3
$\begingroup$

I'm a bit confused when asked to specify the chiral carbons on this sketch of Propranolol as I'm not sure how the $\ce{H3C}$ and $\ce{CH3}$ play in if they are reversed in the name like that...Does that even matter?

I'm pretty sure the only chiral carbon is the one in the middle of those and the $\ce{NH}$. I've heard that you need 4 different groups/atoms surrounding the carbon for it to be chiral but how much truth is there in that, as I've found several with only 3?

Improve this question
$\endgroup$
1
  • $\begingroup$ Note that the nitrogen is not chiral due to amine inversion. $\endgroup$
    – Jan
    Nov 3, 2016 at 18:37

2 Answers 2

Reset to default
5
$\begingroup$

As stated in user208322's answer, $\ce{CH3}$ and $\ce{H3C}$ are exactly the same, they're only written differently for ease of reading. As per your question about how much truth there is to having 4 groups, it's 100% truth. You cannot have a chiral carbon that doesn't have 4 groups attached to it (i.e. must be sp3 hybridized). If you think you've found a chiral carbon that only has 3 groups attached to it, you're probably forgetting the unwritten hydrogen.

In this example specifically, there is only one chiral carbon, the one attached to the secondary alcohol. Although it may look like the carbon is attached to the same group twice, you need to look further than just the first atom. Yes it is attached to two carbons, but one of those carbons is attached to a nitrogen and the other is attached to an oxygen, so the groups are different. Two groups are only the same if you can follow it down every single atom and they match exactly.

Edit: In regards to your question about following the chain all the way down, realistically you only need to follow it until you see that it differs from the other chains. So for chains 1 and 3 once you get to the nitrogen and oxygen atoms you see that they're different and there's no point continuing. And the unwritten hydrogen has to do with the fact that organic chemists use shorthand in their molecular structures. Basically, carbons and hydrogens are extremely common in organic molecules so they just don't write them. All the points are carbons unless otherwise labelled, and they must have 4 bonds, so where you see that the carbon is only bonded to 3 things, it is also bonded to a hydrogen as well. See this wikipedia article for more info (specifically, this section).

Improve this answer
$\endgroup$
6
  • $\begingroup$ When you say "follow the carbon chain all the way" do you mean follow it until you see another atom other than carbon? Cause if I follow the carbon chain all the way from your specified atom I see 3 chains: 1 goes through: carbon, NH, carbon, CH3 or H3C. 2 goes through OH 3 goes through carbon, Oxygen, carbon, carbon, carbon carbon etc. Also there is a 4th there (has to be)? An unwritten hydrogen? I can't seem to visualize that. $\endgroup$
    – Paze
    Feb 4, 2015 at 15:21
  • $\begingroup$ I.e. I thought that they had to be attached group like directly attached to the carbon. Not 4-5 chains away. $\endgroup$
    – Paze
    Feb 4, 2015 at 15:25
  • $\begingroup$ @BjarniJóhannsson I've added more info, but think of it this way: that whole $\ce{CH2-NH-CH-(CH3)2}$ chain is one big group, and the chain with the oxygen and benzene rings is another group. When you look at it that way, they're clearly different, and should be treated as such. $\endgroup$
    – wes3449
    Feb 4, 2015 at 15:32
  • $\begingroup$ @BjarniJóhannsson, chirality is a gnarly beast at first. I strongly recommend you do lots of examples and read up on it to get comfortable with it. It'll come up a lot if you decide to continue with chemistry later on. $\endgroup$
    – wes3449
    Feb 4, 2015 at 16:15
  • 1
    $\begingroup$ @ron It is true. You cannot have an sp2 chiral carbon. 1,3-dochoroallene contains no chiral carbons, though it is a chiral molecule. In my mind that is outside the scope of this question and would only serve to confuse the OP. $\endgroup$
    – wes3449
    Feb 4, 2015 at 17:03
2
$\begingroup$

$\ce{CH3}$ and $\ce{H3C}$ mean the same thing — methyl — But are written that way to show the C is attached directly. It is obvious, but people prefer writing it that way to show attachment.

And I can see only one chiral atom. These are are such that none of the groups directly attached are same.

Example: $\ce{CH3-CH(OH)-C2H5}$ (the $\ce C_2$ is the chiral carbon).

Improve this answer
$\endgroup$
7
  • $\begingroup$ I don't understand your example. What do you mean attacked? Also am I right in my answer? $\endgroup$
    – Paze
    Feb 4, 2015 at 14:24
  • $\begingroup$ Your answer is correct. $\endgroup$
    – nimishxotwod
    Feb 4, 2015 at 14:33
  • 2
    $\begingroup$ @Bjarni Jóhannsson No, your not - chiral is atom connected to OH group. Attach means connect. $\endgroup$
    – Mithoron
    Feb 4, 2015 at 14:47
  • $\begingroup$ I'm confused here, the Carbon connected to the OH group has 2 of the same group connected along with the OH group so how can it be chiral? Could you draw which one you mean to be correct if mine isn't? $\endgroup$
    – Paze
    Feb 4, 2015 at 14:59
  • $\begingroup$ @BjarniJóhannsson, I've updated my answer to explain this. $\endgroup$
    – wes3449
    Feb 4, 2015 at 15:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.