The methyl group in ethanol’s 1H NMR spectrum at 400 MHz splits into 3 peaks, appearing as a triplet due to coupling with the adjacent CH2 protons. This follows the n+1 rule where n=2 neighboring protons on the methylene group cause splitting into 3 lines.

NMR Splitting Basics

Proton NMR splitting arises from spin-spin coupling between magnetically nonequivalent hydrogens on adjacent atoms, governed by the n+1 rule: a proton coupled to n equivalent neighboring protons splits into n+1 peaks. In ethanol (CH3-CH2-OH), the methyl protons (CH3) couple only to the two methylene protons (CH2), ignoring the distant OH proton due to rapid exchange that averages its coupling to zero. The 400 MHz field strength improves resolution but does not alter the splitting pattern, which depends solely on molecular structure and coupling constants.

Ethanol Spectrum Details

Ethanol shows three signals: CH3 as a triplet at ~1.1-1.2 ppm (3H), CH2 as a quartet at ~3.6 ppm (2H), and OH as a broad singlet at ~2-5 ppm (1H, solvent-dependent). The methyl triplet has peak intensity ratios of 1:2:1 from binomial coefficients for n=2. At high resolution and low water content, OH coupling can appear (further splitting CH2 into doublets of quartets), but standard conditions yield clean triplet for CH3.

Common Misconceptions

No options are provided, but typical wrong answers include 1 (ignoring splitting), 2 (mistaking for doublet from n=1), 4 (confusing with CH2 quartet), or 7 (erroneously including OH coupling as n=5 total neighbors). The OH proton does not split CH3 because it is separated by three bonds from CH3 (vicinal coupling) and exchange prevents observable interaction; coupling constants J_CH3-CH2 (~7 Hz) dominate.

The 1H NMR spectrum of ethanol at 400 MHz reveals key structural insights, particularly how the methyl group splits into 3 distinct peaks, making it a classic example for CSIR NET Life Sciences spectroscopy questions.

Understanding Methyl Splitting

In ethanol (CH₃CH₂OH), the methyl protons experience coupling from the two equivalent CH₂ protons across three bonds, following the n+1 rule (n=2 yields 3 peaks: triplet at ~1.2 ppm). High-resolution 400 MHz spectra sharpen this without changing multiplicity, confirming 3 peaks via Pascal’s triangle intensities (1:2:1).

Full Spectrum Breakdown

• CH₃ (triplet, 1.1-1.2 ppm, 3H): Coupled to CH₂.

• CH₂ (quartet, 3.6 ppm, 2H): Coupled to CH₃ (n=3, 4 peaks).

• OH (singlet, 2-5 ppm, 1H): Broadened by exchange, no splitting observed.