| Phosphorous Tribromide |
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Phosphorus tribromide is a clear colourless liquid. It is a Covalent compound of Phosphorus and Bromine with the formula P Br 3. It fumes in air due to decomposition by water and it has a penetrating odour. It is widely used in the laboratory for the conversion of Alcohol s to Alkyl Bromide s. The phosphorus atom has an NMR chemical shift of 227 ppm (downfield of H3PO4 ). CHEMICAL PROPERTIES Phosphorus tribromide, like or Lewis acid in many of its reactions. The most important reaction of PBr3 is with Alcohols , where it replaces an OH group with a bromine atom to produce an Alkyl Bromide . Note that all three bromines can be transferred. PBr3 + 3 ROH → 3 RBr + HP(O)(OH)2 The mechanism (shown for a primary alcohol) involves initial activation of the alcohol oxygen by the electrophilic phosphorus (to form a good leaving group), followed by an SN2 Substitution at the alcohol carbon. Because of the SN2 substitution step, the reaction generally works well for Primary And Secondary Alcohol s, but fails for tertiary alcohols. If the reacting carbon centre is Chiral , the reaction usually occurs with Inversion Of Configuration at the alcohol carbon, as is usual with an SN2 reaction. In a similar reaction, PBr3 also converts Carboxylic Acid s to Acyl Bromide s. PBr3 + 3 RCOOH → 3 RCOBr + HP(O)(OH)2 PBr3 is a reasonably strong Reducing Agent , and the oxidation of PBr3 with Oxygen gas is more vigorous than seen with PCl3. It gives an explosive reaction that forms P2O5 and Br2 . PREPARATION PBr3 is made by the reaction of elemental Phosphorus with Bromine , using PBr3 itself as the solvent (white phosphorus is soluble in PBr3). An excess of phosphorus is used in order to prevent formation of PBr5. P4 + 6 Br2 → 4 PBr3 USES The main use for phosphorus tribromide is for conversion of primary or secondary Alcohol s to Alkyl Bromide s as described Above . PBr3 usually gives higher yields than Hydrobromic Acid , and it avoids problems of Carbocation rearrangement- for example even Neopentyl bromide can be made from the alcohol in 60% yield[6 . Another use for PBr3 is as a catalyst for the α-bromination of Carboxylic Acid s. Although acyl bromides are rarely made in comparison with Acyl Chloride s, they are used as intermediates in the Hell-Volhard-Zelinsky Reaction {Link without Title} . Initially PBr3 reacts with the carboxylic acid to form the acyl bromide, which is more reactive towards bromination. The overall process can be represented as On a commercial scale, phosphorus tribromide is used in the manufacture of Pharmaceutical s such as Alprazolam , Methohexital and Fenoprofen . PRECAUTIONS Highly corrosive, toxic, reacts violently with water and alcohols. Wear gloves and goggles and work in a fume cupboard (hood), use an apron and face shield with larger amounts. Keep away from strong Oxidising Agents . In reactions that produce Phosphorous Acid as a by-product, when working up by distillation be aware that this can decompose above about 160 °C to give Phosphine which can cause explosions in contact with air. {Link without Title} SUPPLIERS/MANUFACTURERS REFERENCES # N. N. Greenwood, A. Earnshaw, ''Chemistry of the Elements'', 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997. # ''Handbook of Chemistry and Physics'', 71st edition, CRC Press, Ann Arbor, Michigan, 1990. # J. March, ''Advanced Organic Chemistry'', 4th ed., p. 723, Wiley, New York, 1992. # ''The Merck Index'', 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960. # R. R. Holmes, ''Journal of Inorganic and Nuclear Chemistry'' 12, 266-275 (1960). # L. G. Wade, Jr., ''Organic Chemistry'', 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005. # George C. Harrison, H. Diehl, in ''Organic Syntheses Collective Volume 3'', p 370, Wiley, New York, 1955. # L. G. Wade, Jr., ''Organic Chemistry'', 6th ed., p. 1051, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005. |
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