Question

For the protons labeled Ha and Hb in the structure below, predict the characteristics of their signals in the 1H NMR spectrum: the approximate chemical shift, the splitting pattern, and the

For the protons labeled Ha and Hb in the structure below, predict the characteristics of their signals in the 1H NMR spectrum: the approximate chemical shift, the splitting pattern, and the integration value.

Answer 1

Concepts and reason

A branched alkane molecule is given and the splitting pattern of the peaks, chemical shifts, and the integration value needs to be given of the two labelled protons ${{\rm{H}}^{\rm{a}}}\;{\rm{and}}\;{{\rm{H}}^{\rm{b}}}$ in its $^1{\rm{H}}$ NMR spectrum. Determine the chemical shifts based on inductive effect and splitting based on adjacent equivalent protons.

Fundamentals

The NMR spectrum is used for identification of compounds based on the nature of hydrogens or carbons present in an organic compound. Two types of NMR spectrum are there - $^1{\rm{H}}$ and $^{13}{\rm{C}}$ . It identifies the chemical environment of the hydrogens and carbons based on the spectrum taken which helps in identifying a compound.

The frequency of a nuclei is measured with respect to a standard in a magnetic field known as chemical shift. The standard used in NMR is tetramethyl silane (TMS).

The inductive effect plays a crucial role in the NMR spectrum. In case of presence of an electron withdrawing substituent, the electron density shifts towards the electronegative element and hence, the signal appears downfield whereas if the proton is attached with an electron releasing substituent, the signal appears upfield.

The protons or carbons give different peak based on the chemical environment they are present in and give a single peak if they are present in same chemical environment.

Magnetically equivalent protons are the protons that give the same peaks. These protons on replacement with another substituent give the same compound.

The spin-spin coupling is another feature of NMR spectrum that helps in identification of structure. Protons present in different chemical environments lead to the splitting of a NMR peak. The number of peaks obtained are given as n+1 where n is the number of equivalent protons.

Thus, the peaks obtained are singlet, doublet, triplets, quartets based on n = 0, 1, 2, 3 and so on respectively.

The integration value or the area under the peak corresponds to the protons that give rise to a particular NMR signal.

The chemical shift of the ${{\rm{H}}^{\rm{a}}}$ proton will be near to 1 ppm, the splitting would be triplet and the integration value will be 6H.

The chemical shift of the ${{\rm{H}}^{\rm{b}}}$ proton will be near to 1.3 ppm, the splitting would be quartet and the integration value will be 4H.

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