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5. TLC Post-Lab Assignment (20 pts.) You MUST turn this paper in to your TA at the END of LAB!

Alexa Gonzalez

YOUR Name: ___________________________

Lab room: ________________

Partner Name: ___________________________

Lab time: _________________

8:30am

Yu-Sheng TA Name: ____________________ Hood #: _________________ If you did not collect any data or are missing some data to complete this post-lab, then give your source of data here and give a brief explanation why you have missing data. If you use your own data you can go straight to question 1, no need to add anything here.

1) (1 pt.) Complete the table below for your first TLC plate. Give the Rf values for all of the spots observed for each sample spotted. Sample Rf Values Solvent mixture used Unknown sample spot 1

0.59

Dichloromethane

Unknown sample spot 2

top= 0.74 bottom=0.41

Dichloromethane

2) (1 pt.) Complete the table below for your second TLC plate. Give the Rf values for all of the spots observed for each sample spotted. Sample Rf Values Solvent mixture used Benzoin

0.69

Dichloromethane 3%

Benzamide

0.417

Dichloromethane 3%

Unknown Sample

Dichloromethane 3%

0.69

3) (1 pt.) Complete the table below for your third TLC plate. Give the Rf values for all of the spots observed for each sample spotted. Sample Rf Values Solvent mixture used

0.68

Co-spot #1

Dichloromethane

4) (1 pts.) Draw the structure of the unknown compound. Explain your choice in ONE or TWO SENTENCES. OH

unknown material is benzoin, when cospotting occurred benzoin and the unknown pure traveled the same distance on the TLC plate

O 15

5) (1 pt.) A sample is run on a TLC plate and gives a single spot shown below, left. However, the solvent was not allowed to run up the plate very far, and so the experiment was repeated with a much larger solvent distance, as shown on the right below. Draw the spot be on the plate on the right using the plate on the left as a guide.

6) (1 pts.) Two different TLC experiments are performed on phenol and anisole, structures given below, with two different mobile phase solvents, as indicated. In each TLC experiment spots are observed, indicated as X and Y. Decide which spot, X or Y is phenol and which is anisole. Give a BRIEF explanation in terms of intermolecular forces (you are not allowed to simply use the term "polarity").

Y

OH

phenol

OMe

97% ethyl acetate 3% methanol

ethyl acetate

X solvent line

solvent line

start line

start line

anisole

H-acceptor

O Et O ethyl acetate

H- Acceptor

H3C OH methanol

h- acceptor t donor

X

Y

X

Y

Y= Phenol Phenol has a stronger IMF force because it's a H-acceptor

X=Anisole

7) (1 pts.) Referring back to question 6 above, in the TLC plate on the left the mobile solvent phase was ethyl acetate, in the plate on the right it was 97% ethyl acetate and 3% methanol. Explain why the Rf values in the plate on the right are larger than those in the plate on the left. Your explanation must be in the terms of intermolecular forces, you are not allowed to only use the word "polarity".

when 3% of methanol is added the solvent mixture will have a stronger IMF resulting the desorption process will occur faster and allow Rf value to get bigger.

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8) (1 pts.) There are THREE dynamical processes that contribute to Rf (three processes that have rates associated with them). What are these THREE processes?

1. rate of absorption 2.rate of desorption 3.rate of solvent motion 9) (1 pts.) The amount of material that is deposited on a TLC plate is very small and it would be impossible to weigh. But let's assume that you deposited 5 micrograms of acetaminophen on the plate. How many MOLES of acetaminophen would 5 micrograms be to 3 significant figures? And how many MOLECULES would 5 micrograms be? Show your work including ALL UNITS.

micrograms = 5 x 10 ^-6 g x mol/g = 3.31 x 10^-8 mol 3.3077 x 10 ^-8 x 6.022 x 10^23 = 1.99 x 10^16 molecules 10) (3 pts.) We will find it is useful to categorize solvents as one of three kinds: POLAR PROTIC, POLAR APROTIC and NONPOLAR (although this category can also include solvents of medium polarity). Characterize each of the following as one of these three kinds of solvent.

polar protic

nonpolar

polar aprotic

polar aprotic

nonpolar

polar aprotic

polar aprotic

polar protic

11) (3 pts) Shown below is a representation of hydrogen bonding between two pairs of molecules as dashed lines. For each pair, clearly indicate which of the molecules are acting as hydrogen bond donors

donor

acceptor donor

polar aprotic

polar protic acceptor polar aprotic

polar protic 17

and which are acting as hydrogen bond acceptors. ALSO, label each of the four structures as polar protic or polar aprotic. 12) (3 pts.) Think about each of the six provided molecules below right when they are interacting with the stationary phase of a TLC plate (shown to the left below at the molecular level). How strongly each molecule adsorbs to the stationary phase is determined by the strength of the intermolecular forces (IMF) between the molecule and the molecular structure of the surface of the stationary phase. Molecules that can both hydrogen bond accept and donate will adsorb more strongly than those that can only hydrogen-bond accept. Assign each structure as POLAR PROTIC or POLAR APROTIC, AND, assign one as H-BOND DONORS AND ACCEPTORS or a H-bond ACCEPTORS ONLY. acceptor

acceptor polar protic

H-donor

donor

polar aprotic H-bond acceptor

polar protic

polar aprotic acceptor

donor acceptor

polar aprotic

polar protic

13) (1 pts.) Is a large Rf value associated with a molecule that moves faster or slower with the solvent in TLC? And is it associated with STRONGER or WEAKER intermolecular forces with the solid H-bonding phase in TLC?

Rf is stronger and IMF is slower

14) (1 pts.) You run a TLC experiment on two compounds A and B, but they have very similar Rf values, compound A has an Rf value of 0.05 and compound B has an Rf value of 0.07, the separation between them is small. Are these Rf values large or small? Would you expect better physical separation if the experiment was to be run again with a more polar solvent so that the Rf values were larger (you can assume that B would still have the larger Rf and that the ratio of the Rf values would not change)?

the Rf is small because it's more polar ,yes it will lead to better separation due to increasing solvent polarity.

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