CHEM 43A Final(QUES)

Which functional group can give a false positive for the Jones test? A. 3° alchohol B. Ketone C. Phenols D. Aldehyde

D. Aldehyde

If you separated a mixture of benzene, toluene, and m-xylene by gas chromatography, what would be the expected order of retention times?

(Shortest retention time)Benzene--Toluene--m-Xylene(Longest retention)

Water was found to be a good solvent for the recrystallization of organic substance X. (a) Which line correctly shows the solubility vs. temperature profile for this substance? - D - B - A - C Suppose 0.1 g of X and 1.0 mL of water were mixed and heated to 80 °C. Would all of substance X dissolve? - No - Yes The solution prepared in (b) is cooled. At what temperature will crystals of X appear? Suppose the cooling described in (c) were continued to 0 °C. How many grams of X would come out of solution?

(a) The correct line is B. Water is a good solvent choice for the recrystallization of X because X has a low solubility in water at low temperatures, but a high solubility in water at high temperatures. (b) All of the substance X would dissolve at 80 °C. A solubility of 17.0 g in 100 mL of water is equivalent to a solubility of 0.17 g in 1 mL of water. Since there is only 0.1 g of X, this is a greater solubility than is required. (c) Crystals will start to form when the solubility is reduced to 0.1 g/mL (equivalent to 10 g/100 mL on the graph). Therefore, crystals of X should appear around 56 °C. (d) The recovery of X would amount to 0.085 g. A solubility of 1.5 g in 100 mL of water at 0 °C is equivalent to a solubility of 0.015 g in 1 mL of water. Therefore, 0.015 g of X would remain dissolved in the water, with the remainder being formed as crystals.

What is the significance of the horizontal line between points X and Y? - A boiling liquid with composition X will produce vapor with composition Y. - The point X gives the vapor composition of the less volatile component and Y gives the vapor composition of the more volatile component. The line gives the difference. - A boiling liquid with composition Y will produce a vapor with composition X. - The length of the line XY corresponds to the difference in boiling point of components A and B.

- A boiling liquid with composition X will produce vapor with composition Y. EXPLANATION: A boiling liquid with composition X will produce vapor with composition Y.

Which of the following statements is true when considering temperature during a recrystallization?(More than one answer may be correct.) - Ideally the hot solvent used will keep the impurities dissolved OR not dissolve them at all. - Enough cold solvent should be added to dissolve the sample BEFORE heating it past its boiling point. - Using ice to cool the sample will speed up the crystallization process so that large crystals will be formed. - It is important that the sample is dissolved in just enough hot solvent.

- Ideally the hot solvent used will keep the impurities dissolved OR not dissolve them at all. - It is important that the sample is dissolved in just enough hot solvent. EXPLANATION: If too much solvent is added, it may not be possible to retrieve the entire sample as it will have a degree of solubility in the cold solvent. If the impurities remain in solution, they can be washed away. If they remain undissolved, they can be filtered off before recrystallization.

Solvents: Which of the following are hazards associated with dichloromethane?(More than one answer may be correct.) - It can dissolve some of the fatty tissues in the skin with prolonged contact, causing chemical burns - Limited evidence of a carcinogenic effect - Harmful by inhalation - Extremely flammable - Corrosive

- It can dissolve some of the fatty tissues in the skin with prolonged contact, causing chemical burns - Limited evidence of a carcinogenic effect - Harmful by inhalation EXPLANATION: Dichloromethane is harmful by inhalation and can dissolve some of the fatty tissues in the skin, causing chemical burns. There is also evidence that suggests it may be carcinogenic (cancer-forming). It is not particularly flammable under ordinary lab conditions; however, you must take care with the vapor, as it is dangerous. Make sure you place the residues into a chlorinated waste container.

SAFETY: Which of the following are advantages of performing experiments at microscale level? (More than one answer may be correct.) - Reduced contact with hazardous materials that may be harmful to health - Reduced production of hazardous and polluting waste - A reduced risk of fire or explosion in the laboratory - Risk-free experiments - No need to wear lab coats or safety goggles - Reduction in the number of laboratory classes

- Reduced contact with hazardous materials that may be harmful to health - Reduced production of hazardous and polluting waste - A reduced risk of fire or explosion in the laboratory EXPLANATION: Microscale experiments use small quantities of chemicals. This reduces the risks involved in handling and heating chemicals; however, strict safety procedures should always be followed, even when using small amounts of chemicals in the laboratory.

SAFETY: Why is it important to handle silica powder in a fume cupboard?(More than one answer may be correct.) - It allows for any spills to be easily contained. - The powder, if inhaled, can cause serious problems to the respiratory tract. - The powder is fine and easily inhaled. - It prevents contamination of the silica powder.

- The powder, if inhaled, can cause serious problems to the respiratory tract. - The powder is fine and easily inhaled. EXPLANATION: Silica powder can be carcinogenic if inhaled. It is a very fine powder that readily forms clouds of dust that are easily inhaled.

What happens if the spots are added below the level of the solvent when preparing a TLC plate for development? - Additional spots will appear on the plate, interfering with analysis. - The spots will saturate the plate, show tailing, and/or diffuse into one another. - The spots will be difficult to see after development. - The spot material will dissolve off the plate.

- The spot material will dissolve off the plate. EXPLANATION: If the spots are placed below the level of the solvent, they will dissolve off the plate and into the solvent.

How long should you run a TLC plate for? - 2 minutes. - Until the solvent front is about half way up. - Until the solvent front is at the top of the plate. - 10 minutes. - Until the solvent front is just below the top of the plate.

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

In a solvent extraction using a separatory funnel, the densities of the two liquids determine whether the organic solvent is the top layer or the bottom layer.Using the table of densities, determine whether dichloromethane is the top layer or the bottom layer when used with water. Solvent Density (g/cm3) Chloroform 1.50 Dichloromethane 1.33 Water 1.00 Ethyl acetate 0.90 Diethyl ether 0.71 - Bottom layer - Top layer

- bottom layer EXPLANATION: Solvents with densities greater than that of water (1.00 g/cm3) are the bottom layer in the separatory funnel (when water is the other solvent). Solvents with densities less than that of water (1.00 g/cm3) are the top layer in the separatory funnel (when water is the other solvent).

Sublimations are generally performed under reduced pressure because: - less heat needs to be supplied for the vapor pressure of the solid to equal that of the external pressure. - solids do not have a vapor pressure at atmospheric pressure. - solids do not melt when heated under reduced pressure. - it prevents the pure product getting contaminated with impurities in the air. - the attractive intermolecular interactions between the molecules in the solid get weaker as the pressure is decreased.

- less heat needs to be supplied for the vapor pressure of the solid to equal that of the external pressure. EXPLANATION: Sublimation occurs when the vapor pressure of a solid equals that of the external pressure; therefore, the lower the external pressure, the more readily a solid will sublime, and the less heat that needs to be supplied. Carrying out sublimation under reduced pressure allows it to be performed at lower temperature. Furthermore, some solids may not sublime at atmospheric pressure, as they will melt when heated.

Sublimation can be used to purify your product if: - your solid product has a relatively high vapor pressure but the impurities have a low vapor pressure. - your product is a volatile liquid containing nonvolatile impurities. - your solid product has a relatively high vapor pressure. - your solid product contains impurities with a very low vapor pressure. - your solid product contains volatile impurities.

- your solid product has a relatively high vapor pressure but the impurities have a low vapor pressure. EXPLANATION: Sublimation is the transition from a solid to gas phase, so your product must be a solid. For purification to be effective, your product should have a relatively high vapor pressure, so that it sublimes at a feasible pressure and temperature. Additionally, the impurities should have significantly lower vapor pressures than your product.

Why might you want to heat a reaction? (More than one answer may be correct.) -To increase the solubility of the reagents. -To evaporate the solvent more quickly. - To decrease the rate of the desired reaction. -To increase the rate of the reaction. -To overcome the activation energy for the reaction.

-To increase the rate of the reaction. - To overcome the activation energy for the reaction. EXPLANATION:In order for some reactions to proceed they require extra energy, which can be supplied in the form of heat, to overcome the activation energy for the reaction. Applying heat also increases the rate of the reaction since the reactive species will have greater kinetic energy, resulting in more successful collisions with the required energy to react. You would not usually want to remove the solvent during the reaction. To make sure this doesn't happen you need to fit the reaction flask with a condenser and check all glass joints are tightly sealed. Adding additional solvent would be preferable to heating a reaction mixture to aid solubility.

Which of the following solvents would be the best to separate a mixture containing benzoic acid, 2,4-dinitrobenzoic acid, and 2,4,6-trinitrobenzoic acid by TLC? (Select all that apply.) - Acetone - Methylene chloride - Hexane - None of the above

-acetone EXPLANATION: Acetone should work well because the mixture is highly polar. An acetone-hexane mixture might work well, too.

Explain this order of elution. Gas chromatography separates compounds depending on their (volatility alone/ polarity and volatility/ polarity alone) . Benzene, m-xylene, and toluene have similar (volatilities/ polarities/ charges), therefore, the main basis for separation is (polarity/ volatility/ charge). The more volatile a component the (lower its solubility/ greater its vapor pressure/ lower its vapor pressure/ greater its solubility) , hence the more time it spends in the (gaseous mobile phase/ liquid stationary phase/ solid stationary phase/ liquid mobile phase), giving it a (longer/shorter) retention time. Therefore, components of a liquid mixture will elute in order of (increasing polarity/ decreasing polarity/ decreasing boiling point/ increasing boiling point)

-polarity and volatility - polarities - volatility - greater its vapor pressure - gaseous mobile phase - shorter -increasing boiling point EXPLANATION: Gas chromatography separates compounds depending on their polarity and volatility. Benzene, m-xylene, and toluene have similar polarities, therefore, the main basis for separation is volatility. The more volatile a component the greater its vapor pressure, hence the more time it spends in the gaseous mobile phase, giving it a shorter retention time. Therefore, components of a liquid mixture will elute in order of increasing boiling point.

Correlate the microscale procedures needed to accomplish the given steps (1-5) to isolate pure isopentyl acetate (banana oil) from the reaction mixture.

1) Aqueous sodium bicarbonate is added to the reaction mixture. This deprotonates unreacted acetic acid, making a water soluble salt. 2)The mixture is stirred, capped and gently shaken, with frequent venting. This ensures that the evolution of carbon dioxide gas is complete. 3) The lower aqueous layer is removed using a Pasteur pipette and discarded. This removes byproducts. 4) The organic layer is dried over granular anhydrous sodium sulfate. This removes water from the product. 5) The dry ester is transferred using a Pasteur pipette to a clean conical vial. This separates the sodium sulfate from the ester.

From the plot of vapor pressures vs temperature above, estimate the boiling point of ethanol when the external pressure is 550 mm Hg.

70 C

Which key reagents would you need to use to distinguish between following two molecules? ( 3 alcohol and 1 alcohol) A. Anhydrous zinc chloride and hydrochloric acid B. Silver nitrate and ammonium hydroxide C. 2,4- dinitrophenyl hydrazine D. Chromic acid

A. Anhydrous zinc chloride and hydrochloric acid

The functional groups in an organic compound can frequently be deduced from its infrared absorption spectrum. A compound contains no nitrogen and exhibits absorption bands at 3300 (s) and 2150 (m) cm-1. Relative absorption intensity: (s)=strong, (m)=medium, (w)=weak. What functional class(es) does the compound belong to? List only classes for which evidence is given here. Attach no significance to evidence not cited explicitly. Do not over-interpret exact absorption band positions. None of your inferences should depend on small differences like 10 to 20 cm-1. The functional class(es) of this compound is(are).(Enter letters from the table below, in any order, with no spaces or commas.) a. alkane (List only if no other functional class applies.) b. alkene h. amine c. terminal alkyne i. aldehyde or ketone d. internal alkyne j. carboxylic acid e. arene k. ester f. alcohol l. nitrile g. ether

C EXPLANATION: 3300 cm-1 is an sp C-H stretch and 2150 cm-1 is a CC triple bond stretch

Which of the following would be expected to form hydrogen bonds with water?Choose all that apply.

EXPLANATION: Hydrogen bonding is usually found in situations represented byD-H---A where both the donor atom, D, and the acceptor atom, A, are one of the highly electronegative elements O, N, or F. The D-H bond is a normal covalent bond, and H---A is a hydrogen bond. Hydrogen bonding is not expected to occur between molecules in which all of the H atoms are covalently bonded to C atoms, even when an acceptor atom is present.However, since water is able to act as both a donor and an acceptor, molecules that have an acceptor atom can form H-bonds with water, even if they are not able to act as H-atom donors. Of the molecules shown, both methyl alcohol and N-ethylpropanamide can act as either a donor or an acceptor in an H-bond with water, while ethyl methyl ketone is able to act as an acceptor. Only cyclopentane is missing both a donor atom and an acceptor atom and cannot form an H-bond with water.

A solid substance has a vapor pressure of 100 mm Hg at the melting point (100 °C). Assuming an atmospheric pressure of 760 mm Hg, describe the behavior of this solid as the temperature is raised from room temperature to its melting point. - The solid will decompose at the melting point. - The solid will melt at a temperature higher than 100 °C. - The solid will sublime instead of melting at 100 °C. - The solid will show a normal solid to liquid transition at 100 °C.

EXPLANATION: Solids sublime if their vapor pressures are greater than atmospheric pressure at their melting points. Here, the vapor pressure of the solid is much lower than atmospheric pressure at the melting point, so the substance melts rather than sublimes.

For each of the following pairs of solutes and solvent, predict whether the solute would be soluble or insoluble.

Malic acid, soluble in water Naphthalene, insoluble in water Amphetamine, soluble in alcohol Aspirin, slightly soluble in water; essentially insoluble Succinic acid, insoluble in hexane Ibuprofen, soluble in diethyl ether 1-Decanol, insoluble in water

Using the phase diagram, determine the molar composition of the vapor in equilibrium with a boiling liquid that has a composition of 60.% A and 40.% B. Percentage vapor composition A =______ % Percentage vapor composition B =_______ % What is the boiling point of this liquid? Boiling point = ______°C

Percentage vapor composition A=90 % Percentage vapor composition B = 10% Boiling point = 105°C

A student is required to perform a crystallization on an impure sample of biphenyl. The sample weighs 0.5 g, and contains about 5% impurity. Based on their knowledge of solubility, they decide to use benzene as the solvent. a Identify the steps that the student must perform, and place them in the correct order. -Dissolve the sample in 5 mL of hot benzene. -Heat a portion of the benzene solvent. - Isolate the biphenyl crystals using gravity filtration. - Add the minimum amount of hot benzene to dissolve the sample. - Isolate the biphenyl crystals by centrifugation. - Add the sample to cold benzene, then heat. - Cool solution, first to room temperature, then in an ice-water bath.

Step 1: A portion of the solvent (in this case benzene) should be heated. Step 2: The minimum amount of hot solvent should be added to the sample to dissolve it. Step 3: The hot solution should be allowed to cool to room temperature and, once no further crystallization is taking place at room temperature, cooled in an ice-water bath. Step 4: The crystals need to be isolated, for example, by centrifugation

The stationary phase in TLC is (polar/acidic/nonpolar) . Therefore, the (more acidic/more polar/less polar) component of the mixture will travel a greater distance up the plate, resulting in a (higher/lower) Rf value for the component. When separating a mixture containing benzoic acid, 2,4-dinitrobenzoic acid, and 2,4,6-trinitrobenzoic acid, the (2,4-dinitrobenzoic /acidbenzoic acid/2,4,6-trinitrobenzoic acid) spot would be expected to travel the greatest distance up the plate as it is the (least polar/most polar/most acidic) component. The (benzoic acid/2,4-dinitrobenzoic acid/2,4,6-trinitrobenzoic acid) would travel the least distance as it is the (least polar/most polar/least acidic) component.

The stationary phase in TLC is polar. Therefore, the less polar component of the mixture will travel a greater distance up the plate, resulting in a higher Rf value for the component. When separating a mixture containing benzoic acid, 2,4-dinitrobenzoic acid, and 2,4,6-trinitrobenzoic acid, the benzoic acid spot would be expected to travel the greatest distance up the plate as it is the least polar component. The 2,4,6-trinitrobenzoic acid would travel the least distance as it is the most polar component.

Predict the trend in boiling points for the following compounds (1 being the lowest boiling point). Water Pentane Ethanol Iso-pentane Sodium n-butoxide

Water 4 Pentane 2 Ethanol 3 Iso-pentane 1 Sodium n-butoxide 5 EXPLANATION: The trend in strength of intermolecular interactions is: Ionic > Hydrogen bonding > Dipole-dipole > London dispersion For organic molecules which exhibit only London dispersion forces, the boiling point will increase with molecular weight and decrease with increasing branching. The greater the molecular weight, the greater the number of electrons there are to produce a temporary dipole, and the greater the distance over which the dipole can be formed. For a given molecular formula, increasing branching will decrease the boiling point, because the surface area available for London dispersion forces decreases.

Consider a sample that is a mixture composed of biphenyl, benzoic acid, and benzyl alcohol. Predict the order of elution of the components in this mixture. Assume that the chromatography uses a silica column and the solvent system is based on cyclohexane, with an increasing proportion of methylene chloride added as a function of time. Elutes first--->Elutes last

When using a nonpolar solvent, such as cyclohexane, you should expect the relatively nonpolar substance to elute first (biphenyl), leading to the following order: biphenyl(elutes first)>benzyl alcohol>benzoic acid(elutes last)

What type(s) of intermolecular forces are expected between CH3CH2CH2CH2CH2CH3 molecules? Indicate with a Y (yes) or an N (no) which apply. dipole forces___Y/N induced dipole forces___Y/N hydrogen bonding___Y/N

dipole forces N induced dipole forces Y hydrogen bonding N EXPLANATION: Induced dipole forces are always present. Induced dipole forces are also called dispersion forces, or London forces. Hydrocarbons are nonpolar. Therefore, CH3CH2CH2CH2CH2CH3 is a nonpolar molecular compound and the only intermolecular forces are induced dipole forces. Hydrogen bonding is found in situations represented by D-H---A where both the donor atom, D, and the acceptor atom, A, are one of the highly electronegative elements O, N, or F. Hydrogen bonding is not expected to be important in compounds in which all of the H atoms are covalently bonded to C atoms, even when an acceptor atom is present in the molecule.

What is the result of boiling the impure product with too much solvent and then cooling on ice? - Small crystals are produced. - No crystals are produced. - Crystals with impurities remaining within them are produced. - Large crystals are produced.

no crystals are produced

SAFETY: When performing a filtration under vacuum, always make sure that: - the collection flask is securely clamped. - the Büchner funnel is securely clamped.

the collection flask is securely clamped.