9. Label 5 small test tubes. Add one drop of 1-butanol to the first tub, one dro
ID: 691847 • Letter: 9
Question
9. Label 5 small test tubes. Add one drop of 1-butanol to the first tub, one drop of 2-butanol to the second tube. Continue appropriately for the other reagents indicated.
10. Add 1ml of acetone to each test tube. Finally, add one drop of the chromic acid/sulfuric acid solution to each test tube. Record any changes within 5 seconds.
11. As part of the analysis section of this lab, compare the structure to function relationship indicated by the series of tests.
These were my observations from the lab:
Test Tubes -Hcolc change to otunge. -batl coles change ohen to ble geer -z-butanol, color charge similion to 2 but much Ouicker 4-. Cych hexane. color ange ( mmkebelcystl lilce structur colos luh par eipitate 6- 7 3-6hand orenge coles cranExplanation / Answer
The reaction of chromic acid/sulphuric acid mixture with alcohols is called Jones Oxidation. In this reaction, alcohols react with the Cr (VI) in chromic acid under acidic conditions further given by sulphuric acid to oxidise the alcohols. Usually since most alcohols with a largely non-polar part are immiscible with water, a less polar and aprotic solvent - acetone is used. In this solvent the oxidant is dark-reddish orange in colour due to inhibited dissociation of protons from the chromic acid. However, when it is added to a protic solvent like water, the chromate anion exists freely in water causing the colour change from reddish-orange in acetone to orange in water.
Jones oxidation of primary alcohols and aldehydes lead to carboxylic acid with no control to stop at aldehyde stage in case of the former substrate. Reaction with secondary alcohols give ketones and the reagent does not oxidise tertiary alcohols, carboxylic acids, alkanes and esters. During oxidation, the Cr (VI) center oxidises the substrate and is in turn reduced to Cr (III) resulting the disappearance of orange colour of Cr (VI) and appearance of bluish-green Cr (III).
For 1-butanol, the colour change occurs due to oxidation to butanoic acid, proceeding through an aldehyde intermediate making the Cr (VI) go to Cr (III).
For 2-butanol, no aldehyde intermediate is formed and being a secondary alcohol, the oxidation stops to give 2-butanone product making the oxidation quicker.
In case of cyclohexane, the absence of hydroxyl groups prevent the formation of a chromic ester intermediate that is similar to the manganate ester intermediate formed in case of potassium permanganate oxidations. This makes the alkane inert to chromic acid oxidations and so there is no colour change to imply oxidations. Even if the substrate was made miscible with the acid, no oxidation shown by a colour change would occur.
Benzaldehyde being an aromatic aldehyde is stabilized by resonance. This makes the formation of the chromate ester intermediate quite facile leading to a very efficient oxidation of this molecule to benzoic acid shown by the precipitation of the sparingly soluble carboxylic acid in the acidic medium.
Ethyl acetate is the ester of ethanol and acetic acid. As mentioned before, chromic acid oxidations do not occur in esters and so the orange acid dissolved in acetone upon mixing with the inert and relatively non-polar ethyl acetate precipitates as such out of the solution. This is also the case with tertiary butanol and in both cases, the clear orange precipitate is chromic acid or if there is any cationic impurity in water of sulphuric acid used, a chromate salt.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.