24.3 Physical Properties As weve seen on a number of occasions, bonds to sp-hybridized carbon are shorter than those to sp-hybridized carbon, and the case of phenols is no exception. The carbon-oxygen bond distance in phenol is slightly less than that in methanol In resonance terms, the shorter carbon-oxygen bond distance in phenol is attrib- uted to the partial double-bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring +Oh +OH H Most stable lewis Dipolar resonance forms of phenol tructure for Many of the properties of phenols reflect the polarization implied by the resonance description. The hydroxyl oxygen is less basic, and the hydroxyl proton more acidic, in phenols than in alcohols. Electrophiles attack the aromatic ring of phenols much faster than they attack benzene, indicating that the ring, especially at the positions ortho and 24,3 PHYSICAL PROPERTIES The physical properties of phenols are strongly influenced by the hydroxyl group, which permits phenols to form hydrogen bonds with other phenol molecules(Figure 24.1a) and cal properties of with water(Figure 241b). Thus, phenols have higher melting points and boiling points and are more soluble in water than arenes and aryl halides of comparable molecular ected in Appendix 1 weight. Table 24.1 compares phenol, toluene, and fluorobenzene with regard to these Some ortho-substituted phenols, such as o-nitrophenol, have significantly lower boiling points than those of the meta and para isomers. This is because the intramolec- ular hydrogen bond that forms between the hydroxyl group and the substituent partially compensates for the energy required to go from the liquid state to the vapor. TABLE 24.1 Comparison of Physical Properties of an Arene, a Phenol, and an Aryl Halide Toluene, Phenol Fluorobenzene Physical property C6H5CH3 C6HsOH Molecular weight Melting point 95°c 43°C -41°C Boiling point (1 atm) 111°C 32°C Solubility in water(25.C) 0.05g/100mL 8.2g/100mL 0.2g/100ml Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteAs we’ve seen on a number of occasions, bonds to sp2 -hybridized carbon are shorter than those to sp3 -hybridized carbon, and the case of phenols is no exception. The carbon–oxygen bond distance in phenol is slightly less than that in methanol. In resonance terms, the shorter carbon–oxygen bond distance in phenol is attrib￾uted to the partial double-bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring. Many of the properties of phenols reflect the polarization implied by the resonance description. The hydroxyl oxygen is less basic, and the hydroxyl proton more acidic, in phenols than in alcohols. Electrophiles attack the aromatic ring of phenols much faster than they attack benzene, indicating that the ring, especially at the positions ortho and para to the hydroxyl group, is relatively “electron-rich.” 24.3 PHYSICAL PROPERTIES The physical properties of phenols are strongly influenced by the hydroxyl group, which permits phenols to form hydrogen bonds with other phenol molecules (Figure 24.1a) and with water (Figure 24.1b). Thus, phenols have higher melting points and boiling points and are more soluble in water than arenes and aryl halides of comparable molecular weight. Table 24.1 compares phenol, toluene, and fluorobenzene with regard to these physical properties. Some ortho-substituted phenols, such as o-nitrophenol, have significantly lower boiling points than those of the meta and para isomers. This is because the intramolec￾ular hydrogen bond that forms between the hydroxyl group and the substituent partially compensates for the energy required to go from the liquid state to the vapor. Dipolar resonance forms of phenol H H H H H OH Most stable Lewis structure for phenol H H H H H OH H H H H H OH H H H H H OH 24.3 Physical Properties 941 The physical properties of some representative phenols are collected in Appendix 1. TABLE 24.1 Comparison of Physical Properties of an Arene, a Phenol, and an Aryl Halide Physical property Molecular weight Melting point Boiling point (1 atm) Solubility in water (25°C) Toluene, C6H5CH3 92 95°C 111°C 0.05 g/100 mL Phenol, C6H5OH 94 43°C 132°C 8.2 g/100 mL Fluorobenzene, C6H5F 96 41°C 85°C 0.2 g/100 mL Compound Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website