Ch. 3 Alkane Structure vs. Reactivity I.Alkane Structure and Radical Halogenation A.R-H reacts with halogens just like methane 1)DH o for Et-H = 98 kcal/mol DH o for Me-H = 105 kcal/mol (  E = -25 kcal/mol) 2)Only one possible product B.Halogenation of Propane: 2 o and 1 o hydrogens 1)6 primary H 2)2 secondary H 3)Statistical product distribution = 3:1 4)But what are bond strengths? a)2 o C-H = 94.5 kcal/mol b)1 o C-H = 98 kcal/mol  E = -27 kcal/mol

5)Secondary H abstraction is easier than Primary H abstraction a)Secondary radical is more stable b)Primary radical is less stable 6) Experimental ratio of products: a)Relative Reactivity = b)Secondary H about 4 time more reactive than Primary H c)Chlorine has a 4:1 Selectivity for Secondary:Primary d)Selectivity depends on X 2, temp, etc…; not always 4:1 43%57%

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

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

4)Br 2 = cheap, selective, liquid a)Reagent of choice in the lab, even though it’s not as reactive as Cl 2 b)NBS = N-bromosuccinimide solid alternative Br radical source II.Ozone Depletion A.Ozone, O 3, blocks harmful nm UV light B.CFC’s (chlorofluorocarbons) Destroy atmospheric ozone 1)Refrigerants, propellants: CCl 3 F, CCl 2 F 2, CCl 2 FCClF 2 2)Radical Chain Mechanism with ozone 3)Worldwide O 3 down 6% since )Replacements: HCFC’s, HFC’s (best) React faster than CFC’s; Destroyed before reaching ozone layer

III.Combustion A.Oxidation of alkanes  E = heat of combustion = -  H o comb = relative energy content of the alkane B.Trends 1)More C’s = larger -  H o comb 2)Isomers do not have the same -  H o comb