14. Conjugated Compounds and Ultraviolet Spectroscopy

Conjugated and Nonconjugated Dienes Compounds can have more than one double or triple bond If they are separated by only one single bond they are conjugated and their orbitals interact The conjugated diene 1,3-butadiene has properties very different from those of the nonconjugated diene, 1,5-pentadiene

Conjugated compounds: common in nature Extended conjugation leads to absorption of visible light, producing color Conjugated hydrocarbon with many double bonds = polyenes Lycopene is a conjugated polyene responsible for red color in tomatoes

Learning Check: Circle any conjugation in the following:

Solution: Circle any conjugation in the following: Nonconjugated

14.1 Conjugated Dienes: Preparation: Typically by elimination of allylic halide Specific industrial processes for large scale production of commodities by catalytic dehydrogenation and dehydration

Stability of Conjugated Dienes: Conjugated dienes are more stable than nonconjugated based on heats of hydrogenation Hydrogenating 1,3-butadiene produces 16 kJ/mol less heat than 1,4-pentadiene

Stability of Conjugated Dienes: Conjugation makes more stable Substitution makes more stable

Description of 1,3-Butadiene The single bond between the conjugated double bonds is shorter and stronger than sp3 Partial Double Bond Character

Molecular Orbital Description Conjugated Double bonds Isolated Double bond The bonding -orbitals are made from 4 p orbitals that provide greater delocalization and lower energy than in isolated C=C electrons in 1,3-butadiene are delocalized over the  bond system Delocalization leads to stabilization

14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations Remember: addition of electrophile to C=C Markovnikov regiochemistry via more stable carbocation

Carbocations from Conjugated Dienes Now: Addition of H+ gives delocalized 2o allylic carbocation

Addition Products to Delocalized Carbocation Nucleophile can add to either cationic site The transition states for the two possible products are not equal in energy

Another Example:

Learning Check: Give the likely products from reaction of 1 equiv HCl with 2-methyl-1,3-cyclohexadiene.

Solution: Give the likely products from reaction of 1 equiv HCl with 2-methyl-1,3-cyclohexadiene.

14.3 Kinetic vs. Thermodynamic Control At completion, all reactions are at equilibrium and the relative concentrations are controlled by the differences in free energies of reactants and products (Thermodynamic Control) If a reaction is irreversible or if a reaction is far from equilibrium, then the relative concentrations of products depends on how fast each forms, which is controlled by the relative free energies of the transition states leading to each (Kinetic Control)

Kinetic and Thermodynamic Control: Example Addition to a conjugated diene at or below room temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct At higher temperature, product ratio changes and 1,4 adduct predominates

_____ Kinetic control _____ Thermodynamic control

14.4 The Diels-Alder Cycloaddition Reaction Conjugate dienes can combine with alkenes to form six-membered cyclic compounds Ring formation involves no intermediate (concerted formation of 2 bonds) Discovered by Otto Paul Hermann Diels and Kurt Alder in Germany in the 1930’s

View of the Diels-Alder Reaction Woodward and Hoffman showed to be an example of the general class of pericyclic reactions Involves orbital overlap, change of hybridization and electron delocalization in transition state The reaction is called a cycloaddition Diene Dieneophile

14.5 Characteristics of Diels-Alder Reaction The alkene component is called a dienophile C=C is conjugated to an electron withdrawing group, such as C=O or CºN Alkynes can also be dienophiles

Stereospecificity of the Diels-Alder Reaction The reaction is stereospecific, There is a one-to-one relationship between stereoisomeric reactants and products

Endo vs Exo positions Endo and Exo denote relative sterechemistry of groups in bicyclic systems Substituent on one bridge is exo if it is anti (trans) to the larger of the other two bridges and endo if it is syn (cis) to the larger of the other two bridges

Regiochemistry of the Diels-Alder Reaction Reactants align to produce endo (rather than exo) product

Example: Endo product formed

Conformations of Dienes in the Diels-Alder Reaction The two double bonds in the diene are “cis” or “trans” to each other about the single bond (being in a plane maximizes overlap) These conformations are called s-cis and s-trans (“s” stands for “single bond”) Dienes react in the s-cis conformation in the Diels-Alder reaction

Not all dienes can rotate to be s-cis

Learning Check: Which can form an s-cis diene?

Solution: Which can form an s-cis diene?

Learning Check: Which would be good Diels-Alder dienophiles?

Solution: Which would be good Diels-Alder dienophiles?

14.6 Diene Polymers: Natural and Synthetic Rubbers Conjugated dienes can be polymerized The initiator for the reaction can be a radical, or an acid Polymerization: 1,4 addition of growing chain to conjugated diene monomer

Natural Rubber A material from latex, in plant sap In rubber repeating unit has 5 carbons and Z stereochemistry of all C=C Gutta-Percha is natural material with E in all C=C Looks as if it is the head-to-tail polymer of isoprene (2-methyl-1,3-butadiene)

Synthetic Rubber Chemical polymerization of isoprene does not produce rubber (stereochemistry is not controlled) Synthetic alternatives include neoprene, polymer of 2-chloro-1,3-butadiene This resists weathering better than rubber

Vulcanization Natural and synthetic rubbers are too soft to be used in products Charles Goodyear discovered heating with small amount of sulfur produces strong material Sulfur forms bridges between hydrocarbon chains (cross-links)

14.7 Structure Determination in Conjugated Systems: UV Spectroscopy Conjugated compounds can absorb light in the ultraviolet region of the spectrum The electrons in the highest occupied molecular orbital (HOMO) undergo a transition to the lowest unoccupied molecular orbital (LUMO) The region from 2 x 10-7m to 4 x 10-7m (200 to 400 nm) is most useful in organic chemistry A plot of absorbance (log of the ratio of the intensity of light in over light transmitted) against wavelength in this region is an ultraviolet spectrum –

Ultraviolet Spectrum of 1,3-Butadiene Example: 1,4-butadiene has four  molecular orbitals with the lowest two occupied Electronic transition is from HOMO to LUMO at 217 nm (peak is broad because of combination with stretching, bending)

Quantitative Use of UV Spectra Absorbance for a particular compound in a specific solvent at a specified wavelength is directly proportional to its concentration You can follow changes in concentration with time by recording absorbance at the wavelength Beers’ law: A = ecl “A” = absorbance “e” is molar absorptivity (extinction coefficient)=amount of UV light absorbed “c” is concentration in mol/L “l” is path of light through sample in cm

14.8 Interpreting UV Spectra: The Effect of Conjugation max: wavelength where UV absorbance for a compound is greatest Energy difference between HOMO and LUMO decreases as the extent of conjugation increases max increases as conjugation increases (lower energy) 1,3-butadiene: 217 nm, 1,3,5-hexatriene: 258 nm

Substituents on  system increase max 1,3-butadiene: H2C=CH-CH=CH2 217 nm

Calculations of lmax The lmax of the p  p* transition for compounds with < 4 conjugated double bonds can be calculated using Woodward-Fieser rules. Start with a base number: To the base add: 30 for each extra conjugated double bond 5 each time a conjugated double bond is an exocyclic double bond 36 for each conjugated double bond frozen s-cis 5 for each alkyl group or halogen bonded to conjugated system of polyene 10 for an a-substituent of a conjugated alkehyde or ketone 12 for a b-substituent of a conjugated aldehyde or ketone

Example: Calculate expected lmax for following: Base = 217 3 alkyl substituents @ 5 each = 15 Calculated = 232 Observed = 232

Learning Check: Calculate expected lmax for following: To the base add: 30 for each extra conjugated double bond 5 each time a conjugated double bond is an exocyclic double bond 36 for each conjugated double bond frozen s-cis 5 for each alkyl group or halogen bonded to conjugated system of polyene 10 for an a-substituent of a conjugated alkehyde or ketone 12 for a b-substituent of a conjugated aldehyde or ketone

Solution: Calculate expected lmax for following: Base = 217 Base = 215 Additional db bd = 30 a substituent = 10 Locked s-cis = 36 b substituent = 12 2 alkyl substituents = 10 Exocyclic db bd = 5 Calculated = 293 Calculated = 242 Observed = 293 Observed = 241

14.9 Conjugation, Color and the Chemistry of Vision Visible region is about 400 to 800 nm Extended systems of conjugation absorb in visible region Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain

11 double bonds in conjugation, max = 455 nm b-Carotene, 11 double bonds in conjugation, max = 455 nm

Vision:

Vision:

Vision: FIND THE MAN IN THE COFFEE BEANS

Vision: Doctors have concluded that if you find the man in the coffee beans in 3 seconds, the right half of your brain is better developed than most people.  If you find the man between 3 seconds and 1 minute, the right half of the brain is developed normally.  If you find the man between 1 minute and 3 minutes, then the right half of your brain is functioning slowly and you need to eat more protein.  If you have not found the man after 3 minutes, the advice is to look for more of this type of exercise to make that part of the brain stronger!!!

Vision: Stare at the eye of the red parrot while you count slowly to 20, then look immediately at one spot in the empty bird cage. The faint, ghostly image of a bluegreen bird will appear in the cage. Try the same thing with the green cardinal. A faint magenta bird will appear in the cage.

The ghostly birds you see here are called afterimages The ghostly birds you see here are called afterimages. An afterimage is an image that stays with you even after you have stopped looking at an object. The back of your eye is lined with light sensitive cells, called cones, which are sensitive to certain colors of light. When you stare at the red bird, your red-sensitive cones adapt to the light and lose their sensitivity. When you shift your gaze to the white background of the bird cage, you see white (minus red) where the red-sensitive cells have become adapted. White light minus red light is blue-green light. That's why the afterimage you see is blue-green and in the shape of a parrot. The same thing happens when you stare at the green bird, but this time it's the green-sensitive cones that adapt. White minus green light is magenta light, so you see the afterimage as a magenta cardinal.

Vision: Look at the flag for about 20 seconds, then look down to the white space below. Notice how the ghostly image of the familar "stars and stripes" appears. This afterimage occurs because red, white and blue are the complementary colors of cyan,black and yellow.

Which of the following is not conjugated? 1. 2. 3. 5. 4. 1 2 3 4 5

How many π (pi) bonding molecular orbitals are present in 1,3,5-hexatriene ? 2 3 4 5 6

Arrange the following alkenes in order of least stable to most stable. A < B < C B < C < A B < A < C A < C < B C < A < B

Predict the expected major organic product of the following reaction. 1. 2. 3. 4. 5. 1 2 3 4 5

Predict the expected major organic product of the following reaction. 1. 2. 3. 4. 5. 1 2 3 4 5

Which of the following dienes would be expected to react fastest in a Diels-Alder reaction? 1. 2. 3. 4. 5. 1 2 3 4 5

Which of the following dienophiles would be expected to react fastest in a Diels-Alder reaction? 1. 2. 3. 5. 4. 1 2 3 4 5

Which of the following dienes will not participate in a Diels-Alder reaction? 1. 2. 3. 4. 1 2 3 4

Select the most appropriate name for the following polymer: poly(methyl fluoride) poly(ethyl fluoride) poly(vinyl fluoride) polyfluoromethane polyfluoroethane

UV/Visible spectroscopy is based on _____ excitation. electronic rotational nuclear vibrational

If one knows the molar absorptivity for a molecule, then one can determine its concentration by taking an ultraviolet/visible absorption spectrum. True False

Which of the following wavelengths can be classified as UV light? 900 nm 725 nm 500 nm 450 nm none of these

Which of the following statements is not true regarding ultraviolet/visible spectroscopy? As the extent of conjugation in a molecule increases, λmax increases. As the extent of conjugation in a molecule increases, the HOMO-LUMO gap increases. The ultraviolet/visible region of the electromagnetic spectrum ranges from 200 – 800 nm. The amount of UV light absorbed by a compound can be expressed by its molar absorptivity, ε. As the path length of the sample increases, its absorbance increases.

Which of the following compounds is most likely to absorb in the visible region of the electromagnetic spectrum? 1. 2. 3. 4. 5. 1 2 3 4 5

The electronic transition that occurs for conjugated molecules in ultraviolet/visible spectroscopy is: σ → π* σ → σ* π → π* σ* → π*