CHEM 233 Unit 3 Exam Questions With
Complete Answers
How many reaction schemes involving the alkene should you have in the
"Reactions" section of your Pre-lab notebook? (1 pt) - ANSWER 3
You will weigh and then add pure molecular bromine to your reaction vessel
when performing the experiment. (1 pt) - ANSWER false
What technique will you use to purify your product? (1 pt) - ANSWER
recrystallization
What technique will you use to identify your product? (1 pt) - ANSWER melting
point
Percent yield is the same as percent recovery. (1 pt) - ANSWER false
In the experiment, you added water to the reaction vessel after the reaction was
complete, but we did not discuss why. Based on what happened after adding the
water, which of the following is the reason for the addition of water? (1 pts) -
ANSWER lower the solubility of the product in solution so it precipitated out
You recrystallize the product of a bromination reaction by dissolving the entire
product in hot solvent, cooling to room temperature, and then collecting the
solid that crystallized. However, you are disappointed with your yield. Select all
of the following actions that would increase the amount of solid recovered from
the recrystallization. (2 pts) - ANSWER cool the recrystallization solution in an
ice bath before collecting the crystals while keeping all impurities dissolved
lower the amount of solvent used in the recrystallization while keeping all
impurities dissolved
Calculate the percent recovery of a recrystallization of a 1.3 g sample of solid to
the tenths place if you recover 0.88 g after the recrystallization. Round to the
nearest tenths. (2 pts) - ANSWER 67.7
After performing a recrystallization, you calculate a percent recovery of 110%.
Select all possible reasons for this result. (1 pt) - ANSWER The crystals were not
dried properly after the recrystallization
After brominating an alkene, you isolate a solid that is orange in color. The
product is known to be a white solid when pure. What do you expect the result of
, performing a melting point analysis on the product to be? (1 pts) - ANSWER The
melting point will be lower than the melting point of the pure compound
In an effort to optimize the bromination reaction, you repeat the reaction three
times with each attempt using a slightly different set of reaction conditions. The
melting points of the crude products of each reaction are shown below. Which
reaction produced the most pure crude product ? (2 pts)
Reaction 1: 110 - 114 °C
Reaction 2: 108 - 114 °C
Reaction 3: 113 - 116 °C - ANSWER Reaction 3
The below compound has a retention factor of 0.35 using a silica gel TLC plate
and a particular solvent system. What would happen if you repeated the TLC
analysis, but used a less polar solvent system? (2 pts) - ANSWER The retention
factor would decrease
The below compound would have strong interactions with the stationary phase
of a Normal Phase TLC plate. (1 pts) - ANSWER True
Trans-cinnamic acid has a molecular weight of 148 g/mol. What mass would you
expect to see in an electrospray ionization mass spectrum if a sodium ion
attaches itself to the molecule? (1 pts) - ANSWER 171
You want to assess the purity of the product of this bromination experiment via
1H NMR spectroscopy. You would be able to see leftover Br2 in a 1H NMR
spectrum. (1 pts) - ANSWER false
You attempt to perform a bromination of trans-cinnamic acid in the hopes of
producing the product shown below. You decide to take a 1H NMR spectrum of
what you hope is the product. Signals with which of the following approximate
chemical shifts would indicate that the product did not form? (2 pts) - ANSWER ~
5.5 ppm
You attempt to perform the bromination of trans-cinnamic acid shown below.
You decide to take an IR spectrum of your product and see a strong, broad
signal at 3500 cm-1. Is this signal definitive proof of a successful reaction? (2
pts) - ANSWER No, because the IR spectra for both the starting material and
product would have that stretch
You are attempting to brominate Compound A, which contains 2 alkenes, using
molecular bromine to make Product B according to the following chemical
equation. - ANSWER 0.075
Complete Answers
How many reaction schemes involving the alkene should you have in the
"Reactions" section of your Pre-lab notebook? (1 pt) - ANSWER 3
You will weigh and then add pure molecular bromine to your reaction vessel
when performing the experiment. (1 pt) - ANSWER false
What technique will you use to purify your product? (1 pt) - ANSWER
recrystallization
What technique will you use to identify your product? (1 pt) - ANSWER melting
point
Percent yield is the same as percent recovery. (1 pt) - ANSWER false
In the experiment, you added water to the reaction vessel after the reaction was
complete, but we did not discuss why. Based on what happened after adding the
water, which of the following is the reason for the addition of water? (1 pts) -
ANSWER lower the solubility of the product in solution so it precipitated out
You recrystallize the product of a bromination reaction by dissolving the entire
product in hot solvent, cooling to room temperature, and then collecting the
solid that crystallized. However, you are disappointed with your yield. Select all
of the following actions that would increase the amount of solid recovered from
the recrystallization. (2 pts) - ANSWER cool the recrystallization solution in an
ice bath before collecting the crystals while keeping all impurities dissolved
lower the amount of solvent used in the recrystallization while keeping all
impurities dissolved
Calculate the percent recovery of a recrystallization of a 1.3 g sample of solid to
the tenths place if you recover 0.88 g after the recrystallization. Round to the
nearest tenths. (2 pts) - ANSWER 67.7
After performing a recrystallization, you calculate a percent recovery of 110%.
Select all possible reasons for this result. (1 pt) - ANSWER The crystals were not
dried properly after the recrystallization
After brominating an alkene, you isolate a solid that is orange in color. The
product is known to be a white solid when pure. What do you expect the result of
, performing a melting point analysis on the product to be? (1 pts) - ANSWER The
melting point will be lower than the melting point of the pure compound
In an effort to optimize the bromination reaction, you repeat the reaction three
times with each attempt using a slightly different set of reaction conditions. The
melting points of the crude products of each reaction are shown below. Which
reaction produced the most pure crude product ? (2 pts)
Reaction 1: 110 - 114 °C
Reaction 2: 108 - 114 °C
Reaction 3: 113 - 116 °C - ANSWER Reaction 3
The below compound has a retention factor of 0.35 using a silica gel TLC plate
and a particular solvent system. What would happen if you repeated the TLC
analysis, but used a less polar solvent system? (2 pts) - ANSWER The retention
factor would decrease
The below compound would have strong interactions with the stationary phase
of a Normal Phase TLC plate. (1 pts) - ANSWER True
Trans-cinnamic acid has a molecular weight of 148 g/mol. What mass would you
expect to see in an electrospray ionization mass spectrum if a sodium ion
attaches itself to the molecule? (1 pts) - ANSWER 171
You want to assess the purity of the product of this bromination experiment via
1H NMR spectroscopy. You would be able to see leftover Br2 in a 1H NMR
spectrum. (1 pts) - ANSWER false
You attempt to perform a bromination of trans-cinnamic acid in the hopes of
producing the product shown below. You decide to take a 1H NMR spectrum of
what you hope is the product. Signals with which of the following approximate
chemical shifts would indicate that the product did not form? (2 pts) - ANSWER ~
5.5 ppm
You attempt to perform the bromination of trans-cinnamic acid shown below.
You decide to take an IR spectrum of your product and see a strong, broad
signal at 3500 cm-1. Is this signal definitive proof of a successful reaction? (2
pts) - ANSWER No, because the IR spectra for both the starting material and
product would have that stretch
You are attempting to brominate Compound A, which contains 2 alkenes, using
molecular bromine to make Product B according to the following chemical
equation. - ANSWER 0.075
