Orgo Lab Midterm – Flashcards
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What effect would poor circulation of the melting point bath liquid have on the observed melting point?
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Depending on the mode of circulation, the observed melting point could be either higher or lower than the true melting point.
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What is the effect of an insoluble impurity, such as sodium sulfate, on the observed melting point of a compound?
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An insoluble impurity like sodium sulfate should not affect the melting point.
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Three test tubes, labeled A, B, and C, contain substances with approximately the same melting points. How could you prove that the test tubes contain different chemical compounds?
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Mixtures of A with B, B with C, and A with C should all show depressed melting points
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One of the most common causes of inaccurate melting points is too rapid heating of the melting point bath. Under these circumstances, how will the observed melting point compare to the true melting point?
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Too-rapid heating of the melting point bath will not allow the mercury (large heat capacity) in the thermometer to warm up to the temperature of the sample (small heat capacity). Therefore, the sample will appear to have a lower melting point than it actually has
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Strictly speaking, why is it incorrect to speak of a melting 'point'?
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All compounds melt over a range of temperature under the conditions used to determine the melting points of organic compounds.
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What effect would the incomplete drying of a sample (for example, the incomplete removal of a recrystallization solvent) have on melting point?
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The solvent remaining in contact with the crystals is an impurity and will normally depress the melting point.
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Why should the melting point sample be finely powdered?
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The melting point sample should be finely powdered to aid heat transfer throughout the sample. A finely powdered sample has good thermal contact with the sides of the melting-point capillary.
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You suspect that an unknown is acetanilide (mp 113.5°C-114°C). Give a qualitative estimation of the melting point when the acetanilide is mixed with 10% by weight naphthalene (mp 80°C).
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The melting point will range from below 80°C (the melting point of naphthalene) to about 105°C.
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You have an unknown with an observed melting point of 90°C-93°C. Is your unknown compound A with a reported melting point of 95.5°C-96°C, or compound B with a reported melting point of 90.5°C-91°C? Explain.
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Since impurities depress the melting point of pure compounds, the unknown is Compound A. Impurities are present in the unknown because it has a melting point of about 93°C.
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Why is it important to heat the melting point bath or block slowly and steadily when the temperature gets close to the melting point?
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It is necessary to heat the melting point bath slowly to allow time for complete transfer of heat from the bath through the capillary to the sample.
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Why is it important to pack the sample tightly in the melting point capillary?
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To allow good heat transfer from the walls of the capillary to the crystals
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An unknown compound is suspected to be acetanilide (mp 113.5°C-114°C). What would happen to the melting point if this unknown were mixed with (a) an equal quantity of pure acetanilide? (b) an equal quantity of benzoic acid (mp 122°C)?
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(a) If the sample were mixed with pure acetanilide, the melting point would remain the same. (b) If the sample were mixed with benzoic acid (mp 122°C), the melting point of the mixture would be below that of acetanilide and the mixture would melt over a large range, perhaps 95- 105°C.
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Which would be expected to have the higher boiling point, t-butyl alcohol (2-methyl-2-propanol) or n-butyl-alcohol (1-butanol)? Explain.
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A roughly spherical molecule such as 2-methyl-2-propanol will not have a large surface area and therefore will not have the intermolecular van der Waals attractions that a linear molecule such as 1- butanol has. Thus, the former molecule boils at 83°C while the latter boils at 118°C.
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What is the purpose of the side arm of the thermometer adapter in a small scale boiling apparatus?
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To relieve pressure built up as the system is heated.
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A sample of naphthalene, which should be pure white, was found to have a grayish color after the usual purification procedure. The melting point was correct, and the melting point range was small. Explain the gray color.
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The gray color of the naphthalene is due to traces of decolorizing charcoal. It can be removed by filtration through fine filter paper.
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How many milliliters of boiling water are required to dissolve 25g of phthalic acid (18g/100mL at 99°C)? If the solution were cooled to 14°C, how many grams of phthalic acid would recrystallize out (0.54g/100mL at 14°C)?
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The amount of water required to dissolve 25 g of phthalic acid (using the solubility information in Experiment 2) is 139 mL. On cooling to 14°C, this solution should deposit 24.25 g of phthalic acid.
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Why should activated carbon be used during a recrystallization?
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Activated carbon is added to adsorb high-molecular-weight impurities, which are often colored and relatively less soluble.
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If a little activated charcoal does a good job removing impurities in a recrystallization, why not use a larger quantity?
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Activated carbon will adsorb some of the product, decreasing the yield.
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Under which circumstances is it wise to use a mixture of solvents to carry out a recrystallization?
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A mixture of solvents is used when no single solvent has the desired property of rendering the substance soluble when hot and insoluble when cold.
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Why is gravity filtration rather than suction filtration used to remove suspended impurities and charcoal from a hot solution?
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During suction filtration of a hot solution, evaporation of the solvent takes place with attendant evaporative cooling. The combination of these two effects causes crystals to form and clog the Bu?chner or Hirsch funnel.
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Why is a fluted filter paper used in gravity filtration?
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A fluted filter paper has a larger surface area through which the solvent can flow than an unfluted filter paper, which presses against the glass side of the funnel.
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Why are stemless funnels used instead of long-stem funnels to filter hot solution through fluted filter paper?
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If a long-stem funnel is used for filtration, the filtrate will cool and deposit crystals as it passes down the stem, clogging the stem
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Why is the final product from the recrystallization process isolated by vacuum filtration rather than gravity filtration?
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Vacuum filtration is a much more efficient method for removing solvent from the crystals than is gravity filtration.
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Why should wood applicator sticks not be used when carrying out a chemical reaction?
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Wood applicator stick will often interact with the reagents being used in the reaction, contaminating the product.
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Why should you never use a beaker for recrystallization?
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The large opening in a beaker allows solvent to evaporate too easily when trying to dissolve a solute. A crust of impure material appears around the edge on cooling. The erlenmeyer flask solves these problems.
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What are the seven steps of recrystallization?
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1. Choose a solvent and solvent pairs 2. Dissolving the solute 3. Decolorizing the solution 4. Filtering suspended solids 5. Recrystallizing the solute 6. Collecting and washing the crystals 7. Drying the crystals
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In either of the distillation experiments, can you account for the boiling point of your product in terms of the known boiling point of the pure components of your mixture? If so, how? If not, why not?
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There will not be much correlation. The distillation will begin at a temperature above the lower boiling component and end below the boiling point of the higher boiling component.
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From the plots of the boiling point versus the volume of distillate in the simple distillation experiments, what can you conclude about the purity of your product?
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The distillate is not pure.
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From the plots of the boiling point versus the volume of distillate in the fractional distillations of cyclohexane-toluene mixture, what conclusion can you draw about the homogeneity of the distillate? Does it have a constant boiling point? If constant, is it a pure substance?
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The distillate consists of a pure cyclohexane fraction and a pure toluene fraction.
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From the plots of the boiling point versus the volume of distillate in the fractional distillations of ethanol-water mixture, what conclusion can you draw about the homogeneity of the distillate? Does it have a constant boiling point? If constant, is it a pure substance?
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Much of the distillate will consist of 95% ethanol and 5% water, a low-boiling azeotrope with bp 78.15°C. The fact that it has a constant boiling point does not mean that it is one substance.
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What is the effect on the boiling point of a solution (e.g. water) produced by a soluble nonvolatile substance (e.g. sodium chloride)? What is the effect of an insoluble substance such as sand or charcoal? What is the temperature of the vapor above these two boiling solutions?
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A soluble, nonvolatile substance such as sodium chloride will raise the boiling point of an aqueous solution. An insoluble substance will have no effect.
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In the distillation of a pure substance (e.g. water), why does all of the water not vaporize at once when the boiling point is reached?
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Heat (of vaporization) must be added to the system to cause the water to vaporize completely.
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In fractional distillation, liquid can be seen running from the bottom of the distillation column back into the distilling flask. What effect does this returning condensate on the fractional distillation?
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This descending liquid, in a slow and careful distillation, is in equilibrium with ascending vapor. From Figure 5.3 on page 92, we can see that the vapor is always richer in the low boiling component. Therefore, this liquid rich in the high boiling component must return to the flask for maximum separation of a mixture.
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Why is it extremely dangerous to attempt to carry out a distillation in a completely closed apparatus (one with no vent to the atmosphere)?
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The application of heat to a cold, sealed, distillation apparatus will cause the gas in the apparatus (primarily air) to expand and build pressure, which may cause the apparatus to come apart. A distillation in progress can, of course, be sealed from the atmosphere because no vapor should escape from the apparatus.
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Why is better separation of two liquids achieved by slow rather than fast distillation?
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The equilibrium between ascending vapor and descending liquid is not established instantaneously. A slow distillation allows time for this equilibrium to become established.
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Explain why a packed fractionating column is more efficient than an unpacked one.
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The packing provides a large surface area on which heat exchange and equilibrium can take place between the descending condensed vapor (richer in the higher boiling component) and the ascending vapor (richer in the lower boiling component).
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In the distillation of the cyclohexane-toluene mixture, the first few drops of distillate may be cloudy. Explain this occurrence.
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The cloudiness is due to an azeotrope of water (often a contaminant) and cyclohexane, which has a lower boiling point than pure cyclohexane. On condensation, the azeotrope separates into two phases and then appears cloudy.
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What effect does the reduction of atmospheric pressure have on the boiling point? Can cyclohexane and toluene be separated if the external pressure is 350 mmhg instead of 760 mmHg?
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Boiling point is a function of pressure. Lowering the pressure over the system will lower its boiling point. Cyclohexane and toluene can be separated at a lower pressure. The boiling point of cyclohexane will still be lower than that of toluene.
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When water-cooled condensers are used for distillation or for refluxing a liquid, the water enters the condenser at the lowest point and leaves at the highest. Why not have the water enter the highest point and exit the lowest?
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If the water entered the condenser at the top it is possible for it to run down the inside of the condenser and not make contact with the inner tube containing the hot vapors to be condensed.
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Bearing in mind that a sealed container filled with steam will develop a vacuum if the container is cooled, explain what would occur if a steam distillation was stopped by turning off the steam valve before opening the screw clamp on the adapter trap.
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If the steam valve is turned off before the trap is opened, the steam in the trap will condense, creating a partial vacuum that will suck liquid from the flask into the trap and up the hose to the steam valve. If the trap is partially opened, this process will create a siphon that will siphon all of the liquid in the flask down the drain.
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A mixture of toluene (bp 110.8°C) and water is steam distilled. Visual inspection of the distillate reveals that there is a greater volume of toluene than water present, yet water (bp 100°C) has the higher vapor pressure. Explain this observation.
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The ratio of moles of water to moles of toluene is equal to the ratio of the vapor pressure of water to that of toluene in the steam distillate. A mole of toluene occupies 106 mL while a mole of water occupies only 18 mL; hence the volume of toluene will be larger than that of water in the distillate.
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Did sublimation aid in the purification of the unknown in part 3? Justify your answer.
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Sublimation should have aided in the purification of the unknown as indicated by appearance (black to white) and perhaps by melting point.
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From your experience and knowledge of the process of sublimation, cite some examples of substances that sublime.
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Some common substances that sublime: snow and ice. Clothes can be hung out to dry even when the temperature is below 0°C. Dry ice, solid carbon dioxide, sublimes to the gas without melting. Any solid that has a strong odor is subliming; for example, moth balls (p-dichlorobenzene or, formerly, naphthalene).
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How might freeze-dried foods be prepared?
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Foods for freeze drying are simply placed in a high vacuum chamber. The food will freeze (without external application of cold) as the first water evaporates because of the high heat of vaporization of water. Once all the water has evaporated, the temperature of the food will rise to room temperature. The water that evaporates is collected on a very cold surface in order to maintain the high vacuum.
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Describe the mercury level in a manometer like that shown in Figure 6.15a if the left arm is 1000cm tall.
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The difference in height between the column of mercury on the left and the one on the right would be 760 mm, atmospheric pressure.
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What criteria would you use to decide whether distillation is an appropriate technique for the purification of a reaction product? What type of distillation method-- simple, fractional, vacuum, or steam--should be used?
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Important criteria are volatility, stability to heat, ease of separation into fractions based on boiling point, and, for steam distillation, vapor pressure and water solubility.
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Why is it necessary to remove the stopper from a separtory funnel when liquid is being drained from it through the stopcock?
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If the stopper is not removed from the separatory funnel, very little liquid will drain from the stopcock because a partial vacuum is created within the funnel.
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The pKa of p-nitrophenol is 7.15. Would you expect this to dissolve in a sodium bicarbonate (pKa = 6.52) solution? The pKa of 2,5-nitrophenol is 5.15. Will it dissolve in bicarbonate solution?
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p-Nitrophenol (Ka = 7 × 10?8, pKa = 7.15) would not be expected to dissolve in bicarbonate because it is a weaker acid than carbonic acid (Ka = 3 × 10? 7). 3,5-Dinitrophenol (Ka = 7.1 × 10?6, pKa = 5.15), a stronger acid than carbonic acid, will dissolve in bicarbonate.
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Draw a flow sheet to show how you would separate the components of a mixture containing an acid substance, tolulic acid, a basic substance, p-bromoanaline, and anthracene, a neutral substance.
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The flow diagram would look like that in Figure 7.10, except that phenol is not present, so an extraction with strong base is not needed
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Write equations showing how acetaminophen might be extracted from an organic solvent such as an ether, if it were soluble.
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Acetaminophen is a phenol and thus a weak acid. It can be extracted from an organic solvent with a strong base such as sodium hydroxide. This aqueous solution upon acidification will regenerate acetaminophen which can be isolated by extraction into an organic solvent.
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Why might it be very difficult to visualize the separation of cis- and trans-2-butene by TLC?
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The butenes are very volatile and will evaporate before they can be detected.
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What error is introduced into determination of an Rf value if the top is left off the developing chamber?
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The solvent evaporates selectively from the top of the TLC plate while the compounds being eluted continue to travel upward. Their Rf values will be too large as a consequence.
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What problem will ensue if the level of the developing liquid is higher than the applied spot in a TLC analysis?
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The mixture being analyzed will dissolve in the elution solvent and will not travel up the TLC plate.
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In what order (from top to bottom) would you expect to find naphthalene, butyric acid,and phenyl acetate on a silica gel TLC plate developed with dichloromethane?
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Naphthalene, a nonpolar hydrocarbon, will be at the top of the plate (have the largest Rf value) while the highly polar butyric acid will be strongly attracted to the adsorbent and have the lowest Rf value. The ester, phenyl acetate, will be in between.
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In carrying out an analysis of a mixture, what do you expect to see when the TLC plate has been allowed to remain in the developing chamber too long, so that the solvent front has reached the top of the plate?
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The spots will grow larger as the compounds diffuse in all directions. If the solvent does not evaporate from the top of the plate, the Rf values should not be affected.
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Arrange the following in order of increasing Rf with TLC: acetic acid, acetaldehyde, 2-octanone, decane, and 1-butanol.
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The smallest Rf value would be given by the substance most strongly adsorbed to the silica gel. This would be the most polar substance, acetic acid. Then, in order of polarity, we might expect to see 1- butanol, acetaldehyde, 2-octanone, and, finally, with the largest Rf value, decane.
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What will be the result of applying too much compound to a TLC plate?
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The spots of various components will overlap to the point where the number of different components cannot be ascertained.
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Why is it necessary to run TLC in a closed container and to have the interior vapor saturated with the solvent?
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The solvent evaporates selectively from the top of the TLC plate while the compounds being eluted continue to travel upward. Incorrect Rf values result.
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What will be the appearance of a TLC plate if a solvent of too low polarity is used for the development? A solvent of too high polarity?
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If a solvent of too low polarity were used, the components would stay adsorbed to the silica gel and would not move from the starting point. If the solvent polarity were too high, the components in the mixture would dissolve readily in the solvent and travel with the solvent front.
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A TLC plate showed two spots with Rf values of 0.25 and 0.26. The plate was removed from the developing chamber, the residual solvent was allowed to evaporate from the plate, and then the plate was returned to the developing chamber. What would you expect to see after the second development was complete?
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Two spots with Rf values (measured from the origin) of about 0.44 and 0.46. It is as if the chromatogram were started anew with the spots from the first chromatogram.
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Which experimental method would you recommend for the preparation of 1-bromooctane? of t-butyl-bromide?
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n-Octyl alcohol is not miscible with water and would require vigorous stirring to complete the reaction. t-Butyl alcohol will react with hydrobromic acid almost instantly at room temperature to form t-butyl bromide through the intervention of the easily formed tertiary carbocation.
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Explain why the crude product is apt to contain certain definite organic impurities.
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Lack of complete reaction will leave unreacted n-butyl alcohol, dehydration will give 1-butene, and a competing reaction will give di-n-butyl ether. Each of the above impurities protonates in concentrated H?SO? to give the acid-soluble oxonium or carbonium ions and will be removed with the acid. n-Butyl bromide does not accept a proton.
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How should the reaction conditions in this experiment be changed to try to produce 1-chlorobutane?
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Sodium bromide should be replaced by sodium chloride and the reaction time and temperature increased slightly.
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Write a balanced equation for the reaction of sodium bisulfite with bromine.
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H?O + HSO?? + Br? ? 2Br- + SO? ²? + 3H?
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What is the purpose of refluxing the reaction mixture for 45 minutes? Why not simple boil the mixture in an Erlenmyer flask?
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The rate of a reaction approximately doubles for each 10°C rise in temperature. Therefore, heat is applied to the reaction to speed it up. If it were heated in an open flask, both the alcohol and the bromide would escape. The reflux condenser returns them to the reaction flask.
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Why is it necessary to remove all water from 1-bromobutane before distilling it?
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Water and n-butyl bromide cannot be separated by distillation because their boiling points are so close. They also co-distill as an azeotrope.
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Why is the resonance of the bromine-bearing carbon atom and the hydroxyl-bearing carbon atom the farthest downfield in Fig. 16.3 and 16.4?
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Bromide and hydroxyl groups are electron withdrawing; they decrease the electron density about the nearby carbon, making it resonate at low field.
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Hydrochloric acid is about as strong a mineral acid as sulfuric acid. Why would HCl not be a satisfactory catalyst in this reaction (Acetylsalicyclic acid)?
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HCl is 20% hydrogen chloride and 80% water. The water will react with the acetic anhydride. Sulfuric acid is 100% sulfuric acid.
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How do you account for the smell of vinegar when an old bottle of aspirin is opened?
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Moisture in the air will hydrolyze aspirin to salicylic acid and acetic acid (vinegar).
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What would be the effect of carrying out the sodium iodide in acetone reaction with alkyl halides using an iodide solution half as concentrated? (Nucleophilic subsitution)
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Since the rate of the SN2 reactions is proportional to the concentrations of both the reactants, the apparent rate of reaction would be expected to decrease if the concentration of iodide ion were reduced.
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The addition of sodium or potassium iodide catalyzes many SN2 reactions of alkyl chlorides or bromides. Explain.
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The iodide ion is a better nucleophile, as well as a better leaving group, than chloride and bromide. A catalytic amount will form an iodide that is then subject to displacement by the nucleophile.
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In SN1 reactions, the intermediate carbocation can eliminate a proton to yield alkenes or react with the solvent to yield ethers. Draw the structures of the byproducts of this type that would be derived from the reaction of the carbocation derived from 2-bromo-2methylbutane in ethanol.
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Two alkenes: 2-methyl-1-butene, 2-methyl-2-butene One ether: 2-ethoxy-2-methylbutane
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In the hypochlorite oxidation of cyclohexanol to cyclohexanone, what purpose does the acetic acid serve?
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The acetic acid acts as a solvent for both the dichromate and the alcohol
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What is the purpose of making derivatives of unknowns? (aldehydes & ketones)
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Solid derivatives of known mp help to distinguish between possible unknowns having very similar mp or bp. they simplify identification of small quantities of liquid unknowns also
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Why are 2,4-dinitrophenylhydrazones better than phenylhydrazones?
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2,4-dinitrophenylhydrazones have a higher MW than corresponding phenylhydrazones and therefore are usually crystalline
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Using chemical tests, how would you distinguish among 2-pentanone, 3-pentanone, and pentanal?
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2-pentanone will give a positive iodoform test and pentanal will give a positive Tollens' test and Schiff's test. 3-pentanone will give negative test to both.
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Hydrocarbons do not dissolve in concentrated sulfuric acid, but methyl benzoate does. Explain this difference and write an equation showing the ions that are produced.
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Methyl benzoate dissolves in concentrated sulfuric acid because the ester oxygens, having unpaired electrons, can be protonated. The ionic oxonium ion produced is soluble in the polar solvent, H2SO4.
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What would you expect the structure of the dinitro ester to be? Consider the directing effects of the ester and the first nitro group upon the addition of the second nitro group. (Methyl benzoate with nitric acid/sulfuric acid --> Methyl 3-nitrobenzene)
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Because both the carbomethoxy and the nitro groups are meta-directors, the second nitro group will enter at the 5 position to give methyl 3,5-dinitro-benzoate
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Write equations to explain why the reaction of 1,4-di-t-butylbenzene with t-butyl chloride and aluminum chloride gives 1,3,5-tri-t-butylbenzene
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Even though the t-butyl group is an ortho-para director, steric hindrance of the bulky t-butyl groups with each other favors the formation of the 1,3,5-tri-t-butyl substiuted compound
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Why must AlCl3 be protected from exposure to air?
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AlCl3 reacts readily with moisture in the air to give hydrogen chloride and various aluminum chlorohydroxides that do not catalyze the FC reaction
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Why is the 1,4 isomer, 1,4-di-t-butyl-2,5-dimethoxybenzene, the major product in the alkylation of dimethoxybenzene? Would you expect either of the following compounds to be formed as side products: 1,3-di-t-butyl-2,5-dimethoxybenzene or 1,4-di-t-butyl-2,5-dimethoxybenzene? Why or why not?
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There is no particular difference with the first alkylation product. The second alkylation can either go on the same side as the first, or on the opposite side, making 3 possible products. As you are adding tert-butyl groups, putting 2 of those together is going to be sterically unfavorable (2,3 di tert butyl product) because the central tert-butyl carbon is stuck in the plane with the sp hybridized carbons. So the options are really the 1-3 and the 1-4 product. The 1-4 product is better because the first alkyl group added is an ortho-para director. The 4 position is open to attack, but the 1 position is taken by the methoxygroup. So, the second alkylation will be favored ortho to the alkyl group, instead of meta to it.
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Suggest two other compounds that might be used in place of t-butyl alcohol to form 1,4-di-t-butyl-2,5-dimethoxybenzene.
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AlCl3 + t-butyl chloride or AlBr3 + t-butyl bromide
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In the first step of the synthesis of lidocaine, the reaction mixture is warmed to 40-50C and subsequently an aq. solution of sodium acetate is added. Explain the rationale of adding sodium acetate.
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As the reaction proceeds, hydrochloric acid is liberated so that 2,6-dimethylaniline is primarily converted to its HCl-salt. This salt will co-precipitate with the product (alpha-chloro-2,6-dimethyl acetamidide) since both are INSOLUBLE in cold acetic acid. The aq. sodium acetate converts the HCl-salt to acetate salt and removes HCl. The water added with the sodium acetate changes the polarity of the solvent and allows the desired product to precipitate.
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In the second step of lidocaine synthesis, toluene is employed as the solvent. Could diethyl ether replace toluene? Explain.
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No. Diethyl ether boils at much lower temperature compared to toluene. The reaction will not proceed at diethyl ether's bp.
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In the second step of lidocaine synthesis, three mole equivalents of diethylamine per mole of acetanilide are used. Why?
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In order to neutralize the Hcl produced in the SN2 reaction, excess diethylamine is added.
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In Grignard synthesis of triphenylmethanol, if the ethyl benzoate is wet what byproduct is formed?
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The phenyl magnesium bromide will react with the water first and form a benzene byproduct
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In the synthesis of benzoic acid, benzene is often detected as an impurity. How does this come about?
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The phenyl magnesium bromide reacts with the frost (ice) that collects on the surface of the dry ice
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Benzoic acid could have been extracted from the ether layer using sodium bicarbonate. What practical reason makes this extraction method less desirable than sodium hydroxide extraction?
answer
CO2 evolved causes the rxn mixture to foam out of the flask
