What is a technique used to purify organic solids?
Recrystallization
When an organic chemist conducts a recrystallization, he chooses a solvent that will ______?
Completely dissolve both the desired compound and any other organic impurities when the solvent is at boiling point.
In addition to the desired product being soluble in the solvent at its boiling point, the desired compound must also be ______?
Insoluble in the solvent at cold temperatures (ice bath), and the organic impurities must remain soluble when the solution is cooled.
What do seed crystals do?
They serve as nucleation sites where more molecules of the desired compound deposit themselves in layers.
What is the purpose of placing a solution containing the desired product in an ice bath?
It induces and maximizes crystal formation.
What is the purpose of hot gravity filtration? What does it do?
It is done to prevent the desired compound from crystallizing along with the impurities. It removes any insoluble impurities from the solution (rather than isolating the desired product).
Why is it good to use the stemless funnel for hot gravity filtration?
With long-stemmed funnels, the crystals come out in the step as the solution cools, blocking up the funnel. Using a stemless funnel prevents this from happening.
In recrystallization, by what method are the crystals of the desired compound collected?
Vacuum filtration. Any remaining impurities are removed from the desired compound.
What is ligroin?
A non-polar solvent. It is a colorless and flammable liquid.
Why is HCl used instead of plain deionized water in p-phenetidine in amide synthesis of acetophenetidin?
HCl is used for the acid-base reaction rather than water becuase HCl’s polarity contributes to a reaction driven to the right. It protonates the amine group, making it easy to dissolve.
What is the purpose of the sodium acetate solution in the amide synthesis of acetophenetidin?
It is used to buffer the reaction mixture because control of acidity is important in maximizing yield. Protonation activates acetic anhydride toward nucleophilic attack, but if the pH is too low the starting amine will will be converted to its conjugate acid.
How do amide synthesis and Williamson Ether synthesis of acetophenetidin differ from each other?
Amide synthesis = formation of the amide bond ; Williamson Ether = formation of the ether linkage.
Why should you always use a boiling stick whenever you heat any liquid in the lab?
Boiling sticks provide a large surface area and give the gas that forms from the hot liquid a place to form and then release. To summarize, it prevents bumping.
How would you define the melting point of a pure substance?
The temperature at which the solid phase is in equilibrium with the liquid phase of the compound.
What was the best recrystallization solvent for the acetophenetidin?
Water because the solute is completely soluble at boiling point, but the crystals reappear when solution is placed in an ice bath.
What does the reflux condenser do? What did we use it for?
Allows the reaction to be carried out at the boiling point of the solvent and yet it prevents loss of solvent or reagent due to evaporation. The vapors condense as they rise up the condenser and thus prevent them from escaping. We used it for Williamson Ether synthesis of acetophenetidin.
What conclusions did you draw from the synthesis of acetophenetidin lab?
Synthesis of acetophenetidin by Williamson Ether synthesis produced purer crystals, but amide synthesis produced a larger crude mass (greener pathway). Recrystallizing the crude product formed from amide synthesis gave purer crystals that were identical to those formed by Williamson Ether synthesis. This is partly due to the fact that water is a good recrystallization solvent.
What are two math equations you need to know after collecting the mass of your product?
*Percent Yield = actual yield/theoretical yield x 100
*Percent Recovery = mass of recovered recrystallized product/mass of crude product used in recrystallization x 100
What is extraction? What types are there?
Extraction is a technique that chemists use to remove selected components from a mixture. The different types are solid-liquid extraction and liquid-liquid extraction.
Why is carbon dioxide used as a solvent for solid-liquid extractions?
It offers a greener alternative to other solvents, it is readily available, nonflammable, nontoxic, and safe for the environment. Ease of removal of solvent at the end of procedure, and it is relatively quick.
What criteria must be met in order for a liquid-liquid extraction to be successful?
The two solvents that are chosen for the extraction must be immiscible (not soluble in each other). In addition, the compound to be extracted must have different affinities for the two solvents.
What is a sufficient base that can be used to deprotonate a carboxylic acid?
Because carboxylic acid is a relatively strong acid, it is strong enough to react with a weak base such as sodium bicarbonate to yield water-soluble carboxylate ions.
What is a sufficient base that can be used to deprotonate a phenol?
Because phenols are much less acidic, it is too weak to react with a weak base so it must react with a strong base such as sodium hydroxide in order to covert phenol to its water-soluble phenolate ion.
After the desired compound has been extracted into the aqueous phase as a water-soluble carboxylate ion and phenolate ion, what must take place? How?
Neutralization must take place to regenerate the neutral organic compound. This is accomplished by adjusting the pH of the aqueous layer with HCl until the solution is acidic.
What do we use to wash an organic layer to remove the majority of dissolved water?
Brine. Even though the organic solvent is usually immiscible with water, they still tend to absorb some water during the mixing that occurs during extractions.
How is the organic solvent dried further? (More water removal)
Placing it in contact with an anhydrous inorganic salt (anhydrous magnesium or sodium sulfate). The salt will absorb the water that is dissolved in the organic solvent.
What is the drying agent removed by in extraction after it is mixed with the organic solvent?
Gravity filtration. (Filtered solution then sits so that organic solvent evaporates and only the extracted compound remains).
After sodium bicarbonate is added to the ether solution, what happens in the separatory funnel?
An aqueous layer (bottom) forms that is immiscible to the organic layer (top) and the aqueous solution.
What is present in the aqueous layer you drained? How do you neutralize it? How do you isolate it?
The carboxylate ion is present. In order to regenerate the carboxylic acid it must be added to HCl in an ice bath to induce crystallization. To isolate the crystals, vacuum filtration is performed.
What is present in the organic layer? What steps do you need to take to isolate the desired product?
The phenol is present. There may be some water present in the solution so it must be dried with brine and anhydrous magnesium sulfate until it no longer clumps together. The drying agent is then separated from the organic solution through gravity filtration. Once this is done, the ether must be evaporated under reduced pressure so that the pure phenol solid is left in the flask.
What is the difference between “extracting” and “washing”?
Extraction is the process of pulling out the desired compound from a solution while washing is cleaning and purifying the extracted compound to eliminate contaminants from the parent solution.
Why phenol is more acidic than cyclohexanol?
When phenol loses a hydrogen, the electron pair will get delocalized and resonance will occur among the pi bonds in the phenyl ring. As a result, it is much more acidic while its conjugate base is weak. Cyclohexanol only consists of sigma bonds so delocalization cannot occur and the molecule is much less stable so it will be a weaker acid while its conjugate base will be much stronger.
What is steam distillation?
It is a process used to isolate essential oils from plants and spices. The spice or herb is suspended in water, and as the water is boiled and vaporizes, it extracts the essential oils and carries them over into the collection flask. (High boiling points of these oils makes them difficult to isolate on their own without risking decomposition of the compound).
Why is it important to soak the cloves in water for several minutes before beginning the distillation?
It will allow the water to penetrate the cloves and should allow more eugenol to be isolated from the cloves.
Describe the practical reasons behind the direction of the flow of water through the condenser of the distillation apparatus.
Vapors will form in the heated distillation flask. The condenser cools the vapors as they travel toward the collection vessel causing them to liquefy again and flow into the vessel. A filled condenser provides maximum cooling allowing maximum recovery of purified liquid during distillation.
Why is dichloromethane (CH2Cl2) added to the solution in the collection vessel? What process is this called?
To extract the eugenol from the water. This process is liquid-liquid extraction and the lower layer will be the dichloromethane along with the desired essential oil.
There might be some water still present in the solution containing the desired product. What measures should you take to remove it?
Add anhydrous magnesium sulfate to the solution.
Carbon dioxide extraction can also be used to isolate essential oil from cloves. Discuss the advantages and disadvantages of each of these processes.
Distillation is more advantageous than carbon dioxide extraction for cloves because a larger product yield is extracted. However, it takes time. Carbon dioxide extraction is also disadvantageous because the pressure build up from sublimation can cause the cap on the test tube to come off.
What is the purpose of crushing magnesium turnings?
Helps expose fresh metal surface by removing some magnesium oxide on the exterior.
How does iodine help facilitate the reaction of the magnesium turnings?
Cleaning the surface of the magnesium metal.
What takes place during a Grignard reaction? KNOW THE MECHANISM FOR THIS REACTION!!!
It is an important carbon-carbon bond forming technique that results in the formation of an alcohol.
Why must the apparatus be flame dried prior to the beginning the reaction?
Grignard reagents are sensitive to water,
Why must bromobenzene be mixed with anhydrous diethyl ether during formation of the Grignard reagent?
Grignard reagents are sensitive to water so if diethyl either containing more water is used it will interfere with the formation of the Grignard reagent.
Why is the reaction mixture added to hydrosulfuric acid and ice?
The acidic solution donates a proton to the negatively charged carbon forming an alcohol.
When the reaction mixture is added to the separatory funnel with diethyl ether what happens? Where is the desired compound?
An aqueous layer forms (bottom) and an organic layer forms (top). The desired compound is dissolved in the organic layer (like dissolves like).
Some water may still be present in the organic solution. What measures should be taken to remove it?
Dry the ether over anhydrous magnesium sulfate and remove the drying agent by gravity filtration.
After all of the ether has evaporated, what can we do to purify the organic solid?
Recrystallization using ethanol. The desired compound is soluble in ethanol at boiling point and is insoluble in ethanol when it cools.
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