Conformational equilibria and structure of 2,3-butadienal and 2,3-butadienoyl halides

The conformational equilibria and stability of 2,3-butadienal and 2,3-butadienoyl fluoride and chloride were investigated using ab initio calculations with the extended 4-31G and 6-31G^* basis sets. The results indicate that the molecules show s-trans ⇌ s-cis conformational equilibria similar to the corresponding allylic molecules. The conformational stability of these molecules was found to be more dependent on the nature of the substituent and less dependent on the type of π system. The replacement of the allylic group by an allenic group slightly influenced the conformational equilibrium between the two stable conformers of the molecules. For 2,3-butadienal the s-trans conformer was predicted to be predominantly more stable than the s-cis conformer at ambient temperatures. For 2,3-butadienoyl halides the energy difference between the two conformers was calculated to be very small with slight thermodynamic stability favouring the s-trans form at room temperature.