Symmetry C3V For MeCl

Methyl chloride belongs to the $C\_{3\mathrm{v}}$ point group. This is because it has a $C\_3$ axis down the $\ce{Cl-C}$ bond. 


However what I do not understand is how it has three vertical mirror planes? It has three planes down each of the $\ce{C-H}$ bonds, but don't these bisect a bond making them dihedral planes? Although I'm wrong I can't fathom why it isn't a $C\_{3\mathrm{d}}$ point group. I have used a molecular viewer to visualise it but I still get the vertical mirror planes bisecting a bond...




You've correctly identified the $C\_3$ axis that contains the $\ce{C-Cl}$ bond. Methyl chloride also has 3 planes of symmetry that **contain** the $C\_3$ axis and each of the $\ce{C-H}$ bonds. Since these planes contain the $C\_3$ axis they are referred to as $\sigma\_{v}$ planes of symmetry. There are no other symmetry elements in methyl chloride so it belongs to point group $C\_{3v}$ (n $\sigma v$ planes plus a $C\_n$ axis). 


There is no $C\_{nd}$ point group. There is a $D\_{nd}$ point group which contains a $C\_n$ axis, n $\sigma\_{v}$ planes (so far methyl chloride fits the description), but also n $C\_2$ axis. Methyl chloride does not contain any $C\_2$ axes, so it does not belong to the $D\_{nd}$ point group. 


 6

If I remember my symmetry correctly then a dihedral plane by definition is a vertical plane that bisects a perpendicular $C\_2$ axis. Since we have a perpendicular $C\_2$ axis then I think that would mean you label it $D\_n$. 


In this case there are no perpendicular $C\_2$ axes so I think for that reason they are just vertical planes until such time that you they bisect a perpendicular $C\_2$. 


I don't know what would be the case if you had $C\_n$ on the principal axis and then $m$ perpendicular $C\_2$ axes, where $m\neq n$, maybe someone could shed some light? My guess would be that it wouldn't be $D$ but would qualify for the $\sigma\_d$ plane. 


 0

The picture shows the vertical symmetry planes along the principal $\ce{C3}$ axis in the $\ce{C\_{3{\mathrm{v}}}}$ molecule $\ce{CH3CN}$ which has a $\ce{CN}$ group instead of Cl.


[![acetonitrile](https://i.stack.imgur.com/83OGkm.png)](https://i.stack.imgur.com/83OGkm.png)


 0