# 26: More on Aromatic Compounds


Generally, the reactivity of a substituent on an aromatic ring is greatly modified from that of its aliphatic counterpart. Likewise, the substituent can influence the reactivity of the ring. We have seen this interplay between ring and substituent in the chemistry of aryl halides (Section 14-6), of arenamines (Sections 23-7C and 23-9F), and in electrophilic substitution reactions of aromatic compounds (Section 22-5). It is particularly manifest in the chemistry of substances that have oxygen attached directly to arene rings. We shall discuss aryl oxygen compounds and some of their oxidation products called quinones in this chapter. We also shall discuss aromatic substances that have carbon substituents in the form of alkyl, haloalkyl (such as $$\ce{-CH_2Cl}$$, $$\ce{-CHCl_2}$$, $$\ce{-CCl_3}$$), aldehyde ($$\ce{-CHO}$$), and carboxylic acid ($$\ce{-CO_2H}$$) groups. We classify such substances as aromatic side-chain derivatives (for want of a better term)

• 26.1: Aryl Oxygen Compounds
Phenols are enols, and enols normally are unstable with respect to the corresponding carbonyl compounds. The situation is different for phenols because of the inclusion of the carbon-carbon double bond into the aromatic ring and the associated aromatic stabilization. Phenol (benzenol) exists exclusively in the enol form. In general, phenols are somewhat more polar than the corresponding saturated alcohols.
• 26.2: Quinones
Quinones are not aromatic compounds but are conjugated cyclic diketones. However, quinones and the related aromatic arenols are readily interconverted, and their chemistry is largely interdependent. A characteristic and important reaction of quinones is reduction to the corresponding arenediols. The reduction products of 1,4-quinones are called hydroquinones. Reduction can be achieved electrochemically and with differing reducing agents.
• 26.3: Tropolones and Related Compounds
The tropolones make up a very interesting class of nonbenzenoid aromatic compound that was discovered first in several quite different kinds of natural products. As one example, the substance called ββ -thujaplicin or hinokitiol has been isolated from the oil of the Formosan cedar and is 4-isopropyltropolone.
• 26.4: Some Aromatic Side-Chain Compounds
We have discussed in this chapter and in previous chapters how the reactivity of halogen, amino, and hydroxy substituents are modified when linked to aromatic carbons rather than to saturated carbons. Other substituents, particularly those linked to an aromatic ring through a carbon-carbon bond, also are influenced by the ring, although usually to a lesser degree. We shall refer to aromatic compounds containing substituents of this type as aromatic side-chain compounds.
• 26.5: Natural Occurrence and Uses of Some Aromatic Side-Chain Compounds
Derivatives of aromatic aldehydes occur naturally in the seeds of plants. For example, amygdalin is a substance occurring in the seeds of the bitter almond. It is a derivative of gentiobiose, which is a disaccharide made up of two glucose units; one of the glucose unites is bonded by a β-glucoside linkage to the OHOH group of the cyanohydrin of benzenecarbaldehyde
• 26.6: Correlations of Structure with Reactivity of Aromatic Compounds
This section is concerned with the quantitative correlation of reaction rates and equilibria of organic reactions with the structure of the reactants. We will restrict the discussion to benzene derivatives. The focus is on a remarkably simple treatment developed by L. P. Hammett in 1935, which has been tremendously influential. Hammett's correlation covers chemical reactivity, spectroscopy and other physical properties, and even the biological activity of drugs.
• 26.E: More on Aromatic Compounds (Exercises)
These are the homework exercises to accompany Chapter 26 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).