to form bonds with protons making them strong nucleophiles while, acids usually donate protons making them weak nucleophiles.

Example Question #8 : Nucleophiles And Electrophiles

Which of the following sets of nucleophiles are correctly listed from strongest to weakest in a protic solvent?

Possible Answers:

None of these

Correct answer:

Explanation:

In a protic solvent, the larger the atom the better the nucleophile. Atomic radius increases as you go down a group on the periodic table. Also, the more electronegative an atom/nucleophile, like fluorine, the higher the capability of forming hydrogen bonds with protic solvents, thus hindering their effectiveness as a nucleophile. 

Example Question #9 : Nucleophiles And Electrophiles

Which of the following is the best nucleophile?

Possible Answers:

Correct answer:

Explanation:

The ordering from best nucleophile to worst nucleophile is as follows:

Smaller molecules are better nucleophiles than larger ones (they are not as sterically hindered).  is a better nucleophile than because nitrogen is less electronegative than oxygen (Look for the the lower electronegativity on the atom holding the lone pair of electrons). Thus, the lone pair of electrons on nitrogen are not as stable as those on oxygen, and will readily donate its pair of electrons to another species. 

Example Question #10 : Nucleophiles And Electrophiles

Which of the following statements are incorrect?

I. The best nucleophiles are negatively charged
II. Smaller molecules are better nucleophiles than larger ones
III. The smaller the atom, the better the nucleophile (protic solvent)

Possible Answers:

II and III

III only

I only

II only

None of these - all statements are correct

Correct answer:

III only

Explanation:

The larger the atom, not molecule, the better the better the nucleophile . Atomic radius increases as you go down a group on the periodic table. Nucleophilicity can also be determined according to strength of the anion as a conjugate base. Remember that  is the strongest acid and thus has the weakest/least stable conjugate base.

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