Formate ester formation

One of the more difficult problems encountered in obtaining a valid assay of formic acid is that of formate ester formation. The formate is derived from the cyclic, hemi-acetal structure which is an equilibrium form of many free sugars in solution. For example, the oxidation of one of the cyclic forms of D-glucose can readily be seen to give a formate ester (as well as a C-formyl group) on the atom originally denoted as C5. It... 

Another side reaction of hydroformylation is formate ester formation (11). The 4% yield of formates did not change in the temperature range examined or with lower CO partial pressure. 

Alcoholysis/Acylation etc. (Acetal Formation, Ester Formation, Amide Formation)... 

Formate Ester Formation. In the case of unmodified cobalt catalyst, the formation of formate ester is observed in minor amovuits. With other catalysts, either trace amovuits or none of the formate esters are fovuid in the product of hydroformylation. 

AUqrl formates are the hydroformylation products of the carbon-oxygen double bond of aldehydes. For more details, see the section Modeling the Formate Ester Formation... 

Modeling the Formate Ester Formation. Formate esters up to 4% of the total products are side products mainly in the unmodified cobalt-catalyzed alkene hydroformylation. Their formation has been explained in analogy to the Heck and Breslow mechanism of olefin hydroformylation (203). By the addition of HCo(CO)4 to the aldehyde carbonyl group, either an a-hydroxyaUgrl- or an alkoxy-cobalt carbonyl is formed. The latter complex converts with carbon monoxide into an (alkoxycarbonyl)cobalt carbonyl. Reduction of this complex gives the formate product (Scheme 15). 

Equation (6) also contains a similar mechanistic influence and explains the frequently encountered side reaction of formate ester formation. 

Claisen condensation Condensation of an ester with another ester, a ketone or a nitrile in the presence of sodium ethoxide, sodium or sodamidc, with the elimination of an alcohol. The result is the formation of a / -ketonic ester, ketone, or nitrile respectively, e.g. 

It condenses with resorcinol and amino-phenols to give phthalein and rhodamine dyestuffs respectively. Esters are used in the formation of polyimides. ... 

HOaQCHjlfiCOiH, CSH14O4. Important dicarboxylic acid obtained by oxidizing ricino-leic acid (from castor oil) also obtained by oxidation of cyclo-octene or cyclo-octadiene formerly obtained from cork. Used in the formation of alkyd resins and polyamides. Esters are used as plasticizers and heavy duty lubricants and oils. 

C7H7CIO1S, p-CHjCjsH SOjCI. Colourless crystals, m.p. 7l°C, formed by the action of chlorosulphonic acid on toluene. Esters of toluenesulphonic acid are frequently called tosylatesand their formation tosylation. Many tosylates are easily obtained crystalline, and the reaction is thus of considerable importance. 

The Claisen condensation is initiated by deprotonation of an ester molecule by sodium ethanolate to give a carbanion that is stabilized, mostly by resonance, as an enolate. This carbanion makes a nucleophilic attack at the partially positively charged carbon atom of the e.ster group, leading to the formation of a C-C bond and the elimination ofan ethanolate ion, This Claisen condensation only proceeds in strongly basic conditions with a pH of about 14. 

The preparation of -butyl bromide as an example of ester formation by Method 1 (p. 95) has certain advantages over the above preparation of ethyl bromide. -Butanol is free from Excise restrictions, and the -butyl bromide is of course less volatile. and therefore more readily manipulated without loss than ethyl bromide furthermore, the n-butyl bromide boils ca. 40° below -butyl ether, and traces of the latter formed in the reaction can therefore be readily eliminated by fractional distillation. 

A considerable amount of the formic acid, however, still remains behind in the distilling-flask as the unhydrolysed monoformate. Therefore, if time allows, dilute the residue in the flask with about an equal volume of water, and then steam-distil, the monoformate ester being thus completely hydrolysed and the formic acid then driven over in the steam. Collect about 400 ml. of distillate. Add this distillate to that obtained by direct heating of the reaction mixture and then treat with lead carbonate as described above. Total yield of lead formate is now about 40 g. 

Note also that if another ester, of general formula R-COOCjHj, were used in place of benzaldehyde in the above reaction, a similar complex would be formed, and on acidification would give an unstable p-hydroxy-P-ethoxy ester, which would very readily lose ethanol with the formation of a 3-keto-ester. 

Note. Methyl oxalate, unlike most other esters, hydrolyses very rapidly in aqueous solution hence it will evolve CO in the above test, owing to the formation of methanol and free oxalic acid. 

Ester formation. Heat gently i ml. of ethanol with 0 5 g. of the acid or one of its salts and a few drops of cone. HjSO for about I minute. Cool and pour into a few ml. of water in a test-tube and note the odour. The test is particularly useful for identifying ... 

As a general guide, however, it may be noted that the following have fairly easily recognisable odours methyl and ethyl formate methyl and ethyl acetate (apples) methyl and ethyl benzoate methyl salicylate (oil of winter-green) and ethyl salicylate methyl and ethyl cinnamate. (It is however usually impracticable to distinguish by odour alone between the methyl and ethyl esters of a particular acid.) Methyl and ethyl o. alate, and methyl and ethyl phthalate are almost odourless. Succinic and tartaric esters have faint odours. 

Hydroxamic acid formation cf. Section 9, p. 334). To a few drops of an ester, add 0 2 g. of hydroxylamine hydrochloride and about 5 ml. of 10% NaOH solution and gently boil the mixture for 1-2 minutes. Cool and acidify with dil. HCl and then add a few drops of ferric chloride solution. A violet or deep red-brown colour develops immediately. 

Ethyl oxalate is the only liquid ester which gives this rapid separation of the amide, which is therefore characteristic. Methyl and ethyl formate react rapidly with ammonia, but the soluble formamide does not separate methyl succinate gives crystalline succinamide after about I hour s standing, other esters only after a much longer time. The solid esters, other than methyl oxalate, are either soluble in water and remain so when treated with ammonia, or alternatively are insoluble in water and hence clearly not methyl oxalate. 

Ester formation. Add carefully 1 ml. of the liquid to i ml. of ethanol and then warm gently for i minute. Pour into water, make alkaline with aqueous Na2C03 solution (to remove HCl and other acid fumes), and note the odour of ethyl acetate or ethyl benzoate. 

Ester formation. Heat under very efficient reflux 1 ml. of diethyl ether, 4 ml. of glacial acetic acid and i ml. of cone. H2SO4 for ro minutes. Distil off 2 ml. of liquid. Use a few drops of this liquid for the hydroxamic add test for esters (p. 334). Use the remainder for other tests for esters (p. 354). 

Aldehydes (including chloral hydrate) formates and lactates some esters chloroform and iodoform reducing sugars some phenols. 

Freshly distilled ethyl formate must be used. Commercial ethyl formate may be purified as follows. Allow the ethyl formate to stand for 1 hour with 16 per cent, of its weight of anhydrous potassium carbonate with occasional shaking. Decant the ester into a dry flask containing a little fresh anhydrous potassium carbonate and allow to stand for a further hour. Filter into a di flask and distil through an efficient fractionating column, and collect the fraction, b.p. 53-54° protect the receiver from atmospheric moisture. 

Strictly speaking the alkyl halides are esters of the halogen acids, but since they enter into many reactions (t.g., formation of Grignard reagents, reaction with potassium cyanide to yield nitriles, etc.) which cannot be brought about by the other eaters, the alkyl halides are usually distinguished from the esters of the other inorganic acids. The preparation of a number of these is described below. 

Ethyl formate. Reflux a mixture of 61 g. (50 ml.) of A.R. formic acid (98/100 per cent.) and 31 g. (39-5 ml.) of absolute ethyl alcohol for 24 hours. Transfer to a Claisen flask with fractionating side arm (or attach a fractionating column to the flask), distil and collect the liquid passing over below 62°. Wash the distillate with saturated sodium bicarbonate solution and saturate with salt before removing the ester layer. Dry with anhydrous sodium or magnesium sulphate, filter, and distil. The ethyl formate passes over at 53-54°. The yield is 36 g. 

Some esters, e.g., methyl formate, methyl oxalate, methyl succinate, methyl and ethyl tartrate, are appreciably soluble In water. These are usually easily hydrolysed by alkali. 

The formation of ethyl acetoacetate is an example of a general reaction knowu as the acetoacetlc ester condensation in which an ester having hydrogen on the a-carbon atom condenses with a second molecule of the same ester or with another ester (which may or may not have hydrogen on the a-carbon atom) in the presence of a basic catalyst (sodium, sodium ethoxide, sodamide, sodium triphenylmethide) to form a p-keto-ester. The mechanism of the reaction may be illustrated by the condensation of ethyl acetate with another molecule of ethyl acetate by means of sodium ethoxide. ... 

Only esters containing two a-hydrogen atoms (ethyl acetate, propionate, n-butyrate, etc.) can be condensed with the aid of sodium alkoxides. For esters with one a-hydrogen atom, such as ethyl tsobutyrate, the more powerful base sodium triphenylmethide PhaC Na leads to condensation with the formation of ethyl a-tsobutyrylisobutyrate ... 

The ester and catalj st are usually employed in equimoleciilar amounts. With R =CjHs (phenyl propionate), the products are o- and p-propiophenol with R = CH3 (phenyl acetate), o- and p-hydroxyacetophenone are formed. The nature of the product is influenced by the structure of the ester, by the temperature, the solvent and the amount of aluminium chloride used generally, low reaction temperatures favour the formation of p-hydroxy ketones. It is usually possible to separate the two hydroxy ketones by fractional distillation under diminished pressure through an efficient fractionating column or by steam distillation the ortho compounds, being chelated, are more volatile in steam It may be mentioned that Clemmensen reduction (compare Section IV,6) of the hj droxy ketones affords an excellent route to the substituted phenols. 

The mechanism of the reaction between aromatic aldehydes and esters probably involves the intermediate formation of an aldol (hence the name— Claisen aldol condensation) ... 

Mix 31 g. (29-5 ml.) of benzyl alcohol (Section IV, 123 and Section IV,200) and 45 g. (43 ml.) of glacial acetic acid in a 500 ml. round-bottomed flask introduce 1 ml. of concentrated sulphuric acid and a few fragments of porous pot. Attach a reflux condenser to the flask and boil the mixture gently for 9 hours. Pour the reaction mixture into about 200 ml. of water contained in a separatory funnel, add 10 ml. of carbon tetrachloride (to eliminate emulsion formation owing to the slight difference in density of the ester and water, compare Methyl Benzoate, Section IV,176) and shake. Separate the lower layer (solution of benzyl acetate in carbon tetrachloride) and discard the upper aqueous layer. Return the lower layer to the funnel, and wash it successively with water, concentrated sodium bicarbonate solution (until effervescence ceases) and water. Dry over 5 g. of anhydrous magnesium sulphate, and distil under normal pressure (Fig. II, 13, 2) with the aid of an air bath (Fig. II, 5, 3). Collect the benzyl acetate a (colourless liquid) at 213-215°. The yield is 16 g. 

Esters of dicarboxyUc acids having hydrogen on tbe 8 or e carbon atoms undergo intramolecular cyclisation when heated with sodium or with sodium ethoxide. This cyclisation is known as the Dieckmann reaction. It is essentially an application of the Claiseu (or acetoacetic eater) condensation to the formation of a ring system the condensation occurs internally to produce s... 

The decomposition of a glycidic ester to an aldehyde and carbon dioxide may involve the formation of a quasi six-membered ring, followed by the shift of three electron pairs ... 

The following mechanism for the reaction has been suggested. It postulates formation of an intermediate paraconic ester (A) the irreversible alkoxlde cleavage of this cyclic ester drives the reaction to completion ... 

---