organic chemistry - Is rotation of a mirror image allowed in order to determine chirality? Doesn't rotation alter the mirror image of the molecule?

So, chiral molecules are those which have non-superposable mirror images.

trans-1,2-Dimethylcyclopentane is chiral; 2-butanol is chiral; both don't superpose on their mirror image.

Then my book cited 2-propanol as achiral. How?

The mirror image doesn't seem to superpose with the molecule. How can it be achiral then?

Then the book provided a pic where the author showed how 2-propanol is achiral; he rotated the mirror image & then superposed! Done! 2-propanol is achiral!

But really how can rotation be allowed? After all the mirror image is a stereo-isomer; if it is rotated, wouldn't it be changed?

I'm not getting, why rotation is allowed before superposition. Not every-time, the mirror image is rotated before superposition.

Can anyone please explain how rotation is allowed before superposition?

Answer

As a rule of thumb, in order for a molecule to be chiral, it must have a chiral center. There are some exceptions. A chiral center is a generalized extension of an asymmetric carbon atom, which is a carbon atom bonded to four different entities.

Carbon-2 in 2-propanol is not a chiral center because it is only bonded to 3 unique substituent groups: ($\ce{-H}$), ($\ce{-OH}$), and 2 ($\ce{-CH3}$).

he rotated the mirror image & then superposed! Done!! 2-propanol is achiral!!

You seem to misunderstand. Mirror images of chiral molecules are non-superposable, and mirror images of achiral molecules are superposable (because they are the same molecule).

Rotating the molecule only means orienting a molecule differently in 3-D space, not changing anything about the molecule itself. I'd recommend playing around with a molecule building set to solidify this in your mind.