Boiling points of aldehydes

Figure 4.2 The boiling points of aldehydes and ketones are higher than those of alkanes and ethers but lower than those of alcohols of comparable molecular weights. 

The polarization of the carbonyl functional group makes the boiling points of aldehydes and ketones higher than those of hydrocarbons of similar size and molecular weight (Table 17-1). Because of their polarity, the smaller carbonyl compounds such as acetaldehyde and acetone are completely miscible with water An aqueous solution of formaldehyde (Section 17-6) has applications as a disinfectant and a fungicide. As the hydrophobic hydrocarbon part of the molecule increases in size, however, water solubility decreases. Carbonyl compounds with more than six carbons are rather insoluble in water. 

High purity acetaldehyde is desirable for oxidation. The aldehyde is diluted with solvent to moderate oxidation and to permit safer operation. In the hquid take-off process, acetaldehyde is maintained at 30—40 wt % and when a vapor product is taken, no more than 6 wt % aldehyde is in the reactor solvent. A considerable recycle stream is returned to the oxidation reactor to increase selectivity. Recycle air, chiefly nitrogen, is added to the air introducted to the reactor at 4000—4500 times the reactor volume per hour. The customary catalyst is a mixture of three parts copper acetate to one part cobalt acetate by weight. Either salt alone is less effective than the mixture. Copper acetate may be as high as 2 wt % in the reaction solvent, but cobalt acetate ought not rise above 0.5 wt %. The reaction is carried out at 45—60°C under 100—300 kPa (15—44 psi). The reaction solvent is far above the boiling point of acetaldehyde, but the reaction is so fast that Httle escapes unoxidized. This temperature helps oxygen absorption, reduces acetaldehyde losses, and inhibits anhydride hydrolysis. 

Formaldehyde is a gas with a boiling point of -21 °C. It is usually supplied as a stabilised aqueous solution ( 40% formaldehyde) known as formalin. When formalin is used as the source of the aldehyde, impurities present generally include water, methanol, formic acid, methylal, methyl formate and carbon dioxide. The first three of these impurities interfere with polymerisation reactions and need to be removed as much as possible. In commercial polymerisation the low polymers trioxane and paraformaldehyde are convenient sources of formaldehyde since they can be obtained in a greater state of purity. 

The lowest members of the series of the aldehydes are colourless liquids of pungent odour and miscible with water the middle members are also liquids, but do not dissolve easily in water the aldehydes of high molecular weight are crystalline solids. The boiling points of the aldehydes are considerably lower than those of the corresponding alcohols ... 

The cooled reaction product is treated with 200 cc. of water, the layer of oil separated, washed once with a second portion of water, and subjected to distillation in vacuo. The first fraction of the distillate contains benzyl alcohol together with unchanged aldehyde, as well as a small quantity of water. The temperature then rises rapidly to the boiling-point of benzyl benzoate, when the receivers are changed. The product boils at 184-185°/15 mm., and analysis by saponification shows it to consist of 99 per cent ester. A yield of 410-420 g. is obtained, which corresponds to 90-93 per cent of the theoretical amount. This benzyl benzoate supercools readily, but after solidifying... 

Table 1.10 compares the boiling points of an alkane, an alcohol, and an aldehyde with the same number of carbon atoms. You can see that the boiling point of an alcohol is much greater than the boiling point of an alkane or an aldehyde. 

In general, esters have sweet odors. For this reason, many are useful in perfumes or as flavorings. The boiling points of esters are similar to those of aldehydes and ketones of compcirable molar masses, which means that the boiling points cire lower than comparable alcohols. 

There is no report of the boiling point of the product in the literature.3 The product has an infrared absorption (CC14 solution) at 1732 cm.-1 (aldehyde C=0). 

The pure ketone has a 1.5240 and a boiling point of 128-130°/14 mm., 150°/30 mm. Insufficient removal of o-meth-oxybenzaldehyde will cause the refractive index to be high to the extent of about 0.0003 for each per cent present. The use of distilled nitroolefin eliminates the aldehyde, but this advantage is offset by the distillation hazard and slightly lower over-all yields. 

Care is essential to insure the complete separation of aldehyde and alcohol because of the fact that any slight continual loss of solvent is multiplied into a serious loss in the long period of heating necessary. The boiling point of chloral is only twenty degrees above that of ethyl alcohol and any loss of the latter would lead to some loss ot the reagent. 

Intermolecular forces between the particles of a liquid also can affect the liquid s boiling point. The graph shows trends in the boiling points of four organic families alkanes, alcohols, aldehydes, and ethers. Use the graph and your knowledge of intermolecular forces to answer the following questions. 

Because the hydrazone and semicarbazone derivatives of carbonyl compounds are often stable, crystalline solids, they used to be used to confirm the supposed identity of aldehydes and ketones. For example, the boiling points of... 

The boiling range of the aldehyde is 177-180° at 20 mm. The lower pressure is preferable to avoid decomposition. The checkers found a boiling point of 139-142 ° at 4 mm. The color of the distillate varies in different runs it may be green, pink, or amber. 

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