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Unformatted text preview: LCU Nucleophilic substitution and elimination competing reactions Alkyl halides (halogenoalkanes) are R-X where X is Cl, Br, I 1 o , 2 o and 3 o refer to the number of carbon atoms attached to the or head carbon that carries the halogen. Alternatively, it can be seen as the number of hydrogens attached to the carbon. 1 o one C attached to C two hydrogens 2 o two C attached to C one hydrogen 3 o three C attached to C no hydrogens F and Cl bonds with C are strongly polar and F and Cl alkyl halides have permanent dipoles. Br and I alkyl halides are less polar. As the halogen becomes bigger, the dipole is due more to the number of electrons present around the halogen atom (with I this is 53, a considerable amount of negativity). The basic reaction of alkyl halides is substitution where a negative ion attacks the alkyl halide. Fluoroalkanes are very unreactive despite having the largest dipole. Several reasons apply: overall reaction is endothermic due to strength of the C-F bond; that F is a poor leaving group because F- is a strong-ish base and forms a weak acid HF etc. The rate of reaction I>Br>Cl is related to activation energy. The C-I bond is weak and so activation energy to break it is low. Substitution reactions involve an attacking group and a leaving group. The attacking group is either a negative ion (anion) e.g. OH- and CN- or another group with an unbonded pair of electrons. There is a displacement series for the nucleophiles and leaving groups. Ones on the left will, as nucleophiles, displace those on the right as leaving groups....
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- Spring '11