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Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: rogixmalix on September 28, 2019, 07:10:30 PM

Title: Infrared Question
Post by: rogixmalix on September 28, 2019, 07:10:30 PM
Acetophenone group C = O has a vibrational frequency of approximately 1685 cm -1. Substitution on the aromatic ring with an amino group in para (II) decreases this value to 1652 cm-1., While replacement with a nitro group (III) increases the value to 1693 cm-1. Explain the change in frequency vibrational in both cases.

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Title: Re: Infrared Question
Post by: hollytara on September 28, 2019, 10:01:54 PM
You can draw resonance structures for acetophenone where the carbonyl becomes a single bond with the O having a negative charge, there is a double bond from ring to carbonyl carbon, and the ring has two double bonds and a positive charge; there are 3 structures, with the charge in different places around the ring (ortho and para to the carbonyl). 

What is the influence of the amino and nitro groups on the contribution of these resonance structures?
Title: Re: Infrared Question
Post by: pgk on October 01, 2019, 12:51:43 PM
Resonance occurs via hybridization of the involved orbitals and therefore, it changes the spring constant k of the involved bonds and consequently, it also changes the wavenumber of vibrational frequency v.

v = (1/2πc)/√(k/μ)

where, μ = m1m2/(m1 + m2)