[vivekfan]

Ok, I've read that methane is superimposable upon its mirror image, but i'm not sure I can completely picture it. Do I rotate it by holding the top carbon fixed? Every time I rotate the mirror image, I end up with the hydrogen (going into the page) and the hydrogen (going out of the page) in different positions. Can somebody explain how to rotate it or at least help me visualize please? I would greatly appreciate it.

As long as it is the same atom (in this case hydrogen) does it matter if the exact hydrogen in the original molecule that was outward becomes inward and the one that was inward becomes outward? Because one has to be inward and one has to be outward anyway, right? But then why would you have to rotate the molecule to get it to superimpose? If they are all hydrogens anyway, how would it matter? This would only matter in cases where the atoms were different, right? For example, if the substituents were bromine, chlorine, fluorine, and hydrogen, then it would matter if an inward and an outward atom switched places because it would switch the configuration, right? Please let me know if I am thinking about this in the correct way and add comments. Thank you very much!

[StarvinMarvin]

When you deal with methane, then whichever way you rotate it, you'll get the same outcome. It's because methane has elements of symmetry. If, however, you had methane with four different subsituents, when you draw its mirror image, you'll obtain a different structure and rotating it won't help. It will just be different

[vivekfan]

When you deal with methane, then whichever way you rotate it, you'll get the same outcome. It's because methane has elements of symmetry. If, however, you had methane with four different subsituents, when you draw its mirror image, you'll obtain a different structure and rotating it won't help. It will just be different

So why would you have to rotate in the first place because aren't all the hydrogens identical? And does it matter that the inside and outside are switched because it is the same substituent? Also how would you rotate it...holding the top carbon fixed?

[Vidya]

Let me help you
First of all when you are looking at the mirror image than you should not rotate it.Place it at one angle and draw its mirror image.As you look into the mirror you can see that your mirror image keeps changing as you change your position.You need to draw mirror image of a fixed position  of the molecule.After that you check whether are they superimposible.

[vivekfan]

Let me help you
First of all when you are looking at the mirror image than you should not rotate it.Place it at one angle and draw its mirror image.As you look into the mirror you can see that your mirror image keeps changing as you change your position.You need to draw mirror image of a fixed position  of the molecule.After that you check whether are they superimposible.

Could you help me figure out how to rotate methane's mirror image to get it to look like the original?

[azmanam]

Something I wrote for this thread: http://www.chemicalforums.com/index.php?topic=28328.0.  It's for determining R/S, but it's also useful for seeing if two molecules drawn differently are superimposable.




There are two operations I always find easiest for reorienting molecules that minimize mental gymnastics: the 120^o rotation and the 180^o rotation.  For each rotation, pick a bond that will remain stationary and that will be the bond around which you rotate - typically one of the in-the-plane bonds.

In the 120^o rotation, you don't change the position of any of the bonds, but the three other substituents (the ones not on the bond you're keeping stationary) rotation positions by one.  A second rotation rotates the three substituents to the next position, and the third rotation brings you back to your original orientation.

In the 180^o rotation, the other in-the-plane substituent stays in the plane, but moves to the other side of the paper.  The substituent in front moves to back, and the substituent in back moves to front.  A second 180^o rotation will bring you back to the original orientation.

You can reorient any molecule to your desired orientation with a combination of only 120^o and 180^o rotations, you just may need more than one rotation.  But you can eventually rotate any molecule to the orientation that's easiest to determine R/S.

Those are really hard to explain without pictures, so hopefully the following diagram will help.