CHEM 233 Unit 3 Questions and
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
Which of the following is an accurate description of the CEC method? (1 pt) - answer
A single enantiomer of alcohol is reacted with both enantiomers of HBTM
Upon being added, the butyric anhydride reacts with what first? (1 pt) - answer
HBTM
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
Which of the following is an accurate description of the CEC method? (1 pt) - answer
A single enantiomer of alcohol is reacted with both enantiomers of HBTM
Upon being added, the butyric anhydride reacts with what first? (1 pt) - answer
HBTM
