Exp 2&3: Recrystallization and Melting Points/TLC of OTC Analgesics Flashcards
What was the recrystallization solvent for acetanilide? Why?
H2O
- most polar solvent with most polar solute
What was the recrystallization solvent for trans-cinnamic acid?
Ethanol/H2O
- medium polarity solvent with medium polarity solute
What was the recrystallization solvent for caffeine salicylate?
Pet ether/Ligroin
- nonpolar solvent with nonpolar solute
Why must you allow recrystallized samples to dry completely before determining the mp?
- if the sample contains unevaporated or unvacuumed solvent, the melting point will reflect an inaccurate melting point that is affected by the contamination of the solvent.
Give two reasons for knowing the literature solubility data when doing a recrystallization.
- Allow students to select a solvent that will allow adequately dissolve the solute.
- Knowing the solubility data will allow students to select other conditions that will aid in recrystallizations
Did the literature values for the solubility of acetanilide in H2O hold true? Which solvent system (20 mL or 40 mL) worked better for yield? For purity?
- yes, literature 5g/L
- 0.5 g in 20 and 50mL was insoluble at room temperature (25g/L and 12.5g/L)
- therefore students had to heat the mixture to boiling in order for the solid to dissolve
- The 20mL system should have worked better for yield and purity???
What is the IUPAC name for acetanilide?
- N-phenylacetamide
What is the IUPAC name for trans-cinnamic acid?
- (E)-3-phenylprop-2-enoic acid
Which solute (acetanilide or cinnamic acid) benefited the most from recrystallization?
Acetanilide
- appearance changed drastically from tan, flaky, and variable sizes to a more uniform sparkly dusty white
- Increased uniformity and the crystals’ ability to catch the light suggests that recrystallization was successfully
What is TLC?
- Thin layer chromatography
- common technique where compounds are separated so that they can be “seen” even if they are part of a mixture
- inexpensive, little sample needed, numerous samples tested at one time
- samples can be recovered using special types of TLC plates if necessary
- good for determining when a reaction is complete by the appearance of product and disappearance of the starting materials
What are the steps for running a TLC?
- dissolve the sample in solvent
- spot the compound onto a TLC plate using a microcapillary tube
- place the plate into a chamber with the developing solvent, place a watch glass over the chamber
- once the solvent has traveled a significant distance (close to the top of the plate), remove the plate, mark the solvent front, and allow it to dry
- Visualize the spots using UV lamp
- Determine Rf values using the formula
What is the Rf formula?
Rf = (Distance compound)/(Distance solvent)
What 3 important factors must be considered in making generalizations about the behavior of the solutes being analyzed by TLC?
- The polarity of the solid support (silica/alumina = immobile phase)
- The polarity of the solvent(s) used to develop the plate (mobile)
- The polarity of the compound/solute itself- how polar/nonpolar is it? What dissolves it well or poorly?
Describe the phases of a TLC plate.
- Immobile (silica gel) phase on the plates is more polar than most organic compounds and solvents
- mobile (solvent) phase is more nonpolar and travels up the plate
Describe the movement of compounds on a TLC plate.
- polar compounds tend to stick on the polar silica gel and travel less (lower Rf)
- nonpolar compounds prefer the solvent and travel more (higher Rf)
What was the order of polarity in the solute compounds tested? (from 1 = most polar)
- Benzoic acid
- Phenol
- Acetophenone
- Anisole
- ## Butyl Phenyl Ether
- Caffeine
- Acetaminophen
- Aspirin
In TLC, what will happen if the initial spot is very large when preparing a plate?
- compound may bleed into other mixtures or compounds being tested or it may streak, leading to uncertainty in the Rf value
In TLC, what will happen if the initial spot is below the level of the solvent in the developing chamber?
- the solvent will wash the solute off the plate, and it is likely that no movement will occur due to lack of solute
Which solvent system in part 2 appeared to give the best spread of Rf values? Why?
The EtOAc:Hexane (10:1) gives the best spread
- more polar solvents accentuate differences in polarity between more polar compounds
- this solvent mixture is less nonpolar due to the high ratio of EtOAc (slightly polar) to hexane (very nonpolar).
- compounds tested were all hydrogen bond donators and acceptors, so they are all somewhat polar
- Therefore the experiment required a mobile phase that was nonpolar enough to move up the stationary phase, but still polar enough to distinguish and separate the somewhat polar compounds.
What is the structure of acetanilide?
benzene ring with amide group (Nitrogen between benzene and carbonyl)
What is the structure of trans-Cinnamic acid?
Benzene ring with carboxylic acid attached via trans hydrocarbon chain (3C total)
What is the structure of acetophenone?
Benzene with ketone substituent
What is the structure of anisole?
Benzene with ether substituent
What is the structure of benzoic acid?
Benzene with carboxylic acid substituent
