Lab Final Flashcards
What is thin layer chromatography used for?
It’s a rapid and inexpensive analytical technique used to separate mixtures.
Why do compounds in a mixture separate in TLC?
Separation occurs because different compounds have different residence times on the stationary phase or mobile phases
In TLC, how do polar compounds interact with polar stationary phases?
The polar compounds interact with the stationary phase’s polar surface through dipole-dipole attraction and hydrogen bonding
In TLC, how do nonpolar compounds interact with polar stationary phases?
They don’t. Instead they remain in the mobile phase, and greater movement is observed.
In TLC, how does the mobile phase move up the plate?
Capillary action
What two things should you focus on when analyzing TLC plates?
1) Appearance of the compound spots
2) Location of the spots
Why do compounds move on TLC plates?
As the solvent passes over the spot, molecules a that spot location move up the plate differently depending on the polarity and molecular properties of both the molecules and the solvent
Why is UV light used in TLC plate analysis?
UV light can reveal defining characteristics (re: colors) of compounds
Why are chemical stains (often inorganic oxidants) used in TLC plate analysis?
Stains react with functional groups in the molecule and permit further visualization
What is Rf?
Ratio to front
Used in place of retention time for TLC chromatography. Defined as the ration of the distance (mm) traveled by a particular component of a mixture to the distance (mm) traveled by the solvent
What range do Rf values occur in?
0.00 to 1.00
What is the purpose of the TLC lab?
To perform TLC and analyze the resulting plate to determine the identity of a mixture of compounds in an unknown sample
How should you handle the silica gel TLC plate?
By its edges only, do not touch the surface!
In the TLC lab, what acted as the stationary phase?
The polar silica gel plate
In the TLC lab, what acted as the mobile phase?
The nonpolar solvent
In the TLC lab, which was more polar, the stationary phase or the mobile phase?
The stationary phase
In the TLC lab, which was less polar, the stationary phase or the mobile phase?
The mobile phase
Rf =
(distance mobile phase, mm)
What experimental techniques were used in the identification of an unknown aldehyde or ketone (IUAK) lab?
Distillation, isolation of a precipitate by vacuum filtration, melting point analysis, and two chemical tests (iodoform and Tollens’)
Why are aldehydes and ketones similar functional groups?
They both have carbonyl groups, which allows them to engage in comparable reactions. They also have similar physical properties
What are the two functions of distillation in the IUAK lab?
1) Helps obtain a pure compounds
2) Provides an important physical attribute of the unknown compound: boiling point range
Bonus: aldehydes tend to contain impurities of their corresponding carboxylic acid, produced by air oxidation. Distillation effectively removes these impurities
What does a Tollens’ test assay for?
Presence or absence of an aldehyde
What does an iodoform test assay for?
The presence or absence of a methyl ketone
What happens in a Tollens’ test?
The aldehyde (should one be present) is oxidized by a silver ion complex [Ag(NH3)2+] into its corresponding carboxylic acid, and the remaining silver(I) salt is reduced to molecular silver and deposited on the vessel in which the reaction was performed.
Silver should deposit as a mirror if the vessel is clean, if the vessel is dirty a colloidal black precipitate is often observed
