Deck 15: Dienes and Aromaticity

Question 1

Which compound should have the greatest _max in its UV spectrum?

A) compound a
B) compound b
C) compound c
D) compound d
E) compound e

Answer 1

D

Question 2

Which compound has UV absorption at the greatest wavelength?

A) compound A
B) compound B
C) compound C
D) compound D

Answer 2

B

Question 3

Circle the sp²-hybridized carbons in the structure.

Answer 3

The central carbon of the cumulated double bond is sp-hybridized.

Question 4

The diene reacts with HBr to give two isomers A and B (C₆H₁₁Br).
​a. Give the structures of A and B.
b. Give the structure of the carbocation intermediate involved in this reaction. Be sure to show any relevant resonance structures.
c. When compounds A and B are each treated under solvolysis conditions with acetone/water, each compound forms a mixture of the same two alcohols C and D. Give the structures of these two alcohols and explain why both are formed from each alkyl halide.

Question 5

Which compound or ion has five  molecular orbitals?

A) compound A
B) compound B
C) compound C
D) compound D

Question 6

Which compound or ion has a UV/visible absorption at the greatest wavelength?

A) compound A
B) compound B
C) compound C
D) compound D
E) compound E

Question 7

1,3-Butadiene, the simplest conjugated diene, has a UV absorption at 217 nm. (One nanometer = 10^-9 meter.)
a. Calculate the energy of UV radiation with this wavelength (in kJ mol^-1). Show your work.
b. Shown is an MO electron-occupancy diagram for the  molecular orbitals of 1,3-butadiene. Draw an arrow on this diagram that corresponds to the energy of this UV radiation.

Question 8

All but one of these molecules are active ingredients in commercial sunscreens. Which compound does not act as a sunscreen?

A) compound A
B) compound B
C) compound C
D) compound D

Question 9

Which compound has the smaller (less positive or more negative) heat of formation?

A) compound A
B) compound B
C) compound C
D) compound D

Question 10

Select all structures that contain conjugated pi bonds.

A) compound A
B) compound B
C) compound C
D) compound D
E) compound E

Question 11

Draw the major organic product of this reaction.

Question 12

Give the structure of the diene and dienophile that would give this product after a Diels-Alder reaction.

Question 13

The Diels-Alder reaction can give two constitutional isomeric products, A and

Question 14

Which of these dienes cannot undergo the Diels-Alder reaction.

A) compound A
B) compound B
C) compound C
D) compound D

Question 15

Draw the endo product formed in the Diels-Alder reaction. Clearly show stereochemistry, where applicable.

Question 16

Deduce the starting materials that would give this compound via the Diels-Alder reaction.

Question 17

The reaction of an allylic alcohol with HBr generates the two products shown. Draw a curved arrow mechanism to explain how both products are formed.

Question 18

Draw the thermodynamic and kinetic product for the reaction. Clearly identify which is the thermodynamic and kinetic product.

Question 19

Which of these compounds is aromatic?

A) compound A
B) compound B
C) compound C
D) compound D

Question 20

Cyclooctatetraene (COT) has eight pi electrons but is a stable compound. Explain why.

Question 21

Draw a Frost circle and determine the molecular orbitals and electron occupancies for furan.

Question 22

Pyridine and pyrrole are both aromatic nitrogen-containing rings. Explain how the lone pair on each nitrogen contributes or does not contribute to the pi electron count.

Question 23

1,3-Cyclopentadiene is unusually acidic, with a pK_a of ~16. On the other hand, 1,3,5-cyclohepatriene is much less acidic, with a pKa of ~36. Explain why the compounds have such differing acidities.

Question 24

Determine whether each compound is aromatic, antiaromatic, or neither. Label each compound with the pi-electron count.

Question 25

An arginine residue and a phenylalanine residue are located close to each other in a protein structure.
Select the most favorable interaction(s) of arginine and phenylalanine.

A) offset stacking
B) edge-to-face interaction
C) pi-cation interaction