Aldehyde reaction with water

The mechanism for the oxidation of 1° alcohols to aldehydes parallels the oxidation of 2° alcohols to ketones detailed in Section 12.12A. Oxidation of a 1 alcohol to a carboxylic acid requires three operations oxidation first to the aldehyde, reaction with water, and then further oxidation to the carboxylic acid, as shown in Mechanism 12.6. 

The zwitterion (6) can react with protic solvents to produce a variety of products. Reaction with water yields a transient hydroperoxy alcohol (10) that can dehydrate to a carboxyUc acid or spHt out H2O2 to form a carbonyl compound (aldehyde or ketone, R2CO). In alcohoHc media, the product is an isolable hydroperoxy ether (11) that can be hydrolyzed or reduced (with (CH O) or (CH2)2S) to a carbonyl compound. Reductive amination of (11) over Raney nickel produces amides and amines (64). Reaction of the zwitterion with a carboxyUc acid to form a hydroperoxy ester (12) is commercially important because it can be oxidized to other acids, RCOOH and R COOH. Reaction of zwitterion with HCN produces a-hydroxy nitriles that can be hydrolyzed to a-hydroxy carboxyUc acids. Carboxylates are obtained with H2O2/OH (65). The zwitterion can be reduced during the course of the reaction by tetracyanoethylene to produce its epoxide (66). 

The initial step is the protonation of the aldehyde—e.g. formaldehyde—at the carbonyl oxygen. The hydroxycarbenium ion 6 is thus formed as reactive species, which reacts as electrophile with the carbon-carbon double bond of the olefinic substrate by formation of a carbenium ion species 7. A subsequent loss of a proton from 7 leads to formation of an allylic alcohol 4, while reaction with water, followed by loss of a proton, leads to formation of a 1,3-diol 3 " ... 

Reaction with water to yield a carboxylic acid Reaction with an alcohol to yield an ester Reaction with ammonia or an amine to yield an amide Reaction with a hydride reducing agent to yield an aldehyde or an alcohol... 

It can also be done by in situ generation of other types of electrophiles. For example, good yields of /V-acyl a-amino acids are formed in a process in which an amide and aldehyde combine to generate a carbinolamide and, presumably, an acyliminium ion. The organopalladium intermediate is then carbonylated prior to reaction with water.254... 

Reaction of the enatiopure aldehyde 2-800, obtained from the corresponding imine by enantioselective hydrogenation, with Meldrum s acid (2-801) and the enol ether 2-802a (E/Z= 1 1) in the presence of a catalytic amount of ethylene diammonium diacetate for 4h gave 2-805 in 90 % yield with a 1,3 induction of >24 1. As intermediates, the Knoevenagel product 2-803 and the primarily produced cycloadduct 2-804 can be supposed the latter loses C02 and acetone by reaction with water formed during the condensation step (Scheme 2.178). 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

A naive look at the product suggests an oxidation to a ketone followed by a Baeyer-Villiger like reaction. The product is best explained by a fragmentation from an intermediate chromate ester, resulting on an aldehyde and a stabilized tertiary carbocation that is transformed into a tertiary alcohol by reaction with water. The hydroxyaldehyde so obtained may evolve to the final lactone either via a lactol or a hydroxyacid. 

Cn>8 aldehydes and heavy side reactions with water Persistence of fluorocarbons... 

Aldehydes and ketones underigo reaction with water to yieM 1,1-dioJr. geminal em> dlols. The hydration reaction is reversible, ond d gem can climinotc water to rsqfencratc a ketorn or aldehyde. 

Both Sc(OTf)3 and Yb(OTf)3 have been employed as Lewis acid catalysts for additions of allylic stannanes to aldehydes. Reactions with the former catalyst can be conducted in a variety of solvents and are not sensitive to water [24]. All four allyl groups of tetraallyltin are consumed in the addition (Table 13). The latter reaction is performed with allyl tributyltin in CH2CI2 (Eq. 16) [25]. 

In some cases this reaction takes place with water alone but usually some other substance is present e.g., calcium hydroxide or carbonate, potassium hydroxide, metallic iron or iron salts which actsasacatalizer. The mono-chlorine derivative of toluene and other benzene homologues may also be used for preparing the aldehydes. In this case the reaction is in two steps, first, reaction with water yielding the alcohol, and second. 

Aldehydes and ketones undergo reaction with water to yield 2,1-dioIs, I geminal (gem) diols. The hydration reaction is reversible, and a gem ( can eliminate w ater to regenerate a ketone or aldehyde. 

The heterocyclic systems of 1,3-oxazinium ions are widely used in the organic synthesis . Thus, 5,6-dihydro-4/f-l,3-oxazines 40, being a cyclic derivative of 1,2-aminoalcohols, can serve as Cj-synthones for the preparation of aldehydes, ketones and carboxylic acids . The 47/-l,3-oxazines 47 can undergo oxidative dehydrogenation by treatment with the trityl salts or benzoquinone to form the 3-azapyrylium salts 48, which then give various jV-acylenamine derivatives 49-52 by reaction with water or with active methylene comjKJunds (equation 18) . 

Recent research in the application of supercritical (sc) fluids and ionic liquids (IL) as solvents in homogeneous catalysis (see Sections 7.3 and 7.4), opened the way to the development of biphasic water/scCOz [171, 172] and water/IL [173] systems for the hydrogenation of various substrates, e.g., alkenes, aldehydes, etc. with water-soluble catalysts. The catalytically highly active, versatile and robust transition metal - N-heterocyclic carbene complexes [174] have also been applied for hydrogenation reactions [175], Given that water-soluble complexes with N-heterocy-clic carbene ligands are known [176], catalytic applications in aqueous systems are also foreseen. 

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