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My textbook says 2-bromobutane is chiral, but it look like it will form superimposition mirror image

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For a molecule to be chiral it must have non-superimposable mirror images. Here is a drawing of the two mirror images for 2-bromobutane.

enter image description here

The chiral carbon atom is denoted by an asterisk. In the case of 2-bromobutane there are 4 different substituents attached to it. The molecule is chiral, you can't pick up one of the mirror images and superimpose it on the other.

Note to think about:

  • 4 different substituents may make an atom chiral, but the molecule as a whole may still be achiral

For example meso compounds

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  • some molecules without 4 different substituents attached to an atom may still be chiral .

For example, compounds with axial chirality such as chiral allenes (1,3-dimethylallene), chiral biphenyls, chiral helicenes, etc.

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Yes, it is chiral because there are 4 different groups on the second carbon, H, Br, methyl and ethyl.

Maybe you are looking at a planar depiction.

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