Carboxylic acid chlorides, cellulose

Acylation of cellulose by carboxylic acid chlorides can be performed under homogeneous or heterogeneous conditions. During this reaction, the HCl released is captured by a base, most commonly pyridine or A,A-dimethylaminopyridine (DMAP) (Fig. 36). 

Cellulose can be modified with organostannane chlorides, such as dibutyl or triphenyl derivatives [91,92], or with organotin halides in the presence of bisethylenediamine copper(II) hydroxide [93]. Epoxy-activated cellulose was prepared by reacting cellulose acetate fibers with sodium methoxide, followed by reacting it with epichlorohydrin in DMSO. This epoxy-activated cellulose has proved to be a useful intermediate to react with substances containing active hydrogen, such as amine, amino acid, or carboxylic acids [94], as shown in Fig. 3. Epoxidized cellulose has also been converted to a thiol derivative via reduction of a thiosulfate intermediate [95], and sulfoethylcellu-[ose has been obtained from sodium chloroethanesulfonate [96]. Cellulose... 

Related to ionic liquids are substances known as deep eutectic solvents or mixtures. A series of these materials based on choline chloride (HOCH2 CH2NMe3Cl) and metal chlorides, carboxylic acids or urea have been reported. The urea-choline chloride material has many of the advantages of better-known ionic liquids e.g. low volatility) but can be sourced from renewable feedstocks and is non-toxic and readily biodegradable. However, it is not an inert solvent and this has been exploited in the functionalization of the surface of cellulose fibres in cotton wool. Undoubtedly, this could be extended to other cellulose based materials, biopolymers, synthetic polymers and possibly even small molecules. 

Methylene blue does not react with aldehyde groups which are characteristic of hydrocellulose or oxidation of cellulose under acid conditions. Thus a sample with a high copper number and low methylene blue affinity indicates either hydrocellulose or acid oxidation. Hydrocellulose is usually accompanied by residual traces of acid. If there is a reduction in the methylene blue absorption brought about by boiling in dilute sodium hydroxide, it indicates the presence of acid, because this treatment has no effect upon the number of carbosyl groups. The latter will only be converted into sodium carbosylate which will revert to sodium chloride and carboxylic acid groups with the hydrogen chloride in the methylene blue hydrochloride. 

Esters of cellulose with interesting properties such as bioactivity and thermal and dissolution behavior can be obtained by esterification of cellulose with nitric acid in the presence of sulfuric acid, phosphoric acid, or acetic acid. Commercially important cellulose esters are cellulose acetate, cellulose acetate propionate, and cellulose acetate butyrate. Cellulose esters of aliphatic, aromatic, bulky, and functionalized carboxylic acids can be synthesized through the activation of free acids in situ with tosyl chloride, iV,iV -carbonyldiimidazole, and iminium chloride under homogeneous acylation with DMA/LiCl or DMSO/TBAF. A wide range of cellulose esters that vary in their DS, various substituent distributions, and several desirable properties can be obtained through these reactions. Recently, a number of enzymes that degrade cellulose esters have been reported. Some of them are acetyl esterases, carbohydrate esterase (CE) family 1, and esterases of the CE 5 [169-172] family. 

The idea of generating a corrqrosite matmal based exclusively on cellulose fibres precursors arose in recent years through the concept of the partial chemical modification of the fibres, not limited this time to their surface OH groups, but, instead, to an in-depth process, itself limited to a modest thickness. In this way, a matmal could be generated in which the inner core maintains the original mechanical propaties of the urrmodified parts of the cellulose fibres, while its outer sleeve could now possess thamoplastic features given a suitable choice of chemical modification. Examples of this approach include esterification with carboxylic acids [17] and etherification with berrzyl chloride [18]. 

DMSO/TBAF is highly efficient as a reaction medium for the homogeneous esterification of cellulose by transesterification and after the in situ activation (see below) of complex carboxylic acids. The acylation using acid chlorides and anhydrides is limited because the solution contains a certain amount of water caused by the use of the commercially available TBAF trihydrate and the residual moisture in the air-dried polysaccharides. Nevertheless, this system has shown a remarkable capacity for the esterification of lignocellulosic mataials, for example, sisal fibres, which contain about 14 per cent hemicellulose [28]. The DS values of cellulose acetate prepared from these fibres with acetic anhydride in mixtures of DMSO/TBAF were found to decrease with increasing TBAF concentration from 6 to 11 per cent (Table 16.4), due to the increased rate of hydrolysis both of the anhydride and the ester moieties. 

The S5mthetic approach of in situ activation of carboxylic acids is based on the preliminary reaction of the carboxylic acid with a specific reagent to give an intermediate reactive daivative which can be prepared prior to the reaction with cellulose or converted directly in a one-pot process. This approach opens the way to a broad variety of new esters, because for numwous acids, for example unsaturated or hydrolytically unstable ones, reactive derivatives such as anhydrides or chlorides simply cannot he synthesized. The mild reaction conditions apphed for the in situ activation prevent common side reactions hke pericychc reactions, hydrolysis, and oxidation. Moreover, due to their hydrophobic charactCT, numa-ous anhydrides are not soluble in organic media used for cellulose modification, resulting in unsatisfactory yields and insoluble products. In addition, the conversion of an anhydride is combined with the loss of half of the acid during the reaction. Consequently, in situ activation is much more efficient. 

One of the early attempts of in situ activation was the reaction of carboxylic acids with sulphonic acid chlorides that was adopted for the homogeneous modification of cellulose. The exclusion of the base simplified the reaction medium and the isolation procedure [29]. There is an ongoing discussion about the mechanism which initiates esterification of cellulose with the carboxylic acids in the presence of p-toluenesulphonic acid chloride (TosCl). The mixed anhydride of p-toluenesulphonic acid (TosOH) and the carboxylic acid is favoured [30]. However, from H NMR experiments with acetic acid/TosCl, it was concluded that a mixture of acetic anhydride (2.21 ppm) and acetyl chloride (2.73 ppm) was responsible for the high reactivity of this system (Figs 16.2 and 16.3). 

Esterification of cellulose dissolved in A,Af-dimethyl acetamide (DMA)/LiCl via the iminium chlorides of different carboxylic acids (adapted from [40]) ... 

Starch esters are synthesized by reaction of a carboxylic acid or an acyl chloride or an acid anhydride with the hydroxyl groups of amylose or amylopectin chains. As in the case of cellulose derivatives, there are organic and inorganic esters depending on the origin of the substituent. 

Cellulose esters of carboxylic acids are prepared by reacting fibrous cellulose with acyl chlorides, usually in an organic base medium, e.g. equation (3)... 

Cellulose esters of carboxylic acids with 7c-electron structures, e.g. where R in the acyl chloride is a benzoyl, cinnamoyl, furoyl or thenoyl group, have been prepared. " The effects of these groups on the properties of cellulose, particularly on the absorption, transfer and localization of energy from ionizing radiations in cellulose, have been reported. Dependent on mechanical restrictions of the fibers during chemical modification and the DS of the products, many of the initial mechanical properties of the fibers are retained. 

The imminium chloride formed was transformed, in-situ, into the corresponding carboxyhc acid derivative, this was added to a solution of cellulose in LiCl/DMAc. Palmitic, stearic, adamantane-1-carboxylic, and 4-nitrobenzoic acids were employed. The DS of the corresponding esters increased as a function of increasing the ratio oxalyl chloride/AGU. The solubihty of the products obtained in aprotic solvents was tested GPC results have indicated negligible degradation of the polymer [200]. 

Cellulose can be esterifled with almost any organic acid [81,82,84], Whereas this is possible in principle with both alkyl chlorides and carboxylic anhydrides, commercial practice has focused on the anhydride option. Although many esters have been described in the literature, industrially manufactured organic esters are prepared only with aliphatic fatty acids with between 2 and 4 carbon atoms in length (i. e., acetates to butyrates, CA to CB). Exceptions are some mixed esters with phthalic acid, which are used for enteric coatings in pharmaceutical applications and a novel carboxymethyl cellulose acetate butyrate (CMCAB), which is used in water-borne coatings applications [81,84],... 

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