P-block elements.pdf

P-Block Elements: P-block elements are those in which the last electron occupies P-orbitals. IIIA GROUP ELEMENTS (BORON FAMILY) Diborane (B2H6): Boron hydrides are known as boranes. The most important borane is Diborane, B2H6. Preparation methods of...

P-Block Elements: P-block elements are those in which the last electron occupies P-orbitals. IIIA GROUP ELEMENTS (BORON FAMILY) Diborane (B2H6): Boron hydrides are known as boranes. The most important borane is Diborane, B2H6. Preparation methods of Diborane: 1. Boron halides are reacted with reducing agents like sodium borohydride and Lithium Aluminium Hydride (LAH) to give diborane. 3NaBH4+ 4BF3→3NaBF4+2B2H6 2. Reduction of B2O3 with H2 at high temperature and pressure in the presence of (Al+AlCl3) catalyst also produces diborane. (Al+AlCl3) 3. B2O3+6H2 B2H6+3H2O Properties:  Diborane is stable in the absence of moisture and grease. It is flammable and easily decomposed in the presence of water. B2H6+6H2O→2H3BO3+6H2  Reaction with Oxygen: Diborane burns in the presence of oxygen to form boron oxide. B2H6+9O2→2B2O3+3H2O+Heat  Reaction with Ammonia:  Diborane reacts with ammonia at high temperature(2000C) to form borazole. Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel   B2H6+6NH3→2B3N3H6+12H2  Diborane reacts with ammonia at low temperature(1200C) to form additive product.  B2H6+2NH3→ B2H6.2NH3 Structure and bonding in Diborane (B2H6): Number of atoms present in diborane = 8 Number of bonds present in diborane = 8-1=7. So, B2H6 molecules require 7X2=14 electrons. But Diborane has 12 electrons. 2B=2X3=6 6H=6X1=6 Total 12 electrons. Two electrons are deficient in Dibornae hence Diborane is an electron-deficient molecule. Boron’s atomic number is five. The electronic configuration is 1S22S22P1 The First excited state is 1S22S12P2 According to Molecular Orbital Theory (MOT), each of the boron atoms is involved in SP 3 hybridization and forms four hybrid orbitals. Which three have one electron each while the fourth one is empty. Two SP3 hybrid orbitals of each boron atom overlap with the 1S orbital of two terminal hydrogen atoms forming two B-H sigma (σ)bonds. One of the remaining Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel filled SP3 hybrid orbitals of one boron atom, the 1S orbital of a H atom and one of the empty SP3 hybrid orbitals of a second boron atom overlap to form three-centered and two-electron bonds. Similar overlapping between a filled SP3 hybrid orbital of one boron atom, the 1S orbital of H atom, and one of the empty SP3 hybrid orbitals of a second boron atom overlap to form one more three-centered and two-electron bonds. Due to repulsions between two nuclei of hydrogens, the bridged bonds are bent away from each other in the middle giving the shape of a banana. Borazole(B3N3H6) Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel The structure of Borazole is similar to Benzene. So Borazole is also known as Inorganic benzene. Preparation: Diborane reacts with ammonia at high temperatures to form Borazole. B2H6+6NH3→2B3N3H6+12H2 Properties: It is a colorless liquid and more reactive than benzene. i)Hydrolysis of Borazole gives boric acid. B3N3H6+9H2O →3H3BO3+3NH3+3H2 ii)Borazole reacts with 3 moles of HCl to form B3N3H9Cl3. B3N3H6+3HCl→B3N3H9Cl3 Structure: The structure of Borazole is similar to Benzene. The Borazole molecule is planar in which B and N atoms are SP2 hybridized. The difference is that in Borazole the π – bonding is dative and it arises from the overlap of empty p-orbitals of Boron with filled p-orbitals of Nitrogen. Thus six electrons in the π - orbitals of Borazole are derived from three nitrogen atoms and not from each of the six atoms of the ring as benzenes. So Borazole is also known as Inorganic benzene. Acidic character of Boron Tri halides Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel Boron trihalides are BF3, BCl3, BBr3, and BI3. In these halides, fluorine is more the electronegative atom. So, as the increase in EN of halogen connected to the Boron atom, increases the acidity, hence acidity is like BF3 ˃BCl3 ˃BBr3 ˃BI3. But the acidic strength of boron tri halides is BF3˂ BCl3˂ BBr3˂ BI3. This is due to the fact that the energies of B and F are the same. So these orbitals overlap together and form a Pπ-Pπ bond. By which it gets a partial double bond character. The resonance forms of BF3 are as follows. Resonance structures of BF3 Due to the formation of the partial double bond, the electron deficiency in BF3 is somewhat nullified. It results in a decrease in acidic character. An increase in the size of the halogen atom decreases the formation of the partial double bond and increases the acidity. So, the order of Boron tri halides acidity is BF3˂ BCl3˂ BBr3˂ BI3. Boron Nitride (Inorganic graphite) Preparation: i) Boron nitride was prepared by heating(10000C) of Nitrogen with Boron. 2B+N2→2BN ii) Boric oxide reacts with carbon to form boron nitride. B2O3+3C+N2→2BN+3CO Structure: Boron nitride has a graphite structure. The hexagons forming the layers are composed of alternate boron and nitrogen atoms. The interatomic distance between the layers is 1.45 A0. The distance between two adjacent layers is 3.33A0. hexagonal rings in BN are stacked directly on top of one another, whereas in graphite the atoms of alternate layers lie between the centers of the hexagonal rings of adjacent layers. The atoms in successive layers are eclipsed unlike in that of graphite where they are staged. Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel ------------------------------------------------The end------------------------------------------------------- Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel  CARBON FAMILY-IV-GROUP ELEMENTS Carbides: Carbon reacts with other elements and forms Carbides. Carbides are divided into three types based on the nature of the bond. 1) Ionic Carbides 2) Covalent Carbides 3) Interstitial Carbides (or) Metallic Carbides Ionic carbides: IA, IIA, IIIA group elements and coinage metals like Cu, Ag, and Au form Ionic carbides. Ionic carbides produce hydrocarbon on hydrolysis. Based on the hydrocarbons formed during hydrolysis, Ionic carbides are subdivided into three types. a) Methanides b) Acetalides c) Allylides Methanides: These carbides produce Methane on hydrolysis. These have C-4 ions. Al4C3+12H2O→3CH4+4Al(OH)3 Acetalides: These carbides produce Acetylene on hydrolysis. These have C2-2 ion. Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel  CaC2+H2O→C2H2+ Ca(OH)2 Allylides: These carbides produce Allene on hydrolysis. These have C3-4 ions. Mg3C2+H2O→C3H4+Mg(OH)2 Covalent carbides: These are also called as molecular carbides Ex: B4C3, SiC. B4C3 is a very hard substance and has a structure similar to graphite. SiC is also known as carborundum. Interstitial Carbides or metallic carbides: Carbide ions occupy the interstitial site of the metal crystal. Ti, Zr, V, Mo, and W elements form these types of carbides Applications of Carbides: 1. Due to the high melting point, carbides are used in the preparation of instruments working at high temperatures. 2. Carbides are used as reductants in the extraction of metals. 3. Carbides are used to line the inner side of furnaces. 4. Al4C3 and CaC2 are used in the preparation of methane and Acetylene. Silicones: Silicones are organo silicon polymers containing Si-O-Si bonds. Preparation methods: 1) Direct Si process: Alkyl halide reacts with silicon in the presence of copper at 3000C to form dimethyl silane which on hydrolysis forms dihydroxy di methylsilane. 2) Aromatic Silylation : Silicon compounds with Si-H bonds are reacted with benzene in the presence of BF3 or BCl3 at 200 to 3000C to form phenyl substituted silanes. Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel  3) From Grignard reagent: Grignard reagent reacts with silicon tetrachloride to form silanes. Types of Silicones: Silicones are classified into three types. 1) Linear Silicones 2) Cyclic Silicones 3) Cross linked Silicones 1) Linear Silicones: Linear Silicones are obtained by the hydrolysis and subsequent condensation of Dimethyl dichloro silane. Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel  2) Preparation of cyclic silicones: cyclic silicones are formed when water is eliminated from the terminal –OH group of the linear silicones. Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel  3) Preparation of Cross linked silicones: Hydrolysis of alkyl trichloro silane CH3SiCl3 gives mono chloro silane triol which then undergoes polymerization to cross linked polymers. Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel Properties of silicones: 1. Silicones are stable to heat at least up to a temperature of 2000C. 2. These are non-toxic. 3. Silicone possess water repellency power. It is due to high content of alkyl or aryl groups. 4. Silicone products may be fluids, greases, resins or rubbers depends on the type of polymer. ------------------------------------------------The end------------------------------------------------------- Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel Hydroxyl amine (NH2-OH): A solution of sodium nitrite (NaNO2) is mixed with a solution of sodium carbonate. Sulphur dioxide is passed into this mixed solution till the solution becomes acidic. Sodium salt of hydroxyl amine disulphuric acid [HO-N(SO3Na)2] is hydrolysed with dilute sulphuric acid to form hydroxyl ammonium bisulphate (H3N(OH)-HSO4). Hydroxyl ammonium bisulphate (H3N(OH)-HSO4) is decomposed by baryta water to give hydroxyl amine. Na2CO3+SO2+H2O→NaHSO3+NaHCO3 NaNO2+3NaHSO3→HON(SO3Na)2 +Na2SO3 + H2O HON(SO3Na)2 + 2H2O→ H3N(OH)-HSO4 + Na2SO4 H3N(OH)-HSO4 + Ba(OH)2 → H2N-OH + BaSO4 + 2H2O Properties: It is a colourless, poisonous solid. It is soluble in water, alcohol and ether. Decomposition: it decomposes at 200C in to ammonia, nitrogen and water. 3NH2OH→NH3 + N2+3H2O Action of halogens: Hydroxyl amine explodes with halogens and reduces them to the corresponding hydracids. 2NH2OH +2Cl2→NH3 + N2O+H2O +4 HCl Reaxn with carbonyl compounds: NH2OH reacts with aldehyde and ketone to give corresponding oximes. NH2OH reacts with acetaldehyde to form acetaldoxime. NH2OH reacts with acetone to form acetonoxime. Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel  Applications of Hydroxyl amine 1. NH2OH is used as analytical reagent for the identification of carbonyl compounds. 2. NH2OH is used for the preparation of caprolactam which is a raw material for nylon- 6. 3. NH2OH is used in the estimation of metal ions. 4. NH2OH is used in the preparation of dimethyl glyoxime. Dr. M. Alivelu, Assistant Professor of Chemistry, Govt. Degree College, Gajwel