Conjugated Compounds and Ultraviolet Spectroscopy

Allene, ${\mathbf{CH}}_{\mathbf{2}}\mathbf{=}\mathbf{C}\mathbf{=}{\mathbf{CH}}_{\mathbf{2}}$ has a heat of hydrogenation of $\mathbf{-}\mathbf{298}\mathbf{ }\mathbf{kJ}\mathbf{/}\mathbf{mol}$

$\mathbf{(}\mathbf{-}\mathbf{71}\mathbf{.}\mathbf{3}\mathbf{ }\mathbf{kcal}\mathbf{/}\mathbf{mol}\mathbf{)}$. Rank a conjugated diene, a nonconjugated diene, and an

allene in order of stability.

Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HCl with 1,3-pentadiene.

Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HBr with the following compound:

The 1,2 adduct and the 1,4 adduct formed by reaction of HBr with 1,3-butadiene are in equilibrium at 40 °C. Propose a mechanism by which the interconversion of products takes place.

Why do you suppose 1,4 adducts of 1,3-butadiene are generally more stable than 1,2 adducts?

Predict the product of the following Diels–Alder reaction:

Which of the following alkenes would you expect to be good Diels–Alder dienophiles?

Which of the following dienes have an s-cis conformation, and which have an s-trans conformation? Of the s-trans dienes, which can readily rotate to s-cis?

Predict the product of the following Diels–Alder reaction:

Draw a segment of the polymer that might be prepared from 2-phenyl-1,3-butadiene.

Show the mechanism of the acid-catalyzed polymerization of 1,3-butadiene.

Calculate the energy range of electromagnetic radiation in the UV region of the spectrum from 200 to 400 nm (see Section 12-5). How does this value compare with the values calculated previously for IR and NMR spectroscopy?

If pure vitamin A has ${\mathbf{\lambda }}_{\max }$ =5 325 ($\mathrm{\epsilon }$= 50,100), what is the vitamin A concentration in a sample whose absorbance at 325 nm is A = 0.735 in a cell with a path length of 1.00 cm?

Which of the following compounds would you expect to show ultraviolet absorptions in the 200 to 400 nm range?

Show the structures of all possible adducts of the following diene with 1 equivalent of HCl:

Show the product of the Diels–Alder reaction of the following diene with 3-buten-2-one, H₂C=CHCOCH₃. Make sure you show the full stereochemistry of the reaction product.

The following Diene does not undergo Diels–Alder reactions. Explain.

The following model is that of an allylic carbocation intermediate formed by protonation of a conjugated diene with HBr. Show the structure of the diene and the structures of the final reaction products.

How can you account for the fact that cis-1,3-pentadiene is much less reactive than trans-1,3-pentadiene in Diels-Alder reaction. 

Would you expect a conjugated diyne such as 1,3-butadiyne to undergo Diels-Alder reaction with a dienophile? Explain.

How could you use Diels-Alder reactions to prepare the following products ? Show the starting diene and dienophile in each case.

a) 

How could you use Diels-Alder reactions to prepare the following products ? Show the starting diene and dienophile in each case.

b) 

How could you use Diels-Alder reactions to prepare the following products ? Show the starting diene and dienophile in each case.

c)

How could you use Diels-Alder reactions to prepare the following products ? Show the starting diene and dienophile in each case.

a) 

Diene polymers contain occasional vinyl branches along the chain.

How do you think these branches might arise?

Tires whose sidewalls are made of natural rubber tend to crack and

weather rapidly in areas around cities where high levels of ozone and

other industrial pollutants are found. Explain.

1,3-Cyclopentadiene polymerizes slowly at room temperature to yield

a polymer that has no double bonds except on the ends. On heating, the

polymer breaks down to regenerate 1,3-cyclopentadiene. Propose a structure for the product.

Arrange the molecules according to where you would expect to find

their wavelength of maximum absorption in UV spectroscopy, from shortest to longest wavelength.

a) 

b) 

c) 

Which of the following compounds would you expect to have a 

UV absorption in the 200 to 400 nm range?

a) 

b) 

c) 

Would you expect allene, ${\mathrm{H}}_2\mathrm{C}=\mathrm{C}={\mathrm{CH}}_2$, to show a UV absorption in

the 200 to 400 nm range? Explain.

The following ultraviolet absorption maxima have been measured:

1,3-Butadiene 217 nm

2-Methyl-1,3-butadiene 220 nm

1,3-Pentadiene 223 nm

2,3-Dimethyl-1,3-butadiene 226 nm

2,4-Hexadiene227 nm

2,4-Dimethyl-1,3-pentadiene 232 nm

2,5-Dimethyl-2,4-hexadiene 240 nm

What conclusion can you draw about the effect of alkyl substitution on

UV absorption maxima? Approximately what effect does each added

alkyl group have?

Draw the resonance forms that result when the dienes below are protonated. If the resonance forms differ in energy, identify the most stable one.

a) 

b) 

c) 

Answer the questions below for 1,3,5-cycloheptatriene.

(a) How many p atomic orbitals are in the conjugated system?

(b) How many molecular orbitals describe the conjugated system?

(c) How many molecular orbitals are bonding molecular orbitals?

(d) How many molecular orbitals are anti-bonding molecular orbitals?

(e) Which molecular orbitals are filled with electrons?

(f) If this molecule were to absorb a photon of UV light an electron would move between which two molecular orbitals (be specific)?

Treatment of 3,4-dibromohexane with strong base leads to loss of 2 equivalents of HBr and formation of a product with formula C6H10. Three products are possible. Name each of the three, and tell how you would use ¹H and ¹³C NMR spectroscopy to help identify them. How would you use UV spectroscopy?

Addition of HCl to 1-methoxycyclohexene yields 1-chloro-1-methoxycyclohexane as a sole product. Use resonance structures to explain why none of the other regioisomer is formed.

Aldrin, a chlorinated insecticide now banned from use in the United

States, can be made by Diels–Alder reaction of hexachloro-1,3-cyclopentadiene with norbornadiene. What is the structure of aldrin?

Norboradiene (Problem 14-55) can be prepared by reaction of chloroethylene with 1,3-cyclopentadiene, followed by treatment of the product with sodium ethoxide. Write the overall scheme and identify the two kinds of reactions.

Hydrocarbon A, , has a UV absorption at ${\mathrm{\lambda }}_{\max }$=236nm and gives hydrocarbon B, C₁₀H₁₈, on catalytic hydrogenation. Ozonolysis of A, followed by zinc/acetic acid treatment, yields the following diketo dialdehyde:

a) Propose two possible structures for A.

Hydrocarbon A, C₁₀H₁₄ , has a UV absorption at ${\mathrm{\lambda }}_{\max }=236\mathrm{nm}$and gives hydrocarbon B, C₁₀H₁₈, on catalytic hydrogenation. Ozonolysis of A, followed by zinc/acetic acid treatment, yields the following diketo dialdehyde:

b) Hydrocarbon A reacts with maleic anhydride to yield a Diels-Alder adduct. Which of your structures for A is correct?

Hydrocarbon A, C₁₀H₁₄, has a UV absorption at${\mathrm{\lambda }}_{\max }=236\mathrm{nm}$ and gives hydrocarbon B, C₁₀H₁₈, on catalytic hydrogenation. Ozonolysis of A, followed by zinc/acetic acid treatment, yields the following di-keto dialdehyde:

c) Write the reactions, showing the starting material and products.

Adiponitrile, a starting material used in the manufacture of nylon, can be prepared in three steps from 1,3-butadiene. How would you carry out this synthesis?

Ergosterol, a precursor of vitamin D, has ${\mathrm{\lambda }}_{\max }$=282nm and molar absorptivity $\mathrm{\epsilon }$= 11900. What is the concentration of ergosterol in a solution whose absorbance A=0.065 with a sample pathlength l=1.0cm?

Dimethyl butynedioate undergoes a Diels-Alder reaction with (2E,4E)-2,4-hexadiene. Show the structure and stereochemistry of the product.

Dimethyl butynedioate also undergoes a Diel-Alder reaction with (2E, 4Z)-2,4-hexadiene, but the stereochemistry of the product is different from that of the (2E,4E) isomer (Problem 14-62). Explain.

Predict the major product (s) from the addition of 1 equivalent of HX and show the mechanism for each reaction below.

Question: In the light of your answer to problem to propose mechanism for the reactions below.

(a) In the light of your answer to problem to propose mechanism for the reactions below.

Luminol, which is used by forensic scientists to find blood, fluoresces as a result of Diels-Alder like process. The dianion of luminal reacts with to form an unstable peroxide intermediate that then loses nitrogen to form a dicarboxylate and emit light. The process is similar to that in 14-21 and 14-22. Propose a mechanism for this process.

An extremely useful diene in the synthesis of many natural products is

known as Danishefsky’s diene. This compound is useful because after

the Diels–Alder reaction it can be converted into a product that could

not be accessed by a typical Diels–Alder reaction. Show the Diels–Alder

adduct and propose a mechanism that accounts for the final products.

Give IUPAC names for the following compounds:

(a)