Aromaticity Lecture Notes

Titration vs. Ideal Gas Law

  • Titration was favored for being an easier lab.
  • Ideal gas law experiment allowed calculation of r from magnesium.

Lecture Plan

  • Aromatic chemistry to be completed soon.
  • Thursday reserved for past exam problems.
  • Students can email specific problems for review.
  • Poll on block three topics for Thursday revision.

Aromaticity Introduction

  • Recap of reactions from previous lecture: addition of substituents across double bonds using bromine.
  • Reaction of bromine with cyclohexane: no reaction.
  • Reaction of bromine with cyclohexene:
    • Bromine (Br-Br) interacts with cyclohexene, forming a bromonium ion and a carbocation.
    • Br^- then adds to the carbocation from the back face.
    • Product: trans-dibromocyclohexane.
  • Cyclohexadiene reacts similarly, with bromine adding to each double bond.

Bromine Test

  • Bromine's orange color fades as it reacts with double bonds.
  • This color change serves as a functional group test for alkenes.
  • Cyclohexatriene (benzene) does not readily undergo this reaction.
  • Toluene (methylbenzene) also does not react with bromine; the color persists.
  • Benzene (cyclohexatriene) doesn't undergo addition across the double bond like other alkenes, despite not being a steric issue.

Stability and Energy

  • Hydrogenation of double bonds: adding hydrogens to each side of a double bond.
  • Question: Why are only certain double bonds hydrogenated in a molecule with multiple double bonds?
  • Energy content: Cyclohexadiene has approximately twice the energy of cyclohexene.
  • Cyclohexatriene should theoretically have around 230 units of energy.
  • Benzene has only 208 units of energy; more stable than expected.

Magnetic Properties of Benzene

  • Electric current runs around the benzene ring.
  • Demonstrated using Nuclear Magnetic Resonance (NMR).
  • NMR: applies magnetic field and analyzes bond interactions.
  • Eddy current effect: magnetic field on a conducting ring induces electron flow.
  • Benzene ring: electrons flow, creating a magnetic field detected by NMR.
  • NMR chemical shifts: Hydrogens inside the ring have lower chemical shifts than those outside, indicating the presence of a magnetic field and electron flow, like in annulene rings.

Bond Lengths in Benzene

  • Expected: alternating short (double) and long (single) bonds.
  • Observed: all bonds are the same length.
  • Scanning tunneling microscopy confirms uniform bond lengths.

Resonance in Benzene

  • Electrons delocalized around the ring due to resonance.
  • Resonance structures: shifting electrons without moving atoms.
  • Conjugated ring system allows electron movement.
  • Electrons not confined to specific double bonds but spread across the ring.
  • Representation: sometimes drawn with a circle inside to indicate delocalization.
  • Half pi bond across every carbon.

Rules for Drawing Resonant Structures

  • Move electrons; don't move atoms.
  • Effective in sp^2 carbons (like benzene).
  • Carboxylate anion: resonance stabilization explains acidity of carboxylic acids.
  • Enolate tautomerization as another example of resonance that moves electrons between two carbons.
  • Resonance works best between sp^2 hybridized carbons. Resonance structures don't work very well in sp^3 hybridized systems.
  • Moving electrons from sp^2 to sp^2 carbons is valid.

Examples of Resonance Structures

  • Example 1: Move electrons in a structure until a double bond is created after losing an H+.
  • Example 2: Following electron movement with arrows to determine product.
    • If two electrons moves to create a double bond, a positive charge is neutralized, but another one will show up down below.

Curly Arrows

  • Revising curly arrows: understanding electron movement.
  • Example: Determine the resulting structure by following the path of curly arrows.
    • Answer example shows 2 electrons from a double bond moving up to create an extra lone pair to neutralize the positive charge.
    • Carbon left behind a positive charge because the electrons that it was sharing have now gone.
    • A, B, C, and D questions with varying number of Hydrogens.
    • Charges must also be balanced so even if the arrows are not followed, with one plus there must be one plus in the answer.

Carbocation Stability

  • Tertiary carbocations more stable than secondary, more stable than primary.
  • Markovnikov's rule: "rich get richer" (more hydrogens).
  • Hydrogen adds to carbon with more hydrogens.
  • Reason: more resonant structures for tertiary carbocations.

Aromatic Systems

  • Compounds with flowing electrons around the ring and high stability.
  • Can be single molecules, macrocycles (CPP), or multiple rings (anthracene).
  • Heteroatoms (O, N, S) can be included (e.g., thiophene).
  • Double bonds in aromatic systems are unreactive towards bromine.
  • Non-aromatic rings (cyclobutadiene, cyclooctatetraene) do react with bromine and are non-planar.
  • Cyclic, planar structure with electron flow is essential for aromaticity.
  • Discovery of a large aromatic compound made up of carbon by Martin Peeks, electrons free to move around a ring which has very interesting magnetic properties.

Huckel's Rules for Aromaticity

  • Cyclic molecule
  • One 2p orbital on each atom
  • 4n + 2 pi electrons (where n is an integer)
  • Planar structure for continuous p orbital overlap
  • Benzene: Aromatic because it is cyclic, planar, with 6 pi electrons satisfying, with each carbon being sp2 hybridized.
  • Anthracene: two, four, six, eight, 10, 12, 14 pi electrons. Does 14 meet the four n plus two rule? Four times three plus two makes 14. Aromatic

Examples

  • Cyclic molecule with carbon: 2, 4, 6, 8, 10, 12, 14 pi electrons which meets the four n plus two rule. Aromatic.
  • Cyclooctatetraene - non aromatic because although it has a p orbital on every carbon (sp2 hybridized), with 8 electrons that does not meet the four n plus two rule. Aromatic.

Non-Aromatic Examples

  • Aromatic: all Carbons have a p orbital and is cyclic, two pi electrons which satisfies four n plus two, so it will be aromatic.
  • Molecule: non-aromatic due to an sp^3 hybridized carbon.
    • However the cation at the top has one hydrogen so there is a vacant p orbital.
  • Molecule with filled sp^3 hybrid orbital - non-aromatic with an sp^3 hybridized carbon. However, the lone pair adopts a planar configuration of adopting this molecule will be aromatic.

Heteroatoms in Aromatic Rings

  • Aromatic system which is pyridine with one lone pair off of the nitrogen.
    • All atoms have a p orbital as all are sp2 hybridized, and with 6 pi electrons with meets the four n plus two rule makes the pyridine aromatic.
  • Another molecule; Furan which is also aromatic.
    • Carbons are sp2 hybridized, and when lone pairs from oxygen are added as sp2 geometry and into the p orbital, both become part of the ring. Six the ring has six pi electrons.
    • An example is pyrrol which adopts sp2 geometry with two four and with the lone pair six two four times one plus two making it aromatic.
    • If a ring contains heteroatoms, it's essential to see if it adopts an sp2 geometry

Planarity, Huckel's Rules, and Pi Electrons

  • Key Factors for Aromaticity that will react quickly with Bromine if not followed
    • A. Molecules which do not have planarity can not be aromatic.
  • Example: Cation tropilium which if aromatic sp2 hybridized.

Practice and Recognition

  • Identify the Non Aromatic, by calculating the electron count.
    • Positive signals can contribute less electrons to the ring which must also be accounted for.
  • Which of these is not aromatic?
    • B can never be aromatic, because carbo cations cannot contribute to a ring.
  • Sulfurs and Carbons donating can have systems with 10 aromatic systems, as carbons donate p orbitals and sulfur donates to pi systems.

Naming Conventions

  • Benzene: not called cyclohexatriene.
  • Ethyl benzene one ethyl benzene.
  • Bromobenzene, nitrobenzene, toluene, phenol, aniline, benzaldehyde, benzoic acid are all special names.
  • Two Substituents: One-Two or One-Three or One-Four. Para means substituents are in a one-four orientation (farthest from each other.
    • Para, dibromobenzene, mata means in the one-three, and ortho means in the one-two.
  • DimethylBenzane is called Ortho-Xylene.
  • One, Three, DimethylBenzane is called Metszyline.
  • One, Four, DimethylBenzane is called Para Xylene.

Importance of Aromaticity

  • Aromaticity affects chemical reactivity.
  • Aromatic rings: cyclic, all atoms sp^2 hybridized (p orbital on each atom), 4n + 2 pi electrons.