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Ch. 3 Alkane Structure vs. Reactivity

Ch. 3 Alkane Structure vs. Reactivity. Alkane Structure and Radical Halogenation R-H reacts with halogens just like methane DH o for Et-H = 98 kcal/mol DH o for Me-H = 105 kcal/mol ( D E = -25 kcal/mol) Only one possible product Halogenation of Propane: 2 o and 1 o hydrogens

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Ch. 3 Alkane Structure vs. Reactivity

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  1. Ch. 3 Alkane Structure vs. Reactivity • Alkane Structure and Radical Halogenation • R-H reacts with halogens just like methane • DHo for Et-H = 98 kcal/mol DHo for Me-H = 105 kcal/mol (DE = -25 kcal/mol) • Only one possible product • Halogenation of Propane: 2o and 1o hydrogens • 6 primary H • 2 secondary H • Statistical product distribution = 3:1 • But what are bond strengths? • 2o C-H = 94.5 kcal/mol • 1o C-H = 98 kcal/mol DE = -27 kcal/mol

  2. Secondary H abstraction is easier than Primary H abstraction • Secondary radical is more stable • Primary radical is less stable • Experimental ratio of products: • Relative Reactivity = • Secondary H about 4 time more reactive than Primary H • Chlorine has a 4:1 Selectivity for Secondary:Primary • Selectivity depends on X2, temp, etc…; not always 4:1 43% 57%

  3. Tertiary H abstraction: Halogenation of (CH3)3CH • DHo Tertiary = 93 kcal/mol DHo Primary = 98 kcal/mol • Statistical Product Distribution = 9:1 • Experimental = 64:36 Selectivity = • 3o:2o:1o = 5:4:1 for Chlorination • Selectivity or F2 and Br2 Halogenations • Reactivity: F > Cl > Br > I • F2 a) Unselective, products = statistical product distribution b) Early TS = reactants = alkanes = very similar reactivities 64% 36%

  4. Br2 • Very Selective • 3o:2o:1o = 1700:80:1 • Late TS = products = radicals = very different stabilities • Synthesis Using Radical Halogenation • Consider: selectivity, cost, convenience • I2 doesn’t work; F2 is expensive, corrosive, unselective • Cl2 is cheap (NaCl), but not very selective; Industrial Use • Lab Use only when you can get only one product • Use safer Cl. generators: Sulfuryl Chloride N-chlorosuccinimide (NCS)

  5. Br2 = cheap, selective, liquid • Reagent of choice in the lab, even though it’s not as reactive as Cl2 • NBS = N-bromosuccinimide solid alternative Br radical source • Ozone Depletion • Ozone, O3, blocks harmful 200-300 nm UV light • CFC’s (chlorofluorocarbons) Destroy atmospheric ozone • Refrigerants, propellants: CCl3F, CCl2F2, CCl2FCClF2 • Radical Chain Mechanism with ozone • Worldwide O3 down 6% since 1978 • Replacements: HCFC’s, HFC’s (best) React faster than CFC’s; Destroyed before reaching ozone layer

  6. Combustion • Oxidation of alkanes DE = heat of combustion = -DHocomb = relative energy content of the alkane • Trends • More C’s = larger -DHocomb • Isomers do not have the same -DHocomb

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