CHE 231

The fractions produced in the first distillation must be ...

Redistilled to produce more pure portions of hexane and toluene

What is the result of applying too much of your sample to your TLC plate?

Reduced separation between spots

Solubility guidelines #1 Like dissolves like

-Any substance (solid/liquid) is soluble in another substance (liquid), only if their chemical nature is similar. -Polar substance is soluble in polar solvent

Criteria for solvent selection (continued)

-Boiling point of the solvent should be low enough to be easily evaporated during drying step -Boiling point of solvent < melting point of solute -Solvent should not react with the solute

Solubility guidelines #3 Branched versus unbranched

-Branched chain compounds are more soluble than corresponding straight chain compounds (branching lowers intermolecular attractions)

Crystallization steps

-Cool filtrate slowly at room temperature -Agitate only if very large crystals form -Cool in ice bath

Solubility guidelines #2 C5 Guidelines

-Organic compounds with less than 5 carbons and a polar function are water soluble

Hot filtration

-Removes insoluble impurities and activated charcoal -Use short stem class funnel with filter paper -Pour near top of paper to maximize filtration -Pour into beaker on hot plate containing small solvent heated near BP

Discuss plates coated with silica gel (SiO2)

-SiO2 has fluorescent indicator added -It is polar -It is a Lewis acid — electron pair acceptor - It is also the stationary phase

Solubility at room temperature

-Solute insoluble in solvent at room temperature -So heat the solvent and test it for complete solubility at high temperature

Solubility at high temperature

-Upon heating the compound dissolved completely in the selected solvent -It is good solvent for recrystallization

Filtration

-Wet filter paper with the right solvent. Filter precipitate by suction with Büchner funnel and vacuum flask. -Use stirring rod to transfer crystals -Release suction after solvent has filtered through the filter paper -Wash crystals with cold solvent then reapply suction

SiO2 (silica gel)

-has fluorescent indicator added -is polar -is a Lewis acid: electron pair acceptor -is also the stationary phase

How much solvent from the list given on the previous slide will be added?

0.2 mL

3 principles of thing layer chromatography (TLC)

1. Chromatography is a separation technique 2. Separation is based on partitioning of a given mixture between stationary phase and mobile phase 3. In TLC, stationary phase used is a solid (silica gel) and mobile phase is a liquid (organic solvent such as chloroform or hexane)

Drying steps

1. Dry the product using vacuum filtration 2. Transfer crystals to clean petridish 3. Press the crystals between filter papers to aid drying 4. Allow crystals to air dry until the next lab 5. Do not put crystals in oven!

Final products from solvent extraction

1. Ethyl 4-amino benzoate 2. Benzoic acid 3. 9-fluorenone

TLC video

1. Lightly draw the origin line near the bottom of the plate 2. Label sample spots 3. Apply samples to labeled positions at origin 4. Plate is ready for chromatography 5. Place TLC plate inside jar containing solvent 6. Replace cap 7. Solvent travels up the plate transporting spots 8. The 3 pure substances move at different rates 9. Mixture is resolved as components move at different rates 10. Remove plate before solvent front reaches top of plate 11. Mark solvent front before solvent evaporates 12. Allow solvent to evaporate 13. Measure distance travelled by solvent front and each spot 14. Determine Rf

Water insoluble compounds: guidelines (3)

1. Molecules without polar functional groups 2. Monofunctional compounds with more than 5 carbon atoms (CH3CH2CH2CH2CH2...) 3. Compounds such as methylene chloride (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4) are insoluble in water; and are commonly used as one of the solvents for liquid-liquid extraction using aqueous solutions

If crystal formation fails...

1. Scratch inside walls of flask 2. Use a seed crystal (from leftover starting material) 3. Heat to evaporate some of solvent and cool the solution for additional time

Outline for recrystallization experiment

1. Select appropriate solvent (Ethanol acetate) 2. Dissolve solid near boiling point of solvent 3. Filter hot solution to remove insoluble impurities if necessary 4. Crystallize solid by cooling 5. Separate crystals out 6. Wash and dry crystals

Water soluble compounds: guidelines (3)

1. Small molecules (5 carbons or less) with at least one polar function group 2. Larger molecules with more than one polar functional group (poly functional compounds, sugars, etc.) 3. Salts (ionic) are water soluble due to ion dipole interactions

Use _____ mL hexane, ______ mL toluene

1.5; 1.5

C-O IR frequency

1000-1250

What is the boiling point of toluene in units of degrees Celsius?

111

Literature reported melting point range of aspirin

134 C - 136 C

C=O IR frequency

1650-1725

Place the solid in ________ Erlenmeyer flask, add a _____ _______

25mL; spin vane

Sp3 C-H IR frequency

2850-3000

How much solvent is added with the boiling chip?

2mL

In this experiment, how many fractions will be collected during the first distillation?

3

How much compound is put in a small test tube?

30mg

O-H IR frequency

3300-3600

What is used as the mixture for solvent of elution for mobile phase in solvent extraction?

3:7 ether and chloroform (CHCl3)

How many moles of hydride ions (H:-) can one mole of NaBH4 provide?

4

If you started recrystallization with 100mg of a sample and you recovered 50 mg after recrystallization, what is the percent recovery

50%

After selecting an appropriate solvent for recrystallization, how much impure aspirin in milligram is expected to be used for recrystallization

500mg

Use ____ mL conical vial for the first distillation; _____ mL conical vial for the redistillation

5; 3

What is the boiling point of hexane in units do degrees Celsius?

69

A solid mixture of three compounds (150mg) is placed in a solution of 3M HCl. Match the compound letter to the compound which is soluble in 3M HCl

A

An overall change in the following reaction is: A. Electrophilic substitution B. Nucleophilic substitution C. Electrophilic addition D. Nucleophilic addition E. Hydrolysis

A

Based on the stoichiometric reaction equation for the reduction of cyclohexane pictured above, balance the equation by matching the red letters A, B, G, H, and J

A = 4 B = 1 G = 4 H = 4 J = 1

Using the reaction equation above, identify the nucleotide and electrophile by matching the correct red letter(s) below

A = electrophile B = nucleophile

Solubility guidelines #4 Water soluble compounds

A. Small molecules 5 (carbons or less) with at least one polar functional group B. Larger molecules with more than one polar functional group (poly functional compounds, sugars, etc...)

Tabulate all solubility data and results that you obtained for selection of an appropriate solvent for recrystallization of aspirin

Acetone -- RT soluble -- HOT soluble Ethyl acetate -- RT insoluble -- HOT soluble Hexane -- RT insoluble -- HOT insoluble

Fractional distillation set up

Added column provides larger surface area for multiple simple distillations

What is the purpose of adding a boiling chip during heating? How does it help?

Adding a boiling chip during heating makes the solvent boil more calmly. Now, the liquid boils smoothly without becoming superheated. It prevents bumping of the solution.

Compounds used in liquid-liquid extraction

All are insoluble in water

Column chromatography uses a stationary and a mobile phase to separate compounds. What is the stationary phase used in this column chromatography experiment?

Alumina

Relating solubility and structure

Any substance (solid/liquid) is soluble in another substance (liquid), only if their chemical nature is similar. Polar substances are soluble in polar solvent Like dissolves likes!!!

To re-precipitate acidic component from aqueous solutions, add ...

Aq HCl

To re-precipitate basic component from aqueous solution add ...

Aq NaOH

Water

Aqueous liquid hazardous chemical waste

Solubility guidelines #5 Salts (ionic)

Are water-soluble due to ion-dipole interactions (NaCl, KCl)

Fried eel-Crafts acylation reaction with ferrocene

Aromatic compound — ferrocene Electrophile — acetyl chloride Lewis acid catalyst — aluminum chloride Solvent — methane chloride

What product and catalyst is used in column chromatography

B is the product AlCl3 is the catalyst

A compound with following number of π electrons is likely to be Hückel aromatic? A. 12 B. 10 C. 8 D. 16 E. 4

B, 4pi + 2

Capillary tubes, pipettes

Broken glass waste

The region towards the right side of an Infrared (IR) spectrum, in the region 1500 cm to 500 cm is known as: A. Functional group region B. Symmetrical stretching region C. Fingerprint region D. Rotational region E. Asymmetrical stretching region

C

Which of the following is a reducing agent? A) Methanol B) Sodium methoxide C) Sodium borohydride D) Cyclohexanone E) Ferrocene

C

The Hickman still has a short column, so a complete separation of hexane and toluene...

Cannot be achieved in one cycle

In the solvent extraction experiment, what is the mobile phase used for the TLC analysis?

Chloroform

What is used as a solvent to develop the chromatographs

Chloroform — CHCl3

Wrap the Hickman still head with aluminum foil to aid

Condensation

During the process of distillation, a liquid is heated to its boiling point and the vapors generated are ....

Condensed by cooling and collected in a receiving vessel, or they return to the boiling mixture

Presence of impurity causes both

Depression of melting point range and melting to occur over wide range

Presence of impurity causes both _______ of melting point range and melting to occur over ________ range

Depression; wide

Which one is more polar, hexane or dichloromethane?

Dichloromethane

Pick the solvent below than can dissolve all three aromatic compounds of the moisture used in solvent extraction experiment

Diethyl ether

The separation is based upon the ...

Differences in their boiling point (and thus the vapor pressure)

What was the method used to separate a mixture of the two compounds?

Distillation

What is the purpose of adding sodium sulfate (Na2SO4) in the solvent extraction experiment?

Drying the organic layer

The peak due to the -OH stretching in the IR spectrum of cyclohexanol would appear at: A) 1680 B) 2250 C) 2931 D) 1710 E) 3345

E

Electropilic

Electron-seeking

Ferrocene can undergo ______ _______ _______ reactions, like benzene

Electrophillic aromatic substitution

What is the name of the organic product obtained in the reaction below?

Ethanol

If ether and water are mixed in equal volumes, the top layer will be:

Ether

True or false: one of the most important criteria of a recrystallization solvent is its ability to completely dissolve the sample at room temperature

False

True or false? Compounds that interact more strongly with silica gel will travel faster on a TLC plate

False

True or false? Glucose is more soluble in hexane than water.

False

True or false? SiO2 is a Lewis base

False

True or false? The melting point of an impure sample is likely to be more than that of the pure sample

False

True or false? The reduction of cyclohexane requires heat.

False

True or false? You will record boiling point in this experiment

False

True or false? Boiling point and vapor pressure are directly proportional.

False, inversely

True or false? When developing a TLC plate the "origin" should be below the solvent level in the developing jar.

False, it is above it

True or false? The first fraction in your distillation experiment will be mostly composed of toluene.

False, it is hexane

True or false? In this experiment, the stationary phase on the TLC is alumina

False, it is liquid gel

True or false? The stationary phase used in the TLC analysis of column chromatography was alumina

False, silica gel

Compounds with weak interaction with silica gel will move up the plate ______

Fast

Following the procedure for the experiment, which compound will be eluted from the column first, ferrocene or acetylferrocene? Why does it elute in this order?

Ferrocene elutes first because ferrocene is less polar than acetylferrocene. Ferrocene elutes first when non-polar hexane is used as a mobile phase

Different colors make it easy to visualize the separation using column chromatography. Choose the correct color for the following compounds

Ferrocene — yellow Acetylferrocene — orange

Since 2 rounds of distillation occur, the process has 2 theoretical plates and is therefore called

Fractional distillation

Ferrocene can react with acetyl chloride under _________conditions to yield _______

Friended-crafts; acetylferrocene

The more polar solvent moves spots _____ up the plate

Further

In this experiment, how would you determine the composition of the fractions collected?

Gas chromatography

Determine the composition of each fraction by recording...

Gas chromatography (GC)

In solvent-solvent extraction, add ______ to the mixture of three compounds in ether

HCl

If compound is insoluble,

Heat the test tube using a hot plate with aluminum block for 3-5 minutes

Which one would elite first on a GC, hexane or toluene? Why?

Hexane elutes first because it has a lower boiling point

The first fraction will contain mostly ______ (it has the lower boiling point). The second will be a mixture of _____ The third fraction will contain mostly _____ (it has the higher boiling point)

Hexane; hexane and toluene; toluene

Toluene (higher BP)

Higher retention time

Visualizing the spots on TLC

Hold the plate under 254 nm ultra-violet light. Spots should become visible immediately. This is due to the fluorescent indicator that has been added to the silica gel. The sample compound either enhances or diminishes the fluorescence of the indicator. In this experiment, the compounds are aromatic and will be visible under the ultra-violet light.

Which solvent did you choose for recrystallizing aspirin? Why?

I choose ethyl acetate for recrystallizing aspirin, because aspirin was insoluble in it at room temperature and soluble at high temperatures. The criteria necessary and desirable for a solvent suitable for recrystallizing a particular organic compound are met.

How would you prepare 10mL of 3M NaOH (aqueous) solution starting with 6M NaOH (aqueous) solution.

I would add 5 mL of 6NaOH and 5mL of water

How would you prepare 100mL of 1:9 Ether:Hexane mixture?

I would combine 10mL ether and 90mL of hexane

If you were running a recrystallization experiment on an unknown compound, and you first used methanol which dissolved the compound at room temperature, which solvent should you try next, a more polar solvent or less polar solvent? Why?

I would try a less polar solvent next, because like dissolves like. Any substance is soluble in another substance only if their chemical nature is similar. Methanol is a polar substance and if the substance dissolves in the methanol that would make it polar. We do not want a substance that dissolves in a solvent at room temperature so I would try the one that is less polar.

What would be the effect on the melting point of a sample if it was not dried completely after filtering the recrystallized sample?

If the sample were not dry it would be impure, depress the melting point, as well as broaden its range.

In chemistry what does TLC stand for and what is its purpose?

In chemistry, TLC stands for thin layer chromatography. It's purpose is to help us separate compounds in a mixture. It can be used to determine the number of components in a mixture, the identity of compounds, and how pure a compound is.

What criteria are necessary and desirable for a solvent suitable for recrystallizing a particular organic compound?

In order for a solvent to be able to recrystallize a particular organic compound, the solute should be soluble in the solvent at high temperature and insoluble at room temperature. Also, the boiling point of the solvent should be low enough to be easily evaporated during the drying step. The boiling point of the solvent should be low so the crystals can be separated from it.

Boiling point and vapor pressure are...

Inversely proportional (the higher the boiling point, the lower the vapor pressure and vice versa)

Retention time

It is the amount of time a component interacts with the stationary phase

Those that are bound ________ tightly to the stationary phase and are more _______ in the mobile phase travel farther than other components of the mixture

Less; soluble

Choose the most powerful reducing agent (NOT used in the experiment) for the following compounds:

LiAlH4

Different types of chromatography techniques can be classified according to physical states of the mobile and stationary phases. Following the procedure for this experiment, how do you classify column chromatography?

Liquid-solid

Hexane (lower BP)

Lower retention time

______ _______ are the most commonly used reagents for reduction

Metal hydrides

Solvent attraction requires the use of two solvents that are immiscible. What solvent what be best used for a liquid-liquid solvent extraction with water?

Methylene chloride

What are the stationary and mobile phases in gas chromatography?

Mobile phase — helium Stationary phase — a high boiling liquid supported on an inert solid support

Most to least polar for ethyl acetate, hexane, and acetone

Most polar — acetone Intermediate — ethyl acetate Least polar — hexane

The reducing agent used in this experiment is:

NaBH4

______ is selective, mild reducing agent; reduces only aldehydes or ketones

NaBH4

Which of the following compounds would you expect to be the most soluble in water?

Needs not be polar

Partition coefficient

When compound X is placed in a mixture of 2 immiscible solvents, it will be distributed between the two phases as a function of its relative solubility in each solvent

What type of reaction is the reduction of cyclohexane to cyclohexanol?

Nucleophillic addition reaction

Nucleophile

Nucleus seeking

What is recrystallization?

One of the most common purification techniques for solids

Micro scale experiment set up

Only one thermometer is used which is the hickman still head. Use Al block instead of sand bath

Acetone, chloroform

Organic liquid hazardous chemical waste

At the end of the experiment (after completing the post-lab assignment), where will the separated fractions be disposed?

Organic liquid hazardous chemical waste container

What formula should be used to calculate % composition after you run your sample on a GC?

Peak area = 1/2 * width * height of peak Percentage of hexane = (peak area of hexane/total area of hexane and toluene) *100 Percentage of toluene = (peak area of toluene/total area of hexane and toluene) * 100

What formula will you use to calculate percent yield of the product after you complete the reaction?

Percent yield = (actual yield)/(theoretical yield) * 100 Actual yield or experimental yield is the amount I measure in lab. Theoretical yield is the amount that would be ideally produced at the end of the experiment.

Heat up the to BP to dissolve (stir constantly) and then

Perform hot filtration if any insoluble solid is observed

Is aspirin polar or nonpolar

Polar

Compounds with strong interaction with silica gel (which is ______ and Lewis _______) will move up the plate _______

Polar; acid; slowly

Separation is achieved by changing the _______ of the mobile phase

Polarity

Product of reduction of an aldehyde using sodium borohydride is:

Primary alcohol

Type of compounds: base

Proton acceptor (Bronsteds definition)

Types of compounds: acid

Proton donor (Bronsted definition)

Measurement of the melting point can provide information about _________. Melting point is usually reported as a _________

Purity; range

Impure solid is dissolved in hot solvent then __________ back as a purified material by ________ the solution.

Recovered; cooling

What is the purpose of recrystallizing an impure sample

Recrystallization is a commonly used method for purifying a solid

Choose solvent such that impurities are more soluble than the substance being ...

Recrystallized at room temperature

Used gloves

Regular trash

Liquid-liquid extraction

Requires that 2 liquids be immiscible when mixed (Ether and H2O) or (Methylene chloride and H2O) The densities determine which layer is on the top, and which is on the bottom.

As the mobile phase progresses, it carries the components of an injected sample. Components that interact strongly with the stationary phase are ______ _______, while those that have weak interactions move out _____ _______

Retained longer; more quickly

Define Rf

Retention factor - The ratio of the distance traveled by the spot over the distance traveled by the solvent front.

Product of reduction of a ketone using sodium borohydride is:

Secondary alcohol

If oil forms, use

Seed crystal or repeat the experiment

Chromatography methods are used for

Separation of multicomponent mixtures

Chromatography is a method for the _______ and identification of substances (solutes) based upon their differential ______ on a supporting medium (________ phase) under the influence of a flowing medium (________ phase), which may be gas or liquid.

Separation; mobility; stationary; mobile

An effective distillation requires that the components of the solution have ...

Significantly different boiling points (preferably between 30-40* C)

In the solvent extraction experiment, what is the stationary phase used for the TLC analysis?

Silica gel

In thin-layer chromatography, a flat solid support such as a sheet of glass, plastic, or aluminum foil is coated with a thin layer of _____ gel or alumina. The mixture of compounds to be separated is applied as small spots of a solution near one edge of the plate. This edge is immersed in a solvent that acts as the ______ phase, or eluent. The solvent moves upwards through the solid support by______ action.

Silica; mobile; capillary

In this experiment, which compound gets oxidized during the reaction?

Sodium borohydride

Impure aspirin, filter paper

Solid hazardous chemical waste

Where would you dispose the left over aspirin sample after the recrystallization experiment

Solid hazardous chemical waste

Where will you dispose TLC plates after you are done with the solvent extraction experiment?

Solid hazardous chemical waste container

Where would you dispose alumina after completing the chromatography separation?

Solid hazardous chemical waste container

Criteria for solvent selection

Solute should be soluble in the solvent at high temperature and insoluble at room temperature

What rises up the plate by capillary action?

Solvent (mobile phase)

Impurities must remain soluble in the

Solvent at room temperature

Which one is on the top? Organic or aqueous? Why?

Solvents arrange based on densities. Ether is less dense than water, so stays on the top. This arrangement would change is you used chloroform instead of ether, as chloroform is more dense than water

Types of compounds: neutral compounds

Some examples: hydrocarbons, aldehydes, ketones, ethers, amides, esters

Each component in a mixture interacts with (adsorbs on) the _______ phase and ________ in the mobile phase to a different extent

Stationary; dissolves

Separation occurs by distributing components of a mixture between two phases:

The mobile phase (for column chromatography, it is liquid — organic solvents of different polarities) and stationary phase (for column chromatography, it is a solid — silica or alumina)

How does the solvent affect Rf values?

The more polar solvent moves spots further up the plate

What would be the effect of an impurity on the observed melting point of a compound?

The presence of impurity causes depression of melting point. Also, the melting point will occur over a wide range.

What is the purpose of adding hydrochloride acid during the work up of this experiment?

To neutralize the excess sodium borohydride

Recrystallization is used when amount of impurities is not

Too large (usually < 10%)

Why minimum amount of solvent?

Too much solvent will reduce recovery; one key word in recrystallization is minimum!

True or false: the boiling point of the solvent chosen must be lower than the melting point of the solute dissolved in it

True

True or false? A compound with higher boiling point would elute slower in a gas chromatography

True

True or false? The greater the number of theoretical plates the better the separation.

True

If two cycles occur, the distillation has

Two theoretical plates

What will be used to detect compounds once they are spotted and ran on a TLC plate?

Ultra violet lamp will be used to detect compounds once they are spotted and ran on a TLC plate. An ultra violet lamp is used because the compounds are aromatic and able to be seen under a UV light.

Distillation is a technique...

Used for separating a mixture of liquids

What is the purpose of using activated charcoal during recrystallization? How does it help?

Using activated charcoal during recrystallization can remove color impurities. The colored impurities get absorbed on the surface of the charcoal.

If compound is pure, it will melt over a ...

Very narrow reproducible range of temperatures

If compound is pure, it will melt over a ....

Very narrow reproducible range of temperatures ~0.5-2.0°C

After determining solubility,

Weigh 500mg of compound and record exact weight

Dry and weigh purified crystals

Weight of compound = weight of container and compound - weight of container

The phase diagram for hexane and toluene helps with the understanding of the simple distillation of 2 volatile components. What is the x-axis and y-axis?

X-axis: mole fraction of hexane Y-axis: temperature

Will aspirin be water soluble

Yes

A theoretical plate represents

one vaporization/condensation cycle

Melting point range can estimate

purity

Solvent extraction

to separate a mixture containing an acidic, basic, and neutral compound by liquid-liquid solvent extraction

What happens during the development of TLC? (4)

—During development, the same place is partitioned between stationary and mobile phases —Solvent (the mobile phase) rises up the plate by capillary action —Compounds with strong interaction with silica gel (which is polar and Lewis acid) will move up the plate slowly — Compounds with weak interaction with silica gel will move up the plate fast

Day 2 column chromatography

• Evaporate solvents. • Get the mass of the separated components. • Record the melting point ranges.

Day 1 column chromatography

• Obtain the ferrocene and acetylferrocene sample mixture from your TA. • Pack the column as per the instructions. • Separate two components from the mixture, label them properly. • Check the TLC of separated components while they are in solution and store the fractions.