Seminary about (aut)oxidation of benzaldehyde

F. Celerse
Ludwig-Maximilians-Universität München, München, Allemagne, & Laboratoire de Chimie Théorique - UMR7616 UPMC & CNRS - Paris
Mercredi 22 Mars 2017, 11h00
bibliothèque LCT, tour 12 - 13, 4^ème étage

Framework

The work presented during this seminary has been performed during my first year in the CAPT master at the University Pierre et Marie Curie, at the University Ludwig-Maximilians of Munich, under the tutelage of Hendrik Zipse and his post-doc student, Sandhyia. Here are the main mail adresses :
zipse@cup.uni-muenchen.de

salach@cup.uni-muenchen.de

frederic.celerse@etu.upmc.fr

This work can be cited as "S. Lakshmanan, F. Celerse, H. Zipse, manuscript in preparation".

1 Introduction

The (aut)oxidation of benzaldehyde is a relevant industrial process, principally used in the synthesis of benzoic acid [1-3]. It is also used in the synthesis of benzyl alcohol, which is the key step in this (aut)oxidation process [2]. The (aut)oxidation of such hydrocarbons is an important process in the oxidative degradation of oils and fats and the oxidative transformation of volatile hydrocarbons in the atmosphere. Most of these reactions proceed in a radical pathway, through their respective initiation, propagation and termination steps.

2 What do we suggest ?

The (aut)oxidation of hydrocarbons proceed via radical chain reactions. The efficiency of the (aut)oxidation reactions depend on the efficiency of the respective initiation, propagation and termination steps. In this regard, the (aut)oxidation of toluene is found to proceed via the bimolecular reactions of benzyl hydroperoxide [4]. As a continuation of this (aut)oxidation scheme, the oxidation of benzyl alcohol into benzoic acid is an important industrial process [2]. The benzaldehyde undergoes (aut)oxidation by a free radical chain mechanism as shown in Figure 1.

[Picture]

Figure 1. Free radical chain mechanism involved in the (aut)oxidation of benzaldehyde.

As noted from Figure 1., the (aut)oxidation cascade of benzaldehyde mainly involves the benzoyl peroxy radical, which further react with benzaldehyde forming perbenzoic acid and benzoyl radical. The benzoyl radical is the key intermediate and again continues the chain reactions. In the absence of any inhibitors or catalysts, the benzoyl oxy radical can directly abstract an H-atom from benzaldehyde yielding benzoic acid and thus the cycle is complete. Furthermore, the benzoyl peroxy radical can add to the carbonyl group yielding an adduct, which again undergoes rearrangement and fragmentation into benzoic acid and benzoyloxy radical.
The initiation reactions of benzaldehyde by triplet oxygen result in benzoyl and hydro peroxy radicals. The homolytic dissociation of perbenzoic acid results in benzoyl oxy and hydroxyl radicals. These radicals further initiate the oxidation of benzaldehyde. The molecule induced homolysis is an effective means of radical initiator in the (aut)oxidation reactions. In this respect, the self-initiation of perbenzoic acid can proceed via H-atom abstraction by the terminal OH-group of one perbenzoic acid from the other, forming benzoyl oxy and benzoyl peroxy radicals with the water co-product. The self-reaction of perbenzoic acid can also lead to the formation of benzoic acid leading a benzoyl peroxy radical and a hydroxyl radical. These initiation reactions are illustrated in Figure 2.

[Picture]

Figure 2. Initiation reactions involved in the (aut)oxidation of benzaldehyde.

Sankar M. et al. suggested in their work that benzoic acid is formed through a radical addition reaction of perbenzoic radical with a benzaldehyde molecule, and then a fragmentation of the O-O bond of the new peroxide formed creates the benzoic acid and gives a benzoyl radical (see Figure 3.), which can react with a perbenzoic acid in order to create a new perbenzoic radical and, in the same line of Figure 2., thus offering another free radical chain cycle (see Figure 1.).
The thermochemistry of the reactions shown in Figures 1 to 3 has been calculated using high level quantum chemical approaches. After determining the thermochemistry, the kinetics of the favourable reactions have been studied and also the all possible bimolecular reactions involved in the (aut)oxidation of benzaldehyde have been elucidated in detail [3].

[Picture]

Figure 3. Propagation reactions for the carbonyl attack reaction.

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References :
[1] I. Hermans, J. Peeters, L. Vereecken, P. A. Jacobs, Chem. Phys. Chem. 2007, 8, 2678.
[2] Sankar M. et al., Nat. Commun. 5:3332 doi: 10.1038/ncomms4332 (2014).
[3] S. Lakshmanan, F. Celerse, H. Zipse, manuscript in preparation.
[4] S. Lakshmanan, H. Zipse, Chem. Eur. J. 2015, 21, 14060.