Carboxylic acids and esters are two important families of organic compounds built around carbonyl chemistry. Carboxylic acids contain the functional group COOH, which makes them weak acids and gives many of them sharp or sour odors. Esters contain the functional group COO between two carbon groups, and many esters have pleasant fruity smells.
Understanding how these groups are related helps explain reactions used in foods, fragrances, medicines, and polymers.
A carboxylic acid can react with an alcohol to form an ester and water in a reversible reaction called esterification. In acid-catalyzed esterification, the carbonyl carbon becomes more reactive, the alcohol adds, and water is removed as the ester bond forms. The general reaction is carboxylic acid + alcohol ⇌ ester + water.
Because the reaction is reversible, chemists often remove water or use excess reactant to increase ester yield.
Key Facts
- Carboxylic acid functional group: RCOOH or RCO2H.
- Ester functional group: RCOOR' or RCO2R'.
- Esterification: RCOOH + R'OH ⇌ RCOOR' + H2O.
- Carboxylic acids are weak acids: RCOOH ⇌ RCOO- + H+.
- In ester names, the alcohol part is named first as an alkyl group, then the acid part ends in -oate, such as ethyl ethanoate.
- Many small esters are volatile and have characteristic odors, such as banana, pineapple, pear, or wintergreen scents.
Vocabulary
- Carboxylic acid
- An organic compound containing a COOH functional group that can donate a proton as a weak acid.
- Ester
- An organic compound containing a COO group bonded between two carbon-containing groups.
- Esterification
- A reaction in which a carboxylic acid and an alcohol form an ester and water.
- Carbonyl group
- A functional group containing a carbon atom double bonded to an oxygen atom, written C=O.
- Hydrolysis
- A reaction in which water breaks a bond, such as converting an ester back into a carboxylic acid and an alcohol.
Common Mistakes to Avoid
- Writing the ester product without removing water is wrong because esterification is a condensation reaction that forms H2O from the acid OH and the alcohol H.
- Naming the ester in the wrong order is wrong because the alkyl group from the alcohol is named first, followed by the carboxylate part from the acid.
- Assuming esterification goes to completion is wrong because the reaction is reversible and reaches equilibrium unless conditions shift the balance.
- Confusing carboxylic acids with alcohols is wrong because carboxylic acids contain COOH, while alcohols contain OH bonded to a saturated carbon.
Practice Questions
- 1 Methanoic acid reacts with methanol to form an ester and water. Write the balanced structural equation and name the ester product.
- 2 A reaction uses 0.250 mol of ethanoic acid and excess ethanol. If the percent yield of ethyl ethanoate is 68.0 percent, how many moles of ester are produced?
- 3 A student says that removing water from an esterification mixture will not affect the amount of ester formed because water is only a product. Explain why this reasoning is incorrect using equilibrium ideas.