CH Compounds containing nitrogen Summary of Group III trends

Transcription

1 CH Compounds containing nitrogen Summary of Group III trends

2 Molecular species containing B-N bonds Aminoboranes, R 3 B NR 3, are isoelectronic with hydrocarbons, R 3 C CR 3. H 3 N-BH 3 is a solid at room temperature, polar molecule H 3 C-CH 3 is a gas that condenses at 89 C, nonpolar The striking difference in the properties is due to hydrogen bonding between H 3 B- NH 3 molecules (recall that strongly hydrogen bonded H 2 O is a liquid under normal conditions, whereas H 2 S is a gas, because of weaker H-bonding) 2

3 Borazine We now extend the discussion to include compounds with boron-nitrogen multiple bonds. Borazine, B 3 N 3 H 6 is a colorless liquid with an aromatic odor and physical properties that resemble those of benzene, C 6 H 6. Borazine can be prepared by different methods, for example: 3H 3 N BH 3 (heating) (HBNH) 3 + 6H 2 3

4 Borazine Borazine, (HBNH) 3, is isoelectronic and isostructural with benzene, C 6 H 6. δ δ δ δ δ δ B N 1.42 Å B H 1.20 Å N H 1.01 Å C C 1.39 Å C H 1.08 Å In borazine, the difference in electronegativities of boron (2.0) and nitrogen (3.0) leads to a charge distribution which makes the B and N atoms susceptible to a nucleophilic and an electrophilic attack, respectively. 4

5 π-molecular orbitals of borazine The π-mo s of borazine are very similar to those of benzene. Bonding MO s have more electron density on N, whereas the antibonding MO s are more concentrated on the less electronegative B atom, as expected. Energy Unoccupied π-antibonding orbitals Occupied π-bonding orbitals 5

6 Borazine Borazine is much more reactive than benzene. The following are representative reactions of borazine: 1. (HBNH) 3 + 3HCl (ClHBNH 2 ) 3 addition reaction 2. 2(ClHBNH 2 ) 3 + 6NaBH 4 2(H 2 BNH 2 ) 3 + 6NaCl + 3B 2 H 6 reduction cyclotriborazane B N 1.57 Å 6

7 Compounds with B=N and B N bonds Compounds with double and triple B N bonds are known and have been structurally characterized. They are more reactive than the corresponding alkenes and alkynes, and are typically stabilized by bulky groups on the atoms of B and N. t-butyl-(t-butylimino)borane Di-isopropylamino-bis(trifluoromethyl)borane 1.26 Å 1.37Å 7

8 B-N bonds Covalent radii: 0.82 Å (B) Å (N) = 1.57 Å B-N bonds in selected species: Species B-N distance / Å Me 3 N BBr Single σ-bond Me 3 N BC Single σ-bond Hexagonal-(BN) n 1.45 σ + some π-bonding B(NMe 2 ) σ + some π-bonding Mes 2 B=NH Double bond Mes 2 B=N=BMes Double bond t BuB N t Bu 1.26 Triple bond Mes = 2,4,6-Me 3 C 6 H 2 8

9 Group III: Some trends 1. The chemistry of boron is quite different from that of the heavier Group III elements. It differs from aluminum in the following ways. a. Boron oxides and hydroxides are acidic whereas those of aluminum are amphoteric. b. Boron is a non-metal and semi-conductor, whereas aluminum is a metal. Boron is very inert and only attacked by hot concentrated oxidizing acids. c. No simple salts of B 3+ are known, whereas those of Al 3+ are numerous. d. Boron forms a wide range of molecular hydrides. AlH 3 has a polymeric structure which resembles that of AlF 3. e. The stereochemistries of many boron compounds are based on trigonal sp 2 and tetrahedral sp 3 geometries. Aluminum forms many compounds with tetrahedral, trigonal bipyramidal, and octahedral geometries. f. π-bonding in B N, B O and B F compounds is much more significant than that for the corresponding aluminum compounds. 9

10 Group III: Some trends 2. Aluminum, gallium, indium, and thallium all form a range of compounds in the +3 oxidation state. However, compounds in the +1 oxidation state become progressively more stable down Group III. 3. The oxides of aluminum and gallium are amphoteric and indium and thallium oxides are more basic. 4. The octahedral aqua-ions [M(OH 2 ) 6 ] 3+ are acidic. The pk a values for the equilibria: [M(OH 2 ) 6 ] 3+ [M(OH 2 ) 5 (OH)] 2+ + H + are Al, ~5; Ga, 3; In, ~4; Tl, The MX 3 compounds are Lewis acids and the Lewis acid strengths decrease in the order: Al > Ga > In. 6. The stability of the hydrides decreases down the Group and there are no stable In-H and Tl-H compounds. Ga 2 H 6 is extremely unstable. 7. Aluminum is resistant to corrosion because of an impermeable oxide layer, but is soluble in non-oxidizing mineral acids (e.g. HCl). Ga and In dissolve readily in acids, but Tl dissolves only slowly in H 2 SO 4 and HCl. 10