Carboxylic acids

Carboxylic acids are organic acids that are characterised by having a carboxyl group, which has the formula , or more commonly written as . In a carboxyl group, an oxygen atom is double-bonded to a carbon atom, which is also bonded to a hydroxyl group. The simplest carboxylic acid, methanoic acid, is shown in Figure 1. The IUPAC suffix for carboxylic acids is -anoic acid.

Figure 1: Methanoic acid

Carboxylic acids are widespread in nature. Methanoic acid (also known as formic acid) has the formula and is found in insect stings. Ethanoic acid , or acetic acid, is the main component of vinegar. More complex organic acids also have a variety of different functions. Benzoic acid for example, is used as a food preservative.

Physical properties

Carboxylic acids are weak acids, in other words they only dissociate partially. Why does the carboxyl group have acidic properties? In the carboxyl group, the hydrogen tends to separate itself from the oxygen atom. In other words, the carboxyl group becomes a source of positively-charged hydrogen ions (). This is shown in Figure 2.

Figure 2: The dissociation of a carboxylic acid

Exercise 1: Carboxylic acids

Problem 1:

Refer to the table below which gives information about a number of carboxylic acids, and then answer the questions that follow.

Table 1

Formula	Common name	Source	IUPAC name	Melting point (℃)	Boiling point (℃)
	formic acid	ants	methanoic acid	8,4	101
		vinegar	ethanoic acid	16,6	118
	propionic acid	milk	propanoic acid	−20,8	141
	butyric acid	butter		−5,5	164
	valeric acid	valerian root	pentanoic acid	−34,5	186
	caproic acid	goats		−4	205
	enanthic acid	vines		−7,5	223
	caprylic acid	goats		16,3	239

1. Fill in the missing spaces in the table by writing the formula, common name or IUPAC name.

2. Draw the structural formula for butyric acid.

3. Give the molecular formula for caprylic acid.

4. Draw a graph to show the relationship between molecular mass (on the x-axis) and boiling point (on the y-axis)

   1. Describe the trend you see.

   2. Suggest a reason for this trend.

Answer 1:

1.

Formula	Common name	Source	IUPAC name	Melting point	Boiling point
	formic acid	ants	methanoic acid	8.4	101
	acetic acid	vinegar	ethanoic acid	16.6	118
	propionic acid	milk	propanoic acid	-20.8	141
	butyric acid	butter	butanoic acid	-5.5	161
	valeric acid	valerian root	pentanoic acid	-34.5	186
	caproic acid	goats	hexanoic acid	-4	205
	enanthic acid	vines	heptanoic acid	-7.5	223
	caprylic acid	goats	octanoic acid	16.3	239

2.

3.

 

4.

i. As the molecular mass increases, so does the boiling point.

ii. A possible reason is that the surface area of the molecules is increasing leading to an increased boiling point. Alternatively the forces holding the molecules in the solid phase are increasing and so the boiling point increases.

Derivatives of carboxylic acids: The esters

When an alcohol reacts with a carboxylic acid, an ester is formed. Most esters have a characteristic smell. In the reaction, the hydrogen atom from the hydroxyl group, and an OH from the carboxlic acid, form a molecule of water. A new bond is formed between what remains of the alcohol and acid. The name of the ester is a combination of the names of the alcohol and carboxylic acid. The suffix for an ester is -oate. An example is shown in Figure 3.

Figure 3: The formation of an ester and water from an alcohol and carboxylic acid