Question

Help me annotate my IR spectrum of 2-​Propanone, 2-​(2,​4-​dinitrophenyl)​hydrazone?

Answer 1

Based on this spectrum of nitrobenzene from SDBS (http://sdbs.db.aist.go.jp/), I don't think the `"1618 cm"^(-1)` is the imine bond. I think that is one of the `"NO"_2` stretches, since they are very similar shapes.

It may be the weak `bb("1689 cm"^(-1))` peak. I don't know how strong or weak it's supposed to be, but it does reasonably match the first reference spectrum I showed.

(I am referring to the weak shoulder to the left of the two nitro stretches.)

If not, it may be the `bb("1587 cm"^(-1))` peak, since the two nitro groups on the same benzene ring may withdraw electron density away from the imine bond, weakening it.

(A weaker bond has a lower force constant and thus a lower fundamental stretching frequency.)

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I searched your compound on http://scifinder.cas.org/, and landed on:

REFERENCE FREQUENCIES

From my text (Techniques in Organic Chemistry 3rd ed. by Mohrig), these are the relevant reference frequencies that I can find off-hand:

• `bb("NO"_2)` stretches (`"1570 - 1490 cm"^(-1), "1390 - 1300 cm"^(-1)`, strong)
• Aromatic
  `"C"="C"` stretch (`"1620 - 1440 cm"^(-1)`, medium to weak),
  `"C"-"H"` stretch (`"3100 - 3000 cm"^(-1)`, medium to weak),
  `"C"-"H"` bend (`"900 - 680 cm"^(-1)"`, strong)
• Amines
  `"N"-"H"` stretch (`"3550 - 3250 cm"^(-1)`, medium;
  `2^@` amine `=>` one band)
• Alkanes
  `"C"-"H"` stretch (`"2990 - 2850 cm"^(-1)`, medium to strong)

Overall, here's what I came up with:

REFERENCE SPECTRA

To compare, here is a spectrum from the Bio-Rad/Sadtler IR Data Collection (Bio-Rad Laboratories, Philadelphia, PA), taken with a BIO-RAD DIGILAB FT-IR:

Since the amine frequencies weren't clear, another reference spectrum is from the Integrated Spectral Database System of Organic Compounds (National Institute of Advanced Industrial Science and Technology [Japan]), taken with a JASCO FT/IR-410:

Lastly, this website reports about `"1660 cm"^(-1)` for the `"C"="N"` stretch.