GATE 2025 CY – Question 59

Organic Chemistry · Organic Synthesis: Synthesis, reactions, mechanisms and selectivity involving the following classes of compounds – alkenes, alkynes, arenes, alcohols, phenols, aldehydes, ketones, carboxylic acids, esters, nitriles, halides, nitro compounds, amines and amides. Uses of Mg, Li, Cu, B, Zn, P, S, Sn and Si based reagents in organic synthesis. Carbon-carbon bond formation through coupling reactions – Heck, Suzuki, Stille, Sonogashira, Negishi, Kumada, Hiyama, Tsuji-Trost reactions; olefin metathesis, McMurry coupling and Buchwald-Hartwig amination reactions. Baylis-Hillman, Henry, Ritter, Sakurai, Tebbe olefination, Pauson-Khand and Nazarov cyclization reactions. Concepts of multistep synthesis – retrosynthetic analysis, strategic disconnections, synthons and synthetic equivalents. Atom economy and green chemistry, Umpolung reactivity – formyl and acyl anion equivalents. Selectivity in organic synthesis – chemo-, regio- and stereoselectivity. Protection and deprotection of functional groups. Concepts of asymmetric synthesis – resolution (including enzymatic), desymmetrization and use of chiral auxiliaries, organocatalysis. Carbon-carbon and carbonheteroatom bond forming reactions through enolates (including boron enolates), enamines and silyl enol ethers. Stereoselective addition to C=O groups (Cram, Prelog and Felkin-Anh models). Asymmetric aldol reactions – Evans reaction and proline catalyzed reaction. · 2 marks · Numerical answer

In the following reaction, 13.4 grams of aldehyde P gave a diastereomeric mixture of alcohols Q and R in a ratio of 2:1. If the yield of the reaction is 80%, then the amount of Q (in grams) obtained is_____ (in integer).

Aldehyde P reacts with methyllithium to give alcohols Q and R.

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: 8

Explanation

**Step 1: moles of the aldehyde P.** Its molar mass is 134 g/mol:
$$n_P=\frac{13.4}{134}=0.100\text{ mol}.$$

**Step 2: moles of alcohol formed.** The yield of the reaction is 80%, and each aldehyde gives one alcohol (methyl lithium adds $CH_3$ and, after work-up, H):
$$n_{alcohols}=0.100\times0.80=0.080\text{ mol}.$$

**Step 3: the share of Q.** The diastereomers Q and R form in a ratio of 2:1, so Q is two-thirds of the product:
$$n_Q=0.080\times\frac23=0.0533\text{ mol}.$$

**Step 4: mass of Q.** The alcohol has molar mass $134+16=150$ g/mol:
$$m_Q=0.0533\times150=\mathbf{8\ g}.$$