Ultrasonic relaxation and the barrier to internal rotation in some halogenated butanes

Abstract

The rates of interconversion of the rotational isomers of the 2-halogeno-2-methylbutanes and of the 2,3-dihalogenobutanes have been measured by means of the ultrasonic relaxation technique. The potential barriers hindering rotation, ΔH⁺_YX, for the less stable → more stable isomeric transitions have been derived from the temperature dependence of these rate constants. These ΔH⁺_YX values (in kcal./mole) are 2-chloro-2-methylbutane, 3·89 ± 0·19; 2-bromo-2-methylbutane, 5·57 ± 0·19; 2-iodo-2-methylbutane, 8·37 ± 0·25; meso-2,3-dichlorobutane, 5·13 ± 0·06; meso-2,3-dibromobutane, 6·43 ± 0·38. The ‘frequency factors’ derived from the rate equations are not consistent with two mechanisms that have been proposed for internal rotation. For the (±)-2,3-dihalogenobutanes a single ultrasonic relaxation was observed in the frequency range 5–300 MHz. This has been interpreted as an average of the relaxation processes associated with the three rotational isomers of this diastereoisomer.