  We present a temperature dependence study on the gas phase reactions of the
C$_2$({\it a}$^3Pi_u$) radical with a selected series of saturated alkanes
(C$_2$H$_6$, C$_3$H$_8$, n-C$_4$H$_10$, i-C$_4$H$_10$, and n-C$_6$H$_14$) by
means of pulsed laser photolysis/laser-induced fluorescence technique. The
bimolecular rate constants for these reactions were obtained between 298 and
673 K. A pronounced negative temperature effect was observed for n-C$_4$H$_10$,
i-C$_4$H$_10$, and n-C$_6$H$_14$ and interpreted in terms of steric hindrance
of the more reactive secondary or tertiary C-H bonds by less reactive CH$_3$
groups. Detailed analysis of our experimental results reveals quantitatively
the temperature dependence of reactivities for the primary, secondary, and
tertiary C-H bonds in these saturated alkanes and further lends support to a
mechanism of hydrogen abstraction.
