6.1.3(a)-(b) - Carboxylic acid properties and reactions

6.1.3(a)-(b) - Carboxylic acid properties and reactions

Carboxylic acids are organic molecules containing the −COOH\mathrm{-COOH} group. In this lesson you will explain why small carboxylic acids dissolve in water, then use the acidic hydrogen in −COOH\mathrm{-COOH} to predict reactions with metals and bases in aqueous conditions. The same chemistry also gives a useful test-tube observation for identifying a carboxylic acid.

Hydrogen bonding with water

The carboxyl group is written as −COOH\mathrm{-COOH}. It contains a carbonyl bond, C=O\mathrm{C=O}, and an O−H\mathrm{O-H} bond attached to the same carbon atom:

RCOOH\mathrm{RCOOH}

The important point for solubility is that the carboxyl group can form hydrogen bonds with water. Hydrogen bonding happens when a δ+\delta^+ hydrogen atom bonded to oxygen is attracted to a lone pair on a δ−\delta^- oxygen atom in another molecule.

Hydrogen Bond

A hydrogen bond is an intermolecular attraction between a δ+\delta^+ hydrogen atom bonded to a very electronegative atom, such as oxygen, and a lone pair on a very electronegative atom in another molecule.

For a carboxylic acid in water, both sides can matter:

  • the carbonyl oxygen in C=O\mathrm{C=O} can accept a hydrogen bond from a water molecule
  • the hydrogen on the carboxylic acid O−H\mathrm{O-H} bond can form a hydrogen bond to the oxygen of a water molecule

[DIAGRAM: asset_name: Lesson 6.1.3a-b: Carboxylic acid properties and reactions - diagram 01; asset_slug: 06_01_03a_carboxylic_acid_properties_and_reactions__diagram_01; recommended_method: drawn_chem; description: structural diagram showing a general carboxylic acid forming hydrogen bonds with water via the carbonyl oxygen as an acceptor and the carboxylic acid O-H hydrogen as a donor, with delta charges, dashed hydrogen bonds, and a caution that H-C is not a hydrogen-bond donor.]
Diagram

These hydrogen bonds are strong enough to let short-chain carboxylic acids, such as methanoic acid and ethanoic acid, mix well with water. As the hydrocarbon part becomes larger, the non-polar part of the molecule becomes more important, so the solubility falls. For OCR, the key explanation is always the hydrogen bonding between the −COOH\mathrm{-COOH} group and water.

A common exam error is to draw a hydrogen bond from the hydrogen attached to carbon. A C−H\mathrm{C-H} hydrogen is not sufficiently δ+\delta^+ for the hydrogen-bonding model used here. Use the hydrogen on O−H\mathrm{O-H}, or a hydrogen from water, not a hydrogen attached to carbon.

Forming carboxylate salts

In aqueous reactions with bases, the acidic hydrogen is the hydrogen in the −COOH\mathrm{-COOH} group. When that hydrogen is removed as a proton, the remaining ion is a carboxylate ion:

RCOOH(aq)⇌RCOO−(aq)+H+(aq)\mathrm{RCOOH(aq) \rightleftharpoons RCOO^-(aq) + H^+(aq)}

The salt formed from a carboxylic acid is called a carboxylate. For example:

acidcarboxylate ionexample sodium salt
methanoic acid, HCOOH\mathrm{HCOOH}methanoate, HCOO−\mathrm{HCOO^-}sodium methanoate, HCOONa\mathrm{HCOONa}
ethanoic acid, CH3COOH\mathrm{CH_3COOH}ethanoate, CH3COO−\mathrm{CH_3COO^-}sodium ethanoate, CH3COONa\mathrm{CH_3COONa}
propanoic acid, CH3CH2COOH\mathrm{CH_3CH_2COOH}propanoate, CH3CH2COO−\mathrm{CH_3CH_2COO^-}sodium propanoate, CH3CH2COONa\mathrm{CH_3CH_2COONa}

The name changes from -oic acid to -oate. The formula changes from −COOH\mathrm{-COOH} to −COO−\mathrm{-COO^-}, then the positive ion balances the charge.

Worked example: writing a carboxylate salt formula

Write the formula of magnesium ethanoate.

Ethanoate is CH3COO−\mathrm{CH_3COO^-}. Magnesium is Mg2+\mathrm{Mg^{2+}}, so two ethanoate ions are needed for each magnesium ion:

(CH3COO)2Mg\mathrm{(CH_3COO)_2Mg}

The brackets matter because the whole ethanoate ion is repeated twice.

Metals and alkalis

Carboxylic acids react in aqueous conditions with reactive metals to form a carboxylate salt and hydrogen gas. With ethanoic acid and magnesium:

2CH3COOH(aq)+Mg(s)→(CH3COO)2Mg(aq)+H2(g)\mathrm{2CH_3COOH(aq) + Mg(s) \rightarrow (CH_3COO)_2Mg(aq) + H_2(g)}

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