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1、939 CHAPTER 24 PHENOLS P henols are compounds that have a hydroxyl group bonded directly to a benzene or benzenoid ring. The parent compound of this group, C6H5OH, called simply phe- nol, is an important industrial chemical. Many of the properties of phenols are anal- ogous to those of alcohols, but
2、 this similarity is something of an oversimplifi cation. Like arylamines, phenols are difunctional compounds; the hydroxyl group and the aromatic ring interact strongly, affecting each others reactivity. This interaction leads to some novel and useful properties of phenols. A key step in the synthes
3、is of aspirin, for exam- ple, is without parallel in the reactions of either alcohols or arenes. With periodic reminders of the ways in which phenols resemble alcohols and arenes, this chapter emphasizes the ways in which phenols are unique. 24.1NOMENCLATURE An old name for benzene was phene, and it
4、s hydroxyl derivative came to be called phe- nol.* This, like many other entrenched common names, is an acceptable IUPAC name. Likewise, o-, m-, and p-cresol are acceptable names for the various ring-substituted hydroxyl derivatives of toluene. More highly substituted compounds are named as deriv- a
5、tives of phenol. Numbering of the ring begins at the hydroxyl-substituted carbon and proceeds in the direction that gives the lower number to the next substituted carbon. Sub- stituents are cited in alphabetical order. OH Phenol OH CH3 m-Cresol OH CH3 Cl 1 2 3 4 5 6 5-Chloro-2-methylphenol *The syst
6、ematic name for phenol is benzenol. BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website The three dihydroxy derivatives of benzene may be named as 1,2-, 1,3-, and 1,4- benzenediol, respectively, but each is more familiarly known by the common name indi- cated in parentheses below the struc
7、tures shown here. These common names are per- missible IUPAC names. The common names for the two hydroxy derivatives of naphthalene are 1-naph- thol and 2-naphthol. These are also acceptable IUPAC names. PROBLEM 24.1Write structural formulas for each of the following compounds: (a) Pyrogallol (1,2,3
8、-benzenetriol)(c) 3-Nitro-1-naphthol (b) o-Benzylphenol(d) 4-Chlororesorcinol SAMPLE SOLUTION(a) Like the dihydroxybenzenes, the isomeric trihydroxy- benzenes have unique names. Pyrogallol, used as a developer of photographic fi lm, is 1,2,3-benzenetriol. The three hydroxyl groups occupy adjacent po
9、sitions on a benzene ring. Carboxyl and acyl groups take precedence over the phenolic hydroxyl in deter- mining the base name. The hydroxyl is treated as a substituent in these cases. 24.2STRUCTURE AND BONDING Phenol is planar, with a COH angle of 109, almost the same as the tetrahedral angle and no
10、t much different from the 108.5 COH angle of methanol: O H 136 pm 109 Phenol O H 142 pm 108.5 CH3 Methanol HOCOH O p-Hydroxybenzoic acid CH3CCH3 O OH 65 41 23 2-Hydroxy-4-methylacetophenone OH OH OH Pyrogallol (1,2,3-benzenetriol) OH OH 1 2 3 4 5 6 1,2-Benzenediol (pyrocatechol) OH OH 1 2 3 4 5 6 1,
11、4-Benzenediol (hydroquinone) OH OH 1 2 3 4 5 6 1,3-Benzenediol (resorcinol) 940CHAPTER TWENTY-FOURPhenols Pyrocatechol is often called catechol. The graphic that opened this chapter is a molecular model of phenol that shows its planar structure and electrostatic po- tential. BackForwardMain MenuTOCS
12、tudy Guide TOCStudent OLCMHHE Website As weve 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 carbonoxygen bond distance in phenol is slightly less than that in methanol. In resonance terms, t
13、he shorter carbonoxygen 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 refl ect the polarization implied by the resonance description. The hyd
14、roxyl 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-ri
15、ch.” 24.3PHYSICAL PROPERTIES The physical properties of phenols are strongly infl uenced 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 m
16、ore soluble in water than arenes and aryl halides of comparable molecular weight. Table 24.1 compares phenol, toluene, and fl uorobenzene with regard to these physical properties. Some ortho-substituted phenols, such as o-nitrophenol, have signifi cantly 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 st
