Aluminum is important in industrial economies as the metal and as a component of alloys. Its compoun — Organic Chemistry Chemistry Question
Structure of aluminum halides
Aluminum is important in industrial economies as the metal and as a component of alloys. Its compounds are widely used as catalysts in the production of organic compounds and polymers. For example, aluminum chloride (AlCl3) is a catalyst in Friedel-Crafts alkylations. Organoaluminum compounds, such as Al2(CH3)6 and [(C2H5)2AlCl]2, are used in organic synthesis and as components of Ziegler-Natta polymerization catalysts.
A. Aluminum halides
2.1 In the solid state, aluminum chloride, AlCl3, has a layer lattice with six-coordinate aluminum (m.p. = 192 °C; sublimes at 180 °C), but a luminum chloride in the vapor state is a dimer, Al2Cl6. Draw the Lewis structure for the dimer and describe the bonding in this compound using Lewis and VSEPR (valence shell electron pair repulsion) theories.
Model Answer
Aluminum chloride in the vapor phase and aluminum bromide in the solid state are both dimeric. Both have AlX3 units bound by Al–X–Al bridges. You can imagine the Al atoms are sp3 hybridized. Three of the sp3 hybrid atomic orbitals (HAOs) are used in sigma bonding to chloride or bromide. The fourth HAO is empty but can interact with a lone pair of electrons from a bridging X atom in a neighboring AlX3 molecule.
2.2 Aluminum bromide, AlBr3, is a low melting solid (m.p. = 98 °C, sublimes at 255 °C), whereas aluminum fluoride, AlF3, has a very high melting point (m.p. = 1291 °C). I s the structure and bonding in aluminum fluoride and aluminum bromide likely to be similar to aluminum chloride?
Model Answer
Aluminum fluoride is a high melting solid, which implies that it is an ionic compound. Each Al3+ ion is surrounded octahedrally by F– ions. Each F– ion bridges between two Al3+ ions. (Structure of AlF3 taken from Wikipedia Commons.)
B. An organoaluminum halide
If [(C2H5)2AlCl]2 is treated with NaF, the air-sensitive fluorine analog, [(C2H5)2AlF]x, is isolated. As noted in question A above, aluminum halides are at least dimeric under many conditions, as is (C2H5)2AlCl. Is [(C2H5)2AlF]x also dimeric or could it be monomeric, trimeric, tetrameric, and so on?
2.3 The molar mass of [(C2H5)2AlF]x was determined by measuring the freezing point depression of a solution in benzene. A 1.097 g sample of the compound dissolved in 65.26 g of benzene had a freezing point of 5.276 °C . (In this experiment, the freezing point of benzene was 5.500 °C, and the calibrated f reezing point depression constant was –5.57 °C /molal.) What is the value of x in [(C2H5)2AlF]x?
Model Answer
Structure of diethylaluminum fluoride was originally investigated by A. W. Lauben-gayer and G. F. Lengnick, Inorg. Chem., 1966, 5, 503-507.
Change in freezing point = –0.224 °C = (–5.57 °C/molal)
Molal concentration = 0.0402 mol kg–1
Amount of organoaluminum fluoride = 0.0402 mol kg–1 × 0.06526 kg = 0.00262 mol
Molar mass of unknown compound = 418 g mol–1
Molar mass of (C2H5)2AlF = 104 g mol–1
Unknown compound is [(C2H5)2AlF]4. It is a tetramer.
2.4 Sketch a possible Lewis structure for [(C2H5)2AlF]x.
Model Answer
The most likely structure is an 8-sided polygon with F atoms bridging Al atoms (as in AlF3).