Which of the following bonds will have the lowest IR stretching frequency? (a) C

Question

Which of the following bonds will have the lowest IR stretching frequency?

(a) C–C (b) C–P (c) C–Se (d) C–I (e) Si–I

The stretching motion of which of the indicated bonds absorbs at a lower frequency in an IR spectrum and why? CH3CH2CH2-NH2 , CH3CH2CN

(a) The C-N bond in the molecule on the left because it is the stronger bond. (b) The C-N bond in the molecule on the left because it is the weaker bond. (c) The CN bond in the molecule on the right because it is the weaker bond. (d) The CN bond in the molecule on the right because it is the stronger bond.

The answer to these question are e and b, can you explain the reason? thanks

Explanation / Answer

* Greater the difference in electronegativities of bonded atoms, greater the dipole which inturn absorbs greater amount of radiation. Smaller the polarity smaller the stretching frequency.Hence

(b) C - C bond is non polar so has lowest IR stretching frequency.

* CH3CH2CH2NH2 absorbs a lower frequency than CH3CH2CN due to its lower polarity.

i.e (b) The C-N bond in the molecule on the left because it is the weaker bond

(d) The CN bond in the molecule on the right because it is the stronger bond.

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