CH 236 Final Exam

The value of retention factors are always between what numbers?

0 and 1

What are the steps to prepare a TLC plate? (3)

1. A plane is drawn lightly on the TLC plate with a pencil about 1 cm rom the end of the plate 2. A micro capillary tube is used to draw a small amount of the organic samples to be tested 3. A small dot of ash sample is placed on the pencil line

In extraction, what are emulsions?

1. A suspension of insoluble droplets of one liquid in another 2. There is no clear separation between layers and they do not easily separate

Describe how a sublimation apparatus is assembled:

1. A test tube is filled with ice and placed in the mouth of the flask 2. A filter adapter around the test tube prevents the loss of vapor from the flask 3. When the flask is heated in the sand bath, the desired compound vaporizes while the impurities do not 4. The cold test tube serves as a surface for condensation of the vapors

In the hydrolysis of t-butyl chloride, why was acetone used to rinse the glassware?

1. Acetone is a strong cleaning agent 2. Acetone is a strong drying agent because it binds to water

How is the identity of a compound determined by using mixed melting points? (3)

1. An unknown compound can be mixed with a known pure compound and the melting point measured 2. If the two compounds in the mixture are identical, the melting point will be the same as in each individually tested 3. If the two compounds are not identical, they will act as impurities to one another and the melting point of the mixture will change

How is the extraction of caffeine done? (2)

1. By steeping tea with hot water, caffeine passed freely through the filter due to its solubility in water 2. The other components of the tea were also soluble in water so another extraction had to be done

In solid liquid chromatography, what determines the distance traveled along the plate by the compounds? (2)

1. Depending on how soluble each organic compound is in the solvent (mobile phase) 2. The strength at which is stuck to the stationary phase

What are the steps in the recrystallization of a compound? (5)

1. Find a suitable solvent for recrystallization 2. Dissolve the impure solid in a minimum volume of hot solvent 3. Remove any insoluble impurities by filtration 4. Slowly cool hot solution to crystallize the desired compound from solution 5. Filter the solution to isolate the purified solid compound

In TLC, list the common solvents (lowest to highest polarity): (9)

1. Hexane 2. Cyclohexane 3. Toluene 4. Dichloromethane 5. Diethyl ether 6. Ethyl acetate 7. Acetone 8. Ethanol 9. Methanol

In fractional distillation, why does additional distillation occur in the fractioning column? (2)

1. It occurs due to the constant condensing and revaporization of the component on the copper threading 2. Because of the repeated condensations and vaporizations, the vapor is repeatedly undergoing a series of simple distillation as it travels up the column to the condenser

In determining melting point, what can the presence of impurities in a sample do? (2)

1. Lower or depress the melting temperature 2. Broaden the melting range

In chromatography, what are the two phases?

1. Mobile 2. Stationary

Why is sublimation a valuable purification technique?

1. No solvent is involved 2. Can be performed at low temperature 3. Loss of material due to handling is minimum 4. It readily separates nonvolatile impurities

In recrystallization, what can be done if crystallization does not happen? (2)

1. Scratch the inner wall of the Erlenmyer flask with a glass stirring rod to release small particles of glass which act as nuclei for crystal growth 2. Addition of a small crystal of desired compound (a seed crystal) to serve as a template for crystallization of the rest of the compound

In solid liquid chromatography what are the most common stationary phases? (2)

1. Silica gel 2. Alumina

What are the two major types of distillation used in organic chemistry lab?

1. Simple 2. Fractional

In dealing with recrystallization, what are properties of a good solvent? (4)

1. The compound should be very soluble at the boiling point of the solvent and only sparingly soluble the solvent at room temperature 2. The unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent 3. The solvent should not react with the compound being purified 4. The solvent should be volatile enough to be easily removed from the solvent after the compound has crystallized

How can emulsions be avoided? (2)

1. The funnel should be slowly inverted and swirled with the addition of dichloromethane 2. Avoid vigorously shaking

In recrystallization, how can colored impurities be removed? (3)

1. The impure solvent is dissolved in hot solvent, a small amount of decolorizing carbon is added 2. The solution is tired and heated a few minutes 3. Then the solution is filtered to removed colorized carbon

How is distillation carried out? (4)

1. The liquid mixture is initially heated until it boils 2. The vapors rise above the liquid and enter a condenser 3. Once the vapors reach the condenser, they are condensed into a liquid form and the liquid is collected 4. The differences in boiling point of the two components allowed for only one of the components to vaporize while the other remained in liquid form

What must be done to carry out chromatography on the TLC plate? (2)

1. The plate just be placed in a developing chamber 2. A small mount of solvent is placed in the chamber and the chamber is lined with a piece of filter paper

After there was a mixture of NaCl, H2O and caffeine, what was done? (3)

1. The solution was chilled and poured into a separatory funnel 2. 3 5 mL portions of dichloromethane was added one portion at a time to the separatory funnel 3. After all three portions were added and the funnel was carefully turned upside down to mix the two solutions, two distinct layers formed

What are the two processes that distillation involves?

1. Vaporization 2. Condensation

In the sublimation of caffeine, what must not the sand bath exceed?

220° C

What can mixed melting points be used as?

A a way to determine the identity of a compound

What is an azeotrope?

A constant-boiling mixture of definite proportion

In recrystallization what should be used if it is hard to remove a colored impurity from a solid mixture?

A decolorizing carbon is used to remove the impurities

What type of substitution is the hydrolysis of t-butyl chloride?

A first order nucleophilic substitution (SN1)

What is distillation?

A process in which components of a liquid mixture can be separated

What is extraction?

A process which allows for the separation of one compound from a mixture

What is needed in order to carry out an extraction?

A proper solvent

What determines the type of solvent used in TLC?

A solvent of an intermediate polarity is used to ensure that all the various compounds are pulled up the plate to varying degrees

What is chromatography?

A tool that can be used to separate components in mixtures

In sublimation of caffeine, what happens when caffeine begins to sublime?

A white solid will form on the test tube, the solid is then scraped off

In distillation, what is bumping?

An instantaneous violent explosion of vapor from the liquid

In distillation, why is it important to use the rheostat?

Because as the adjustment of the resistances allows for the controlling of the amount of heat delivered by the sand bath

Why was caffeine extracted from the water layer to the dichloromethane?

Because caffeine has a higher affinity for dichloromethane than it does for water

Why does dichloromethane form on the bottom layer of the separatory funnel?

Because it is more dense than water

Why can an ultraviolet lamp be used to mark the separation of the compounds on a TLC plate?

Because the TLC plates are treated with a fluorescent indicator that glows green when under UV light

Why do precipitation experiments have to undergo an additional final purification step to give the pure compound?

Because the amorphous solid formed can contain many trapped impurities

In distillation, how can bumping be avoiding?

Boiling chips are added to the distilling flask to prevent hot spots from developing in the liquid

How is the efficiency of a distillation measured?

By using theoretical plates

In a two compound mixture, what will one of the compounds do?

Depending on the melting properties of each of the compounds, one of them, as its concentration increases, will act as an impurity on the other until a minimum melting point is reached

In recrystallization, what happens to the solubility of the compound?

For most compounds, the solubility of the compound increases as the temperature of the solvent increases

In distillation, why should a sample not be distilled to dryness?

If no liquid remains to carry away the heat, the flask will become hot and can crack

Why should distillations never be carried out in a sealed apparatus?

If the apparatus is sealed, pressure could build up and cause an explosion

Describe the main idea of recrystallization:

Impure solid is dissolved in a solvent and then allowed to slowly crystallize out as the solution cools

What compounds does sublimation occur in?

In compounds that do not display strong intermolecular forces

In distillation, what happens if the thermometer is not properly placed?

Inaccurate readings could occur

In distillation, what does a rheostat act as?

It acts as a resistor in a circuit

In TLC, what does the value of the retention factor determine?

It can be used to determine the identity of unknown compounds by comparing the retention factor of it to the retention factors of known pure substances

In TLC, what does the polarity of the solvent determine?

It determines the distance traveled by the compounds

In sublimation, what does the outer vessel (flask) hold?

It holds the impure solid

In recrystallization, what does crystallization of a solid involve?

It involves a slow, selective formation of the crystal framework to give a pure solid

In recrystallization, what does precipitation involve?

It involves the rapid formation of a solid from a solution that usually produces an amorphous solid containing many trapped impurities in the crystal framework

What is melting point and why is it useful?

It is a characteristic physical property that can be useful in identification of a compound

In distillation, what does stopcock grease do?

It is a lubricant used to ensure the tapered joints of the glassware do not become frozen together

What is one theoretical plate equal to?

It is equal to one simple distillation

How is melting point measured?

It is measured as a range form the point the solid begins to melt to the point the solid has completely melted and is a clear liquid

Why is a fractional distillation needed in fractional distillation?

It is needed when a mixture is composed of two liquids with very similar boiling points

In distillation, what should the volume of the mixture being distilled be?

It should fill the distilling flask between one-half and two-thirds volume to avoid loss of distillate as vapor in flasks too large and the splashing up of liquid in a flask too small

In distillation, where should the bulb of the thermometer be placed?

Just below the opening of the sidearm to the condenser to ensure the bulb is bathed in vapor before the vapor reaches the condenser

What is distillate?

Liquid collected in distillation after the vapors reach the condenser and are condensed back into liquid

What compound can be used to break up emulsions?

Magnesium sulfate

What is an azeotrope made of?

Mixtures of miscible liquids that form non ideal solutions in which the volatility of each component in the mixture is affected by the presence of another compound

What was dissolved in the caffeine/water mixture for the initial extraction?

NaCl

In recrystallization, are impurities included in the framework of the crystal?

No

Is a simple comparison of melting points a good way of determining identity?

No because there are a vast number of different organic compounds and may will have the same melting point

Is thin-layer chromatography a reliable way to identify a compound?

No, because so many compounds can have the same retention factor

Can the two components of an azeotrope be separated using fractional distillation?

No, never, regardless of the number of theoretical plates in the column

In distillation, what is separation based on?

On the differences in the boiling points of the components

When is the TLC plate removed from the chamber?

Once the solvent has traveled near the top of the plate

In TLC, what is the equation for the retention factor?

Rf = (distance traveled by the compound) / (distance traveled by the solvent)

In chromatography, the more _____ a compound is absorbed on the ______ phase, the slower that particular compound will move down the _____

Strongly Stationary Plate

In TLC, what happens when the solvent chamber is at equilibrium?

The TLC plate is carefully stood up inside the chamber and the lid is replaced

In solid liquid chromatography, what happens when organic compounds are placed on the gel?

The compounds will stick with varying degrees of strength

How does fractional distillation differ from simple?

The fractional distillation apparatus has an additional fractionating column between the distillation flask and distillation head

What is a eutectic point?

The minimum melting point in a mixture of two compounds

What happens as more theoretical plates are added?

The more plates added in a fractionating column, the more simple distillations that occur and the better the separation of liquids

In chromatography, what does separation depend on?

The polar and non-polar traits of each of the components

What is sublimation?

The process by which compounds go directly from the solid phase to the gas phase, skipping the liquid intermediate

In distillation, what happens when the power settings on the rheostat are decreased?

The resistance of the rheostat is increased and therefore less voltage is delivered to the sand bath, causing it to operate at a lower voltage and lower heat

How is an accurate melting point of a compound obtained?

The solid must be heated slows

What is melting point defined as?

The temperature at which the solid phase is in equilibrium with the liquid phase

In a two compound mixture, what can each of the compounds act as?

They can act as an impurity to the other, lowering the melting temperature and broadening the melting range

In the hydrolysis of t-butyl chloride, how many times was each experiment carried out?

Three

In the hydrolysis of t-butyl chloride, why was each experiment done three times?

To allow for a realization of consistency or error in the procedure

What do the properties of azeotropes cause them to do?

To behave as a single pure compound even though they are actually mixtures

Why are copper threads placed in the fractioning column?

To increase the surface area because the vapor from the distillation flask must pass through the fractionating column before reaching the condenser, additional distillation occurs

In sublimation of caffeine, why should the flask be tilted?

To reduce the amount of caffeine subliming on the sides of the flask

What was the purpose of the hydrolysis of t-butyl chloride?

To study the rate of reaction and the effects on the rate that concentration and temperature had

For the extraction of caffeine, what makes a good solvent and why?

Water makes a good solvent because caffeine is highly soluble in water

In a two compound mixture, what happens when a minimum melting point is reached?

When a minimum melting point is reached, the eutectic point, the other compound begins to act as an impurity

In TLC, when can separation be analyzed?

When the plate dries

Can many organic compounds have the same retention factor?

Yes

Is sublimation a valuable purification technique?

Yes

Can an emulsion disperse?

Yes, if the funnel is allowed to sit for a few minutes