chapter_6_alkyl_halides_nucleophilic_substitutions_and_eliminations

Chapter_6_alkyl_halides_nucleophilic_substitutions_and_eliminations

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Chapter 6:Alkyl Halides Nucleophilic Substitution and Elimination
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Nomenclature (6-2) Alkyl Halides are also known as Haloalkanes h Molecules containing: F, Cl, Br, I h Nomenclature RCHX R RCX R RCH 2 X 247 R 2 o halide 3 o halide 1 o halide Dihalides: R X X R C X C X geminal vicinal
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Structure of Alkyl Halides (6-4) h Because X is more electronegative than C, the C-X bond is strongly polarized 248
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Synthesis of Alkyl Halides (6-6) We have already seen that alkyl halides are available via the corresponding alkanes by radical substitution. The reaction normally favours the alkyl halides formed from 249 the most stable radical, however the yields are usually low.
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h However, free radical haloganation of alkenes is a high yield process since it takes place via a very stable allylic radical (assuming that allylic hydrogen are present). The allylic position is the carbon directly attached to the alkene. 250
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h The mechanism of the reaction follows the same 3 steps (initiation, propagation and termination) that we have seen before, but the high yield in these reactions is due to the very stable allylic radical produced in the process. 251
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h Large quantities of Br 2 must be avoided as this reagent will also add to the double bonds of alkene (chapter 8). A good way to make large amounts of allylic bromide is by using NBS (N-bromosuccinimide), as this reagent keeps the concentration of 252 Br 2 low.
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Practice Question h Complete the following reactions. 253
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h There are many other ways to prepare alkyl halides from other functional groups. These will be covered later in other chapters: 254
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Reactions of Alkyl Halides (6-7) Alkyl halides can give 2 major types of reactions which often compete with each other Nucleophilic Substitutions ( S N 2 or S N 1 ) and Eliminations ( E 2 and E 1 ) 255
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h Because of this competition that is always present, it is often difficult to identify the major product in their reactions. h Analysis of reaction conditions is very 256 Analysis of reaction conditions is very important to decide on the product formed
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Second Order Nucleophilic Substitutions (S N 2) (6-8) General Mechanism: S N 2 257
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h In S N 2 the mechanism of this reaction is concerted ” ( i.e. bonds are formed and broken at the same time). In other words, no reactive intermediates are formed. 258
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