Q.50 The total number of multiplet peaks in the 1 H NMR spectrum of 1,3,5-tri-isopropylbenzene in
CDCl 3 is ________.

The total number of multiplet peaks in the ¹H NMR spectrum of 1,3,5-triisopropylbenzene in CDCl₃ is 2.

This compound exhibits high symmetry due to its three identical isopropyl groups at positions 1,3,5 on the benzene ring, resulting in only two distinct types of protons that produce multiplet signals. The aromatic protons appear as one multiplet, while the methine protons of the isopropyl groups form another.

Molecular Structure

1,3,5-Triisopropylbenzene (C₁₅H₂₄) features a benzene ring substituted with three -CH(CH₃)₂ groups at symmetric meta positions. This C₃ symmetry makes the three aromatic CH protons (at positions 2,4,6) chemically equivalent and the nine methine CH protons (one per isopropyl) equivalent to each other. The eighteen methyl CH₃ protons are also equivalent but appear as a distinct signal.

Proton Environments

• Aromatic protons (3H): Located ortho to isopropyl groups, these experience deshielding and couple to methine protons (³J ≈ 7-8 Hz), producing a septet-like multiplet around 6.9 ppm.

• Methine protons (9H): The -CH- protons in isopropyl groups couple to six equivalent methyl protons each (³J ≈ 7 Hz) and weakly to aromatic protons, resulting in a complex septet multiplet near 2.9 ppm.

• Methyl protons (18H): These twelve equivalent CH₃ groups produce a clean doublet at ~1.2-1.3 ppm due to coupling with methine protons, not a multiplet.

NMR Peak Analysis

Actual spectra in CDCl₃ show signals at ~6.91 ppm (aromatic multiplet), ~2.95-2.80 ppm (methine multiplet cluster), and ~1.29 ppm (methyl doublet). The methine region spans multiple Hz positions (e.g., 264-250 Hz at 90 MHz), confirming one broad multiplet despite fine structure. Only these two signals qualify as multiplets; the methyl is a simple doublet.

The 1H NMR spectrum of 1,3,5-triisopropylbenzene in CDCl3 is a key topic for CSIR NET Life Sciences and IIT JAM Biotechnology aspirants, highlighting molecular symmetry in spectroscopy. This compound’s high symmetry simplifies its spectrum to essential signals, making it ideal for understanding multiplet peaks in 1H NMR.

Key Signals in the Spectrum

Actual 90 MHz data shows three main regions: aromatic multiplet (6.9 ppm, 3H), methine multiplet (2.8-2.95 ppm, 9H), and methyl doublet (1.29 ppm, 18H). The total number of multiplet peaks is 2, as only aromatic and methine protons split into multiplets due to n+1 rule coupling.

• Aromatic: Septet from methine coupling

• Methine: Complex septet from methyl (6H) and aromatic

• Methyl: Doublet (not multiplet)

Why Only 2 Multiplets?

Symmetry equates all three aromatic H, all nine CH, and all eighteen CH₃, reducing signals. CDCl₃ solvent ensures clean peaks without interference. Exam questions test this equivalence for numerical answers like “total multiplet peaks in 1,3,5-triisopropylbenzene CDCl3”.

Exam Relevance

For competitive exams, recognize symmetry in 1,3,5-trisubstituted benzenes yields 2 multiplets + 1 doublet. Practice with real spectra confirms the answer as 2.