# In infrared spectroscopy, why is there no C=C reading for E alkenes?

For the (E)-2-pentene, there was no C=C reading, about 1600. For the (Z)-2-pentene, there is a C=C. Why is there a reading on the Z but not on the E?

The $\ce{C=C}$ stretch is responsible for this ir peak. For an ir absorbtion to occur, the absorption must result in a change in dipole moment. If we examine the $\ce{C=C}$ stretch in cis- and trans-2-butene we find that
• cis-2-butene has a dipole moment (0.33 D) and upon stretching the double bond the dipole moment will change; therefore we would expect to see an ir peak for the $\ce{C=C}$ stretch in this molecule
• trans-2-butene does not have a dipole moment (0 D) and upon stretching the double bond the dipole moment will remain zero, there is no change; therefore we would not expect to see an ir peak for the $\ce{C=C}$ stretch in this molecule
A general rule based on these dipole moment arguments is that, for symmetrically di-substituted olefins $\ce{C=C}$ stretches in trans olefins are non-existent or very weak, the absorption is much stronger in the cis isomers.