Question

Both \[HCHO\] and \[C{H_3}CHO\] gives positive tests with:
A. Fehling’s reagent
B. Ammoniacal silver nitrate
C. Schiff’s reagent
D. \[{I_2} + KOH\]

Answer 1

Hint: The common name for \[HCHO\] and \[C{H_3}CHO\] is formaldehyde and acetaldehyde respectively. All the aldehydes (having alpha hydrogen) give positive Fehling’s test, Tollen’s test and Schiff’s test. The compounds which give positive iodoform tests are the ones with alpha methyl groups.

Complete step by step answer:
Both the compounds belong to the class of aldehydes. The first compound i.e. formaldehyde have an alpha hydrogen and the second compound i.e. acetaldehyde have both alpha hydrogen and alpha methyl group.
Now, we will look at each reagent mentioned in the options and determine with which regent both the compounds will show a positive test.
The first reagent is Fehling’s reagent, a blue coloured basic solution of bistartratocuprate(II) complex. It detects the presence of aldehydes by reduction of the deep blue solution of copper(II) to a red precipitate of insoluble copper oxide.
Any aldehydic compound having an alpha hydrogen will show positive Fehling’s test.
Formaldehyde and acetaldehyde both have alpha hydrogen. Thus, both compounds will show positive Fehling’s test.
The second reagent is ammoniacal silver nitrate, commonly called Tollen’s reagent. It is a basic, aqueous solution containing silver ions coordinated to ammonia: \[{\left[ {Ag{{\left( {N{H_3}} \right)}_2}} \right]^ + }\]. The corresponding test is also called the silver mirror test. It exploits the fact that aldehydes are readily oxidized, whereas ketones are not. The compound with alpha hydrogen will show a positive Tollens test. Thus, both the given aldehydes will show a positive Tollens test.
The third reagent is Schiff’s reagent.
 p-rosaniline hydrochloride solution decolourised with sulphurous acid is known as Schiff's reagent. Whenever an aldehyde (with alpha hydrogen) is added to the Schiff’s reagent, it will turn from colourless to magenta colour. Thus, both formaldehyde and acetaldehyde will show positive Schiff’s test.
The fourth reagent is \[{I_2} + KOH\].
This reagent is used in iodoform test. Whenever this reagent (iodine and potassium hydroxide) is added to a compound that contains either a methyl ketone or a secondary alcohol with a methyl group in the alpha position, a yellow coloured precipitate of iodoform (\[CH{I_3}\]) is formed.
Acetaldehyde contains an alpha methyl group, but formaldehyde does not have the same. Thus, only acetaldehyde will show positive iodoform test.
Considering all the above points, both the given compounds give positive test to Fehling’s reagent, Tollen’s reagent, Schiff’s reagent but not to the reagent in option D.

Thus, the correct answer is option A, B and C.

Note: Alpha hydrogen is the one adjacent to the carbonyl carbon similarly, alpha methyl group is the methyl group next to the carbonyl carbon.
Whenever ketones are added to any of the reagent in first three options i.e. Fehling’s reagent, Tollen’s reagent, Schiff’s reagent, they do not show any kind of reaction at all but ketones show positive iodoform test because of the presence of alpha methyl group.