Ochem midterm

Match each compounds to its role in today's reaction. 1.tert-Butyl chloride 2.Calcium chloride 3.water 4.tert-Butyl alcohol 5.Sodium bicarbonate 6.Hydrochloric acid

1.Organic product 2.Drying reagent 3.Washing reagent 4.Organic starting material 5.Base 6.Inorganic starting material

After running a TLC you visualized the TLC plate and observed the distance travelled by the organic compound is 10.0cm and calculated the retention factor (Rf) of that organic compound is 0.83. What is the distance travelled by the eluent?

12.0cm

Using the following whole number atomic weights, what is the whole number formula weight of the compound that will be used in the crystallization experiment. H = 1, C=12, N=14, O=16

122

A hydrocarbon with formula C4H10 has a boiling point of 0 °C. Another hydrocarbon with formula C6H14 has a boiling point of 69 °C. What would you expect the boiling point of C8H18 to be?

125 °C

In your SN1 reaction, you took 5.5mL tert-butyl alcohol and 17mL hydrochloric acid to get a 60% yield of tert-butyl chloride. How much tert-butyl chloride did you get?

3.75 mL

You may have noticed that % composition vs Melting Point graphs are given in % Mole not % Weight. This is because melting point depression is a colligative property (review in your general chemistry text) that depends on the moles of impurity, the same number of moles of impurity of two different compounds will cause the same melting point depression! Before the advent of mass spectrometry, this was one of the techniques used to determine the molecular weight of compounds. Using the melting point depression constant of camphor given in your text (38.5 °C kg/mole), what is the molecular weight (g/mole) of a pure compound that causes an 18 °C melting point depression in the melting point of camphor when 42.284 g of the compound is mixed with 280 g of camphor? Give a whole number answer with no decimal point or units.

323

Which sample would you predict to have the broadest melting range?

50% 2-Naphthol + 50% Benzoic Acid

The following figure shows a temperature vs. % composition diagram for a mixture of acetone and ethyl acetate at 760 mmHg. You have performed a fractional distillation of a 50/50 mixture of acetone and ethyl acetate. Assuming you have achieved three vaporization-condensation cycles in your fractional distillation, answer the following questions: (i) At what temperature the distillate will boil and (ii) what is the % composition of the mixture in the distillate?

57.5 °C and (ii) 94 mol% acetone + 6 mol% ethyl acetate

On the top of Rocky Mountain, the atmospheric pressure is 650 mm Hg. Considering the boiling point of Methanol is 65 °C, what would be the boiling point of Methanol on the top of Rocky Mountain?

60

The vapor pressure of pure cyclohexane is 125 mmHg and the vapor pressure of pure toluene is 40 mmHg at 32 °C. Calculate the vapor pressure of a solution of 0.6 mol fraction of toluene and 0.4 mol fraction of cyclohexane at 32 °C and determine whether the solution will boil?

74 mmHg and the solution will NOT boil

Let's assume the distribution coefficient of caffeine between dichloromethane and water is 3. You have a 10mL aqueous solution of caffeine that contains 100mg of caffeine dissolved in it. You also have 10mL dichloromethane. If you do just one extraction using 10mL dichloromethane, how much caffeine you can recover from the aqueous solution?

75mg

If Solvent A gave a compound an Rf = 0.35 and solvent B gave the same compound an Rf = 0.96, which statement is more true

Solvent A is less polar than solvent B

What is the name of the phase transition that occurs when a solid is converted directly into a gas without going through the liquid phase?

Sublimation

You are running TLC for the caffeine and salicylamide in 30/70 solvent mixture of ethylacetate and hexane. As described in the textbook about the polarity trend of these two compounds, which of the following TLC plates does make sense to you?

TLC plate # 2

In a SN2 reaction of an alcohol with hydrochloric acid, what would be the most unstable, highest energy intermediate?

A transition state where chloride attacks the protonated alcohol from the backside

Which of these solvents will be in the lower layer of a liquid-liquid extraction with water? (Negative credits will be applied to incorrect answers. Select 2 correct answers to get full credits.)

Dichloromethane Chloroform

During the tea-brewing step, you have added sodium carbonate. Sodium carbonate reacts with which of the following compounds?

During the tea-brewing step, you have added sodium carbonate. Sodium carbonate reacts with which of the following compounds?

A SN1 reaction is associated with which of the following types of side reaction?

E1 elimination

Tetrahydrofuran (THF) is an example of what kind of solvent? (Negative credits will be applied to incorrect answers. Select 2 correct answers to get full credits.)

Moderately polar Aprotic

Which of the following statements is true if the solvent exceeds the top of the TLC plate?

The Rf calculation will not be accurate.

A very common problem in crystallization experiment that some of you might encounter is difficulty in forming benzoic acid crystals from the filtrate. What are the most common reasons for poor crystallization? (Negative credits will be applied to incorrect answers. Select 3 correct answers to get full credits.)

The concentration of benzoic acid in filtrate is too low to form crystals. You threw away a lot of benzoic acid crystals on the filter paper during gravity filtration. You added too much water when making benzoic acid solution and/or during gravity filtration.

Why vacuum and not gravity filtration was used for collecting the purified crystals at the end?

The crystals need to be completely separated and dried from the mother liquor.

Which of the following statement are correct for a temperature vs. composition diagram of a mixture of A and B? (Negative credits will be applied to incorrect answers. Select 2 correct answers to get full credits.) The diagram is as follows:

The eutectic mixture of A and B is 60 mol% A + 40 mol% B. The eutectic point of A and B is 62 °C.

What type of reaction you are going to perform today?

substitution

Which of the following materials are commonly used as stationary phase in thin layer chromatography? (Negative credits will be applied to incorrect answer. Select 2 correct answers to get full credits.)

silica gel alumina

What is the hybridization of the central carbon (color coded in blue) in tert-butyl carbocation intermediate? See the figure below.

sp2

The most important reason for adding boiling chips to a liquid is

to ensure even boiling and prevent boiling over

Why gravity filtration of hot benzoic acid solution is required?

to remove insoluble impurities

You were given samples of toluene, benzyl alcohol, benzoic acid and benzaldehyde. You spotted these 4 compounds on a silica gel TLC plate and run that plate in dichloromethane solvent. Predict the order of elution for these 4 compounds in terms of Rf. (Hint: Figure out the polarity order of these 4 compounds)

toluene > benzaldehyde > benzyl alcohol > benzoic acid

Tea bags contain cellulose, caffeine and tannin. How are you going to remove cellulose in your caffeine extraction experiment?

vacuum filtration

The melting point is the temperature at which a solid and its liquid have the same___

vapor pressure

What is the name of the solvent that will be used in the crystallization experiment? One word answer, lower case.

water

Which solvent will be used for today's crystallization experiment?

water

During the crystallization experiment, if too much solvent is added, the solution will ____(1)____. In that case, solvent needs to be ____(2)____ by ____(3)____ the solution until the solution is ____(4)____. Fill the blanks with appropriate words.

(1) not be saturated (2) evaporated (3) heating (4) saturated

In case of minor accidents which may require medical treatment, we must always call which number?

(915) 747-5611 (Campus Police)

The following figure shows a temperature vs. % composition diagram for a mixture of acetone and ethyl acetate at 760 mmHg. You have performed a simple distillation of a 50/50 mixture of acetone and ethyl acetate. (i) At what temperature the distillate will boil and (ii) what is the % composition of the mixture in the distillate?

(i) 63.5 °C and (ii) 75 mol% acetone + 25 mol% ethyl acetate

Calculate the mole fraction of each compound in a mixture containing 15g of cylclohexane and 5g of toluene.

0.77 mole fraction of cyclohexane and 0.23 mole fraction of toluene

Match the steps for this lab experiment in correct order: 1.Making hot benzoic acid solution 2.Gravity filtration of hot benzoic acid solution 3.Wait for crystallization 4.Vacuum filtration of benzoic acid crystals

1 2 3 4

Match the following materials to its proper waste disposal treatment: 1- Paper towel that you used to dry your hands after washing 2- You spilled some benzoic acid and need to dispose it 3- Some unused benzaldehyde from your laboratory experiment 4- Saturated sodium bicarbonate aqueous solution portion after a liquid-liquid extraction 5- You accidentally broke a round-bottom flask and need to dispose the broken glasses 6- The NMR tube that you used to take NMR spectrum of your synthesized compound 7- Some unused dichloromethane from your laboratory experiment

1- Trash can 2- solid waste container 3- non halogenated waste 4- aqueous waste 5- broken glass waste 6- you should not dispose it 7- halogenated waste

Match a compound or mixture on the left to its melting point in Celsius on the right. (Perfect Match) 1. water 2. ethylene glycol 3.Eutectic point temperature of water and ethylene glycol. A mixture called anti-freeze, used in car radiators 4.Eutectic point temperature of water and sodium chloride. The temperature of most freezers, the old fashion way of making ice cream, the reason why salt is spread on roadways when it is cold, and why the center of an ice cube made in an ice tray is cloudy.

1. 0 2. -13 3. -60 4. -20

Predict the melting points of the following compounds: 1.C6H6 2.C10H8 3.C14H10 4.C18H12

1. 5 °C 2. 80 °C 3. 215 °C 4. 355 °C

Thin layer chromatography (TLC) falls in to what type of chromatography?

liquid chromatography

You may have noticed that % composition vs Melting Point graphs are given in % Mole not % Weight. This is because melting point depression is a colligative property (review in your general chemistry text) that depends on the moles of impurity, the same number of moles of impurity of two different compounds will cause the same melting point depression! Before the advent of mass spectrometry this was one of the techniques used to determine the molecular weight of compounds. Using the melting point depression constant of camphor given in your text (38.5 °C kg/mole), what is the molecular weight (g/mole) of a pure compound that causes a 1 °C melting point depression in the melting point of camphor when 1.392 g of the compound is mixed with 285 g of camphor? Give a whole number answer with no decimal point or units.

188

At 50 °C, the vapor pressure of methanol and water are 400 torr and 92 torr, respectively. Given a mixture at 50 °C that contains 10mL of methanol and 5mL of water, calculate the partial pressure of methanol and water.

188 torr for methanol and 49 torr for water

Fill the green box with proper number so that the following equation is fully balanced.

2

In a SN1 reaction between 2-methyl-2-butanol and hydrochloric acid, common side products are alkenes, formed via an E1 elimination. Which of the following alkenes are the most possible side products in this reaction? (Negative credits will be applied to incorrect answers. Select 2 correct answers to get full credits.)

2-Methyl-1-butene 2-Methyl-2-butene

Which of the following alcohols would be best suitable for SN1 type substitution reaction with hydrochloric acid?

2-Methyl-2-butanol

In a SN1 reaction between tert-butyl alcohol and hydrochloric acid, a common side product is an alkene, formed via an E1 elimination. Which of the following alkenes is the most possible side product in this reaction?

2-Methylpropene

If you measured out 0.508 g of impure benzoic acid but recovered only 0.112 g after recrystallization, what is your percent recovery?

22

Let's assume the distribution coefficient of caffeine between dichloromethane and water is 3. You have a 10mL aqueous solution of caffeine that contains 100mg of caffeine dissolved in it. You also have 10mL dichloromethane. Instead of doing just one extraction, you decided to do two extractions with 5mL portions of dichloromethane. How much caffeine you can recover from the aqueous solution?

84mg

The solubility of compound X in water is 0.53g/100mL at 0°C and 5.5g/100mL at 100 °C. What would be the maximum theoretical % recovery from recrystallization of 5g of solid X from 100mL water assuming the water is chilled to 0°C?

89.4

In a SN1 reaction of an alcohol and hydrochloric acid, what would be the most unstable, highest energy intermediate?

A carbocation

What is the molecular formula of the compound that we will use in the crystallization experiment? Use the Hill method with proper capitalization.

C7H6O2

According to the polarity trend described in your textbook about the components in common analgesic, which of the following are true. (Negative credits will be applied to incorrect answers. Select all the correct answers to get full credits.)

Caffeine is the most polar component. Phenacetin is more polar than salicylamide.

What activities can endanger your laboratory environment? (Negative credits will be applied to incorrect answers. Select all the correct answers to get full credits.)

Entering lab if you don't see your TA in the lab Entering lab without long pant Entering lab without lab coat Entering lab without safety goggles Not asking your TA if you are unsure of your waste treatment Pouring down a clear, water-like liquid to the sink without knowing the identity of the liquid Walking around the lab wearing a flip flop Pipetting any liquid by your mouth Leaving the chemical bottle uncapped in the balance room Quickly throwing away the broken glass after breaking an glassware, without telling your TA Smelling your product as it smells good Using a round bottom flask with a visible crack for a high heat refluxing experiment Following your TA's instructions even if you don't understand fully

Which pair of solvents would NOT be good for liquid-liquid extraction?

Ethyl acetate / Acetone

Which of the following statements are true about the eutectic point on a two component (compounds A and B) phase diagram? (Negative credits will be applied to incorrect answers. Select 2 correct answers to get full credits.)

Eutectic point is lower than the melting points of either of the individual compounds. Melting point depression is maximum at eutectic point.

You will be turned away from the lab and receive a zero in attendance for missing all but one item

Experimental Organic Chemistry A Small-Scale Approach, 2nd Edition, Wilcox & Wilcox

A solid will sublime if the vapor pressure of the solid is lower than the atmospheric pressure.

False

Crystallization is another word for 'freezing point' because the substance goes from liquid to solid.

False

Each condensation-vaporization cycle will yield a purer solution of the less volatile component.

False

In TLC, it is important that the solvent front would reach the top of the TLC plate quicker. And that is why it is also important that the solvent level in the TLC chamber must be above the location of the spot on the TLC plate as shown in the following figure.

False

It is only necessary to read the experimental section of the textbook and not the full chapter for background information.

False

Rapid heating is more efficient than slow heating when you are trying to achieve separation of a mixture of two liquids with close boiling points.

False

Since the melting points are the same for benzoic acid and 2-naphthol, there will not be any melting point depression if the two are mixed.

False

The Rf value of compound A is 0.35 when TLC plate is developed in hexane and 0.45 when the plate is developed in diethyl ether. Compound B has an Rf value of 0.45 in hexane and 0.75 in diethyl ether. Hexane would be better solvent choice for separating a mixture of compounds A and B by TLC.

False

The sublimation temperature of any organic solid will be higher than the melting point under reduced pressure.

False

Which item is missing from our lab? A copy of the Safety Agreement is linked on the main page.

Fire blanket

The following figure is a typical crystallization set-up of benzoic acid. In which container the benzoic acid crystals will be formed?

Flask

Which of the following phenolic compounds would you expect to have the lowest Rf value (Solvent: 90:10 Dichloromethane:Methanol).

Gallic acid

In recrystallization of benzoic acid, if you add benzoic acid to room-temperature water, it does not dissolve. You have to heat the mixture to make a benzoic acid solution. Why you need to heat to dissolve benzoic acid in to water?

Heat increases molecular movements in water which helps to make a solution.

What effects do impurities have on the melting-point range of an organic compound? (Negative credits will be applied to incorrect answers. Select 2 correct answers to get full credits.)

It broadens the melting point range. It lowers the melting point range.

In crystallization experiment, which method you will use in the laboratory to determine the purity of the final benzoic acid crystals?

Melting point determination

Melting point of pure ice 0 °C and pure salt is 800 °C. The eutectic mixture of ice and salt contains 77% ice by mass and the eutectic point of this mixture is -21 °C. Which of the following statements are TRUE? (Negative credits will be applied to incorrect answer. Select 2 correct answers to get full credits.)

Melting point of a 50:50 salt and ice mixture is in between -21 °C and 800 °C Melting point of a 10:90 salt and ice mixture is in between 0 °C and -21 °C

Why are melting points reported as a range of temperatures instead of a specific temperature?

Melting points are measured by determining when melting first begins and when the compound is finally fully melted. Thus a range is reported as melting point.

In extraction of caffeine, you will be adding Na2CO3 to tea bags in boiling water to perform the extraction. If you do not find Na2CO3 in your lab, which alternate chemicals you can use to do the extraction without affecting the outcome of the result? Hint: Think about the role Na2CO3 plays in the extraction process.

NaHCO3 CaCO3

We know that both melting point and boiling point increases with increase in atmospheric pressure. For a particular compound, do you think the rate of increase of melting point will be same as the rate of increase of boiling point when atmospheric pressure increases?

No

If you were asked to purify your reaction product via distillation, what should be temperature range that you would apply for collecting your purest fraction?

None of the above

Which of the following compounds is NOT a component of common analgesics?

None of the above

Two universities purchase the same reagent from the same company and the same batch number (production cycle). A student from each university takes the melting point but the results do not match! Which is probably the least likely reason why this happened?

One measurement is done in El Paso, the other in Corpus Christi

Why are polar solvents preferred in SN1 reactions?

Polar solvents stabilize the carbocation intermediate, thus increases the rate of the reaction.

Which of the following chemicals can NOT be used as drying agent in your reaction?

Potassium hydroxide

When the solid and liquid phase are in equilibrium, the temperature

Remains constant

'Backside attack' is a term associated with which of the following types of reaction mechanism?

SN2 Substitution

You will be doing several types extraction in caffeine experiment. Which of the following types of extraction will NOT be performed?

Solid phase extraction

TLC is often used in organic lab to monitor organic reaction. By analyzing the Rf of the reaction mixture compare to the starting materials, one can make sure whether the reaction is complete. Here is a TLC plate of a NaBH4 reduction reaction of benzophenone after 15 min (shown below). Looking at the TLC plate shown below, should you conclude that the reaction is complete?

The reaction is not complete.

What are the properties of a good crystallization solvent? (Negative credits will be applied to incorrect answers. Select 4 correct answers to get full credits.)

The solvent must be inexpensive. The solvent should be inert. The solvent should have a low boiling point. The solvent should dissolve the compound at boiling temperature.

What is missing from the following distillation set-up?

Thermometer adapter

When you were asked to create the melting point capillary loaded with the sample, you made sure that the sample is finely powdered. Why do you think the sample needs to be finely powdered? (Negative credits will be applied to incorrect answers. Select 2 correct answers to get full credits.)

To prevent the formation of air pockets and uneven heating of the sample To get a very sharp melting point range

A densely packed fractionating column can achieve better separation of two liquids with close boiling points than a loosely packed fractionating column.

True

Caffeine is more soluble in dichloromethane than water.

True

Learning organic chemistry with chemical structure drawings and not just words (names) is required for this course.

True

Never add boiling chips to a liquid which is already hot and boiling.

True

The vapor pressure of a liquid can NOT be greater than 1 atm.

True

You are trying to extract an organic compound which is dissolved in 10mL water. You have 30mL diethyl ether. Three multiple extractions with 10mL portions of ether is more efficient than just one extraction with 30mL diethyl ether.

True

the overall yield of this reaction will be affected by how fast the product is washed with aqueous reagents because this reaction is reversible.

True

Which of the following methods are commonly used to visualize the TLC plates after an elution if the organic compounds are colorless? (Negative credits will be applied to incorrect answer. Select 4 correct answers to get full credits.)

UV light Iodine chamber para-Anisaldehyde solution Potassium permanganate solution

While developing of the TLC plate, you have observed that your solvent front is uneven. What are the most common reasons for this problem? (Negative credits will be applied to incorrect answer. Select 3 correct answers to get full credits.)

You dropped your plate rather than carefully placing it in the TLC chamber, causing splash on the plate. You moved the TLC chamber while you were developing the plate. Your TLC plate is unevenly cut at the bottom where you spotted the compounds.

After you run your TLC plate, you observed long smears on the plate instead of discrete spots as shown below. What is the most common reason for this problem?

You overloaded the spots as the concentration of the compound is too high.

Which of the following solvents do you think is the most polar?

acetic acid

What is the name of the compound that will be used in the crystallization experiment (section 8.4B)? Use all lower case letters.

benzoic acid

Chromatography operates on the principle that some components will be more strongly interacting to the stationary phase than other components. Which of the following types of interaction is NOT responsible for chromatographic separation?

covalent interaction

Which of the following components is NOT required in a simple distillation set-up?

fractionating column

The identity of an unknown compound can be confirmed by...

mixing the unknown with the suspected known compound and measuring a sharp melting range

In a crystallization procedure, the liquid that is left over after the crystals are filtered off is called

mother liquor

Which of the following compound pairs could NOT be separated by simple distillation?

n-butanol and butyl acetate

What are the units for Retention factor (Rf)?

none of the above

Fractional distillation is more suitable and needed than simple distillation for some purification processes. Which of the following separation processes must need fractional distillation?

obtaining gasoline from crude oil

The major difference between a melting point tube and a capillary tube is:

one has both ends open and the other only has one end open

Which of the following is NOT a step in a process of distillation?

precipitation

Complete solubility of a solid in a solvent depends on: (Negative credits will be applied to incorrect answers. Select all the correct answers to get full credits.)

temperature of the solvent polarity of the solid polarity of the solvent dielectric constant of the solvent amount of the solid amount of the solvent

In your SN1 reaction between tert-butyl alcohol and hydrochloric acid that you have performed in the lab, what is the limiting reagent?

tert-Butyl alcohol

In a SN1 reaction of alkyl halide, which of the following is the correct order for relative reactivity?

tertiary > secondary > primary > methyl

Used melting point tubes are to be placed in ... (Select all that apply)

the broken glass container sharps container