Benzene and the Concept of Aromaticity other representations: two equivalent resonance forms:

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Presentation transcript:

Benzene and the Concept of Aromaticity other representations: two equivalent resonance forms:

Bond Lengths in Benzene All carbon-carbon bond lengths in benzene are EQUAL (139 pm) This is intermediate between a typical C-C (single) bond (154 pm) and a typical C=C (double ) bond (134 pm) This is consistent with  electron delocalization (We usually draw benzene with alternating single and double bonds for ease of electron counting, but we must remember that the electrons are actually delocalized around the ring)

(Un)Reactivity (stability) of Benzene Highly unsaturated (r + db = 4), yet neither Br 2 nor HBr adds across its multiple bonds as with alkenes. Reacts with Br 2 in presence of FeBr 3 catalyst by SUBSTITUTION rather than by addition (which is the way alkenes react with Br 2 ).

Stability of Benzene (same product) Hypothetical ‘cyclohexatriene’ Actual Benzene

Pi Bonding in Benzene

Delocalization in Benzene Note complete delocalization of  electrons!

Hückel Definition of Aromaticity For a system to be aromatic, it must have: 4n + 2  electrons (for n = any integer: 0, 1, 2, etc.) in the periphery of a monocyclic planar delocalized (conjugated) system (Hückel’s # = (4n+2) = 2, 6, 10, 14, 18, etc. for various integral values of n)

Examples of Aromatic Systems #  e =

Examples of Aromatic Systems 1 more res. form 3 more res. forms 5 more res. forms

Some Non-Aromatic Systems #  e = (not planar!)

Some Heteroaromatic Systems pyridine pyrrole imidazole (has a lp in sp 2 ) (lp is in p orbital) (has a lp in sp 2 hybrid orbital + a lp in p orbital)

Bonding in Pyrrole & Imidazole 4 electrons in  bonds plus 2 lp electrons = 6  electrons (6 is a Hückel #)

Polycyclic Aromatic Compounds 10  e- 14  e- 14  e- naphthalene anthracene phenanthrene All three are found in coal. Note that in this Kekule resonance form they obey Hückel’s rule; try others!

Spectra of Aromatic Compounds IR: Ar C-H3030 cm -1 Ar C=C1600, 1500, (1450) cm -1 (2 or 3 sharp bands) 1 H-NMR: Ar H  benzene = 7.27  ) 13 C-NMR:Ar C  benzene =  UV:n -  *205 nm (strong)  -  * nm (weak)

Summary: Attributes of Aromatics Exceptional Stability Due to extended system of delocalized  electrons (conjugation) Special (un)Reactivity Electrophilic substitution rather than addition Bond Length Equalization All C-C bond lengths are equal in benzene; there are NOT single bonds and double bonds. Magnetic Properties The circulating  electrons establish a ‘ring current’ which causes large downfield shifts of aryl protons.