Chemistryhard

How Many Peaks Are Expected in the 1H NMR Spectra of 1,3,5-Tri[((4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-2H-indazol-2-yl)methyl]-2,4,6-trimethylbenzene?

Question

How many peaks are expected in the 1H NMR spectra of 1,3,5-tri[((4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-2H-indazol-2-yl)methyl]-2,4,6-trimethylbenzene?

A complete walkthrough: from molecular symmetry to peak count.

Expected ¹H NMR Peaks

12 distinct proton environments — scroll down for the full analysis

1Recognize the Molecular Symmetry

Before counting individual protons, always look for symmetry — it dramatically reduces the number of unique environments.

This molecule has a **mesitylene core** (1,3,5-trisubstituted 2,4,6-trimethylbenzene) with **three identical substituent arms**. The C₃ rotation axis means all three arms are equivalent.

C₃ Symmetry

Arm A indazolyl

←

Mesitylene Core

→

Arm C indazolyl

↓
Arm B indazolyl

Arm A ≡ Arm B ≡ Arm C → count protons in ONE arm only

Key Principle

With C₃ symmetry, we only need to find unique proton environments in **one arm + the core**. Every environment in one arm appears identically in the other two — they give the same NMR peak, just with 3× the integration.

2Break Down the Structure

The molecule has three distinct regions:

A) Mesitylene Core

2,4,6-trimethylbenzene with substituents at positions 1, 3, 5. All six ring positions are substituted (three CH₃, three CH₂N groups), so there are **no aromatic ring protons**. The three methyl groups are equivalent by C₃ symmetry.

B) Methylene Linker (–CH₂–)

Each arm is connected via an N–CH₂–Ar group. The three CH₂ groups are equivalent by symmetry. However, within each CH₂, the two protons are **diastereotopic** — the adjacent chiral centers at C-4 make Hₐ and Hᵇ inequivalent.

Diastereotopic Protons

When a CH₂ group is adjacent to a stereocenter (or attached to a prochiral center in a chiral molecule), the two protons experience different chemical environments. They appear as **two separate signals**, often as an AB quartet. This is critical for correct peak counting.

C) The Indazolyl Arm

The (4S,7R)-7,8,8-trimethyl-4,5,6,7-tetrahydro-4,7-methano-2H-indazol-2-yl group is essentially a **camphor-skeleton fused to a pyrazole ring**. The 4,7-methano bridge with gem-dimethyl at C-8 creates a rigid norbornane-type bicyclic framework.

    Pyrazole ring         Methano bridge
         ╱   ╲                 ╱
  H─C3═C3a──C4(H)──────C8(CH₃)₂
       ║      │              ╱
       N1    C7a──C7(CH₃)──╱
       ║                │
       N2          C6(H₂)
       │                │
      CH₂          C5(H₂)
       │
  Mesitylene

  Green = protons to count

3Count Every Unique Proton Environment

Click any row to see the reasoning behind each assignment:

#	Assignment	Region	Total H
1	Ar–CH₃ (mesitylene)	Core	9H
2	N–CH₂–Ar (Hₐ)	Linker	3H
3	N–CH₂–Ar (Hᵇ)	Linker	3H
4	H-3 (pyrazole =CH)	Pyrazole	3H
5	H-4 (bridgehead)	Ring	3H
6	H-5 endo	Ring	3H
7	H-5 exo	Ring	3H
8	H-6 endo	Ring	3H
9	H-6 exo	Ring	3H
10	C7–CH₃	Bridge	9H
11	C8–CH₃ (α)	Bridge	9H
12	C8–CH₃ (β)	Bridge	9H

Core: Ar–CH₃9H

Linker: N–CH₂ (2 peaks)6H

Pyrazole: H-33H

Bridgehead: H-43H

Ring: H-5endo + H-5exo6H

Ring: H-6endo + H-6exo6H

Bridge: C7–CH₃9H

Bridge: C8–CH₃ × 2 (2 peaks)18H

TOTAL PROTONS60H

UNIQUE ENVIRONMENTS12 peaks

4Expected ¹H NMR Spectrum

Here's a schematic representation of the expected spectrum. Peak heights reflect relative integration (number of equivalent protons). Hover over peaks for details.

Simulated ¹H NMR (schematic)δ / ppm

Note: Exact chemical shifts are approximate and would depend on solvent, concentration, and spectrometer frequency. The schematic illustrates the expected *number* and *relative intensity* of signals.

5Key Concepts Used

C₃ Symmetry Reduction

Three identical substituents on a symmetric core means every proton environment repeats 3× — count it once, integrate ×3.

Diastereotopic Protons

CH₂ groups adjacent to stereocenters have non-equivalent protons. The N–CH₂ linker and the C5/C6 endo/exo pairs each produce separate peaks due to the rigid chiral norbornane framework.

Gem-Dimethyl Diastereotopicity

The two methyl groups on C-8 (bridge carbon) are diastereotopic because the bridge connects two different bridgehead carbons (C-4 with H vs C-7 with CH₃). They give two separate singlets.

No Aromatic Ring Protons

The mesitylene core is fully substituted at all 6 positions — a common trap in exam questions. Don't accidentally count ring H's that aren't there.

Quiz

Test your understanding with these questions.

What symmetry does this molecule possess that reduces the number of NMR peaks?

Are the ArCH₃ groups on the mesitylene core chemically equivalent?