why are achiral molecules optically inactive?

[sweetydnic51]

Why are chiral molecules optically active (rotate polarized light) while achiral molecules are optically inactive (do not rotate polarized light)?

I know it's something to do with the light interacting with the electron clouds... that's all I've got though haha.

can someone explain what property about the chiral molecule makes it rotate polarized light that achiral molecules do not possess? thanks in advance!

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[Blueprint Med School]

In a nutshell, chiral molecules rotate in one specific direction (R or S) while achiral molecules will rotate in both directions. Those rotations will cancel each other out, making them optically inactive. This isn't the case with chiral molecules, which will rotate in only one direction and "turn" the light.

The answer can get significantly more in depth than this, but that will be beyond the scope of the MCAT.

Kevin W, MCAT Tutor
Med School Tutors

 

[sweetydnic51]

In a nutshell, chiral molecules rotate in one specific direction (R or S) while achiral molecules will rotate in both directions. Those rotations will cancel each other out, making them optically inactive. This isn't the case with chiral molecules, which will rotate in only one direction and "turn" the light.

The answer can get significantly more in depth than this, but that will be beyond the scope of the MCAT.

Kevin W, MCAT Tutor
Med School Tutors

Thanks for the reply, I appreciate it! Just trying to understand it better so it’ll stick, rather than just rote memorization of this fact. I didn’t know chiral molecules could only turn one way...

 

[Blueprint Med School]

Thanks for the reply, I appreciate it! Just trying to understand it better so it’ll stick, rather than just rote memorization of this fact. I didn’t know chiral molecules could only turn one way...

Sorry for the late response- remember that R and S stuff you probably learned in organic chemistry and how you would order the atoms around the carbon based on priority to determine what direction it spins in? So chiral molecules can rotate in both directions, but each enantiomer can only rotate in one direction. Like achiral molecules, if you had a mixture of 50/50 of both enantiomers (a racemic mixture), then it would be optically inactive because the rotations would cancel out.

 

[sweetydnic51]

Sorry for the late response- remember that R and S stuff you probably learned in organic chemistry and how you would order the atoms around the carbon based on priority to determine what direction it spins in? So chiral molecules can rotate in both directions, but each enantiomer can only rotate in one direction. Like achiral molecules, if you had a mixture of 50/50 of both enantiomers (a racemic mixture), then it would be optically inactive because the rotations would cancel out.

ah, thanks! sounds vaguely familiar. It's been a while since I took orgo (10 years). I've got a ways to go with my review haha. thanks for your help!!