Chem 334 - Summer 1998 - Organic Chemistry I Dr. Carl C. Wamser

Chapter 3 - Alkanes. Reactions

homolytic and heterolytic bond dissociations

bond dissociation energies (DHo)
order of radical stability: 3o > 2o > 1o > CH3
reactivity/stability correlations
structure of alkyl radicals (sp2)
hyperconjugation

petroleum

composition, cracking - pyrolytic reactions

relative stabilities of alkanes

heats of combustion
heats of formation

Halogenation of alkanes

CH4 + Cl2 --(hv)--> CH3Cl + HCl

R-H + X2 --(hv)--> R-X + HX

overall substitution - replacement of H by halogen

initiated by light (or heat)

works best for Cl2
slow for Br2
unreactive with I2
explosive with F2

usually multiple products (not very useful synthetically)

e.g., CH4 --> CH3Cl --> CH2Cl2 --> CHCl3 --> CCl4

mechanism: free radical chain reaction

initiation (generation of radicals)

propagation (reaction of radicals)

products are generated in the propagation steps
the overall net reaction is the sum of these steps

termination (loss of radicals)

chlorination of methane

Initiation:

Cl-Cl + hv ----> 2 Cl

Propagation:

Cl + CH4 ----> CH3 + H-Cl

CH3 + Cl-Cl ----> CH3-Cl + Cl

(repeat these two propagation steps)

Termination:

e.g., 2 CH3 ---> CH3CH3 (minor product)

 

calculation of H for each step

propagation step 1 is the rate-determining step

transition state structures, potential energy diagrams

control of monosubstitution (use excess methane)

halogen reactivity

F2 > Cl2 > Br2 > I2

comparative kinetics of propagation step 1
late vs early transition states (Hammond Postulate)

alkane reactivity (removal of different H's)

3o > 2o > 1o > CH3-H

comparative kinetics of propagation step 1
selectivity/reactivity correlations

selectivity of halogenation reactions

calculations of product mixtures

synthetic halogenation reactions

Br2 very selective, works well
chlorination usually by SO2Cl2