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 


![enter image description here](https://i.stack.imgur.com/0PNJJ.png)


[image source](http://socratic.org/questions/how-can-i-draw-cis-and-trans-isomers)


* *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.


We can extend this reasoning to molecules with dipole moments, if the dipole moment is very small. *Trans*-2-pentene does have a dipole moment but it is very small and it is less than the dipole moment in the *cis*-isomer **and** the dipole moment in the *trans*-isomer (being smaller to begin with) will change less upon stretching the double bond than it will for the *cis* isomer. Therefore we would expect a very weak absorption in *trans*-2-pentene, while the *cis* isomer should show a stronger absorption.


 12