Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen and nitrogen, although other elements are found in the side chains of certain amino acids. About 500 occurring amino acids are known and can be classified in many ways, they can be classified according to the core structural functional groups' locations as alpha-, beta-, gamma- or delta- amino acids. In the form of proteins, amino acid residues form the second-largest component of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. In biochemistry, amino acids having both the amine and the carboxylic acid groups attached to the first carbon atom have particular importance, they are known as α-amino acids. They include the 22 proteinogenic amino acids, which combine into peptide chains to form the building-blocks of a vast array of proteins.
These are all L-stereoisomers, although a few D-amino acids occur in bacterial envelopes, as a neuromodulator, in some antibiotics. Twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as "standard" amino acids; the other two are selenocysteine, pyrrolysine. Pyrrolysine and selenocysteine are encoded via variant codons. N-formylmethionine is considered as a form of methionine rather than as a separate proteinogenic amino acid. Codon–tRNA combinations not found in nature can be used to "expand" the genetic code and form novel proteins known as alloproteins incorporating non-proteinogenic amino acids. Many important proteinogenic and non-proteinogenic amino acids have biological functions. For example, in the human brain and gamma-amino-butyric acid are the main excitatory and inhibitory neurotransmitters. Hydroxyproline, a major component of the connective tissue collagen, is synthesised from proline. Glycine is a biosynthetic precursor to porphyrins used in red blood cells.
Carnitine is used in lipid transport. Nine proteinogenic amino acids are called "essential" for humans because they cannot be produced from other compounds by the human body and so must be taken in as food. Others may be conditionally essential for medical conditions. Essential amino acids may differ between species; because of their biological significance, amino acids are important in nutrition and are used in nutritional supplements, fertilizers and food technology. Industrial uses include the production of drugs, biodegradable plastics, chiral catalysts; the first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus, subsequently named asparagine, the first amino acid to be discovered. Cystine was discovered in 1810, although its monomer, remained undiscovered until 1884. Glycine and leucine were discovered in 1820; the last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth.
The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. Usage of the term "amino acid" in the English language is from 1898, while the German term, Aminosäure, was used earlier. Proteins were found to yield amino acids after enzymatic acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "peptide". In the structure shown at the top of the page, R represents a side chain specific to each amino acid; the carbon atom next to the carboxyl group is called the α–carbon. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as alpha amino acids; these include amino acids such as proline which contain secondary amines, which used to be referred to as "imino acids". The alpha amino acids are the most common form found in nature, but only when occurring in the L-isomer.
The alpha carbon is a chiral carbon atom, with the exception of glycine which has two indistinguishable hydrogen atoms on the alpha carbon. Therefore, all alpha amino acids but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other. While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amin
Carbon tetrachloride known by many other names is an organic compound with the chemical formula CCl4. It is a colourless liquid with a "sweet" smell, it has no flammability at lower temperatures. It was widely used in fire extinguishers, as a precursor to refrigerants and as a cleaning agent, but has since been phased out because of toxicity and safety concerns. Exposure to high concentrations of carbon tetrachloride can affect the central nervous system, degenerate the liver and kidneys. Prolonged exposure can be fatal. Carbon tetrachloride was synthesized by the French chemist Henri Victor Regnault in 1839 by the reaction of chloroform with chlorine, but now it is produced from methane: CH4 + 4 Cl2 → CCl4 + 4 HClThe production utilizes by-products of other chlorination reactions, such as from the syntheses of dichloromethane and chloroform. Higher chlorocarbons are subjected to "chlorinolysis": C2Cl6 + Cl2 → 2 CCl4Prior to the 1950s, carbon tetrachloride was manufactured by the chlorination of carbon disulfide at 105 to 130 °C: CS2 + 3Cl2 → CCl4 + S2Cl2The production of carbon tetrachloride has steeply declined since the 1980s due to environmental concerns and the decreased demand for CFCs, which were derived from carbon tetrachloride.
In 1992, production in the U. S./Europe/Japan was estimated at 720,000 tonnes. In the carbon tetrachloride molecule, four chlorine atoms are positioned symmetrically as corners in a tetrahedral configuration joined to a central carbon atom by single covalent bonds; because of this symmetrical geometry, CCl4 is non-polar. Methane gas has the same structure, making carbon tetrachloride a halomethane; as a solvent, it is well suited to dissolving other non-polar compounds and oils. It can dissolve iodine, it is somewhat volatile, giving off vapors with a smell characteristic of other chlorinated solvents, somewhat similar to the tetrachloroethylene smell reminiscent of dry cleaners' shops. Solid tetrachloromethane has two polymorphs: crystalline II below −47.5 °C and crystalline I above −47.5 °C. At −47.3 °C it has monoclinic crystal structure with space group C2/c and lattice constants a = 20.3, b = 11.6, c = 19.9, β = 111°. With a specific gravity greater than 1, carbon tetrachloride will be present as a dense nonaqueous phase liquid if sufficient quantities are spilled in the environment.
In organic chemistry, carbon tetrachloride serves as a source of chlorine in the Appel reaction. One specialty use of carbon tetrachloride is in stamp collecting, to reveal watermarks on postage stamps without damaging them. A small amount of the liquid was placed on the back of a stamp, sitting in a black glass or obsidian tray; the letters or design of the watermark could be seen. Carbon tetrachloride was used as a dry cleaning solvent, as a refrigerant, in lava lamps. In case of the latter, carbon tetrachloride is a key ingredient that adds weight to the otherwise buoyant wax, it once was a popular solvent in organic chemistry, because of its adverse health effects, it is used today. It is sometimes useful as a solvent for infrared spectroscopy, because there are no significant absorption bands > 1600 cm−1. Because carbon tetrachloride does not have any hydrogen atoms, it was used in proton NMR spectroscopy. In addition to being toxic, its dissolving power is low, its use has been superseded by deuterated solvents.
Use of carbon tetrachloride in determination of oil has been replaced by various other solvents, such as tetrachloroethylene. Because it has no C-H bonds, carbon tetrachloride does not undergo free-radical reactions, it is a useful solvent for halogenations either by the elemental halogen or by a halogenation reagent such as N-bromosuccinimide. In 1910, the Pyrene Manufacturing Company of Delaware filed a patent to use carbon tetrachloride to extinguish fires; the liquid was vaporized by the heat of combustion and extinguished flames, an early form of gaseous fire suppression. At the time it was believed the gas displaced oxygen in the area near the fire, but research found that the gas inhibits the chemical chain reaction of the combustion process. In 1911, Pyrene patented a portable extinguisher that used the chemical; the extinguisher consisted of a brass bottle with an integrated handpump, used to expel a jet of liquid toward the fire. As the container was unpressurized, it could be refilled after use.
Carbon tetrachloride was suitable for liquid and electrical fires and the extinguishers were carried on aircraft or motor vehicles. In the first half of the 20th century, another common fire extinguisher was a single-use, sealed glass globe known as a "fire grenade," filled with either carbon tetrachloride or salt water; the bulb could be thrown at the base of the flames to quench the fire. The carbon tetrachloride type could be installed in a spring-loaded wall fixture with a solder-based restraint; when the solder melted by high heat, the spring would either break the globe or launch it out of the bracket, allowing the extinguishing agent to be automatically dispersed into the fire. A well-known brand was the "Red Comet,", variously manufactured with other fire-fighting equipment in the Denver, Colorado area by the Red Comet Manufacturing Company from its founding in 1919 until manufacturing operations were closed in the early 1980s. Prior to the Montreal Protocol, large quantities of carbon tetrachloride were used to produce the chlorofluorocarbon re
Water is a transparent, tasteless and nearly colorless chemical substance, the main constituent of Earth's streams and oceans, the fluids of most living organisms. It is vital for all known forms of life though it provides no calories or organic nutrients, its chemical formula is H2O, meaning that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. Water is the name of the liquid state of H2O at standard ambient pressure, it forms precipitation in the form of rain and aerosols in the form of fog. Clouds are formed from suspended droplets of its solid state; when finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is water vapor. Water moves continually through the water cycle of evaporation, condensation and runoff reaching the sea. Water covers 71% of the Earth's surface in seas and oceans. Small portions of water occur as groundwater, in the glaciers and the ice caps of Antarctica and Greenland, in the air as vapor and precipitation.
Water plays an important role in the world economy. 70% of the freshwater used by humans goes to agriculture. Fishing in salt and fresh water bodies is a major source of food for many parts of the world. Much of long-distance trade of commodities and manufactured products is transported by boats through seas, rivers and canals. Large quantities of water and steam are used for cooling and heating, in industry and homes. Water is an excellent solvent for a wide variety of chemical substances. Water is central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, sport fishing, diving; the word water comes from Old English wæter, from Proto-Germanic *watar, from Proto-Indo-European *wod-or, suffixed form of root *wed-. Cognate, through the Indo-European root, with Greek ύδωρ, Russian вода́, Irish uisce, Albanian ujë; the identification of water as a substance Water is a polar inorganic compound, at room temperature a tasteless and odorless liquid, nearly colorless with a hint of blue.
This simplest hydrogen chalcogenide is by far the most studied chemical compound and is described as the "universal solvent" for its ability to dissolve many substances. This allows it to be the "solvent of life", it is the only common substance to exist as a solid and gas in normal terrestrial conditions. Water is a liquid at the pressures that are most adequate for life. At a standard pressure of 1 atm, water is a liquid between 0 and 100 °C. Increasing the pressure lowers the melting point, about −5 °C at 600 atm and −22 °C at 2100 atm; this effect is relevant, for example, to ice skating, to the buried lakes of Antarctica, to the movement of glaciers. Increasing the pressure has a more dramatic effect on the boiling point, about 374 °C at 220 atm; this effect is important in, among other things, deep-sea hydrothermal vents and geysers, pressure cooking, steam engine design. At the top of Mount Everest, where the atmospheric pressure is about 0.34 atm, water boils at 68 °C. At low pressures, water cannot exist in the liquid state and passes directly from solid to gas by sublimation—a phenomenon exploited in the freeze drying of food.
At high pressures, the liquid and gas states are no longer distinguishable, a state called supercritical steam. Water differs from most liquids in that it becomes less dense as it freezes; the maximum density of water in its liquid form is 1,000 kg/m3. The density of ice is 917 kg/m3. Thus, water expands 9% in volume as it freezes, which accounts for the fact that ice floats on liquid water; the details of the exact chemical nature of liquid water are not well understood. Pure water is described as tasteless and odorless, although humans have specific sensors that can feel the presence of water in their mouths, frogs are known to be able to smell it. However, water from ordinary sources has many dissolved substances, that may give it varying tastes and odors. Humans and other animals have developed senses that enable them to evaluate the potability of water by avoiding water, too salty or putrid; the apparent color of natural bodies of water is determined more by dissolved and suspended solids, or by reflection of the sky, than by water itself.
Light in the visible electromagnetic spectrum can traverse a couple meters of pure water without significant absorption, so that it looks transparent and colorless. Thus aquatic plants and other photosynthetic organisms can live in water up to hundreds of meters deep, because sunlight can reach them. Water vapour is invisible as a gas. Through a thickness of 10 meters or more, the intrinsic color of water is visibly turquoise, as its absorption spectrum has
Thiocyanate is the anion −. It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium sodium thiocyanate. Organic compounds containing the functional group SCN are called thiocyanates. Mercury thiocyanate was used in pyrotechnics. Thiocyanate is analogous to the cyanate ion, −, wherein oxygen is replaced by sulfur. − is one of the pseudohalides, due to the similarity of its reactions to that of halide ions. Thiocyanate used to be known as rhodanide because of the red colour of its complexes with iron. Thiocyanate is produced by the reaction of elemental sulfur or thiosulfate with cyanide: 8 CN− + S8 → 8 SCN− CN− + S2O2−3 → SCN− + SO2−3The second reaction is catalyzed by thiosulfate sulfurtransferase, a hepatic mitochondrial enzyme, by other sulfur transferases, which together are responsible for around 80% of cyanide metabolism in the body. Organic and transition metal derivatives of the thiocyanate ion can exist as "linkage isomers". In thiocyanates, the organic group is attached to sulfur: R−S−C≡N has a S–C single bond and a C≡N triple bond.
In isothiocyanates, the substituent is attached to nitrogen: R−N=C=S has a S=C double bond and a C=N double bond: Organic thiocyanates are valuable building blocks in organic chemistry and they allow to access efficiently various sulfur containing functional groups and scaffolds. Several synthesis routes exist, the most basic being the reaction between alkyl halides and alkali thiocyanate in aqueous media. Organic thiocyanates are hydrolyzed to thiocarbamates in the Riemschneider thiocarbamate synthesis. Thiocyanate is known to be an important part in the biosynthesis of hypothiocyanite by a lactoperoxidase, thus the complete absence of thiocyanate or reduced thiocyanate in the human body, is damaging to the human host defense system. Thiocyanate is a potent competitive inhibitor of the thyroid sodium-iodide symporter. Iodine is an essential component of thyroxine. Since thiocyanates will decrease iodide transport into the thyroid follicular cell, they will decrease the amount of thyroxine produced by the thyroid gland.
As such, foodstuffs containing thiocyanate are best avoided by Iodide deficient hypothyroid patients. In the early 20th century, thiocyanate was used in the treatment of hypertension, but it is no longer used because of associated toxicity. Sodium nitroprusside, a metabolite of, thiocyanate, is however still used for the treatment of a hypertensive emergency. Rhodanese catalyzes the reaction of sodium nitroprusside with thiosulfate to form the metabolite thiocyanate. Thiocyanate shares its negative charge equally between sulfur and nitrogen; as a consequence, thiocyanate can act as a nucleophile at either sulfur or nitrogen — it is an ambidentate ligand. − can bridge two or three metals. Experimental evidence leads to the general conclusion that class A metals tend to form N-bonded thiocyanate complexes, whereas class B metals tend to form S-bonded thiocyanate complexes. Other factors, e.g. kinetics and solubility, are sometimes involved, linkage isomerism can occur, for example Cl2 and Cl2. If − is added to a solution with iron ions, a blood-red solution forms due to the formation of 2+, i.e. pentaaquairon.
Lesser amounts of other hydrated compounds form: e.g. Fe3 and −. Co2+ gives a blue complex with thiocyanate. Both the iron and cobalt complexes can be extracted into organic solvents like diethyl ether or amyl alcohol; this allows the determination of these ions in coloured solutions. The determination of Co in the presence of Fe is possible by adding KF to the solution, which forms uncoloured stable complexes with Fe, which no longer react with SCN−. Phospholipids or some detergents aid the transfer of thiocyanatoiron into chlorinated solvents like chloroform and can be determined in this fashion. Sulphobes Greenwood, Norman N.. Chemistry of the Elements. Butterworth-Heinemann. ISBN 978-0-08-037941-8
Chlorine is a chemical element with symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are intermediate between them. Chlorine is a yellow-green gas at room temperature, it is an reactive element and a strong oxidising agent: among the elements, it has the highest electron affinity and the third-highest electronegativity on the Pauling scale, behind only oxygen and fluorine. The most common compound of chlorine, sodium chloride, has been known since ancient times. Around 1630, chlorine gas was first synthesised in a chemical reaction, but not recognised as a fundamentally important substance. Carl Wilhelm Scheele wrote a description of chlorine gas in 1774, supposing it to be an oxide of a new element. In 1809, chemists suggested that the gas might be a pure element, this was confirmed by Sir Humphry Davy in 1810, who named it from Ancient Greek: χλωρός, translit. Khlôros, lit.'pale green' based on its colour.
Because of its great reactivity, all chlorine in the Earth's crust is in the form of ionic chloride compounds, which includes table salt. It is the second-most abundant halogen and twenty-first most abundant chemical element in Earth's crust; these crustal deposits are dwarfed by the huge reserves of chloride in seawater. Elemental chlorine is commercially produced from brine by electrolysis; the high oxidising potential of elemental chlorine led to the development of commercial bleaches and disinfectants, a reagent for many processes in the chemical industry. Chlorine is used in the manufacture of a wide range of consumer products, about two-thirds of them organic chemicals such as polyvinyl chloride, many intermediates for the production of plastics and other end products which do not contain the element; as a common disinfectant, elemental chlorine and chlorine-generating compounds are used more directly in swimming pools to keep them clean and sanitary. Elemental chlorine at high concentrations is dangerous and poisonous for all living organisms, was used in World War I as the first gaseous chemical warfare agent.
In the form of chloride ions, chlorine is necessary to all known species of life. Other types of chlorine compounds are rare in living organisms, artificially produced chlorinated organics range from inert to toxic. In the upper atmosphere, chlorine-containing organic molecules such as chlorofluorocarbons have been implicated in ozone depletion. Small quantities of elemental chlorine are generated by oxidation of chloride to hypochlorite in neutrophils as part of the immune response against bacteria; the most common compound of chlorine, sodium chloride, has been known since ancient times. Its importance in food was well known in classical antiquity and was sometimes used as payment for services for Roman generals and military tribunes. Elemental chlorine was first isolated around 1200 with the discovery of aqua regia and its ability to dissolve gold, since chlorine gas is one of the products of this reaction: it was however not recognised as a new substance. Around 1630, chlorine was recognized as a gas by the Flemish chemist and physician Jan Baptist van Helmont.
The element was first studied in detail in 1774 by Swedish chemist Carl Wilhelm Scheele, he is credited with the discovery. Scheele produced chlorine by reacting MnO2 with HCl: 4 HCl + MnO2 → MnCl2 + 2 H2O + Cl2Scheele observed several of the properties of chlorine: the bleaching effect on litmus, the deadly effect on insects, the yellow-green color, the smell similar to aqua regia, he called it "dephlogisticated muriatic acid air" since it is a gas and it came from hydrochloric acid. He failed to establish chlorine as an element. Common chemical theory at that time held that an acid is a compound that contains oxygen, so a number of chemists, including Claude Berthollet, suggested that Scheele's dephlogisticated muriatic acid air must be a combination of oxygen and the yet undiscovered element, muriaticum. In 1809, Joseph Louis Gay-Lussac and Louis-Jacques Thénard tried to decompose dephlogisticated muriatic acid air by reacting it with charcoal to release the free element muriaticum, they did not succeed and published a report in which they considered the possibility that dephlogisticated muriatic acid air is an element, but were not convinced.
In 1810, Sir Humphry Davy tried the same experiment again, concluded that the substance was an element, not a compound. He announced his results to the Royal Society on 15 November that year. At that time, he named this new element "chlorine", from the Greek word χλωρος, meaning green-yellow; the name "halogen", meaning "salt producer", was used for chlorine in 1811 by Johann Salomo Christoph Schweigger. This term was used as a generic term to describe all the elements in the chlorine family, after a suggestion by Jöns Jakob Berzelius in 1826. In 1823, Michael Faraday liquefied chlorine for the first time, demonstrated that what was known as "solid chlorine" had a structure of chlorine hydrate. Chlorine gas was first used by French chemist Claude Berthollet to bleach textiles in 1785. Modern bleaches resulted from further work by Berthollet, who first produced sodium hypochlorite in 1789 in his laboratory in the town of Javel, by passing chlorine gas through a solution of sodium carbonate; the resulting liqu
The haloform reaction is a chemical reaction where a haloform is produced by the exhaustive halogenation of methyl aldehyde or a methyl ketone, in the presence of a base. The reaction can be used to transform acetyl groups into carboxyl groups or to produce chloroform, bromoform, or iodoform. In the first step, the halogen disproportionates in the presence of hydroxide to give the halide and hypohalite: Br 2 + 2 OH − → Br − + BrO − + H 2 O If a secondary alcohol is present, it is oxidized to a ketone by the hypohalite: If a methyl ketone is present, it reacts with the hypohalite in a three-step process: 1. Under basic conditions, the ketone undergoes keto-enol tautomerization; the enolate undergoes electrophilic attack by the hypohalite. 2. When the α position has been exhaustively halogenated, the molecule undergoes a nucleophilic acyl substitution by hydroxide, with −CX3 being the leaving group stabilized by three electron-withdrawing groups. In the third step the −CX3 anion abstracts a proton from either the solvent or the carboxylic acid formed in the previous step, forms the haloform.
At least in some cases the reaction may stop and the intermediate product isolated if conditions are acidic and hypohalite is used. Substrates are broadly limited to methyl ketones and secondary alcohols oxidizable to methyl ketones, such as isopropanol; the only primary alcohol and aldehyde to undergo this reaction are ethanol and acetaldehyde, respectively. 1,3-Diketones such as acetylacetone give the haloform reaction. Β-ketoacids such as acetoacetic acid will give the test upon heating. Acetyl chloride and acetamide don't give this test; the halogen used may be chlorine, iodine or sodium hypochlorite. Fluoroform cannot be prepared by this method as it would require the presence of the unstable hypofluorite ion; however ketones with the structure RCOCF3 do cleave upon treatment with base to produce fluoroform. This reaction forms the basis of the iodoform test, used in history as a chemical test to determine the presence of a methyl ketone, or a secondary alcohol oxidizable to a methyl ketone.
When iodine and sodium hydroxide are used as the reagents a positive reaction gives iodoform, a solid at room temperature and tends to precipitate out of solution causing a distinctive cloudiness. In organic chemistry, this reaction may be used to convert a terminal methyl ketone into the analogous carboxylic acid, it was used to produce iodoform and chloroform industrially. Water chlorination can result in the formation of haloforms if the water contains suitable reactive impurities. There is a concern that such reactions may lead to the presence of carcinogenic compounds in drinking water; the haloform reaction is one of the oldest organic reactions known. In 1822, Georges-Simon Serullas added potassium metal to a solution of iodine in ethanol and water to form potassium formate and iodoform, called in the language of that time hydroiodide of carbon. In 1831, Justus von Liebig reported the reaction of chloral with calcium hydroxide to form chloroform and calcium formate; the reaction was rediscovered by Adolf Lieben in 1870.
The iodoform test is called the Lieben haloform reaction. A review of the Haloform reaction with a history section was published in 1934
Thiol is an organosulfur compound of the form R-SH, where R represents an alkyl or other organic substituent. Thiols are the sulfur analogue of alcohols, the word is a portmanteau of "thion" + "alcohol," with the first word deriving from Greek θεῖον = "sulfur"; the -- SH functional group itself is referred to as either a sulfhydryl group. Many thiols have strong odors resembling that of rotten eggs. Thiols are used as odorants to assist in the detection of natural gas, the "smell of natural gas" is due to the smell of the thiol used as the odorant. Thiols are sometimes referred to as mercaptans; the term "mercaptan" was introduced in 1832 by William Christopher Zeise and is derived from the Latin mercurium captāns because the thiolate group bonds strongly with mercury compounds. Thiols and alcohols have similar connectivity; because sulfur is a larger element than oxygen, the C–S bond lengths – around 180 picometres in length – is about 40 picometers longer than a typical C–O bond. The C -- S -- H angles approach 90 °.
In the solid or liquids, the hydrogen-bonding between individual thiol groups is weak, the main cohesive force being van der Waals interactions between the polarizable divalent sulfur centers. The S-H bond is much weaker than the O-H bond as reflected in their respective bond dissociation energy. For CH3S-H, the BDE is 366 kJ/mol. Due to the small difference in the electronegativity of sulfur and hydrogen, an S–H bond is polar. In contrast, O-H bonds in hydroxyl groups are more polar. Thiols have a lower dipole moment relative to the corresponding alcohol. There are several ways to name the alkylthiols: The suffix -thiol is added to the name of the alkane; this method is nearly identical to naming an alcohol and is used by the IUPAC, e.g. CH3SH would be methanethiol; the word mercaptan replaces alcohol in the name of the equivalent alcohol compound. Example: CH3SH would be methyl mercaptan, just as CH3OH is called methyl alcohol; the term sulfanyl or mercapto is used as e.g. mercaptopurine. Many thiols have strong odors resembling that of garlic.
The odors of thiols those of low molecular weight, are strong and repulsive. The spray of skunks consists of low-molecular-weight thiols and derivatives; these compounds are detectable by the human nose at concentrations of only 10 parts per billion. Human sweat contains /-3-methyl-3-sulfanylhexan-1-ol, detectable at 2 parts per billion and having a fruity, onion-like odor. Methanethiol is a strong-smelling volatile thiol detectable at parts per billion levels, found in male mouse urine. Lawrence C. Katz and co-workers showed that MTMT functioned as a semiochemical, activating certain mouse olfactory sensory neurons, attracting female mice. Copper has been shown to be required by a specific mouse olfactory receptor, MOR244-3, responsive to MTMT as well as to various other thiols and related compounds. A human olfactory receptor, OR2T11, has been identified which, in the presence of copper, is responsive to the gas odorants ethanethiol and t-butyl mercaptan as well as other low molecular weight thiols, including allyl mercaptan found in human garlic breath, the strong-smelling cyclic sulfide thietane.
Thiols are responsible for a class of wine faults caused by an unintended reaction between sulfur and yeast and the "skunky" odor of beer, exposed to ultraviolet light. Not all thiols have unpleasant odors. For example, furan-2-ylmethanethiol contributes to the aroma of roasted coffee, whereas grapefruit mercaptan, a monoterpenoid thiol, is responsible for the characteristic scent of grapefruit; the effect of the latter compound is present only at low concentrations. The pure mercaptan has an unpleasant odor. Natural gas distributors were required to add thiols ethanethiol, to natural gas after the deadly New London School explosion in New London, Texas, in 1937. Many gas distributors were odorizing gas prior to this event. Most gas odorants utilized contain mixtures of mercaptans and sulfides, with t-butyl mercaptan as the main odor constituent in natural gas and ethanethiol in liquefied petroleum gas. In situations where thiols are used in commercial industry, such as liquid petroleum gas tankers and bulk handling systems, an oxidizing catalyst is used to destroy the odor.
A copper-based oxidation catalyst neutralizes the volatile thiols and transforms them into inert products. Thiols show little association both with water molecules and among themselves. Hence, they have lower boiling points and are less soluble in water and other polar solvents than alcohols of similar molecular weight. For this reason thiols and corresponding thioether functional group isomers have similar solubility characteristics and boiling points, whereas the same is not true of alcohols and their corresponding isomeric ethers; the S-H bond in thiols is weak compared to the O-H bond in alcohols. For CH3X-H, the bond enthalpies are 365.07 for X = S and 440.2 kcal/mol for X = O. H-atom abstraction from a thiol gives a thiyl radical with the formula RS. where R = alkyl or aryl. Volatile thiols are and unerringly detected by their distinctive odor. S-specific analyzers for gas chromatographs are useful. Spectroscopic indicators are the D2O-exchangeable SH signal in the 1H NMR spectrum; the νSH band appears near 2400 cm−