1.3.2 Vanilin
Vanillin (4‐hydroxy‐3‐methoxybenzaldehyde) is an organic compound consisting of a benzene ring substituted with three functional groups: aldehyde –CHO, hydroxyl –OH, and methoxy –O–CH3 (Figure 1.18a).
It is a naturally occurring compound (in the form of its β‐D‐glucoside, Figure 1.18b) that can be directly obtained in the extraction process from the bean or seed pods of Vanilla planifolia, the tropical orchid presently cultivated in a number of tropical countries. Although this method has been known for centuries and it is still used, actually less than 1% of vanilla produced in the world is obtained in such a way [79]. Almost all vanillin is now synthesized much more cheaply through chemical processes. Synthetic vanillin is commercially available and is commonly used in both food and nonfood applications, in fragrances, as a flavoring in pharmaceutical preparations, as an intermediate in the chemical and pharmaceutical industries for the production of herbicides, antifoaming agents or drugs, and in household products, such as air fresheners and floor polishes. Synthetic or semisynthetic vanillin can be derived from two compounds: guaiacol and eugenol, both available from petrochemical sources or of natural origin.
Figure 1.16 Route of the synthesis epoxy monomers from selectively hydrodeoxygenated lignin.
Figure 1.17 Lignin modification and cross‐linking: (a) ozone oxidation of Kraft lignin and (b) synthesis of multiple carboxylic acid derivatives.
Figure 1.18 Chemical structures of (a) vanillin and its naturally occurring precursors: (b) vanillin glucoside, (c) guaiacol, (d) eugenol, and (e) coniferyl alcohol.
Figure 1.19 Synthesis of vanillin from guaiacol.
The first one, guaiacol (2‐methoxyphenol) (Figure 1.18c), is a naturally occurring organic compound present in an aromatic oil from flowering plants Guaiacum. Guaiacol can also be gained from creosotes formed by distillation of various tars and pyrolysis of plant‐derived material, such as wood. Semisynthetic vanillin can be obtained from guaiacol through the Reimer–Tiemann reaction of phenols formylation (Figure 1.19) [80].
The reaction is carried out using chloroform deprotonated by a strong base (hydroxide typically) to form the chloroform carbanion and finally the dichlorocarbene, which is the principal reactive specie in nucleophilic substitution also occurred in deprotonated phenol. Another method of vanillin synthesis from guaiacol is its reaction with glyoxylic acid (Figure 1.20a), leading to the formation of 2‐hydroxy‐2‐(4‐hydroxy‐3‐methoxyphenyl)‐acetic acid (Figure 1.20b) [81]. The obtained vanillylmandelic acid is converted via 2‐(4‐hydroxy‐3‐methoxyphenyl)‐2‐oxoacetic acid (Figure 1.20c) to vanillin by the oxidative decarboxylation [82].
Eugenol (2‐methoxy‐4‐(prop‐2‐en‐1‐yl)phenol) (Figure 1.18d) present in an essential oil extracted from the clove plant Syzygium aromaticum is the next important natural raw material for the vanillin synthesis (Figure 1.21).
The synthesis consists of two steps: the basic isomerization of the double bond in eugenol leading to the formation of isoeugenol and oxidation of the rearranged double bond to vanillin [83, 84]. The process can be carried out with or without isolation of the intermediate product which is isoeugenol [85].
Figure 1.20 Synthesis of vanillin from guaiacol using glyoxylic acid.
Figure 1.21 Synthesis of vanillin from eugenol.
Lignin from softwood is still one of the most important sources of raw materials for the synthesis of vanillin. Three‐dimensional network structures of lignin are composed of three types of monolignols: p‐coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. Coniferyl alcohol (Figure 1.18e) is the main intermediate in the pathways to vanillin from a softwood (coniferous) lignin, as well as the precursor for eugenol in its biosynthesis. Vanillin can be produced from the lignin‐containing waste manufactured by the sulfite pulping process for preparing wood pulp for the paper industry [86]. This first developed method of vanillin synthesis from lignin lost its relevance primarily for environmental reasons (the need to safely get rid of alkaline‐based liquid waste). However, thanks to the results of work on the process optimization [87], it was possible to achieve an increase in the yield of vanillin and a reduction in waste stream volume. Therefore, the volume of vanillin production from lignin is still estimated at around 15% of the total world vanillin production.
There are also known for years [88] and still developed biotechnological processes of vanillin synthesis [89]. They are promising for the industrial‐scale production of vanillin due to the use of natural raw materials, renewable and readily available in large quantities, such as rice bran [90] or corn sugar [91]. However, actually bio‐synthesized vanillin is still very expensive [92] and its high cost of production are justified only for specific applications in the food, cosmetics, and pharmaceutical industries. Nowadays, the biotechnological production is not suitable and profitable source of vanillin for the synthesis of polymeric materials.