Acrylic, methacrylic, fumaric, maleic and muconic acids

For acrylic and methacrylic acids, the yields of the product based on ozone consumption are compiled in Table 6.4.

Mechanistically, acrylic and methacrylic acids follow the same pattern [reactions (53)–(59) and reactions

The second zwitterions are given in brackets as they may not be intermediates, but they indicate the directions the ozonides will take upon their decay. The absence of formic and acetic acids in the case of acrylic and methacrylic acids, respectively, shows that the other conceivable route, for example, reactions (60)–(62) is not taken in these acids.

C

Table 6.4 Product yields (mol per mol O₃consumed) in the reaction of O₃with acrylic and methacrylic acids according to Leitzke & von Sonntag (2009)

Product Acrylic acid

Anion (free acid)

Methacrylic acid Anion (free acid)

Formaldehyde^(a) 0.52 (0.72) 0.43 (0.62)

Hydroxymethylhydroperoxide 0.43 (ND) see text (ND)

1-Hydroperoxypropan-2-one – 0.57 (ND)

Glyoxylic acid 0.54 (ND) –

2-Oxopropanoic acid – 0.41 (ND)

Gylcol aldehyde 0.48 (ND) –

Methylglyoxal – 0.54 (ND)

Formic acid absent (absent) –

Acetic acid – absent (absent)

Total Peroxide 1.01 (1.0) 1.04 (1.0)

H₂O₂ 0.58 (ND) 0.46 (ND)

(a)Includes the yield of hydroxymethylhydroperoxide that decomposes into formaldehyde during the assay, ND=not determined,–product cannot be formed.

Moreover, cleavage along the O–O and C–C bonds of the envisaged dioxetane intermediate [cf. reaction (57)] does not take place.

Depending on the protonation state of the carboxylic group, there is some bias in the branching between reactions (55) and (56). As expected, the deprotonated carboxyl group decarboxylates more readily favouring reaction (56). Formaldehyde, which is produced from reaction (55), can be measured with some accuracy. A plot of the formaldehyde yield as a function of pH is shown in Figure 6.3.

The inflection point is at pH 3, markedly distant from the pKavalue of acrylic acid, 4.25 (dashed line). It may reflect the pKavalue of the ozonide, as the zwitterion is most likely not an intermediate, but if it were it would be too short-lived for the pKaequilibrium to become established during its lifetime.

The reactions of methacrylic acid ozonide are depicted in reactions (63)–(67).

base catalysed

Figure 6.3 The pH dependence of the formaldehyde yield in the ozonolysis of acrylic acid. The dashed line indicates how this dependence would look if it were governed by the pK_avalue of acrylic acid. From Leitzke &

von Sonntag, 2009 with permission.

Interestingly, hydroxymethylhydroperoxide, formed in reaction (64), cannot be detected by its slow reaction with molybdate-activated iodide. It is too short-lived here, as the buffering properties of methacrylic acid that decompose hydroxymethylhydroperoxide into formaldehyde and H2O2 [reaction (60)] are much more pronounced than in the case of acrylic acid. This has been shown by production of hydroxymethylhydroperoxide in the reaction of buten-3-ol with ozone [reaction (31)] and subsequent addition of methacrylic acid. The latter rapidly catalyses reaction (66) and hydroxymethylhydroperoxide is no longer detected [note that the somewhat lower value of hydroxymethylhydroperoxide as compared to that of formaldehyde in the case of acrylic acid (Table 6.4) may be due to the, albeit much lower, buffering effects of acrylic acid. Due to its higher pKavalue, methacrylic acid is a better buffer, and thus a more efficient catalyst, than acrylic acid in close to neutral solutions].

The hydroperoxide attributed to 1-hydroperoxypropan-2-one formed in reaction (65) reacts much faster with molybdate-activated iodide (k=0.15 M⁻¹s⁻¹) than hydroxymethylhydroperoxide (k=3.4×10⁻³ M⁻¹s⁻¹). Interestingly, the methyl substituent in this hydroperoxide has a marked effect on its decay.

Compared to aldehydes, ketones form hydrates (and thus also hydroxyalkylhydroperoxides) less efficiently [in water, formaldehyde is practically fully hydrated, CH2(OH)2, acetaldehyde ∼50%

hydrated, acetone,0.1% hydrated (Bell, 1966)]. Thus, the reaction analogous to reaction (57) will be inefficient, and water elimination occurs instead [reaction (67)]. The buffer properties of methacrylic acid may speed up this reaction. There are other α-hydroxyalkylhydroperoxides, where water elimination rather than H2O2 elimination is observed. A case in point is the muconic compound formed in the ozonolysis of phenol (Mvula & von Sonntag, 2003). Reasons for this are as yet not known, but one may recall that H2O2elimination is reversible while water elimination is not. A more detailed study of H2O2

with aldehydes would be required to elucidate this interesting aspect.

The reactions of O3with maleic and fumaric acids in aqueous solution has found some attention (Gilbert, 1977; Leitzke & von Sonntag, 2009; Ramseier & von Gunten, 2009), but mechanistic details were only understood recently (Leitzke & von Sonntag, 2009; Ramseier & von Gunten, 2009). These two isomeric acids are available in high purity. In aqueous solution, cis⇄trans isomerisation takes place. This may lead to an artefact, that is, the other isomer increases linearly with the ozone dose, when the work-up of the samples follows the sequence of ozone doses (Leitzke, 2003). It is now clear, that ozone does not inducecis⇄trans isomerisation. These two isomers give rise to the same products, but their yields differ remarkably (Table 6.5).

With fumaric acid, glyoxal is formed in 27% yield, while with maleic acid it is only 3%. This is a strong indication that these two isomers give rise to two ozonides that differ with respect to their preferred decay routes. This is in line with the concept that ozone addition to the double bond is a concerted reaction

Table 6.5 Product yields (mol per mol O3 consumed) in the reaction of O₃with fumaric and maleic acids at pH 5.3 according to Leitzke & von Sonntag (2009)

Product Fumaric acid Maleic acid

Formic acid 0.77 0.92

Glyoxylic acid 0.85 1.05

Glyoxal 0.27 0.03

Total peroxide 0.31 ND

ND=not determined.

[reactions (68) and (69)]. In any case, a zwitterionic intermediate, if it is an intermediate, must be too short-lived to allow a rotation around the former C–C double bond.

Maleic acid

We are not yet in a position to present a mechanistic explanation for this difference, but the general mechanism suggested by reactions (70)–(74) accounts for the observed products.

(70)

The ozonolysis oftrans,trans-muconic acid in aqueous solution has been studied under the conditions of extensive ozonation (Gilbert, 1980). No distinction between primary and secondary products has been made. The primary products and their yields in the ozonolysis of cis,cis-muconic acid (Leitzke et al., 2003) are compiled in Table 6.6, and additional information on primary as well as secondary products is also available (Ramseier & von Gunten, 2009) [reactions (75)−(79)].