Question

Following are balanced equations for fluorination of propane to produce a mixture of 1-fluoropropane and 2-fluoropropane. CH_3CH_2CH_3 + F_2 rightarrow CH_3CH_2CH_2F + HF CH_3CH_2CH_3 + F_2 rightarrow CH_3^FCHCH_3 + HF Assume that each product is formed by a radical chain mechanism. (a) Calculate lambda H^0 for each reaction. (b) Propose a pair of chain propagation steps for each reaction and calculate lambda H^0 for each step. (c) Reasoning from Hammond's postulate, predict the regioselectivity of radical fluorination relative to that of radical chlorination and bromination.

Answer 1

process same

H Br Solved Prohlems 733 CH, H,C CH3 Br The order of reactivity of halogen in halogenation reaction of alkanes is CH3 Fluorination reaction is too violent and iodine actually does not react at all. The reaction of halogens with alkanes show a wide spread of relative reactivities. Fluorine is very reactive, in fact reaction is violent. Chilorine is much more reactive than bromine. Iodine is so unreactive that under ordinary conditions no reaction takes place. The wide variation in reactivity of the halogens towards alkanes can be explained by a careful examination of AF and Exe for each step CHj HC H BrBr CH, Br CH3 CHy Fluorination: Step O 2F Step () F CHCH,HF Step (iii) CH+FCHP+F Br CH3 + 159 +159 Br H 134 + 50 H Br CH3 CH, H,C Chlorination (R. R)-2,3-Dibromobutane Step ) C2 Step (ii) .CI + CH4_→ HCI + CH3 Step (iii) CH, +Cl2-CHC1 + Cl 99. Write the order of reactivity of halogens in the halogenation of hydrocarbons. Ans. The halogenation of alkanes involves the replacement of hydrogen atom (s), from +243 + 243 an alkane molecule, by halogen atom(s). 106 Net H-102 The mechanism of halogenation involves three steps (0) chain initiation: Bromination: Br-→ 2Br. + 192 Step (i) Step (ii) Step (iii) Chi +BrCH,Br+ B +192 (ii) chain propagation: .Br +CH4_→ CH3 + HBr +69 78 100 H-31 Halogen radical abstracts a hydrogen atom from the alkane molecule thereby producing an alkyl radical. The alkyl radical abstracts a halogen atom from halogen molecule, thus producing alkyl halide and a halogen atom