1.3 Carboxylic acids, amines and carboxlyic acid derivatives

Carboxylic acids

Carboxylic acids are organic acids that are characterised by having a carboxyl group, which has the formula -(C=O)-OH, or more commonly written as -COOH. 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 [link] . The IUPAC suffix for carboxylic acids is -anoic acid.

Carboxylic acids are widespread in nature. Methanoic acid (also known as formic acid ) has the formula HCOOH and is found in insect stings. Ethanoic acid (CH ${}_3$ COOH), or acetic acid , is the main component of vinegar. More complex organic acids also have a variety of different functions. Benzoic acid (C ${}_6$ H ${}_5$ COOH) 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 (H ${}^{+}$ ). This is shown in [link] .

Carboxylic acids

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

   Formula	Common name	Source	IUPAC name	melting point ( ${}^0$ C)	boiling point ( ${}^0$ C)
   	formic acid	ants	methanoic acid	8.4	101
   CH ${}_3$ CO ${}_2$ H		vinegar	ethanoic acid	16.6	118
   	propionic acid	milk	propanoic acid	-20.8	141
   CH ${}_3$ (CH ${}_2$ ) ${}_2$ CO ${}_2$ H	butyric acid	butter		-5.5	164
   	valeric acid	valerian root	pentanoic acid	-34.5	186
   CH ${}_3$ (CH ${}_2$ ) ${}_4$ CO ${}_2$ H	caproic acid	goats		-4	205
   	enanthic acid	vines		-7.5	223
   CH ${}_3$ (CH ${}_2$ ) ${}_6$ CO ${}_2$ H	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.

Derivatives of carboxylic acids: the esters

When an alcohol reacts with a carboxylic acid, an ester is formed. Most esters have a characteristic and pleasant 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 [link] .

The amino group

The amino group has the formula -NH ${}_2$ and consists of a nitrogen atom that is bonded to two hydrogen atoms, and to the carbon skeleton. Organic compounds that contain this functional group are called amines . One example is glycine . Glycine belongs to a group of organic compounds called amino acids , which are the building blocks of proteins.