Question

A- You have a compound which you know is either an aldehyde or a ketone. Specify a test you could run to determine which it is.   What is the reagent and what are the visible results for a positive test?

B- You have a compound you know is an alcohol. How can you tell whether the alcohol is primary, secondary, or tertiary? What reagents would you use and what are the visible results for a positive test?

POSSIBLE TESTS:

-Ceric ammonium nitrate test

-Chromic acid/anhydride test

- Lucas test (HCl, ZnCl₂)

- Tollen’s test

- 2.4-DNP test

- Bromine in water

- Bromine in DCM

- Ferric chloride test

Answer 1

A. If the compound is aldehyde and it gives positive test with both Tollens reagent (Ammonical silver nitrate) which gives silver mirror on walls of the test tube and test with 2,4-DNP (Brady's reagent) gives a orange or red solid precipitate.

Aldehyde will reacts with Chromic acid and forms acid while ketone not responds.

While if it is ketone, gives only 2,4-DNP test and it won't give Tollens test

B. Lucas Test which differentiate the 3 types of alcohols by reacting with Lucas reagent

Tertiary alcohol gives turbidity (formation of alkyl halide) with Lucas reagent immediately since it involves the formation of stable tertiary cation while secondary alcohol forms precipitate in 5 minutes where as primary alcohol takes more than 10 minutes and also heating needed.

Chromic acid is also used as it oxidizes the primary alcohol to aldehyde initially and later to acid while secondary alcohol to ketone.

Tertiary alcohol won't reacts with Chromic acid.