The effect of aldehyde structure on pyrolysis reactions

•Pyrolysis products of isovaleraldehyde and pivaldehyde are identified.•Aldehydes with branched alkyl groups decompose similarly to unbranched aldehydes.•Pivaldehyde produces fewer unique products than isovaleraldehyde.

The thermal decomposition of gas-phase isovaleraldehyde, (CH3)2CHCH2CHO, and pivaldehyde, (CH3)3CCHO, was studied in the 1270-1470 K range to determine the effect of alkyl chain structure on the decomposition mechanism of aliphatic aldehydes. Pyrolysis was performed with a pulsed hyperthermal nozzle and the products were identified via matrix-isolation Fourier transform infrared spectroscopy. The products of isovaleraldehyde pyrolysis were carbon monoxide, isobutene, water, 3-methyl-1-butyne, isopropylketene, vinyl alcohol, propene, acetaldehyde, acetylene, propyne, methane, ethylene, and ketene. The products of pivaldehyde pyrolysis were carbon monoxide, isobutene, propene, acetaldehyde, acetylene, propyne, methane, ethylene, and ketene. The reduced number of products from pivaldehyde, as compared to isovaleraldehyde, is due to the fully branched chain structure of the pivaldehyde backbone. The differences between the results for isovaleraldehyde and pivaldehyde lend support to the proposed reactions occurring early in the pyrolysis mechanism. However, the presence of acetylene, propyne, methane, ethylene, and ketene cannot easily be explained and points to the need for further exploration of the mechanisms.