Physical properties of carboxylic acids

The properties of salts of long-chain carboxylic acids that make them useful as soaps will be discussed in Section 18-2F. 

General methods for the preparation of carboxylic acids are summarized in Table 18-5, at the end of the chapter. 

18-1 PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS 18-1A Hydrogen Bonding 

The carboxyl function does absorb ultraviolet radiation, but the wavelengths at which this occurs are appreciably shorter than for carbonyl compounds such as aldehydes and ketones, and, in fact, are out of the range of most commercial ultraviolet spectrometers. Some idea of how the hydroxyl substituent modifies the absorption properties of the carbonyl group in carboxylic acids can be seen from Table 18-2, in which are listed the wavelengths of maximum light absorption (Amax) and the extinction coefficients at maximum absorption (emax) of several carboxylic acids, aldehydes, and ketones. 

Wavelengths tor Maximum Ultraviolet Absorption of Some Carboxylic Acids, Aldehydes, and Ketones (n--- n ) 

The first three members of the saturated monocarboxylic acids are colorless liquids with a sharp odor. Carboxylic acids with carbon numbers between 4 and 9 smell of butter and almond oil. The smell of spoilt butter and cheese and the unpleasant odor of sweat is due acid formation. For example, butyric acid smells like butter because it is found in butter. 

Carboxylic acids having more than ten carbons are odorless, wax-like solids. The acids from C14 to C22 are found in vegetable oils and fats and are known as fatty acids. 

Some of the physical and chemical properties of carboxylic acids are dependent upon the polarity of the carboxyl group. Acid molecules can form strong hydrogen bonds with each other and as a result carboxylic acid molecules can form dimers. 

Because of the strong hydrogen bonds, carboxylic acid molecules form dimers. 

The boiling points of carboxylic acids are approximately 30-40 °C higher than those of hydrocarbons, alcohols, ethers, aldehydes and ketones of comparable molar mass. For example, ethyl alcohol boils at 78 °C and formic acid at 100.5 °C. 

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Table 1.12 lists some of the physical properties of carboxylic acids. Notice that carboxylic acids have even stronger hydrogen bonding than alcohols. 

Polarity of functional group Because the nitrogen atom attracts electrons more strongly than carbon or hydrogen atoms, the C-N and N-H bonds are polar. As a result, the physical properties of amides are similar to the physical properties of carboxylic acids. 

Physical properties of carboxylic acids and derivatives include solubility, melting point, boiling point, and a few other characteristics. In this section we examine each class and discuss the most important physical properties. (In the upcoming section Considering the Acidity of Carboxylic Acids, we discuss the most important chemical property of Ccirboxylic acids — acidity.)... 

Problem 16.3 Account for the following physical properties of carboxylic acids, (a) Only RCOOH s with five or fewer C s are soluble in water, but many with six or more C s dissolve in alcohols. (f>) Acetic acid in the vapor state has a molecular weight of 120 not 60. (c) Their boiling and melting points are higher than those of corresponding alcohols. M... 

Structure and Nomenclature of Acid Derivatives 982 21-3 Physical Properties of Carboxylic Acid Derivatives 988 21-4 Spectroscopy of Carboxylic Acid Derivatives 991 21-5 Interconversion of Acid Derivatives by Nucleophilic Acyl Substitution 997... 

Q Draw and name carboxylic acids and dicarboxylic acids, and use spectral information to determine their structures. Q Describe the trends in the acidity and physical properties of carboxylic acids, and explain how their acidity varies with their substituents. 

How these intermolecular forces affect the physical properties of carboxylic acids is summarized in Table 19.2. 

In Sec. 18.4 we contrasted physical properties of carboxylic acids with those of their salts amines and their salts show the same contrast. Amine salts are typical ionic compounds. They are non-volatile solids, and when heated generally decompose before the high temperature required for melting is reached. The halides, nitrates, and sulfates are soluble in water but are insoluble in non-polar solvents. 

Checking Out Some Physical Properties of Carboxylic Acids ana OeriPatiOes... 

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