{"id":1512,"date":"2017-10-12T14:11:43","date_gmt":"2017-10-12T14:11:43","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/?post_type=chapter&p=1512"},"modified":"2018-10-05T19:06:51","modified_gmt":"2018-10-05T19:06:51","slug":"the-nomenclature-of-monosubstituted-benzenes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/chapter\/the-nomenclature-of-monosubstituted-benzenes\/","title":{"raw":"The Nomenclature of Monosubstituted Benzenes","rendered":"The Nomenclature of Monosubstituted Benzenes"},"content":{"raw":"
\u00a0Benzene, C6<\/sub>H6<\/sub>, is an organic aromatic compound with many interesting properties. Unlike aliphatic (straight chain carbons) or other cyclic organic compounds, the structure of benzene (3 conjugated \u03c0 bonds) allows benzene and its derived products to be useful in fields such as health, laboratory, and other applications such as rubber synthesis.<\/div>\r\n
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Introduction<\/h3>\r\nBenzene derived products are well known to be pleasantly fragrant. For this reason, organic compounds containing benzene rings were classified as being \"aromatic\" (sweet smelling) amongst scientists in the early 19th century when a relation was established between benzene derived compounds and sweet\/spicy fragrances. There is a misconception amongst the scientific community, however, that all aromatics are sweet smelling and that all sweet smelling compounds would have a benzene ring in its structure. This is false, since non-aromatic compounds, such as camphor, extracted from the camphor laurel tree, release a strong, minty aroma, yet it lacks the benzene ring in its structure (Figure 1). On the other hand, benzene itself gives off a rather strong and unpleasant smell that would otherwise invalidate the definition of an aromatic (sweet-smelling) compound. Despite this inconsistency, however, the term aromatic continues to be used today in order to designate molecules with benzene-like rings in their structures. For a modern, chemical definition of aromaticity, refer to sections Aromaticity<\/a> and H\u00fcckel's Rule<\/a>.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"103\"]\"Camphor.png\" Figure 1. Top-view of camphor, along with its monoterpene unit. Notice how camphor lacks the benzene ring to be \"aromatic\".[\/caption]\r\n\r\nMany aromatic compounds are however, sweet\/pleasant smelling. Eugenol, for example, is extracted from essential oils of cloves and it releases a spicy, clove-like aroma used in perfumes. In addition, it is also used in dentistry as an analgesic.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"211\"]\"Eugenol.png\" Figure 2. Eugenol, an aromatic compound extracted from clove essential oils. Used in perfumes and as an analgesic. The benzene ring is labeled in red in the eugenol molecule.[\/caption]\r\n\r\n
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Is it cyclohexane or is it benzene?<\/h4>\r\nDue to the similarity between benzene <\/strong>and cyclohexane<\/strong><\/a>, the two is often confused with each other in beginning organic chemistry students.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"550\"]\"CyclohexanevsBenzene.png\" Figure 3. Structure comparison between cyclohexane and benzene[\/caption]\r\n\r\nIf you were to count the number of carbons and hydrogens in cyclohexane<\/strong>, you will notice that its molecular formula is C6<\/sub>H12<\/sub><\/strong>. Since the carbons in the cyclohexane <\/strong>ring is fully saturated<\/a> <\/strong>with hydrogens (carbon is bound to 2 hydrogens and 2 adjacent carbons), no double bonds are formed in the cyclic ring. In contrast, benzene <\/strong>is only saturated<\/a> with one hydrogen per carbon, leading to its molecular formula of C6<\/sub>H6<\/sub><\/strong>. In order to stabilize this structure, 3 conjugated \u03c0 (double) bonds are formed in the benzene ring in order for carbon to have four adjacent bonds.\r\n
In other words, cyclohexane is not the same as benzene! These two compounds have different molecular formulas and their chemical and physical properties are not the same. The hydrogenation<\/strong><\/a> technique can be used by chemists to convert from benzene to cyclohexane by saturating the benzene ring with missing hydrogens.<\/blockquote>\r\nA special catalyst is required to hydrogenate benzene rings due to its unusual stability and configuration. Normal catalytic hydrogenation techniques will not hydrogenate benzene and yield any meaningful products.\r\n\r\n<\/div>\r\n
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What about Resonance?<\/h4>\r\nBenzene can be drawn a number of different ways. This is because benzene's conjugated pi electrons freely resonate within the cyclic ring, thus resulting in its two resonance forms.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"224\"]\"BenzeneResonance.png\" Figure 4. The Figure to the left shows the two resonance forms of benzene. The delocalized electrons are moved from one carbon to the next, thus providing stabilization energy. Ring structures stabilized by the movement of delocalized electrons are sometimes referred to as arenes.[\/caption]\r\n\r\n\"Benzene1.png\"\"Benzene2.png\"\"BenzeneCirc\"BenzeneCircDashed.png\"\r\n\r\nAs the electrons in the benzene ring can resonate within the ring at a fairly high rate, a simplified notation is often used to designate the two different resonance forms. This notation is shown above, with the initial three pi bonds (#1, #2) replaced with an inner ring circle (#3). Alternatively, the circle within the benzene ring can also be dashed to show the same resonance forms (#4).\r\n\r\n<\/div>\r\n
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The Formation of the Phenyl Group and its Derivatives<\/h4>\r\nThe phenyl group can be formed by taking benzene, and removing a hydrogen from it. The resulting molecular formula for the fragment is C6<\/sub>H5<\/sub>. NOTE: Although the molecular formula of the phenyl group is C6<\/sub>H5<\/sub>, the phenyl group would always have something attached to where the hydrogen was removed. Thus, the formula is often written as Ph-R, where Ph refers to the Phenyl group, and R refers to the R group attached to where the hydrogen was removed.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"369\"]\"PhGroup.png\" Figure 5. Figure demonstrating the removal of hydrogen to form the phenyl group.[\/caption]\r\n\r\nDifferent R groups on the phenyl group allows different benzene derivatives to be formed. Phenol, Ph-OH, or C6<\/sub>H5<\/sub>OH, for example, is formed when an alcohol (-OH) group displaces a hydrogen atom on the benzene ring.\u00a0Benzene, for this very same reason, can be formed from the phenyl group by reattaching the hydrogen back its place of removal. Thus benzene, similar to phenol, can be abbreviated Ph-H, or C6<\/sub>H6<\/sub>.\r\n\r\n\"PhenylPhenolDesc.png\"<\/a>\r\n\r\n\"EGCG.png\"\r\n\r\nAs you can see above, these are only some of the many possibilities of the benzene derived products that have special uses in human health and other industrial fields.\r\n\r\n<\/div>\r\n<\/div>\r\n
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Nomenclature of Benzene Derived Compounds<\/h3>\r\nUnlike 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. In these sections, we will analyze some of the ways these compounds can be named.\r\n
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Simple Benzene Naming<\/h4>\r\nSome common substituents, like NO2<\/sub>, Br, and Cl, can be named this way when it is attached to a phenyl group. Long chain carbons attached can also be named this way. The general format for this kind of naming is:\r\n\r\n(positions of substituents (if >1)- +\u00a0# (di, tri, ...) +\u00a0substituent)n<\/sub> +\u00a0benzene<\/strong>.\r\n\r\nFor example, chlorine (Cl) attached to a phenyl group would be named chlorobenzene (chloro +\u00a0benzene)<\/strong>. Since there is only one substituent on the benzene ring, we do not have to indicate its position on the benzene ring (as it can freely rotate around and you would end up getting the same compound.)\r\n\r\n\"Chlorobenzene\"Nitrobenzene\r\nFigure 8. <\/strong>Example of simple benzene naming with chlorine and NO2<\/sub> as substituents.<\/em>\r\n\r\n\"13dichlorobenzene.png\"\"1chloro3ethylbenzene.png\"\r\nFigure 9. <\/strong>More complicated simple benzene naming examples - Note that standard nomenclature priority rules are applied here, causing the numbering of carbons to switch. See Nomenclature of Organic Compounds<\/a> for a review on naming and priority rules.<\/em>\r\n\r\n<\/div>\r\n
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Ortho-, Meta-, Para- (OMP) Nomenclature for Disubstituted Benzenes<\/h4>\r\nInstead of using numbers to indicate substituents on a benzene ring, ortho- (o-), meta- (m-), or para (p-)<\/em>\u00a0<\/strong>can be used in place of positional markers when there are two<\/strong> substituents on the benzene ring (disubstituted benzenes). They are defined as the following:\r\n