JEE (Advanced) 2026, Paper 1, Chemistry, Section 1 — single correct option, +3 for the correct option, 0 if unanswered, −1 otherwise.
Q.4 Considering LiBH4 reduces an ester group to the corresponding alcohol and does not reduce a carboxylic acid group, the correct statement about the major products P, Q, R and S is
A reaction flask and an overlay bench. Press START — for each substrate the experiment runs LiBH4 then H+, and BH3 then H+, builds both lactones in 3D, lays one on top of the other in the best possible orientation, then tries the mirror image. Then it names each pair and finds the matching option.
Two substrates wait on the bench. The question's structures are already loaded — press START EXPERIMENT.
Pre-set to the structures printed in the question. Move the CH3 — or take it away — and see what changes.
The ester arm is always the wedge and the acid arm the hash, as printed.
0 Å = the two lie exactly on top of each other. Controls check the test itself.
The rotation is found by least squares; the curve shows the mismatch shrinking as it is applied.
Nothing measured yet.
Run the experiment — the solution opens with the answer.
The experiment has not finished yet. Press START and let it run to the end — or use Reveal anyway in the header if you are revising.
LiBH4 reduces the ester (given in the question) and leaves the acid alone. BH3 is the classic reagent for the opposite job: it reduces a carboxylic acid quickly and an ester only slowly. So the two routes reduce different arms of the same molecule.
In each product a CH2CH2OH and a CH2C(=O)X sit on the same carbon. Counting O–CH2–CH2–C1–CH2–C(=O) gives a six-membered ring, so acid catalyses ring closure (loss of H2O from the hydroxy acid, or of EtOH from the hydroxy ester) to a spiro δ-lactone. All four products have the same formula, C9H14O2, and the same connectivity in each pair.
In the lactone the spiro carbon C1 carries two different ring branches (–CH2–C(=O)– and –CH2–CH2–O–), so swapping which face each branch is on inverts C1. The CH3 carbon is untouched by either reagent.
C1 and the CH3 carbon next to it are both stereocentres. One is inverted, the other is not: P and Q are diastereomers — both chiral, not mirror images. (CIP: the CH3 carbon is R in both; the spiro carbon is R in P and S in Q — one descriptor changes, not both.)
With the CH3 directly across the ring, a mirror plane runs through C1, the CH3 carbon and the lactone ring, so R and S are each achiral. They differ in whether the C=O branch sits on the same face as the CH3 (cis) or the opposite face (trans): R and S are cis/trans diastereomers.
(A), (B) and (D) each call a pair "identical". That would need the two routes to give the same lactone, which happens only if C1 is not a stereocentre in the product — for example with no CH3 on the ring at all. Remove the CH3 from a substrate in the experiment and that pair does come out identical. Neither pair can be enantiomers: only C1 is inverted, never the CH3 carbon.
For a student opening it for the first time. The whole experiment runs by itself after one press.
One substrate, two reagents. LiBH4 gives P (or R); BH3 gives Q (or S). Are the two products the same molecule, mirror images, or something else?
The printed structures — CH3 next to the quaternary carbon on top, across the ring below, both on a wedge — are already set. You do not need to change anything.
Move either CH3, flip it to a hash, or remove it. The badge switches to ⚙ USING CUSTOM VALUES; ↺ RESET TO QUESTION VALUES puts everything back.
Press ▶ START EXPERIMENT. There is nothing else to click — the experiment runs from beginning to end on its own (under a minute; 2× makes it faster).
Hydride hits one arm, the new OH swings round and closes the lactone, and the leaving group drifts away. Then the two products are laid on top of each other: green rings mark atoms that fit, red rings atoms that do not. Then the second one is reflected and tried again.
After both pairs are classified the experiment says ✓ EXPERIMENT COMPLETE and reveals the answer. The header answer box unlocks at the same time.
Scroll to Detailed solution. Each step points back to what you saw in the experiment.
Press ▶ REPLAY EXPERIMENT, or set the top CH3 to none and press ⚙ RUN WITH CUSTOM VALUES — which option would be right then? CLASSROOM MODE enlarges everything for a projector.
| Exam | JEE Advanced 2026, Paper 1, Chemistry, Section 1, Q.4 |
|---|---|
| Chapter | Organic Chemistry - Some Basic Principles and Techniques |
| Topic | Stereoisomerism — diastereomers from chemoselective reduction and lactonisation |
| Difficulty | Moderate-Hard |
| Answer | — run the experiment |