Organic Chemistry Lab Exam
4 acetaldehyde + 1 sodium borohydride + 3 H2O → 4 ethanol + 1 NaH2BO3 If 2.25 g of ethanol were actually formed in the reaction, determine the percent yield of ethanol
92.2%
What is meant by 'salting out'? How does it help separation?
"salting out" = the separation of an organic phase from an aqueous phase by the addition of a salt (makes the aqueous layer even more polar)
What is the flash point ( degrees C) of each of the following compounds? (a) 1-pentanol (b) 2-pentanone
(a) 49 oC (b) 10 oC
A mixture contains 80 % hexane and 20 % pentane. (a) Use the phase diagram in Figure 12.4 of your prelab reading to estimate the composition of the vapor overthis liquid. (b) This vapor is condensed and the resulting liquid is heated. What is the composition of the vapor above the second liquid?
(a) 58% hexane, 42% pentane (b) 32% hexane, 68% pentane
Each of the following TLC plates has been improperly spotted. Describe what is wrong in each case
(a) one spot not spotted on baseline (will give incorrect Rf) (b) Spots below solvent level (will dissolve into solvent) and below baseline (Rf will not be correct) (a) Spots are too close together
Indicate which of the two distillation techniques (S=Simple, F=Fractional) would be more suitable for each of the following scenarios. (3 points) (a) Preparing drinking water from sea water (b) Removing diethyl ether, bp 35 degrees C (760 torr), from a solution containing p-dichlorobenzene, bp 174 degrees C (760 torr) (c) Separating benzene, bp 80 degrees C (760 torr), from toluene, bp 111 degrees C (760 torr)
(a) simple distillation (b) simple distillation (c) fractional distillation
What is the flash point for acetone?
-20°C (closed cup)
What is the maximum rate at which temperature should be rising when you determine the melting point? Why is this important?
. 1 - 2 ⁰C/min; if rate is faster, melting point range will not be accurate (by the time melting is observed, the temperature reading will have increased; observed values will be higher than actual values)
4 acetaldehyde + 1 sodium borohydride + 3 H2O → 4 ethanol + 1 NaH2BO3 Calculate the mass (in grams) of the excess reagent remaining after the reaction is complete.
0.0133 mol NaBH4 required; 0.0132 mol NaBH4 excess = 0.499 g NaBH4 excess
4 acetaldehyde + 1 sodium borohydride + 3 H2O → 4 ethanol + 1 NaH2BO3 Calculate the number of moles of sodium borohydride before the reaction.
0.0264 mol NaBH4
Calculate the number of moles of acetaldehyde present before the reaction. 4 acetaldehyde + 1 sodium borohydride + 3 H2O → 4 ethanol + 1 NaH2BO3
0.053 mol acetaldehyde (C2H4O)
When an unknown liquid is steam distilled at 93ºC (vapor pressure of water at this temperature is 589 mm Hg) and barometric pressure (760 mm Hg), the distillate contains 6.0 g of water and 11.0 g of oil. Calculate the molecular weight of the oil.
113.7 g/mol
The vapor pressure of aniline near 100ºC is approximately 46 mm Hg. Estimate the amount of aniline that would steam distill if 500 g of water (steam) is collected at barometric pressure of 760 mm Hg.
166.4 g aniline
How 'high' should your sample be in a melting point tube?
2-3 mm
4 acetaldehyde + 1 sodium borohydride + 3 H2O → 4 ethanol + 1 NaH2BO3 Calculate the theoretical yield (in grams) of ethanol.
2.4 g ethanol
4 acetaldehyde + 1 sodium borohydride + 3 H2O → 4 ethanol + 1 NaH2BO3 Determine the volume of 2.25 g of ethanol.
2.85 mL
What is the molecular weight of (triphenylphosphoranylidene)acetaldehyde?
304.33 g/mol
A mixture of 40 mol % n-propylcyclohexane and 60 mol % n-propylbenzene is distilled through a simple distillation apparatus; assume that no fractionation occurs during the distillation. The standard vapor pressures of n-propylcyclohexane and n- propylbenzene are known to be 769 torr and 725 torr, respectively, at 157 degrees C. Calculate the percentage of each of the two components in the first few drops of distillate. You must show your work on this question in order to receive full credit.
A = n-propylcyclohexane B = n-propylbenzene PA =(769torr)(0.40)=307.6torr PB =(725torr)(0.60)=435.0torr Ptotal=742.6torr %A =PA/Ptotal=307.6torr/742.6torr=41.4% %B =PB/Ptotal=435.0torr/742.6torr =58.6%
4 acetaldehyde + 1 sodium borohydride + 3 H2O → 4 ethanol + 1 NaH2BO3 Determine whether acetaldehyde or sodium borohydride is the limiting reagent.
Acetaldehyde is the limiting reagent
What is the largest available unit size and price per gram (in USD) of (triphenylphosphoranylidene)acetaldehyde?
Acros 100g @ $413.93, Sigma-Aldrich 25 g 166.50
What is the smallest available unit size and price per gram (in USD) of (triphenylphosphoranylidene)acetaldehyde?
Acros 5 g @ $38.76, Sigma-Aldrich 5 g @45.80
When two layers form during a petroleum ether/water extraction, what would be an easy, convenient way to tell which layer is which if the densities were not available?
Add a drop of H2O and observe which layer it goes into
What fire extinguishing media should be used to extinguish an acetone fire?
Alcohol resistant foam. Powder. Carbon dioxide (CO2).
Why is the boiling point of a steam distillation always less than 100ºC?
Because of the high vapour pressure of water
What is the molecular formula of (triphenylphosphoranylidene)acetaldehyde?
C20H17OP
What is the CAS (Chemical Abstracts Service) number for (triphenylphosphoranylidene)acetaldehyde?
CAS# 2136-75-6
What is a catalyst?
Catalyst = a substance that speeds up a reaction, but is not itself consumed in the reaction
What is meant by "steam distillation"?
Co-distillation with water
What is an emulsion? What should you do initially if an emulsion forms?
Emulsion = mixtures of 2 or more liquids that are normally immiscible (seen as a cloudy, milky mix between layers) that prevents full separation of the layers - If emulsion forms, at first be patient, and give the layers a few minutes to settle
What does the pictogram, flame picture, found on the SDS mean?
Flammable liquid
When 2-propanol was used as the developing solvent, two substances moved with the solvent front (Rf = 1) during TLC analysis on a silica gel plate. Can you conclude that the substances are identical? If not, what additional experiments would you perform?
Having the same Rf doesn't conclusively prove 2 substances are the same. If both have Rf = 1, the eluent is too polar and the TLC should be repeated using a less polar eluent. (you could also run additional tests such as IR, NMR, GC, bp, mp, etc)
Why should a TLC plate be removed from the solvent before the solvent front reaches the top of the plate?
If solvent front reaches top of plate, distance travelled by solvent cannot be accurately determined, and so Rf values can't be calculated
What is the LD50 for acetone? Is this value for ingestion or inhalation? What animal was used to determine this value?
LD50 Oral=5,800 mg/kg (rat) LC50 Inhalation=76 mg/L 4.00 hours rat LD50 Dermal = 5800 mg/kg rabbit
When one has a mixture of immiscible components A and B, what is the relationship of the total pressure Ptot to the vapor pressures (Pº) of the individual components?
Ptotal = P⁰A + P⁰B (sum of partial pressures)
Calculate the Rf value of the following compound
Rf = 21mm/29mm = 0.724
Name one of two common adsorbents used in TLC
Silica gel, alumina
Which compound will have a smaller Rf on silica gel: one that is polar, or one that is nonpolar? On alumina?
Silica: polar compound has smaller Rf alumina: polar compound has smaller Rf
Explain how the rate of crystal growth can affect the purity of a recrystallized compound.
Slower crystal growth yields larger crystals with a higher purity. Rapid crystal growth traps impurities in the crystal lattice.
Page 163, Question 1 (in your Prelab Reading)
Student drained the bottom layer, which was the aqueous layer (density of diethyl ether = 0.713 g/mL, density of water = 1 g/mL); student then added ether to ether, so there was nothing to separate
When doing the extraction, which layer will be on the bottom in the separatory funnel, the aqueous water layer or the organic dichloromethane layer? How do you know?
The organic dichloromethane layer will be below the aqueous water layer, because the density of dichloromethane (1.33 g/mL) is greater than the density of water (1 g/mL)
Why is it important that the catalyst not be allowed to dry (on the filtration apparatus)?
The reactive hydrogen adsorbed on the catalyst surface may spontaneously ignite if exposed to air
In a fractional distillation, why is it important to keep the fractionating column as vertical as possible?
The vapour needs to travel straight up the column, condense on the plates and drip back down. If the column isn't vertical, the liquid will run down the sides of the column and the multiple minidistillations will not occur.
A student carried out a simple distillation on a compound known to boil at 124ºC and reported an observed boiling point of 116-117ºC. Gas chromatographic analysis of the product showed that the compound was pure, and a calibration of the thermometer indicated that it was accurate. What procedural error might the student have made in setting up the distillation apparatus?
Thermometer was in the wrong place.
Explain the role of the stirbar normally added to a liquid that is to be heated to boiling.
To ensure smooth boiling and prevent bumping by increasing surface area for bubble formation
Why would it be important to purge the system of air at the beginning of the reaction?
To prevent any ignition of hydrogen adsorbed on the catalyst surface if it is splashed on side of flask; ensures that the solution is exposed to the H2 gas (H2 is lighter than air)
Why is the rate of heating during a fractional distillation extremely important?
Too much heat: column heats up too quickly, won't get multiple mini-distillations Too little heat: column loses heat faster than it can be warmed up, so no distillation occurs
Describe two ways in which colorless compounds can be located on a TLC plate.
UV light, iodine visualization, dipping or spraying reagent (chemical stain)
Define the term "essential oil"
Volatile oil that exists naturally and can be extracted from flowers or plants
The solubility of a compound is 59 g per 100 mL in boiling methanol and 30 g per 100 mL in cold methanol, whereas its solubility in water is 7.2 per 100 mL at 95oC and 0.22 g per 100 mL at 2oC. Which solvent would be better for recrystallization of the compound? Explain.
Water is the better solvent, as its percent recovery (97%) is better than that of methanol (49%)
When performing an extraction of a wash with a separatory funnel, at what point should you stop the process of shaking and venting?
When no more gas is vented upon venting of the separatory funnel
When is it desirable to separate components by steam distillation rather than by normal distillation?
When the compounds you want to separate are insoluble in H2O, or when the compounds decompose at high temperatures
How will you know when you have added enough drying agent to the solution?
When the drying agent no longer clumps at the bottom, and floats freely in the flask ("snow globe" effect)
What does b.p. measure?
b.p. = boiling point in ⁰C (at 760 mmHg unless otherwise stated)
What does d or Den measure?
d = density in g/cm3
List 2 characteristic features of a melting point that would tell you that you have an impure sample.
depressed (lowered) melting point, wider melting point range
You have 100 mL of a solution of benzoic acid in water; the amount of benzoic acid in the solution is estimated to be 0.30 g. The distribution coefficient of benzoic acid in diethyl ether and water is approximately 10. Calculate the amount of benzoic acid that would be left in the water solution after four 20-mL extractions with ether. Do the same calculation using one 80-mL extraction with ether to determine which method is more efficient. You must show all your work for this question (all calculations) to get any credit.
final solute mass in water = ( V2)n initial solute mass in water V2+V1K (a) x = ( 100mL )4 x = 0.0037gremaining 0.30g 100mL+20mL(10) (b) 0.33 g remaining
What is the indication of the onset of melting?
first appearance of liquid
A mixture of water and nitrobenzene distills at 99.3ºC when the barometric pressure is 760 mm Hg. The vapor pressure of water at 99.3ºC is 740 mm Hg. Calculate the weight ratio of water to nitrobenzene that will distill.
gH2O/gnitrobenzene = 5.41 (note: Pnitrobenzene = 760 mmHg - 740 mmHg = 20 mmHg)
What is the melting point of (triphenylphosphoranylidene)acetaldehyde?
mp = 185 - 188 ⁰C
What does nD measure? What does the subscript D signify?
nD = refractive index; D indicates measured at l=589 nm (sodium D-line)
List three solvents in which the compound, 3-methylbutanoic acid, is soluble.
water, ethanol, ethyl ether, chloroform
