The potential reversibility of the aromatic sulfonation reaction was noted earlier. The following equation illustrates how this characteristic of the sulfonic acids may be used to prepare the 3-bromo derivative of ortho-xylene. Direct bromination would give the 4-bromo derivative.
Direct nitration of phenol hydroxybenzene by dilute nitric acid gives modest yields of nitrated phenols and considerable oxidative decomposition to tarry materials; aniline aminobenzene is largely destroyed. Bromination of both phenol and aniline is difficult to control, with di- and tri-bromo products forming readily. Because of their high nucleophilic reactivity, aniline and phenol undergo substitution reactions with iodine, a halogen that is normally unreactive with benzene derivatives.
The mixed halogen iodine chloride ICl provides a more electrophilic iodine moiety, and is effective in iodinating aromatic rings having less powerful activating substituents. By acetylating the heteroatom substituent on phenol and aniline, its activating influence can be substantially attenuated.
For example, acetylation of aniline gives acetanilide first step in the following equation , which undergoes nitration at low temperature, yielding the para-nitro product in high yield. The modifying acetyl group can then be removed by acid-catalyzed hydrolysis last step , to yield para-nitroaniline.
The following diagram illustrates how the acetyl group acts to attenuate the overall electron donating character of oxygen and nitrogen. The non-bonding valence electron pairs that are responsible for the high reactivity of these compounds blue arrows are diverted to the adjacent carbonyl group green arrows.
It should now be apparent that an extensive "toolchest" of reactions are available to us for the synthesis of substituted benzenes. Six proposed syntheses are listed in the following diagram in rough order of increasing complexity. You should try to conceive a plausible reaction sequence for each. Once you have done so, you may check suggested answers by clicking on the question mark for each.
Compounds in which two or more benzene rings are fused together were described in an earlier section , and they present interesting insights into aromaticity and reactivity.
The smallest such hydrocarbon is naphthalene. Naphthalene is stabilized by resonance. Three canonical resonance contributors may be drawn, and are displayed in the following diagram.
The two structures on the left have one discrete benzene ring each, but may also be viewed as pi-electron annulenes having a bridging single bond. The structure on the right has two benzene rings which share a common double bond. As expected from an average of the three resonance contributors, the carbon-carbon bonds in naphthalene show variation in length, suggesting some localization of the double bonds. The C1—C2 bond is 1.
This contrasts with the structure of benzene, in which all the C—C bonds have a common length, 1. Naphthalene is more reactive than benzene, both in substitution and addition reactions, and these reactions tend to proceed in a manner that maintains one intact benzene ring. The following diagram shows three oxidation and reduction reactions that illustrate this feature. Electrophilic substitution reactions take place more rapidly at C1, although the C2 product is more stable and predominates at equilibrium.
Examples of these reactions will be displayed by clicking on the diagram. The kinetically favored C1 orientation reflects a preference for generating a cationic intermediate that maintains one intact benzene ring. By clicking on the diagram a second time , the two naphthenonium intermediates created by attack at C1 and C2 will be displayed. The structure and chemistry of more highly fused benzene ring compounds, such as anthracene and phenanthrene show many of the same characteristics described above.
The chief products are phenol and diphenyl ether see below. This apparent nucleophilic substitution reaction is surprising, since aryl halides are generally incapable of reacting by either an S N 1 or S N 2 pathway.
The presence of electron-withdrawing groups such as nitro ortho and para to the chlorine substantially enhance the rate of substitution, as shown in the set of equations presented on the left below. To explain this, a third mechanism for nucleophilic substitution has been proposed. This two-step mechanism is characterized by initial addition of the nucleophile hydroxide ion or water to the aromatic ring, followed by loss of a halide anion from the negatively charged intermediate. When there is a single substituent on a benzene ring and the substituent contains six or fewer carbons, the substituent is included as a prefix to benzene.
Alkyl groups are named according to the alkane series convention ending with -yl: methyl for a single carbon , ethyl for two carbons , propyl for three carbons , etc. If the substituent contains more than six carbons, the alkane portion is named first, and the aromatic ring portion is added as a suffix.
For instance, an aromatic ring bonded to an 8-carbon chain would be 1-phenyloctane, and not octylbenzene. When there are multiple substituents, ring atoms are numbered to minimize the numbers assigned to the substituted positions.
Disubstituted benzene rings can be named based on the relative positions of the substituents: the prefix ortho — is used if the substituents occupy adjacent positions on the ring 1,2 , meta — is used if the substituents are separated by one ring position 1,3 , and para — if they are found on opposite sides of the ring 1,4.
This involves heating a C6-C10 alkane fraction of petroleum with hydrogen in the presence of a catalyst to modify the molecular structure of its components. Some amazing transformations take place, and the C6-C7 alkanes can be converted to cycloalkanes, which, in turn, are converted to arenes.
Benzene, and methylbenzene toluene are produced primarily in this way. Unlike aliphatic organics, nomenclature of benzene-derived compounds can be confusing because a single aromatic compound can have multiple possible names such as common and systematic names be associated with its structure. Common names are often used in the nomenclature of aromatic compounds.
IUPAC still allows for some of the more widely used common name to be used. These common names take the place of the benzene base name.
Methylbenzene is commonly known with the base name toluene, hydroxyphenol is known as phenol etc. It is very important to be able to identify these structures as they will be utilized in the nomenclature of more complex compounds.
Mono-substituted benzene rings, with a substituent not on the list above, are named with benzene being the parent name. If the alkyl group attached to the benzene contains seven or more carbons the compounds is named as a phenyl substituted alkane. The name phenyl C 6 H 5 - is often abbreviated Ph and comes from the Greek word pheno which means "I bear light". This name commemorates the fact that benzene was first isolated by Michael Faraday in from the residue left in London street lamps which burned coal gas.
If the alkyl substituent is smaller than the benzene ring six or fewer carbons , the compound is named as an alkyl-substituted benzene following the rules listed above. The benzyl group abbv. Nomenclature of benzyl group based compounds are very similar to the phenyl group compounds. For example, a chlorine attached to a benzyl group would simply be called benzyl chloride, whereas an OH group attached to a benzyl group would simply be called benzyl alcohol. With disubstituted benzenes there are three distinct positional isomers which can occur and must be identified in the compounds name.
Although numbering can be used to indicate the position of the two subsituents it is much more common for the compounds to be named using prefixes. These prefixes are italicized and are often abbreviated with a single letter.
They are defined as the following:. When three or more substituents are present the ortho, meta, para positional prefixes become inadequate and a numbering system for the ring must be applied. Also, this substituent is given position one in the numbering system. The other substituents are numbered such that they get the lowest possible sum. In the compound's name the subsituents are given their position number and listed alphabetically.
Remember that di-, tri, tetra- prefixes are still used to indicate multiple of the same substituent being present but are ignored for alphabetical listing.
Benzene unusual stability is caused by how many conjugated pi bonds in its cyclic ring? Menthol, a topical analgesic used in many ointments for the relief of pain, releases a peppermint aroma upon exposure to the air.
Based on this conclusion, can you imply that a benzene ring is present in its chemical structure? Why or why not? No, a substance that is fragrant does not imply a benzene ring is in its structure. See camphor example figure 1. No reaction, benzene requires a special catalyst to be hydrogenated due to its unusual stability given by its three conjugated pi bonds.
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