Midterm Orgo Chem

Which statements are true about the principles of thin‑layer chromatography (TLC)? Select all that apply. The separation will be achieved if one component adheres to the stationary phase more than the other component does The stationary phase will always be more polar than the mobile phase High compound concentration facilitates visualization The solvent moves by capillary action If the concentration of the component is too high, the spot will become a smear or begin to tail.

1. The separation will be achieved if one component adheres to the stationary phase more than the other component does 2. If the concentration of the component is too high, the spot will become a smear or begin to tail. 3. The solvent moves by capillary action

Acetanilide was separated from benzoic acid through extraction. What statement best describes the procedure followed after the 1:1 mixture of benzoic acid and acetanilide are dissolved in dichloromethane?

An aqueous solution of NaOH was added to the organic layer in order to extract benzoic acid

Which of the following is NOT a requirement a substance must satisfy in order to be isolated by steam distillation. A. Po is more or equal to 5 torr at 100 oC B. Miscible with water C. Does not decompose at 100 οC D. Does not react with water E. All of the answers are correct

B. Miscible with water

Assuming the dielectric constant of acetone and isopropanol is near equal, which intermolecular force is primarily responsible for the lower equilibrium vapor pressure of isopropanol? A. dipol-dipole B. induce dipole-induce dipole C. hydrogen bonding D. ion-dipole E. none of the forces

C. hydrogen bonding

Which of the following could not be used as a stationary phase for thin-layer chromatography?

Carbowax

Which dye isolated from spinach leaves is most polar? Chlorophyll a Chlorophyll b Pheophytin a Pheophytin b β-carotene

Chlorophyll b

What type(s) of distillation could be appropriate to fully separate a mixture of benzene (bp = 80 oC) and toluene (bp = 111 oC)? Simple distillation Fractional distillation Steam distillation Simple & fractional distillation Fractional distillation & steam distillation

Fractional distillation

Which of the following errors would prevent you from calculating the Rf value of a sample on a TLC plate? Spots are seen only under UV light Omitting the co-spot Spotting more than one sample on the same plate Having the eluent (solvent) level below the level of the spots in the TLC chamber Letting the eluent (solvent) run past the end of the TLC plate

Letting the eluent (solvent) run past the end of the TLC plate

Which of the following are different between enantiomers of carvone? IR spectra Boiling point Magnitude of optical rotation Smell Retention factor

Smell

How is it possible to separate two liquids with the exact same boiling point by gas chromatography (GC) when it cannot be done by fractional distillation? The use of a nonpolar GC column will separate the liquids based on differences in volatility Decreasing the length of the GC column will allow the more volatile component to elute faster The use of a polar GC column will separate the liquids based on differences in polar interactions and volatility Decreasing the diameter of the GC column will allow the more volatile component to elute faster Increasing the carrier gas flow rate will allow the more volatile component to elute faster

The use of a polar GC column will separate the liquids based on differences in polar interactions and volatility

What is the purpose of adding sodium sulfate to the dichloromethane layer after the aqueous extraction? To facilitate precipitation of the product To neutralize the solution To remove the organic solvent To remove residual water To prevent the product from precipitating from the solution

To remove residual water

When is steam distillation used?

When the boiling point of the substance extracted is greater than that of water

When should you start collecting Fraction 2 in a fractional distillation experiment?

When the temperature begins to increase rapidly

Compound X has a melting point of 87°C, the compound was not dried and a melting point was taken. The melting point will be: above 87°C below 87°C exactly 87°C it is not possible to tell

below 87°C because water lowers melting point

Which is NOT a requirement of a recrystallization solvent? A. easily removed by evaporation or boiling B. has a boiling point lower than the melting point of the solid C. impurities are nearly insoluble at all temperatures D. the recrystallized solid is soluble at high temperatures, but nearly insoluble at low temperatures E. not reactive with a purified compound

has a boiling point lower than the melting point of the solid

When you perform an extraction 3 times with 10 mL of extracting solvent instead 1 time with 30 mL of extracting solvent you get: more compound extracted less compound extracted the same amount of compound extracted It is not possible to tell

more compound extracted

Which best describes the relationship between pheophytins and chlorophylls

pheophytins are conjugate acids of chlorophylls

Binary solvents of intermediate polarity can be made by mixing a polar solvent with a nonpolar solvent. Why would a binary mixture of polar water and nonpolar petroleum ether not be a suitable binary solvent for chromatography

water and petroleum ether are immiscible