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1、2.The elements of the boron familyAllotrope:Amorphous boron crystalline boron Brown powder,black grey High chemical activity high hardness High melting point and boiling point.-Diamond boron(B12)Regular dodecahedron 2.1 Bonding characteristics of boron(1)Covalent bond(main)B:The atomic radius is sma
2、ll，I1、I2、I3 are lager B:hybrid sp2,as BX3,B(OH)3 hybrid sp3,as BF4-,BH4-,B(OH)4-(2)Electron deficient properties of B（ns2 np1）Valence orbital number:4 2s 2px 2py 2pz Valence electron number:3 2s2 2p1 Example BX3,B(OH)3 BF3 +F-=BF4-BF3：Lewis acid The oxidation state is+3,however,ns2 tends to be stabl
3、e with the increase of atomic number.Tl:Oxidation state of+1 is characteristic.(3)Form polyhedron with multi-center electron-deficient bonds B-B-B is found in boron crystal,B-B-B,or B-H-B(3c-2e bond)in borane(4)B is an F-and O-friendly element Bond energy/kJmol-1 B-O 561690 Si-O 452 B-F 613 Si-F 565
4、(5)Similarity between B and Si Diagonal rulei Be B C Na Mg Al Si2.2 Boron element(1)Allotrope of boron Amorphous boron(brown powder)Boron element Crystal boron:the most important is rhombic boron(black gray)-rhombic boron structure Atomic crystal,structural unit:B12 Number of edges:in B12 cell,each
5、B is connected with another 5 B,which are related with 5 edges.The total number of edges is 5 12/2=30B12 structure:Icosahedron,12 vertex B atoms,3 12=36valence electrons 2.3 Chemical properties of boronCrystal boron is inert (1)Amorphous boron is more active and can react with N2,O2,S,X2 at high tem
6、perature,showing reducibility.At R.T.2B(s)+3F2(g)2BF3 (B is friendly to F)At 973K 4B(s)+3O2(g)2B2O3rH298 =-2887 kJmol-1rG298 =-2368 kJmol-1 B-O Si-O C-O bond energy/kJmol-1 560-690 452 358 (B is friendly to O)B is used as deoxidizer in steelmaking.2B(s)+N2(g)2BNBN:graphite structure,B-N bond polarit
7、y,insulator(2)Amorphous B is oxidized by hot concentrated H2SO4 or HNO3 2B(s)+3H2SO4(concentrated)2H3BO3+3SO2(g)B(s)+3HNO3(concentrated)H3BO3+3NO2(g)(3)In the presence of oxidant under strong heat condition,it reacts with alkali 2B+2KOH+3KNO3 3KNO2+2KBO2+H2O 2.4.Preparation of boron -Diamond boron 1
8、2 BI3 B12(C)+18 I2(g)Amorphous boron Na2B4O710H2O+2HCl=4H3BO3+2NaCl+5H2O2H3BO3 =B2O3 +3 H2O(800K)B2O3+Mg=3MgO+2B(800K)3.Boron compoundsuHydrides of boronuOxygen containing compounds of boron (1)Boron trioxide(2)Borax（Na2 2B4 4H7 7 10H2 2O）(3)Boric acid（H3 3BO3 3）uHalides of boronBoron hydrides-boran
9、e Classification：BnHn+4 and BnHn+6 Sun as:B2H6 B4H10 Diborane Tetraborane There is CH4，but no BH4 The simplest borane:B2H6(1)Composition BnHn+4 BnHn+6 (2)Nomenclature:same as carbon alkane BnHn+4:B2H6 B5H9 Pentaborane-9 B16H20 BnHn+6:B5H11 Pentaborane-11 Diborane,B1B10,methyl,ethyl,propyl,butyl,octa
10、ne,decane,etc.Hexadecaborane(More than B11：11)The number of boron atoms is the same,but the number of H atoms is different B5H9 Pentaborane-9,B5H11:Pentaborane-11(3)Borane method Hydrogenation method 2BCl3+6H2=B2H6+6HCl Proton replacement method 2BMn+6H+=B2H6+Mn3+Hydrogen anion replacement method 3N
11、aBH4+4BF3=2B2H6+3NaBF4(4)The structure of borane In the early 1960s,Lipscomb solved the problem,which has puzzled the chemical field for decades.DiboraneB2H6 Valence electron number:32+16=12IfThere is no connection between the 2 BH3,and each B has only 6e after the bonding.Unreasonable！The following
12、 structure is assumed.Molecular structure diagram of B2H6 B-H bond Bond length:119 pm B-H-B 3c-2e bond,bridge H The bond with electron deficient propertiesThe B2H6 molecule has a“multi center electron deficient bond”，that is 3c-2e bond(3 center-2 electron bond.In the early 1960s,William n.Lipscomb(H
13、arvard University)proposed the type of borane bond,and won the Nobel Prize of chemistry in 1976 Diamagnetism of B2H6 molecule(electrons are paired)Experimental determination 1H NMR Two kinds of H,ratio 4:2(4 H and 2H)Two B and four H are coplanarStructure of boraneB：Using sp3 hybridized orbital,three centers and two electron bonds are formed with hydrogen.Key points:Hybrid mode of B,three center two electron bond,hydrogen bridge.Hydrogen bridgeRecord as：