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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 α or head carbon that carries the halogen. Alternatively, it can be seen as the number of hydrogens attached to the α carbon. 1 o one C attached to α C two hydrogens 2 o two C attached to α C one hydrogen 3 o three C attached to α 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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This note was uploaded on 09/24/2011 for the course BIO 100 taught by Professor John during the Spring '11 term at Harvard.
- Spring '11