Which of the following functional groups acts most like an acid in water? A) carboxyl B) amino C) carbonyl D) hydroxyl

Question

I got answer D wrong on my homework and would like to know why and could you please explain briefly why the correct answer is right. 

Answer 1

Hello Lexi

 

The answer is A) carboxyl as J.R.S stated in his response.

 

An acid is a substance that donates protons (H⁺ ions) when added to water to form a hydronium ion and the conjugate base, the carboxylate salt.  The dissociation reaction of a carboxylic acid can be written as:

 

R-COOH + H₂O ⇔ H₃O⁺ + R-COO^-

 

Alcohols are also acidic, but extremely weak when compared to carboxylic acids.  Alcohols combine with water to form the hydronium ion and its conjugate base, an alkoxide anion.  The dissociation reaction of an alcohol can be written as:

 

R-OH + H₂O ⇔ H₃O⁺ + R-O^-

 

So why is the carboxylic acid a stronger acid??  The answer is because of the conjugate base that is formed as a product from the dissociation reaction.  The carboxylate anion is more stable than the alkoxide ion because it exists as 2 resonance structures.  The negative charge is distributed equally between both oxygen atoms whereas in the alkoxide ion, the negative charge is localized on the lone oxygen atom.  This makes the alkoxide ion more reactive compared to the carboxylate anion.  As a result, the reverse reaction will predominate and the alcohol will be present in solution as a neutral molecule.

 

FYI

 

b) Amino or amine.  This compound is a base as it accepts a proton more easily from water to form an ammonium salt and the hydroxide ion.

 

R-NH₂ + H₂O ⇔ R-NH₃⁺ + OH^-

 

c) carbonyl.  A carbonyl functional group characterized by a carbon-oxygen double bond (C=O) is present in 2 types of organic compounds called aldehydes and ketones.  This functional group does not contain any hydrogens to donate, hence, they are considered to be "neutral" when added to water.

 

Aldehyde   R-CHO + H₂O ⇔ R-CHO (aq)   **water solubility depends on the structure

Ketone      R₂-C=O + H₂O ⇔ R₂-C=O (aq)   **water solubility depends on the structure

Answer 2

Since a carboxyl group (-COOH) is called a carboxylic acid, I would guess that it (A) would be the correct choice.  The H on the -COOH, is able to be released somewhat easily because the electrons in the bond are pulled away from this H by the O which is electronegative.