Chem 363, Section 2
Exam 2, May 5, 2008
Name: Answer Key Student ID#:_________
1) (4 pts.) Give the correct IUPAC name for the following compound.
Answer:
2-Bromo-4-nitrobenzoic acid
2) (8 pts.) Draw structures which correspond to the following names.
a. 4-Amino-2-chlorobenzaldehyde b. ortho-Propylbenzonitrile
Answer:
a. b.
3) (20 pts.) Give short answers to the following questions.
a. Describe the Huckel 4n + 2 rule.
Answer:
A molecule is aromatic if it has a planar, monocyclic system of conjugation and contains a total of
4n + 2 π electrons, where n is an integer. Only molecules with 2, 6, 10, 14, 18,… π electrons can be aromatic.
b. Circle any conjugated portions in the molecules below.
a. b.
Answer:
a. b.
c. Give a general example for a Claisen condensation reaction.
Answer:
d. Give a general example for an aldol reaction.
Answer:
e. Why is UV spectroscopy less commonly used than the other three spectroscopic techniques, IR, NMR, and MS? Explain ultraviolet excitation.
Answer:
Because it only gives information about the nature of conjugated π electron systems. As result of irradiation with ultraviolet light the compounds absorb energy at a certain wavelength and a π electron is promoted from the HOMO to the LUMO. The process in which an electron is promoted from a bonding π molecular orbital to an antibonding π* molecular orbital is called π → π* excitation.
4) (4 pts.) An electrophilic addition reaction to a conjugated diene is shown below. Briefly explain how can you control the reaction conditions that either the 1,2 adduct or the 1,4 adduct be the major product in the reaction.
1,2 adduct 1,4 adduct
Answer:
The 1,2 adduct is the major product if the reaction is under kinetic control. In this case the reaction needs to be run at lower temperatures. The product depends only on relative rates and equilibrium is not reached. The 1,4 adduct is the major product if the reaction is under thermodynamic control. In this case the reaction needs to be run at higher temperature. The product depends only on thermodynamic stability and equilibrium is reached.
5) (8 pts.) For each of the structures below, predict whether it would be expected to show aromatic character or not. Explain your answer in each case.
a.
Answer:
Not aromatic. This compound, [16]annulene, appears to be a cyclic, planar, conjugated molecule with 16 pi electrons. However, 16 is not a Huckel number, so the compound is probably not planar, and is not aromatic.
b.
Answer:
Aromatic. The cycloheptatrienyl cation is planar and conjugated molecule with 6 pi electrons. 6 is a Huckel number, so the compound is aromatic.
6) (24 pts.) Complete the following reactions.
a.
Answer:
b.
Answer:
c.
Answer:
d.
Answer:
e.
Answer:
f.
Answer:
7) (6 pts.) Indicate which spectral technique, NMR, IR, UV, or MS, would most readily allow differentiation between compounds in each pair of compounds below. Explain your answer in each case.
a.
Answer:
NMR. These isomers have the same molecular weight and will show no strong IR and no UV absorptions. The compound on the left will have a single peak in its 1H-NMR spectrum while the compound on the right will have three multiplets.
b.
Answer:
UV. These compounds will have similar NMR, IR, and MS. The UV spectra will be significantly different since the compound on the left is conjugated.
c.
Answer:
MS. These compounds have different molecular weights but similar NMR, IR and UV spectra.
8) (9 pts.) Write the complete stepwise mechanism for the following reaction. Show all intermediate structures and the electron flow using the curved arrow convention.
Answer:
9) (14 pts.) Using acetylene (ethyne, C2H2) and any alkyl halides as starting materials, synthesize the following compound. More than one step will be required. Include a Claisen condensation reaction in your synthetic scheme. Show all reagents and all intermediate compounds.
Answer:
10) (3 pts.) Briefly describe three important uses of compounds that contain conjugated systems in our everyday lives.
Answer:
Structures of molecules that contain conjugated systems are used as colorants, are found in biologically important molecules (ex. hormones), and are used in the manufacture of different polymers.
Bonus Question
11) (5 bonus pts.) In 1839 Charles Goodyear discovered a process by which both natural and synthetic rubbers are hardened. What is this process called?
Answer:
Vulcanization