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Organic Chemistry II - Aromatic Compounds

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Study GuideOrganic Chemistry II–Aromatic Compounds1.Benzene–Structure, Stability, and ResonanceBenzene is one of the most important molecules in organic chemistry. At first glance, its formulaseems simple. But understanding its structure took many years of scientific discovery.Let’s go step by step and make everything clear.1. Discovery of Benzene’s FormulaIn1834, Eilhardt Mitscherlich determined the molecular formula of benzene to be:Now compare this with a saturated six-carbon alkane:That means benzene is missing8 hydrogenscompared to a saturated compound.What does this tell us?This indicatesfour degrees of unsaturation, which could mean:•Double bonds•A ring•Or a combination of bothSo benzene must contain multiple unsaturations.2. Kekulé’s Proposed Structure (1866)In 1866, August Kekulé proposed a structure for benzene based on three key ideas:•Benzene has formulaC₆H₆•Each carbon formsfour bonds•All hydrogens areequivalent

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Study GuideHis ConclusionKekulé suggested:•Six carbons arranged in aring•Three double bonds alternating with three single bondsThis structure explains:•Four degrees of unsaturation (1 ring + 3 double bonds)•Each carbon having four bondsAt the time, this was a brilliant idea.3. A Problem with Kekulé’s StructureScientists noticed something strange.If benzene truly had alternating double bonds, then substituting groups at:•1,2 positions•1,6 positionsshould producedifferent compounds.But experimentally, these compounds were thesame.This meant something was missing in Kekulé’s model.

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Study Guide4. Kekulé’s Modification–Rapid InterconversionTo explain this, Kekulé suggested:There aretwo structuresthat differ only in the positions of the double bonds.These two structures:•Rapidly convert into each other•Interchange so fast that they appear identicalThis was an early hint of something deeper—what we now callresonance.Resonance in BenzeneModern chemistry explains benzene usingresonance theory.What is Resonance?When more than one valid Lewis structure can be drawn for a molecule:

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Study Guide•None of them alone is completely correct.•The real structure is ahybridof all possible structures.•This hybrid ismore stablethan any single structure.The extra stability gained is calledresonance energy.Resonance in BenzeneThe two Kekulé structures are actuallyresonance structures.Instead of three fixed double bonds:•Theπelectrons aredelocalized•They move around the entire ringWe represent this by drawing acircle inside the hexagon.That circle shows:•πelectrons are shared equally over all six carbons•All carbon–carbon bonds are equivalent

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Study GuideEvidence from Bond LengthsExperimental measurements show:•All C–C bonds in benzene are1.40 pm•A single bond is 1.46 pm•A double bond is 1.34 pmBenzene’s bond length is exactlyin between.This proves:•Bonds are neither purely single nor purely double.•All bonds are identical.Also:•All bond angles are 120°•The molecule isplanar (flat)Resonance EnergyBenzene is more stable than expected for a molecule with three double bonds.The extra stability is about:This extra stability is called itsresonance energy.The greater the resonance energy, the more stable the compound.Orbital Picture of BenzeneNow let’s look at the modern molecular explanation.Because:•Benzene is planar•All bond angles are 120°

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Study GuideEach carbon is:What does this mean?Each carbon has:•Three sp² hybrid orbitals (formingσbonds)•One unhybridized p orbitalThe sp² orbitals form:•C–C sigma bonds•C–H sigma bondsThe p orbitals overlap sideways (above and below the ring).Figure 1πMolecular OrbitalsWhen six p orbitals overlap:They formsix molecular orbitals:•3 bonding orbitals (low energy)•3 antibonding orbitals (high energy)

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Study GuideThe bonding orbitals are:π₁, π₂, π₃The antibonding orbitals are:π₄*, π₅*, π₆*Some orbitals have equal energy. These are calleddegenerate orbitals.Figure 2Why Is Benzene So Stable?Benzene has:•Sixπelectrons•All electrons fill thebonding orbitals only•All electrons are paired•A completely filled set of bondingπorbitalsThis creates aclosed shell of delocalizedπelectrons, which gives:•Exceptional stability•Large resonance energyFinal Big PictureHere’s what you should remember:

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Study Guide•Benzene has formula C₆H₆.•It contains four degrees of unsaturation.•Kekulé proposed alternating double bonds in a ring.•Modern theory explains benzene using resonance.•All C–C bonds are equal in length.•The molecule is planar and sp² hybridized.•Sixπelectrons are delocalized over the ring.•All bondingπorbitals are filled.•This gives benzene very high stability (36 kcal/mol resonance energy).2.Hückel’s Rule, Nomenclature, and Reactions of BenzeneLet’s now connect three important ideas about benzene:1.Why benzene is aromatic (Hückel’s Rule)2.How benzene compounds are named3.How benzene reacts1. Hückel’s Rule–Why Benzene Is AromaticIn 1931, Erich Hückel proposed a rule to predict whether a ring compound isaromatic.What kind of molecules does the rule apply to?The compound must be:•Cyclic (ring-shaped)•Planar (flat)•Fully conjugated (each atom has a p orbital)•Containπelectrons
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Document Details

University
Texas A&M University
Course
Organic Chemistry II
Subject
Chemistry

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