Carboxylic halogenated

Most commercial microporous membranes are hydrophobic and relatively inert. If the selected basic membrane does not possesses the functional groups necessary for spacers arms and hgand couphng, it can be activated to acquire reactive groups such as hydroxyl, carboxyl, halogen, or amine groups using similar methods as for particulate materials. The major methods employed to modify the porous membranes (some of them aheady mentioned) are based on... 

Hunsdiecker Reactions. The Hunsdiecker reaction is the de-carboxylative halogenation of metal carhoxylate salts. The reaction of a,/3-unsaturated carboxylic acids with NCS and catalytic lithium acetate in acetonitrile-water provides the corresponding /3-halostyrenes in moderate yields under mild conditions. The reaction proceeds with a good degree of stereospecificity. For example, the reaction of 3-(4-methoxy-phenyl)-acrylic acid with NCS with a catalytic amount of lithium acetate at room temperature provides l-(2-chloro-vinyl)-4-methoxybenzene in good yield (eq 23). 3... 

Regio- and stereo-selective functionalization of allylic and homoallylic alcohols (37 n = 1 or 2) to give 1,2- or 1,3-diols, respectively, has been reported via the carboxylation-halogenation sequence shown in Scheme 30. ... 

The ethylene oligomerization catalyst cycle in the presence of nickel catalyst, HNiYL (where, Y = acyl, carboxyl, halogen L = phosphine), is shown in the following reaction series [71] ... 

Three closely related reports have appeared on the stereoselective functionalization of allylic and homoallylic alcohols via iodo-carbonates (38) (Scheme 25). In an extension of earlier work (6,171), carboxylation-halogenation of allylic and homoallylic alcohols has been used as a route to triols (39). Homoallylic t-butyl carbonates (40) have also been found to undergo halogenation to afford (38b) in acetonitrile (to capture the t-butyl cation) subsequent conversions lead to diols (41) or triol monoethers (42). The iodination of homoallylic carbamates (43) also leads to (38b), and hence to diols (41). In all of these methods, the carbonates (38b) from homoallylic alcohols are formed with a preferred, 3-syn arrangement of the oxygen atoms. 

Nitroamlines. Acetyl derivatives (p. 388), Benzoyl derivatives (p. 388). Diamines. Diacet> l derivatives (p. 388), Dibenzoyl derivatives (p. 388). Halogeno-hydrocarbons, a-Naphthyl ethers (from reactive halogen compounds, p. 391, and their Picratcs, p. 394), Nitro-derivatives (p.39i). Carboxylic acid (if oxidisable side chain) (p. 393). 

The conversion of an aliphatic carboxylic acid into the a-bromo- (or a-chloro ) acid by treatment with bromine (or chlorine) in the presence of a catal3rtic amount of phosphorus tribromide (or trichloride) or of red phosphorus is known as the Hell-Volhard-Zelinsky reaction. The procedure probably involves the intermediate formation of the acyl halide, since it is known that halogens react more rapidly with acyl haUdes than with the acids themselves ... 

The most widely used method for the preparation of carboxylic acids is ester hydrolysis. The esters are generally prepared by heterocyclization (cf. Chapter II), the most useful and versatile of which is the Hantzsch s synthesis, that is the condensation of an halogenated a- or /3 keto ester with a thioamide (1-20). For example ethyl 4-thiazole carboxylate (3) was prepared by Jones et al. from ethyl a-bromoacetoacetate (1) and thioformamide (2) (1). Hydrolysis of the ester with potassium hydroxide gave the corresponding acid (4) after acidification (Scheme 1). 

Carboxylate ion (RC—O ) An ester IS formed when the negatively charged oxygen of a carboxylate re places the halogen of an alkyl halide... 

Methyl ketones are cleaved on re action with excess halogen in the presence of base The products are a trihalomethane (haloform) and a carboxylate salt... 

An electronegative substituent particularly if it is attached to the a carbon increases the acidity of a carboxylic acid As the data m Table 19 2 show all the mono haloacetic acids are about 100 times more acidic than acetic acid Multiple halogen sub stitution increases the acidity even more trichloroacetic acid is 7000 times more acidic than acetic acid ... 

Esterification of carboxylic acids involves nucleophilic addition to the carbonyl group as a key step In this respect the carbonyl group of a carboxylic acid resembles that of an aldehyde or a ketone Do carboxylic acids resemble aldehydes and ketones m other ways Do they for example form enols and can they be halogenated at their a carbon atom via an enol m the way that aldehydes and ketones can ... 

The enol content of a carboxylic acid is far less than that of an aldehyde or ketone and introduction of a halogen substituent at the a carbon atom requires a different set... 

Section 19 16 Halogenation at the a carbon atom of carboxylic acids can be accom plished by the Hell-Volhard-Zehnsky reaction An acid is treated with chlorine or bromine m the presence of a catalytic quantity of phospho rus or a phosphorus trihalide... 

Hell-Volhard-Zelmsky reaction (Section 19 16) The phos phorus tnhahde catalyzed a halogenation of a carboxylic acid... 

Acyl Halides. Acyl halides, in which the hydroxyl portion of a carboxyl group is replaced by a halogen, are named by placing the name of the corresponding halide after that of the acyl radical. When another group is present that has priority for citation as principal group or when the acyl halide is attached to a side chain, the prefix haloformyl- is used as, for example, in fiuoro-formyl-. 

Typical nonsieve, polar adsorbents are siUca gel and activated alumina. Kquilihrium data have been pubUshed on many systems (11—16,46,47). The order of affinity for various chemical species is saturated hydrocarbons < aromatic hydrocarbons = halogenated hydrocarbons < ethers = esters = ketones < amines = alcohols < carboxylic acids. In general, the selectivities are parallel to those obtained by the use of selective polar solvents in hydrocarbon systems, even the magnitudes are similar. Consequendy, the commercial use of these adsorbents must compete with solvent-extraction techniques. 

A number of compounds of the types RSbY2 and R2SbY, where Y is an anionic group other than halogen, have been prepared by the reaction of dihalo- or halostibines with lithium, sodium, or ammonium alkoxides (118,119), amides (120), azides (121), carboxylates (122), dithiocarbamates (123), mercaptides (124,125), or phenoxides (118). Dihalo- and halostibines can also be converted to compounds in which an antimony is linked to a main group (126) or transition metal (127). 

Electron-attracting substituents in the coupling components such as halogen, nitro, sulfo, carboxyl, and carbonyl, are deactivating and tend to retard coupling. 

This normally occurs at a /3 position, but in some cases displacement of an a substituent such as carboxyl, acyl or halogen takes place (Scheme 6). 

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