(i) \(\underset { Propanal }{ { 2CH }_{ 3 }-{ CH }_{ 2 }-CHO } \overset { Dil\quad NaOH }{ \underset { \quad Aldol\\ Condensation }{ \longrightarrow } } \underset { 3-hydroxy-2-methyl \ pentanal }{ { CH }_{ 3 }-{ CH }_{ 2 }-\underset { \overset { | }{ OH } }{ CH } -\underset { \overset { | }{ { CH }_{ 3 } } }{ CH } -CHO } \)
(ii) (CH₃)C-CHO:
\({ CH }_{ 3 }-\overset { \underset { | }{ { CH }_{ 3 } } }{ \underset { \overset { | }{ { CH }_{ 3 } } }{ C } - } CHO\overset { dil.NaOH }{ \underset { Cannizzao\\ reaction }{ \longrightarrow } } \underset { 2,2diemthy-l-propanol }{ { CH }_{ 3 }-\overset { \underset { | }{ { CH }_{ 3 } } }{ \underset { \overset { | }{ { CH }_{ 3 } } }{ C } } -{ CH }_{ 2 }OH+ } \underset { 2,2-dimethyl \ sodium \ propionate }{ CH_{ 3 }-\overset { \underset { | }{ { CH }_{ 3 } } }{ \underset { \overset { | }{ { CH }_{ 3 } } }{ C } } -COONa } \) 

(i) Perkin's Reactions:
When an aromatic aldehyde is heated with an aliphatic acid anhydride in the presence of the sodium salt of the acid corresponding to the anhydride, condensation takes place and an α, β unsaturated acid is obtained
This reaction is known as Perkin's reaction.

(ii) Knoevengal Reaction:
(a) Benzaldehyde condenses with malonic acid in the presence of pyridine forming cinnamic acid
(b) Catalyst - Pyridine
(c) Carbanion formed from malonic acid.

Primary alcohols and aldehydes can easily be oxidised to the corresponding carboxylic acids with oxidising agents such as potassium permanganate (in acidic or alkaline medium), potassium dichromate (in acidic medium)
Example:
\(\underset{Ethyl \ alcohol}{ { CH }_{ 3 }{ CH }_{ 2 }OH}\overset { { H }^{ + }/{ K }_{ 2 }{ Cr }_{ 2 }{ O }_{ 7 } }{ \underset { (O) }{ \longrightarrow } } \underset{Acetaldehyde}{{ CH }_{ 3 }CHO}\underset { (O) }{ \longrightarrow } \underset{Acetic \ acid}{{ CH }_{ 3 }COOH}\)

(i) Cannizzaro reaction is a redox reaction which takes place only in aldehydes which do not have α- hydrogen atom.
(ii) Formaldehyde (HCHO) and benzaldehyde (C₆H₅CHO) do not have a-hydrogen atom and so they undergo Cannizzaro reaction.
(iii) But acetaldehyde CH₃CHO contains 3 α-H atoms and it does not undergo Cannizzaro reaction.

When alkylcyanides are reduced using SnCl₂ HCl, imines are formed, which on hydrolysis gives corresponding aldehyde.
\({ CH }_{ 3 }-C\equiv N\overset { { SnCl }_{ 2 }/HCl }{ \longrightarrow } { CH }_{ 3 }-CH={ NH }\overset { { H }_{ 3 }{ O }^{ + } }{ \longrightarrow } { CH }_{ 3 }-CHO\)