''tert''-Butylthiol, also known as 2-methylpropane-2-thiol, 2-methyl-2-propanethiol, ''tert''-butyl mercaptan (TBM), and ''t''-BuSH, is an organosulfur compound with the formula (CH₃)₃CSH. This

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

is used as an

odorant An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently vol ...

for

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon di ...

, which is otherwise odorless. It may also have been used as a flavoring agent.

Preparation

At least one publication has listed ''tert''-butylthiol as a very minor component of cooked potatoes, but because the ''tert''-butyl moiety is very rare in natural products, other sources doubt the existence of natural sources of the compound. It was first prepared in 1890 by Leonard Dobbin by the reaction of zinc sulfide and ''t''-butyl chloride. The compound was later prepared in 1932 by the reaction of the Grignard reagent, ''t''-BuMgCl, with sulfur to give the corresponding thiolate, followed by hydrolysis. This preparation is shown below: :''t''-BuMgCl + S → ''t''-BuSMgCl :''t''-BuSMgCl + H₂O → ''t''-BuSH + Mg(OH)Cl It is currently prepared industrially by the reaction of isobutylene with hydrogen sulfide over a clay (silica alumina) catalyst.

Reactions

''tert''-Butylthiol can react with metal

alkoxide In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, wh ...

s and

acyl chloride In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...

s to form thiol esters, as shown in the equation: : Thallium(I) 2-methylpropane-2-thiolate synthesis

Thallium(I) 2-methylpropane-2-thiolate reaction

In the reaction above, thallium(I) ethoxide converts to thallium(I) ''t''-butylthiolate. In the presence of diethyl ether,

thallium Thallium is a chemical element with the symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes and ...

(I) ''t''-butylthiolate reacts with

s to give the corresponding tert-butyl thioesters. Like other thioesters, it reverts to tert-butylthiol by hydrolysis. Lithium 2-methylpropane-2-thiolate can be prepared by treatment of ''tert''-butylthiol with

lithium hydride Lithium hydride is an inorganic compound with the formula Li H. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not solub ...

in an aprotic solvent such as

hexamethylphosphoramide Hexamethylphosphoramide, often abbreviated HMPA, is a phosphoramide (an amide of phosphoric acid) with the formula This colorless liquid is a useful reagent in organic synthesis. Structure and reactivity HMPA is the oxide of the highly basic te ...

(HMPA). The resulting thiolate salt is a useful demethylating reagent. For example, treatment with

7-methylguanosine 7-Methylguanosine (m7G) is a modified purine nucleoside. It is a methylated version of guanosine and when found in human urine, it may be a biomarker of some types of cancer. In the RNAs, 7-methylguanosine have been used to study and examine the ...

gives guanosine. Other ''N''-methylated nucleosides in tRNA are not

demethylated Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen ato ...

by this reagent. : T-BuSH reaction tRNA

Metal complexes

The anion derived from ''tert''-butylthiol forms complexes with various metals. One example is tetrakis(''tert''-butylthiolato)molybdenum(IV), Mo(''t''-BuS)₄. This complex was prepared by treating MoCl₄ with ''t''-BuSLi: :MoCl₄ + 4 ''t''-BuSLi → Mo(''t''-BuS)₄ + 4 LiCl Mo(''t''-BuS)₄ is a dark red diamagnetic complex that is sensitive to air and moisture. The molybdenum center has a distorted tetrahedral coordination to four sulfur atoms, with overall ''D''₂ symmetry. : Mo(t-BuS)4

Safety

Even in well ventilated areas, extreme caution must be made when handling ''tert''-butylthiol as it is a highly odorous chemical with an odor threshold of <0.33 ppb. Extreme caution is not due to toxicity, but due to the significant odor and the concern this odor could cause to individuals that might be exposed. The PEL for thiols of most types is 500 ppb, primarily due to reaction of nausea at levels of 2–3 ppm.

Commercial use

''tert''-Butylthiol is the main ingredient in many gas odorant blends. It is always utilized as a blend of other compounds, typically dimethyl sulfide,

methyl ethyl sulfide In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in man ...

, tetrahydrothiophene or other mercaptans such as

isopropyl mercaptan In organic chemistry, propyl is a three-carbon alkyl substituent with chemical formula for the linear form. This substituent form is obtained by removing one hydrogen atom attached to the terminal carbon of propane. A propyl substituent is often ...

, ''sec''-butyl mercaptan and/or ''n''-butyl mercaptan, due to its rather high melting point of . These blends are used only with natural gas and not propane, as the boiling points of these blends and propane are quite different. Because propane is delivered as a liquid and vaporizes to gas when it is delivered to the appliance, the vapor liquid equilibrium would substantially reduce the amount of odorant blend in the vapor. ''tert''-Butylthiol has been listed on the

European Food Safety Authority The European Food Safety Authority (EFSA) is the agency of the European Union (EU) that provides independent scientific advice and communicates on existing and emerging risks associated with the food chain. EFSA was established in February 2002, ...

(FL-no: 12.174) as a flavor additive. There is no indication of what flavor(s) it may have been used in. It has been removed from this list.

References