With ONLY the IR spectrum I need help determining if this unknown is either a Cr

Question

With ONLY the IR spectrum I need help determining if this unknown is either a Crotonaldehyde or Valeraldehyde. The other possible option was Chloral (Trichloroacetaldehyde) but I couldn't detect any halogen presence.

Unknown is a yellow liquid, extremely strong sweet (vanilla/almond) odour, boiling point = 100-102°C, soluble in NaOH, not soluble in H2O & HCl.

Created a 2-4DNP derivative but was unsuccessful only creating a melting point of 98C. The closest this was to was Valeraldehyde (2,4DNP derivitive =107°C).

20 30 40 50 60 70 80 90 100 Transmittance 8 3 | 2973.46- 2814.16- 2733.25 1683.55 1640.28 1443.63 1393.04 1279.55 1146.53 , 1078.36 1044.05 8 9 716.60 Cn 544.36 461.39

Explanation / Answer

I will answer the question based on the IR spectrum. Note the structures of crotonaldehyde and valeraldehyde as shown below.

crotonaldehyde

valeraldehyde

The IR spectrum shows a strong peak at 1683 cm-1due to the C=O group. The C=O stretching frequency occurs at a lower wavelength than normal aldehydes (1740-1720 cm-1). This is definitely due to conjugation with the olefinic double bond of crotonaldehyde. The C=C stretching frequency of crotonaldehyde occurs as a medium intensity peak around 1640 cm-1.

–C-H stretching frequency of alkyl groups occur as strong peaks around 2850-3000 cm-1. The absence of a strong peak in the said region leads us to rule out valeraldehyde as the possible compound. The compound is therefore, crotonaldehyde.

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