Aromatic Chemistry PDF

Medical Chemistry Aromatic compounds Dr/ Wael Sobhy Darwish 1ST stage&2024  Introduction Aromatic word is derived from the Greek word „aroma‟ which means fragrant smell. For such reason, benzene and its derivative were originally classify as aromatic compounds as many of them have d...

Medical Chemistry Aromatic compounds Dr/ Wael Sobhy Darwish 1ST stage&2024  Introduction Aromatic word is derived from the Greek word „aroma‟ which means fragrant smell. For such reason, benzene and its derivative were originally classify as aromatic compounds as many of them have distinctive odors. Not correct: later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition). Currently, the term aromatic is used to refer to the class of compounds related structurally to benzene.  Four conditions must be satisfied for compound to be Aromatic Completely Obeys Cyclic Planner conjugated HÜckels Rule HÜckels Rule is used in order to estimate the aromatic qualities of any planar ring-shaped molecule. The molecule must have 4n+2 π-electrons, where n is an integer, that is, n = 0, 1, 2, 3, 4, etc., should possess aromatic stability  How does the 4n+2 Rule Work? Molecules that doesn’t follow any of the first three aromatic criteria are called Non- Aromatic. To apply the 4n+2 rule, first count the number of π electrons in the molecule. Then, set this number equal to 4n+2 and solve for n. If n turns out to be 0 or any positive integer (1, 2, 3,...), the rule has been met. For; 4n+2 π electrons (n = 0,1,2,3,……= 2, 6, 10, 14, 18 ) is Aromatic Aromatic compounds are broadly divided into two categories 1. Benzenoids (one containing benzene ring) 2. Non-benzenoids (those not containing a benzene ring) for example, furan. Benzene Benzene has a high degree of unsaturation but is unreactive. It does not undergo the usual addition reactions of alkenes and alkynes. It does react under more vigorous conditions(heat or catalysts) to form products of substitution. This difference in chemical reactivity is due to the structure of benzene.  H H H H H H H H H H H H Benzene, C6H6 , is represented as a six carbon ring with 3 double bonds. Two possible isomers can be drawn to show benzene in this form  Naming Aromatic Compounds Aromatic compounds are named according to IUPAC. When one group is attached to benzene, the compound is named by placing the name of the group as prefix to the benzene. Monosubstituted benzenes, are systematically named with benzene as the parent name and the substituent is indicated by a prefix :-  Aromatic compounds have many common names that have been accepted by IUPAC: Toluene = methylbenzene Phenol = hydroxybenzene Aniline = aminobenzene  Disubstituted benzenes Relative positions on a disubstituted benzene ring: ortho- (o) on adjacent carbons (1,2 disubstituted) meta- (m) separated by one carbon (1,3 disubstituted) para- (p) separated by two carbons (1,4 disubstituted)  The ortho- (o), meta- (m), and para- (p) nomenclature is useful to describe reaction patterns Example: “Reaction of toluene with Br2 occurs at the para position”  Multisubstituted benzenes Multisubstituted benzenes (more than two substituents) are named as follows: Choose the sequence when the substituents have the lowest possible number List substituents alphabetically with hyphenated numbers Use common names, such as “toluene”, as parent name (as in TNT)  Use common names, such as “toluene”, as parent name  The principal substituent is assumed to be on C1 Practice Problem: Tell whether the following compounds are ortho-, meta-, or para-disubstituted (a) Meta (b) Para (c) Ortho  Structure and Stability of Benzene Benzene is very stable It undergoes substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier Example: Benzene reacts slowly with Br2 to give bromobenzene (where Br replaces H)  Benzene’s Unusual Structure All its C-C bonds are the same length Electron density in all six C-C bonds is identical Structure is planar, hexagonal All C–C–C bond angles are 120° Benzene has 6 p electrons, delocalized over the ring  Reactions of Benzene Even though benzene is highly unsaturated it does not undergo any of the regular reactions of alkenes such as addition or oxidation Substitution Reactions Of Aromatic Compounds Chlorine Methyl chloride Acetyl chloride Nitric acid Sulfuric acid