CHEM 238 TEST PAPER 2025/2026 QUESTIONS AND
SOLUTIONS MARKED A+
✔✔When is recrystallization not a suitable method to purify an organic material? -
✔✔When the organic material is a liquid instead of a solid.
✔✔Which of the following are requirements for a suitable solvent when performing a
recrystallization? - ✔✔The solvent should dissolve the compound while hot.
The solvent should not chemically react with the compound.
✔✔What is the purpose of the initial hot vacuum filtration in the recrystallization
process? - ✔✔To remove insoluble solid impurities.
✔✔What effect will result if the filtrate solution is cooled too rapidly? - ✔✔Small impure
crystals will form.
✔✔Use clamps to secure the filter flask to avoid tipping and unnecessary exposure to
hot liquids or the solute. - ✔✔True
✔✔Recrystallization requires the material that is to be purified is a solid. - ✔✔True
✔✔For hot vacuum filtration, the filter paper should be completely dry when pouring the
hot solution into the Buchner funnel to filter. - ✔✔False
✔✔Recrystallization is the process of dissolving a solid material into a suitable solvent
or mixture of solvents and allowing the material to crystallized from this solution. -
✔✔True
✔✔For recrystallization, rapid cooling gives the best crystals. - ✔✔False
✔✔Why should the lids not be put back on the waste containers? - ✔✔The reaction
waste is still generating hydrogen gas that would get trapped in the container.
✔✔Procedural steps for the sodium borohydride reduction of camphor to isoborneol. -
✔✔Dissolve camphor in ethanol in an Erlenmeyer flask.
Carefully add an excess of sodium borohydride in small portions.
Let the reaction mixture stand for 10 minutes.
Heat to a gentle boil for 10 minutes.
Pour the reaction mixture in a beaker of ice water.
Collect the precipitate by vacuum filtration, and blot dry.
Recrystallize from methanol and water.
✔✔What is sublimation? - ✔✔The transition directly from the solid phase to gas phase.
, ✔✔Which of the following statements about the ratio of products formed by a reaction
under kinetic control are true? - ✔✔The ratio of products is not determined by the
energies of products.
The ratio of products is determined by the relative energies of transition states.
✔✔Which of the following statements relates the value and magnitude of delta Hf to
stability of a product - ✔✔A large negative value of delta Hf indicates a stable product
✔✔Which of the following statements about semi-empirical methods are true: -
✔✔Some experimentally measured parameters are substituted into the calculation.
It is good at predicting structural properties that arise from subtle interactions of orbitals.
✔✔1,2-addition product - ✔✔Kinetic product (higher energy product)
Secondary allylic carbocation (low energy carbocation)
Lower energy transition state
✔✔1,4-addition product - ✔✔Thermodynamic product (lower energy product)
Primary allylic carbocation (higher energy carbocation)
Higher energy transition state
✔✔Which of the following experimental mistakes will decrease the effectiveness of a
Grignard reaction? - ✔✔The magnesium ribbon was not wiped clean before use.
The glassware was not dried before use.
✔✔What was the purpose of the saturated sodium chloride solution in the preparation of
a tertiary alcohol by a Grignard reaction? - ✔✔Saturated sodium chloride decreases the
solubility of the organic product in water.
✔✔Procedural steps required to convert 2-butanone into 3-methyl-3-hexanol using a
Grignard prepared from 1-bromopropane and magnesium metal. - ✔✔Dry all glassware
and reagents prior to starting the experiment.
Add 1-bromopropane to clean magnesium.
Wait for reaction to turn cloudy. Sonicate if necessary.
Add 2-butanone dropwise to the Grignard reagent, allowing a gentle reflux.
Add aqueous ammonium chloride to quench the reaction.
Wash the reaction mixture with sodium bicarbonate and sodium chloride solutions in a
separatory funnel.
Dry over magnesium sulfate and distill product from THF.
✔✔Why are ethers commonly used as a solvent for Grignard reagent preparations? -
✔✔Ether's high vapor pressure helps to prevent moisture from entering the reaction
vessel.
Ether excludes moisture from the reaction, because water is not very soluble in it.
SOLUTIONS MARKED A+
✔✔When is recrystallization not a suitable method to purify an organic material? -
✔✔When the organic material is a liquid instead of a solid.
✔✔Which of the following are requirements for a suitable solvent when performing a
recrystallization? - ✔✔The solvent should dissolve the compound while hot.
The solvent should not chemically react with the compound.
✔✔What is the purpose of the initial hot vacuum filtration in the recrystallization
process? - ✔✔To remove insoluble solid impurities.
✔✔What effect will result if the filtrate solution is cooled too rapidly? - ✔✔Small impure
crystals will form.
✔✔Use clamps to secure the filter flask to avoid tipping and unnecessary exposure to
hot liquids or the solute. - ✔✔True
✔✔Recrystallization requires the material that is to be purified is a solid. - ✔✔True
✔✔For hot vacuum filtration, the filter paper should be completely dry when pouring the
hot solution into the Buchner funnel to filter. - ✔✔False
✔✔Recrystallization is the process of dissolving a solid material into a suitable solvent
or mixture of solvents and allowing the material to crystallized from this solution. -
✔✔True
✔✔For recrystallization, rapid cooling gives the best crystals. - ✔✔False
✔✔Why should the lids not be put back on the waste containers? - ✔✔The reaction
waste is still generating hydrogen gas that would get trapped in the container.
✔✔Procedural steps for the sodium borohydride reduction of camphor to isoborneol. -
✔✔Dissolve camphor in ethanol in an Erlenmeyer flask.
Carefully add an excess of sodium borohydride in small portions.
Let the reaction mixture stand for 10 minutes.
Heat to a gentle boil for 10 minutes.
Pour the reaction mixture in a beaker of ice water.
Collect the precipitate by vacuum filtration, and blot dry.
Recrystallize from methanol and water.
✔✔What is sublimation? - ✔✔The transition directly from the solid phase to gas phase.
, ✔✔Which of the following statements about the ratio of products formed by a reaction
under kinetic control are true? - ✔✔The ratio of products is not determined by the
energies of products.
The ratio of products is determined by the relative energies of transition states.
✔✔Which of the following statements relates the value and magnitude of delta Hf to
stability of a product - ✔✔A large negative value of delta Hf indicates a stable product
✔✔Which of the following statements about semi-empirical methods are true: -
✔✔Some experimentally measured parameters are substituted into the calculation.
It is good at predicting structural properties that arise from subtle interactions of orbitals.
✔✔1,2-addition product - ✔✔Kinetic product (higher energy product)
Secondary allylic carbocation (low energy carbocation)
Lower energy transition state
✔✔1,4-addition product - ✔✔Thermodynamic product (lower energy product)
Primary allylic carbocation (higher energy carbocation)
Higher energy transition state
✔✔Which of the following experimental mistakes will decrease the effectiveness of a
Grignard reaction? - ✔✔The magnesium ribbon was not wiped clean before use.
The glassware was not dried before use.
✔✔What was the purpose of the saturated sodium chloride solution in the preparation of
a tertiary alcohol by a Grignard reaction? - ✔✔Saturated sodium chloride decreases the
solubility of the organic product in water.
✔✔Procedural steps required to convert 2-butanone into 3-methyl-3-hexanol using a
Grignard prepared from 1-bromopropane and magnesium metal. - ✔✔Dry all glassware
and reagents prior to starting the experiment.
Add 1-bromopropane to clean magnesium.
Wait for reaction to turn cloudy. Sonicate if necessary.
Add 2-butanone dropwise to the Grignard reagent, allowing a gentle reflux.
Add aqueous ammonium chloride to quench the reaction.
Wash the reaction mixture with sodium bicarbonate and sodium chloride solutions in a
separatory funnel.
Dry over magnesium sulfate and distill product from THF.
✔✔Why are ethers commonly used as a solvent for Grignard reagent preparations? -
✔✔Ether's high vapor pressure helps to prevent moisture from entering the reaction
vessel.
Ether excludes moisture from the reaction, because water is not very soluble in it.
