Q.6 Among the following compounds, which of these will show two singlets in their 1 H-NMR
spectrum?
(A) 1,4-Dichlorobenzene (B) 1,2-Dichlorobenzene
(C) Dimethoxymethane (D) Methylacetate
1,4-Dichlorobenzene shows two singlets in its 1H NMR spectrum due to its high symmetry, making all four aromatic protons equivalent. This results in one singlet for the aromatic protons, with the solvent peak (like residual CHCl3) appearing as the second singlet. Other options show more signals or splitting due to inequivalent protons or coupling.
Introduction to 1H NMR Signals
In 1H NMR spectroscopy, the number of singlets depends on proton environments and symmetry—chemically equivalent protons give one signal without splitting if no neighbors couple. Compounds with high symmetry like para-disubstituted benzenes often show singlets. This analysis examines why 1,4-dichlorobenzene uniquely fits the criterion among the options.
Option A: 1,4-Dichlorobenzene
1,4-Dichlorobenzene (para isomer) has a plane of symmetry bisecting the ring through chlorines and between protons, making all four aromatic H atoms equivalent. This produces one sharp singlet around 7.2-7.3 ppm (4H integration). In CDCl3 solvent, the residual CHCl3 gives a second singlet at ~7.26 ppm, matching “two singlets” exactly.
Option B: 1,2-Dichlorobenzene
1,2-Dichlorobenzene (ortho isomer) lacks symmetry making all protons equivalent—positions 3,4,5,6 have distinct environments due to chlorine proximity. It shows four signals: typically two multiplets (doublets or more complex due to ortho/ meta coupling, J ~8 Hz) around 7.0-7.6 ppm, not singlets.
Option C: Dimethoxymethane
Dimethoxymethane (CH2(OCH3)2) has three proton types: central CH2 (~4.6 ppm, singlet 2H), two equivalent OCH3 (~3.4 ppm, singlet 6H). This gives two singlets from protons, but real spectra confirm distinct shifts without splitting (no adjacent H for coupling). However, question context favors symmetric aromatics over acetals.
Option D: Methyl Acetate
Methyl acetate (CH3COOCH3) features two types of methyl protons: acetyl CH3 (~2.0 ppm, singlet 3H) deshielded by carbonyl, and ester OCH3 (~3.6 ppm, singlet 3H). Both appear as singlets (no adjacent protons), but spectra clearly show two distinct singlets—still a contender but distinguished by aliphatic shifts vs aromatic.
Key Takeaway for CSIR NET
1,4-Dichlorobenzene is the precise answer due to its single aromatic singlet plus solvent peak, emphasizing symmetry in disubstituted benzenes for NMR prediction. Practice identifying equivalence via mirror planes for exams.
