How to identify a compound as chiral or achiral

I read my book and learned everything about enantiomers, racemization, $\mathrm{S\_N1}$ and $\mathrm{S\_N2}$ reactions, but when I'm trying to identify molecules as chiral or achiral, I get confused.


Consider the two molecules below:



```
1.CH3-CH-OH
      |
      Br

2.CH3-CH-Br
      |
      Br

```

How can I tell if a compound is chiral?




Look for carbons with four **different** groups attached to identify potential chiral centers.


Draw your molecule with wedges and dashes and then draw a mirror image of the molecule. If the molecule in the mirror image is the same molecule, it is achiral. If they are different molecules, then it is chiral.


Here you're particularly interested in the C in the CH. However, take a look at the C in CH3. Why isn't it a chiral carbon?


 11

Chirality can be determined by visualising the molecule in space. Once you do that, you can check to see if there exists a non-superimposable mirror image. If this is indeed the case, then your molecule is chiral.


You can look for an asymmetric carbon atom--because this often results in chirality. 


An asymmetric carbon (or chiral carbon) is, very simply, a carbon atom attached to four different groups.


With this in mind, consider your examples: 1) should thus be chiral and 2) is achiral.


Hope this makes sense.


 8

1. Draw both molecules. Don't draw them flat. Alternatively, model them using [Avogadro](http://avogadro.cc).
2. Exchange the positions of $\ce{Br}$ and $\ce{OH}$ in the first case. Is the new molecule identical to the original one?
3. Do the same for the dibromo-substituted compound. How is the result here?


 8

The first method I used to understand chirality was to grab some different colored zip ties and make a couple small tetrahedral structures with them, assigning a color to each different radical (say blue for $\ce{CH3}$, white for $\ce{H}$, red for $\ce{OH}$ and yellow for $\ce{Br}$.)


Then look at it by rotating the first to see if I can get the second molecule, if you can't then that molecule is chiral.


 5