Reaction with na yields the trialkyl eg 3meda12c12 6na

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Unformatted text preview: e maximum photon flux in sunlight occurs at 75-95Mmol-' and GaAs shows promise, though other factors make Cu2S-CdS cells more attractive commercially at the present time. 1.. 736 Organometalliccompounds 259 products: trace of 2A1+ 3RC1- 12 AICI3 or AIR3 R3A12C13 & TR4A12C12 1 + tR2A12C14 Addition of NaCl removes R2A12C14 as the complex (2NaAlC13R) and enables R4A12C12 to be distilled from the mixture. Reaction with Na yields the trialkyl, e.g.: 3MedA12C12 + 6Na --+ 2A12Me6 + 2 A1+ 6NaCl Higher trialkyls are more readily prepared on an industrial scale by the alkene route (K. Ziegler et al., 1960) in which H2 adds to A1 in the presence of preformed AlR3 to give a dialkylaluminium hydride which then readily adds to the alkene: In each case A1-C, is about 10% longer than A1-C, (cf. Al2X6, p. 235; B2H6, p. 157). The enthalpy of dissociation of into monomers is 84kJmol-'. A12Eh (mp -53") and A12PG (mp -107") are also dimeric at room temperature but crystalline trimesitylaluminium (mesityl = 2,4,5-trimethylphenyl) is monomeric with planar 3-coordinate Al; the mesityl groups adopt a propeller-like configuration with a dihedral angle of 56" between the aromatic ring and the AlC3 plane and with A1-C 199.5 pm.(73) As with Al(BH4)3 and related compounds (p. 230), solutions of A12Me6 show only one proton nmr signal at room temperature due to the rapid interchange of bridging and terminal Me groups; at -75" this process is sufficiently slow for separate resonances to be observed. can be prepared on a laboratory scale by the reaction of HgMe2 on A1 at -90°C. A12Ph6 can be prepared similarly using HgPh2 in boiling toluene or by the reaction of LiPh on A12C16. On the industrial (kilotonne) scale A1 is alkylated by means of RX or by alkenes plus H2. In the first method the sesquichloride R3A12C13 is formed in equilibrium with its disproportionation 7 3 J . J. JERIUS, J. M. HAHN, . F. M. M. RAHMAN, . MOB, A 0 W. H. ISLEY and J. P. OLIVER, rganometallics 5 , 1812-14 O (1986). 2A1+ 3H2 150" + 2A12Ek +{6Et2AlH) 6CH2CH2 70" 3A12Eb Similarly, Al, H2 and Me2C=CH2 react at 100" and 2OOatm to give AlBui in a singlestage process, provided a small amount of this compound is present at the start; this is required because AI does not react directly with H2 to form AlH3 prior to alkylation under these conditions. Alkene exchange reactions can be used to transform AlBu; into numerous other trialkyls. AlBui can also be reduced by potassium metal in hexane at room temperature to give the novel brown compound K2A12Bud (mp 40") which is notable in providing a rare example of an Al- A1 bond in the diamagnetic anion [B~iAlAlBui]~.(74) Al2R6 (or AlR3) react readily with ligands to form adducts, LAlR3. They are stronger Lewis acids than are organoboron compounds, BR3, and can be considered as 'hard' (or class a) ? I t is interesting to note that the reaction of EtI with AI metal to give the sesqui-iodide "EgA1213" was the first recorded preparation of an organoaluminium compound (W. Hallwachs and A. Schafarik, 1859). 74 H. HOBERC S . KRAUSE, ngew. Chem. Int. Edn. Engl. and A 17, 949-50 (1979). Aluminium, Gallium, lndium and Thallium...
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This note was uploaded on 10/12/2012 for the course CHEMISTRY CHM1040 taught by Professor Jhon during the Spring '12 term at Florida A&M.

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