Aromatic Compounds

Topics: Aromaticity, Delocalized electron, Benzene Pages: 12 (1933 words) Published: January 19, 2011
CH 16

- the name of this functional group class is historically derived from the pleasant aromas associated with many of its constituent compounds discovered in past centuries - although not all aromatic compounds known today share the same distinction (many are in fact, odorless or even unpleasant), they are the principle components in such pleasant smelling substances as cinnamon, licorice, wintergreen, clove & vanilla

- also known as arenes, many aromatic compounds possess the benzene ring functional group: G's FG = aromatic compound or arene benzene ring or aryl grp

- other compounds which do not contain a benzene ring are also referred to as “aromatic” based on their similarity in physical & chemical behavior to benzene

- there are several characteristics of benzene which, although initially seemed contradictory, helped finally to determine its enigmatic structure a) Unsaturated Molecular Formula – - benzene ‘s molecular formula (MF = C6H6 ), indicates a high level of unsaturation - recall the calculation of centers of unsaturation (CU): CU = (2n + 2) - # H's 2 [2(6) + 2] - 6 2 where n = #C's

For C6H6: CU =

CU = 4

benzene is a highly unsat'd HC

- from the many possible combinations of rings and multiple bonds consistent with four centers of unsaturation, F.A. Kekule’ (1866) reckoned correctly that benzene is comprised of one sixmembered ring and three alternating (conjugated) double bonds

b) Resistance to Addition Reactions – - although benzene has double bonds, it does not undergo typical alkene reactions such as electrophilic addition - benzene is, in fact, very stable towards hydrogenation, halogenation, hydration & addition of hydrogen halides: NR



c) Preference for Substitution Reactions – - instead of addition reactions like alkenes, benzene readily undergoes substitution reactions typical of alkanes: EX. C6H6 + Cl2 Fe C6H5 Cl + HCl

- thus, although it is highly unsaturated, benzene behaves like a saturated hydrocarbon

d) Three Isomers of Disubstituted Benzene – - a benzene ring with two substituents can exist as three distinct regioisomers - an addition product would have only one isomer ± the two groups would be adjacent EX. Dichlorobenzene, C6H4Cl2, has three structural isomers

e) All Equal Bond Lengths – - all carbon-carbon bond lengths are ~ 1.40D which is between the length of a C!C single bond (1.54D) & a C=C double bond (1.34D) - all bond angles in the benzene ring are also equal (bond Ê’s = 120o) f) Heat of Hydrogenation, ∆HH o – 2

- the predicted value of ∆HH o for a six-membered ring with three double bonds (“cyclohexatriene”) is 90 kcal/mol; the actual ∆HH o value for benzene is 54 kcal/mol 2 2

- therefore, benzene is more stable than the theoretical “cyclohexatriene” by 36 kcal/mol

Kekule’ Structure – - to rationalize the preceding experimental evidence, Kekule’ proposed a very fast equilibrium between two structural isomers:

- he reasoned that the isomers interchanged so fast, they averaged out on a reaction time scale, explaining facts a,b,c & d, but not e & f

Resonance Description – - the Kekule’ structures describe what is now known as the resonance condition % only electrons are displaced in the contributing structures:

Resonance Contributors RC's

Resonance Hybrid RH

- either one of the RC’s or the RH may be used to describe benzene structurally (note that each carbon has one bond remaining)

- the stabilized resonance hybrid can be constructed using hybrid orbitals in the usual manner: - the sigma (σ) bond “framework” is made up of sp2 hybrid orbitals (sp2 HO’s) from each carbon - hence, each carbon has a trigonal planar molecular geometry with bond angles of 120o - this correctly predicts the planar regular hexagon which is the observed structure for benzene σ2 H σ2 H C σ1 σ1 C H σ2 C σ1 σ1 C H σ2 C σ2 H 3 "e- pairs" on ea C HO's = sp2 MG = trigonal...
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