The below mentioned article provides a short note on the synthetic auxins in plant.
After the isolation and characterisation of Indole Acetic Acid as the natural auxin a number of other chemical compounds have been synthesized which also exhibit auxin activity. They are called as synthetic auxins. Some of the commonly known synthetic auxins are for example, Indole-3-propionic acid, Indole-3-pyruvic acid, 2, 4 Dichloro phenoxy acetic acid (2, 4-D), Phenoxyacetic acid, 2-Methyl-4-ChIorophenoxy acetic acid (Methoxone), β-Naphthoxyacetic acid, α-Naphthalene acetic acid, β-Naphthalene acetic acid, Anthracene acetic acid and 2 methoxy-3,6-dichlorobenzoic acid (dicamba). The degree of auxin activity depends on their structural peculiarities and configurations.
According to Koepfli et al (1938), a chemical compound should fulfill the following structural requirements in order to exhibit the auxin activity:
(i) Aromatic unsaturated ring with an aliphatic side chain.
(ii) A—COOH group or a group which can easily be converted into—COOH group.
(iii) At least one carbon atom between the ring and the —COOH group in the side chain.
(iv) A particular spatial configuration between the ring and the side chain.
(v) A molecular distance of about 0.5 nm between fractional positive charge on the aromatic ring and a negatively charged carboxylic group is common feature of all biologically active auxins.
The structures of common synthetic auxins are given in Fig. 17.39.
(Some chemical compounds which were previously included in synthetic auxins Such as IBA (Indole-3-butyric acid) and 4-C1-IAA (4-chloro indole-3-acetic acid) have now been isolated from plant parts and are now considered as natural auxins and are therefore, not included in synthetic auxins).