1 2008 Lectures 10 11 12 Alkyl halides - Chem 2203/2207...

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Chem 2203/2207 Answers to Test 1 and Marks have been posted on 1 http://http-server.carleton.ca/~pbuist/
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Alkyl Halides 2 Chapter 10 and 11
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Alkyl Halides (R X ) Nomenclature Physical Properties Mechanisms of Alkyl Halide reactions 3 (Stereochemistry and Kinetics) Top Ten Reactions
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Introduction I Organohalides do occur naturally but most are of synthetic origin Alkyl halides: 4 Thyroxine Halothane (2-bromo-2-chloro- 1,1,1-trifluoroethane) Bromomethane Aromatic halides:
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Introduction II Chlorinated aromatic hydrocarbons persist in the environment because they are resistant to degradation by atmospheric oxygen 5 PCB
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Degrees of Unsaturation For organic molecules containing only CHOS, degree of unsaturation = [Max # hydrogens (2n +2) - Obs # hydrogens] / 2 6 For organohalides, Add the number of halogens in the formula to the number of obs. hydrogens. • E.g. C 6 H 13 Cl n = 6 carbons, ((2 x 6) +2) – (13 + 1))/2 = 0 degrees of unsaturation Cl
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Nomenclature I Structure IUPAC other Methane CH 3 H CH 3 F Fluoromethane Methyl fluoride 7 CH 3 Cl CH 3 Br CH 3 I Chloromethane Methyl chloride Bromomethane Methyl Bromide Iodomethane Methyl Iodide
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Nomenclature II • Primary alkyl halide Secondary alkyl halide 8 • Tertiary alkyl halide
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Bond lengths of Alkyl Halides • C-X bond length increases in the series X = F, Cl, Br, I (1.4 Å to 2.1Å) • compare: C-H (1.1 Å) 9 • As bonds lengthen, bonds become easier to break Recall: 1 Angstrom ( Å) = 100 pm = 0.1 nm
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Alkyl Halide Properties 10 Alkyl halides are polar molecules Recall: dipole moment = Q (charge separation) x r (distance between atoms)
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Electronegativity of Halogens 11
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C-X bond is polarized 12 Affects physical properties and chemistry!
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Physical Properties: Alkyl Halides (R X ) Physical Properties are influenced by three types of intermolecular forces: • Van Der Waals (induced dipole) ipole ipole 13 • dipole-dipole • hydrogen bonding O-H---X
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Physical Properties II • Bp and mp increase with increasing atomic weight of halide ( Vander Waals and dipole/dipole interactions) mp ( ° C) bp ( ° C) CH -182 -164 C X δ δ + 14 4 CH 3 F -141 -78 CH 3 Cl -97 -24 CH 3 Br -94 -3.5 CH 3 I -66 42 δ + δ - Dipole/Dipole Interactions (Intermolecular forces) X C δ δ + -
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Physical Properties III Density: Many alkyl halides are heavier than water! • Important polar solvents: CH 2 Cl 2 , CHCl 3 15 Not soluble in water (typically) – H-bonding is not efficient (except with F, which is small) due to size of halide. X R O H H .. .. : .. ..
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Physical Properties of Typical Alkyl Halides: Summary 1. Bp and Mp > corresponding alkane 16 2. Higher Density than water 3. Insoluble in water
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Chemistry of Alkyl halides • “Typical” alkyl halide behaviour is exhibited by alkylchlorides and alkyl bromides and is dominated by heterolytic cleavage of C-X bond 17 • Fluorides and Iodides tend to behave somewhat anomalously: • (C-F bond very strong (Teflon)) • (C-I bond very weak)
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A cid No
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1 2008 Lectures 10 11 12 Alkyl halides - Chem 2203/2207...

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