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.

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.

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.

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.

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.

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.

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.

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.

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.

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