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Q) Which of the following is chiral?

a) 1-Pentanol

b) 3-Pentanol

c) 3-Methyl-2-butanol

d) 3-Methyl-1-butanol

Answer: c)

If structure and it's mirror image are not superimposable then it's chiral.

enter image description here

But aren't 1st, 2nd and 4th options also not superimposable ?

I drew the 1st one '1- Pentanol' and took a mirror image of it. If I keep the second one on the 1st image it's no way superimposable. I may be wrong.

Also, is there any rule like if the molecule is optically active then it's chiral?

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    $\begingroup$ You need a 3D diagram to judge if its mirror image is superimposable. You are allowed to rotate the mirror also. You are also allowed to rotate the sigma bonds about the axis connecting the two centres of the bond. $\endgroup$
    – DHMO
    Commented Jan 14, 2017 at 6:52
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    $\begingroup$ "optically active" is a synonym of "chiral". $\endgroup$
    – DHMO
    Commented Jan 14, 2017 at 6:52
  • $\begingroup$ @DHMO examples 3 and 4 both have one chiral carbon which implies both are optically active. Then aren't they both chiral? $\endgroup$
    – user406653
    Commented Jan 14, 2017 at 6:55
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    $\begingroup$ Option (d) does not have chiral carbons, nor is it chiral as a whole. $\endgroup$
    – DHMO
    Commented Jan 14, 2017 at 6:56

1 Answer 1

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As DHMO said, you need 3D diagrams, so here they are (all taken from PubChem):

a) 1-pentanol:

A good rule of thumb is that a chiral carbon has four distinct substituents (you can't have more than four anyway). This structure has no such carbon: every carbon has at least two hydrogen atoms attached. None is chiral.

1-pentanol

b) 3-pentanol:

This one has none either. The carbon in the middle of the chain, bonded to oxygen, has two ethyl groups attached, so not chiral.

3-pentanol

c) 3-methyl-2-butanol:

The carbon attached to the oxygen has four distinct groups: hydroxyl, hydrogen, methyl and tert-butyl. So this carbon is chiral.

3-methyl-2-butanol

d) 3-methyl-1-butanol:

Here every single carbon has either two hydrogen atoms or two methyl groups attached, so not chiral.

3-methyl-1-butanol

Since the only chiral carbon found is in structure c), this is the correct answer.

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