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Homework answers / question archive / 1) You attempt to make an oxime derivative for what you think is a ketone, however you don't get any crystals
1) You attempt to make an oxime derivative for what you think is a ketone, however you don't get any crystals. What might you do before you repeat the entire derivative synthesis again in exactly the same way? (choose all that apply)
Group of answer choices
A) Check that for your candidate ketone, the oxime derivative is known - if it's not known, it's likely either unstable or a liquid
B) Consider adjusting reaction conditions - if the reaction is water sensitive, consider adding a drying tube. If it's a slow reaction, consider gentle heating.
C) Consider a different derivative, or in the case of amines/carboxylic acids, a titration
D) Evaporate all of the solvent you used for the recrystallization. That will tell you whether it was the reaction that failed or the recrystallization specifically
E) Confirm that you really do have a ketone - if you have an ester, the oxime reaction won't work
F) Get a melting point of the crude product you saved from BEFORE the recrystallization. That way you at least have some data that might rule out some compounds.
2) What might you do if 2 tests appear to contradict each other (choose all that apply)
Group of answer choices
A) Consider the possibility that your compound could have 2 functional groups (e.g. benzoin has both an alcohol and a ketone)
B) Consider if 1 of the tests is prone to false positives or false negatives and consider omitting that test
C) Try a 3rd test to break the tie
D) Consider if 1 of the tests could have given a wrong result because of a contaminant (such as acetone you just used to rinse the glassware) and retry the test under more rigorous conditions
Answer:
Condensation of aldehydes or ketones with hydroxylamine gives oxime products. These compounds belong to imines. The aldoximes are produced from aldehydes and ketoximes are produced from ketones.
Step-by-step explanation
During the synthesis process of oxime, one should be careful about following steps-
One must be clear about the oxime derivative. For example, preparation of cyclohexanone oxime from the condensation reaction of cyclohexanone and hydroxylamine must be known.
Oximes exist as crystals and are poorly soluble in water. Therefore, the reaction conditions must be adjusted. If the reaction is water sensitive, consider adding a drying tube. Consider a different derivative, or in the case of amines/carboxylic acids, a titration
For recrystallization appropriate solvent must be used. It is a method of purifying the desired product. Crystallization requires evaporating the solvent which can be used in minimal quantity for the product to form. Evaporate all of the solvent that is used for the recrystallization. This will ensure whether the reaction of recrystallization occurred or not. Oxime is specifically the product of condensation of a ketone. It is required to be confirmed about the reactant species. An ester doesn't result in the oxime formation.
The melting point determination is one of the confirmatory processes of obtaining the desired product.
Get a melting point of the crude product ensures about the purity of the product during the recrystallization.
Hence, options (A), (B), (D), (E), and (F) are correct.
2) If two tests contradict in their results, it might be because of various factors that could lead to such contradiction. Factors such as reaction conditions, temperature, concentrations of the reactants, presence of contaminants, different functional groups etc. The compounds could have different functional groups. For example, benzoin has both alcohol and a ketone. If one of the tests is prone to false results then it must not be carried further.
One can conduct a third test to get confirmation. If one of the tests goes wrong because of a contaminant then simply rinse the glassware and retry the test under more rigorous conditions.
Thus, options (A), (B), (C), and (D) are correct.
