Sulphur-Nitrogen compounds | Chemistry Optional Notes for UPSC PDF Download

Heterocyclic compounds of S-N

The chemistry of Sulphur-Nitrogen compounds has several general features which are of great interest and importance, namely, stability of Sulphur-nitrogen bonds, tendency to make five and six membered rings, ring contraction, polymerization, and negative ion formation. Several heterocyclic compounds of S-N have been prepared in last century.
Some of the important compounds are as following:

Tetrasulphur tetranitride (S₄N₄)

Tetrasulfur tetranitride is an inorganic compound with the formula S₄N₄. This gold-poppy coloured solid is the most important binary sulfur nitride, which are compounds that contain only the elements sulfur and nitrogen. It is a precursor to many S-N compounds and has attracted wide interest for its unusual structure and bonding.

Preparation 

• By the reaction of SCl₂ and NH₃
  
  S₄N₄ was first prepared by M. Gregory by the reaction of disulfur dichloride with ammonia.
  
• Reaction of Sulphur with anhydrous liquid ammonia
  

Physical properties

• Solid
• M.P. = 178°C
• Thermochromic; It changes color with temperature 
  • At liquid ammonia temperature ₌ Colorless 
  • At room temperature ₌ Orange-yellow 
  • At 100°C ₌ Red
• Crystals are reasonably stable to attack by air, but explosively sensitive to shock and friction.

Chemical properties

• Reduction
   
• Oxidation
  
• Fluorination
  

Hydrogenation

Structure 

Cradle shaped structure
Average S-N bond length = 1.62 Å
S-N bond = partial double bond character
S…S distance = 2.58Å (weak bonding)
All bond lengths are equal so there is delocalization.

Disulfur dinitride (S₂N₂) 

Disulfur dinitride is the chemical compound S₂N₂ with a cyclic square planar structure.

Preparation 

 Passing gaseous S₄N₄ over silver metal wool at 250–300 °C at low pressure (1mm Hg) yields cyclic S₂N₂. The silver reacts with the sulfur produced by the thermal decomposition of the S₄N₄ to form Ag₂S, and the resulting Ag₂S catalyzes the conversion of the remaining S₄N₄ into the four-membered ring S₂N₂.

Physical properties 

• Colorless
• Crystalline 
• Soluble in diethyl ether 
• Sublimes readily 
• Molar mass = 92.144g/mol

Structure

The S₂N₂ molecule is virtually square and planar. The S-N bond lengths are 165.1Å and 165.7Å and the bond angles are very close to 90°.

Polythiazyle (SN)_x

The compound was first reported as early as 1910 by F.P. Burt, who obtained it by heating tetrasulfur tetranitride in vacuum over silver wool.

Properties

• Polythiazyl is a metallic-golden and shiny, crystalline but fibrous material. The polymer is mostly inert to oxygen and water, but decomposes in air to a grey powder. 
• At temperatures above 240 °C explosive decomposition can occur. The compound also explodes on impact. It is highly conductive at room temperature due to delocalization of pi electrons.

Structure

The material is a polymer. The S and N atoms on adjacent chains align. Several structures resonance can be written.

The structure of the crystalline compound was resolved by X-ray diffraction. This showed alternating SN bond lengths of 159Å and 163Å and SNS bond angles of 120 °C and NSN bond angles of 106 °C.

Uses

Due to its electrical conductivity, polythiazyl is used in LEDs, transistors, battery cathodes, and solar cells.