Structure and Properties of Carboxylic Acids

Carboxylic acids are similar in some respects to both ketones and alcohols. Like ketones, the carboxyl carbon is sp -hybridized, and carboxylic acid groups are therefore planar with C-C=0 and 0=C-0 bond angles of approximately 120° (Table 20.2). 

A list of Ka values for various carboxylic acids is given in Table 20.3. For most, Ka is approximately to 10 . Acetic acid, for instance, has i a = 1.75 X 10 , which corresponds to a p/Cg of 4.76. In practical terms, aiCg value near 10 means that only about 0.1% of the molecules in a 0.1 M solution are dissociated, as opposed to the 100% dissociation found with strong mineral acids like HCl. 

Although much weaker than mineral acids, carboxylic acids are nevertheless much stronger acids than alcohols and phenols. The of ethanol, for example, is approximately 10 , making ethanol a weaker acid than acetic acid by a factor of 10  

Experimental evidence for the equivalence of the two carboxylate oxygens comes from X-ray crystallographic studies on sodium formate. Both carbon-oxygen bonds are 127 pm in length, midway between the C=0 bond (120 pm) and C-O bond (134 pm) of formic acid. An electrostatic potential map of the formate ion also shows how the negative charge (red) is dispersed equally over both oxygens. 

Problem 20.3 Assume you have a mixture of naphthalene and benzoic acid that you want to separate. How might you take advantage of the acidity of one component in the mixture to effect a separation  

Like Other Bronsted-Lowry acids discussed in Section 2.7, carboxylic acids dissociate slightly in dilute aqueous solution to give and the corresponding carboxylate anions, RC02. The extent of dissociation is given by an acidity constant, K.  

Why are carboxyhc acids so much more acidic than alcohols, even though both contain —OH groups An alcohol dissociates to give an alkoxide ion, in 

1 Provide both an lUPAC name and a common name for each of the following compounds  

The carbonyl group as well as the two atoms attached to the carbonyl carbon all reside in a plane. 

Carboxylate ions play an important role in how drugs are distributed throughout the body, as we saw in the Medically Speaking box at the end of Section 3.3 entitled Dmg Distribution and pKg . 

Compounds containing the -C=N functional group are called nitriles and undergo some chemistry similar to that of carboxylic acids. Simple open-chain nitriles are named by adding -nitrile as a suffix to the alkane name, with the nitrile carbon numbered Cl  

Nitriles can also be named as derivatives of carboxylic acids by replacing the -ic acid or -oic acid ending with -onitrile or by replacing the -carboxylic acid ending with -carbonitrile. The nitrile carbon atom is attached to Cl but is not itself numbered. 

Draw structures corresponding to the following lUPAC names  

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Structure and Properties of Carboxylic Acids 755 Table 20.2 Physical Parameters for Acetic Acid... 

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