Ncert Books Class 12 Chemistry Chapter 12 Free Download

The most frequently asked named reactions in board papers are the Aldol Condensation and the Cannizzaro Reaction. Students are expected to know the conditions, write the complete balanced reaction, and name the products. Other important reactions include Clemmensen Reduction, Wolff-Kishner Reduction, and Hell-Volhard-Zelinsky (HVZ) Reaction. These often appear as a part of a larger 5-mark question.

In the CBSE board exam, distinguishing test questions usually provide a pair of organic compounds (e.g., ethanal and propanone, or pentan-2-one and pentan-3-one). You are required to name a specific chemical test, describe the procedure, and state the expected observation for each compound. For example, to distinguish an aldehyde from a ketone, Tollen's Test (silver mirror) is a very common and important question.

The alpha-hydrogen in aldehydes and ketones is acidic due to the strong electron-withdrawing effect of the adjacent carbonyl group (-C=O) and the resonance stabilization of the resulting conjugate base (enolate ion). This concept is the fundamental principle behind the Aldol Condensation, a very important question for the boards, where the enolate ion acts as a nucleophile.

Aldehydes are more reactive than ketones towards nucleophilic addition for two main reasons:

• Electronic Factors: The carbonyl carbon in aldehydes is more electrophilic because it has only one electron-donating alkyl group, unlike ketones which have two.
• Steric Hindrance: The presence of two relatively larger alkyl groups in ketones hinders the approach of the nucleophile to the carbonyl carbon, compared to the one alkyl group and one small hydrogen atom in aldehydes. This difference in reactivity is a frequent topic for conceptual questions.

Carboxylic acids are more acidic than phenols because the conjugate base of a carboxylic acid, the carboxylate ion, is highly stabilised by resonance. The negative charge is delocalised over two electronegative oxygen atoms. In contrast, the negative charge on the phenoxide ion is delocalised over only one oxygen atom and the less electronegative carbon atoms of the benzene ring. Exam questions often ask to arrange these compounds in increasing or decreasing order of their acidic strength (pKa value).

For higher marks, expect multi-step conversion questions. Important conversions include:

• Starting from an alkene or alkyne to prepare an aldehyde or ketone.
• Using a Grignard reagent to step-up the carbon chain and produce an alcohol, which is then oxidised.
• Converting a carboxylic acid to an amide or ester.

These questions test your knowledge of reagents like LiAlH₄, NaBH₄, PCC, and K₂Cr₂O₇ and their specific functions.

A frequent error is forgetting the crucial final step of acidic hydrolysis (H₃O⁺) after the nucleophilic addition of the Grignard reagent to the carbonyl compound. The initial adduct formed is an alkoxide, and adding a proton source is essential to form the final alcohol product. Missing this step can lead to a significant loss of marks in a conversion or mechanism question.

A 5-mark question from this chapter is typically not a single question but a composite one, often structured as (a), (b), (c) or (i), (ii), (iii). It might include:

• A named reaction (2 marks)
• A question on distinguishing tests (1 mark)
• A conceptual reason-based question (e.g., comparing boiling points or acidity) (1 mark)
• A single-step conversion (1 mark)

To score full marks, it is crucial to identify each part and answer it precisely and separately.