What specific feature determines whether a given molecule is chiral ?

Look at a single carbon atom. If each of the four moieties (groups) surrounding that carbon atom is different (different number of carbon, hydrogen, or something else), the molecule is chiral. All four must be different.

Im studying halothane-desflurane-isoflurane...

Im being asked which of these molecules are chiral and to draw two representations of the structure which arise because the molecule is chiral...

With the answer you have given none have four different atoms coming off the carbon... What am I doing wrong ?

Halothane is CF3 - C(H)(Br)(Cl)
Desflurane is CF3 - C(H)(F)-O-CF2H
Isofluane is CF3-C(H)(Cl)-OCF2H

The rightmost carbon of halothane is chiral because it has four different groups attached to it: -CF3, -H, -Br, and -Cl

The second carbom from the left of desflurane and isoflurane is the "chiral center" for the same reasons.

When you draw the chiral center, you have to show the three dimensional structure or it doesn't make any sense. Unfortunately, I can't draw ;-) but Wikipedia has a page on chirality that shows some drawings:

http://en.wikipedia.org/wiki/Chirality_(chemistry))

File:Chiral.svg - Wikipedia, the free encyclopedia

The two different structrures are called "enantiomers". They are designated "L", "l", Levorotatory, or "+" and "D", "d", Dextrororatory, or "-". When you pass a polarized light through a solution of the molecule, the plane of polarization is rotated either to the left "L", or to the right, "D".

Levorotation and dextrorotation - Wikipedia, the free encyclopedia

Hope this helps.

Perito I had to spread the rep but your answer is very good.

Thank you Perito you are most helpful.I will try and understand it and take it all in.

I know understand which parts are chiral but am having trouble drawing two representations which arise because the molecule is chiral...

I am presuming the mean mirror image of the chiral part ?

If so does the CF3 and the CF2H stay in there original place?

In other words is it just the chiral parts that has the mirror image ?

The two enantiomers are mirror images. They represent two different molecules -- one left-handed and the other right-handed.

You can think of -CF3 and -CF2H as simply rotating groups. They will rotate, in reality. You say, "stay in their original places", and I guess that is correct.

The "chiral center" is the carbon atom at the center of it all. Yes, the chiral center has the mirror image. If you created a three-dimensional model of one of the non-chiral centers, and if you tried to make a mirror image of it, you would find that you could rotate one of the images so it would be superimposed on the remaining one. In other words, there is no mirror image of a non-chiral center.

Look at the Wikipedia pictures. It's not possible to superimpose the images of chiral molecules.