CHEM 43A

You are told never to add a solid to a solution that is at or near its boiling point as this can causing "bumping". What is "bumping" ?

"Bumping" refers to an uncontrolled, violent boiling of a solution, characterized by the formation of large, erratic bubbles that can splatter the solution, sometimes even causing spills and burns. Adding a solid to a solution that is at or near its boiling point can trigger this phenomenon, which can lead to loss of product and potential safety hazards.

How does solid-liquid extraction work

- boil tea in water - filter the tea by centrifuge and vacuum filtration

what are the steps of rescrystallization exp1

- dissolution in hot solvent - insoluble impurities are removed through hot gravity filtration -crystallization - macroscale and microscale crystal recovery via vacuum traps and centrifuge

Explain the steps to find the partition coefficient

- liquid/liquid extraction - dry with MgSO -filter with cotton - evaporate and weigh

How does the caffeine extraction work

- use the separatory funnel to separate the organic and aqueous layers - boil the organic layer - sublimate

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

-The flask's smaller opening reduces the amount of solvent that escapes during heating - The rounded bottom and tapered sides of the Erlenmeyer flask provide an increased surface area for the solvent to evaporate and promote crystallization. - The narrow neck and rounded bottom of an Erlenmeyer flask make it easier to swirl the solution, promoting uniform mixing and improving the rate of crystallization.

what are the 5 types of interactions for solids

1. covalent 2. ionic 3. hydrogen bonds 4. dipole-dipole 5. van der waals

give the steps of an amine extraction

1. add the compound to MTBE 2. add HCL and wash to make an aqueous layer with only the amine 3. add NaOH so it becomes deprotinated from the positive cation it used to be 4. combine with more MTBE so that the amine goes back since it is not soluble in the organic layer anymore 5. add magnesium sulfate and dry it out 6. you got it!

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

1. Sublimation can be a faster purification technique: Sublimation involves going directly from a solid to a gas, bypassing the liquid phase. This can make sublimation a faster purification process compared to recrystallization, which involves dissolving the solid in a solvent, then re-precipitating it. 1. Sublimation is less damaging to heat-sensitive materials: Since sublimation does not involve a liquid phase, it is less likely to damage heat-sensitive materials compared to recrystallization. Recrystallization requires heating the solid in a solvent, which can cause thermal degradation of the material. Sublimation can be carried out at lower temperatures, making it a gentler purification technique for heat-sensitive materials.

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

Activated carbon has a high surface area and a porous structure, which makes it highly effective at adsorbing impurities from the solution

(1) How does charcoal work?

Activated carbon removes impurities from a solution through a process called adsorption. Adsorption occurs when impurities in the solution are attracted and held onto the surface of the activated carbon particles. The high surface area and porous structure of activated carbon provide a large number of active sites where impurities can be adsorbed, and the chemical and physical properties of the activated carbon can be tailored to target specific impurities. When impurities are adsorbed onto the surface of the activated carbon, they are removed from the solution and do not interfere with the crystallization process. Over time, the activated carbon can become saturated with impurities, and it may need to be replaced or regenerated in order to maintain its effectiveness. Adsorption is a physical process, as opposed to a chemical reaction, so the impurities remain attached to the activated carbon particles but are not chemically altered. This makes activated carbon a highly effective and convenient method for removing impurities from solutions in a laboratory setting, as it is non-toxic and does not generate waste products.

(1) What is an emulsion and how can it be avoided?

An emulsion is a mixture of two immiscible liquids, one of which is dispersed as small droplets within the other. Emulsions can form when mixing two immiscible liquids, such as water and oil, which do not mix uniformly. The droplets of one liquid are suspended in the other and the mixture appears cloudy or milky in appearance. You can avoid it by mixing and stirring gently. You can also use brine

Why does "bumping" occur?

At the boiling point, the liquid is just able to maintain its temperature and turn into vapor. The addition of a solid increases the internal energy of the solution, causing it to rapidly boil and produce large, unpredictable bubbles. These bubbles can rise rapidly to the surface and burst, causing splashing and spilling of the solution.

How does a boiling chip work? Why is it not a good idea to add boiling chips to a hot solution

Boiling chips provide a surface on which bubbles can form and escape the solution more easily, preventing the buildup of internal pressure that can lead to "bumping." By providing a more uniform release of pressure, boiling chips help to ensure a smoother, more controlled boiling of the solution. The addition of boiling chips to a hot solution can suddenly increase the internal pressure of the solution, causing it to boil more vigorously and unpredictably. This can result in the solution boiling over the sides of the container

You 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?

By cooling the flask first to room temperature and then to 0°C, the solution is cooled gradually, which promotes the formation of larger, well-formed crystals and maximizes the recovery of the substance.

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?

Contamination of the product with impurities can lead to a higher percent yield if the impurities are not removed during the recrystallization process. If the impurities are included in the final weight calculation, they will artificially increase the yield.

Draw the structures of diethyl ether and MTBE. Explain why these solvents and water are immiscible.

Diethyl ether and MTBE (methyl tert-butyl ether) are immiscible with water because they are non-polar solvents and water is a polar solvent

melting point and polarity

If the substance is polar then its melting point is higher

Which of the following tend to have the higher melting points, organic compounds or inorganic compounds?

Inorganic compounds tend to have higher melting points because many of them have giant structures with strong ionic or covalent bonding. Organic compounds are mostly molecular and lower temperatures are required to overcome the relatively weak intermolecular forces.

Is an ionic compound more soluble in polar or nonpolar solvents

Ionic compounds are generally more soluble in polar solvents, such as water, due to the ability of polar solvents to interact with the charged ions in an ionic compound and effectively reduce the attraction between the positive and negative ions.

(1) 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?

It makes the caffeine non-polar which won't mix with the aqueous phase as well

What is oiling out and what are the two main causes of oiling out?

Oiling out refers to the phenomenon in which a non-polar solvent (such as an oil) begins to separate from an aqueous solution, forming a separate layer on top of the solution. This can occur during the filtration process and can interfere with the desired outcome of the experiment. 1. Insufficient Solubility: Oiling out can occur when a non-polar solvent has low solubility in the aqueous solution being filtered. As the solution is passed through a filter, the solvent may begin to separate and collect on the filter surface. 2. High Vacuum Pressure: When a high vacuum pressure is applied to the filtration process, it can cause the non-polar solvent to boil, which can result in oiling out. This can occur because the high vacuum pressure decreases the boiling point of the solvent, causing it to separate from the aqueous solution.

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 and why should you use a small amount of solvent and why should it be cold?

Prevents re-dissolution: Washing the crystals with a small amount of cold solvent helps to prevent the crystals from re-dissolving back into the solution, which can result in a loss of the purified product. Removes any residual impurities: The wash step helps to remove any residual impurities that may be present in the crystals, thereby increasing their purity.

(1) 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 (pages 2-3, point 4). 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 (so we won't discuss this here), can you explain why we omitted this step in Part A (determination of the partition coefficient)?

Salt water is more polar than normal water, decreasing the water solubility of organic solvent and drives the two solutions apart, this also decreases the water solubility of desired organic compounds and increases the partition coefficient

in recrystallization, how should the solute react to hot and cold solvent

Solvents for recrystallization must be carefully selected so that the compound being purified is soluble in the hot solvent but insoluble in the cold solvent.

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?

The deprotonated form of a carboxylic acid, also known as the carboxylate ion, is more soluble in water because it has a negatively charged ionic species, which is attracted to the positively charged hydrogen ions in water. This interaction between the negatively charged ion and the positively charged water molecules allows the carboxylate ion to dissolve in water. The protonated form of an amine, the amine cation, is a stronger electrolyte compared to the parent amine, which is a weak electrolyte. As a result, the amine cation is more soluble in water due to the increased solubility of strong electrolytes in water.

When adding drying agent, how do you know when you have added enough?

The powder no longer forms clumps.

(1) 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?

The salt water works to pull the water from the organic layer to the water layer. This is because the concentrated salt solution wants to become more dilute and because salts have a stronger attraction to water than to organic solvents.

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

The slower the crystals form, the more pure they are likely to be because slower crystal growth allows for more efficient removal of impurities from the solution. When a solution is slowly cooled, the concentration of solute in the solution decreases gradually, and the rate at which impurities can be dissolved and incorporated into the growing crystals decreases as well.

In a recrystallization, after the crystals have formed (with cooling on standing), the solution can be cooled with ice. Why is this done?

The solubility is lower at lower temperatures, so the cooler the solution, the more material will crystallize out.

You are instructed to use a vacuum trap between the filter flask and vacuum line. What is the purpose of this trap?

The trap serves as a barrier that catches any liquids or solids that might be carried over from the filtration process, ensuring the purity of the material being transferred to the next step in the experiment.

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.

When performing a liquid-liquid extraction using MTBE (methyl tert-butyl ether) and water, the gas that is released during venting is most likely air or oxygen. MTBE is a volatile organic solvent that has a low boiling point and can easily evaporate, creating a pressure buildup in the extraction vessel. Venting allows the release of this pressure, preventing damage to the vessel. The release of gas decreases as mixing continues because the pressure buildup decreases as the MTBE evaporates and leaves the mixture. Additionally, as the extraction progresses, the distribution of the MTBE between the two layers becomes more established, reducing the amount of pressure buildup and the need for frequent venting.

What is the result of boiling the impure product with too much solvent and then cooling on ice?

When too much boiling solvent is used, both the product and the impurities remain in solution. On cooling, the solution will be too dilute to reach saturation, and no crystals will form.

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?)

When you are boiling water at home, "bumping" does not typically occur because the amount of heat being supplied to the water is relatively low and consistent. The rate at which the water is heated is controlled by the stove's heating element or burner, and the energy being lost through the process of boiling is relatively constant. As a result, the boiling of the water is relatively uniform and stable, and there is no buildup of internal pressure that could cause "bumping."

higher density means top or bottom?

bottom

What does MgSO4 do?

dries out water

(1) 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?

drop some water to see the aqueous

why does fluorene have a higher melting point than benzhydrol

due to its planar and highly conjugated structure

how do you isolate and recover macroscale vs microscale crystals

for macroscale, use a vacuum trap for microscale, use a centrifuge

An inexperienced chemist adds too little boiling solvent to an impure sample of product and then leaves this mixture to cool. The crystals:

form with product coated onto impure crystals.

what is tannin broken into and why

glucose and gallic acid, they are both soluble in water

how can solid impurities be removed

hot gravity filtration using folded/fluted filter paper

if a compound has more than 4-5 carbons is it miscible or imiscible

imiscible

what happens if you are under the solubility curve

it will dissolve

is meta more acidic or ortho para

ortho para

(1) 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"?

polar

why do we pre warm the filter

pre-warming the flask can also help to ensure a uniform temperature throughout the filtration process, which can help to prevent any potential problems such as precipitation of the solute or clogging of the filter. Additionally, it can help prevent thermal shock

what does fast cooling do to crystals

produces fast crystals

how will cellulose be filtered out

solid-liquid extraction

what can weak bases be used on

strong acids

What happens if you use too much solvent in recrystallization

the concentration will be too low, cooling will not yield a supersaturated solution (think about solubility curve)

what is flash point

the minimum temperature at which liquid fuel releases sufficient vapors to form an ignitable mixture with air near the liquids surface

(1) 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 (Vollhardt, chapter 9-4 and 19-9). Suggest what could happen if you were to attempt recrystallization of a carboxylic acid with ethanol or an ester with water.

using ethanol as a solvent for recrystallization of a carboxylic acid could result in the formation of an ester instead of purifying the carboxylic acid. water can also react with esters, causing hydrolysis and breaking the ester bond. This could also lead to the formation of an alcohol and a carboxylic acid, which would alter the structure of the original ester.

what type of filtration should you use for fast filtration

vacuum

what happens if crystallization happens on the filter paper during hot gravity filtration

wash again with hot solvent

what can strong bases be used on

weak acids

Sublimation can be used to purify your product if:

your solid product has a relatively high vapor pressure but the impurities have a low vapor pressure. Sublimation is the transition from a solid to gas phase, so your product must be a solid. For purification to be effective, your product should have a relatively high vapor pressure, so that it sublimes at a feasible pressure and temperature. Additionally, the impurities should have significantly lower vapor pressures than your product. high vapor pressure= low boiling point