How does acetaldehyde react with the following reagents?
(A) Dilute aqueous caustic soda
(B) Hydrazine
(C) Ammoniacal silver nitrate
(D) Phosphorus pentachloride

Hint: Acetaldehyde is an aldehyde that is highly reactive and toxic. It contains 2 carbon atoms, 1 of which is a methyl group and the other is an aldehyde group.
-When an aldehyde is reacted with dilute aqueous caustic soda it undergoes an aldol reaction.
-When it is reacted with hydrazine it undergoes Wolff-Kishner reaction.
-When it is reacted with ammoniacal silver nitrate it undergoes Tollens reaction.
-And when it is reacted with phosphorus pentachloride it gives a gem-chloride as a product.

Complete step by step solution:
Acetaldehyde is an organic compound that has a short life. The IUPAC name of this compound is ethanal. The chemical formula of acetaldehyde is $C{H}_{3}CHO$. Let us see the reactions of acetaldehyde with the given reagents.
(a) Dilute aqueous caustic soda: Caustic soda is the common name for the very important base $NaOH$. Aldehyde and ketones which have at least one $\alpha$-hydrogen atom are known to react with dilute aqueous caustic soda. As a result, they produce $\alpha$-hydroxy aldehydes. These compounds are known as aldols and reaction is known as aldol reaction. The reaction is given as follows.
$ \underset { Acetaldehyde }{ 2C{ H }_{ 3 }-CHO } \quad \xrightarrow { Dil.NaOH } \quad \underset { 3-Hydroxy\quad butanal }{ C{ H }_{ 3 }-CH(OH)-C{ H }_{ 2 }-CHO } \quad \xrightarrow [ { H }^{ + } ]{ \Delta } \quad \underset { But-2-enal }{ C{ H }_{ 3 }-CH=CH-CHO }$
(b) Hydrazine: The chemical formula of hydrazine is $N{H}_{2}-N{H}_{2}$. In this reaction, the carbonyl group of ethanal reacts with hydrazine. As a result, it produces a hydrazone in which the carbon is attached with the nitrogen atom with the means of a double bond. The reaction is given as follows.
$ \underset { Acetaldehyde }{ C{ H }_{ 3 }-CHO } \quad +\quad \underset { Hydrazine }{ N{ H }_{ 2 }-N{ H }_{ 2 } } \quad \longrightarrow \quad \underset { Hydrazone }{ C{ H }_{ 3 }-CH=NN{ H }_{ 2 } } \quad +\quad \underset { Water }{ { H }_{ 2 }O }$
(c) Ammoniacal silver nitrate: This reaction is known as Tollens reaction. In this, the acetaldehyde is warmed up with freshly prepared ammoniacal silver nitrate solution. As this reaction takes place, a shining silver mirror is produced. This is due to the reason that acetaldehyde gets oxidized to an acetate ion and silver ion is reduced to metallic silver which appears as a shining silver mirror. The reaction is given as follows.
$ \underset { Acetaldehyde }{ C{ H }_{ 3 }-CHO } \quad +\quad \underset { Silver\quad nitrate\quad ion }{ 2{ [Ag{ (N{ H }_{ 3 }) }_{ 2 }] }^{ - } } \quad +\quad 3O{ H }^{ - }\quad \longrightarrow \quad \underset { Acetate\quad ion }{ C{ H }_{ 3 }CO{ O }^{ - } } \quad +\quad \underset { Silver\quad mirror }{ 2Ag\downarrow } \quad +\quad \underset { Water }{ 2{ H }_{ 2 }O } \quad +\quad \underset { Ammonia }{ 4N{ H }_{ 3 } }$
(d) Phosphorus pentachloride: The chemical formula of this compound is $P{Cl}_{5}$. When acetaldehyde is reacted with phosphorus pentachloride, it gives gem-dichloride as a product. The reaction is given as follows.
$ \underset { Acetaldehyde }{ C{ H }_{ 3 }-CHO } \quad +\quad \underset { Phosphorus\quad pentachloride }{ P{ Cl }_{ 5 } } \quad \longrightarrow \quad \underset { Gem-dichloride }{ C{ H }_{ 3 }CH{ Cl }_{ 2 } } \quad +\quad PO{ Cl }_{ 3 }$

Note: Students tend to confuse between formaldehyde and acetaldehyde. Make sure you do not confuse between the two. Formaldehyde is $HCHO$ and acetaldehyde is $C{H}_{3}CHO$. Formaldehyde is the aldehyde of methane and acetaldehyde is the aldehyde of ethane. If you confuse between the two in this question, you will end up getting the products which are not desired for the question.