Methyl Mercaptan

OVERVIEW

Methyl mercaptan (METH-uhl mer-KAP-tan) is a colorless, highly flammable, foul-smelling gas with the odor of rotten cabbage released from decaying animal and vegetable matter. It is also produced in the intestinal tract by the action of bacteria on a variety of proteins known as the albumins.

Methyl mercaptan belongs to a class of organic compounds called mercaptans or thiols in which one or more sulfhydryl (-SH) groups are attached to a carbon atom. Methyl mercaptan has only one carbon atom, but some mercaptans contain up to twenty carbon atoms. Like methyl mercaptan, other mercaptan compounds are known for their disagreeable odors. For example, allyl mercaptan has the characteristic smell of garlic, while butyl mercaptan occurs in the spray that skunks release to protect themselves from predators.

KEY FACTS

OTHER NAMES:

Methanethiol; mercaptomethane; thiomethyl alcohol; methyl sulfhydrate

FORMULA:

CH₃SH

ELEMENTS:

Carbon, hydrogen, sulfur

COMPOUND TYPE:

Mercaptan; thiol (organic)

STATE:

Gas

MOLECULAR WEIGHT:

48.11 g/mol

MELTING POINT:

−123°C (−189°F)

BOILING POINT:

5.9°C (43°F)

SOLUBILITY:

Slightly soluble in water; soluble in ethyl alcohol and ether

HOW IT IS MADE

Methyl mercaptan is made by the direct reaction between methanol (methyl alcohol; CH₃OH) and hydrogen sulfide gas (H₂S). The hydroxyl group (-OH) from the alcohol combines with one hydrogen from hydrogen sulfide to form water, leaving the methyl mercaptan behind as the major product.

COMMON USES AND POTENTIAL HAZARDS

Methyl mercaptan's primary use is in the synthesis of other organic compounds, especially pesticides, fungicides, jet fuel components, plastics, and the amino acid methionine [CH₃SCH₂CH₂CG(NH₂)COOH]. Amino acids are the building-block compounds from which proteins are made. The compound is also used as an odorant, a substance with a noticeable and usually offensive odor added to odorless compounds for the purpose of safety. For example, propane and natural gas are two widely used gases that are very flammable, but odorless. If these gases were used without having an odorant added, consumers might not be aware of a leak until the gas caught fire or exploded. The presence of the odorant, such as methyl mercaptan, makes a leak obvious and allows it to be repaired before an accident occurs.

In spite of the fact that it is a natural product, methyl mercaptan is a health and environmental risk. When inhaled, it can irritate the eyes, skin, nose, throat, and lungs, producing dizziness, headache, vomiting, muscle weakness, and loss of coordination. In large concentrations, methyl mercaptan can damage the central nervous system, leading to respiratory failure and even death. The U.S. Occupational Safety and Health Administration (OSHA) has set maximum exposure limits at 20 milligrams of the compound per cubic meter of air per eight-hour work day.

Interesting Facts

• Methyl mercaptan may be responsible for some of the unpleasant odors produced by humans, such as bad breath, flatulence, and stinky feet. When bacteria attack proteins in the body, they release several gases, methyl mercaptan among them. These gases are responsible for bad breath and periodontal disease (inflammation of the gums) in the mouth, and unpleasant odors from other parts of the body. For example, old or dirty socks and shoes are ideal breeding sites for the bacteria responsible for the production of methyl mercaptan and similar foul-smelling organic compounds. One popular product designed to deal with this problem is shoe insoles that contain activated charcoal. Activated charcoal is a form of charcoal consisting of very fine grains that absorbs large volumes of gases, such as methyl mercaptan and its bad-smelling cousins.
• Why does eating asparagus produce bad-smelling urine? Asparagus contains the amino acid methionine, which is metabolized in the body to produce methyl mercaptan. The peculiar odor of urine produced after consuming asparagus is caused by the methyl mercaptan excreted from the body in urine.

FOR FURTHER INFORMATION

"Bad Breath: What Usually Is the Source of a Person's Bad Breath?" Animated-Teeth.com. http://www.animated-teeth.com/bad_breath/t3_causes_of_halitosis.htm (accessed on October 17, 2005).

Emsley, John. "Molecules at an Exhibition." http://www.nytimes.com/books/first/e/emsley-molecules.html (accessed on October 17, 2005).

"Safety Data Sheet: Methyl Mercaptan." Air Liquide. http://www.airliquide.com/safety/msds/en/083_AL_EN.pdf (accessed on October 17, 2005).

"ToxFAQs for Methyl Mercaptan." Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/tfacts139.html (accessed on October 17, 2005).