General lab techniques
1. Vacuum filtration is most useful for separating:
a. solids from solids b. solids from liquids
c. liquids from liquids d. substances of different polarities
2. Separatory funnels are most useful for separating:
a. solids of different polarities
b. solids of similar polarity
c. liquids of different polarities
d. liquids of similar polarity
3. What could potentially happen if you never vent your separatory funnel?
4. Why should you pre-weigh round-bottom flasks, filter papers, etc.?
5. When should you use anhydrous Na 2 SO 4 ?
6. Look up the boiling points for the following solvents: hexane, diethyl ether, dimethyl sulfoxide, and toluene. Rank the solvents from LEAST to MOST at risk of bumping (on the rotary
7. If you want to isolate your desired compound, hexanal, in the rotary evaporator, why might it be a bad idea to use dimethyl sulfoxide as the reaction solvent? Hint: there are two reasons that are much so related.
8a. You are separating aniline (liquid) from benzoic acid (solid). For structures, see below. Assume they do not react. You choose to use vacuum filtration but retrieve very low yields. What might have happened?
b. To increase yield, you might have chosen to wash the filter paper down with solvent. What are the potential problems with this action?
c. In light of low yields via vacuum filtration, you then choose to separate them via extraction.
You mix the two compounds and dissolve both in diethyl ether and try to wash with water. However, no separation occurred. Where would the molecules be, in the ether or aqueous layer, and why? Hint: assess whether the benzene ring is polar or non-polar.
d. Is aniline acidic or basic? In order to separate aniline from benzoic acid, what must be done to change the physical properties of aniline? After this change has been made, in which layer would the altered aniline reside?
e. Is benzoic acid acidic or basic? Likewise, if you wanted to separate benzoic acid from
aniline, what must be done to change the physical properties of benzoic acid? Where would the altered benzoic acid lie?
f. Recall from Experiment 2 where acetaminophen, aspirin, and caffeine were separated. Why was strong acid (4M HCl) added to Solution B? What would you do to the altered aniline and altered benzoic acid in order to isolate the compounds?
9. You are trying to separate estrogen from testosterone via Thin Layer Chromatography (TLC). Considering a solvent system of 10% ethyl acetate in hexanes (9:1 Hex:EtOAc), which would move faster, and why?
10. The surfactant 4-nonylphenol is used in some pesticides and is considered to be an
estrogen mimic, potentially disrupting the hormone levels of organisms affected. Between
Estrogen and 4-nonylphenol, which would move faster in a 10% ethyl acetate in hexanes
solvent system and why?
11a. Say you are running a reaction where you convert a secondary alcohol to a secondary
acetate. You check the progress of the reaction at various points in time via TLC with 10% ethyl acetate in hexanes. Rank each TLC plate chronologically, i.e. from the beginning (1) of the reaction to completion
b. Considering a mixture of both the alcohol and acetate, which compound would elute first in column chromatography with the same solvent system?
c. If the solvent system was more polar, what would happen to the separation in column
chromatography? Assume both the alcohol and acetate are soluble in a more polar solvent
d. TLC plates are typically constructed with very polar SiO 2 units. Imagine if TLC plates were
instead constructed with Si(CH 3 ) 3 units. How might the order of elution change in this newly
constructed TLC plate? Explain.
11a. Recrystallization will only work if the solid is ___________________ in a solvent at room temperature.
b. Describe the protocol for recrystallization of a solid and the subsequent isolation of the
c. What could happen if you agitate the solution during the recrystallization process? What could happen if you cool the solution rapidly? For each, describe what the overall effect on purity would be.
d. Would it be practical to use recrystallization as a method to isolate a solid from your desired liquid phase? Explain why. Think about this carefully — we always want to use the best method!
12. Consider the vapor phase diagram between two compounds, A and B:
a. What is the approximate composition of the vapor at T1? of the liquid?
b. What is the approximate composition of the vapor at T2? of the liquid?
c. Which is the lower boiling compound? Which compound would distill first?
d. What problems would be faced if the mixture was heated to T2 at the very beginning of the experiment?
13. Consider the vapor phase diagram between two compounds, C and D:
a. Judging from the vapor phase diagram, the mixture of C and D produces a/n ____________.
b. Label where the azeotrope is. What is the approximate composition of the azeotrope?
c. If you’ve collected 20 mols of the distillate at T1, roughly how many moles are D?
e. What are the advantages of collecting distillate at T1 as opposed to T2? the disadvantages?
14. Select all the Infrared Spectroscopic signals that would be expected for the following
compounds. *On the actual lab final, I doubt you will have the IR absorptions table with cm -1 and functional group. Compound Signals
a. Estrogen______________________ i. Alkane v. Hydroxyl
b. Testosterone___________________ ii. Alkene C=C vi. Alkyne, C=C
c. 2-hexanone____________________ iii. Alkene C-H vii. Alkyne, C-H
d. limonene______________________ iv. Carbonyl viii. Amine, N-H
15. For the following IR spectra (a-d), match the spectrum with the listed compounds: n-butanol, 3-butenoic acid, 1-decene, 2-hexanone, cyclopentenone, propanamide.
16. Draw a reasonable IR spectrum for 3-hexyn- 1-ol. Remember, “useful” IR information is contained within 3800 cm -1 to 1500 cm -1 .
17 a. The melting point literature value for benzoic acid is 122 deg C. The melting point of your isolated sample is about 100 deg C. Your partner obtains a melting point of about 120 deg C.
Whose sample is more pure and why?
b. Another lab member obtains a melting point of 140 deg C. What could have caused this?
What can you imply about the purity of their sample from this melting point measurement alone?
c. What are some general indications that your product may not be pure? Think about all
methods of characterization.
18a. You are running a reaction with 2.00 g of benzoic acid and excess ethanol to produce ethyl benzoate. How many moles of the limiting reagent are you working with and what is the theoretical yield of ethyl benzoate?
b. You transfer your reaction solution to a round-bottom flask (34.74 g) and rotary evaporate. The mass of your RBF with product is 36.96 g. What is the percent yield?
19. You are running a reaction with 25.0 mmol of limonene and an excess of other reagents.
How many milliliters of limonene would you measure out for this reaction?
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