Boiling Points of Aldehydes and Ketones

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. 

Problem 14.10. Although the boiling and melting points of aldehydes and ketones are considerably lower than those of alcohols, there is little difference in their solubilities in water. Explain why aldehydes and ketones are as soluble in water as are alcohols. 

Physical constants such as melting point boiling point and solubility in water are collected for a variety of aldehydes and ketones in Appendix 1... 

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. 

The polarity of the carbonyl group is manifest in the physical properties of carbonyl compounds. Boiling points for the lower members of a series of aldehydes and ketones are 50-80° higher than for hydrocarbons of the same molecular weight this may be seen by comparing the data of Table 16-2 (physical... 

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... 

These imines are solids and are useful for the characterization of aldehydes and ketones. For example, infrared and nmr spectroscopy may indicate that a certain unknown is acetaldehyde. It is difficult to determine the boiling point of a few milligrams of a liquid, but if it can be converted to a solid derivative the mp can be determined with that amount. The 2,4-dini-trophenylhydrazones are usually the derivatives of choice because they are nicely crystalline compounds with well-defined melting or decomposition points and they increase the molecular weight by 180. Ten milligrams of acetaldehyde will give 51 mg of 2,4-DNP ... 

Esters are mildly polar and have pleasant aromas. The boiling points and melting points of esters are comparable to those of aldehydes and ketones. Esters are formed from the reaction between a carboxylic acid and an alcohol. They can undergo hydrolysis back to the parent carboxylic acid and alcohol. The base-catalyzed hydrolysis of an ester is called saponification. 

Water-soluble hydrazones of aldehydes and ketones are obtained by means of iV-methylnicotinhydrazide /7-toluenesulfonate (9),928 the components being boiled for 15 min in anhydrous ethanol the products have sharp melting points and can be purified easily it is also easy to cleave them by acid and to convert them into the 2,4-dinitrophenylhydrazones in acid solution. 

Therefore, boiling points of pure aldehydes and ketones are expected to be lower than those of alcohols with similar molecular weights. Remember, alcohols can form hydrogen bonds with one another. % Table 4.2 shows that the boiling points of propanal and acetone are 49°C and 56°C, respectively, whereas the alcohol of comparable molecular weight, 1-propanol, has a boiling point of 97°C. 

Also notice in Table 4.2 that the boiling points of the aldehyde and ketone are higher than that of the comparable alkane. This can be explained by differences in polarity. Whereas alkanes are nonpolar, the carbonyl group of aldehydes and ketones is polar due to the more electronegative oxygen atom ... 

Because the carbonyl groups of aldehydes and ketones interact with water molecules by hydrogen bonding, low-molecular-weight aldehydes and ketones are more soluble in water than are nonpolar compounds of comparable molecular weight. Table 12.3 lists the boiling points and solubilities in water of several low-molecular-weight aldehydes and ketones. 

Melting and Solidifying-Points—Boiling-Point and Distillation— Determination of Esters—Determination of Alcohols— Tables for the Calculation of Esters and Alcohols—Separate DMermination of Citronellol in Presence of Gwaniol—Determination of Aldehydes and Ketones— Miscellaneous Processes—Determination of Phenols—Detection of Chlorine—Determination of Hydrocarbons—Detection of some Common Adulterants. 

The polyhydric alcohols of Solubility Group II are liquids of relatively high boiling point and may be detected inter alia by the reactions already described for Alcohols (see 6). Compounds containing two hydroxyl groups attached to adjacent carbon atoms (1 2-glyeols), a-hydroxy aldehydes and ketones, and 1 2-diketones may be identified by the periodic acid test, given in reaction 9. 

Hydrogen bonding m carboxylic acids raises their melting points and boiling points above those of comparably constituted alkanes alcohols aldehydes and ketones... 

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