Aromatic aldehyde

Aromatic aldehydes are compounds in which the aldehyde functional group O 

As previously mentioned, the first reactions between phosphine and aromatic aldehydes were carried out by Messinger and Engels althougli the 

When a stream of phosphine is passed into an ether solution of benzal-dehyde saturated with HCl, a colourless, crystalline solid, insoluble in the reaction mixture is formed. The infra-red spectrum of this product shows an absorption band at 1145 cm which is typical for the P=0 group of a tertiary phosphine oxide. This, together with the chemical behaviour, indicates that the product is benzyl-bis(ot-hydroxybenzyl)phosphine oxide, 6, 

When methanol is used as reaction solvent for the treatment of benzal-dehyde with phosphine, however, the product is tris(o -methoxybenzyl)phos-phine, 7, 

The use of ethanol or isopropanol as solvent results in the formation of tris (a-ethoxybenzyl)- or tris(a-isopropoxybenzyl)-phosphine, respectively, in yields of up to 85%  

Kinetic investigations of the reactions discussed here have been made by Ettel and Horak.  

The aldehyde group deactivates the ring and is meta directing. There are few useful examples, since not only is electrophilic attack more difficult than for benzene, but also the aldehyde group is prone to oxidation during the attack. Substituted benzaldehydes are therefore usually synthesized by functional group transformations or by direct formylation. 

It is not uncommon to add a trace of acid to promote these reactions. 

Under these conditions, the carbonyl group is protonated, effectively increasing the electron deficiency of the carbon atom. 

A major structural difference between aromatic aldehydes and most aliphatic analogues is that the former lack an a-hydrogen atom. As a consequence, they are unable to enolize and so enolates/carbanions cannot be generated from them. Nevertheless, aromatic aldehydes can react with carbanions derived from, for example, aldehydes, ketones, esters and anhydrides, and so undergo a range of condensation reactions. 

Q Outline a general scheme to show how carbanions can be derived from substrates containing an a-hydrogen atom and how the car-j bunions react with an aromatic aldehyde. 

FIGURE 2.12. p-Chlorobenzophenone. The M peak is arbitrarily set in the table above at intensity 100% for discussion of the molecular ion cluster. 

By side-chain chlorination of the hydrocarbon (Section IV,23), followed by hydrolysis of the dichloro compound, say, with water at 95-100° in the presawe of iron as a catalyst, for example  

By oxidation of the methyl derivative of an aromatic hydrocarbon with a solution of chromic anhydride in acetic anhydride and acetic acid. The aldehyde formed is immediately converted into the gem-diacetate, which is stable to oxidation. The diacetate is collected and hydrolysed with sulphuric add, for example  

By passing a mixture of carbon monoxide and hydrogen chloride into the aromatic hydrocarbon in the presence of a mixture of cuprous chloride and aluminium chloride which acts as a catalyst (Gattermann - Koch reaction). The mixture of gases probably reacts as the equivalent of the unisolated acid chloride of formic acid (formyl chloride)  

The Gattermann-Koch formylation was found unsuited to the preparation of aldehydes from phenols and phenol ethers such aldehydes may be obtained by Gattennann s aldehyde reaction. 

By interaction of hydrogen cyanide and hydrogen chloride with an aromatic compound (hydrocarbon, phenol or phenol ether) in the presence of aluminium chloride (or zinc chloride). This is known as the Gattermann 

Formylation is the direct introduction of the formyi group CH=0, into a rtolecule. 

In this chapter in particular, [oany reactions will l re met which are named after Ihei discoverers, AIttiough it can be helpful to remember the names, it is far more important to understand why the reactions occur and the mechanisnis by which they proceed, 

Hydrazones and oximes are oi value in itie synthesis of heterocycles. For example, the acid-catalysed cydization of phenylhydrazones gives indoles. 

3-Diketones react with hydroxylamine 10 give rsoxazoles. 

There appears to have been no attention given to the low temperature gas phase uncatalysed oxidation of benzaldehyde, although various studies 

In the latter synthesis, as the benzaldehyde is formed, it is converted into its diacetate by the acetic anhydride, so preventing further oxidation subsequent hydrolysis generates the aldehyde group. The benzaldehyde has thus been protected from oxidation. Benzyl alcohol can 

---

Leuckart reaction The conversion of ketones and aromatic aldehyde,s to primary amines by reaction with ammonium methanoale at a high temperature. 

Perkin reaction A condensation between aromatic aldehydes and the sodium salts of fatty acids or their aromatic derivatives. The reaction between benzaldehyde and sodium ethanoate in the presence of ethanoic anhydride leads to sodium cinnamate... 

SchifT s bases A -Arylimides, Ar-N = CR2, prepared by reaction of aromatic amines with aliphatic or aromatic aldehydes and ketones. They are crystalline, weakly basic compounds which give hydrochlorides in non-aqueous solvents. With dilute aqueous acids the parent amine and carbonyl compounds are regenerated. Reduction with sodium and alcohol gives... 

SchifT s reagent A solution of rosaniline in water decolorized with sulphurous acid. Aliphatic aldehydes and aldose sugars give a magenta colour with this reagent with aromatic aldehydes and aliphatic ketones the colour develops more slowly aromatic ketones do not react. 

This reaction is given by most aromatic aldehydes having the aldehyde group directly joined to the benzene ring it is also given by formaldehyde, with the formation of methanol and formic acid. Other aliphatic aldehydes do not give Cannizzaro s reaction under these conditions. 

Many aromatic aldehydes (having the -CHO group joined directly to the benzene ring) undergo polymerisation when heated with a solution of potassium cyanide in aqueous ethanol. Thus benzaldehyde gives benzoin, a compound of double function, since it contains both a secondary alcoholic and a ketonic... 

A) most aromatic aldehydes giving Cannizzaro s reaction. 

With paraldehyde and the aromatic aldehydes (being insoluble in water), it is advisable to warm the mixture gently on a water-bath, shaking the tube vigorously from time to time to break up the oily globules of the aldehyde. 

Almost insoluble in cold water. Higher alcohols (including benzyl alcohol), higher phenols (e.g., naphthols), metaformaldehyde, paraldehyde, aromatic aldehydes, higher ketones (including acetophenone), aromatic acids, most esters, ethers, oxamide and domatic amides, sulphonamides, aromatic imides, aromatic nitriles, aromatic acid anhydrides, aromatic acid chlorides, sulphonyl chlorides, starch, aromatic amines, anilides, tyrosine, cystine, nitrocompounds, uric acid, halogeno-hydrocarbons, hydrocarbons. 

For the preparation of oximes, pheuylhydrazones and p nitrophenyl-hydrazones (where applicable), see under Aromatic Aldehydes, Section IV,135,4-d. 

Perkin reaction. The condensation of an aromatic aldehyde with an acid anhydride in the presence of the sodium or potassium salt of the acid... 

Benzoin condensation. Aromatic aldehydes when treated with an alkali cyanide, usually in aqueous solution, undergo condensation to the -hydroxyketone or benzoin. The best known example is the conversion of benzaldehyde to benzoin ... 

Claisen-Schmidt reaction. Aromatic aldehydes condense with aliphatic or mixed alkyl-aryl ketones in the presence of aqueous alkali to form ap-unsaturated ketones ... 

The above are examples of the Claisen - Schmidt reaction. The formation of p-nitrostyrenes by reaction of nitroalkanes with aromatic aldehydes in the presence of aqueous alkali may be included under the Claisen- hmidt condensation ... 

Clalsen aldol condensation. This consists in the condensation of an aromatic aldehyde and an ester R—CHjCOOCjHj in the presence of finely divided sodium and a trace of alcohol at a low temperature. The catalyst is the alkoxide ion aqueous alkalis caimot be employed since they will hydrolyse the resulting ester. The product is an ap-unsaturated ester, for example ... 

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

Aromatic aldehydes usually have relatively high boiling points, but distil with little or no decomposition. The vapours burn with a smoky flame. They are easily oxidised on standing in the air into the corresponding acids the odours are often pleasant and characteristic. Aromatic aldehydes, by virtue of their high molecular weight, yield... 

Aromatic aldehydes react with the dimedone reagent (Section 111,70,2). All aromatic aldehydes (i) reduce ammoniacal silver nitrate solution and (ii) restore the colour of SchifiF s reagent many react with sodium bisulphite solution. They do not, in general, reduce Fehling s solution or Benedict s solution. Unlike aliphatic aldehydes, they usually undergo the Cannizzaro reaction (see Section IV,123) under the influence of sodium hydroxide solution. For full experimental details of the above tests, see under Ali-phalic Aldehydes, Section 111,70. They are easily oxidised by dilute alkaline permanganate solution at the ordinary temperature after removal of the manganese dioxide by sulphur dioxide or by sodium bisulphite, the acid can be obtained by acidification of the solution. 

The melting points of the various derivatives of a number of typical aromatic aldehydes are collected in Table IV,135,... 

By catalytic reduction of a p-unsaturated ketones, prepared from aldehydes by the Claisen - Schmidt reaction (see under Aromatic Aldehydes), for example ... 

The preparation of crystaUine derivatives, including 2 4-dinitro-phenyl hydrazones, semicarbazones, oximes, phenylhydrazones and p-nitrophenylhydrazones can be carried out as described under Aromatic Aldehydes, Section IV, 135. 

Only half of the aldehyde is reduced to the alcohol, the other half being oxidised to the acid. By using a slight excess (say, 1 -3 mols) of aqueous formaldehyde, practically the whole of the aromatic aldehyde is converted into the alcohol the formaldehyde Is simultaneously oxidised to formic acid. This is sometimes termed a crossed Cannizzaro reaction. The example given is ... 

Aromatic alcohols are insoluble in water and usually burn with a smoky flame. Their boiling points are comparatively high some are solids at the ordinary temperature. Many may be oxidised by cautious addi-tion of dilute nitric acid to the corresponding aldehyde upon neutralis-tion of the excess of acid, the aldehyde may be isolated by ether extraction or steam distillation, and then identified as detailed under Aromatic Aldehydes, Section IV,135. 

The above reversible equation indicates that one mol of aluminium iso-propoxlde will reduce directly three mols of the carbonyl compound. It is generally desirable to use excess of the reductant except for aromatic aldehydes for the latter side reactions (e.g., 2RCHO-----> RCOOCH R Tischenko re-... 

This preparation illustrates the reduction of an aromatic aldehyde. 

Azlactones (anhydrides of a-acylamino acids) are formed by the condensation of aromatic aldehydes with acyl derivatives of glycine in the presence of acetic anhydride and anhydrous sodium acetate. Thus beiizaldehyde and acetyl-... 

Aromatic aldehydes, and also aliphatic aldehydes containing no a-hydrogen atom, undergo the Cannizzaro reaction (see Section IV,123) when treated with aqueous alkali, for example ... 

---