I didn't understand the fact that chiral objects are non-superimposable on their mirror images. I mean, if you put your right hand (apparently an achiral object) in front of a mirror, the mirror image will be like your left hand, but, when you make your hand touch the mirror, isn't it superimposable on its mirror image (and hence achiral)? Doesn't that mean that everything is achiral?
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$\begingroup$ The mirror image of your hand does not superimpose on your hand. If the palm of your hand is facing away from you, the mirror image palm will be facing towards you. If you rotate the mirror image so that the palm faces away from you, it's thumb will be on the opposite side. Thus your hands are chiral. $\endgroup$– canadianerOct 21, 2014 at 0:11
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3 Answers
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Your hands are chiral, that is why you need two different leather gloves, one that only fits your right hand, and one that only fits your left hand. If your hands were superimposable, then you would only need one kind of glove and it would fit both hands.
answered Oct 20, 2014 at 15:46
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if you put your right hand (apparently an achiral object)
No, your right and left hand are both chiral objects, as we will see below.
in front of a mirror, the mirror image will be like your left hand,
Yes.
but, when you make your hand touch the mirror, isn't it superimposable on its mirror image
No, that mirror image is not superimposable. Just because the bottoms of your right hand and left hand can overlay one another does not make them superimposable. If you could take that 3-dimensional mirror image out of the mirror and place it next to your hands and compare it to your right and left hand, it would look like your left hand. And, just like your real left hand, you couldn't pick the mirror image up and place it next to your right hand, look at them both and say, "they are the same" because, for one thing, the thumbs are on opposite sides. Therefore, they are non-superimposable mirror images.
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$\begingroup$ Would our hands have been achiral if they were just plane having outline as our original hands and with all fingers pointing in same direction? $\endgroup$– Raksh23Apr 5, 2016 at 9:49
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1$\begingroup$ Our hands would be achiral if they were 2-dimensional (planar) and could still be manipulated through 3 dimensions. If these planar hands existed in a 2-dimensional world, they would still be chiral. $\endgroup$– ronApr 5, 2016 at 13:59
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I have to say that when they taught Chiral molecules to me in Chemistry, they always used confusing molecules to demonstrate. The easiest to visualise the mirror symmetry is with spirenes (spiral molecules).
Think of a spiral staircase, it can spiral clockwise, or it can spiral anticlockwise, but if you tried to put one of each in the same space (superimpose them), nobody could climb the stairs !
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$\begingroup$ You're talking about molecules like
CHFClBr, right? They use those because they are the simplest molecules that are chiral - not because they are the molecules for which the chirality is most visualizable. $\endgroup$– zwolOct 20, 2014 at 20:59 -
7$\begingroup$ I think @RogUpton's point is that it's very helpful to also have a visualizable example, not just a simple one. Some students learn better by visualization and it's important to provide multiple examples. $\endgroup$ Oct 22, 2014 at 12:52