Benzene ring, phenols

Aromatic alcohols are called phenols. The simplest phenol, also called phenol, forms when a hydroxyl group replaces a hydrogen atom in the benzene ring. Phenol (carbolic acid) was used as an antiseptic in the 1800s. Today other phenol derivatives are used in antiseptic mouthwashes and in cleaning disinfectants such as Lysol. Phenols are easily oxidized, and this makes them ideal substances to use as antioxidants. By adding phenols such as BHT (butylated hydroxy toluene) and BHA (butylated hydroxy anisole) to food, the phenols oxidize rather than the food. 

Phenol(s) Aromatic molecules in which one or more hydroxyl groups (-OH) are directly attached to a benzene ring. Phenols are powerful antibacterial compounds and essential oils containing phenols need to be used with caution. Examples of phenols in essential oils include thymol, carvacrol and eugenol. Phenol is the name for the parent compound, the simplest of the phenols (C6H5OH). 

Phenol (hydroxybenzene, 1) has a hydroxyl group attached directly to the benzene ring. Phenol is a stable enol and, although there are some obvious similarities, the hydroxyl group exhibits sufficiently different properties from an alcoholic hydroxyl group to merit a separate classification. 

Phenols contain an OH group attached to the sp carbon of a benzene ring. Phenol itself has no other substituent on the benzene ring ... 

In addition to being used to synthesize substituted benzenes, arenediazonium ions can be used as electrophiles in electrophilic aromatic substitution reactions. Because an arenediazonium ion is unstable at room temperature, it can be used as an electrophile only in reactions that can be carried out well below room temperature. In other words, only highly activated benzene rings (phenols, anilines, and N-alkylanilines) can undergo electrophilic aromatic substitution reactions with arenediazonium ion electrophiles. The product of the reaction is an azo compound. The N=N linkage is called an azo linkage. 

The alcohol function is found in simple aliphatic molecules, in acyclic and cyclic ter-penes, and in molecules containing benzene rings. Phenols are also contained in this group of aroma chemicals. 

Alcohols and phenols are characterized by the hydroxyl group, —OH. In alcohols, the hydroxyl group is bound to an sp hybridized carbon atom. If this C atom also has only one other C atom (and two H atoms) bonded to it, the alcohol is a primary alcohol. If it has two other C atoms (and one H atom), the alcohol is a secondary alcohol. Finally, if it bonded to three other C atoms (and no H atoms), it is a tertiary alcohol. The systematic naming of alcohols uses the suffix -ol and was discussed in Chapter 3. In phenols, the hydroxyl group is attached to a benzene ring. Phenols are more acidic than alcohols because the anion formed by a phenol is stabilized by resonance but the anion formed by an alcohol is not (see page 772). 

The most important reaction of the diazonium salts is the condensation with phenols or aromatic amines to form the intensely coloured azo compounds. The phenol or amine is called the secondary component, and the process of coupling with a diazonium salt is the basis of manufacture of all the azo dyestuffs. The entering azo group goes into the p-position of the benzene ring if this is free, otherwise it takes up the o-position, e.g. diazotized aniline coupled with phenol gives benzeneazophenol. When only half a molecular proportion of nitrous acid is used in the diazotization of an aromatic amine a diazo-amino compound is formed. 

One of the characteristic properties of phenol is the ease with which it gives substitution products, this property being particularly well shown by the ready nitration, sulphonation and bromination which the benzene ring in the phenol molecule undergoes. 

These materials will then slowly react with further formaldehyde to form their own methylol derivatives which in turn rapidly react with further phenol to produce higher polynuclear phenols. Because of the excess of phenol there is a limit to the molecultir weight of the product produced, but on average there are 5-6 benzene rings per molecule. A typical example of the many possible structures is shown in Figure 23.11. 

In this case the formation of phenol-alcohols is rapid but their subsequent condensation is slow. Thus there is a tendency for polyalcohols, as well as monoalcohols, to be formed. The resulting polynuclear polyalcohols are of low molecular weight. Liquid resols have an average of less than two benzene rings per molecule, while a solid resol may have only three to four. A typical resol would have the structure shown in Figure 23.12. 

An 5-(l-m-nitrophenyl-2-benzoyl)ethyl thioether was used to protect thio-phenols during electrophilic substitution reactions of the benzene ring. ... 

The acid-catalyzed reaction occurs by an electrophilic substitution where formaldehyde is the electrophile. Condensation between the methylol groups and the benzene rings results in the formation of methylene bridges. Usually, the ratio of formaldehyde to phenol is kept less than unity to produce a linear fusible polymer in the first stage. Crosslinking of the formed polymer can occur by adding more formaldehyde and a small amount of hexamethylene tetramine (hexamine. 

Benzene rings in both the skeleton structure and on the side groups can be subjected to substitution reactions. Such reactions do not normally cause great changes in the fundamental nature of the polymer, e.g. they seldom lead to chain scission or cross linking. (N.B. The phenolic resins provide an important exception here.)... 

The compound MON-0585 is a nontoxic, biodegradable larvicide that is highly selective against mosquito larvae. Synthesize MON-0585 using either benzene or phenol as a source of the aromatic rings. 

In phenol, aniline, and other compounds in which an atom with unshared electron pairs is attached directly to the benzene ring, structures such as = 0 H or especially... 

The oxidation of benzene to phenol and 1,4-dihydroxybenzene (Figure 2.11a) (Hyman et al. 1985), both side chain and ring oxidation of ethyl benzene, and ring-hydroxylation of halogenated benzenes and nitrobenzene (Keener and Arp 1994). 

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