7. Haloalkanes - 2010 Department of Chemistry, The...

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© 2010 Department of Chemistry, The University of Western Ontario 7.1 7. Haloalkanes (text 7.1 – 7.9) A. Structure and Nomenclature Like hydrogen, the halogens have a valence of one. Thus, a halogen atom can replace a hydrogen in a molecule. The general formula of a saturated mono-halo acyclic compound is C n H 2n+1 X. So, treat X as an H when calculating unsaturation units. e.g. C 3 H 4 Br 2 = one unit of unsaturation Non-IUPAC, two-word names are commonly used for simple compounds. The first word identifies the alkyl group, the second identifies the halogen. Cl Br IUPAC: 2-chlorobutane bromocyclohexane Common: sec- butyl chloride cyclohexyl bromide The reactions that alkyl halides undergo strongly depend on the nature of the alkyl position. It’s essential that we recognize the “type” of alkyl halide.
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7.2 1. Primary halide (1°). The carbon bearing the halogen is bonded to one alkyl group and has two H. CH 3 CH 2 Br (CH 3 ) 3 CCH 2 Br CH 2 I 2. Secondary halide (2°). The carbon bearing the halogen is bonded to two alkyl groups and one H. Cl Br 3. Tertiary halide (3°). The carbon bearing the halogen is bonded to three alkyl groups (and no H). CH 3 Cl Br All the above have halogens bonded to sp 3 carbon (alkyl). We need to know that there can be aryl or vinyl halides, which have rather different chemistry because the halogen is bonded to an sp 2 carbon (covered later on).
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7.3 B. Physical Properties The halogens are more electronegative than carbon, so an sp 3 C–X bond is polar. The polar bond does not cause alkyl halides to be H 2 O-soluble. Halogenated organic compounds rarely occur in nature, and never in mammalian cells. This is why many organic halides are highly toxic and are very slowly destroyed, e.g. DDT. We study alkyl halides because they are very useful in laboratory syntheses. As well, they serve as excellent examples of natural reaction mechanisms. C. Characteristic Reactions Haloalkanes can undergo substitution reactions, where a nucleophile replaces the halogen (the leaving group). Nucleophiles can be either neutral or negatively charged, but they must have at least one nonbonding pair of electrons. Leaving groups are all atoms/groups that create a + charge on an sp 3 carbon atom. CH 3 CH 2 Br + -
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7.4 The above reaction is a template for a wide range of reactions differing in Nu and LG. If a neutrally charged nucleophile is used, there is usually a deprotonation step after the substitution reaction. Because nucleophiles have nonbonding electrons, they can also act as bases. Therefore, substitution and -elimination are frequently competing reactions.
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7.5 D. Substitution Mechanisms How does substitution occur? That is, how does the nucleophile replace the leaving group? Two possibilities: 1. Nu waits until LG departs C on its own , after which Nu comes and takes the empty spot; or 2. Nu kicks out the LG through backside attack.
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7. Haloalkanes - 2010 Department of Chemistry, The...

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