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Topic: OPTICAL ISOMERISM  (Read 10029 times)

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NISHANT

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OPTICAL ISOMERISM
« on: January 06, 2005, 04:54:53 AM »
WE ALL KNOW THAT ETHENE WITH IT'S CARBONS HAVING ALL DIFFERENT GROUPS ATTACHED TO THE IS OPTICALLY INACTIVE.BUT IS PROPENE WITH DIFFERNT GROUPS ATTACHED TO THE DOUBLE BONDED CARBONS ALSO OPTICALLY INACTIVE?

Offline AWK

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Re:OPTICAL ISOMERISM
« Reply #1 on: January 06, 2005, 07:17:44 AM »
this is the same problem. In propene one of the substituents is a methyl group.
AWK

NISHANT

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Re:OPTICAL ISOMERISM
« Reply #2 on: January 06, 2005, 07:53:46 AM »
NO,BOTH THE CASES R DIFFERENT,in ehtene case the molecule can be cut into 2 by a plane.But in the propene case the molecule cannot be cut into 2 by a plane because the geometry of a carbon in propene in SP3 also?

Offline movies

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Re:OPTICAL ISOMERISM
« Reply #3 on: January 06, 2005, 12:43:24 PM »
Suppose you had ethene.  Then you substituted ethene so that the four substituents around the double bond were all different.  Suppose you picked to put a methyl group and an ethyl group off of one carbon and just one methyl group off of the other carbon, leaving one hydrogen (giving the compound 3-methyl-2-pentene, when properly named).

Now suppose you started with propene.  Then you subsituted the CH2 carbon with a methyl group and an ethyl group.  You would get the same molecule as above, so the question is essentially the same.

All that said, you need a chiral center (or chiral axis) for bulk optical activity.  3-methyl-2-pentene doesn't have a chiral center, so it won't be optically active.

Offline AWK

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Re:OPTICAL ISOMERISM
« Reply #4 on: January 07, 2005, 01:29:33 AM »
In propene, the methyl gruop can turn round a single bond. As a result we will obtain population of conformations that will show the same plane of symmetry as double bond.
But each individual conformation can be opticalyy active if fixed.
AWK

dexangeles

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Re:OPTICAL ISOMERISM
« Reply #5 on: January 07, 2005, 01:57:48 AM »
don't you have to have a chiral center for it to be optically active?

double carbons in double bonds cannot be chiral centers

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Re:OPTICAL ISOMERISM
« Reply #6 on: January 07, 2005, 02:51:36 AM »
As AWK alluded to, optical activity is a little more complicated than just having a chiral center.  For example, single molecules (even achiral ones) are optically active because the interaction with the polarized light is dependent on the orientation of the molecule relative to the plane of the light (such as parallel to the plane versus perpendicular to the plane).  When organic chemists measure optical activity they are dealing with many millions of molecules, so essentially every orientation has an equal population and they all cancel each other out.  When you have a chiral center, however, you can't populate some of the mirror image orientations needed to even everything out, so you can observe an optical rotation.

I hope that makes sense....

dexangeles

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Re:OPTICAL ISOMERISM
« Reply #7 on: January 07, 2005, 07:10:19 PM »
i do understand the optical activity part and racemic mixtures, meso forms, etc....

i'm just trying to give the person asking the question some clues/ideas to reach an answer, instead of giving it directly
at least make the person asking the question think a lil, i thought  :)
« Last Edit: January 08, 2005, 06:31:36 PM by dexangeles »

nocommentes

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Re:OPTICAL ISOMERISM
« Reply #8 on: January 10, 2005, 07:58:55 AM »
Don't confuse optical and geometry ISOMERISM!!!
 an essential condition for optical active molecule is presence rotation-reflecting axis higher then 2 order/. How could you find in any alkene this axis?
Alkens are planar, so alken molecule with 4 different functions will be agree with it's  mirror image. People why you do't like reading books?

dexangeles

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Re:OPTICAL ISOMERISM
« Reply #9 on: January 11, 2005, 12:23:57 AM »
who is confusing what?

well, here is what my book says about optical activity

Quote
To be optically active, the sample must contain a chiral substance and one enantiomer must be present in excess of the other.  A substance that does not rotate the plane polarized light is said to be optically inactive.  All achiral substances are optically inactive."



nocommentes,

Quote
People why you do't like reading books?
i think insults to everyone is not good in this forum
i took your comment very personal
i do read books, that's all i do, that's why i finish first in all my classes
even on my spare time that's all i do, read

plus read around the forums, i believe the mods and admins in here deserve more respect than that
i'd bet they know more than you think they do
« Last Edit: January 11, 2005, 12:24:56 AM by dexangeles »

NISHANT

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Re:OPTICAL ISOMERISM
« Reply #10 on: January 11, 2005, 07:33:47 AM »
I totally agrre with u dexangles.we r here to help each other and not to make fun.besides that noncommentes it is symmytry that is involved in optical activity,mirror image is just a mere way to express that,infact u haven't even understood my question!!!

Offline Mitch

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Re:OPTICAL ISOMERISM
« Reply #11 on: January 11, 2005, 02:48:23 PM »
an essential condition for optical active molecule is presence rotation-reflecting axis higher then 2 order

That is the perfect answer(aside from grammar), but would be beyond someone without a little background in group theory specifically symmetry operations. In the future, best to leave the ego at home, we're here to help.
« Last Edit: January 11, 2005, 02:49:53 PM by Mitch »
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