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As I understand Enantiomers have similiar physical properties but they do show different chemical properties with optically active reagents. This point is something which I am not able to logically comprehend. Why would a compound which being only different in rotation of direction of PPL behave differently in a chiral environment ?

I tried searching and found through the following answer Why are optically active compounds abundant in nature?

here in comments it has been crudely mentioned that one of the enantiomers may react to some enzyme that specifically works on it.... Which makes me even more confused.

I will be glad if this could be cleared up. Regards

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    $\begingroup$ I would be also cautious saying that they have the same physical properties. First, the main difference is what you are aware of. Second, at least in principle, you can imagine more. Like the friction on a chiral surface, or the viscosity in a chiral medium, etc. Again, as per Tyberius answer. $\endgroup$ – Alchimista Jun 4 at 8:50
  • $\begingroup$ @Alchimista Thanks for your inputs :) Yes I do get the differences now.. $\endgroup$ – Rishi Kirti Jun 4 at 11:42
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    $\begingroup$ Suppose you are introduced to someone, and you want to shake their hand. They extend their right hand to do so. No matter how you position your body or hands, you cannot grasp their right hand with your own left in the manner appropriate for a conventional handshake. Your two hands have approximately the same kind of geometric relationship as a pair of opposite enantiomers, and the effect that only one of them has the right shape to complete that handshake is actually pretty analogous to the fact that sometimes only one of a pair of enantiomers has the right shape for a particular reaction. $\endgroup$ – John Bollinger Jun 4 at 15:57
  • $\begingroup$ @RishiKirti yet, as pure compounds themselves, then yes, all physical properties are the same except for the obvious one. This I add to prevent you getting troubles in the class ;) $\endgroup$ – Alchimista Jun 5 at 10:03
  • $\begingroup$ @Alchimista. Thanks :) $\endgroup$ – Rishi Kirti Jun 6 at 10:48
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I think a simplified version of the enzyme example is helpful here. Consider this image of binding a molecule to a receptor:

Enantiomers binding to a receptor

Using a model of enantiomers where the attached groups are just differently shaped blocks, it is clear to see that if one enantiomer fits, the other one can not, regardless of the way it is oriented. The different rotation of light is a consequence of this different arrangement of attached groups, which in turn has an effect on reactivity in chiral environments.

So while certain physical properties that just depend on the composition and arrangement of the groups in space will remain the same for each enantiomer, any property that depends on the relative positioning of these groups will differ. This includes optical rotation, but also chemical interactions between chiral species or more physical interactions or more exotic effects like the interaction with the electroweak field.

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It is worth pointing out what exactly enantiomers are: exact mirror images of each other. Most of what enantiomers do and don't do can be understood by remembering that basic definition and applying it logically.

First, let's compare a chiral compound to an achiral one. Imagine an achiral molecule in front of a (molecular-sized) mirror. The orientation does not matter; the mirror will show a mirror image. Take a screenshot of the picture in the mirror. By definition, it will be possible to take that achiral molecule and rotate it so that it looks exactly like the screenshot you just took, except that you aren't using the mirror.

Whichever macroscopic property of the pure material you decide to measure, the resulting value is typically one of two things: either a simple number with a unit (i.e. it is a non-directional property; e.g. melting point) or a directional vector (e.g. rotation of polarised light. Each individual molecule will contribute identically to the simple-number properties; whereas the directional properties will typically be averaged out as there will always be a molecule oriented exactly opposite in a large enough sample.

Enantiomers on the other hand cannot be mapped onto their mirror image. Thus, it will be impossible to find a molecule oriented in exactly the opposite way in a pure sample of a single enatiomer. However, you are able to produce an equally pure sample of the opposite enantiomer, i.e. the exact mirror image. Now, the simple-number type of macroscopic properties will remain unaffected by which enantiomer you are measuring but those properties that are vector-based will be opposite, as they cannot be internally compensated.

Therefore it follows that unlike your first sentence enantiomers do not have similar properties but identical ones save those where direction actually matters. The most common example of an experiment where direction matters is the rotation of polarised light.

However, this difference is a consequence of the molecular structure; not the other way around. It is worth remembering that.

Now we move on to the interaction between a chiral compound and another compound, be it achiral or chiral. You can do some macroscopic experiments to investigate this, if the thought experiment isn't conclusive enough. For example, take a (small!) figure or sculpure or whatever of a human being or animal whose arms or legs are doing different things. This could be your chiral molecule. As an achiral object, you could take e.g. a mug. Play around with the two to see how they interact. You will be able to get 'better fits' one way but not the other way. For example, if your figure has one arm bent at an angle while the other is straight, this angled arm can probably easily lock around the mug's handle while they are both standing on the table; not, however, using the other arm. This is akin to a chiral molecule reacting with an achiral one in a stereospecific manner.

To understand how two chiral molecules interact, use two such asymmetric figures and see what you can do. One way will 'fit' notably 'better' than the other way. Finally, image one of the figures being a mirror image of itself. Now, the interaction will be markedly different and they may not even fit together at all. (Of course, it would be ideal if you had some kind of mirror images of figures but I'm not sure if that's all that common. Some kind of irl 3D tetris blocks perhaps?)

This is the gist behind the image in Tiberius' nice and succinct answer. The ramblings herein only serve to further illustrate the point and to allow a thought path to be re-understood.

Finally, I will briefly address that Alchemista's comment can also be understood in this context but I will argue that Alchemista's examples are not physical properties of the pure compound and thus not really applicable to the first part of this answer.

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  • $\begingroup$ Thanks for clearing up things :) Yes the finger example does make sense. $\endgroup$ – Rishi Kirti Jun 4 at 14:58

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