1. Delocalized electrons and pKa values

2. Book: Bruice
Chapter 7

3. Electrons – restricted to a particular region called localized electrons
Belong either to a single atom
Or confined to a bond between two atoms
Many organic compounds contain delocalized electrons
Neither belong to a single atom nor are confined to a bond
Shared by three or more atoms

4. The benzene structure C6H6
Has eight fewer hydrogens than a noncyclic alkane with six C atoms
The number of rings and pi-bonds in benzene must total four – all hydrogens must be identical

5. Neither of these structures is consistent with the formation of 3 compounds
1865 – Kekule proposed benzene not as single compound but as a mixture of 2 compounds in rapid equillibrium
Only 3 disubstituted products are obtained when a monosubstituted benzene undergoes a second substitution

6. 1930s – benzene is a planar molecule and the 6 C-C bonds have the same length of 1.39 Å
Shorter than C-C single bond (1.54) and longer than C-C double bond (1.33)
Benzen does not have alternating single and double bonds
Same length of bonds – same number of electrons between the C atoms

7. Bonding in benzene Each C atom is sp2 hybridized Bond angels 120°
Each of the carbons uses two sp2 orbitals to bond to two other carbons
Third sp2 orbital overlaps the s orbital of a hydrogen
Each carbon- p orbital at right angles to the sp2 orbitals.
Because benzene is planar-p orbitals are parallel

8. Each of the six pi-electrons, therefore, is localized neither on a single carbon nor in a bond between two carbons
Each pi-electron is shared by all six carbons
Other words – pi-electrons are delocalized
Roam freely over and under the ring of C atoms

9. Resonance contributors and the resonance hybrid
Dashed lines-delocalized electrons are shared equally by all 6 C atoms
Also: all the carbon–carbon bonds have the same length, but they do not show how many pi-electrons are in the ring
Alternative: structures that portray the electrons as localized
Approximate structure with localized electrons – resonance contributor
Actual structure with delocalized electrons – resonance hybrid

10. Double headed arrow – the structures are not in equilibrium
Actual structure is in between
Resonance contributors are
imaginary, not real
Resonance hybrid is real

11. Drawing resonance contributors
organic compound with delocalized electrons is generally represented as a structure with localized pi-electrons
To know how many electrons are present in the molecule
two nitrogen–oxygen bonds are identical; they each have the same bond length
more accurate description of the molecule’s structure is obtained by drawing the two resonance contributors

12. nitrogen–oxygen double bond and a nitrogen–oxygen single bond, but they indicate that the electrons are delocalized
by depicting the double bond in one contributor as a single bond in the other

13. The resonance hybrid, in contrast, shows that the p orbital of nitrogen overlaps the p orbital of each oxygen
shows that the two pi-electrons are shared by three atoms
also shows that the two nitrogen–oxygen bonds are identical and that the negative charge is shared equally by both oxygen atoms

14. Rules for drawing resonance contributors
1. Only electrons move. Atoms never move.
2. Only π-electrons (π -electrons in bonds) and nonbonding electrons can move; σ electrons never move.
3. The total number of electrons in the molecule does not change. Therefore, each of the resonance contributors for a particular compound must have the same net charge. If one has a net charge of zero, all the others must also have net charges of zero.

15. Additional – Peptide bonds
Every third bond in a protein is a peptide bond. A resonance contributor can be drawn for a peptide bond by moving the lone pair on nitrogen toward the sp2 carbon.
Because of the partial double bond character of the peptide bond, the carbon and nitrogen atoms and the two atoms bonded to each are held rigidly in a plane

16. Predicted stabilities of resonance contributors
All resonance contributors do not necessarily contribute equally to the resonance hybrid.
depends on its predicted stability
resonance contributors are not real - stabilities cannot be measured but predicted
The greater the predicted stability of the resonance contributor, the more it contributes to the structure of the resonance hybrid; and the more it contributes to the structure of the resonance hybrid, the more similar the contributor is to the real molecule.

17. Example Two resonance contributors
B has has two features that make it less stable than A:
one of its oxygen atoms has a positive charge (not a comfortable situation for an electronegative atom) and the structure has separated charges ->
positive charge and a negative charge can be neutralized by the movement of electrons
Contributors relatively unstable -> energy needed to keep the opposite charges separated
A predicted to be more stable than B

18. C and D predicted to be equally stable
Expected to contribute equally to the resonance hybrid

19. Example 2 E has negative charge on C atom
F has negative charge on O atom
Oxygen is more electronegative than carbon -> accomodates better the negative charge
F is predicted to be more stable than E

20. Delocalization energy
Delocalized electrons stabilize a compound
Extra stability from these electrons – delocalization energy
Electron delocalization – resonance
Delocalization energy = resonance energy
Conclusion: resonance hybrid is more stable than any of its resonance contributors is predicted to be

21. The delocalization energy depends on the number and predicted stability of the resonance contributors
The greater the number of relatively stable resonance contributors, the greater is the delocalization energy
Example: delocalization energy of a carboxylate ion (with two relatively stable resonance contributors) is significantly greater than the delocalization of a carboxylic acid with only one relatively stable resonance contributor

22. Delocalized electrons affect...
Stability
Product of reaction
pKa

23. pKa Acid dissociation konstant Ka
For practical purposes we use logarithmic constant pKa
pKa=-log Ka
Can be affected by delocalized electrons

24. Example pKa (carboxylic acid)=4.76 pKa (EtOH)=15.9
Conjugate base of acid is weaker -> more stable than conjugate base of alcohol
factor most responsible for the increased stability of the carboxylate ion is its greater delocalization energy relative to that of its conjugate acid
Reason: ion has two equivalent resonance contributors that are predicted to be relatively stable, whereas the carboxylic acid has only one

26. In contrast all electrons in alcohol are localized
Loss of proton is not accompanied by an increase in delocalization energy

27. Ultraviolet and visible spectroscopy
UV/Vis
Provides information about compounds that have conjugated double bonds
UV: wavelengths ranging from 180 to 400 nm
Visible (Vis): wavelengths ranging from 400 to 780 nm
Stronger wavelenght -> greater the energy of the radiation
UV greater energy than Vis

28. If a compound absorbs ultraviolet light, a UV spectrum is obtained
if it absorbs visible light, a visible spectrum is obtained
λmax= is the wavelength corresponding to the highest point of the absorption band

29. Chemistry link to health: UV light and sunscreens
Exposure to ultraviolet light stimulates specialized cells in the skin to produce a black pigment melanin which causes the skin to look tan
Melanin absorbs UV light, so it protects our bodies from the harmful effects of the sun
If more UV light reaches the skin than the melanin can absorb, the light will burn the skin and cause photochemical reactions that can result in skin cancer

30. Applying a sunscreen can protect skin against UV light
The amount of protection provided by a particular sunscreen is indicated by its SPF (sun protection factor)
Higher SPF -> greater the protection
contain an inorganic component, such as zinc oxide, that reflects the light as it reaches the skin
Others contain a compound that absorbs UV light

31. UV-A is the lowest-energy UV light (315 to 400 nm) and does the least biological damage.
Fortunately, most of the more dangerous, higher-energy UV light, UV-B (290 to 315 nm) and UV-C (180 to 290 nm), is filtered out by the ozone layer in the stratosphere
we need to worry about the apparent thinning of the ozone layer!!!

32. λmax and conjugated double bonds
More conjugated double bonds -> the longer is wavelength at which λmax occurs
If there are enough conjugated double bonds in the compound -> absorb visible light and will be colored

33. Absorbance of visible light
White light – mixture of all wavelengths of visible light
Removing one of them – remaining light is colored
a compound that absorbs visible light is colored - color depends on the color of the wavelengths of the absorbed light
wavelengths that the compound does not absorb are reflected back to the viewer, producing the color the viewer sees

34. Example Azobenzenes (benzene rings connected by an N=N bond)
have an extended conjugated system that causes them to absorb light from the visible region of the spectrum
Used as dyes

35. Example 2
Margarine production – adding of butter yellow to make it look more like butter (years ago)
Dye was cancerogenic
β-carotene is used nowadays to color margarine
Methyl orange – used as acid-base indicator

36. Anthocyanins highly conjugated compounds
responsible for the red, purple, and blue colors of many flowers, fruits and vegetables
In a neutral or basic solution, the monocyclic fragment is not conjugated with the rest of the molecule, so the anthocyanin does not absorb visible light and is therefore a colorless compound

37. Solve the problems

38. Summary Localized/delocalized electrons Resonance contributors
Resonance hybrid
Delocalization energy
UV/Vis