It is the commercial method for preparing benzaldehyde. Benzal chloride can be obtained by photochlorination of toluene i.e., chlorination of toluene in presence of sunlight. 
Then, benzal chloride on heating with boiling water produces benzaldehyde as shown below.

\((i)\ ({ { CH }_{ 3 }) }_{ 2 }{ C }{ = }{ CHCOCH }_{ 3 } \) 
\((ii){ \quad CH }_{ 3 }{ - }{ CH(OCH }_{ 3 }){ - }{ CO }{ - }{ CH(OCH }_{ 2 }{ CH }_{ 3 }){ CH }_{ 3 }\) 

Propanal and 2-methylpropanal both have a-hydrogens and hence can undergo cross aldol condensation in which each one of them can act either an electrophile or a nucleophile thereby giving two cross aldol products. Thus, 

Besides, cross aldol condensation each aldehyde can also undergo selfot simple aldol condensation giving two self or simple aldol products.

Since Cd (E.N. = 1.7) is less electropositive than Mg (E.N. = 1.2), therefore, dialkyl cadmium's are less reactive than Grignard reagents towards nucleophilic addition reactions. As such, dialkyl cadmium's react with the more reactive acid chlorides to give ketones but do not react further with the less reactive ketones thus formed to give tert-alcohols. In contrast, Grignard reagents being more reactive not only react with the acid chlorides but also with the ketones so formed to give tert-alcohols. \(\underset { Ketone }{ { { R }^{ ' } }{ - }\overset { O\\ || }{ C } { - }{ R } } \overset { { R }_{ 2 }Cd }{ \underset { -RCdCl }{ \longleftarrow } }\underset { Acid\quad chloride }{ { R }^{ ' }{ - }\overset { O\\ || }{ C } { - }{ Cl } } \quad \overset { { R }MgX }{ \underset { -Mg(Cl)X }{ \longleftarrow } }\underset { Ketone }{ { R }^{ ' }{ - }\overset { O\\ || }{ C } { - }{ R } } \quad \overset { { (i)R }MgX }{ \underset { (ii){ H }^{ + }/{ H }_{ 2 }O }{ \longleftarrow } }\underset { 3°Alcohol }{ { R }^{ ' }{ - }\overset { OH\\ | }{ \underset { |\\ R }{ C } } { - }{ R } } \) 

It is ammoniacal silver nitrate (agNO₃ + NH₄OH) solution. It is used as mild oxidising agent to test the presence of aldehyde group.

 (i) Conversion of Propanone to Propane:

 (ii) Conversion of Benzoyl chloride to benzaldehyde:

(iii) On treatment with dilute alkali, ethanol produces 3-hydroxybutanal gives But-2-enal on hheating.

\((i)\ { C }_{ 5 }{ H }_{ 5 }{ NH }^{ + }{ CrO }_{ 3 }{ Cl }^{ - }(pcc)\ in\ { CH }_{ 2 }{ Cl }_{ 2 }\) 
\( (ii)\ { K }_{ 2 }{ Cr }_{ 2 }{ O }_{ 7 }\ in\ acidic\ medium\) 
\((iii)\ Cr{ O }_{ 2 }\ in\ the\ presence\ of\ acetic\ anhydride\ (273-283K)\ or\ Cr{ O }_{ 2 }{ Cl }_{ 2 }, { H }_{ 2 }O\) 
\((iv)\ DIBALH\:\ (Diisobutyl)\ aluminium\ hydride, 78°C,{ H }_{ 2 }O\) 
\((v)\ PCC\ in\ { CH }_{ 2 }{ Cl }_{ 2 }\) 
\((vi)\ { O }_{ 3 }/{ H }_{ 2 }O-Zn\ dust\) 

(i) \(2CH_{ 3 }COONa\overset { Electrolysis }{ \longrightarrow } 2CH_{ 3 }COO^{ - }+2Na^{ + }\)

At anode : \(2CH_{ 3 }COO^{ - }\longrightarrow \overset { CH_{ 3 } }{ \underset { CH_{ 3 } }{ | } } +2CO_{ 2 }+2e^{ - }\)
At Cathode : \(2H^++2e^-\rightarrow H_2(g)\)
(ii) \(_{ CH_{ 3 }COO }^{ CH_{ 3 }COO }{ _{ \diagup }^{ \diagdown }{ Ca } }\quad \overset { Dry }{ \underset { distillation }{ \longrightarrow } } \underset { Acetone }{ CH_{ 3 }COCH_{ 3 } } +CaCO_{ 3 }\)
(iii) \(\underset { Sodium\quad benzoate }{ C_{ 6 }H_{ 5 }COONa } +\underset { Soda\quad lime }{ NaOH(CaO) } \overset { \triangle }{ \longrightarrow } \underset { Benzene }{ C_{ 6 }H_{ 6 } } +\underset { sodium\\ carbonate }{ Na_{ 2 }CO_{ 3 } } \) 

During acetylation, one H-atom (at. mass = Iamu) of the OH group is replaced by an acetyl group, i.e., CH₃CO (mol. mass = 43 amu). .
-OH + (CH₃CO)₂O ⇾ -O-COCH₃ + CH₃COOH
In other words, acetylation of each OH group increases the mass by 43 - I = 42 amu. Now the mol. mass of C₄H₁₀O = 106 amu while that of the acetylated product is 190 amu, therefore, the number of OH groups III the compound\(={190-106\over 42}=2\)

(i) Loss of a proton from an oxime gives a conjugate base which is stabilized by resonance but the conjugate base of NH₂OH is not.
(ii) To prepare iodoform from acetone, I⁺ is required which is supplied by IO^- ion but not by I^- ion as shown below:
(iii) During oxidation of toluene with CrO/H₂SO₄, the intermediate benzaldehyde formed readily undergoes oxidation to form benzoic acid due to the presence of H₂O in H₂SO₄,
\(\underset{Toluene}{C_6H_5-CH_3}\xrightarrow[\Delta]{CrO_3/H_2SO_4}\underset{Benzaldehyde}{C_6H_5-CHO}\xrightarrow{H_2O}\underset{Benzaldehyde\ hydrate}{C_6H_5CH(OH)_2}\xrightarrow{CrO_3/H_2SO_4}\underset{Benzalde\ acid}{C_6H_5COOH}\)
However, with CrO ₃3 in (CH₃CO)₂O, due to the absence of H₂O as soon as benzaldehyde is formed, it reacts with acetic anhydride to form benzylidene diacetate which does not undergo further oxidation. In this way, oxidation of benzaldehyde to benzoic acid is prevented. The gem-diacetate thus formed upon subsequent hydrolysis with alkali or acid gives benzaldehyde
\(\underset{Toluene}{C_6H_5CH_3}\xrightarrow[273-283K]{CrO_3/H_2SO_4}\underset{Benzylidene diacetate\\ (gem-diacelate)}{C_6H_5CH(OOH_3)_2}\xrightarrow[(Hydrolysis)]{H_3O^+}\underset{Benzaldehyde}{C_6H_5CHO}+\underset{Acetic\ acid}{2CH_3COOH}\)