Essential Organic Chemistry Paula Yurkanis Bruice Chapter 6 Delocalized Electrons and Their Effect on Stability, Reactivity, and pKa

Localized Electrons vs. Delocalized Electrons Electrons that are restricted to a particular region. Delocalized electrons: Neither belong to a single atom nor are confined to a bond between two atoms, but are shared by three or more atoms.

6.1 Delocalized Electrons: Structure of Benzene The Lewis representation of benzene suggests that we deal with a six-membered ring of carbon atoms that are held together by alternating single and double bonds. C: 6 • 4 = 24 valence electrons H: 6 • 1 = 6 valence electrons ---------------------------------------- Total = 30 electrons = 15 bonds

Benzene This implies that we should observe alternating short (1.33 Å) and long (1.54 Å) bond lengths. Modern analytical techniques showed that all bond lengths are the same (1.39 Å). Thus, they must have the same number of electrons between the carbon atoms, this can be so only if the π electrons are delocalized around the ring.

Benzene Since both representations are equal, in average each C-C bond has a bond character in between a single and a double bond. Measured bond length is: 1.39 Å

Benzene H 2 x X A B This also explains why replacing two of the six hydrogens produces only three isomers. In case of alternating double and single bonds, A and B should be different and we should have four isomers.

6.2 The Bonding in Benzene C-C and C-H sigma bonds in benzene Benzene is planar and six p orbitals are parallel. P orbitals are close enough for side-to-side overlap. Overlapping p orbitals form a doughnut-shaped cloud of electrons above and below the benzene ring. All C-C bonds have the same electron density.

6.3 Resonance Contributors and Resonance Hybrid When it is possible to draw more than one valid Lewis diagram for a molecule or ion, that species is said to have resonance (electrons are delocalized). The molecule or ion is said to be a resonance hybrid of the structures drawn. For species with resonance, no single Lewis diagram will suffice to describe them correctly. Dashed lines inside benzene do not show how many π electrons are in the ring.

Resonance Contributors and Resonance Hybrid Chemists prefer to use approximate structures. The approximate structure with localized electrons is called a resonance contributor. The actual structure with delocalized electrons is called a resonance hybrid.

Resonance Contributors and Resonance Hybrid Electron delocalization occurs only if all the atoms sharing the delocalized electrons lie in or close to the same plane, so that their p orbitals can effectively overlap.

6.4 Drawing Resonance Contributors Delocalized electrons result from a p orbital overlapping the p orbitals of more than one adjacent atom.

Rules for Drawing Resonance Contributors Only electrons move. The nuclei of the atoms never move. The only electrons that move are p electrons or lone-pair electrons. The total number of electrons in the molecule doesn’t change. All resonance contributors have the same net charge.

Rules for Drawing Resonance Contributors Electrons are always moved toward an sp2 carbon. An sp2 carbon is either a positively charged carbon or a double-bonded carbon. Electrons cannot be moved toward an sp3 carbon, because an sp3 carbon cannot accept any more electrons, it has a complete octet.

Rules for Drawing Resonance Contributors Moving lone-pair electrons toward an sp2 carbon. The sp2 carbon can accommodate the new electrons by breaking a π bond.

Resonance We move only the electrons All atoms are sp2 hybridized

6.5 Predicted Stabilities of Resonance Contributors All resonance contributors do not necessarily contribute equally to the resonance hybrid. The greater the predicted stability of the resonance contributor, the more it contributes to the structure of resonance hybrid, and the more similar the contributor is to the real molecule.

Predicted Stabilities of Resonance Contributors Structure B is less stable than A because: 1. One of its oxygen has a positive charge. 2. It has “separated charges” (it takes energy).

Predicted Stabilities of Resonance Contributors Structures C and D are equally stable.

6.6 Resonance Stabilization Delocalized electrons stabilize a compound. The extra ability gains from having delocalized electrons is called resonance stabilization or resonance energy. The greater the number of relatively stable resonance contributor and the more nearly equivalent they are, the greater is the resonance stabilization.

6.7 The Effect of Delocalized Electrons on Stablility Dienes are hydrocarbons with two double bonds. Conjugated dienes have conjugated double bonds, each separated by one single bond. Isolated double bonds are separated by more than one single bond. πElectrons in an isolated diene are localized, while πelectrons in a conjugated diene are delocalized. Conjugated dienes are more stable than isolated dienes.

The Effect of Delocalized Electrons on Stablility Because allylic cation and benzylic cation have delocalized electrons, they are more stable than other primary carbocations.

The Effect of Delocalized Electrons on Stablility Not all allylic and benzylic carbocations have the same stability.

6.8 The Effect of Delocalized Electrons on the Nature of the Product Formed in a Reaction The reactions of isolated dienes are like the reactions of alkenes.

The Effect of Delocalized Electrons on the Nature of the Product Formed in a Reaction When a conjugated diene reacts with a limited amount of HX, only two addition products are formed: One is a 1,2-addition product, the other is a 1,4-addition product.

6.9 The Effect of Delocalized Electrons on pKa An increase in resonance stabilization means an increase in stability. Loss of proton from a carboxylic acid is accompanied by an increase in resonance stabilization.

The Effect of Delocalized Electrons on pKa- Phenol

The Effect of Delocalized Electrons on pKa- Aniline

The Effect of Delocalized Electrons on pKa