End of Experiment Questions

Pentadecane has a vapor pressure of 1 mm Hg at 91.6 C. Can this chemical be steam distilled and if so are there any practical difficulties?

It would steam distill but there would be many difficulties. A lot of water would be required to distill pentadecane because it has such a low vapor pressure

A student typically uses n-butyl alcohol instead of t-butyl alcohol in the reaction with HCl. What is the probable outcome of her reaction? Would the results change if she added ZnCl2?

-The reaction would not occur because it is a primary alcohol. ZnCl2 is not enough to catalyze the reaction

What two essential properties must a compound have for steam distillation to be useful in its purification?

-immiscible liquids -volatile

Arrange compounds in decreasing order of reactivity in Sn2 reaction. Most reactive to least.

1-chlorobutane 1-chloro-2-methylpropane 1-chloro-2,2-dimethylbutane 2-chloro-2-methyl-butane

Describe how you would carry out the recrystallization of biphenyl from ethanol.

1. Dissolve in warm solvent 2. Decolorizing carbon if needed 3. Hot filtration to filter out insoluble material 4. cool slowly and then in ice bath to cause crystallization of the precipitate 5.Filter with Buchner funnel to collect crystals 6. wash and dry crystals

You need to choose a solvent for a projected recrystallization. List 4 important criteria for choosing the right solvent.

1. solvent cannot react with the solute. 2. solvent should allow the solute to dissolve 3. solvent should have a lower boiling point than the solute's melting point 4. solvent should be volatile

What is a racemic mixture?

A mix of both S and R configurations

You have a grossly impure compound that refuses to crystallize and does not sublime. How can you initially purify the compound enough so that you can finish purification by recrystallization?

Chromatography - allows the separation of a solution by having the components travel together along a stationary until each one stops at different levels. Components of the impure mixture will then be able to be extracted and purified by recrystallization.

Why don't we use n-butyl fluoride instead of n-butyl bromide as the starting material for preparing n-butyl iodide

F = poor leaving group - small and electronegative so more strongly bonded to carbon and requires more energy to break bond

When aqueous HCL neutralizes a benzoate anion, one of the actual acid species present is the hydronium ion H 3 O + . Which is the more prevalent acid, HCl (pKa -2.2) or H3O+ (pKa -1.74)? Write an equation that shows how H3O+ forms from HCl.

HCL is more prevalent because it has a lower pKa which means it is the stronger acid and has a larger Ka. HCl + H2O ---> H3O+ + Cl-

Air potentially causes a problem during the synthesis of ferrocene. Why is this?

Iron (II) chloride is easily oxidized by air, which forms iron (III) chloride. Iron (III) chloride cannot form ferrocene.

If you distill methanol away from sand here in the lab, will it boil at 65C or will it boil at a higher temp?

It will boil at 65C because it is unaffected by insoluble material

Why is the crude ester mixture washed with aqueous NaHCO3?

NaHCO3 is a base that when added to an ester mixture with strong acid will create a salt that is soluble in water and will neutralize the solution while dissolving any alcohol left to purify the ester

If you concentrate sugar water by boiling off some of the water, will it boil at 100C?

No, it will boil above 100C because sugar is soluble and requires more energy to break the bonds between sugar and water

If you distill pure methanol on the top of mount washington, will the boiling point be 65C?

No, it will decrease because atmospheric pressure will decrease and thus decrease vapor pressure

Two of the steps in the recrystallization of biphenyl can be left out. Which steps can be left out and when is this advisable?

Steps 2 and 3. If the crystals are not colored there is no need for decolorizing carbon and thus no need for hot filtration to get rid of the decolorizing carbon

A student assembles a simple distillation apparatus. He uses a 25-mL round-bottomed flask for the pot, adds 10 mL of liquid to be distilled and a boiling chip, and attaches a head to the pot. He attaches a thermometer adapter to the head and inserts a thermometer. Next he attaches a condenser to the head and a receiver tube to the condenser. Finally, he attaches condenser hoses to the condenser, turns the water on, and starts heating. What big mistake did this student make and what might be expected to happen?

The student forgot to insert the tube adaptor in between the condenser and receiver which allows for relief of pressure. The student also turns on the water after attaching the tubes but should see how fast the water is coming out first. Last, he did not use keck clips to ensure proper attachment.

You wish to distill 45 mL of a liquid. Should you use a 50 mL, 100 mL or 250 mL round-bottomed flask for the pot flask?

Use a 100 mL flask so that there is enough room for the molecules to move when heated. This prevents bumping and explosion of the liquid

Ethyl acetate is another common extraction solvent. To substitute ethyl acetate for dichloromethane in the tea experiment that you just completed, what would you need to know about the solvent?

We would need to know if it dissolves caffeine, is not miscible with water, easily volatile and does not react with caffeine

In the past, this experiment had two sections, parts a and b. In part a, the reaction of n-butyl bromide with sodium iodide was performed as described here. In part b, we attempted the SN2 reaction of t-butyl chloride with sodium iodide under the same conditions a. What was the outcome for reaction of t-butyl chloride with sodium iodide? Explain. What organic product was typically isolated?

a) reaction does not occur because Sn2 doesnt use 3 alkyl halides b) initial t-butyl chloride

What is the major advantage of steam distillation over simple distillation?

distills at a temperature below boiling point as long as the material is present

structure of main component of clove oil

eugenol

1-Butanol is water soluble. But if you add salt to an aqueous solution of 1-butanol, you can extract the 1-butanol in dichloromethane. Explain how this works.

inorganic salts are more dissolvable in water than 1-butanol and can thus extract the organic solvent

Based on the relative Rf values of ferrocene, acetylferrocene, and 1,1'-disacetylferroce, which is most polar? Which is least polar?

most polar = 1,1'-diacetylferrocene because smallest Rf and ferrocene = least polar because large Rf. polar compounds adhere to the polar silica gel which causes it to travel slower

Arrange compounds in decreasing order of reactivity in Sn2 reaction. Most reactive to least.

n-butyl iodide n-butyl bromide n-butyl chloride n-butyl fluoride

Arrange the following compounds in order of elution from first to last from a silica gel column using ethyl acetate/hexanes as eluent.

octane, 2-pentanone, butanol, acetic acid

The structure of ferrocene is different in the gas phase than in the solid phase. The rings are staggered in the solid phase and eclipsed in the gas phase. Which form is shown in figure 12.1?

staggered - opposite points of cyclopentadiene eclipsed - same points going down of cyclopentadiene