Chapter 5 - Alkynes and Compounds Containing C≡C Groups

Abstract

Substitution of a halogen atom on the 3-carbon atom splits the degeneracy and both in plane and out-of-plane ≡C–H bending modes are observed in the infrared (IR). With the exception of 3-iodopropyne the in-plane bending mode occurs at a higher frequency than the out-of-plane bending modes. 3-iodopropyne exhibits the C–H bending modes at 637 cm-1, while the Cl, Br, and F analogs exhibit the in-plane bending mode at 652, 649, and 674 cm-1, respectively, and the out-of-plane bending mode occurs at 637, 639, and 636cm-1. Substitution of deuterium for hydrogen ≡C–H to ≡C–D helps in establishing the fundamental vibrations that result from this portion of the molecule. In the case of 3-chloropropyne-1-d and 3-bromopropyne-l-d the v≡C–D modes occur at 2618 and 2607 cm-1, respectively. In the case of ≡C–D bending, the Cl and Br analogs exhibit the in-plane mode at 516 and 512cm-1, respectively, while the out-of-plane mode occurs at 502 and 503cm-1, respectively.