CHEM 334L FINAL PRACTICAL QUESTIONS AND CORRECT
DETAILED ANSWERS (VERIFIED ANSWERS) ALREADY
GRADED A+
Which of the following is NOT true? Ans✓✓✓Sodium borohydride
(NaBH4) can easily reduce a carboxylic acid to an alcohol
NaBH4 is hygroscopic - it absorbs moisture quickly and forms a brittle
solid. What step(s) can be employed to ensure accurate amount of
NaBH4 is used in this reaction? Ans✓✓✓All of the above
At the end of the reaction period, what reagent will be used for
neutralizing unreacted sodium borohydride in the reaction mixture?
Ans✓✓✓1 Molar hydrochloric acid solution
Which of the following techniques will not be used for product
characterization in this experiment? Ans✓✓✓UV-Vis spectroscopy
In the reduction of 9-fluorenone, why should the temperature of the
reaction mixture be monitored in this experiment? Ans✓✓✓As the
reaction proceeds, heat is evolved and the reaction rate increases. The
temperature of the reaction mixture should be kept below boiling point
of methanol (65 °C).
Which molecular orbital in 9-fluorenone interacts with the hydride ion
from NaBH4? Ans✓✓✓pi antibonding orbital
,The reaction of sodium borohydride with hydrochloric acid causes...
Ans✓✓✓evolution of hydrogen, H2
Reduction of benzoin benzoate with NaBH4 is chemoselective. This
particular reduction is also diastereoselective (producing unequal
amounts of diastereomers). How many possible stereoisomers can be
generated from the reduction of racemic benzoin benzoate with NaBH4?
Ans✓✓✓4
What key absorption is present in the infrared spectrum of your starting
material? Ans✓✓✓1715 cm^-1
How many signals should be present in the 13C NMR spectrum of 9-
hydroxyfluorene? Ans✓✓✓7 signals
The organic portion of a Grignard reagent has a partial negative charge
and thus behaves as a(n) Ans✓✓✓carbanion
In the phenylmagnesium bromide + ethyl benzoate (lab 2), in this
experiment, TLC will be used to check the product for the presence of
an impurity. What impurity is this? Ans✓✓✓biphenyl
What is likely to happen if anhydrous solvents and conditions are not
used for the formation of phenylmagnesium bromide? Ans✓✓✓All of
the above
,In lab 2, a fresh surface of magnesium metal will facilitate the formation
of phenylmagnesium bromide. What will be done to ensure that
magnesium used has a fresh surface? Ans✓✓✓Buff the magnesium
ribbon with sandpaper and/or cut it into small pieces.
The TLC plate shown below was developed using 20% ethyl
acetate/80% hexanes mixture as the solvent. (two spots very close
together and with small Rf values)What change could be made to
increase the Rf value of the spots? (Keep in mind that silica gel is the
polar stationary phase.) Ans✓✓✓change the developing solvent to 30%
ethyl acetate/70% hexanes
Compared to the p orbital, the s orbital is better able to stabilize negative
charge. Therefore, based on the orbital hybridization, which Grignard
reagent should be the most stable? Ans✓✓✓c-triple bond-c-MgBr
What product(s) should be expected from the reaction shown below?
(benzoic acid and cyclohexylmagnesium bromide, then H3O+)
Ans✓✓✓benzoic acid and cyclohexane mixture
Given the developed TLC plate shown below, in what order would you
expect the three components (A, B, and C) to elute from the column?
Ans✓✓✓largest Rf will elute faster
How would the the 1H NMR spectrum of the product differ from that of
the starting material for the following reaction?
, (2-butanone to 2-butanol) Ans✓✓✓The protons on C-4 of the product
will give rise to a singlet with an integration of 6H.
In a Wittig reaction, the oxaphosphetane intermediate generated is a
compound that contains a ________ -member ring. Ans✓✓✓four
Which statement is true?
a.
The Jab coupling constant for a trans alkene is 11-18 Hz whereas that of
a cis alkene is 30-40 Hz.
b.
The Jab coupling constant for a trans alkene is the same as that for a cis
alkene (11-18 Hz)
c.
The Jab coupling constant for a trans alkene is 11-18 Hz whereas that of
a cis alkene is 0 Hz.
d.
The Jab coupling constant for a trans alkene is 11-18 Hz whereas that of
a cis alkene is 5-10 Hz. Ans✓✓✓d. The Jab coupling constant for a
trans alkene is 11-18 Hz whereas that of a cis alkene is 5-10 Hz.
The reaction of an aldehyde with a phosphonium ylide results in the
formation of _________ in the final product of interest. Ans✓✓✓a new
carbon-carbon double bond (C=C)
DETAILED ANSWERS (VERIFIED ANSWERS) ALREADY
GRADED A+
Which of the following is NOT true? Ans✓✓✓Sodium borohydride
(NaBH4) can easily reduce a carboxylic acid to an alcohol
NaBH4 is hygroscopic - it absorbs moisture quickly and forms a brittle
solid. What step(s) can be employed to ensure accurate amount of
NaBH4 is used in this reaction? Ans✓✓✓All of the above
At the end of the reaction period, what reagent will be used for
neutralizing unreacted sodium borohydride in the reaction mixture?
Ans✓✓✓1 Molar hydrochloric acid solution
Which of the following techniques will not be used for product
characterization in this experiment? Ans✓✓✓UV-Vis spectroscopy
In the reduction of 9-fluorenone, why should the temperature of the
reaction mixture be monitored in this experiment? Ans✓✓✓As the
reaction proceeds, heat is evolved and the reaction rate increases. The
temperature of the reaction mixture should be kept below boiling point
of methanol (65 °C).
Which molecular orbital in 9-fluorenone interacts with the hydride ion
from NaBH4? Ans✓✓✓pi antibonding orbital
,The reaction of sodium borohydride with hydrochloric acid causes...
Ans✓✓✓evolution of hydrogen, H2
Reduction of benzoin benzoate with NaBH4 is chemoselective. This
particular reduction is also diastereoselective (producing unequal
amounts of diastereomers). How many possible stereoisomers can be
generated from the reduction of racemic benzoin benzoate with NaBH4?
Ans✓✓✓4
What key absorption is present in the infrared spectrum of your starting
material? Ans✓✓✓1715 cm^-1
How many signals should be present in the 13C NMR spectrum of 9-
hydroxyfluorene? Ans✓✓✓7 signals
The organic portion of a Grignard reagent has a partial negative charge
and thus behaves as a(n) Ans✓✓✓carbanion
In the phenylmagnesium bromide + ethyl benzoate (lab 2), in this
experiment, TLC will be used to check the product for the presence of
an impurity. What impurity is this? Ans✓✓✓biphenyl
What is likely to happen if anhydrous solvents and conditions are not
used for the formation of phenylmagnesium bromide? Ans✓✓✓All of
the above
,In lab 2, a fresh surface of magnesium metal will facilitate the formation
of phenylmagnesium bromide. What will be done to ensure that
magnesium used has a fresh surface? Ans✓✓✓Buff the magnesium
ribbon with sandpaper and/or cut it into small pieces.
The TLC plate shown below was developed using 20% ethyl
acetate/80% hexanes mixture as the solvent. (two spots very close
together and with small Rf values)What change could be made to
increase the Rf value of the spots? (Keep in mind that silica gel is the
polar stationary phase.) Ans✓✓✓change the developing solvent to 30%
ethyl acetate/70% hexanes
Compared to the p orbital, the s orbital is better able to stabilize negative
charge. Therefore, based on the orbital hybridization, which Grignard
reagent should be the most stable? Ans✓✓✓c-triple bond-c-MgBr
What product(s) should be expected from the reaction shown below?
(benzoic acid and cyclohexylmagnesium bromide, then H3O+)
Ans✓✓✓benzoic acid and cyclohexane mixture
Given the developed TLC plate shown below, in what order would you
expect the three components (A, B, and C) to elute from the column?
Ans✓✓✓largest Rf will elute faster
How would the the 1H NMR spectrum of the product differ from that of
the starting material for the following reaction?
, (2-butanone to 2-butanol) Ans✓✓✓The protons on C-4 of the product
will give rise to a singlet with an integration of 6H.
In a Wittig reaction, the oxaphosphetane intermediate generated is a
compound that contains a ________ -member ring. Ans✓✓✓four
Which statement is true?
a.
The Jab coupling constant for a trans alkene is 11-18 Hz whereas that of
a cis alkene is 30-40 Hz.
b.
The Jab coupling constant for a trans alkene is the same as that for a cis
alkene (11-18 Hz)
c.
The Jab coupling constant for a trans alkene is 11-18 Hz whereas that of
a cis alkene is 0 Hz.
d.
The Jab coupling constant for a trans alkene is 11-18 Hz whereas that of
a cis alkene is 5-10 Hz. Ans✓✓✓d. The Jab coupling constant for a
trans alkene is 11-18 Hz whereas that of a cis alkene is 5-10 Hz.
The reaction of an aldehyde with a phosphonium ylide results in the
formation of _________ in the final product of interest. Ans✓✓✓a new
carbon-carbon double bond (C=C)
