Silicates and SIlicones.pdf

Silicates and SIlicones Silicon Although compounds are similar in stoichiometry like carbon they do not share structural or chemical similarity. CO2 is a gas, SiO2 is a network forming molecule with each Si atom bonded to four O, why?. CCl4 don’t react with water while SiCl4 do? Silane...

Silicates and SIlicones Silicon Although compounds are similar in stoichiometry like carbon they do not share structural or chemical similarity. CO2 is a gas, SiO2 is a network forming molecule with each Si atom bonded to four O, why?. CCl4 don’t react with water while SiCl4 do? Silane alcohols when dehydrated give polymers, not like the alkenes from carbon.  Predominance of Silicate Minerals in the Earth’s Crust CRUST MOSTLY Oxygen O and Silicon Si 27% of all known minerals are silicates 40% of common minerals are silicates >90% minerals in the earth’s crust are silicates  Silicates Majority of silicates are water insoluble as they have infinite polymeric structure and great strength of Si—O bond The basic building block of all silicate minerals is the [SiO4]4− tetrahedron. (O.S: Si = +4, O = -2)  Silicates Soluble silicate can also be formed by fusing metal carbontes with sand at 1400 oC. Na2CO3  CO2 + Na2O Na2O + SiO2  Na4SiO4, (Na2SiO3)n , etc. Used in the preparation of liquid detergents to maintain high pH to dissolve the grease ad fat. Couldn’t be used with hard water as it will form insoluble calcium silicate Principles of silicate Structure Majority of silicates are water insoluble as they have an infinite ionic structure (polymer) of highly stable Si—O bond Electronegativity difference of O and Si = 3.5 – 1.8= 1.7 Hence, Si—O bond is 50% ionic and 50% covalent Si is sp3 hybridized, hence SiO44- is tetrahedron in shape Ionic radius of Si+4= 0.26 Å; O = 1.36 Å Ionic Si:O Radius Ratio (4-fold) = 0.21 Structures of the silicate anions Ortho- or Nesosilicatesor Island silicates (isolated tetrahedra) X2(SiO4) Unit Composition X often +2 valence Isolated, but tightly packed (SiO4)4- tetrahedral In this group the oxygen atoms are shared with octahedral groups that contain other cations like Mg+2, Fe+2, or Ca+2. Forms silicate minerals with high density and hardness Equi-dimensional habits and poor cleavage Low degree of Al substitution with Si Olivine (Mg, Fe)SiO4  Sorosilicates/pyrosilicates (Double Island Silicates) One of the corner oxygens is shared with another tetrahedron, referred to as the double island group In this case, the basic structural unit is Si2O7-6. < Si—O—Si = 133o. Example of a sorosilicate is the mineral hemimorphite - Zn4Si2O7(OH)2.H2O and throtveitite – Sc2(Si2O7) Some sorosilicates are a combination of single and double islands, epidote - Ca2(Fe+3,Al)Al2(SiO4)(Si2O7)(OH)  Meta- or Cyclosilicates (Ring Silicates) If two of the oxygens/unit are shared and the structure is arranged in a ring, General formula (SiO3)n2n- Three membered rings, Si3O96- (Wollastonite), four membered rings, Si4O128-, and five membered rings Si5O1510- and Six membered rings Si6O1812- are commonly sobserved. Example of a cyclosilicate is the mineral Beryl - Be3Al2Si6O18. Most important source of beryllium and also form gem quality stones  Pyroxenes/Inosilicates (Single Chain Silicates) If two of the oxygens are shared in a way to make long single chains of linked SiO4 tetrahedra, The basic structural unit is SiO3-2. Structural Formula: (SiO3)n2n- This group is the basis for the pyroxene group of minerals, like the orthopyroxenes (Mg,Fe)SiO3 or the clinopyroxenes Ca(Mg,Fe)Si2O6, Spodumene LiAl(Si2O6): Mineral source of Li  Amphiboles/Inosilicates (Double Chain Silicates) If two chains are linked together so that each tetrahedral group shares 3 of its oxygens, with the basic structural group being Si4O11-6. General Formula: (Si4O11)n6n- The amphibole group of minerals are double chain silicates, for example the tremolite - ferroactinolite series - Ca2(Mg,Fe)5Si8O22(OH)2 Crocidolite [blue asbestous] Na2Fe5Si8O22(OH)2  Phyllosilicates (Sheet Silicates) If 3 of the oxygens from each tetrahedral group are shared such that an infinite sheet (2D str) with basic structural group is Si2O5-2 is formed. General Formula: (Si2O5)n2n- The micas, clay minerals, chlorite, talc, and serpentine minerals are all based on this structure. Example is Talc - Mg3(OH)2Si4O10, Kaolinite – Al2(OH)4Si2O5, biotite - K(Mg,Fe)3(AlSi3)O10(OH)2. Note that in this structure, Al is substituting for Si in the tetrahedral groups. Phyllosilicates: e.g. Mica  Tectosilicates (3D-Framework Silicates) All of the four oxygen atoms are shared with another SiO4 tetrahedron to form 3 D lattice The basic structural group then becomes SiO2. The minerals quartz, cristobalite, and tridymite all are based on this structure. Tectosilicates (3D-Framework Silicates) Quartz (SiO2) - three-dimensional framework silicate Summary  Role of Al in Silicate Minerals Al+3 may occur in tetrahedral (substituting for Si+4) or octahedral coordination Ionic radius of Al+3 = 0.39 Å (4-fold) (Si+4 = 0.26 Å) = 0.54 Å (6-fold) Ionic Al:O Radius Ratio for Td sites= 0.39/1.36 = 0.286 Ionic Al:O Radius Ratio for Oh sites= 0.54/1.36 = 0.39 (Upper limit of tetrahedral coordination RR = 0.225) (Upper limit of octahedral coordination RR = 0.414) Electrovalence = 3/4 for Al+3 in tetrahedral coord. = 3/6 = 1/2 in octahedral coord.  Aluminosilcates/Borosilicates Replacement of ½ or ¼ Si with Al and B is quite common with formula of MI[AlSi3O8] and MII[Al2Si2O8]. Example: AlSi3O8-, 1/4th Si+4 is replaced with Al+3, and larger cations viz., K+, Ca+2, Ba+2. Smaller cations, which are common in sheet silicates do not occur in 3D silicates as the cavities in later are too large Due to replacement the framework acquires extra negative charges These negative charges are balanced by presence of extra cations in the cavities Such replacements give rise to three group of minerals: 1. Feldspars 2. Zeolites 3.Ultramarines Zeolite Applications Cement Mixture of Ca-slicates and aluminates They hydrolyze: Ca2SiO4 +4H2O = 3CaO.2SiO2.3H2O + Ca(OH)2 Settling cement 3CaO.2SiO2.3H2O Portland Cement: CaO (~70%), SiO2(~20%), Al2O3 (~5%)and Fe2O3 (~3%) Hydrolyzed hydrared silicates, aluminates and Fe2O3 slowly crystalize Calcium hydroxides convert to carbonates Glass Composite mixture of Na, K, Alkaline earth (eg. Ca) or heavy metal (Pb) aluminosilicates Supercooled molten aluminosilicates Isotropic amorphous materials These materials are transparent Gradually softens on heating and becomes plastic They can be molded or rolled like sheets Can be drawn into short fine fibers to make fiber glasses or glass wools Borosilicate glasses are harder and heat-resistant They possess very low thermal expansion coefficient Used to make laboratory glassware or home utensils Silica Gel Normal and Reverse phase chromatography  Silicone Polymers J Stanley Kipping (British; 1863-1949) coined the word silicone. In 1937, after 30 years of research, on the occasion of his retirement (Bakrerian lecture of the Royal Society) said “The prospect of any immediate and important advance in this section of organic chemistry does not seem to be very hopeful." In 1943 Dow Corning company took up industrial production of silicone polymers  Silicone Polymers When SiCl4 undergoes hydrolysis SiO2 was obtained SiCl4 + 4H2O  Si(OH)4 – 2H2O  SiO2 Kipping performed the similar reaction of chlorosilane, expecting ketone analogue: R2CCl2 + H2O  ?  ? R2SiCl2 + H2O  ?  ? Silicone Polymers His research on optically active compounds led him to study organic silicon compounds from 1900. His findings were published in a series of 51 papers. Because of their exceptional water resistance and high temperature stability, silicones eventually found nearly universal applications as synthetic rubber, water repellents, hydraulic fluids, and greases.  Early synthesis of organosilanes First report 1863 Friedel and Craft using alkylzinc reagents SiCl4 + m/2 ZnR2 RmSiCl4-m + m/2 ZnCl2 F. Stanley Kipping used first organozinc, followed by Wurtz coupling (Na) and later Grignard reagents to make organosilicon compounds. In 57 research papers Kipping created the basis of organosilicon chemistry. He made mostly ethyl and phenyl organochlorosilanes as methyl chloride is a gas difficult to handle. First examples of silicone polymers (having ethyl and or phenyl groups) was reported by Kipping in 1904 and he described the products of his reaction as “sticky messes of no particular use!”  Silicones/organosilicones : Synthesis Si—C bond is as strong as C—C bond but has less tendency to catenate Si does not form pπ—pπ while C—C can Silicon can form pπ—dπ bond Preparation: By Grignard reagent- SiCl4 + CH3MgI  CH3SiCl3 +MgCl2 - - - (CH3)4Si By Organolithium compounds: LiR + SiCl4  SiR4 + 4 LiCl (CH3)4Si volatile compound used as NMR reference compound The Direct synthesis: Rochow-Mueller Process (1940) The economically viable alternative for Grignard method: Modern synthesis of organochlorosilanes  Silicone Polymers Silicon polymers have wide variety of application as fluids, oils, elastomers and resins Three types of polymers are commonly used: Cross linked (3D) silicone polymers Chain silicone Ring silicone When SiCl4 undergoes hydrolysis SiO2 was obtained SiCl4 + 4H2O  Si(OH)4 – 2H2O  SiO2 Linear Silicones: By hydrolysis of organohalosilinaes Me Me Me2SiCl2 + H2O HO Si OH + HO Si OH Me Me H2O sticky messes ! Me Me Me Si O HO Si O Si OH Me Me Me n Show the preparation of cross linked silicones?  Silicone Polymers Me Me H2O Si O Me Me2SiCl2 + Trimer (0.5%) O Si Pentamer (7%) 4 lit 12 lit Me Me Hexamer (2%) Si O Me O Si 42% Me Me Me Me Me Si O Me Me3SiOSiMe3 O Si Me3SiO Si O SiMe3 Me Thermal and/or by Me Si O acid or base catalyst (eg. conc. H2SO4) Me n Me O Si Me Me Poly dimethyl siloxane PDMS Lowest glass transition temperature, Tg = -127 °C  Types of silicone polymers Silicone fluids: Linear chain silicones with varying substituents on silicon Silicone rubbers or elastomers: Crosslinked silicone fluids with or without a filler Silicone resins: Highly branched Uses and properties of Silicones Silicones are highly stable & non volatile even at high temp; used as high temp heating bath and as vaccum pump oil Don’t become viscous and hence, could be used as low temperature lubricants Insulator; and could be used to cover the electric wires Don’t become brittle at low temperature; could be used to form pipes for low temperature application High thermal stability and chemical resistance; could be used for high temp application and to store the chemicals  Industrial success of Silicones First human Hydrophobicity Oxygen permeability foot print on moon made Wide temp range Low glass transition temp with silicone Antifrothing Non toxicity soled shoes Moon temperature (-153 to +107 °C)