chem 43a midterm 2

SAFETY: Why is it important to handle silica powder in a fume cupboard?More than one answer may be correct. (E4)

-The powder, if inhaled, can cause serious problems to the respiratory tract.

-The powder is fine and easily inhaled.

Silica powder can be carcinogenic if inhaled. It is a very fine powder that readily forms clouds of dust that are easily inhaled.

When setting up a column, what is the reason for adding a layer of sand on top of the adsorbent column? (E4)

-To prevent disruption of the surface when solvent is added

It is important that the surface of the silica is level and not disturbed. If the surface is disturbed, the compounds will not travel through the column as cylindrical bands; instead, these bands will be uneven. This may give a poorer separation, especially if the compounds elute from the column close together.

Why shouldn't the start line, where the solution mixtures are initially placed, be drawn in ink? (E4)

-The ink is likely to dissolve in the solvent and will add extra spots to the chromatogram.

Pen inks tend to be soluble in organic solvents of most polarities and would contaminate the chromatogram.

SAFETY: Which of the following statements about the procedure for column chromatography are true? More than one answer may be correct. (E4)

-Apply enough pressure to increase the flow rate to a reasonable degree.

-Check for cracks in the glass before pressurising the system.

The column should not be over pressurised. Releasing the pressure suddenly can cause cracks in the column. The column should be securely clamped, but not too tightly so as to damage the column.

SAFETY: How should the silica be disposed of at the end of the experiment? (E4)

-In the appropriate waste solid container.

The silica is a solid and should be disposed of in the appropriate manner. Check with your instructor where to dispose of the silica before starting the procedure.

In silica gel chromatography, which compounds elute first? (E4)

-The least polar

In silica gel chromatography, polar compounds are retained on the column, as they have a greater affinity for the silica than the more non-polar compounds that are eluted first.

What is the reason for the layer of sand at the bottom of the column? (E4)

-To prevent the adsorbent from passing through to the tap

The sand provides a base on which the adsorbent column can sit and prevents it from running through to the tap.

What is a good R f value in order to ensure that a compound will elute over a reasonable timeframe? (E4)

-Around 0.3

Too low and the compound won't elute. Too high and it will run through the column too quickly.

What can you do to elute a compound that is not coming off the column? (E4)

-Change to a more polar solvent

A standard silica or alumina column with a polar stationary phase polar compound will elute slowly with non-polar solvents. A more polar solvent will compete with the compound to adsorb to the stationary phase, so it will start to elute.

SAFETY: TLC spotters are sharp. The MAJOR safety concern when using spotters is that: (E4)

-they can cause stab injuries.

SAFETY: Which of these are safety concerns related to the use of UV light to expose spots on a TLC plate? (E4)

-It can damage your eyes if looked at directly.

If using UV light to visualize the spots on a TLC plate, do not look directly into the light. UV light can damage your eyes.

Why should the solvent level in the elution tank be below the level of the baseline of the TLC plate? (E4)

-The compound in the spots would dissolve into the solvent if it were higher.

If the spots dissolve into the solvent, they will not move up the plate and be resolved.

The R f values of the spots on this TLC plate? value is calculated as the ratio of the distance travelled by the spot to the distance travelled by the solvent front.

-refer to E4 lab manual

How long should you run a TLC plate for? (E4)

-Until the solvent front is just below the top of the plate.

You should maximize the length of the run in order to get good separation; however, be aware that the longer the plate is run, the more diffuse the spots will become.

When setting up a fractional distillation, the main difference from a simple distillation is: (E5)

-You need to add a column between the flask and the distillation head.

The addition of a fractionating column allows a more controlled separation of components.

SAFETY: When performing a distillation, the collection end should be in some way open to the atmosphere. (E5)

-True

There should always be an outlet to prevent pressure build-up. Do not create a closed system, which could explode when heated.

Which is the best description of a fractional distillation? (E5)

-A technique to separate liquid mixtures with boiling points close to each other.

In distillation, the usual objective is to separate one component from a mixture. A mixture of liquids with boiling points that differ by less than 30°C can be separated using fractional distillation.

A gas chromatograph can be used to (more than one answer may be correct) (E5)

-separate nonvolatile compounds.

-determine the purity of a product.

A gas chromatograph can be used to separate vaporizable compounds. They can determine the purity of products by accurately detecting the presence of any impurities. Furthermore, the percentage composition of a mixture can be determined. Products can be identified by their retention times if there is a known or literature value for comparison.

A gas chromatograph needs to be connected to another detector such as a mass spectrometer in order to determine the mass on the individual peaks. This technique is known as gas chromatography-mass spectrometry or GC-MS.

While some GC instruments can be used to purify large quantities of compounds, this is not common practice and they are almost exclusively used as an analytical technique as they are not suitable for large injection volumes.

SAFETY: When derivatizing an alcohol with trimethylsilyl chloride, you should: (E5)

-Wear a lab coat, glasses and gloves.

Reacting trimethylsilyl chloride (TMSCl) with alcohols is a common example of derivatization.

Trimethylsilyl chloride is highly flammable, reacts violently with water, and causes severe burns. Therefore, TMSCl should not be used in the presence of ignition sources, in the presence of water, or without the appropriate protective clothing. The reaction produces HCl, and should be carried out in a fume cupboard.

You are required to separate a mixture of two solvents. One has a boiling point of 45°C, and the other has a boiling point of 120°C.Which of the following is the most appropriate type of distillation to use? (E5)

Simple distillation

There is no need to use anything other than simple distillation to separate these components.

In a vacuum distillation, what is the effect of using reduced pressure? (E5)

The boiling point of the fractions are decreased

Reducing the pressure reduces the boiling point.

SAFETY: How much should you fill your distillation pot? (E5)

Less than half full.

Do not overfill your reaction flask. It should be less than half full. If you have too much sample, you should use a larger flask.

Where should the thermometer bulb be during a distillation? (E5)

The bulb should be at the mouth of the condenser to measure the temperature of the condensate.

Which of the following is an appropriate use for distillation apparatus? More than one answer may be correct. (E5)

-To measure a boiling point of a liquid.

-To remove a liquid product from a reaction mixture.

If the objective is to collect a solid from a liquid, use filtration, not distillation. Distillation can be used to remove a liquid from a reaction mixture, and can be used to measure the boiling point of a liquid. Fractional distillation is often used to separate a mixture of solvents.

wo liquids have boiling points which are less than 20 °C apart. How can a simple distillation be modified to improve the separation? (E5)

Use a fractionating column.

Two closely boiling liquids can be separated by adding a fractionating column between the flask and the still head.

Why should the cold end of the apparatus be open to the atmosphere in a distillation set-up? (E5)

To prevent pressure build up, which could cause an explosion.

The collection end is either fitted with an adaptor to connect to a round bottom flask clipped to it or the liquid drips into a collection vessel below the adaptor. The adaptor should have a side arm on it to prevent pressure build up.

What rate should the liquid come across in a fractional distillation? (E5)

A few drops per minute.

In fractional distillation, it is extremely important to distill slowly, as it is more likely that you will get mixed fractions distilling over unless you take care.

When performing a fractional distillation, how can you tell when your liquid is beginning to distill over? More than one answer may be correct. (E5)

-The temperature remains constant after initially rising.

-Liquid droplets start to condense on the thermometer bulb.

The temperature will begin to rise when you start heating your sample, but the liquid will only start to distill when it is boiling and vapor is rising up to the still head and thermometer bulb. A tell-tale sign is to see droplets forming on the thermometer bulb.

Which of the following factors affect boiling point? More than one answer may be correct. (E5)

-The pressure.

-Whether a compound is polar or not.

The boiling point depends on the pressure, molecular weight, structure, and purity of the liquid.

The vapor pressure of the liquid must equal the external pressure in order for boiling to occur, therefore, the boiling point decreases with decreasing pressure and less energy is required to make the liquid boil.

The structure of a compound determines the strength of intermolecular interactions that hold the molecules in the liquid phase: polar compounds have dipole-dipole and, potentially, hydrogen bonding, whereas, nonpolar compounds have van der Waals interactions.

A gas chromatograph can be used to (more than one answer may be correct) (E5)

-identify compounds based on their retention times.

-determine the purity of a product.

A gas chromatograph can be used to separate vaporizable compounds. They can determine the purity of products by accurately detecting the presence of any impurities. Furthermore, the percentage composition of a mixture can be determined. Products can be identified by their retention times if there is a known or literature value for comparison.

A gas chromatograph needs to be connected to another detector such as a mass spectrometer in order to determine the mass on the individual peaks. This technique is known as gas chromatography-mass spectrometry or GC-MS.

While some GC instruments can be used to purify large quantities of compounds, this is not common practice and they are almost exclusively used as an analytical technique as they are not suitable for large injection volumes.

What features make a sample suitable for analysis by gas chromatography? (More than one answer may be correct.)

High boiling point.

Thermal instability.

High vapor pressure.

Appreciable volatility.

Thermal stability. (E5)

-High vapor pressure.

-Appreciable volatility.

-Thermal stability.

Gas chromatography involves a gaseous mobile phase and stationary phase, therefore, the sample must be volatile enough that it can be vaporized and carried into the column by the carrier gas.

Samples that are too involatile to be analyzed by GC can be derivatized.

The compounds in the samples must be thermally stable at the temperature required for analysis by GC. Certain functional groups are less thermally stable than others, therefore, samples can be derivatized to increase thermal stability.

Which of the following statements concerning the principle of separation in gas chromatography is true when using a liquid stationary phase? More than one answer may be correct. (E5)

-The extent to which a molecule interacts with the stationary phase depends on its size.

-When the sample enters the column, the compounds dissolve to varying extents in the stationary phase.

Compounds are separated in the column based on the level of interaction with the stationary phase. The greater the interaction with the stationary phase the slower the compound will travel through the column.

When the gaseous sample enters the column, it condenses and dissolves in the liquid stationary phase until its equilibrium vapor pressure has been reached.

The lower the vapor pressure of a compound, the greater its interaction with the stationary phase. Therefore, factors that affect vapor pressure, such as size, affect the level of interaction with the stationary phase.

The higher the vapor pressure of a compound, the greater the proportion that remains in the mobile gas phase, hence the faster the compound gets carried through the column by the carrier gas.

What is the consequence of the gas flow rate being too high? (E5)

-The sample won't have a chance to equilibrate with the stationary phase, leading to poor separation.

The flow rate must be slow enough for the compounds to equilibrate with the stationary phase so that they can be separated based on their equilibrium vapor pressures. However, if the flow rate is too slow, the peaks become broad (poor resolution) due to the diffusion of the compounds in the gas phase.

What is the effect of using a longer column? (E5)

-The separation will be more effective, as the sample undergoes more partitioning between the mobile and stationary phases.

In a longer column, the compounds undergo greater partitioning between the mobile and stationary phases, hence the difference in the interaction of the compounds with the stationary phase is accentuated and separation is more effective.

SAFETY: When injecting a sample solution it is important to (E5)

hold the syringe with two hands; one hand holding the barrel and the other positioned behind the plunger.

When manually injecting your sample it is important to remember to clean the syringe with an appropriate solvent, and rinse it with a portion of your sample before using it.

The sample should be drawn up into the barrel so that no solution remains in the needle; any solution left in the needle would boil upon contact with the hot metal needle guide. The syringe should be held with two hands; one hand holding the barrel and the other positioned behind the plunger.

When injecting samples, be careful not to touch the metal needle guide and not to press the plunger until the needle has pierced the septum.

SAFETY: Which statement best describes how the system should be heated? (E6)

-Heat gently so that the solvent boils and the vapors extend no more than half way up the condenser.

The height of the vapor can be seen as a ring within the condenser. This should not extend beyond half way up the condenser and preferably lower to ensure no vapor is lost from the system.

Which of the following is an appropriate use for reflux? (E6)

-To carry out a reaction that needs heating.

Use reflux for reactions that require heating and contain volatile solvents, reactants, or products.

IR spectroscopy can be used to determine which of the following? More than one answer may be correct. (E6)

-The presence of impurities in a product

-The presence of certain bonds within a molecule.

-The absence of certain groups from a molecule.

SAFETY: When using an aluminum heating block with an electric hot plate it is important to (more than one answer could be correct) (E6)

1. hover your hand above the heating block to check whether it is hot.

2. not leave the reaction unattended until the set temperature has been reached and the temperature remains stable.

3. select a size that snugly fits the round bottom flask to ensure even heating.

It is important to select the appropriately sized aluminum block to fit the round bottom flask snugly and also to secure glassware using a clamp and clamp stand.

Glass mercury thermometers should not be used as they can break when in direct contact with the heating block releasing toxic mercury vapours. A digital thermometer should be used instead to monitor the temperature.

Never touch the heated aluminum block directly with your hand, you can check to see if it's hot by passing your hand over it.

It is very important to remain with your reaction until it has reached the temperature that you have set to ensure that the temperature is stable and doesn't become too low or too high.

You shouldn't leave the reaction flask to cool in the aluminum block. As the heating block cools down it can contract and break the flask.

SAFETY: When using a heating mantle make sure (more than one answer might be correct) (E6)

1. you turn the heating mantle off as soon as you have finished using it.

2. that there is a temperature control unit connected before plugging the heating mantle into mains electricity.

3. you use a lab jack to elevate and lower the heating mantle to and away from the glassware in use.

4. you choose a heating mantle that can accommodate your flask snugly.

If you are using a heating mantle that doesn't have a built in controller, you must never plug it directly into the mains because you will have no way of regulating the temperature.

The heating mantle should fit your round bottom flask snugly to ensure even heating of the contents.

A lab jack should be used to raise and lower the heating mantle as this enables the heat source to be removed quickly if overheating occurs.

You should always turn off the heating mantle as soon as you have finished using it.

Which of the following would give you the most accurate mass of your compound? (E6)

-Using an analytical balance and weighing by difference.

Top-loading balances give mass to the nearest 0.1-0.001 g, depending on the model, whereas analytical balances give mass to the nearest 0.0001 g. Analytical balances should be used for accurate mass measurements. Weighing by difference removes the error associated with a particular balance, hence increasing the accuracy.

SAFETY: Where should the reflux equipment be clamped? (E6)

-Securely at the neck of the flask

The equipment should be secured at the neck of the flask. Any extra clamping of the condenser may cause the joint between the flask and the condenser to be compromised, allowing vapor to escape.

Which of the following is the best description of a reflux? (E6)

-Reflux is a technique used to heat volatile materials. The reaction vessel is fitted with a condenser so that any vapors from the flask fall back into the heated mixture.

Correct. Reflux is often used in reactions that require heating over an extended period of time. Reflux allows the contents of the flask to be heated without a build-up of pressure (as the system is open to the atmosphere), while preventing the contents of the flask from escaping.

If you wanted to prevent moisture from the air getting into your reaction, how could this be achieved? (E6)

-Attach a tube containing a drying agent, such as Drierite, in the top of the condenser.

A drying tube can be attached to the top of the condenser to keep the atmosphere free of water vapor. The top of the condenser should never be stoppered, as this creates a sealed system that will pressurize when the liquid starts to boil and could cause an explosion.

SAFETY: How far should you fill the reaction vial before refluxing? (E6)

-Less than two thirds full

Do not overfill your reaction vessel. Flasks should be less than half full. Microscale vials should be less than two thirds full.

SAFETY: When removing solvent using open-vessel evaporation,

(more than one answer may be correct) (E6)

-you do not need to stir the solution.

-the procedure must be carried out in a fume cupboard.

-you can evaporate solutions to dryness.

In open-vessel evaporation the solvent vapors escape into the air, therefore, the procedure must always be performed in a fume cupboard to prevent inhalation of harmful vapors.