Mechanisms of Exchange Reactions of Primary and Secondary Alkyl Iodides with Elementary Iodine
Several primary and secondary alkyl iodides exchange thermally with I/ sup 131/ in hexachlorobutadiene between 130 and 200 deg . If the solutions are saturated with oxygen at one atmosphere, rates of exchange fit the kinetic expression k/sub b/STARI! STAl/sub 2/!1/2. Degassed solutions always exchange faster than oxygen saturated ones, but methyl, ethyl, and n-propyl iodides show the same kinetics as with oxygen. Exchange rates of degassed isopropyl and neopentyl iodides also show contributions from a k/sub a/STARI! term. Exchange in degassed ethylene dichloride is 3 to 4 times as fast as in degassed hexachlorobutadiene. Activation energies for k/sub b/ are usually about 27 to 31 kcal/mole. Effects of substitution on alpha carbon are illustrated by the rate sequence methyl < ethyl < i-propyl = sec-butyl. Effects of substitution on beta carbon are illustrated by the rate sequence ethyl < npropyl>> neopentyl. Since the rates of exchange of methyl, ethyl, and i-propyl iodides vary in the opposite direction from the sequence for bimolecular nucleophilic substitution, the explanation proposed suggests that for nucleophilic substitution the effect of added methyl groups on an alpha carbon is a steric hindrance to solvation by solvent dipoles rather than a steric hindrance to the group attacking the carbon atom itself.
- Research Organization:
- Columbia Univ., New York; and Univ. of Oregon, Eugene
- Sponsoring Organization:
- USDOE
- NSA Number:
- NSA-15-016885
- OSTI ID:
- 4019865
- Journal Information:
- Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 7 Vol. 83; ISSN 0002-7863
- Publisher:
- American Chemical Society (ACS)
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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