Top 10 Most Important Naming Chemical Reactions for Class 12th Chemistry that are useful for various Board Exams.
Top 10 Most Important Chemical Reactions – Class 12 Chemistry
This blog post covers the 10 most important chemical reactions for Class 12 Chemistry, including their mechanisms, examples, and importance.
1. Cannizzaro Reaction
Description: The Cannizzaro reaction is a redox reaction that occurs with non-enolizable aldehydes. In the presence of a strong base like sodium hydroxide, one aldehyde is oxidized to a carboxylate ion, and the other is reduced to an alcohol.
Chemical Reaction:
2 HCHO + NaOH → CH3OH + HCOONa
(Example: Formaldehyde)
Mechanism:
First, the hydroxide ion attacks the aldehyde carbonyl, generating a hydride shift that reduces one aldehyde and oxidizes the other:
HCHO + OH⁻ → HCOO⁻ + H⁻ (Hydride transfer)
H⁻ + HCHO → CH3OH
Importance: This reaction is useful for synthesizing alcohols and carboxylic acids from aldehydes lacking α-hydrogen atoms.
2. Claisen Condensation
Description: Claisen condensation is the formation of a β-keto ester by the reaction of two esters or one ester and a ketone in the presence of a strong base like sodium ethoxide.
Chemical Reaction:
CH3COOEt + CH3COOEt → CH3COCH2COOEt + EtOH
(Example: Ethyl acetate)
Mechanism:
In the first step, sodium ethoxide deprotonates the ester to form an enolate ion. The enolate ion then attacks the carbonyl group of another ester, followed by elimination of ethanol:
CH3COOEt + NaOEt → CH2COOEt (Enolate formation)
CH2COOEt + CH3COOEt → β-keto ester
Importance: Claisen condensation is important in organic synthesis for producing β-keto esters, which are intermediates in various reactions.
3. Clemmensen Reduction
Description: Clemmensen reduction reduces aldehydes and ketones to alkanes using zinc amalgam (Zn/Hg) and hydrochloric acid (HCl).
Chemical Reaction:
RCO + Zn(Hg) + HCl → RCH2
(Example: Acetone to Propane)
Mechanism:
The carbonyl group is protonated and subsequently reduced by zinc, which delivers electrons, transforming the carbonyl group into a hydrocarbon:
RCO → RCH2 (Zinc reduces the carbonyl carbon)
Importance: Clemmensen reduction is used in converting carbonyl compounds into hydrocarbons, particularly under acidic conditions.
4. Fittig’s Reaction
Description: Fittig’s reaction is a coupling reaction that occurs between aryl halides and sodium in dry ether, forming biaryl compounds.
Chemical Reaction:
C6H5Br + 2Na + C6H5Br → C6H5-C6H5 + 2NaBr
(Example: Bromobenzene)
Mechanism:
Sodium metal reduces the aryl halide to form an aryl radical, which then couples with another aryl radical:
ArBr + 2Na → Ar⁻ + NaBr
2Ar⁻ → Ar-Ar
Importance: Fittig’s reaction is widely used for synthesizing biaryl compounds, key in many pharmaceuticals and polymers.
5. Friedel-Crafts Alkylation
Description: Friedel-Crafts alkylation introduces an alkyl group onto an aromatic ring using an alkyl halide and a Lewis acid, such as AlCl3.
Chemical Reaction:
C6H6 + CH3Cl + AlCl3 → C6H5CH3 + HCl
(Example: Methylation of Benzene)
Mechanism:
AlCl3 polarizes the alkyl halide, forming a carbocation that attacks the aromatic ring, resulting in alkylation:
CH3Cl + AlCl3 → CH3⁺ + AlCl4⁻
C6H6 + CH3⁺ → C6H5CH3
Importance: Friedel-Crafts alkylation is essential for synthesizing alkylated aromatic compounds used in the production of detergents and dyes.
6. Hundsdiecker Reaction
Description: The Hundsdiecker reaction involves the conversion of silver carboxylates to alkyl halides using halogens such as bromine or chlorine.
Chemical Reaction:
RCOOAg + Br2 → RBr + CO2 + AgBr
(Example: Silver acetate to ethyl bromide)
Mechanism:
The halogen reacts with the silver carboxylate, forming a carboxyl radical that releases carbon dioxide, yielding the alkyl halide:
RCOOAg + Br2 → RCOO∙ + AgBr
RCOO∙ → R∙ + CO2
R∙ + Br2 → RBr
Importance: This reaction is significant for converting carboxylic acids into halides, which can be used in further organic transformations.
7. Reimer-Tiemann Reaction
Description: The Reimer-Tiemann reaction is used for the ortho-formylation of phenols to form salicylaldehyde, using chloroform (CHCl3) and sodium hydroxide (NaOH).
Chemical Reaction:
C6H5OH + CHCl3 + NaOH → C6H4(OH)CHO + HCl + NaCl + H2O
(Example: Phenol to Salicylaldehyde)
Mechanism:
Chloroform reacts with sodium hydroxide to form dichlorocarbene (:CCl2), which inserts into the ortho position of the phenol, leading to formylation:
CHCl3 + OH⁻ → :CCl2 + Cl⁻
C6H5OH + :CCl2 → C6H4(OH)CCl2
C6H4(OH)CCl2 + OH⁻ → C6H4(OH)CHO
Importance: This reaction is crucial for the synthesis of aromatic aldehydes, which are important intermediates in organic chemistry.
8. Schiemann Reaction
Description: The Schiemann reaction is the conversion of aromatic amines to aryl fluorides by diazotization followed by treatment with fluoroboric acid (HBF4).
Chemical Reaction:
C6H5NH2 + HNO2 + HBF4 → C6H5F + N2 + HBF3 + H2O
(Example: Aniline to Fluorobenzene)
Mechanism:
First, the amine undergoes diazotization to form the diazonium salt, which then reacts with fluoroboric acid to release nitrogen gas and form the aryl fluoride:
C6H5NH2 + HNO2 → C6H5N2⁺Cl⁻
C6H5N2⁺BF4⁻ → C6H5F + N2
Importance: This reaction is significant for producing aryl fluorides, which are used in the pharmaceutical and agrochemical industries.
9. Ullmann Reaction
Description: The Ullmann reaction is a copper-catalyzed coupling of aryl halides, leading to the formation of biaryl compounds.
Chemical Reaction:
2 C6H5I + Cu → C6H5-C6H5 + 2 CuI
(Example: Iodobenzene)
Mechanism:
The copper catalyst facilitates the reductive coupling of aryl halides by forming an organocopper intermediate, which undergoes a coupling reaction:
C6H5I + Cu → C6H5Cu + I⁻
C6H5Cu + C6H5I → C6H5-C6H5
Importance: This reaction is widely used in the synthesis of biphenyl compounds, which are valuable in organic electronics and material sciences.
10. Wurtz Reaction
Description: The Wurtz reaction is the formation of alkanes through the coupling of alkyl halides in the presence of sodium metal.
Chemical Reaction:
2 R-X + 2 Na → R-R + 2 NaX
(Example: Ethyl bromide to Butane)
Mechanism:
The sodium metal donates electrons to the alkyl halide, forming alkyl radicals, which couple to form the alkane:
R-X + 2Na → R∙ + NaX
R∙ + R∙ → R-R
Importance: Wurtz reaction is an important method for the preparation of higher alkanes from alkyl halides.
FAQs on Important Chemical Reactions
1. What is the Cannizzaro Reaction?
The Cannizzaro reaction involves the disproportionation of an aldehyde without an alpha-hydrogen into a carboxylate and an alcohol in the presence of a strong base.
2. Which aldehydes undergo the Cannizzaro Reaction?
Aldehydes without alpha-hydrogen, such as formaldehyde, undergo the Cannizzaro reaction.
3. What products are formed in the Claisen Condensation?
Claisen condensation produces a beta-keto ester and an alcohol from two ester molecules.
4. What base is used in the Claisen Condensation?
Typically, a strong base like sodium ethoxide is used to initiate the Claisen condensation.
5. What is the Clemmensen Reduction?
The Clemmensen reduction converts carbonyl compounds, like ketones or aldehydes, into hydrocarbons using zinc amalgam and hydrochloric acid.
6. Can Clemmensen Reduction be applied to alcohols?
No, the Clemmensen reduction is specific to aldehydes and ketones, not alcohols.
7. What type of compounds undergo Fittig’s Reaction?
Fittig’s reaction is used for the coupling of aryl halides, leading to the formation of biaryl compounds.
8. What catalyst is used in Friedel-Crafts Alkylation?
Friedel-Crafts alkylation uses a Lewis acid catalyst, commonly aluminum chloride (AlCl3).
9. What is the major limitation of Friedel-Crafts Alkylation?
Friedel-Crafts alkylation may lead to polyalkylation and carbocation rearrangements.
10. What is the Hundsdiecker Reaction used for?
The Hundsdiecker reaction is used to convert silver carboxylates to alkyl halides.
11. What is the product of the Reimer-Tiemann Reaction?
The Reimer-Tiemann reaction produces ortho-formylated phenols, such as salicylaldehyde.
12. What reagent is essential for the Schiemann Reaction?
Fluoroboric acid (HBF4) is a key reagent in the Schiemann reaction, which converts diazonium salts to aryl fluorides.
13. What is the Ullmann Reaction?
The Ullmann reaction is a copper-catalyzed coupling reaction of aryl halides, leading to biaryl formation.
14. Which metal is used in the Ullmann Reaction?
Copper is the metal catalyst used in the Ullmann reaction.
15. What is the purpose of the Wurtz Reaction?
The Wurtz reaction is used to couple alkyl halides to form higher alkanes in the presence of sodium metal.
16. What are the byproducts of the Wurtz Reaction?
The byproducts of the Wurtz reaction are sodium halides (NaX), such as sodium chloride (NaCl).
17. Why is the Claisen Condensation important?
The Claisen condensation is important for forming carbon-carbon bonds, crucial in organic synthesis.
18. What type of aldehydes undergo the Reimer-Tiemann Reaction?
Phenolic compounds, like phenol, undergo the Reimer-Tiemann reaction to form aldehydes.
19. Can Friedel-Crafts Alkylation be used on deactivated aromatic rings?
No, Friedel-Crafts alkylation does not work well on deactivated aromatic rings, such as nitro-substituted rings.
20. What is a key limitation of the Cannizzaro Reaction?
The key limitation of the Cannizzaro reaction is that it only works on aldehydes without alpha-hydrogens.
