CSULB ochem 223A lab final review

TLC stationary phase

- silica gel, very fine powder; highly polar

- stationary phase more polar than mobile phase

TLC liquid phase

- le solvent

TLC uses

- establish two compounds are identical

- determine # of components in a mixture

- determine the appropriate solvent for a column-

chromatographic separation

- check effectiveness of saperation

- monitor progress of rxn

TLC steps

1. injection: spot the TLC plate; sample applied to plate

2. separation: development or "running" the plate; as the solvent ascends the plate

3. UV light

TLC solvents

1. CH2Cl2 & toulene: good choice of solvents for a wide variety of functional groups

2. hexane & petroleum: good choices for hydrocarbon materials

3. More polar materials may require ethyl acetate, acetone, or methanol

TLC choice of solvent

- solvent that causes spots to move with solvent front is too polar

- solvent that does not create movement in any material is not polar enough

TLC calculate Rf

Rf = (distance traveled by substrate)/(distance traveled by solvent front)

TLC effects of polarity

more polar move more slowly. less polar have higher Rf values.

IR spectroscopy is related to ____

the stretching and bending of bonds in a molecule.

Pure substances A & B have exact melting points Ta and Tb. If they are mixed, the melting point is ____.

What can be judged by the melting point?

Lower in all cases

purity

Polar molecules have a _______ boiling point that nonpolar molecules with similar molar mass.

higher.

In recrystallization, a compound ideally should be ______ at high temperature and ______ at low temperature

soluble at high temp

insoluble at low temp

A mixture of aspirin and acetanilide was dissolved in dichloromethane and extracted with the appropriate aqueous solution to remove the aspirin from the mixture. The remaining dichloromethane solution of acetanilide was washed with saturated aqueous sodium chloride. The purpose of this step is to:

In organic layer a little amount of aspirin would be present. So the residual aspirin will dissolve in water and removed by NaCl solution.

The wavenumber and the energy required to cause bond oscillation are:

Wave number is directly proportional to the energy.

E = hv

The intensity of the absorption peaks in IR spectroscopy is related to the:

Intensity of a ir peak depends on the absorption of it light by particular bond. So more the bond dipole, more light absorption occurred.

Which of the following statements about extraction is false?

A) The extraction solvent should form an emulsion with the aqueous phase

B) The extraction solvent should be immiscible with the aqueous phase

C) The extraction solvent should have a different density than the aqueous phase

D) Multiple extractions are more productive than one extraction

Option A) is the false statement about the extraction. Extraction should not form any emulsion as it complicates the separating procedure by having dispersion of one solvent over another.

7) Among the following, the best solvent for cleaning the IR salt plate is:

A) Water

B) Ethanol

C) Dichloromethane

D) Dimethylsulfoxide

Option C) Dichloromethane is the best solvent for washing IR salt plate as it is a polar solvent as well as having low boiling point and after the cleaning all the solvents will be evaporated.

Option A and B, water and ethanol are very polar solvents and has the tendency to dissolve the ionic salt (KBr, NaBr) IR plates, so should not be used for cleaning. Option D, even though very polar solvent, having a very high boiling point, difficult to remove the solvent after cleaning, so should not be used for IR plate cleaning process.

8) Which of the following procedure is incorrect when you crystallize a solid compound?

A) Using as much as solvent as possible to make a dilute solution

B) Using a solvent that dissolves the solid at room temperature

C) Using a solvent that does not dissolve the solid at boiling temperature

D) Heating up the flask containing solid fast before introducing solvent

E) All of these

All of these options are incorrect. When we crystallize a solid, we need to use minimum of solvent and also need to choose proper solvent so that the compound should not dissolve at room temperature and should be soluble after heating the solution.

Lower the _____, the lower the retention time of compound in GC

Boiling point.

It is likely to spend more time in the gas phase

Compound X was dissolved in water for recrystallization. The solubility of compound X was determined as shown in the following table.

temp 0C 25C 50C 100C

Solubility(Xg / 100mL H2O) 0.05 0.335 1.0 3.5

The minimum volume of water that you should use to recrystallize 4.55 g of compound X is:

100 mL of water ---> 3.5g

X? of water ---> 4.5g

X = (4.5g * 100mL)/(3.5g)

X = 130mL volume of water

Given the solubility data below (in g of solute per 100 mL of solvent), which solvent would be the best choice for recrystallizing Compound Y?

Solvent Hexane H2O EtOH CH2Cl2

Solubility(cold) 0.5 0.5 0.1 0.2

Solubility(Hot) 0.8 3.0 6.0 1.0

A) Hexane

B) Ethanol

C) Dichloromethane

D) Water

The solubility of compound X in EtOH at High temp is high and its solubility at low temp is way less.

So loss of yield with crystallization is less. So EtOH is best choice

An organic compound X was dissolved in water for recrystallization. The solubility of compound X in water at various temperatures is listed in the following table:

Temp 0C 25C 50C 100C

Solubility(Xg / 100mL water) 0.25 0.335 1.0 4.20

3.50g X was dissolved in 120 ml water. What the maximum amount of pure crystallized product, X, recovered after cooling in ice water?

1) The solubility of X at 0oC = 0.250 g/100 mL. The solubility of X at 0oC in 120 mL = (0.250/100 )120 = 0.300g.

Total amount dissolved in 120 mL = 3.500g

Thus maximum amount of compound which can be recovered at 0oC is 3.50g - 0.300g = 3.200g

((double check answer))

Vacuum filtration is the correct technique for which of the following?

A) removal of decolorizing carbon

B) removal of drying agent

C) isolation of crystals

D) all of the above

...

A student left a mixture of alkyl halides (RCl and RBr) from the competing nucleophiles experiment in an uncapped vial for 30 minutes. The halide composition will change such that:

A) The amount of RCl will increase relative to RBr

B) The amount of RCl will decrease relative to RBr

C) The proportion of RCl and RBr will remain constant

D) The amount of RBr will decrease relative to RCl

Bromine is a better leaving group compared chlorine in nucleophilic substitution reactions.

So if we left the mixture which is uncapped, slowly the bromine groups are replaced with another groups.

So the amount RBr will becressed relative to RCl.

2-Butanol can be converted to a mixture of trans-2-butene, cis-2-butene, and 1-butene by dehydration with a mixture of H3PO4 and H2SO4. What is the order of elution of the products in a gas chromatogram of this reaction?

Elution order in gas-liquid chromatography depends on two factors: the boiling point of the solutes, and the interaction between the solutes and the stationary phase.

They don't give boiling point but you can determine BP from polarity. More polar typically means higher BP. So the order is trans-2-butene will elite first followed by 1-butene then cis butene then 2-butanol will elute last

.....

In a certain TLC experiment the following data were obtained. (All measurements listed are from the bottom of the plate). What is the Rf for spot A?

Original Solvent depth: 1.00 cm

Final Solvent front: 8.00 cm

Original Position of spots: 2.00 cm

Final Position of spot A: 5.00 cm

Distance traveled by solvent = 8-2cm = 6cm

Distance traveled by A = 5-2cm = 3cm

Rf = 3/6 = 0.5

What pieces of laboratory equipment are needed to perform a simple distillation?

A ring stand, clamps, round bottom flask, distillation head, condenser, heating mantle and receiving flask

Two organic solid compounds, A and B are observed to have the same melting point. What should be done to prove that these substances are the same compound?

A) look up the literature m.p. values of compound A and B

B) recrystallize compound A and re-take melting point

C) dissolve compound B and take boing point

D) perform a mixed melting point of compounds A and B

B

a. no bc it still leaves you in the confusion that why are they different

b. taking melting point may correct any previous errors

c. Boiling point will not tell about the comparison with A

d. It will again be nearby the two values already known, hence will not reassure anything new

10

WIP

The percent yield of an experiment is influenced by several unavoidable systematic losses. Which of the following is not a potential systematic loss?

A) Material stuck to a piece of filter paper

B) Partial solubility of a compound in a recrystallization solvent

C) The pot reside left behind when performing a distillation

D) Material used for melting points or other analysis

D

Which of the following will lead to an inaccurate value of the melting point?

A) Sample is wet

B) The sample formed large crystals

C) The rate of heating on the melting point apparatus is increasing at 20°C/minute.

D) A and C

E) A and B

E) A and B

bc to measure the melting point of a compound, the compound should be pure means crystal form and heating of sample rate should be low then only we get an accurate measurement

Plate A" below, represents the TLC chromatogram of an unknown compound developed in hexane. The same compound was then spotted on a longer TLC plate and again run in hexane. The longer TLC plate that most closely resembles what you would predict from this experiment is:

A) Not enough information provided

B) Plate B

C) Plate C

D) Plate D

https://www.chegg.com/homework-help/questions-and-answers/plate-represents-tlc-chromatogram-compound-run-hexanes-compound-spotted-large-tlc-plate-ru-q12280657

Plate C

- If we run the same spot on the longer TLC then we have to allow elution of TLC for more time.

- Spot will move further than the shorter plate

- Option B eliminated

- Option D has spot near solvent front

Suppose caffeine has a partition coefficient of 2.5 between dichloromethane and water (2.5/1 = dichloromethane/water). You are given a solution of 10 grams of caffeine dissolved in 100 mL water. You extract your caffeine solution 1 x 100 mL dichloromethane, and evaporate your dichloromethane layer. How much caffeine would you expect to isolate?

partition coefficient = 2.5

"how much x grams in 100mL"

2.5 = [ (x/100) ] / [ (10-x)/100 ] = [ (x/100) / (100/ (10-x) ] = [x/(10-x)]

2.5 = [x/(10-x)]

x = 25 - 2.5x --> 3.5x = 25

x = 7.14g

For Questions 28 - 30, you synthesized acetylsalicylic acid (aspirin)

Table:

Reagent Mol Weight Density (g/mL)

Salicylic acid 138.1g/mol Solid

Acetic Anhydride 102.1 1.08

Sulfuric Acid 98.1 1.84

28) The molecular weight of acetylsalicylic acid is 180.15. Calculate the theoretical yield if you started with 3.12 grams of salicylic acid.

29) What is the percent yield if you isolated 2.5 grams of acetylsalicylic acid from this reaction, starting with 3.12 grams of salicylic acid?

30) When this reaction is carried out, the actual yield (best yield you can get) is 80%. Calculate how much acetic anhydride and salicylic acid you would need to make 4 grams of acetylsalicylic acid, considering a maximum yield of 80%.

28) mol of salicylic acid = 3.12g/138.1g/mol = 0.022 mol

molar weight of aspirin = 180.15

180.15g/mol * 0.0226 = 4.07g of acetylsalicylic acid

29) 2.5g of acetylsalicylic acid: (2.5) (1/4.07) 100% = 61.42%

or

(2.5g / 180g/mol) = 0.014 && (3.12g / 138.1 g/mol) = 0.022mol

0.014/0.022 ~ 61% (rounded to much got 62 to 63%)

30) % yield = 4g

theoretical = 4g / 80% = 5g

theotetical in mol = 5/180.16 = 0.0277 mol

Acetic Anhydride: (0.0277 mol)(102g/mol)(mL/1.08g) = 2.6mL

Salicylic acid: (0.0277 mol)(138.1g/mol) = 3.8g

1) A mixture of biphenyl, benzyl alcohol, and benzoicacid is spotted on a silica gel TLC plate and developed using Ethyl Acetate with 0.5% acetic acid as the solvent. Predict the relative Rf(highest to lowest)

- Less polar have higher Rf values

Biphenyl > benzy alc > benzoic acid

If you ran an unknown sample on a TLC and observed only one spot with a Rf value of 0.95, is it safe to assume that it is a pure compound? If not, how can you check it out using TLC ?

0.95 is a bad TLC. Rf should be around 0.5 to interpret it correctly. Such high Rf can mislead. Use less polar solvent to extract more accurate info

A mixture of 1-hexanol, hexane, and 1-chlorohexane is injected on a gas chromatography column.

a) Predict the sequence of retention times indicating which compound will have the shortest, middle, and longest retention time

b) The sample vial of (a) was not carefully sealed and left partially open for a couple hours. A GC is taken again with only two peaks realized. The first peak to appear has an area of 24 mm2and the second peak 40 mm2. What mole% of each compound is most likely present?

The retention time depends on the following factors:

(1) Higher the intermolecular forces present in the molecule higher is the retention time.

(2) Among the three compounds hexane being a linear chain alkane has only weak vanderwaals forces of attraction. Its molecular weight is less than 1- hexanol and 1-chlorohexane. So, hexane is the most volatile ( lowest boiling point) and it runs faster in GC column. So, hexane has the lowest retention time.

(3) Between 1-hexanol and 1-chlorohexane, 1- hexanol has the strongest intermolecular forces ( H-bonding). Hence, 1-hexanol retains in the column for longer time than 1- chlorohexane. The retention time of 1- chloro hexane is less than 1- hexanol.

hexane<1-chlorohexane<1-hexanol

A mixture of 1-hexanol, hexane, and 1-chlorohexane is injected on a gas chromatography column.

b) The sample vial of (a) was not carefully sealed and left partially open for a couple hours. A GC is taken again with only two peaks realized. The first peak to appear has an area of 24 mm2and the second peak 40 mm2. What mole% of each compound is most likely present?

(b) If you keep the vial open, the most volatile compound evaporates completely and there is no peak for the compound in the GC spectra.

As hexane is the most volatile compound, the peak for hexane disappears.

The two peaks are for 1-chlorohexane and 1- hexanol. The first peak (with lower retention time) is for 1 - chlorohexane (24 mm2). Second peak (with higher retention time) is for 1- hexanol (40 mm2)

Mole percent of 1-hexanol = (40 * 100/ (40+24)) = 62.5%

Mole percent of 1- chlorohexane = (100-62.5)% = 37.5%

1. The table below shows the solubility data (in g of solute per 100 mL solvent) for compound X in several different solvents

Solvent water EtOH Toulene

Solubility(0C) 2.2 4.2 0.45

Solubility(hot) 2.5 8.3 5.0

How much of that solvent would you need to recrystallize 3.3 g of the compound X ?

When the solutionis cooled down to 0°C during the recrystallization process, how many grams will NOTbe recovered? Where is the missing compound X?

for m = 3.3 g of compound...

(5-0.45) = 4.55 g will precipitate

4.55 g --> 100 mL

3.3 --> x mL

x = 3.3/4.55*100 = 72.52 mL are required at hot temperature and at 0°C in order to form 3.3 g

we will NOT recover:

72.52 mL * 0.45 g / 100 mL = 0.32634 g

since those will be in solution

The solvent pairs below are immiscible. Circle the solvent that would be on top.

(a)Carbon tetrachloride and water

(b)Diethyl ether and water

a. density of CCl4 = 1.59g/cm^3

density of H2O = 1g/cm^3

H2O takes place on top

b. diethyl ether density is lower than water so it takes place on the top

In the IR spectrography graph, what is the y-axis

The y-axis is % transmittance•100% transmittance.

It means that all the light shone on the sample is transmitted and none is absorbed•0% transmittance means that none of the shone light on the sample is transmitted and all is absorbed

The_______ the change in dipole moment, the more ______ the absorption

The GREATER the change in dipole moment, the more INTENSE the absorption

The different types of stretching and bending vibrations induced by the absorption of infrared energy

- symmetric vibration

- asymmetric vibration

- stretching vibration

- in-plane and out-plane stretching

IR amines

•1o amines give two peaks

•2o amines give one peak

•3o amine have no N-H bonds and do not absorb in this region

IR ether/ester stretches

1000-1300 strong sword

ester:) will have a C=O stretch of course