Final material E4-7

Question 1

The activities of chromatography adsorbents (stationary phases) such as silica and alumina are greatly affected by their water content. The most active adsorbents (silica gel/aluminum) are those that contain the least amount of moisture. Why is this the case?

Answer 1

water’s strong interaction w/ the polar stationary phase reduces the availability of adsorption sites for other polar compounds, diminishing the separation efficiency

essentially, silica is deactivated

Question 2

You are told to mark the TLC plate with a pencil, NOT with a pen. Why should you NOT use a pen to mark the TLC plate?

what is ink soluble in?

Answer 2

the ink, given its soluble in organic solvents, will travel up the TLC plate w/ the solvent, interfering w/ the results

Question 3

As silica would be deactivated by water, what real-life experiences tell you that it is best to keep
silica TLC plates stored in a desiccator? That it, where in real life have you purchased a product that
was stored with silica? What is the use of silica gel in those products?

Answer 3

Silica gel packets can be found in shoe boxes to absorb moisture aka water.

Question 4

The melting points of the two constitutional isomers you used in Part A of the experiment are as
follows: ortho hydroxyacetophenone: 4-6 °C; para-hydroxyacetophenone: 109-111 °C. Explain the large difference in melting points exhibited by these two compounds.

inter vs intra, which is whuch

Answer 4

In ortho, an INTRAmolecular Hbond, can be formed making it less prone to react w/ nearby molecules thus, its melting point is lower

In para, can form INTERmolecular Hbond w/ nearby molecules thus, requiring higher energy + a higher melting point

Question 5

Your TLC results for ortho-hydroxyacetophenone versus para-hydroxyacteophenone will indicate
that one compound is significantly less polar than the other. Based on an analysis of polarity (using
only dipole moments and electronegativities), which compound would you expect to be more polar? Draw structures of the two compounds and indicate the polarity using the cross-arrow to explain your answer (remember that dipoles are vector values).

Answer 5

Para has a larger dipole moment meaning its more polar than ortho meaning more acidic meaning higher bp

Question 6

Now, when you perform the experiment, you will find that the ortho-hydroxyacetophenone is much LESS polar than para-hydroxyacetophenone. Discuss why the observed polarity of the two isomers in the TLC experiment is different from your analysis above. Think of your analysis used in question (4) above. Think about how the TLC result can be explained by the expected interaction between each isomer and the silica gel. Think of a qualitative approach of determining polarity by analyzing the expected distribution of charges within a molecule

what does the o isomer react w/ instead: mobile or stationary?

Answer 6

o < p in polarity

this is bc o interacted w/ the mobile phase instead of the stationery given the solvent used for the mobile phase was nonpolar

When looking at the structure of the o-isomer, the OH group has the ability to form an INTRAmolecular Hbond w/ the carbonyl group, making it LESS prone to interacting w/ the molecules of the silica plate. This allows it to pass through the plate faster due to less INTERmolecular forces occurring, resulting in a larger Rf value

Question 7

In column chromatography, the particle size of the stationary phase makes a significant difference in separation efficiency. Why is this?

what does it increase?

Answer 7

Smaller particles in the stationary phase increases surface area for separation

Question 8

What is the disadvantage to smaller particle size of the stationary phase?

2 points: increased what?

Answer 8

Smaller sample capacity and increased backpressure

backpressure = the resistance or force opposing the flow of the mobile phase (solvent or gas) through the column

aka, flow of the column is restricted

Question 9

You have a mixture that is soluble only in 100% MTBE. You want to run your column in 5% MTBE/hexanes. Which method of column loading should you use in this case and why?

dry vs wet loading? is hexane non/polar

Answer 9

dry loading - use when mixture is only soluble in solvents that are MORE polar than the eluent of choice for the column

–> MTBE is MORE polar than hexane

Dry Loading is used when the solvent you dissolve your powdered mixture in is MORE POLAR than the mobile phase

Wet Loading is used when the solvent you dissolve your powdered mixture in is LESS POLAR than the mobile phase

Question 10

You are performing a gradient elution and are instructed to begin with a low polarity solvent
(petroleum ether) and continue to increase solvent polarity w/ MTBE. Why are the mobile phases
added in order of increasing polarity rather than in the order of decreasing polarity?

2 points

Answer 10

achieve better separation of the components in your sample.

the compound of lowest polarity is eluted first and the most polar is eluted last

Question 11

After running the column and separating the three components, you can evaporate the solvent to isolate the compounds, then add up the total mass recovered and compare to the total mass you began with. Why is this not 100%

over + below 100%

Answer 11

below 100% –> some of the solvent may still be attached to the stationary phase

over 100% –> not all solvent has been evaporated

Question 12

In this experiment, you were able to “see” the separation of the compounds on the column due to
the fact that they are colored. However, most organic compounds are not colored making it more
difficult to run the column. Since many organic compounds do absorb UV light, it would seem like
we could just hold a UV light up to the glass column to observe the separation. However, this is not
possible. Why? (Consider whether or not you get a sun tan with sunlight shining through a window.)

what does the glass column do?

Answer 12

The glass of the column absorbs the UV rays making it unable to reach the product itself

Question 13

A chemist makes a 1 M solution of compound A and another 1 M solution of compound B and
spots both on a TLC plate and runs the TLC. Upon visualizing the plate under UV light, the chemist
notices that the spot corresponding to “A” is much darker than the spot corresponding to “B”. Since
identical amounts were applied to the plate, how can the chemist explain the differences in spot
intensity? 2 POINTS

Answer 13

1) B could have absorbed into the TLC paper more

2) more uv absorbance = darker spot

Question 14

(1) You are instructed to insulate the fractionating column for fractional distillation but not the
condenser. Why don’t you wrap the condenser with a cotton insulation?

(2) You are told to wrap the fractional distillation column with cotton. Why is this done?

Answer 14

(1) You dont wrap the condenser bc it needs to be cold so the vapor can be converted back into liquid

(2) to contain the heat of the column for vaporization

Question 15

You are told that distillations should never be conducted to dryness. Why do you think this is important?

what will expand?

Answer 15

If done to dryness, no water will be left in the flask and heat will start expanding the glass which can lead to it shattering

Question 16

Describe, using the ideal gas law, why heating a liquid in a closed system can lead to an explosion.
How do we make sure this does not happen in this experiment?

Answer 16

PV=nRT

Increased temp + pressure = constant volume

increased pressure causes an explosion

To avoid this, an opening into the external environment is needed to release the buildup of pressure the system is heated up.

Question 17

If the thermometer in a distillation apparatus were positioned too high, what effect would this have
on the experimental results? What if it is positioned too low?

Answer 17

in both scenarios readings would NOT be accurate

too high = they would be slightly lower

too low = they would be slightly higher

Question 18

You are instructed to tightly cap the vials containing your distillate. Why is this important and what
would happen if you did not cap the vials?

Answer 18

volatile compounds would evaporate + composition would change

if not capped, then sample evaporates (loss of sample)

Question 19

For fractional distillation, you add copper mesh to the column. What is the purpose of the copper
mesh?

what is increased and why?

Answer 19

copper mesh increases the surface area, allowing for multiple repeated vaporizations + condensations cycles

Question 20

You are instructed to use the same GC instrument for all your analyses and to record a GC trace of
the original mixture of unknowns on each day of analysis. Why is this important? 3 points

Answer 20

bc each GC instrument has different response factor, retention times and peak areas differ trial to trial, even for the same sample

Question 21

You are instructed to rinse the syringe several times first with the mixture to be analyzed before
taking a sample for analysis. Why is this done?

Answer 21

You rinse the syringe to get rid of any leftover mixture that may have been remaining

Question 22

Why is it important to press the start button immediately after injecting your sample of the GC?

2 points

Answer 22

to minimize the disturbance of gas flow

to make sure the entire sample reaches the column at the same time. (you dont want overlapping peaks!)

Question 23

In GC, what is the most typical factor that determines the order of elution for different components
in a mixture

its a physical characteristic

Answer 23

boiling points, compounds with lower bp elude first

Question 24

You are instructed to estimate the boiling points of the unknown compounds using data from the plots of the vapor temperature vs. volume of distillate.

Suppose the compositions indicate that the estimation is not accurate, **how do you expect the accurate boiling point to differ from the estimation? **

For each component (higher boiling and lower boiling), do you expect the accurate boiling point of the pure substance to be higher or lower than the estimation, and why?

Answer 24

increased/decreased bp from estimation

**lower boiling ** - accurate bp is expected to be lower than the estimated bp bc of contamination by higher boiling component

higher boiling - accurate bp is expected to be higher than the estimated bp bc of contamination by the lower boiling component

Question 25

You are instructed to use the recorded atmospheric pressure to correct the values of the estimated boiling points of your unknown components. (1) Why is it necessary to do so? (2) Does the direction for the correction make sense based on your understanding of vapor pressure and boiling (i.e. how the value of boiling point changes with external pressure)? | to account for what?

Answer 25

(1) Correcting boiling points is necessary to account for deviations in external pressure during your experiment (2) YES, bc bp of a liquid is the temp @ which its vapor pressure equals the external (atmospheric) pressure bc the bp of a liquid is dependent on the atmospheric pressure, the direct for correction makes sense Lower Pressure = Lower Bp Higher Pressure = Higher Bp

Question 26

Identify all the aldehydes in the unknown list and draw their structures. (1) What type(s) of intermolecular forces hold the molecules of these aldehydes together in their liquid phase? (2) How does their bp relate to these intermolecular forces? Explain the trend of bp for these aldehydes | C chains + branching in terms of bp ???

Answer 26

(1) London Dispersion Forces (Van der Waals Forces; present in ALL molecules), Dipole-Dipole Interactions bc of carbonyl ***NO Hbonds bc lack of OH (2) makes aldehydes have a lower boiling point bc of lack of Hbond intermolecular forces. --> higher bp are due to longer C chains; stronger LF --> lower bp are due to branching and shorter C chains; weaker LF Branching - non linear molecules; as you increase branching, you will increase the melting point, but decrease the boiling point.

Question 27

In general chemistry, you learned about the effect of branching on boiling points. Draw the structures of 3-methyl-1-butanol and 1-pentanol. What type of isomers are these compounds? Rank the boiling points of these two compounds based on your knowledge

Answer 27

structural (constitutional) isomers - have the same molecular formula but diff atom connectivity 1-pentanol > 3-methyl-1-butanol --> bc of branching in 3-methyl-1-butanol

Question 28

Fisher esterification mechanism

Question 29

In this experiment, you are using the carboxylic acid (acetic acid) in excess. What technique is used to remove the excess following the reaction? Write out the reaction that the carboxylic acid undergoes

Answer 29

by adding sodium carbonate, then the base (the carbonate ion) will deprotenate carboxylic acid, then add MTBE (separation technique) (deprotonated carboxylic acid will move to the aq layer) This must be done because at the end of the fisher reaction, there is still some carboxylic acid that needs to be removed, and there deprotonation ensures that the excess carboxylic acid moves into the aq layer

Question 30

In most cases of Fischer esterification, the carboxylic acid is the more “valuable” piece and the alcohol is used in excess. Let’s say you are making the ethyl ester of an acid and use the ethanol in excess. What technique would be used to remove the excess ethanol? | protonation or deporotonation?

Answer 30

If ethanol in excess, we an do workup by adding an aq acid, which would polarize the alcohol by protonating and removing the OH group, allowing excess ethanol to move to aq layer and be removed Organic - ester Aqueous - carboxylic acid

Question 31

You are instructed to reflux the reaction for 1 hr. If the instructions simply told you to reflux until the reaction was complete, how would you know when this was?

Answer 31

you can't know

Question 32

You are told to cool the reaction and add sodium carbonate cautiously to the reaction. Why must you be cautious?

Answer 32

CO2 will evolve as the base (sodium carbonate) neutralizes the excess acid Add sodium carbonate slowly + carefully so that the contents do NOT bubble out of the reaction vessel

Question 33

Following reflux you are instructed to add aqueous carbonate and MTBE to the reaction and mix. This is known as an “extractive workup”. What compound(s) should be present in the organic layer? What should be present in the aqueous layer?

Answer 33

Carboxylic acid (acetic acid) = aq layer ester = organic layer

Question 34

Why do you need to continue to wash the organic layer with sodium carbonate until the aqueous layer remains basic?

Answer 34

to make sure no carboxylic acid remains in the solution

Question 35

Why do we wash the organic with aqueous base? Is acetic acid water soluble (how do you know)? If it is water soluble, why don’t we just wash with water instead of aqueous base?

Answer 35

Wash organic w/ base to transfer carboxylic acid from organic to aq Acetic acid is water-soluble because carboxylic acid is polar and can form H-bonds If you add water it will push back to the reactants side bc esterification is reversible

Question 36

You are told to remove the solvent completely following the silica plug (filtration). If the evaporation of solvent is not complete + solvent is present, what effect would this have on the percent yield? What effect would this have on the boiling point determination? What effect would this have on the IR spectra?

Answer 36

If not entirely evaporated it will negatively affect the purification of ester.- % yield is only accurately when there is 100% conversion of the starting reactants to products. If evaporation is not complete, there will be more product than expected bc there will still be some excess methylene chloride and % yield will be HIGHER than should be + impurities will show up Impurities increase bp of ester (this occurs bc the presence of impurities decreases the # of water molecules available to become vaporized during boiling) IR will have inaccurate peaks/ functional groups

Question 37

IR can be used to demonstrate the success of a chemical transformation. In the case of a Fischer esterification, the functional groups of the two starting materials are a carboxylic acid and an alcohol. The product is an ester. List all the characteristic IR peaks that one would observe for each of these three species. Note their differences and similarities. How can the IR specrTra help to distinguish three species assuming that they are separate from one another (i.e., not mixed together)?

Answer 37

Carboxylic Acid: OH (~3000); C=O (~1700) Alcohol: OH (~3000) Ester: C=O (~1700)

Question 38

Although IR can be very useful in distinguishing among the 3 species of a Fischer esterification as discussed in the previous question, it may not be the best analytical method to use in monitoring a reaction as it is occurring. This is because at any one time, up to three species may be present in a sample of the reaction. Critically evaluate the utility of the various analytical techniques that you are familiar with (bp; mp; TLC; GC; IR) in monitoring a Fischer esterification. Assume that the technique will be required to analyze a three-component mixture composed of carboxylic acid, alcohol, and ester. Conduct the same analysis assuming that the alcohol used is methanol or ethanol.

Answer 38

For monitoring a reaction as its occurring boiling point, melting point, and TLC would be the best for the various analytical techniques we have learned so far. For distinguishing among, IR and GC are most useful

Question 39

The Fischer esterification reaction is thermodynamically controlled and completely reversible. What is the reverse of this reaction called? How do we control in the laboratory which reaction will predominate?

Answer 39

ester hydrolysis --> reverse rxn you can adjust the catalyst or add a different product to make the equilibrium go the different way (ie add water will make it go the other way)

Question 40

Assume that you react 100 mg of benzoic acid with 10 mL of methanol and 10 mL of sulfuric acid to produce methyl benzoate. Write a balance chemical equation for this reaction. Determine the limiting reagent and calculate a theoretical yield of both the ester and water. If you isolate 75 mg of methyl benzoate, what is the actual yield of the reaction? | just know the equations

Answer 40

limitting reagent moles = mass(g) / mw moles = (density)(mL) / mw theoretical yield = Moles of limiting reagent × mw acutal yield observed mass / expected mass (100)

Question 41

Oftentimes, reaction mechanisms are determined using labeling experiments. That is, an isotope of one of the atoms in a reactant is utilized and then its location in the product is determined. If you understand the mechanism of the Fischer esterification thoroughly, you will be able to determine where a labeled oxygen (oxygen-18 is most commonly used) would end up in the ester product if the oxygen of the alcohol were so labeled. In addition, if one used O-18 labeled water for the reverse reaction (ester hydrolysis), you should also be able to say where the label would end up.

Answer 41

Labelled oxygen ends up in the ester product O as either double bond O or as O that is attached to the R' group

Question 42

For the Fischer esterification in this experiment, you are instructed to add about half of your unknown alcohol and then weigh it. Explain why we don’t just measure the amount using volume? Why don’t we just use the density of the unknown alcohol to determine its mass?

Answer 42

Density is NOT known bc unknown is NOT known Volume measurement is less accurate bc of reading errors Mass measurement gives a more precise result for the amount of alcohol used

Question 43

Why do we want an accurate mass for the starting alcohol?

Answer 43

to calculate % yield

Question 44

When you are refluxing the reaction in the Fischer esterification, is the temperature of the reaction the same as the temperature of the metal bead bath? (Hint: when you are boiling water, is the temperature of the water the same as the temperature of the stove top?)

Answer 44

No. the metal bead bath is hotter bc it

Question 45

You are told to record the viscosity of your unknown. What is viscosity?

Answer 45

The state of being thick, sticky, and semifluid in consistency due to internal friction or resistance to flow

Question 46

You should carry out the boiling point determination carefully. What factors can influence the accuracy of a boiling point measurement? 3 points

Answer 46

Purity air/room temp altitude

Question 47

On the table of ketones and derivatives, some derivatives list the mp and in parentheses (di) or (mono). What do these descriptors refer to (hint: look up the structures of the compounds first)?

Answer 47

these prefixes refer to the level of substitution that the derivative has Mono = 1 (substituents are on diff sides) Di = 2 (the substituents are on same side)

Question 48

You are told that you need to have a strategy to solve your unknown’s structure in an efficient way. You are instructed to obtain an IR spectrum before conducting chemical tests, why? If you do not have an IR instrument and need to rely on chemical tests alone, which chemical test would you perform first?

Answer 48

In order to determine which functional groups are present: IR shows if you have an alcohol or an aldehyde/ketone 1st, do 2,4-DNP to distinguish alcohols from aldehydes/ketones

Question 49

You will be using analytical techniques to determine the identity of your unknown. You have learned other analytical techniques this quarter which may or may not be helpful to you but are not called for you to do in this experiment. What would be the best analytical tool that would be helpful but which is not used in this experiment? In order to use this technique, what else would be required (that is what else would you need that is not available to you)?

Answer 49

mixed melting point technique In order to do this students would need access to the standards of the derivatives of their unknown

Question 50

(1) The Lucas test must be conducted and interpreted with some serious thought. One important consideration is that the unknown must be miscible with water, Why is this the case? (2) What rule of thumb can be used to predict the water solubility of organic molecules? (3) Based on this assessment what would be the most important auxiliary test to perform when doing a Lucas test: (a) a positive control or (b) a negative control or (c) test the compound’s solubility in water

Answer 50

(1) If it's not miscible, it will be a false positive (2) less than 5 = good solubility (3) (+) control --> use 2 or 3 alcohol (-) control --> use aldehyde or ketone testing solubility --> use water

Question 51

Be sure that you know ALL the chemical tests available to you. Understand the chemical reactions and the visual readouts. Know what compounds will test positive and negative and especially when an exception can occur. Make sure you know how to use the chemical tests to distinguish among possibilities such as an aldehyde and ketone. Also, be sure to know the structures and chemical reactions for ALL of the derivative preparations and what function each reagent performs in the reaction (for example, what is the pyridine for in the preparation of 3,5-dinitrobenzoates?).

Answer 51

Lucas test (+) for 1/2 alcohols + aldehydes turns from red/orange to blue green Jones oxidation test (+) for 3 (instant)/2 (slow) alcohols 2 layers of precipitate (cloudy) formation 2,4-Dinitrophenyl hydrazine (2,4-DNP) (+) for aldehydes + ketones orange to orange-red Tollens Test (+) for aldehydes clear solution to silver (mirror) solution