[deutdeut]

I have a question in my book. There is a chiral molecule, the CH2=CH group is on the left. Another molecule also has the same kinds of atoms, and the CH2=CH group is also on the left. Since they are not mirror images, I think, they should not be considered as optical isomers. But my book said that it is. Why?

[english]

The definition of an optical isomer is if your molecule cannot be superimposed on its reflection. 

[Dan]

Can you draw them out?

[deutdeut]

I don't know how to draw then out on screen. But the molecule has a tetrahedral shape with the carbon atom at centre, bromine atom at the top, the CH2=CH group on the left, a hydrogen atom pointing out of paper and a methl group pointing out of paper, both on the right of CH2=CH group. The other molecule has the same kind of atoms, but the bromine atom is replaced by hydrogen atom and vice versa. I cannot see why the two are mirror images to each other though they are chiral. Can anyone help?

[english]

I cannot possibly draw those because you said that the H and CH₃ are both coming out of plane.  That makes no sense.

All you need to realize is that you have 4 unique groups generating an asymmetric center, a.k.a. stereogenic center.

You have Br, H, a vinyl group and a methyl group.


You do not need software to draw them.  You can draw them by hand and scan them onto your computer, uploading them here.


But because I am an amazingly nice person, I will point you to this good sketching freeware:

http://www.acdlabs.com/download

[deutdeut]

Sorry, the methyl group should be into the plane.

[english]

The easiest way to tell if something simple like this is chiral is to look at what the carbon is bonded to.

If it is bonded to 4 unique groups, it is chiral.

Unique meaning different groups.  If a carbon is bonded to —OH, —NH₂, H, and —CH₂CH₃, it is a chiral molecule.


With two centers it gets trickier.

[deutdeut]

I know it's chiral, but the problem is the two given structures seem not to be mirror images to each other as the corresponding atoms are not facing each other. The problem is question11c.

[english]

I know it's chiral, but the problem is the two given structures seem not to be mirror images to each other as the corresponding atoms are not facing each other.

You can have chiral compounds by showing the reflection of that compound (mirror image), OR by keeping the same orientation as that compound and reversing the two stereobonds.

For example, keep both compounds facing the same direction (not as mirror images), and reverse the dash and wedge of one of them, now you have the other one.


We can draw them as reflections, as in image 1.  Or we can have them face the same direction and just switch the stereobonds, as in image 2.


Either way, they are both right in representing the two enantiomers of this compound.  Both enantiomers are chiral.

[deutdeut]

The problem is scanned as below. Please help to analyse!

[english]

I've already explained your question.  If you have any more questions just PM me. 

[deutdeut]

But can you show me the arrangement of atoms of the molecule at the right of question11c (the page that I've attached)after it's been rotated 180 degrees, because the molecule at the right do not shift bonds as you've mentioned?

[Dan]

You just have to rotate them around to see the mirror plane relationship.

[deutdeut]

Can you show more examples of molecules like you've rotated in the first picture?

[Dan]

These rotations are just moving the structure around without breaking any bonds. Essentially, you are just drawing the structure from a different viewpoint. You can rotate around single bonds (like my first rotation), and rotate the entire structure (like the second one). This applies to any molecule. Just practice. Stereochemistry at a glance by Eames and Peach is a good book to look at with lots of examples and definitions. If you have trouble doung 3D rotations in your head, get yourself a model kit (expensive), or use plasticine and toothpicks or something similar (cheap)