# Benzotriazole

Benzotriazole (BTA) is a heterocyclic compound with the chemical formula C$_6$H$_5$N$_3$. Its five-membered ring contains three consecutive nitrogen atoms. This bicyclic compound may be viewed as fused rings of the aromatic compounds benzene and triazole. This white-to-light tan solid has a variety of uses, for instance, as a corrosion inhibitor for copper.

## Structure

Benzotriazole features two fused rings. Its five-membered ring can in principle exist as tautomers A and B:
Various structural analyses with UV, IR and $^1$H-NMR spectra indicate that tautomer A is the dominant form at room temperature.

## Synthesis and reactions

Synthesis of benzotriazole involves the reaction of o-phenylenediamine, sodium nitrite, and acetic acid. The conversion proceeds via diazotization of one of the amine groups:
The synthesis can be improved when the reaction is carried out at low temperatures (5–10 °C) and briefly sonicated in an ultrasonic bath. Typical batch purity is 98.5% or greater.

### Reactions

BTA is a weak acid with a pK$_a$ = 8.2
It is a very weak Brønsted base with pK$_a$ < 0.
Not only can it act either as an acid or base, it can also bind to other species, utilizing the lone pair electrons. Applying this property, the BTA can form a stable coordination compound on a copper surface and behave as a corrosion inhibitor.
Benzotriazole (BtH) reacts with aromatic aldehydes (ArCHO) in the presence of ethanol to give benzotriazole-based N,O-acetals:
    ArCHO + BtH + EtOH → ArCH(OEt)(Bt) + H$_2$O
These acetals are susceptible to deprotonation, giving access to acylsilanes and acylboranes.
N-amination of BtH with hydroxylamine-O-sulfonic acid gives 1-aminobenzotriazole. Oxidation of this amine with lead(IV) acetate affords benzyne, which rapidly dimerises to biphenylene.

## Applications

Benzotriazole has been known for its great versatility. It has already been used as a restrainer (or anti-fogging agent) in photographic emulsions or developing solutions, and as a reagent for the analytical determination of silver. More importantly, it has been extensively used as a corrosion inhibitor in the atmosphere and underwater. Also, its derivatives and their effectiveness as drug precursors have been drawing attention. Besides all the applications mentioned above, the BTA can be used as antifreezes, heating and cooling systems, hydraulic fluids, and vapor-phase inhibitors as well.

### Corrosion inhibition

Benzotriazole is an effective corrosion inhibitor for copper and its alloys by preventing undesirable surface reactions. It is known that a passive layer, consisting of a complex between copper and benzotriazole, is formed when copper is immersed in a solution containing benzotriazole. The passive layer is insoluble in aqueous and many organic solutions. There is a positive correlation between the thickness of the passive layer and the efficiency of preventing corrosion. BTA is used in heritage conservation, notably for the treatment of bronze disease. The exact structure of the copper-BTA complex is controversial and many proposals have been suggested.

### Drug precursor

Benzotriazole derivatives have chemical and biological properties that are versatile in the pharmaceutical industry. Benzotriazole derivatives act as agonists for many proteins. For instance, vorozole and alizapride have useful inhibitory properties against different proteins. Benzotriazole esters are used as mechanism-based inactivators to treat severe acute respiratory syndrome (SARS) by inhibiting the SARS 3CL protease of the SARS-CoV-1 virus. The methodology is not only limited to heterocyclization but was also successful for polynuclear hydrocarbons of small carbocyclic systems.

## Environmental relevance

Benzotriazole is fairly water-soluble, not readily degradable and has a limited sorption tendency. Hence, it is only partly removed in wastewater treatment plants and a substantial fraction reaches surface water such as rivers and lakes. It is considered to be of low toxicity and a low health hazard to humans although exhibiting some antiestrogenic properties.

## Related compounds

Tolyltriazole is a mixture of isomers or congeners that differ from benzotriazole by the addition of one methyl group attached somewhere on the benzene ring. Tolyltriazole has similar uses, but has better solubility in some organic solvents.