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.

 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 ^.

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.

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.

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. 

 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. 

Note that many of these carbonyl groups are actually part of various functional groups. For example, the gold is an aldehyde, the green and purple are both ketones, the red is an amide, and the blue is an ester. We know the electrophilicity of carbonyl-containing functional groups is as follows:

Thus, our aldehyde, in gold, is the most electrophilic.

A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor.

 is an ionic molecule and is neutral.  and  have positive charges and are electron pair acceptors, as they are more stable when they are neutral.

The correct answer, , has a lone pair on the nitrogen atom that can be donated to form bonds with other atoms, so it is a Lewis base.

Electrophiles are substances that accept an electron pair to form a covalent bond, and nucleophiles are those that donate an electron pair to form a covalent bond. The chloride and iodide ions are both nucleophiles, as they each have a charge of  and would thus be willing to donate their extra electron. Ammonia () is also a nucleophile, as the nitrogen has a lone pair of electrons to donate. The methyl carbocation (carbon attached to three hydrogen atoms, with a positive charge) is an electrophile. The positive charge on the carbon makes it willing to accept an electron pair to form a covalent bond.

This question tests your knowledge about electron donation, as well as acidity. The question asks you to identify the species with the highest pK_a, which means you need to look for the R group that will be the most donating. Electron donation will destabilize the conjugate base anion, localized partially on the oxygen of the hydroxyl moiety; the result is a less acidic acid, and a higher pKa.

Of the answer choices, three are electron-withdrawing. These include the nitro (-NO₂), the ketone (-CCOMe), and the carboxyl (-COOH). The only answer choices that are electron-donating are the methyl (-Me), and the ether (-OMe). As the ether can push its lone pairs into the pi system of the ring and the carboxyl group, it is the stronger electron-donating group. The methyl can only donate electrons through inductive effects, or electronic polarization of sigma bonds, which is not as strong of an electron donation effect. Thus, -OMe is the correct answer.