SILANES H Hydrides of Silicon are known as Silanes. Sin H2n+2 (n ups 7) Silanes (also known as saturated hydrosilicons) consist only of Si and H atoms and are bonded through single bonds . Silanes are analogous to alkanes. Each silicon atom has 4 bonds (either Si-H or Si-Si bonds), and each hydrogen atom is joined to a silicon atom (H-Si bonds). A series of linked silicon atoms is known as the silicon skeleton or silicon backbone. H Si H H H

SILANES Preparation By the action of acids on metallic silicides. Mg2 Si + 4 HCl  2 MgCl2 + SiH4 By reacting SiCl4 with LiAlH4 in ether at low temperature. SiCl4 + LiAlH4  SiH4 + LiCl + Alcl3 3. All Silanes are prepared by the reduction of corresponding chlorides using Lithium Aluminum hydride in ether solution. 2Si2 Cl6 + 3 LiAlH4  2 Si2 H6 + 3LiCl + 3 AlCl3

SILANES Properties All the silanes are colourless volatile covalent compounds. The first two silanes are gases while other are liquids Action of alkali solution : Silanes are soluble in strong alkali solution in presence of air and evolve H2 2 SiH4 + 4 NaOH + O2  2 Na2 SiO3 + 6H2 4. Action of HCl & HBr SiH4 when treated with HCl or HBr at 1000 C in presence of a catalyst Al2 Cl3 results in the substitution of H-atom by Cl/Br atom. Al2 Cl3 SiH4 + 4 HCl SiH3 Cl + H2

SILANES Hydrocarbons Silanes 5. Action of heat : when heated to v. high temperatures, silanes break into its elements 670k 470k SiH4 Si + 2H2 Si2 H6 2Si + 3H2 ∆ ∆ 6. Action of salts : Ag is pptd when SiH4 reacts with Agcl in heated flow reactor 200 0 C SiH4 + 2 AgCl SiH3 Cl + HCl + 2 Ag Silanes have structure similar to alkanes i.e. to those of the corresponding saturated hydrocarbons. Difference between alkanes and silanes Hydrocarbons Silanes Its number of carbon atoms in a chain can have any value CH4 does not get hydrolysed . maximum covalency of carbon is four which is satisfied in CH4 hence it resists the attack of e- donating molecules Maximum number of silicon atom is seven. SiH4 gets hydrolysed. Mximum covalency of Si is six, but only 4 bonds are present, so Si accepts ions pair of e- s from e- donating molecules

SILANES Structure Si 3s 3p Tetrahedra SiH4 sp3 hybridisation H H H Si Ground state H H 3s 3p H Si Excited state Tetrahedra sp3 1e from each H – 1S1 SiH4 sp3 hybridisation