M1 - Experiments 0-3 + safety

Question 1

Types of interactions for solids: intramolecular forces - electrostatic attractions within the same molecule

Are they stronger than intermolecular forces?

Answer 1

(1) Covalent bonds
(2) Ionic bonds

***NO

Question 2

Types of interactions for solids: intermolecular forces - attractions between different molecules

which is the strongest, suggesting higher melting and boiling points? which are the weakest?

Answer 2

(1) Dipole-Dipole
(2) H bonds - STRONGEST
(3) London Dispersion - WEAKEST

Question 3

Characteristics that promote a HIGHER melting point?

4 total

Answer 3

(1) Forces - Ionic Bonds > Hydrogen Bonds > Dipole-Dipole Interactions > London Dispersion Forces (LDF)

(2) More Symmetry = Higher Melting Point

(3) Larger Size and Mass = Higher Melting Point

(4) Functional Groups (e.g., -OH, -NH₂, C=O) = Higher Melting Point

Question 4

(7) Suppose you have an unknown that melts at 124 - 126 °C (corrected result). You make a mixture of your unknown with benzamide (mp 125 – 128 °C) and the mixture melts at 100-102 °C. You then realize that the second measurement is made on a new instrument that has not been calibrated. You wonder if you have a eutectic mixture or if the new instrument has a large systematic error. Describe the possible outcomes and the conclusions you can make if you (a) make a new mixture of benzamide and the unknown using a different ratio and take the mp on the same new instrument; or (b) calibrate the
new instrument with a standard

Answer 4

(a) If the depression remains close to 100-102 °C, you likely have a eutectic mixture - combining them makes them melt easier and faster

(b) it will reveal if there was a systematic error

Question 5

8) You have learned that impurities will lower and broaden melting point ranges. You have an unknown that melts at 61 - 62 °C. Then you mix it with Vanillin and find the melting point to be 70 - 75 °C. Although the range has broadened, the temp is
HIGHER. Explain. Consider the melting point of a mixture that is 95% B relative to the melting point of compound A

Answer 5

vanillin must have a much higher melting point than the unknown, and/or more abundant in the mixture

Question 6

9) You learned the general concept that given similar molar masses, the compound with the stronger intermolecular forces has molecules held stronger together, resulting in a higher melting point. Recall that different types of intermolecular forces are still electrostatic interactions of different strengths (but not as strong as bonding forces).

What happens if different parts of the same molecule experience favorable electrostatic interactions (but still not as strong as bonding forces)? How would the presence of intramolecular interactions within each molecule affect the intermolecular interactions between different molecules? Would it make intermolecular interactions stronger or weaker? How may that in turn affect properties like melting point?

Answer 6

presence of intramolecular interactions usually makes intermolecular interactions weaker

lower melting and boiling points

Question 7

You are told to NEVER add a solid to a solution that is @ or near its boiling point as this can causing “bumping”. What is “bumping”? Why does it occur (chemical reason)?

Answer 7

(1) Hot liquids can form large bubbles of superheated vapor that can suddenly boil over in a violent eruption

(2) Why: a lack of nucleation sites that initiate bubble formation

Question 8

You are told to add boiling chips to your recrystallization mixture. What are boiling chips made of? Why are boiling chips used? How does a boiling chip work? Why is it not a good idea to add boiling chips to a hot solution?

why –> think about what forms to prevent bumping

Answer 8

(1) porous carborundum or calcium carbonate
(2) provides nucleation site
(3) chips help small bubbles form quickly, prevent bumping
(4) can cause bumping

Question 9

When you are cooking at home and you boil water, why don’t you need to add boiling chips? (i.e. why doesn’t “bumping” occur?)

Answer 9

because the pots and pans used for cooking have enough nucleation sites on the surface to prevent bumping

Question 10

what is a nucleation site?

Answer 10

A nucleation site is a small imperfection or irregularity on a surface

Question 11

You are told that you have the option to add activated carbon (charcoal) to your recrystallization solution. Why is it used? How does charcoal work?

Answer 11

(1) to remove colored soluble impurities

(2) Bc colored molecules have
very large numbers of conjugated double bonds and will adsorb (stick) to the surface of charcoal
—> it is good at trapping other carbon based impurities

Question 12

You are told that the slower the crystals form, the more pure they are likely to be. Why is this?

Answer 12

because when crystals grow slowly, the impurities are rejected

Question 13

u are instructed to cool the recrystallization flask to room temperature and then to
0 °C “to maximize the recovery”. How does this maximize the recovery? Use the solubility curve

Answer 13

Solubility curves typically show that a compound’s solubility decreases as temperature decreases

as the solution cools, solubility decreases, compound begins to crystallize out of solution

compound’s solubility is now @ its lowest practical point - minimal product remains dissolved in the solvent

Question 14

You are instructed to vacuum filter the purified crystals and to wash them with a small amount of cold solvent. Why should you do this wash? why should you use a
small amount of solvent? why should it be cold?

Answer 14

(1) to remove surface Impurities
(2) excess solvent can dissolve some of the desired product
(3) helps prevent them from dissolving back into solution during the wash, maximizing crystallization

Question 15

A student conducted a recrystallization and obtained a 103% recovery. Assuming all
measurements and calculations were done correctly, what is the most reasonable explanation for this?

2 explanations

Answer 15

(1) Incomplete Removal of Impurities
(2) excess solvent

Question 16

you are instructed to use a vacuum trap between the filter flask and vacuum line.
(1) What is the purpose of this trap?
(2) does it matter how you connect the two tubings on the trap?
(3) We do not want any water or solvent to get inside a vacuum pump. So how does the vacuum trap help in this case?

Answer 16

(1) vacuum trap prevents any solution from being pulled into the vacuum line

(2) Yes

(3) The vacuum trap helps prevent water or solvent from getting into the vacuum pump by providing a physical barrier

Question 17

Why is an Erlenmeyer flask preferred over a beaker for recrystallization (there are 3
main reasons)?

Answer 17

1. the conical shape of an Erlenmeyer flask decreases amount of solvent lost to evaporation during heating
2. prevents the formation of a crust around the sides of the glass
3. makes it easier to swirl the hot solution while dissolving the solid without splashing it out of the flask.

Question 18

Your text (p. 235; 4th ed.) discusses the complication of “oiling out” during attempted recrystallization. What is oiling out and what are the two main causes of oiling out?

Answer 18

Oiling out: during recrystallization, the product comes out as a liquid instead of crystals

Main causes of oiling:
1. Desired compound has large impurity → prevents crystallization bc it gets in the way of solute molecules from finding each other to form crystals

2. crystals mp < solvent bp - When it precipitates, it precipitates as a liquid

Question 19

Choosing the correct solvent for recrystallization is the most critical step for achieving success with this technique. However, some common solvents can react with the compounds that you are trying to purify and should be avoided. The most common solvents that can cause undesired reactions are alcohols and water. In experiment 6 we will perform a Fischer Esterification. Suggest what could happen if you were to attempt
recrystallization of a carboxylic acid with ethanol or an ester with water

Answer 19

By Le Chatelier’s, the addition of ethanol or water would push the reaction backwards, thus affecting product production

***see what is being added, where is it (either products or reactants) and infer that the shift is opposite of where the added solvent is located

Question 20

4 key regions of IR

Question 21

Given diethyl ether and MTBE, Explain why these solvents and water are immiscible.

Answer 21

Immiscible - don’t mix
water - polar
MTBE/diethyl ether - non polar

Question 22

In a liquid-liquid extraction, how can you experimentally tell which layer is organic and which layer is aqueous? Can you always rely on the relative density of the pure organic solvent and water?

Answer 22

(1) drop some water, whichever it dissolves in is aq layer

(2) No bc there are immiscible solvents w/ very similar densities + actual densities can shift depending on the extraction process

Question 23

In the overview of this experiment for liquid-liquid extraction, washing with a brine solution is mentioned as one way to dry the organic solution. No brine wash is actually used in this experiment, but it will be used in Experiment 3 and later. While the reasons for this are complicated for the isolation of caffeine, can you explain why we omitted this step in Part A (determination of the partition coefficient)?

think about the solvent used and its polarity

Answer 23

brine may have affected K by removing some of the benzoic acid since brine removes water + water soluble impurities

MTBE is sparingly miscible in water

Question 24

Not all organic solvents are suitable for liquid-liquid extraction with an aqueous solution. List at least three such solvents; make sure each of the three bears a different functional group. Your answer arises from a general rule regarding miscibility of
organic molecules with water. What is the “rule of thumb”?

Answer 24

Acetone, methanol, ammonia

Rule of thumb is that like dissolves like so nonpolar solvents will be immiscible in water while polar solvents will dissolve in water

Question 25

We can consider a liquid-liquid extraction to be efficient if >90% of the desired compound can be recovered. Presuming (i) the desired compound is soluble in the organic solvent and (ii) we use equal volumes of the organic solvent and water, what is the minimum value of the partition coefficient (K) to get an efficient extraction with only one extraction step?

Answer 25

K= [solute in aqueous layer] / [solute in organic layer] ​ K / (1+K) > % of desired recovery Organic layer (target): 90% of the compound Aqueous layer (leftover): 10% of the compound

Question 26

The partition coefficient of a compound between a specific organic solvent and water is often smaller than the value you found in question (5). If the value is not too small, it is still possible to reach an efficient extraction. What is the general strategy?

Answer 26

Multiple Extractions

Question 27

What is an emulsion and how can it be avoided? | how has two answers? think about what u can add

Answer 27

(1) a suspension of insoluble droplets of one liquid in another liquid (2) gentle mixing of two phases may reduce or eliminate emulsion formation. Also they can sometimes be resolved by adding salt (sodium chloride) to the aqueous layer or by centrifugation.

Question 28

When performing an extraction, venting of the vessel should be done almost immediately after initial mixing. This often results in the release of a gas (pressure). After subsequent mixing, each venting releases less gas, so venting does not need to be done as frequently. What is the gas (pressure) that is being released during venting? Why does the gas form and why is it less prominent as mixing is continued? Assume that it is the mixing of MTBE and water. | think about what decreases

Answer 28

gas released during venting = primarily dissolved air + vaporized MTBE forms bc mixing reduces solubility of gases in liquids and allows MTBE to vaporize. as the gases are released = equilibrium is approached = pressure buildup decreases, requiring less frequent venting

Question 29

Write an equation describing the hydrolysis of one ester group in tannins by Na 2 CO 3 . The core structure of tannin is given on page 4 of the lab manual

Answer 29

RCOOR’+Na 2CO3 +H2O→RCOO - Na++R’OH+HCO 3

Question 30

In the discussion of the caffeine extraction, it is stated that the sodium carbonate serves to hydrolyze the tannins. It also indicates that the base converts caffeine molecules that may be present as salts to the free base. Why is the conversion of caffeine to its free base important in this experiment?

Answer 30

The basic solution converts any caffeine molecules that may be present as salts (protonated and charged, with higher water solubility) into the free-base form (uncharged, with lower water solubility).

Question 31

Saponins are amphiphiles that have soap-like properties. Look up the structures of saponins and identify the functional groups providing the molecules with their amphiphilic properties. Next, look up the structures of simple soaps and do the same. Describe how soaps work to remove grease from skin and clothes. Rationalize why the basic hydrolysis of fatty acid esters is called “saponification”

Answer 31

Saponins can increase the solubility of organic molecules in water (like soaps and detergents). Esters can be cleaved back into a carboxylic acid and an alcohol by reaction with water and a base. The reaction is called a saponificationfrom the Latin sapo which means soap. The name comes from the fact that soap used to be made by the ester hydrolysis of fats

Question 32

How would the roasting time of coffee beans affect their caffeine content?

Answer 32

The longer coffee beans are roasted, the more caffeine is burned off.

Question 33

Using the phase diagram on page 4 of the lab manual, explain why sublimation only occurs at temperatures and pressures below the triple point of the substance. If the triple point is above room temperature but below atmospheric pressure, is it still possible to perform sublimation? | what must exceed the atmospheric pressure

Answer 33

Sublimation occurs in solids with vapor pressures that exceed atmospheric pressure at or near room temperature YES. The atmospheric pressure MUST be BELOW the triple point pressure so that the substance doesn’t melt When the vapor pressure of the solid exceeds the atmospheric pressure, sublimation occurs

Question 34

Recrystallization and sublimation are both purification techniques for solids. Suggest two advantages of sublimation over recrystallization (hint: think of material and time).

Answer 34

(1) Sublimation can be carried out @ lower temps, making it a gentler purification technique for heat-sensitive materials; decrease risk of decomposition, higher % recovery (2) Time Efficiency

Question 35

You are told that the ionic compound deriving from the deprotonated form of an organic acid or the protonated form of an organic base has much different solubility properties than the parent acid or base. What are the differences and why is this?

Answer 35

The salt of an organic acid or base is much more soluble in water than the parent acid or base bc the salt is a charged, ionic compound whereas the parent is strictly covalently bound. Salts are polar and so is water, like dissolves like

Question 36

Is an ionic compound more soluble in polar or nonpolar solvents?

Answer 36

more soluble in polar solvents ionic = charged/polar

Question 37

Which is more acidic, 3'-hydroxyacetophenone or 4'-hydroxyacetophenone and why? Which is more basic, benzyl amine or aniline and why? Which is more basic, ethylamine or diethylamine and why?

Answer 37

4'-Hydroxyacetophenone is more acidic than 3'-hydroxyacetophenone --> para position Benzyl amine is more basic than aniline --> benzyl group > phenyl ring Diethylamine is more basic than ethylamine --> 2 ethyl groups

Question 38

When converting sodium carboxylate dissolved in water to its acid form, you are instructed to adjust the pH well into the acidic range (pH = ~2). Why don’t you just take the pH to neutral? You do the same for adjusting the pH of a solution containing an ammonium salt taking it well into the basic range rather than neutral. Why are you instructed to do this? | what are you trying to maximize via complete protonation/deprotonation?

Answer 38

To maximize product recovery An excess of acid or base is used to ensure that the water soluble salt is completely converted back to the starting carboxylic acid or amine which improves extraction efficiency. taking the pH well into the acidic or basic range ensures complete protonation or deprotonation

Question 39

For the separation of the unknown mixture, you are instructed to isolate the amine followed by the acid and finally the neutral. Does the order matter? If not, what other orders could be followed?

Answer 39

The neutral has to be last because it is "left over." You could switch the order of the acid and amine/base.

Question 40

In the overview of experiment 2, you are told that brine removes the bulk of water from the organic solution. In this experiment we use brine to “wash” the organic solution. How does brine remove water from the organic solution? | what is the name of this phenomenon? what increases what decreases?

Answer 40

salting out --> adding brine to aq layer to help w/ extraction as it decreases solubility of organic compound in aq. phase distribution coefficient increases allowing more of organic compound to be transferred to organic layer Also separates water and water soluble organic compound into two phases

Question 41

In the first two steps of the separation of the unknown mixture, after extraction with either an aqueous acid or aqueous base, you are told to then extract the combined aqueous layers with organic solvent. Why is this done? | what exactly are you trying to retreave

Answer 41

To maximize recovery/isolation: This step removes any neutral and carboxylic acid molecules that were dissolved in the aqueous solution during the extractions.

Question 42

You are instructed to test the pH of the solution by taking a small sample and touching it to the pH paper rather than dipping the pH paper in the solution to be tested. Why? | 2 points

Answer 42

(1) because some of the pH paper could affect the solution (contamination by the indicating compounds would result) (2) Also, dipping the pH paper directly will absorb some of the solution. Using a stir rod to lightly touch the pH paper minimizes the amount lost and prevents contamination

Question 43

In this experiment we use liquid-liquid extraction to achieve the separation of three unknowns in a given mixture. How is separation different from purification? How can we determine if the separated compounds are pure? What purification technique(s) can we use if the compounds are not pure?

Answer 43

Separation is used to separate compounds based on chemical properties, in this case solubility. It does not selectively select for our compounds and therefore may contain impurities of your desired compound Melting pint range is broad. Or IR or Chromatography Sublimation, recrystallization, distillation OR it might be useful to dissolve the compound in a small amount of MTBE, dry again over magnesium sulfate, filter, and remove the solven