1. 17.10 Sigmatropic Rearrangements
Sigmatropic Rearrangements – a pericyclic reaction in which one sigma bond is replaced with another
Note that the pi bonds move their location
Copyright 2012 John Wiley & Sons, Inc.

2. 17.10 Sigmatropic Rearrangements
The notation for sigmatropic rearrangements is different from reactions we have seen so far
Count the number of atoms on each side of the sigma bonds that are breaking and forming
This is a [3,3] sigmatropic rearrangement
Copyright 2012 John Wiley & Sons, Inc.

3. 17.10 Sigmatropic Rearrangements
The reaction below is a [1,5] sigmatropic rearrangement
Practice with conceptual checkpoint 17.25
Copyright 2012 John Wiley & Sons, Inc.

4. 17.10 Sigmatropic Rearrangements
The Cope rearrangement is a [3,3] sigmatropic reaction in which all 6 atoms in the cyclic transition state are CARBONS
In general, what factors affect the spontaneity of the reaction (product favored vs. reactant favored)?
Practice with conceptual checkpoint 17.26
Copyright 2012 John Wiley & Sons, Inc.

5. 17.10 Sigmatropic Rearrangements
The Claisen rearrangement is a [3,3] sigmatropic reaction in which one of the 6 atoms in the cyclic transition state is an OXYGEN
In general, what factors affect the spontaneity of the reaction?
Practice with conceptual checkpoints and 17.28
Copyright 2012 John Wiley & Sons, Inc.

6. 17.10 Sigmatropic Rearrangements
Two pericyclic reactions occur in the biosynthesis of Vitamin D
Copyright 2012 John Wiley & Sons, Inc.

7. 17.11 UV-Vis Spectroscopy
If light with the necessary energy strikes a compound with pi bonds, an electron will be excited from the HOMO to the LUMO
Light energy is converted into potential energy
Copyright 2012 John Wiley & Sons, Inc.

8. 17.11 UV-Vis Spectroscopy
In general, the necessary energy to excite an electron from π  π* (HOMO to LUMO) is either in the UV or visible region of the spectrum
What might happen after the electrons is excited?
Copyright 2012 John Wiley & Sons, Inc.

9. 17.11 UV-Vis Spectroscopy
UV-Vis spectroscopy gives structural information about molecules
A beam of light ( nm) is split in two
Half of the beam travels through a cuvette with the analyte in solution
The other half of the beam travels through a cuvette with just the solvent (used as a negative control)
The intensities of the light that pass through the cuvettes are compared to determine how much light is absorbed by the analyte
Copyright 2012 John Wiley & Sons, Inc.

10. 17.11 UV-Vis Spectroscopy
UV-Vis spectroscopy gives structural information about molecules
The resulting data is plotted to give a UV-Vis absorption spectrum
Compounds require specific wavelengths of energy to excite
WHY?
Copyright 2012 John Wiley & Sons, Inc.

11. 17.11 UV-Vis Spectroscopy
More conjugation gives a smaller π  π* energy gap
The smaller the energy gap, the greater the lamda max
Copyright 2012 John Wiley & Sons, Inc.

12. 17.11 UV-Vis Spectroscopy
The group of atoms responsible for absorbing UV-Vis light is known as the chromophore
Woodward and Fieser developed rules to predict λmax for chromophore starting with butadiene as the base
Copyright 2012 John Wiley & Sons, Inc.

13. 17.12 Color
The visible region of the spectrum ( nm) is lower energy than UV radiation
Lycopene is responsible for the red color of tomatoes
Β-carotene is responsible for the orange color of carrots
Copyright 2012 John Wiley & Sons, Inc.

14. 17.12 Color
The color observed by your eyes will be the opposite of what is required to cause the π  π* excitation. WHY?
Practice with conceptual checkpoint 17.31
Copyright 2012 John Wiley & Sons, Inc.

15. 17.12 Color
Bleaching agents generally work by breaking up conjugation through an addition reaction
Destroying long range conjugation destroys the ability to absorb colored light. WHY?
Does bleach actually remove stains?
Copyright 2012 John Wiley & Sons, Inc.

16. 18.1 Introduction to Aromatic Compounds
Aromatic compounds or arenes include benzene and benzene derivatives
Many aromatic compounds were originally isolated from fragrant oils
However, many aromatic compounds are odorless
Aromatic compounds are quite common
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17. 18.1 Introduction to Aromatic Compounds
8 of the 10 best-selling drugs have aromatic moieties
(Esomeprazole)
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18. 18.1 Introduction to Aromatic Compounds
Coal contains aromatic rings fused together and joined by nonromantic moieties
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19. 18.2 Nomenclature of Benzene Derivatives
Benzene is generally the parent name for monosubstituted derivatives
Copyright 2012 John Wiley & Sons, Inc.

20. 18.2 Nomenclature of Benzene Derivatives
Many benzene derivatives have common names.
For some compounds, the common name becomes the parent name.
Copyright 2012 John Wiley & Sons, Inc.

21. 18.2 Nomenclature of Benzene Derivatives
If the substituent is larger than the ring, the substituent becomes the parent chain
Aromatic rings are often represented with a Ph (for phenyl) or with a φ (phi) symbol
Copyright 2012 John Wiley & Sons, Inc.

22. 18.2 Nomenclature of Benzene Derivatives
The common name for dimethyl benzene derivatives is xylene
What do ortho, meta, and para mean?
Copyright 2012 John Wiley & Sons, Inc.

23. 18.2 Nomenclature of Benzene Derivatives
Identify the parent chain (the longest consecutive chain of carbons)
Identify and Name the substituents
Number the parent chain and assign a locant (and prefix if necessary) to each substituent
Give the first substituent the lowest number possible
List the numbered substituents before the parent name in alphabetical order
Ignore prefixes (except iso) when ordering alphabetically
Copyright 2012 John Wiley & Sons, Inc.

24. 18.2 Nomenclature of Benzene Derivatives
Locants are required for rings with more than 2 substituents
Identify the parent chain (generally the aromatic ring)
Often a common name
can be the parent chain
Identify and Name the substituents
Copyright 2012 John Wiley & Sons, Inc.

25. 18.2 Nomenclature of Benzene Derivatives
Number the parent chain and assign a locant (and prefix if necessary) to each substituent
A substituent that is part of the parent
name must be assigned locant NUMBER 1
List the numbered substituents before the parent name in alphabetical order
Ignore prefixes (except iso) when ordering alphabetically
Complete the name for the molecule above
Practice with SkillBuilder 18.1
Copyright 2012 John Wiley & Sons, Inc.

26. 18.2 Nomenclature of Benzene Derivatives
Name the following molecules
Copyright 2012 John Wiley & Sons, Inc.

27. 18.3 Structure of Benzene
In 1866, August Kekulé proposed that benzene is a ring comprised of alternating double and single bonds
Kekulé suggested that the exchange of double and single bonds was an equilibrium process
Copyright 2012 John Wiley & Sons, Inc.

28. 18.3 Structure of Benzene
We now know that the aromatic structures are resonance contributors rather than in equilibrium
HOW is resonance different from equilibrium?
Sometimes the ring is represented with a circle in it
WHY?
Copyright 2012 John Wiley & Sons, Inc.

29. 18.4 Stability of Benzene
The stability that results from a ring being aromatic is striking
Recall that in general, alkenes readily undergo addition reactions
Aromatic rings are stable enough that they do not undergo such reactions
Copyright 2012 John Wiley & Sons, Inc.

30. 18.4 Stability of Benzene
Heats of hydrogenation can be used to quantify aromatic stability. Practice with conceptual checkpoints 18.6 and 18.7
Copyright 2012 John Wiley & Sons, Inc.

31. 18.4 Stability of Benzene
MO theory can help us explain aromatic stability
The 6 atomic p-orbitals of benzene overlap to make 6 molecular orbitals
Copyright 2012 John Wiley & Sons, Inc.

32. 18.4 Stability of Benzene
The locations of nodes in the MOs determines their
shapes based on high-level mathematical calculations
Copyright 2012 John Wiley & Sons, Inc.

33. 18.4 Stability of Benzene
The delocalization of the 6 pi electrons in the three
bonding molecular orbitals accounts for the stability of benzene
Copyright 2012 John Wiley & Sons, Inc.

34. Study Guide for Sections 17.10-17.12, 18.1-18.4
DAY 12, Terms to know:
Sections , Cope rearrangement, Claisen rearrangement, UV-Vis Spectroscopy, aromatic, arenes, ortho, meta, para
DAY 12, Specific outcomes and skills that may be tested on exam 2:
Sections ,
Be able to predict possible products from sigmatropic rearrangements including Cope and Claisen reactions
Be able to predict the favored side of a sigmatropic rearrangement equilibrium and explain
Be able to explain how UV-Vis Spectroscopy is used to identify substances and measure concentrations and why extended pi systems are often used as dyes
Be able to name aromatic compounds using both common and IUPAC naming systems
Be able to explain at least two pieces or evidence that suggest aromatic C=C are significantly more stable than other alkenes
Be able to use MO theory to explain why aromatic characteristics make a molecule stable

35. Extra Practice Problems for Sections 17.10-17.12, 18.1-18.4
Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period

36. Prep for Day 13 Must Watch videos: Other helpful videos:
(aromatic, nonaromatic, antiaromatic, FLC)
(aromatic examples FLC)
(Huckel’s Rule, Dave)
(Huckel’s Rule, ChemistNATE)
(heteroaromatics, AK lectures)
(benzylic reactions, Khan)
Other helpful videos:
(properties of aromatics, UC-Irvine)
(carbon nanotubes, The Universe)
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