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Aromatic Nomenclature

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  1. Aromatic Nomenclature Naming substituted benzenes

  2. Monosubstituted Benzenes • Most monosubstituted aromatics are named using -benzene as the parent name preceded by the substituent name (as a prefix; all one word): nitro ethyl fluoro fluorobenzene nitrobenzene ethylbenzene

  3. Alkyl-substituted Benzenes • Alkyl substituted benzenes are named according to the length of the carbon chain of the alkyl group. • With six carbons or fewer in the alkyl chain, they are named as ‘alkylbenzene.’ • e.g., propylbenzene:

  4. Alkyl-substituted Benzenes • With more than six carbons in the alkyl chain, they are named as a ‘phenylalkane,’ where the benzene ring is named as a substituent (phenyl) on the alkane chain • e.g., 4-phenylnonane 4-phenylnonane

  5. Common Names of Subs. Benzenes • There are a number of nonsystematic (common) names commonly used for certain monosubstituted benzenes (see next slide) • These ten common names should be memorized. • These common names are used as base names when naming more their more highly substituted derivatives. Examples of these will be given later.

  6. Common Names of Subs. Benzenes

  7. The Benzyl Group • The benzyl group is a common name for a methyl substituted benzene (toluene) having substitution for one of the hydrogens on the methyl group. the benzyl group benzyl bromide benzyl alcohol

  8. Disubstituted Benzenes • Disubstituted benzenes can be named in one of two ways. Each method describes the relative positions of the two groups on the benzene ring. • Systematic numbering of the aromatic ring. • Using the prefixesortho-, meta-, or para-. • When numbering the ring carbons, carbon # 1 is always a substituted carbon. • The substituents are listed alphabetically.

  9. Disubstituted Benzenes ortho- (abbreviated o- ) = 1,2-disubstituted (two groups on adjacent carbons on the ring)

  10. Disubstituted Benzenes meta- (abbreviated m- ) = 1,3-disubstituted (two groups having one unsubstituted carbon between them)

  11. Disubstituted Benzenes para- (abbreviated p- ) = 1,4-disubstituted (two groups on opposite sides of the ring)

  12. Disubstituted Benzenes • When one of the substituents changes the base name, either o-, m-, and p- or numbers may be used to indicate the position of the other substituent. • Carbon # 1 is always the carbon bearing the substituent that changes the base name. 4 1 2 3 2 1 p-bromoaniline or 4-bromoaniline o-chlorophenol or 2-chlorophenol

  13. Common Names of Disubs. Benzenes • There are a few nonsystematic (common) names for disubstituted benzenes that you should be familiar with:

  14. Polysubstituted Benzenes • Polysubstituted benzenes must be named by numbering the position of each substituent on the ring (with more than two substituents, o-, m-, and p-can NOT be used.) • The numbering is carried out to give the substituents the lowest possible numbers. Carbon #1 always has a substituent. • List the substitutents alphabetically with their appropriate #s. 2 1 3 4 2-ethyl-1-fluoro-4-nitrobenzene

  15. Polysubstituted Aromatics having a Common base name • Common names of the monosubstituted benzenes are used as parent names for polysubstituted aromatics when one of the substituents changes the base name. • For such rings with common names, the carbon bearing the substituent responsible for the common name is always carbon #1. • The substitutents are listed in alphabetical order. toluene 1 chloro 2 bromo 3 5 4 5-bromo-2-chlorotoluene

  16. Polysubstituted Benzenes 4 1 3 2 3 2 5 1 4 4-bromo-2-ethyl-1-nitrobenzene 5-bromo-2-chlorophenol

  17. Polysubstituted Benzenes 2 1 1 2 3 6 3 6 4 5 5 4 2-bromo-6-chloro-4-nitrotoluene 1-bromo-3-chloro-2-ethyl-5-nitrobenzene

  18. Polycyclic Aromatic Hydrocarbons (PAH) Metabolic byproducts of benzo [a] pyrene react with DNA to form adducts, leading to carcinogenesis (cancer).