lecture 21 - Benzene and and the Concept of Aromaticity...

Info iconThis preview shows pages 1–5. Sign up to view the full content.

View Full Document Right Arrow Icon
Benzene and Benzene and and the Concept of and the Concept of Aromaticity Aromaticity Chapter 21 Chapter 21 ¾ The term aromatic aromatic was originally use to classify benzene and its derivatives ¾ They had distinctive odors They had distinctive odors ¾ They do not undergo addition, They do not undergo addition, oxidation, and reduction oxidation, and reduction reactions like alkenes reactions like alkenes Chemical Properties of Benzene ± When benzene is treated with Br 2 /FeCl 3 only one compound is formed (a substitution reaction) ± All 6 hydrogens of benzene must be equivalent Chemical Properties of Benzene Benzene Benzene - Kekul Kekul é ² The first structure for benzene was proposed by August Kekulé in 1872 C H C H C H C H C H C H C C C C C C H H H H H H Remember that electrons were not discovered until 1897 ² This structure, however, did not account for the unusual lack of chemical reactivity of benzene Benzene does not react the same way alkenes do
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Benzene Benzene - Resonance Model Resonance Model ± The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure The carbon skeleton is a regular hexagon All C-C-C and H-C-C bond angles 120° All C-C are sp2 hybridized Benzene - The Resonance Model The Resonance Model ± The pi system of benzene the carbon framework with the six 2 p orbitals MO Representation of the pi bonding in Benzene Higher energy 3 MO’s Lower energy 3 MO’s Benzene Benzene - The Resonance Model The Resonance Model ± The pi system of benzene (b) overlap of the parallel 2 p orbitals forms one torus above the plane of the ring and another below it this represents the lowest-lying pi-bonding molecular orbital
Background image of page 2
Benzene Benzene – The Resonance Model The Resonance Model ± We often represent benzene as a hybrid of two equivalent Kekulé structures each makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in between Benzene - Resonance Resonance ± Resonance energy: Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds one way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene Heats of Hydrogenation of Benzene and Cyclohexene Benzene Benzene Other Aromatic Hydrocarbons Are these compounds aromatic?
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Concept of Concept of Aromaticity Aromaticity ± The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations Concept of Aromaticity ± To be aromatic, a compound must be cyclic have one p orbital on each atom of the ring be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring have a closed loop of (4 n
Background image of page 4
Image of page 5
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 09/06/2009 for the course CHEM chem 12 AB taught by Professor Valentin during the Spring '09 term at Evergreen Valley.

Page1 / 19

lecture 21 - Benzene and and the Concept of Aromaticity...

This preview shows document pages 1 - 5. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online