Question

Order the following chemicals according to the position of their ¹H-NMR signal from low to high ppm values

CH₂Cl₂ , CH₂Br_2, CH_4,CH₂F₂

Answer 1

The position of their ¹H-NMR signal depends on the electronegativity of atom attached to adjacent carbon atom.

we know electronegativity order of given set as F > Cl > Br > H ( Do not confuse with electron affinity, in this case Cl > F > Br > H). What will happen when a proton is adjacent to more electronegative atom is the electronegative atom reduces electron density around proton due to its electronegative character and this will in turn removes the secondary induced magnetic fields by electrons. Hence the proton will resonate at much lower fields (deshielded). Generally we show NMR scale from right(0 ppm) to left (14 ppm). Downfield means left side (increasing ppm value).

In view of above the correct order should be as follows

CH₄, CH₂Br₂, CH₂Cl₂ ,CH₂F₂

For your reference purpose i have also included 1H NMR chemical shift table also.

Thank You!

Please rate this answer by clicking on "Thumbs Up" icon to support me, if this answer finds correct and helpful.