# Sulfur Compounds

## Sulfur forms stable compounds with all elements except the noble gases.

#### Key Points

• Hydrogen sulfide is mildly acidic in water and is extremely toxic. Sulfur can form chains with itself (catenation). Polysulfides are formed by reduction of elemental sulfur. Polysulfanes are protonated polysulfides. Reduction of sulfur gives sulfide salts.

• Burning sulfur forms the principal sulfur oxides. The sulfur oxides form numerous oxyanions, which are related to numerous acids. Oleum is a solution of pyrosulfuric acid and sulfuric acid. Peroxides convert sulfur into unstable sulfoxides.

• Sulfur compounds with halogens include sulfur hexafluoride,sulfur dichloride, and chlorosulfuric acid. Thionyl chloride is a common reagent in organic synthesis. Tetrasulfur tetranitride and thiocyanates are compounds of sulfur and nitrogen. Phosphorus sulfides are numerous.

• The principal ores of many metals are sulfides. They are formed by the reaction of hydrogen sulfide with metal salts. Tarnishing is the process of metal corrosion by sulfur.

• Sulfur-containing organic compounds include thiols (the sulfur analogs of alcohols) and thioethers (the sulfur analogs of ethers). Compounds with carbon–sulfur bonds are uncommon. Organosulfur compounds are responsible for the some of the unpleasant odors of decaying organic matter.

• Sulfur-sulfur bonds are a structural component of proteins, providing them with rigidity. Vulcanization is the process of heating rubber and sulfur untilI disulfide bridges form between isoprene units of the polymer. This increases rigidity of rubber.

#### Terms

• The ability of a few elements, most especially carbon, to yield chains and rings by forming covalent bonds with atoms of the same element.

• A process by which rubber is hardened using heat and sulphur.

#### Examples

• For some organic sulfur compounds, smell depends on their concentration. The sulfur-containing monoterpenoid grapefruit mercaptan in small concentrations has the characteristic scent of grapefruit, but has a unpleasant thiol odor at larger concentrations.

#### Figures

1. ##### Lapis lazuli

Lapis lazuli owes its blue color to a sulfur radical.

2. ##### Peroxydisulfuric acid

Peroxydisulfuric acid (H2S2O8) is made from the action of H2SO4 on concentrated H2O2.

Common oxidation states of sulfur range from −2 to +6. Sulfur forms stable compounds with all elements except the noble gases.

Treatment of sulfur with hydrogen produces hydrogen sulfide. When dissolved in water, hydrogen sulfide is mildly acidic:

$H_2S \leftrightarrow HS^– + H^+$

Hydrogen sulfide gas and the hydrosulfide anion are extremely toxic to mammals because they inhibit of the oxygen-carrying capacity of hemoglobin and certain cytochromes in a manner similar to cyanide and azide.

Reduction of elemental sulfur produces polysulfides, which consist of chains of sulfur atoms terminated with S centers:

$2 Na + S_8 \rightarrow Na_2S_8$

This reaction highlights arguably the single most distinctive property of sulfur: its ability to catenate (bind to itself by formation of chains). Protonation of these polysulfide anions gives the polysulfanes, H2Sx where x = 2, 3, and 4. Ultimately, reduction of sulfur gives sulfide salts:

$16 Na + S_8 \rightarrow 8 Na_2S$

The interconversion of these species is used in sodium-sulfur batteries.

The radical anion S3 gives the blue color of the mineral lapis lazuli (Figure 1). With very strong oxidants, S8 can be oxidized, for example, to give bicyclic S82+.

The principal sulfur oxides are obtained by burning sulfur:

$S + O_2 \rightarrow SO_2$

$2 SO_2 + O_2 \rightarrow 2 SO_3$

Other oxides are known—sulfur monoxide and disulfur mono- and dioxides—but they are unstable. The sulfur oxides form numerous oxyanions with the formula SOn2–. Sulfur dioxide and sulfites (SO32−) are related to the unstable sulfurous acid (H2SO3). Sulfur trioxide and sulfates (SO42−) are related to sulfuric acid. Sulfuric acid and SO3 combine to give oleum, a solution of pyrosulfuric acid (H2S2O7) in sulfuric acid.

Peroxides convert sulfur into unstable compounds such as S8O, a sulfoxide. Peroxymonosulfuric acid (H2SO5) and peroxydisulfuric acids (H2S2O8) Figure 2 are made from the action of SO3 on concentrated H2O2, and H2SO4 on concentrated H2O2, respectively. Thiosulfate salts(S2O32−), sometimes referred as "hyposulfites" are used in photographic fixing (HYPO) and as reducing agents. These salts feature sulfur in two oxidation states. Sodium dithionite, (S2O42−), contains the more highly reducing dithionite anion. Sodium dithionate (Na2S2O6) is the first member of the polythionic acids (H2SnO6), where n can range from 3 to many.

There are two main sulfur fluorides. Sulfur hexafluoride is a dense gas used as a nonreactive and nontoxic propellant. Sulfur tetrafluoride is a rarely used organic reagent that is highly toxic. Their chlorinated analogs are sulfur dichloride and sulfur monochloride. Sulfuryl chloride and chlorosulfuric acid are derivatives of sulfuric acid; thionyl chloride (SOCl2) is a common reagent in organic synthesis.

An important S–N compound is the cage tetrasulfur tetranitride (S4N4). Heating this compound gives polymeric sulfur nitride ((SN)x), which has metallic properties even though it does not contain any metal atoms. Thiocyanates contain the SCN group. Oxidation of thiocyanate gives thiocyanogen, (SCN)2 with the connectivity NCS–SCN. Phosphorus sulfides are numerous.

The principal ores of copper, zinc, nickel, cobalt, molybdenum, and other metals are sulfides. These materials tend to be dark-colored semiconductors that are not readily attacked by water or even many acids. They are formed by the reaction of hydrogen sulfide with metal salts. The mineral galena (PbS) was the first demonstrated semiconductor. It was used as a signal rectifier in the cat's whiskers of early crystal radios. Upgrading these ores, usually by roasting, is costly and environmentally hazardous. Sulfur corrodes many metals via the process called tarnishing.

The following are some of the main classes of sulfur-containing organic compounds.

Thiols or mercaptans (as they are mercury capturers as chelators) are the sulfur analogs of alcohols; treatment of thiols with base gives thiolate ions

Thioethers are the sulfur analogs of ethers. Sulfonium ions have three groups attached to a cationic sulfur center. Dimethylsulfoniopropionate (DMSP) is one such compound, important in the marine organic sulfur cycle.

Sulfoxides and sulfones are thioethers with one and two oxygen atoms attached to the sulfur atom, respectively. The simplest sulfoxide, dimethyl sulfoxide, is a common solvent; a common sulfone is sulfolane. Sulfonic acids are used in many detergents.

Compounds with carbon–sulfur bonds are uncommon except for carbon disulfide, a volatile colorless liquid that is structurally similar to carbon dioxide. Unlike carbon monoxide, carbon monosulfide is only stable as a dilute gas, as in the interstellar medium. Organosulfur compounds are responsible for the some of the unpleasant odors of decaying organic matter.

Sulfur-sulfur bonds are a structural component to stiffen rubber, similar to the biological role of disulfide bridges in rigidifying proteins. In the most common type of industrial "curing" or hardening and strengthening of natural rubber, elemental sulfur is heated with the rubber until chemical reactions form disulfide bridges between isoprene units of the polymer. Because of the heat and sulfur, the process was named vulcanization, after the Roman god of the forge and volcanism.

#### Key Term Glossary

acid
an electron pair acceptor; generally capable of donating hydrogen ions
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alcohol
class of organic compounds containing a hydroxyl functional group
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anion
A negatively charged ion, as opposed to a cation
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atom
the smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons
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base
A proton acceptor, or an electron pair donor.
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battery
a device that produces electricity by a chemical reaction between two substances
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bond
a link or force between neighboring atoms in a molecule
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chemical reaction
A process involving the breaking or making of interatomic bonds, in which one or more substances are changed into others.
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compound
A substance made from any combination elements.
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concentration
the proportion of a substance in a mixture
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copper
A reddish-brown, malleable, ductile metallic element with high electrical and thermal conductivity. Its symbol is Cu and its atomic number is 29.
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crystal
A solid composed of an array of atoms or molecules possessing long-range order and arranged in a pattern which is periodic in three dimensions.
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element
Any one of the simplest chemical substances that cannot be decomposed in a chemical reaction or by any chemical means, and are made up of atoms all having the same number of protons.
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elemental
of, relating to, or being an element (as opposed to a compound)
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ether
compound containing an oxygen atom bonded to two hydrocarbon groups
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gas
Matter in a state intermediate between liquid and plasma that can be contained only if it is fully surrounded by a solid (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly.
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group
A vertical column in the periodic table, which signifies the number of valence shell electrons in an element's atom.
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heat
Heat is defined as the energy transferred from one system to another by thermal interaction.
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hemoglobin
The iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globulin), and haem (a porphyrin ring with an atom of iron at its centre).
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ion
An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
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liquid
A substance that flows and keeps no definite shape, such as water. A substance whose molecules, while not tending to separate from one another like those of a gas, readily change their relative position, and which therefore retains no definite shape, except that determined by the containing receptacle; an inelastic fluid.
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matter
The basic structural component of the universe. Matter usually has mass and volume.
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metal
Any of a number of chemical elements in the periodic table that form a metallic bond with other metal atoms; generally shiny, somewhat malleable and hard, often a conductor of heat and electricity
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ore
a type of rock that contains minerals with important elements including metals
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organic compound
any member of a large class of gaseous, liquid, or solid chemicals whose molecules contain carbon
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oxidation
a reaction in which the atoms of an element lose electrons and the valence of the element increases
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Oxidation
Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
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oxidation state
the state of an atom having a particular oxidation number.
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Oxidation State
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound.
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oxide
a binary chemical compound of oxygen with another chemical element
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oxygen
a chemical element (symbol O) with an atomic number of 8 and relative atomic mass of 15.9994
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polymer
A long or larger molecule consisting of a chain or network of many repeating units, formed by chemically bonding together many identical or similar small molecules called monomers. A polymer is formed by polymerization, the joining of many monomer molecules.
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propellant
fuel, oxidizer, reaction mass or mixture for one or more engines (especially internal combustion engines or jet engines) that is carried within a vehicle prior to use
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protonation
the addition of a proton (hydrogen ion) to an atom, molecule or ion, normally to generate a cation
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A group of atoms joined by covalent bonds, but lacking a bond at one position, that take part in reactions as a single unit. They are characterized by a free, unpaired electron that imparts their reactivity.
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reducing agent
any substance that donates electrons to another; in so doing, it becomes oxidized
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reduction
a reaction in which electrons are gained and valence is reduced, often by the removal of oxygen or the addition of hydrogen
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salt
An ionic compound that is composed of cations and anions The product is electrically neutral. The two ions are held together by ionic bonds, not covalent bonds.
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semiconductor
a substance with electrical properties between  those of a good conductor and those of a good insulator
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solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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solvent
a substance that dissolves a solute, resulting in a solution
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state
The physical property of matter as solid, liquid, gas or plasma
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toxic
having a chemical nature that is harmful to health or lethal if consumed or otherwise entering into the body in sufficient quantities
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unstable
Fuctuating; not constant.
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volatile
evaporating or vaporizing readily under normal conditions; having a low boiling point
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vulcanization
A process by which rubber is hardened using heat and sulphur.
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zinc
a chemical element (symbol Zn) with an atomic number of 30.