Speed is a subject.
JEE isn't only about knowing — it's about time economics. This trainer measures what every solve costs: Time-Efficiency Ratio (TER), Topper-Velocity zones, time leaks, and guess-vs-concept diagnostics.
Crack JEE Spectroscopy in 20 Seconds
Your velocity map
Greed = wrong answer with a first tap faster than the question's guess threshold (a guess-flag, not a judgement). Concept-gap = wrong with a genuine attempt.
The bromine coin flip
Bromine comes in two flavors — ⁷⁹Br and ⁸¹Br — about 50/50 each. Flip a pair of bromine coins 40 times and watch the mass-spec peaks build themselves:
Two bromines give combinations (79,79), (79,81)/(81,79), (81,81) → peaks at M, M+2, M+4 in ratio 1 : 2 : 1. You don't memorize the table — you just built it. Same idea for chlorine: ³⁵Cl : ³⁷Cl is 3 : 1, so two chlorines square it → 9 : 6 : 1.
Fold the benzene
p-Dibromobenzene has a centre of symmetry: all four ring protons live identical lives → one singlet (4H) near δ 7.5. The ortho isomer is crowded and asymmetric → multiple signals. Para with different groups → two doublets (2H each).
Spectroscopy Hack Sheet
| Halogen pattern | Peaks | Ratio | Origin |
|---|---|---|---|
| 1 × Cl | M, M+2 | 3 : 1 | ³⁵Cl : ³⁷Cl ≈ 75% : 25% |
| 2 × Cl | M, M+2, M+4 | 9 : 6 : 1 | (3+1)² expansion |
| 1 × Br | M, M+2 | 1 : 1 | ⁷⁹Br : ⁸¹Br ≈ 50% : 50% |
| 2 × Br | M, M+2, M+4 | 1 : 2 : 1 | two-coin combinations |
| Benzene substitution | ¹H NMR |
|---|---|
| 1,4-disubstituted, identical groups (para) | 1 singlet, 4H |
| 1,4-disubstituted, different groups | 2 doublets, 2H each |
| 1,2-disubstituted (ortho) | multiple signals |
- Trap 1: the singlet must integrate to 4H — a 1H singlet is a different beast.
- Trap 2: low-resolution spectra can smear ortho's mess into a fake blob.
- Shortcut: read halogen counts from isotope ratios FIRST — it slaughters options before any arithmetic.