A secondary amine is an amine (nitrogen atom) that is attached to two carbon-containing groups (alkyl groups or aryl groups). The nitrogen in ephedrine is attached to two alkyl groups, making it a secondary amine.

Primary amines are generally written as . Secondary amines are generally written as . A tertiary amine will be bound to three different R-groups. Quaternary amines require a positive charge on the nitrogen atom to accommodate a fourth R-group.

Carboxylic acids are able to create hydrogen bonds with one another. This forms a dimer, which doubles the effective molecular weight and greatly increases the boiling point of carboxylic acids. Aldehydes and ketones are not able to form hydrogen bonds with one another, so their boiling points are dependent on each individual molecule's molecular weight. As a result, their boiling points are not as high as the corresponding carboxylic acids'. Note that carboxylic acids cannot form intramolecular hydrogen bonds.

All of the options, except for the carboxylic acid itself, are derivatives of a carboxylic acid. These derivatives differ in their reactivity when creating new compounds. Acid chlorides are the most reactive compounds, and amides are the least reactive out of these option. Acid chlorides and acid anhydrides frequently participate in reactions, while amides are less likely to react.

Ketones, like compound A, contain an internal carbon-oxygen double bond. Alcohols, like compound B, contain a hydroxyl group (-OH). In this case, compound A is a secondary ketone and compound B is a tertiary alcohol.

Alkenes, like compound C, contain a carbon-carbon double bond.

Esters have the general molecular formula of , where and are carbon groups. The rightmost region of compound A shows an ester.

Ketones have the formula of , with a carbon-oxygen double bond. Ethers have the formula of , with an oxygen linked by single bonds within a carbon chain. An ester resembles adjacent ketone and ether groups. Carboxylic acids have the formula , resembling an ester with a hydrogen in place of a second carbon chain. Finally, the nitrile group has a formula of , with a triple bond between nitrogen and carbon.

Compound A also contains an aromatic function group (the benzene ring) and a nitro group, , on the far left side.

Aldehydes have lower acidity than carboxylic acids, but are more reduced and thus have higher overall bond energies.

Carboxyl groups, or carboxylic acids, are good acids due to the resonance between the two oxygen atoms, allowing for greater stability of the conjugate base upon removal of a proton. Acetals and aldehydes can act as weak acids, but carboxyl groups will be deprotonated first.

Aspartic acid is the only viable answer. Its R-group contains a second carboxylic acid that is deprotonated in most physiological conditions. The negatively charged aspartate will interact favorably with the positively charged cations moving through the pump. Lysine is a basic amino acid that contains an amine group at the terminal end of its R-group. Isoleucine is an aliphatic and hydrophobic amino acid. Oleic acid is an example of a fatty acid, which can carry a negative charge in some conditions, but it is not an amino acid and thus cannot be incorporated into a protein.

Nitro groups and amide groups both contain oxygen components, and cannot be found in the compound described. We also know that the benzene ring only has a single constituent, meaning that it cannot be a methylamine. The compound must be benzylamine, a benzene ring with a -CH2NH2 substituent.

Bicarbonate is the product of deprotonation of carbonic acid. Anything that stabilizes this product will encourage the deprotonation reaction, and resonance is a key stabilizing factor for bicarbonate. Stability of the conjugate base is a major contributing factor to the strength of an acid and its ability to deprotonate.