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After having been acquainted with enantiomerism for nearly six months, I have one standing doubt.

Consider the structure of alanine. Suppose I have a pure mixture of exclusively 'S-alanine'(left in the picture) in a flask.

S-alanine & R-alanine:Wikipedia

There are hundreds, if not, thousands of molecular collisions in the flask per second. Why can not a bump to one of the S-alanine change it to R-alanine? This way, the mixture should racemise itself if left standing for some time. But this does not happen. Why? Can someone please explain?

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    $\begingroup$ If I somehow had a tank full of left hands, and I agitated the tank, you might get lots of different conformations such as a tight fist, an open palm, etc... but there's no way I'm going to turn a left hand into a right hand (a different configuration) without chopping off the fingers and gluing them back the wrong way. Similarly, racemisation (at a chiral centre) requires the breakage of a single bond, which is not very likely to happen. When it does occur, there is often some mechanism to explain it, e.g. keto-enol tautomerism for alpha carbons. Sorry for the morbid analogy. $\endgroup$ – orthocresol Aug 31 '15 at 15:51
  • $\begingroup$ @orthocresol You should write this up into an answer. $\endgroup$ – bon Aug 31 '15 at 17:06
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Because the activation energy for the enantiomeric H-Atom to jump to the other side of the carbon atom's sp3 tetraeder (by whatever mechanism) is so high that it is just not available.

You're right, statistically, every energy is available (with very bad odds). But very likely other processes, and more destructive ones, have a lower activation energy. Your compound will likely degrade and rot away long before it racemises. But even that, for alanine at room temperature, does not happen on any timescale that you can observe.

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