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Introduction to Thiols & Sulfides

Sulfur and Phosphorus Compounds

Thiols or mercaptans are sulfur analogs of alcohols, and sulfides are the ether analogs. The chemical behavior of sulfides and thiols different with that of alcohols and ethers in some significant ways. Because hydrogen sulfide (H2S) is a much stronger acid than water (by more than ten million fold), we suppose and find, thiols to be stronger acids than equal phenols and alcohols. Thiolate conjugate bases are simply formed, and have confirmed to be excellent nucleophiles in SN2 reactions of alkyl tosylates and halides.

R-S(-) Na(+)   +   (CH3)2CH-Br   →   (CH3)2CH-S-R   +   Na(+) Br(-)

Even though the basicity of ethers is approximately a hundred times greater than that of equal sulfides, the nucleophilicity of sulfur is much greater than that of oxygen, leading to a number of interesting and helpful electrophilic substitutions of sulfur that are not generally observed for oxygen. Sulfides, for an instance, react with alkyl halides to give ternary sulfonium salts (equation no.1) in identical way that 3º-amines are alkylated to quaternary ammonium salts. Even though equivalent oxonium salts of ethers are known, they are only prepared under intense conditions, and are exceptionally reactive. Noticeably, sulfoxides (equation no.2), sulfinate salts (no.3) and sulfite anion (no.4) also alkylate on sulfur, in spite of the partial negative formal charge on oxygen and partial positive charge on sulfur.

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Oxidation States of Sulfur Compounds

Oxygen supposes only two oxidation states in its organic compounds (-1 in peroxides and -2 in other compounds). Sulfur, alternatively, is found in oxidation states ranging from -2 to +6, as displayed in the following table (some simple inorganic compounds are displayed in orange).

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In these compounds try drawing Lewis-structures for the sulfur atoms. most of the higher Oxidation states will have formal charge separation, like in equation no.2 in the above table, If you restrict your formulas to valence shell electron octets. The formulas written here neutralize this charge separation by double bonding that enlarges the valence octet of sulfur. Certainly, the S=O double bonds do not contain the customary σ & π-orbitals found in carbon double bonds. Like a third row element, sulfur has five empty 3d-orbitals which may be employed for p-d bonding in a fashion identical to p-p (π) bonding. In this way sulfur may expand an argon-such as valence shell octet by two (for example sulfoxides) or four (example- sulfones) electrons. Sulfoxides have a rigid pyramidal shape (the sulfur non-bonding electron pair occupies one corner of a tetrahedron with sulfur at the center). Subsequently, sulfoxides having two different aryl or alkyl substituents are chiral. Enantiomeric sulfoxides are stable and may be isolated.

In their oxidation chemistry thiols also vary noticeably from alcohols. Oxidation of 1º and 2º-alcohols to aldehydes and ketones changes the oxidation state of carbon but not oxygen. The oxidation of thiols and other sulfur compounds alters the oxidation state of sulfur rather than carbon. We can see some representative sulfur oxidations in the following illustrations. In the first example, mild oxidation converts thiols to disufides. An equal oxidation of alcohols to peroxides is not generally observed. The reasons for this dissimilar behavior are not difficult to recognize. The S-S single bond is nearly twice as strong as the O-O bond in peroxides and O-H bond is more than 25 kcal/mole stronger than an S-H bond. So, thermodynamics favors disulfide formation over peroxide. 
Disufides' Mild oxidation with chlorine gives alkylsulfenyl chlorides, but stronger oxidation forms sulfonic acids (2nd example). At last, with hydrogen peroxide (or per-acids) oxidation of sulfides leads first to sulfoxides and then to sulfones.

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The nomenclature of sulfur compounds is usually simple. The prefix thio represents replacement of functional oxygen by sulfur. So, C=S is a thione and -SH a thiol and. The prefix thia represents replacement of a carbon atom in a chain or ring by sulfur, even though single ether-like sulfur is generally entitled as a sulfide. For an instance, C2H5SC3H7 is ethyl propyl sulfide and C2H5SCH2SC3H7 may be termed as 3,5-dithiaoctane. Sulfonates are sulfonate acid esters and sultones are the equal of lactones.

Nucleophilicity of Phosphorus Compounds

Phosphines are the phosphorous analogs of amines. The chemistry of phosphines and the related phosphite esters is dominated by their strong nucleophilicity and reducing character. When such types of compounds are treated with reactive alkyl halides, the nucleophilicity of trivalent phosphorus results in rapid formation of phosphonium salts. For an instance, even though resonance delocalization of the nitrogen electron pair in triphenylamine, (C6H5)3N, renders it comparatively unreactive in SN2 reactions, the subsequent phosphorus compound, triphenylphosphine, goes through a rapid and exothermic reaction to give a phosphonium salt, as displayed below in the first equation. Phosphite esters react in identical manner, but the resultant phosphonium salts (shaded box) are frequently unstable, and on heating yield dialkyl phosphonate esters by way of a second SN2 reaction (equation 2 below).

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Oxidation States of Phosphorus Compounds

The variation in oxidation states between phosphorus and nitrogen is less pronounced than between sulfur and oxygen. Organophosphorus compounds having phosphorus oxidation states ranging from -3 to +5, as displayed in the following table, are well known (some simple inorganic compounds are shown in green in the diagram). Like in the example of sulfur, the P=O double bonds drawn in some of the formulas do not contain the customary pi-orbitals & sigma found in carbon double bonds. Phosphorus is a third row element and has five empty 2d-orbitals that migh be used for p-d bonding in a fashion identical to p-p (π) bonding. In this way phosphorus may increase an argon-like valence shell octet by two electrons (for example phosphine oxides).

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