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CHEM 2331H
T.Andrew Taton

Chapter 6 Book Notes Alkyl Halides: Nucleophilic Substitution and Elimination 6.1) Introduction • Halogenated organic compounds have 3 major classes o alkyl halide: halogen atom bonded to one of the sp hybrid C atoms of alkyl group o vinyl halide: halogen atom bonded to one of sp hybrid C in alkene 2 o aryl halide: halogen atom bonded to one of sp hybrid C of aromatic ring o Chemistry of vinyl and aryl halide different from alkyl halide b/c bonding & hybridization = different • C-halogen bond in alkyl halide = polar b/c halogen = more electronegative than C o most rxns involve breaking this polarized bond o nucleophile can attack C (electrophilic)  halogen leaves as halide ion, takes bonding electron pair w/ it o halogen (leaving group) can be eliminated/substituted (replaced) by functional group  this versatility allows alkyl halides to = intermediates in synthesis of other functional groups 6.2) Nomenclature ofAlkyl Halides • Haloalkane: alkane name and prefix (fluoro-, chloro-, bromo-, iodo-) before it as substituent • Common/ “trivial” names: name of alkyl group + halide  alkyl halide (ex: isopropyl bromide) • Other names not related to structure o methylene halide: has structure CH X (m2t2ylene group + 2 halogens) o haloform: formula CHX 3 o carbon tetrahalide: CX 4 • Alkyl halides classified by nature of C bonded to halogen o o primary halide: when halogen-bearing C bonded to 1 C (C is primary 1 ) o secondary halide: ……2 C (…secondary 2 ) o o tertiary halide:……3 C (…tertiary 3 ) o o methyl halide: when halogen-bearing C bonded to methyl group (bonded to no other C atoms) • Geminal dihalide: has 2 halogen atoms bonded to same C • Vicinal dihalide: 2 halogens bonded to adjacent C’s 6.3) Common Uses of Alkyl Halides • 6.3A) Solvents (industrial and household) o Carbon tetrachloride (CCl ) o4ce used for dry cleaning, spot removing, other domestic cleaning; toxic & carcinogenic (causes cancer)  dry cleaners use other solvents now (1,1,1-trichloroethane) o Methyl chloride (CH Cl )2an2 chloroform (CHCl ) = goo3 solvents for cleaning & degreasing  methyl chloride once used to dissolve caffeine from coffee beans  decaffeinated coffee, but safety concerns about methyl chloride residue  liquid CO 2  chloroform = more toxic & carcinogenic than methyl chloride  methyl chloride/other solvents preferred o All halogenated solvents (even the safest) = potentially toxic & carcinogenic, dissolve fatty oils that protect skin  form of dermatitis • 6.3B) Reagents o Used in many syntheses as starting materials to make more complex molecules o ex: converting alkyl halides  organometallic reagents (compounds w/ C-metal bonds) = particularly important tool for organic synthesis • 6.3C) Anesthetics o Chloroform (CHCl ) use3 to produce general anesthesia, but toxic and carcinogenic  diethyl ether used instead o Mixed alkyl halide (CF CHC3Br aka Halothane) = less toxic halogenated anesthetic o Ethyl Chloride used as topical anesthetic for minor procedures; has low temp bp, so when sprayed on skin, evaporates and cools area  better numbing effect • 6.3D) Freons: Refrigerants and FoamingAgents o Freons (aka chlorofluorocarbons or CFCs): fluorinated haloalkanes that replace ammonia (toxic, leaking refrigerators killed people working/sleeping nearby) as refrigerant gas  Freon-12 (R), CF C2 o2ce very widely used o Low boiling freons (ex: Freon-11 (R), CCl F) u3ed to = foaming agents added to plastic  vaporize  form froth  hardens into plastic foam o Freons released into atmosphere (stratosphere)  Cl catalyzes decomposition of ozone (O 3 into oxygen (O )2 depletes ozone/ozone hole o Now international treaties limit future ozone-destroying Freon production   Freon-12 in aerosol cans replaced by hydrocarbons/carbon dioxide  Freon-12 in refrigerators replaced by Freon-22 (CHClF ) b2c have C-H bonds (generally called HCFCs) destroyed @ lower altitudes before reaching stratosphere  Freon-11 replaced in plastic foams by propane, CO , a2d HCFC-123 (CHCl C2 )3 • 6.3E) Pesticides o Before, famine, disease, plagues caused death and prevented exploration o Arsenic compounds, nicotine, and crude insecticides used but harmful to humans and other animals too o DDT : extremely toxic to insects, but not as much for humans/mammals  can dust on people/rodent burrows to prevent diseases  But residues accumulate in environment  Banned by U.S. EPAbut still used in places w/ human life-threatening insect-borne diseases o Other chlorinated insecticides: some accumulate in environment, others = safer  rarely used in agriculture  used when potent insecticide needed for protection (ex: lindane: lice shampoo, chlordane: protecting wooden buildings from termites) 6.4) Structure of Alkyl Halides • alkyl halides: halogen bonded to sp hybridized C  halogen more electronegative than C  polarized • electronegativities of halogens: I
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