Organic Chemistry Lab exercises
If 50% of a compound can be removed using 1 extraction, how many extractions are needed to remove 75% of the compound. What is the partition coefficient?
2; 75%
Assume that the partition coefficient for compound A between ethyl ether and water is 3 (i.e., A preferentially partitions into ether). Given 400 mL of aqueous solution containing 12 g of A, how many grams of A could be removed from the solution by one extraction with 200mL of ethyl ether; how much can be extracted from 2 100ml extractions with ether
7.2g; 8.08g
Which is the bottom layer: acetone or water (2)
Acetone
Many common drying agents remove water by a chemical reaction that converts the anhydrous form of the drying reagent into a hydrated form. However, calcium hydride (CaH2), which is commonly used to dry hydrocarbons and other inert solvents, removes water by a reaction that destroys the water molecule and produces a gaseous product. Write an equation for a chemical reaction between water and CaH2 that explains this observation. (2)
CaH2 (aq) + 2H2O (l) --> 2H2 (g) + Ca (OH)2 (aq)
Which is the bottom layer: water or chloroform (2)
Chloroform because it has a larger density
Inaccurate melting points are obtained when the sample is heated too rapidly in the melting point apparatus. Under such conditions, would the recorded melting point be higher or lower than the true melting point? Explain why. (3)
Higher because due to human error, we could miss the actual melting point.
Instead of using conc. HCl, would it be possible to carry out the synthesis of t-amyl chloride from t-amyl alcohol using a solution of NaCl in conc. aqueous H2SO4? Explain your answer. (7)
It is possible because the sulfuric acid provides the hydrogen to protonate the t-amyl chloride, which creates a good leaving group and a carbocation. The Cl- can then attack the carbocation and create the desired product.
Strong acids such as H2SO4 are usually used as the catalysts in dehydration reactions. However, HCl or HBr are also strong acids and therefore excellent sources of H+. Would it be possible to use HBr, for example, for the dehydration of an alcohol to produce an alkene as the major product? Explain your answer. (4)
It will not be possible to use HBr because when an alcohol is treated with HBr it will experience a nucleophilic substitution reaction and will generate an alkyl halide and water. Also the conjugate base of HBr (Br-) is a better nucleophile than base compare to H2SO4 and H3PO4.
Assuming that the equilibrium constant for esterification of p-aminobenzoic acid with ethanol is 3.5, calculate the theoretical yield of benzocaine based on the molar amounts of the starting materials employed in this experiment. (9)
K= X2/[NH2COOH-x][CH3CH2OH-x] 3.5= X2/ (0.00729mol-x)(0.171mol-x)
Which is the bottom layer: methanol or pentane (2)
Methanol
Is it possible for the double bond in cyclohexene to exist as the trans isomer? Why? (4)
No because cyclohexene does not have the flexibility to twist to create the trans isomer.
Would you expect benzene to give a positive Baeyer test? Why? (4)
No because since all six carbons hold a p orbital, there is extensive delocalization across the molecule. Due to this delocalization, an electrophile cannot target a specific carbocation in the benzene molecule which makes it difficult to attack.
Explain the role of boiling chips that are normally added to a liquid being distilled. (1)
Provides a rough surface for boiling to take place. Keeps the boiling of the solvent to a minimum
Suppose that in an extraction experiment, the organic compound of interest distributes between ethyl acetate and water with a partition coefficient of only 2. Suggest a simple method to increase this partition coefficient to make the extraction more efficient. (2)
Salting out
What is the role of acetic acid in the electrophilic aromatic substitution reaction you did in the laboratory? (6)
Solvate the 1,4-dimethoxybenzene. Solvation is preferred in this reaction because it leads to a stabilization of the final product more quickly.
Based on a general knowledge of double bond structure, would you expect an alkene to act as an electrophile or a nucleophile? Why? (4)
The double bond will act as an nucleophile because there is an extra pair of electrons there to attack an electrophilic center leaving with a carbocation
Why is it necessary to remove the drying agent from organic liquids prior to distillation? (4)
The drying agent removes most of the water from the distillate, and this allows the equilibrium to shift to the right which creates more product, maximizing the yield.
In the electrophilic aromatic substitution reaction you did in the laboratory, the substitution of the second t-butyl group on the ring is faster than the first substitution. Explain why this is true. (6)
The first t-butyl alcohol group is an electron doanting group, and it makes the benzene ring more reactive. Since the ring is more reactive, it's willing to react with another t-butyl alcohol group to reach a more stable state.
When the Lucas test is run on 1-octanol, a separate layer forms immediately which leads to the erroneous conclusion that 1-octanol is a tertiary alcohol. What is the source of error in this experiment? (7)
The reason why the substitution of the second t-butyl group on the ring is faster than the first substitution is because the first t-butyl alcohol group in an electron donating ring while causes the benzene ring to be more reactive. A more reactive ring is more likely to react with another t-butyl group,allowing it to reach more stability.
Succinic anhydride has the same structure as maleic anhydride without the double bond. If succinic anhydride is used for the experiment in this week's lab, the reaction will work to give an analogous product. However, the second step of this reaction is exceedingly slow. why does this observation make sense. (11)
The second step of the process with succinic anhydride will be exceedingly slower because the single bond on the succinic causes there to be a greater degree of freedom. On the malice anhydride the double bond causes there to be a fixed geometry causing it to react quicker.
Explain in detail why vacuum filtrations should never be used to filter hot solutions. (3)
The suction of the vacuum creates a lower pressure beneath the hot solution, and this lower pressure will cause the filtrate to boil. If the filtrate starts to boil, the solution will splatter everywhere and will skew the final results.
Why do finely powdered samples give more accurate melting points than samples with large crystals or chunks? (3)
There's more surface area for the heat to transfer to the substance and properly melt it.
How many products would be expected from the acid-catalyzed dehydration reaction of 2-butanol? (4)
Two products: 1-butene and 2-butene
The conversion of maleic anhydride to the amide in the first step of the reaction in this week's experiment requires on equivalent of aniline. The conversion of acetyl chloride to the amide using aniline requires two equivalents of aniline. Explain this observation mechanistically. (11)
Typically in acid chloride reactions, hydrochloric acid is produced during the reaction. If HCl was produced in this reaction, it would react with anything basic which includes the aniline and the product. Two equivalents are needed so one equivalent will create the product, and the other will create PhNH3CL instead of hydrochloric acid.
Which is the bottom layer: water or ethyl ether (2)
Water
Benzoic acid is much more soluble in acetone than in water. Can acetone be used to extract benzoic acid out of water? Why?
no, because acetone is soluble in water so there will be no distinct layers
Concentrated sulfuric acid is added as a catalyst in the esterification procedure even though the reaction mixture is already acidic due to the presence of p-aminobenzoic acid. Why is it necessary to add sulfuric acid? (9)
the hydrogen from the acid is used to 1) protonate the carbonyl oxygen allowing the carbonyl carbon to be able tot react with the Nu- quicker 2) the hydrogen is also used to attach to one of the hydroxyl groups converting to water to make it a better leaving 3) p-ABA is not strong enough to do so.
Using aqueous NaHCO3 instead of aqueous NaOH to extract benzoic acid presents a serious safety problem caused by foaming and pressure buildup in the separatory funnel. What is the chemical explanation for this issue?
the reaction with nahco3 produces co2 as a byproduct
The purpose of neutralizing the reaction mixture with sodium carbonate during the work-up procedure is two-fold. What are the reasons for these steps in the procedure? (9)
the salt of the amino group was formed, and to get the desired product, the salt must be exposed to a base so it will lose one of its protons and be converted into the desired product.
If the heating rate is too high in taking a melting point, what type of error does this cause? Explain your answer
the sample's melting point range will be lower and wider than expected. This occurs because heating the sample too fast causes the sample's temp. and the thermometer's temp. to not correlate due to the heat reaching the sample before reaching the thermometer.
Analyze the procedure you used in the laboratory to recrystallize benzoic acid and identify a specific step that if omitted would lead to a more nearly pure sample of recrystallized material.
the slower cooling process is better for crystallization of the product. Ice bath should be omitted for an purer recrystallization
