19.6 Substituents and acid Strength disrupts the water structure least. Because their surfaces are negatively charged, two micelles repel each other rather than clustering to form higher aggregates It is the formation of micelles and their properties that are responsible for the cleansing action of soaps. Water that contains sodium stearate removes grease by enclos- ing it in the hydrocarbon-like interior of the micelles. The grease is washed away with the water not because it dissolves in the water but because it dissolves in the micelles that are dispersed in the water. Sodium stearate is an example of a soap; sodium and potassium salts of other C12-Ci8 unbranched carboxylic acids possess similar properties Detergents are substances, including soaps, that cleanse by micellar action. A large number of synthetic detergents are known. One example is sodium lauryl sulfate Sodium lauryl sulfate has a long hydrocarbon chain terminating in a polar sulfate ion and forms tic potenti f sodium lau soap-like micelles in water. on Learning By Modeling. Sodium lauryl sulfate (sodium dodecyl sulfate) Detergents are designed to be effective in hard water, meaning water containing calcium salts that form insoluble calcium carboxylates with soaps. These precipitates rob the soap of its cleansing power and form an unpleasant scum. The calcium salts of synthetic deter- gents such as sodium lauryl sulfate, however, are soluble and retain their micelle-forming ability in water. 19.6 SUBSTITUENTS AND ACID STRENGTH Alkyl groups have little effect on the acidity of a carboxylic acid. The ionization con- stants of all acids that have the general formula CnH2n+I CO2H are very similar to one another and equal approximately 10(pKa 5). Table 19.2 gives a few examples An electronegative substituent, particularly if it is attached to the a carbon, increases the acidity of a carboxylic acid. As the data in Table 19.2 show, all the mono- haloacetic acids are about 100 times more acidic than acetic acid. Multiple halogen sub- stitution increases the acidity even more; trichloroacetic acid is 7000 times more acidic than acetic acid! The acid-strengthening effect of electronegative atoms or groups is easily seen as an inductive effect of the substituent transmitted through the g bonds of the molecule According to this model. the g electrons in the carbon-chlorine bond of chloroacetate ion are drawn toward chlorine, leaving the a-carbon atom with a slight positive charge The a carbon, because of this positive character, attracts electrons from the negativel harged carboxylate, thus dispersing the charge and stabilizing the anion. The more sta ble the anion, the greater the equilibrium constant for its formation H contains molecular models of CH3 COz(acetate)and Cl3Cco Chloroacetate anion is hese two ions with respect to tabilized by electron- the amount of negative charge withdrawing effect of on their oxygens. Back Forward Main MenuToc Study Guide ToC Student o MHHE Websitedisrupts the water structure least. Because their surfaces are negatively charged, two micelles repel each other rather than clustering to form higher aggregates. It is the formation of micelles and their properties that are responsible for the cleansing action of soaps. Water that contains sodium stearate removes grease by enclos￾ing it in the hydrocarbon-like interior of the micelles. The grease is washed away with the water, not because it dissolves in the water but because it dissolves in the micelles that are dispersed in the water. Sodium stearate is an example of a soap; sodium and potassium salts of other C12–C18 unbranched carboxylic acids possess similar properties. Detergents are substances, including soaps, that cleanse by micellar action. A large number of synthetic detergents are known. One example is sodium lauryl sulfate. Sodium lauryl sulfate has a long hydrocarbon chain terminating in a polar sulfate ion and forms soap-like micelles in water. Detergents are designed to be effective in hard water, meaning water containing calcium salts that form insoluble calcium carboxylates with soaps. These precipitates rob the soap of its cleansing power and form an unpleasant scum. The calcium salts of synthetic deter￾gents such as sodium lauryl sulfate, however, are soluble and retain their micelle-forming ability in water. 19.6 SUBSTITUENTS AND ACID STRENGTH Alkyl groups have little effect on the acidity of a carboxylic acid. The ionization con￾stants of all acids that have the general formula CnH2n1CO2H are very similar to one another and equal approximately 105 (pKa 5). Table 19.2 gives a few examples. An electronegative substituent, particularly if it is attached to the  carbon, increases the acidity of a carboxylic acid. As the data in Table 19.2 show, all the mono￾haloacetic acids are about 100 times more acidic than acetic acid. Multiple halogen sub￾stitution increases the acidity even more; trichloroacetic acid is 7000 times more acidic than acetic acid! The acid-strengthening effect of electronegative atoms or groups is easily seen as an inductive effect of the substituent transmitted through the bonds of the molecule. According to this model, the electrons in the carbon–chlorine bond of chloroacetate ion are drawn toward chlorine, leaving the -carbon atom with a slight positive charge. The  carbon, because of this positive character, attracts electrons from the negatively charged carboxylate, thus dispersing the charge and stabilizing the anion. The more sta￾ble the anion, the greater the equilibrium constant for its formation. Cl C C H O O H   Chloroacetate anion is stabilized by electron￾withdrawing effect of chlorine. O O S O Na O 2 Sodium lauryl sulfate (sodium dodecyl sulfate) 19.6 Substituents and Acid Strength 745 Compare the electrosta￾tic potential maps of sodium lau￾ryl sulfate and sodium stearate on Learning By Modeling. Learning By Modeling contains molecular models of CH3CO2 (acetate) and Cl3CCO2 (trichloroacetate). Compare these two ions with respect to the amount of negative charge on their oxygens. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website