19.3 Physical Properties Additionally, sp- hybridization of the hydroxyl oxygen allows one of its electron pairs to be delocalized by orbital overlap with the T system of the roup(Figure 19.1). In resonance terms, this electron delocalization is represented as H →>H—( OH OH Lone-pair donation from the hydroxyl oxygen makes the carbonyl group less elec trophilic than that of an aldehyde or ketone. The graphic that opened this chapter is an GURE 19.1 Carbon electrostatic potential map of formic acid that shows the most electron-rich site to be the and both oxygens g acid oxygen of the carbonyl group and the most electron-poor one to be, as expected, the OH The T component of the c=o group and the p or- Carboxylic acids are fairly acid, and benzoic acid have dipol system that includes carbo and the two oxygens 19.3 PHYSICAL PROPERTIES The melting points and boiling points of carboxylic acids are higher than those of hydro- carbons and oxygen-containing organic compounds of comparable size and shape and ndicate strong intermolecular attractive forces ng By Modeling and notice how much more intens carbon 2-Methyl-l-butene 2-Butanone 2-Butanol bp(I atm) 99°C A unique hydrogen-bonding arrangement, shown in Figure 19. 2, contributes to erties these attractive forces. The hydroxyl group of one carboxylic acid molecule acts as a proton donor toward the carbonyl oxygen of a second. In a reciprocal fashion, the n Appendix hydroxyl proton of the second carboxyl function interacts with the carbonyl oxygen of the first. The result is that the two carboxylic acid molecules are held together by two hydrogen bonds. So efficient is this hydrogen bonding that some carboxylic acids exist as hydrogen-bonded dimers even in the gas phase. In the pure liquid a mixture of hydrogen-bonded dimers and higher aggregates is present. In aqueous solution intermolecular association between carboxylic acid molecules is replaced by hydrogen bonding to water. The solubility properties of carboxylic acids are similar to those of alcohols. Carboxylic acids of four carbon atoms or fewer are mis cible with water in all proportions. FIGURE 19.2 Attrac positive(blue)and negative (red)electrostatic potentia ecular hydrogen bonding between two molecules of acetic acid Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteAdditionally, sp2 hybridization of the hydroxyl oxygen allows one of its unshared electron pairs to be delocalized by orbital overlap with the system of the carbonyl group (Figure 19.1). In resonance terms, this electron delocalization is represented as: Lone-pair donation from the hydroxyl oxygen makes the carbonyl group less elec￾trophilic than that of an aldehyde or ketone. The graphic that opened this chapter is an electrostatic potential map of formic acid that shows the most electron-rich site to be the oxygen of the carbonyl group and the most electron-poor one to be, as expected, the OH proton. Carboxylic acids are fairly polar, and simple ones such as acetic acid, propanoic acid, and benzoic acid have dipole moments in the range 1.7–1.9 D. 19.3 PHYSICAL PROPERTIES The melting points and boiling points of carboxylic acids are higher than those of hydro￾carbons and oxygen-containing organic compounds of comparable size and shape and indicate strong intermolecular attractive forces. A unique hydrogen-bonding arrangement, shown in Figure 19.2, contributes to these attractive forces. The hydroxyl group of one carboxylic acid molecule acts as a proton donor toward the carbonyl oxygen of a second. In a reciprocal fashion, the hydroxyl proton of the second carboxyl function interacts with the carbonyl oxygen of the first. The result is that the two carboxylic acid molecules are held together by two hydrogen bonds. So efficient is this hydrogen bonding that some carboxylic acids exist as hydrogen-bonded dimers even in the gas phase. In the pure liquid a mixture of hydrogen-bonded dimers and higher aggregates is present. In aqueous solution intermolecular association between carboxylic acid molecules is replaced by hydrogen bonding to water. The solubility properties of carboxylic acids are similar to those of alcohols. Carboxylic acids of four carbon atoms or fewer are mis￾cible with water in all proportions. bp (1 atm): 2-Methyl-1-butene 31°C O 2-Butanone 80°C OH 2-Butanol 99°C O OH Propanoic acid 141°C H OH C O H C O OH H O C OH 19.3 Physical Properties 739 FIGURE 19.1 Carbon and both oxygens are sp2 - hybridized in formic acid. The component of the CœO group and the p or￾bital of the OH oxygen over￾lap to form an extended system that includes carbon and the two oxygens. A summary of physical prop￾erties of some representative carboxylic acids is presented in Appendix 1. Examine the electrostatic potential map of butanoic acid on Learning By Modeling and notice how much more intense the blue color (positive charge) is on the OH hydrogen than on the hydrogens bonded to carbon. FIGURE 19.2 Attrac￾tions between regions of positive (blue) and negative (red) electrostatic potential are responsible for intermo￾lecular hydrogen bonding between two molecules of acetic acid. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website