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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

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Conjugated and nonconjugated dienes
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
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
Learning Check:

  • Circle any conjugation in the following:


Solution
Solution:

  • Circle any conjugation in the following:

Nonconjugated

Nonconjugated


14 1 conjugated dienes
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
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 dienes1
Stability of Conjugated Dienes:

  • Conjugation makes more stable

  • Substitution makes more stable


Description of 1 3 butadiene
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
Molecular Orbital Description

Conjugated Double bonds

Isolated Double bond

The bonding -orbitals are made from 4 porbitals 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
14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

  • Remember: addition of electrophile to C=C

    • Markovnikovregiochemistry via more stable carbocation


Carbocations from conjugated dienes
Carbocations from Conjugated Dienes Carbocations

  • Now: Addition of H+ gives delocalized 2oallyliccarbocation


Addition products to delocalized carbocation
Addition Products to Delocalized Carbocation Carbocations

  • Nucleophile can add to either cationic site

The transition states for the two possible products are not equal in energy


Another example
Another Example: Carbocations


Learning check1
Learning Check: Carbocations

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


Solution1
Solution: Carbocations

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


14 3 kinetic vs thermodynamic control
14.3 Kinetic vs. Thermodynamic Control Carbocations

  • 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
Kinetic and Thermodynamic Control: Example Carbocations

  • 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 Carbocations

_____ Thermodynamic control


14 4 the diels alder cycloaddition reaction
14.4 The Diels-Alder Cycloaddition Reaction Carbocations

  • Conjugate dienescan 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
View of the Diels-Alder Reaction Carbocations

  • 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
14.5 Characteristics of Diels-Alder Reaction Carbocations

  • 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
Stereospecificity of the Diels-Alder Reaction Carbocations

  • The reaction is stereospecific,

    • There is a one-to-one relationship between stereoisomeric reactants and products


Endo vs exo positions
Endo vs Exo positions Carbocations

Endoand 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
Regiochemistry of the Diels-Alder Reaction Carbocations

  • Reactants align to produce endo (rather than exo) product


Example
Example: Carbocations

Endo product formed


Conformations of dienes in the diels alder reaction
Conformations of CarbocationsDienes 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-cisand s-trans(“s” stands for “single bond”)

  • Dienes react in the s-cisconformation in the Diels-Alder reaction



Learning check2
Learning Check: Carbocations

  • Which can form an s-cis diene?


Solution2
Solution: Carbocations

  • Which can form an s-cis diene?


Learning check3
Learning Check: Carbocations

  • Which would be good Diels-Alder dienophiles?


Solution3
Solution: Carbocations

  • Which would be good Diels-Alder dienophiles?


14 6 diene polymers natural and synthetic rubbers
14.6 Diene Polymers: Natural and Synthetic Rubbers Carbocations

  • 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
Natural Rubber Carbocations

  • 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
Synthetic Rubber Carbocations

  • 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
Vulcanization Carbocations

  • 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
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
Ultraviolet Spectrum of 1,3-Butadiene Spectroscopy

  • 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
Quantitative Use of UV Spectra Spectroscopy

  • 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
14.8 Interpreting UV Spectra: The Effect of Conjugation Spectroscopy

  • 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
Substituents on Spectroscopy system increase max

1,3-butadiene: H2C=CH-CH=CH2 217 nm


Calculations of l max
Calculations of Spectroscopylmax

  • The lmax of the p p* transition for compounds with < 4 conjugated double bonds can be calculated using Woodward-Fieserrules.

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


Example1
Example: Spectroscopy

  • Calculate expected lmax for following:

Base = 217

3 alkyl substituents @ 5 each = 15

Calculated = 232

Observed = 232


Learning check4
Learning Check: Spectroscopy

  • 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


Solution4
Solution: Spectroscopy

  • 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
14.9 Conjugation, Color and the Chemistry of Vision Spectroscopy

  • 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


b Spectroscopy-Carotene,

11 double bonds in conjugation, max = 455 nm


Vision: Spectroscopy


Vision: Spectroscopy


Vision: Spectroscopy

FIND THE MAN IN THE COFFEE BEANS


Vision: Spectroscopy

  • 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: Spectroscopy

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. 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: 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 (

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

2. 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 (

1.

3.

5.

4.

Which of the following is not conjugated?

  • 1

  • 2

  • 3

  • 4

  • 5



Arrange the following alkenes in order of least stable to most stable
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. most stable.

2.

3.

4.

5.

Predict the expected major organic product of the following reaction.

  • 1

  • 2

  • 3

  • 4

  • 5

°C


Predict the expected major organic product of the following reaction1

2. most stable.

1.

3.

4.

5.

Predict the expected major organic product of the following reaction.

  • 1

  • 2

  • 3

  • 4

  • 5


Which of the following dienes would be expected to react fastest in a diels alder reaction

2. most stable.

1.

3.

4.

5.

Which of the following dienes would be expected to react fastest in a Diels-Alder reaction?

  • 1

  • 2

  • 3

  • 4

  • 5


Which of the following dienophiles would be expected to react fastest in a diels alder reaction

1. most stable.

2.

3.

5.

4.

Which of the following dienophiles would be expected to react fastest in a Diels-Alder reaction?

  • 1

  • 2

  • 3

  • 4

  • 5


Which of the following dienes will not participate in a diels alder reaction

1. most stable.

2.

3.

4.

Which of the following dienes will not participate in a Diels-Alder reaction?

  • 1

  • 2

  • 3

  • 4


Select the most appropriate name for the following polymer
Select the most appropriate name for the following polymer: most stable.

  • poly(methyl fluoride)

  • poly(ethyl fluoride)

  • poly(vinyl fluoride)

  • polyfluoromethane

  • polyfluoroethane


Uv visible spectroscopy is based on excitation
UV/Visible spectroscopy is based on _____ excitation. most stable.

  • 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
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
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.


1. ultraviolet/visible spectroscopy?

2.

3.

4.

5.

Which of the following compounds is most likely to absorb in the visible region of the electromagnetic spectrum?

  • 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*


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