C₄H₆

From the molecular formula, the compound has "2 degrees of unsaturation" (two double bonds, carbonyls or rings).

3400 cm-1:	no OH or NH peak present		3100 cm-1:	sharp peak suggesting sp2 CH
2900 cm-1:	no peak indicating sp3 CH		2200 cm-1:	no unsymmetrical triple bonds
1700 cm-1:	no carbonyl absorbance		1600 cm-1:	strong band consistent with C=C

The spectrum seems to be consistent with a simple alkene containing no saturated C-H. The small peak at 1920 cm^-1 is not a carbonyl; it is not intense enough to be one and appears at too large a wave number.

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Structure:

IUPAC Name: 1,3-butadiene