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Use of Bond Equations in Solving Problems Ray A. Gross Jr Prince George’s Community College 1 How many bonds are present in benzene, C6H6? 2 Consider Water, H2O 1 2 1 H O H normal covalence B = 1/2(1 + 2 + 1) = 2 B = 1/2NVV 3 For C6H6 • B = ½[(6 x 4) + (6 x 1)] • B = ½(24 + 6) • B = 15 H H H H H H 4 Alternative Solution Hydrocarbons • Total Bonds = Bonds (corresponding saturated, acyclic compound) minus number of p bonds and rings in the structure. 5 C6H14 B = 19 C6H6 B = 15 Lose one bond for every p bond or ring. Let B = bond deficiency, BD. BD = p + r B = Bsat - BD 6 BD for Hydrocarbons Alkane: CnH2n + 2 Hydrocarbon: CnH2n + 2 - 2p - 2r h = 2n + 2 - 2p - 2r p + r = n +1 - h/2 BD = n + 1 - h/2 7 B = Bsat - BD B = 3n + 1 - [n + 1 - h/2] B = 2n + h/2 C6H6 B = 2(6) + 6/2 = 15 8 BD for All Compounds BD = 1 – ½(X – N – 2C – 3P – 4S) X = number of monovalent atoms N = number of trivalent atoms C = number of tetravalent atoms P = number of pentavalent atoms S = number of hexavalent atoms 9 Taxol, C47H51NO14 , has 7 rings. How many s bonds does it have? p + r = 1 – ½(X – N – 2C – 3P – 4S) p + r = 1 – ½[(51 – 1 – 2(47)] = 23 p = 23 – 7 = 16 B = ½[47(4) + 51 + 3 + 14(2)] = 135 s = 135 – 16 = 119 10 How many bonds are in HPO4-2? B = ½[1 + 5 + 4(2) – 2] = 6 O H O P OO11 What’s all of this good for? • Increases students knowledge of structure and bonding. • Possibly use B to classify reactions. 12 Derive an Eqn for Btot in an Alkyne. B = Bsat – BD B = 3n + 1 – 2 B = 3n - 1 13 Dehydration of Cyclohexanol O H H B = -2 + B = 2 C6H12O C6H10 B = 19 B = 17 H O H 14