Properties of carboxylic acids
Chemical properties
Composition of carboxylic acids
Carboxylic acids are compounds containing a carboxyl group -COOH, a carbon atom attached to an atom oxygen O and hydroxyl group OH.
In order to understand the structures of the carboxyl group and the carboxylate ion, it is necessary to turn to the theory of resonance. Resonance theory is the assumption that the distribution of electrons in a molecule is a combination of different configurations of two-electron covalent bonds.
There are two possible configurations of the carboxyl group:
O=C/R-O-H
O--C/R=O+-H
Resonant structure: Oδ--C/R-Oδ+-H
And two configurations of the carboxylate ion:
Two equivalent structures:
O=C/R-O-
O--C/R=O δ-
Resonant structure: Oδ--C/R-Oδ-
So, according to the resonance theory, structures without a separated charge are more stable, therefore the resonant structure of the carboxylate ion is more stable than that of the carboxyl group, which implies that the ionization of the carboxyl group is advantageous in terms of the energy of the molecule.
Acidity of carboxylic acids
If an electrophilic component is present in carboxylic acid, for example, Cl, then the proton (hydrogen atom) is more easily cleaved off from the hydroxyl group, hence the acid is stronger. The more electrophilic components in the carbon group, the stronger the acid: the presence of three chlorine atoms gives carboxylic acid an acidity similar to HCl.
Similarly, nucleophiles act on the molecule, making the splitting of hydrogen more energy-consuming, lowering the acidic properties of the molecule.
Physical properties
The presence of a C=O double bond and a hydroxyl group endow carboxylic acid molecules with polar properties. Carboxylic acids interact well with water, alcohols and with each other using hydrogen bonding. Carboxylic acids with up to 10 carbon atoms in the chain are liquids with a characteristic odor, more than 10 atoms are odorless solid waxy substances.
Derivatives of carboxylic acids
The group R=C-O is called acyl, in the acyl group there is one free bond to which various elements can be attached, halogen - a halohydride is formed, COOR - an acid anhydride is formed, OR - an ester, NH2(R,R) - amides.
Basic carboxylic acids
| Traditional name | Systematic name | Formula | Being in nature |
|---|---|---|---|
| Acetic acid | Ethanol acid | CH3-COOH | Wine fermentation product |
| Formic acid | Methane acid | H-COOH | Red ants, bees, nettles and needles |
| Benzoic acid | C6H5-COOH | Incense resin (benzoin resin) | |
| Succinic acid | Ethane-1,2-dicarboxylic acid | NOOS-CH2-CH2-COOH | In amber |
| Salicylic acid | 2-hydroxybenzoic acid | C6H4(HE)UNSD | Willow bark |
| Oxalic acid | Ethanedienoic acid | NOOS-UNV | In sorrel, rhubarb, carambola and some other plants |
| Lactic acid | 2-hydroxypropanoic acid | CH3-CH(OH)-COOH | It is formed during lactic acid fermentation of sugars, in particular, in sour milk, during fermentation of wine and beer. |
| Malic acid | Hydroxybutanedioic acid | NOOS-CH2-CH(OH)-UNV | It is found in unripe apples, grapes, mountain ash, barberry and raspberries. |
| Citric acid | 2-hydroxypropane-1,2,3-tricarboxylic acid | HOOC-CH2-C(OH)COOH-CH2-COOH | Juice of unripe lemons |
| Acetylsalicylic acid | 2-acetyloxybenzoic acid | C9H8O4 | In the bark of young branches of white willow |
| Table 1. Basic carboxylic acids | |||
What do carboxylic acid molecules look like
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