Answer
The IR spectrum of trans-oct-2-enoic acid, shows the following peaks;
O-H and C-H stretch: ~2980-2800 $cm^{-1}$, broad and strong
C=O stretch: ~1700-1650 $cm^{-1}$, broad and strong
C-O stretch: ~1300-1280 $cm^{-1}$
O-H in-plane bend: ~1400 $cm^{-1}$
O-H out-of-plane bend: ~960-920 $cm^{-1}$
If the carboxylic acid is saturated, then the C=O stretching frequency occurs in the range of 1750-1710 $cm^{-1}$. Due to unsaturation, the peak is shifted towards lower frequency. Moreover, there is conjugation between C=O and C=C bonds in the compound. So, the C=O stretching frequency is in the range of ~1700-1650 $cm^{-1}$
Work Step by Step
The stretching frequency is affected by conjugation and unsaturation. Due to conjugation, the frequency basically shifts to lower wave number.
