Question

What functional group can act as acid?

Answer 1

Functional groups are specific groupings of atoms within molecules that have their own characteristic properties, despite the opposite atoms present in a molecule. Common examples are alcohol, amines, carboxylic acids, ketones, and ethers. An acid is any hydrogen-containing substance that's capable of donating a proton (hydrogen ion) to a different substance. A base may be a molecule or ion ready to accept a proton from an acid. Acidic substances are usually identified by their sour taste.

Complete answer:
The sulfonic, phosphoric, and carboxylic acid groups are the strongest acids. Many functional groups behave as weaker acids.
A functional group is an acid if it can donate a proton to a base.
Here is a list of functional groups that can behave as acids, along with their approximate $pKa$ values.
$R - S{O_3}H$- Sulfonic Acids- $pKa = - 1$(important in industry)
$R - P{O_4}{H_4}$- Phosphoric acids- $pKa = 2$(important in biochemistry)
$R - COOH$- Carboxylic acid- $pKa = 4$
$R - SH$- Thiols- $pKa = 13$
$R - OH$- Alcohols- $pKa = 17$
$R - N{H_2}$- Amines- $pKa = 35$
$C - H$ Bonds can also behave as acids. Their $pKa$ values range from $13 - 50$, depending on the groups that are next to the C atom.

Note:
The chemistry of acids and bases and buffers is a crucial area. For instance, the relative strengths of acids influence the formation of nitronium ions within the nitration of benzene, and therefore the understanding of pH and buffers is important in biology. According to the Lewis definition, acids are molecules or ions capable of coordinating with unshared electron pairs, and bases are molecules or ions having unshared electron pairs available for sharing with acids. To be acidic within the Lewis sense, a molecule must be electron deficient.