Alkyl Halides & Radical Rx’s chapter 8 Alkyl Halides & Radical Rx’s Structure Nomenclature Physical Properties Halogenation of Alkanes Mechanism of Halogenation Allylic Halogenation Examples Radical addition Note the Chapter Summary and Key Rx’s ss18 12

free radical substitution mechanism Chapter 8 synthesis free radical substitution mechanism Chapter 7 synthesis allylic benzylic Hammond radical addition

free radical substitution Chapter 8 synthesis classification nomenclature mechanism and details Hammond postulate free radical substitution

8.1 Structure of ‘R-X’ haloalkane R-X H3C-Cl haloalkene sp3 haloarene (a vinyl halide) sp3 (alkyl halide) haloarene sp2 (aryl halide)

Haloalkane (alkyl halide) sp3 8.1 Structure of ‘R-X’ Haloalkane (alkyl halide) sp3 R-X H3C-Cl methyl chloride X R C " ' 3o halide R C H X ' 2o halide R C H X 1o halide

8.2 Nomenclature #-haloalkane #-halocycloalkane IUPAC - halides (X) are substituents Substituent names: halo fluoro, chloro, bromo, iodo #-haloalkane #-halocycloalkane (R)-4-bromo-1-chloro-4-fluoro-1-cyclopentene structure ?

Common Names: (alkyl halide) or (special names) (isopropyl bromide) vs 2-bromopropane H 3 C B r (chloroform) vs trichloromethane H-CCl3

8.3 Physical Properties “polar covalent bond” - dipole - mismatch of electronegativity -size H 3 C    ) H C B r H 3 C

8.4 Halogenation of Alkanes substitution of X for H hv = ultraviolet light,  = heat X2 = Cl2, Br2 seldom F2 (too reactive - exothermic) or I2 (endothermic, unreactive)

Substitution, products and by-products + other R-X’s hv Cl2 hv Cl2

A few rx’s are useful, e.g.: Generally halogenation not useful - mixtures (separate) A few rx’s are useful, e.g.: + Br2  Br + HBr + other Br's Others - allylic & benzylic

Substitution, products and by-products monobromination + HBr + diBr + etc + HBr + diBr +... (92) (8) + HBr + diBr +... bromination favors 3o > 2o > 1o

initiation propagation and/or terminations

Regioselective for 3o > 2o > 1o C-H (92) (8) (57) (43)

radical stability like carbocations - R. is electron deficient (not charged)

Order of stability of R(+) / R. same order

free radical substitution mechanism Chapter 8 synthesis free radical substitution mechanism Chapter 7 synthesis Hammond allylic benzylic radical addition

Selectivity 3o > 2o > 1o, but Cl and Br are different major mono-X product Br2 1600 80 1 Cl2 5 4 1 3o 2o 1o Cl. more reactive less selective than Br.

Hammond’s Postulate Cl vs Br Hammond’s Postulate: the structure of the transition state: for an exothermic reaction looks more like the reactants of that step - for an endothermic reaction looks more like the products of that step

Hammond’s Postulate In halogenation of an alkane, the rate-limiting step is hydrogen abstraction this step is endothermic for bromination and exothermic for chlorination.  H° (kcalmol)

Hammond’s Postulate For chlorination (hydrogen abstraction is exothermic): transition state resembles the alkane and chlorine atom little radical character on carbon in t.s. regioselectivity only slightly influenced by radical stability

Hammond site of collision important PE stability of R. important early t.s. - like SM site of collision important PE SM prog rx Prod SM prog rx Prod PE late t.s. stability of R. important

(free radical substitution) Halogenation (free radical substitution)

Allyl Radical - resonance

initially H-Br with progress rx

NBS for Br2

Free Radical Stability

Free Radical Stability allylic benzylic

Radical ADDITION - note rx conditions!!! Add H-X in the presence of peroxides reverses normal addition “anti-Markovnikov” Markovnikov “normal”

Radical ADDITION - note rx conditions!!! Add H-X in the presence of peroxides reverses normal addition “anti-Markovnikov” X = Cl, Br, I end