# Distinguish between symmetric and asymmetric molecules with IR

I get increasingly nervous as I get my organic lab course reports. For, I often get asked whether the peak in the IR comes from an asymmetric or symmetric stretch.

I've looked at about ten books and turned google upside down. But I still can't answer the question in every case, though our molecules are quite simple.

Can you help me out with a table or something equivalent?

The most urgent example is a $\ce{C-H}$ streching at $3006~ \mathrm{cm}^{-1}$ from p-tbutyl-(2-ethanol)-benzene (don't mind the nomenclature, please).

• Is that peak for the hydrogens on the benzene ring, the t-butyl group, or the ethanol group? – LDC3 May 7 '14 at 1:02
• But are you talking about symmetric and asymmetric molecules or symmetric and asymmetric vibration modes?9 – G M May 7 '14 at 13:49

## 2 Answers

Whether the stretch of a bond in a molecule is asymmetric or symmetric depends on the relative movement of the atoms connected by the bond. Lets take a $\ce{CH2}$ group as an example. If the two hydrogen atoms connected to the carbon move in the same direction during stretching, it s a symmetric stretch, if they are going in opposite directions, the stretch is asymmetric (see also here):

Left: symmetric stretch, right: asymmetric stretch

A bond vibration like stretching will only be IR-active (i.e. give a band in the IR spectrum) if it is accompanied by a change of dipole moment. In the case of the $\ce{C-H}$ bond, carbon is slightly negatively and hydrogen slightly positively polarized. A symmetric stretch increases and decreases the distance between these partial charges periodically, and therefore also changes the dipole moment, because the latter is dependent from the distance between charges (source). During an asymmetric stretch, the elongation of one $\ce{C-H}$ bond is compensated by the shortening of the other, so the overall change of dipole moment should be weaker.

In the case of 2-(4-tert-butylphenyl)ethanol (the correct name for your compond), the band at 3006 $\mathrm{cm}^{-1}$ can either be assigned to the $\ce{CH2}$ groups of the substituted ethanol or to $\ce{C-H}$ bonds in the benzene ring. In the first case, you can distinguish between symmetrical and asymmetrical stretch (symmetrical seems more likely to me), while in the latter case, a single hydrogen is attached to a carbon, and a distinction of the stretching, like in the case of two substituents at the same carbon, cannot be made.

first of all I have no practical experience with this field (yet), but we had covered this in theory (and also I don't have chemistry in English). I hope that I can help.

From what I recall, you have to first write the structural diagrams of the suspected molecules and compare them as it can be seen on the picture bellow
(I had izopropanol and propanol in my book, but I think that you will get this from this). Then you search in tables for appropriate absorption frequencies and write them down.

Then you check the experimental graph that you obtained in order to find out with what molecule you are dealing. So for example the corresponding graphs for these two substances are:

• thanks, but this is not quite my problem. i know how to figure out if i got my compound from an ir. my problem is, that i have no idea how to know if a strech is smetric or not... – Bulawa May 7 '14 at 10:53
• @Bulawa I cannot help you with that. The title mislead me. Cheer up! Somebody from the community is bound to know the answer :) – studen May 7 '14 at 14:02