Para-Hydroxy Benzaldehyde

Colorless needles, d,4 ° 1.129 Soluble in alcohol, ether and hot water Mol. formula C6H4OHCHO Mol wt. 122 

Traditionally para-hydroxy benzaldehyde has been produced as a co-product during production of ortho-hydroxy 

Direct oxidation of p-cresol to p-hydroxy benzaldehyde is a difficult reaction. However, experimental investigations conducted in various industrial R D centers and research and educational institutions revealed that unlike the other two isomers, p-cresol can be directly oxidized to p-hydroxy benzaldehyde by selecting a proper catalyst 

Cobalt acetate or NaOH/MeOH, Fe-Porphine catalyst-CuSO at 1.5 bar/ 50°C 

Significant work has been done in Japan on liquid and vapor phase oxidation of p-cresol. Similarly Professors Sheldon and Jihad Dakka of Delft University of Technology, Holland have reported use of metal alumino-phosphate sieves (MeAPos) more particularly, CoAPO, for selective oxidation to p-hydroxy benzaldehyde with molecular oxygen in alkaline methanolic solution at 50°C [33]. 

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Source Vanillin [wt.%1 Vanillic acid [wt.%1 para-Hydroxy benzaldehyde [wt.%1 para-Hydroxy benzoic acid [wt.%1... 

Reimer-Tiemann Reaction. —Both the ortho- and para-hydroxy benzaldehydes are important. They may both be synthesized by what is known as the Reimer-Tiemann reaction. This consists of the interaction between a salt of a phenol and chloroform in the presence of an excess of alkali. The result is the introduction of the aldehyde group, (—CHO), into the benzene ring of the phenol as follows ... 

The advantage of this synthesis is that the para compound only is formed and that phenol ethers undergo the reaction also. The para-hydroxy benzaldehyde is not volatile with steam, is quite soluble in... 

It is the methyl ether of para-hydroxy benzaldehyde, CeHK... 

Para-hydroxy benzaldehyde is an important building block for a number of key organic compounds including p-anisic aldehyde, 3,4,5-trimethoxy benzaldehyde veratral-dehyde, bromoxylin and others. Important uses have been shown in Fig. 5.3. 

Oxidation of aldehydes with peroxy-acids is not so synthetically useful as oxidation of ketones and generally gives either carboxylic acids or formate esters. However, reaction of ortho- and para-hydroxy-benzaldehydes or -acetophenones with alkaline hydrogen peroxide (the Dakin reaction) is a useful method for making catechols and quinols. With benzaldehyde itself, only henzoic acid is formed, but orr/jo-hydroxy-benzaldehyde (salicylaldehyde) gives catechol almost quantitatively (6.65) and 3,4-dimethylcatechol was obtained by oxidation of 2-hydroxy-3,4-dimethylacetophenone. 

The Reimer-Tiemann reaction of phenol with chloroform, in the presence of 50% NaOH, gives a much higher ratio of para to ortho hydroxy benzaldehyde when polyethylene glycol is used as compared to the established procedure without any addition of a hydrotope. A para to ortho ratio of 1 1 has been realized (Neumann and Sasson, 1986). 

Working across the preceding pages from left to right, we may derive from the four aldehydes at least one of the chemical names for each of the remaining products. (These are only shown when commonly used.) Anisaldehyde, for example, is sometimes referred to as para-methoxy benzaldehyde, and vanillin can be referred to aspara-hydroxy-/raeta-methoxy benzaldehyde, or 4-hydroxy-3-methoxybenzaldehyde. (Not surprisingly the common name is the one preferred.)... 

The reaction is a general one for phenols both mono- and poly- and for the ethers of poly-phenols. It results always in a mixture of the ortho and para compounds. In the preparation of hydroxy benzaldehyde the two may be easily separated as the ortho compound is volatile with steam, while the para compound is not. 

A formyl substituent, like a nitro group, is strongly electron-withdrawing and acidstrengthening, especially when ortho or para to the hydroxyl group. p-Hydroxy-benzaldehyde, with a of 2.4 x 10 , is a stronger acid than phenol. 

The second-order rate constants for the reaction of OH radical with meta- and para-isomers of hydroxy-, methoxy-, chloro- and nitro-benzaldehydes are in the range (2.8 to 12) x 10 dm mol s . The higher rate in methoxybenzaldehyde is attributed to the activation of the ring by the electron-donating -OCH3 group. 

The inexpensive and easy to handle sodium percarbonate in aqueous THF allowed the conversion of various hydroxylated benzaldehydes or acetophenones to the corresponding hydroxy phenols through the so-called Dakin reaction. orfho-Hydroxybenzaldehyde is more reactive than the para isomer while the meta isomer is unreactive [204]. 

Dakin and Dakin-like Reactions Cinnamaldehydes or aromatic structures containing carbonyl substituents para to a free phenolic hydroxyl group (e.g., p-hydroxy-acetophenones) react with stabilized alkaline peroxide to form benzaldehydes and hydroquinones, respectively [37,38], Structures containing aryl-a-carbonyl substituents para to an etherilied phenolic group are degraded more slowly [39]. 

Readily available chiral amines related to the Betti base [phenyl(2-hydroxy-l-naphthyl)methanamine] catalyse enantioselective addition of diethylzinc to aldehydes in moderate to excellent ee Observed enantioselectivities in addition of diethylzinc to aldehydes catalysed by a series of (5)-proline-derived pyrrolidines have been explained in terms of steric effects. New 2,5-diazabicyclo[2.2.1]heptanes have been applied to enantioselective addition of diethylzinc to benzaldehyde. (S)-2-(3-Methyl-2-pyridyl)-3,5-di-r-butylphenol (76) has been used as an enantioselective catalyst of diethylzinc addition to benzaldehydes. Reaction in toluene shows a significant variation in % ee with temperature, including observation of an inversion temperature with maximum ee. This value varies with the nature of the para-substituent in the aldehyde, and the overall behaviour may be due to a shift in the rate-determining step of the reaction. Other reports of zinc reagents include enantioselective addition of diethylzinc to aldehydes addition of diphenylzinc to aldehydes using a chiral ferrocene-based hydroxyoxazoline catalyst in up to 96% ee and 3-exo-morpholinoisoborneol has been proposed as a more convenient and efficient enantioselective catalyst of alkylzincs than Noyori s original 3-exo-dimethylamino catalyst. ... 

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