Search This Blog

Exhaustive Guide: Nucleophilic Addition Reactions | Class 12 Chemistry

Exhaustive Guide: Nucleophilic Addition Reactions | Class 12 Chemistry | ChemCA

Nucleophilic Addition Reactions

Module 2 | CBSE Class 12 Chemistry | Organic Chemistry

1. Physical Properties

Methanal is a gas at room temperature. Ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature.

  • Boiling Points: The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses. This is due to weak molecular association arising from dipole-dipole interactions (since the >C=O group is polar).
  • Comparison with Alcohols: Their boiling points are lower than those of alcohols of similar molecular masses. This is because aldehydes and ketones cannot form intermolecular hydrogen bonds among themselves.
  • Solubility: Lower members (methanal, ethanal, propanone) are miscible with water in all proportions because they form hydrogen bonds with water. Solubility decreases rapidly as the length of the hydrophobic alkyl chain increases.

2. Mechanism of Nucleophilic Addition

Unlike alkenes which undergo electrophilic addition, aldehydes and ketones undergo nucleophilic addition reactions. This is because the highly electronegative oxygen atom pulls the pi-electrons towards itself, leaving the carbonyl carbon electron-deficient (δ+).

Nu- + >Cδ+=Oδ- → Intermediate Alkoxide →(H+) Addition Product

Step-by-Step Mechanism:

  1. A nucleophile (Nu-) attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridized orbitals of carbonyl carbon.
  2. The hybridization of carbon changes from sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced.
  3. This intermediate captures a proton from the reaction medium to give the electrically neutral addition product.

3. Reactivity: Aldehydes vs. Ketones (Board Favorite)

Crucial Concept: Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons.
  • Steric Factor: The presence of two relatively large alkyl groups in ketones hinders the approach of the nucleophile to carbonyl carbon. In aldehydes, there is only one alkyl group and one small hydrogen atom. Thus, aldehydes offer less steric hindrance.
  • Electronic Factor (+I Effect): Alkyl groups are electron-releasing (+I effect). Ketones have two alkyl groups that push electron density towards the carbonyl carbon, reducing its electrophilicity (positive charge). Aldehydes have only one alkyl group, keeping the carbonyl carbon more positive and highly susceptible to nucleophilic attack.
Reactivity of Aromatic Aldehydes: Aromatic aldehydes (like benzaldehyde) are less reactive than aliphatic aldehydes. This is because the +R (resonance) effect of the benzene ring delocalizes electron density onto the carbonyl carbon, reducing its electrophilicity.

4. Important Nucleophilic Addition Reactions

4.1 Addition of Hydrogen Cyanide (HCN)

Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction occurs very slowly with pure HCN. Therefore, it is catalyzed by a base.

The base removes a proton from HCN to generate the powerful nucleophile CN- ion.
HCN + OH- ⇔ CN- + H2O

>C=O + CN- → >C(O-)(CN) →(H+) >C(OH)(CN) (Cyanohydrin)

Application: Cyanohydrins are very useful synthetic intermediates because the -CN group can be easily hydrolyzed to a carboxylic acid (-COOH) or reduced to a primary amine (-CH2NH2).

4.2 Addition of Sodium Bisulphite (NaHSO3)

Aldehydes and ketones (mostly aliphatic and unhindered) react with saturated aqueous sodium hydrogensulphite to form addition products. It is an equilibrium reaction.

>C=O + NaHSO3 ⇔ >C(OH)(SO3-Na+) (Bisulphite addition compound)

Separation & Purification: The bisulphite addition compounds are crystalline solids. Because they are water-soluble and can be converted back to the original carbonyl compound by treating them with dilute mineral acid or alkali, this reaction is widely used for the separation and purification of aldehydes.

4.3 Addition of Alcohols (Acetals & Ketals)

Aldehydes react with one equivalent of monohydric alcohol in the presence of dry HCl gas to yield alkoxyalcohol intermediates known as hemiacetals. A further reaction with another molecule of alcohol gives gem-dialkoxy compounds known as acetals.

R-CHO + R'OH ⇔(Dry HCl) R-CH(OH)(OR') [Hemiacetal]
R-CH(OH)(OR') + R'OH ⇔(Dry HCl) R-CH(OR')2 [Acetal] + H2O

Ketals: Ketones do not react easily with monohydric alcohols. Instead, they react with dihydric alcohols (like ethylene glycol) in the presence of dry HCl gas to form cyclic products known as ethylene glycol ketals.

Role of Dry HCl: Dry hydrogen chloride gas protonates the oxygen of the carbonyl group, increasing the electrophilicity of the carbonyl carbon and facilitating the nucleophilic attack of the alcohol molecule.

4.4 Addition of Ammonia and its Derivatives

Nucleophiles, such as ammonia and its derivatives (H2N-Z), add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalyzed by acid.

>C=O + H2N-Z ⇔(H+) >C=N-Z + H2O

Important Derivatives (Must Memorize):

Reagent (H2N-Z) Name of Reagent Product (>C=N-Z) Name of Product
H2N-OH Hydroxylamine >C=N-OH Oxime
H2N-NH2 Hydrazine >C=N-NH2 Hydrazone
H2N-NH-C6H5 Phenylhydrazine >C=N-NH-C6H5 Phenylhydrazone
H2N-NH-C6H3(NO2)2 2,4-Dinitrophenylhydrazine (Brady's Reagent) >C=N-NH-C6H3(NO2)2 2,4-DNP derivative
H2N-NH-CO-NH2 Semicarbazide >C=N-NH-CO-NH2 Semicarbazone
Brady's Test (2,4-DNP Test): 2,4-DNP derivatives are yellow, orange, or red solids. Their formation is used as a characteristic chemical test for the identification of the carbonyl group (aldehydes and ketones).

5. NCERT Solved Examples (Step-by-Step)

NCERT Example 12.3: Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reactions:
Ethanal, Propanal, Propanone, Butanone.

Solution:
Reactivity decreases as the number of alkyl groups attached to the carbonyl carbon increases (due to both steric hindrance and the +I effect making the carbonyl carbon less electrophilic).
Therefore, Aldehydes are more reactive than Ketones.
- Among ketones: Butanone has a larger alkyl group (ethyl) than propanone (methyl). So, Butanone < Propanone.
- Among aldehydes: Propanal has an ethyl group, ethanal has a methyl group. So, Propanal < Ethanal.
Order: Butanone < Propanone < Propanal < Ethanal.

NCERT Example 12.4: Predict the products of the following reaction:
Cyclohexanone + H2N-OH →(H+) ?

Solution:
H2N-OH is Hydroxylamine. When a ketone reacts with hydroxylamine, it loses a water molecule to form an Oxime.
Product: Cyclohexanone oxime. (The oxygen of the carbonyl group and two hydrogens from the -NH2 group leave as H2O, and a double bond forms between C and N).

6. Previous Year Questions (PYQs) & Exhaustive Question Bank

Part A: Conceptual (1-2 Marks)

[CBSE 2018, 2021]

Q1. Why is benzaldehyde less reactive than propanal towards nucleophilic addition reactions?

Answer: In benzaldehyde, the carbonyl group is attached to a benzene ring. The resonance effect (+R) of the benzene ring delocalizes the pi-electron density from the ring towards the carbonyl carbon. This drastically reduces the magnitude of the positive charge (δ+) on the carbonyl carbon, making it less susceptible to attack by a nucleophile compared to propanal.
[CBSE 2017, 2019]

Q2. Although semicarbazide (H2N-NH-CO-NH2) has two -NH2 groups, only one is involved in the formation of semicarbazones. Why?

Answer: Semicarbazide has one -NH2 group attached to an -NH- group, and another -NH2 group attached to a -CO- group. The lone pair of electrons on the -NH2 group adjacent to the carbonyl group is involved in resonance with the carbonyl group (it is delocalized). Hence, it is not available to act as a nucleophile. The other -NH2 group (attached to -NH-) is far from the carbonyl group, its lone pair is localized and freely available for nucleophilic attack on the aldehyde/ketone.

Part B: Assertion-Reason Type (1 Mark)

[CBSE Sample Paper 2023]

Q3. Assertion (A): The reaction of aldehydes with ammonia derivatives is carried out in a weakly acidic medium (pH 3.5).
Reason (R): In a highly acidic medium, ammonia derivatives get protonated and lose their nucleophilic character.

Answer: Both Assertion and Reason are correct, and Reason is the correct explanation for Assertion. A trace of acid is required to protonate the carbonyl oxygen, making the carbonyl carbon more electrophilic. However, if the medium is too acidic, the ammonia derivative (which is a base) will itself get protonated to form an ammonium salt (R-NH3+), leaving it with no lone pair to act as a nucleophile. Hence, optimum pH (around 3.5) is strictly maintained.

Part C: Synthesis and Application (3 Marks)

[CBSE 2016, 2022]

Q4. Give the chemical reaction and structural product when ethanal reacts with:
(i) HCN
(ii) Excess of ethanol in presence of dry HCl
(iii) 2,4-Dinitrophenylhydrazine

Answer:
(i) Ethanal (CH3CHO) reacts with HCN in the presence of base to form Acetaldehyde cyanohydrin: CH3CH(OH)(CN).

(ii) Ethanal reacts with excess ethanol (C2H5OH) and dry HCl to first form a hemiacetal and then a stable Acetal: CH3CH(OC2H5)2 (1,1-Diethoxyethane).

(iii) Ethanal reacts with 2,4-DNP to form Acetaldehyde 2,4-dinitrophenylhydrazone, which is a yellow/orange precipitate. The >C=O oxygen and the two H's from the terminal -NH2 of DNP combine to release water, forming the >C=N-NH-Ar linkage.

© 2026 ChemCA.in - The Premier Digital Resource for Chemistry.
This module is strictly mapped to the latest rationalised NCERT syllabus for Class 12 Chemistry.
Coming up in Module 3: Oxidation, Reduction, and Important Distinguishing Tests (Tollens', Fehling's, and Iodoform Test).

Powered by

📚 Also Read

Lecture Notes

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)

Featured Post

H₂O as a Ligand: Weak vs Strong Field Cases