Question

Propose the structure of a monosubstituted geminal alkene that fits the following NMR spectrum.

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Answer 1

NMR spectra of compound gives following information:

1) The relation between number of signals (peakss) and number of different kinds of hydrogens.

2)The area underneath each signal are in the same ratio as the number of hydrogen atoms causing signals.

3) The splitting patter depends on the number of neighbouring non-equivalent hydrogen atoms. Splitting pattern can be predicted by n+1 rule, where n is the number of neighbouring protons.

4) The chemical shift value tells the sheilding ot deshielding of protons.

5)  The hydrogen atoms bound to a carbon consisting of a double bond are found in low field of the NMR spectrum, which is the left side, and the hydrogens are said to be deshielded. The cause for this is due to the movement of the electrons in the pi bond of the carbon-carbon double bond.

In the given question:

The mono substituted geminal alkenes are basically terminal alkene. So the proposed structure of molecular formula C₅H₁₀ may be as :

CH₂=CH-CH₂-CH₂-CH₃

If we observe the structure, we have six different types of hydrogen.

a) -CH₃ - Highly shielded, will appear around 0.9 ppm, as a triplet, as spin spin splitting due to two neighbouring hydrogens

b) -CH₂- : Will appear around1.4ppm, as a sextate, due to splitting by 5 neighbouring protons.

c) -CH₂- ; Will appear around 2.0 ppm, as a quaderate, due to splitting by 3 neighbouring protons.

d) =CH - Will appear around 5.8 ppm, as a multiplet, due to splitting by neighbouring protons.

e) =CH_e : Will appear around 4.8 ppm, as a triplet, due to splitting by neighbouring protons.

f) =CH_f : Will appear around 4.9 ppm, as a multiplet, due to splitting by neighbouring protons.