Presence of Acids

The mildest type of reaction of the sugars induced by acids is the interconversion between a- and /3-isomers or between ring isomers. This type of change has been discussed above under the general subject of mutarotation. Dilute acids at room temperatures have little or no additional action on the sugars, but hot concentrated acids produce profound changes. 

Yields of hydroxymethylfurfural as high as 54 % and of levulinic acid as high as 69 to 79 % have been reported by the use of sucrose as the initial 

Ketohexoses react with acids much more rapidly than aldohexoses and, hence, give better yields of 5-hydroxymethylfurfural. The reaction proceeds at an appreciable rate in aqueous solutions of fructose, and even glucose, at 100 to 150° without added acids. Since furfurals readily undergo further reactions with the formation of brown-colored products, it is probable that many of the brown colors produced in food processing and in autoclaved solutions result from the intermediary formation of furfurals (60). (See also p. 446). When concentrated hydrochloric or hydrobromic acid is used, the corresponding 5-halogenomethylfurfural is produced. 

The enediol (V), as in the alkaline rearrangements, is a likely intermediate. The great difference in the rate of conversion of glucose and fructose to 

On the other hand, mechanisms differing from the above in the order of dehydration have been proposed by Hurd and Isenhour, by Isbell (68), and by Wolfrom, Schuetz, and Cavalieri (69). The mechanism of the latter involves in the first two dehydrations the production of adouble bonds in a manner generally characteristic of jS-hydroxy carbonyl compounds. To be noted also is the apparent greater ease of dehydration (indicated by the Dische diphenylamine test, presumably a measure of the hydroxylevulinic aldehyde formed) of arabinal and xylal as compared to 2-deoxyribose (70), although the former are known to have pyranose rings. 

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This extremely reactive substance rearranges, in the presence of acids, with the production of />-aminophenol ... 

In the presence of acids, hydrazobenzene rearranges to give about 0 per cent, of benzidine (4 4 -diaminodiphenyl) and about 30 per cent, of diphenyline (2 4 -diaminodiphenyl) ... 

The reaction probably proceeds as follows. Crotonaldehyde is first formed by oondensation of the depolymerised acetaldehyde in the presence of acid ... 

B. Mix 1 drop or several small crystals (ca. 0 05 g.) of the compound with 1 ml. of 0-5 V hydroxylamine hydrochloride in 95 per cent, ethanol and add 0-2 ml ot aqueous sodium hydroxide. Heat the mixture to boiling and, after the solution has cooled slightly, add 2 ml. of N hydrochloric acid. If the solution is cloudy, add 2 ml. of 95 per cent, ethyl alcohol. Observe the colour produced when I drop of 6 per cent, ferric chloride solution is added. If the resulting colour does not persist, continue to add the reagent dropwise until the observed colour pervades the entire solution. Usually only 1 drop of the ferric chloride solution is necessary. Compare the colour with that produced in test. 4. A positive test will be a distinct burgundy or magenta colour as compared with the yellow colour observed when the original compound is tested with ferric chloride solution in the presence of acid. 

Reagents with carbonyl type groupings exhibit a or (if n. S-unsaturated) a properties. In the presence of acidic or basic catalysts they may react as enol type electron donors (d or d reagents). This reactivity pattern is considered as normal . It allows, for example, syntheses of 1,3- and 1,5-difunctionaI systems via aldol type (a -H d or Michael type (a + d additions. 

Terminal alkyne anions are popular reagents for the acyl anion synthons (RCHjCO"). If this nucleophile is added to aldehydes or ketones, the triple bond remains. This can be con verted to an alkynemercury(II) complex with mercuric salts and is hydrated with water or acids to form ketones (M.M.T. Khan, 1974). The more substituted carbon atom of the al-kynes is converted preferentially into a carbonyl group. Highly substituted a-hydroxyketones are available by this method (J.A. Katzenellenbogen, 1973). Acetylene itself can react with two molecules of an aldehyde or a ketone (V. jager, 1977). Hydration then leads to 1,4-dihydroxy-2-butanones. The 1,4-diols tend to condense to tetrahydrofuran derivatives in the presence of acids. 

The achiral triene chain of (a//-rrans-)-3-demethyl-famesic ester as well as its (6-cis-)-isoiner cyclize in the presence of acids to give the decalol derivative with four chirai centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Escherunoser, 1959 W.S. Johnson, 1968, 1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric acid catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such acids activate oxygen functions selectively (K.B. Sharpless, 1970). 

Selective oxidation of secondary alcohols to ketones is usually performed with CrOj/HjSO, I I in acetone (Jones reagent) or with CrOjPyj (Collin s reagent) in the presence of acid-sensitive groups (H.G. Bosche, 1975 C. Djerassi, 1956 W.S. Allen, 1954). As mentioned above, a,)S-unsaturated secondary alcohols are selectively oxidized by MnOj (D.G. Lee, 1969 D. Arndt, 1975) or by DDQ (D. Walker, 1967 H.H. Stechl, 1975). 

Carbonyiation of butadiene gives two different products depending on the catalytic species. When PdCl is used in ethanol, ethyl 3-pentenoate (91) is obtained[87,88]. Further carbonyiation of 3-pentenoate catalyzed by cobalt carbonyl affords adipate 92[89], 3-Pentenoate is also obtained in the presence of acid. On the other hand, with catalysis by Pd(OAc)2 and Ph3P, methyl 3,8-nonadienoate (93) is obtained by dimerization-carbonylation[90,91]. The presence of chloride ion firmly attached to Pd makes the difference. The reaction is slow, and higher catalytic activity was observed by using Pd(OAc) , (/-Pr) ,P, and maleic anhydride[92]. Carbonyiation of isoprcne with either PdCi or Pd(OAc)2 and Ph,P gives only the 4-methyl-3-pentenoate 94[93]. 

Nitriles are classified as carboxylic acid derivafives because fhey are convened fo car boxylic acids on hydrolysis The condifions required are similar fo fhose for fhe hydrol ysis of amides namely healing m aqueous acid or base for several hours Like fhe hydrolysis of amides nilrile hydrolysis is irreversible m fhe presence of acids or bases Acid hydrolysis yields ammonium ion and a carboxylic acid... 

In the presence of acid, solutions of iodide are oxidized by hydrogen peroxide... 

Reaction (5.N) describes the nylon salt nylon equilibrium. Reactions (5.0) and (5.P) show proton transfer with water between carboxyl and amine groups. Since proton transfer equilibria are involved, the self-ionization of water, reaction (5.Q), must also be included. Especially in the presence of acidic catalysts, reactions (5.R) and (5.S) are the equilibria of the acid-catalyzed intermediate described in general in reaction (5.G). The main point in including all of these equilibria is to indicate that the precise concentration of A and B... 

The protein fraction is filtered and dried to become high (60%) protein content com gluten meal. The starch slurry can be dewatered and dried to produce regular com starch. Dry starch can be sold as is or heat treated in the presence of acid catalysts to produce dextrins. Or, it is chemically modified before dewatering and drying to produce modified starches used in food and industrial appHcations. Lasdy, it can be hydroly2ed to produce corn sweeteners. 

Pyronines. Pyronines are diphenylmethane derivatives synthesized by the condensation of y -dialkylarninophenols with formaldehyde, followed by oxidation of tiie xantiiene detivative (12) to the coiiesponding xanthydiol (13) which in the presence of acid forms the dye (14). If R is methyl, the dye produced is... 

Polymerization. Paraldehyde, 2,4,6-trimethyl-1,3-5-trioxane [123-63-7] a cycHc trimer of acetaldehyde, is formed when a mineral acid, such as sulfuric, phosphoric, or hydrochloric acid, is added to acetaldehyde (45). Paraldehyde can also be formed continuously by feeding Hquid acetaldehyde at 15—20°C over an acid ion-exchange resin (46). Depolymerization of paraldehyde occurs in the presence of acid catalysts (47) after neutralization with sodium acetate, acetaldehyde and paraldehyde are recovered by distillation. Paraldehyde is a colorless Hquid, boiling at 125.35°C at 101 kPa (1 atm). 

Reactions with Ammonia and Amines. Acetaldehyde readily adds ammonia to form acetaldehyde—ammonia. Diethyl amine [109-87-7] is obtained when acetaldehyde is added to a saturated aqueous or alcohoHc solution of ammonia and the mixture is heated to 50—75°C in the presence of a nickel catalyst and hydrogen at 1.2 MPa (12 atm). Pyridine [110-86-1] and pyridine derivatives are made from paraldehyde and aqueous ammonia in the presence of a catalyst at elevated temperatures (62) acetaldehyde may also be used but the yields of pyridine are generally lower than when paraldehyde is the starting material. The vapor-phase reaction of formaldehyde, acetaldehyde, and ammonia at 360°C over oxide catalyst was studied a 49% yield of pyridine and picolines was obtained using an activated siHca—alumina catalyst (63). Brown polymers result when acetaldehyde reacts with ammonia or amines at a pH of 6—7 and temperature of 3—25°C (64). Primary amines and acetaldehyde condense to give Schiff bases CH2CH=NR. The Schiff base reverts to the starting materials in the presence of acids. 

Acetic anhydride acetylates free hydroxyl groups without a catalyst, but esterification is smoother and more complete ia the presence of acids. For example, ia the presence of -toluenesulfonic acid [104-15-4], the heat of reaction for ethanol and acetic anhydride is —60.17 kJ/mol (—14.38 kcal/mol) (13) ... 

The rate of hydrolysis of DMAC is very low, but increases somewhat in the presence of acids or bases. DMAC is a stable compound, but is mildly hygroscopic and desiccation and/or dry nitrogen blanketing of storage vessels are sometimes used to reduce water pick-up. In the absence of water, acids, or bases, DMAC is stable at temperatures up to its hoiling point at atmospheric pressure. Its greater stability enables more economical recovery by distillation relative to that of other similar solvents. 

In the presence of acid catalysts, butynediol and aldehydes (47) or acetals (48) give polymeric acetals, useful intermediates for acetylenic polyurethanes suitable for high energy soHd propellants. 

When butyrolactone and alcohols are heated for long times and at high temperatures in the presence of acidic catalysts, 4-alkoxybutytic esters are formed. With sodium alkoxides, sodium 4-alkoxybutyrates are formed (150). 

The addition of alcohols to form the 3-alkoxypropionates is readily carried out with strongly basic catalyst (25). If the alcohol groups are different, ester interchange gives a mixture of products. Anionic polymerization to oligomeric acrylate esters can be obtained with appropriate control of reaction conditions. The 3-aIkoxypropionates can be cleaved in the presence of acid catalysts to generate acrylates (26). Development of transition-metal catalysts for carbonylation of olefins provides routes to both 3-aIkoxypropionates and 3-acryl-oxypropionates (27,28). Hence these are potential intermediates to acrylates from ethylene and carbon monoxide. 

Formamide is hydroly2ed very slowly at room temperature. The rate of hydrolysis increases rapidly in the presence of acids or bases and is further accelerated at elevated temperatures. 

Syntheses of Aliphatic Carboxylic Acids and Derivatives. Alkenes are carbonylated in the presence of acid catalysts at 75-100°C and under pressures of 60—90 MPa (600—900 atm) to give carboxyUc acids (186). 

Miscellaneous Reactions. Some hydantoin derivatives can serve as precursors of carbonium—immonium electrophiles (57). 5-Alkoxyhydantoins are useful precursors of dienophiles (17), which undergo Diels-Alder cycloadditions under thermal conditions or in the presence of acid catalysis (58). The pyridine ring of Streptonigrine has been constmcted on the basis of this reaction (59). 

The reaction of thioethers with ethyleneimine in the presence of acid yields sulfonium compounds. The reaction is reversible under alkaline conditions (125). Compounds in which double-bonded sulfur can exist in tautomerism with a form having a free SH group, such as thiourea (126,127), thiocarboxyhc acids (128), and thiophosphates (129), react to give aminoaLkylated products. The P-aminoethyl thiocarboxylate rearranges to give the amide. 

Isoprene reacts with a-chloroalkyl ethers in the presence of ZnCl in diethyl ether from 0—10°C. For example,a-chloromethyl methyl ether at 10°C gives a 6 1 ratio of the 1,4-adduct, (F)4-chloro-l-methoxy-2-methyl-2-butene, to the 1,2-adduct, 2-chloro-l-methoxy-2-methyl-3-butene. Other a-chloroalkyl ethers react in a similar manner to give predominately the 1,4-addition product. A wide variety of aHyUc chlorides and bromides and a-chloroethers and esters add primarily 1,4- to isoprene in the presence of acid catalysts (8). 

Six-membered heterocycles with two heteroatoms are prepared by reaction of diketene with a substrate containing a C—O or C—N multiple bond. With carbonyl compounds diketene reacts in the presence of acids to give l,3-dioxin-4-ones. The best known is 2,2,6-trimethyl-4H-l,3-dioxin-4-one [5394-63-8] (15), the so-called diketene—acetone adduct, often used as a diketene replacement that is safer to handle and to transport, albeit somewhat less reactive than diketene itself (103,104), forming acetylketene upon heating. 

Ma.nufa.cture. Mesityl oxide is produced by the Hquid-phase dehydration of diacetone alcohol ia the presence of acidic catalysts at 100—120°C and atmospheric pressure. As a precursor to MIBK, mesityl oxide is prepared ia this manner ia a distillation column ia which acetone is removed overhead and water-saturated mesityl oxide is produced from a side-draw. Suitable catalysts are phosphoric acid (177,178) and sulfuric acid (179,180). The kinetics of the reaction over phosphoric acid have been reported (181). 

PTMEG is a polymeric ether susceptible to both thermal and oxidative degradation. It usually contains 300—1000 ppm of an antioxidant such as 2,6-di-/ f2 -butyl-4-hydroxytoluene (BHT) to prevent oxidation under normal storage and handling conditions. Thermal decomposition in an inert atmosphere starts at 210—220°C (410—430°E) with the formation of highly flammable THE. In the presence of acidic impurities, the decomposition temperature can be significantly reduced contact with acids should therefore be avoided, and storage temperatures have to be controlled to prevent decomposition to THF (261). 

Acetone also reacts with diphenylamine, in the presence of acid, to form a variety of condition-dependent products (5). Excess amine and a small amount of strong acid catalyst at 100—150°C give 2,2-[4,4 -(dianilino)diphenyl]-propane [2980-26-9] (6). With a large amount of hydrochloric acid at 250°C in the presence of excess diphenylamine, the main product is 9,9-dimethylacridan [6267-02-3]. 

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