Unformatted text preview: The species formed by a homolytic bond cleavage of a neutral molecule are free radicals.
Therefore, we measure energy cost of homolytic bond cleavage to gain information about
stability of free radicals. The more stable the free-radical products, the weaker the bond, and the
lower the bond-dissociation energy. Table 4.3 For example: Bond-dissociation energy measurements tell us that isopmpyl radical is 13 kJ/mol
more stable than propyl. two he Hoﬁcm m L‘s-7mm. m Pyn;d>* 1209”» re exact“ :r was»: Propyl radical (primary) 1
CH-ﬁlli‘h’; + H. 33 k5/1710} lsopropyl radical (secondary)
( 3 kcalluaul) —-—1—-——-—-—-—-— (1135mm + H-
4l0kjimcl 1 (98 kcalimol} 39? kl/mol
(95 keel/incl) __L CH3CH2CH3
Propane 4.17 Mechanism of Chlorination of Methane
Free-radical chain mechanism. Radical substitution. 3R Ste 1: Dissociation Of a chlorine molecule into me chlorine atoms: kWehEm amour: L‘ER'lmPDQ’TW)
Cl: _—_» 3121:} Energy ——-> ; . :21): .
Chlorine molecule Two chlorine atoms The initiation step "gets the reaction going"by producing free radicals—«chlorine atoms from chlorine molecules in this case. Initiation step is followed by propagation steps. Each propagation step consumes one free radical but generates another one. Wt Wm ﬂ ...
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- Summer '08
- Organic chemistry