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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: bigfranz1 on December 01, 2021, 08:05:28 AM

Title: Is this molecule chiral or achiral?
Post by: bigfranz1 on December 01, 2021, 08:05:28 AM
here: Nc2nnc(c1cccc(Cl)c1Cl)c(N)n2
I have coursework and have to talk about the optical activity of an assigned molecule, the molecule in the picture is lamotrigine, I'm a bit rusty on chirality and the lecturers always show really easy examples then throw you right in the deep end with stuff like this so I'm a bit stuck. One website I looked at said that all molecules that are asymmetrical are chiral, clearly the molecule is asymmetrical so I thought it must be chiral right? I looked for potential chiral centres though and can't find any, the amine group looks like it could be but because the lone pair momentarily occupies a p orbital its state interchanges constantly from sp3 to sp2 so they don't count as being chiral. I thought maybe the dichlorobenzene ring could be too where the carbon binds to chlorine and two other carbons on each side of it, but carbons on double bonds don't count as being chiral either, from here I don't really know where else to go though.

(mod edit put in structure in case image breaks)
Title: Re: Is this molecule chiral or achiral?
Post by: mjc123 on December 01, 2021, 09:52:01 AM
A chiral molecule does not need to have a chiral centre (in the sense of a single atom bound to 4 different groups); sometimes a larger structural feature of the molecule is responsible, with no individual chiral atom. Consider for example a substituted allene
 R1R2C=C=CR1R2
(I hope you are aware that the two =CRR groups are perpendicular to each other, not coplanar as in an alkene.) This molecule has no plane of symmetry, and is not superimposable on its mirror image.
Now consider your molecule. If the two rings were coplanar, the molecule would have a plane of symmetry (neglecting the lone pairs on the amino groups; we assume free rotation about the C-N bonds, and the Ns can invert). But the steric hindrance between the ortho Cl and NH2 groups prevents the rings from being coplanar. What effect does that have?

To say that all "asymmetrical" molecules are chiral is too vague; what is meant by "asymmetrical"? Is this molecule "clearly" asymmetrical?
The most general definition is that a molecule is achiral if it has an improper rotation (rotation-reflection) axis Sn. (S1 is the same as a mirror plane; S2 is the same as a centre of inversion.)
Title: Re: Is this molecule chiral or achiral?
Post by: mjc123 on December 01, 2021, 09:58:19 AM
Not here anyway. Don't cross-post. Read forum rules.
Title: Re: Is this molecule chiral or achiral?
Post by: rolnor on December 01, 2021, 10:10:13 AM
https://en.m.wikipedia.org/wiki/Chirality
Title: Re: Is this molecule chiral or achiral?
Post by: billnotgatez on December 01, 2021, 10:44:56 AM
I moved and merged both topics
please do not cross post
Title: Re: Is this molecule chiral or achiral?
Post by: Orcio_87 on December 01, 2021, 01:29:07 PM
Quote
But the steric hindrance between the ortho Cl and NH2 groups prevents the rings from being coplanar. What effect does that have?
True, but do the rings cannot rotate one versus another ?

I mean - in the solid state - OK. But - in the solution ? I think that the rings will rotate one versus another.
Title: Re: Is this molecule chiral or achiral?
Post by: bigfranz1 on December 01, 2021, 02:04:39 PM
A chiral molecule does not need to have a chiral centre (in the sense of a single atom bound to 4 different groups); sometimes a larger structural feature of the molecule is responsible, with no individual chiral atom. Consider for example a substituted allene
 R1R2C=C=CR1R2
(I hope you are aware that the two =CRR groups are perpendicular to each other, not coplanar as in an alkene.) This molecule has no plane of symmetry, and is not superimposable on its mirror image.
Now consider your molecule. If the two rings were coplanar, the molecule would have a plane of symmetry (neglecting the lone pairs on the amino groups; we assume free rotation about the C-N bonds, and the Ns can invert). But the steric hindrance between the ortho Cl and NH2 groups prevents the rings from being coplanar. What effect does that have?

To say that all "asymmetrical" molecules are chiral is too vague; what is meant by "asymmetrical"? Is this molecule "clearly" asymmetrical?
The most general definition is that a molecule is achiral if it has an improper rotation (rotation-reflection) axis Sn. (S1 is the same as a mirror plane; S2 is the same as a centre of inversion.)
If I knew the answer to what you're asking I probably wouldn't have needed to make this post, I know to say that all asymmetrical molecules are chiral is vague, but that's what one site said so it confused me. I'm gonna assume it's achiral, correct me if I'm wrong.
Title: Re: Is this molecule chiral or achiral?
Post by: Babcock_Hall on December 01, 2021, 07:15:27 PM
@OP, mjc123 gave you some valuable information.  I suggest you read up on atropisomerism.  The molecule in question might be chiral, but it depends on one thing.  What information would you need to be certain?
Title: Re: Is this molecule chiral or achiral?
Post by: rolnor on December 02, 2021, 01:11:49 AM
But it can not be chiral, it lacks orto-substituents? Even if rotation is restricted.

Edit, The hydrogen is also a substituent so this is wrong
Title: Re: Is this molecule chiral or achiral?
Post by: Orcio_87 on December 02, 2021, 01:56:43 AM
@rolnor Because of the steric hinderence, the second ring (the one with two -Cl substitutents) can be lie in or out of plane of the first ring. If further rotation is restricted then this will be two different isomers.
Title: Re: Is this molecule chiral or achiral?
Post by: rolnor on December 02, 2021, 04:41:49 AM
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5967358/

The hydrogen is to small to give rotational barrier
Title: Re: Is this molecule chiral or achiral?
Post by: Babcock_Hall on December 02, 2021, 09:33:14 AM
https://knowleslab.princeton.edu/wp-content/uploads/presentations/2018-06-09-AJM_Atropisomerism_GM_website.pdf

This is a good general discussion of atropisomerism.
Title: Re: Is this molecule chiral or achiral?
Post by: bigfranz1 on December 02, 2021, 12:47:28 PM
@Babcock_Hall I'm not saying the information isn't useful, it's just I don't understand it well enough to get any use out of it, I'd rather he actually explained it rather than answering my question with a question.
Title: Re: Is this molecule chiral or achiral?
Post by: Babcock_Hall on December 02, 2021, 01:35:34 PM
It is a forum rule (see red link above) that you must show your attempt at answering a question before we can help you.  It is a judgment call whether or not your opening post clears that bar.  I would focus on bond that connects the two rings in your molecule and ask the question how quickly does rotation around this bond occur.  If you have not looked at the links that Rolnor and I provided, please do so now.
Title: Re: Is this molecule chiral or achiral?
Post by: rolnor on December 02, 2021, 04:56:31 PM
It is not strange that you find the answers complicated, this is not trivial and at low temperature the molecule probably behaves like its chiral and the isomers can theoreticaly be separated, when these are warmed the chirality will be lost. Its hard to say how cold this is and how fast the chirality is lost. Maybe its close to 0 Kelvin. If you realy want to understand this you need to do some reading, whe can not force-feed this into you brain.
Title: Re: Is this molecule chiral or achiral?
Post by: rolnor on December 02, 2021, 05:19:14 PM
You need a rotational barrier of 23kcal/mol to see this type if chirality. Its easy to calculate the rotational barrier with molecular mechanichs software.
https://en.m.wikipedia.org/wiki/Chirality_(chemistry)
Title: Re: Is this molecule chiral or achiral?
Post by: mjc123 on December 03, 2021, 10:17:31 AM
Quote
@Babcock_Hall I'm not saying the information isn't useful, it's just I don't understand it well enough to get any use out of it, I'd rather he actually explained it rather than answering my question with a question.

I'm sorry if you didn't find my response helpful, but that's the kind of forum this is. We don't simply give people answers, but try to help them learn by giving them pointers to help them work it out for themselves. Like you, I am bound by the forum rules (please read them if you haven't already), so I can't just tell you the answer (especially if, as some people think, it's the wrong answer!). There are other forums that will do that if that's what you want.

Of course this depends both on where you are in your learning and my ability to explain at the right technical level, so it doesn't always succeed, but we are trying to do something more constructive than simply spoonfeed answers.
Title: Re: Is this molecule chiral or achiral?
Post by: bigfranz1 on December 03, 2021, 11:14:10 AM
@Babcock_Hall I think maybe you underestimate my chemistry ability, in order to get into uni to do this course I did a course that was mostly biology and physics and only a bit of chemistry (only spend 1 term doing organic chem), it wasn't anywhere near in depth as what I'm learning now. I feel like I made an attempt and gave my reasoning, but I seriously don't know how to answer his question. Those papers that were linked still don't really help me too much and I don't have the time to read it, understand it and make notes as I have other modules to do work in too. It's not like the things I'm learning about are completely alien to me, and I can get a grip on most of it, it's just as I mentioned my lecturers explain it in a very simple way with piss easy examples and from what I learn from them and some independent learning I can't find a way to apply it.
Title: Re: Is this molecule chiral or achiral?
Post by: Babcock_Hall on December 03, 2021, 02:06:12 PM
I hope that you learned one thing, that a molecule does not need a center of chirality to be chiral.  I would hope that you learned at least that the size of the rotational barrier is the key question.  I suspect that Rolnor is correct concerning your molecule.
Title: Re: Is this molecule chiral or achiral?
Post by: kriggy on December 06, 2021, 04:59:41 AM
The issue witht your  molecule is quite complicated. I did my PhD with atropoisomeric compounds and there are some good answers already in this thread.

The link to the princeton lab looks very usefull esp. page 9.

I suppose (as was already mentioned) at low temperature, you might be able to separate the enantiomers by suitable method. You have ortho-ortho  monosubstituted biphenyl, thats not exactly stable with regards to the bond rotation but I think it could be done and be observed in some kind of LC separation.

I dont think you can separate the enantiomers and keep like one vial with P and one vial with M enantiomer, they are going to racemize quickly.

a) does the molecule have non-superimposable mirror images?
b) are the enantiomers stable?
c) Can we call it chiral if it only exists as a racemic mixture?

Those are probably the questions you are suppose to ask and answer for your homework assignment.
Title: Re: Is this molecule chiral or achiral?
Post by: rolnor on December 06, 2021, 12:23:00 PM
There is an name for chiral compounds that have fast interconversion of the enantiomers, I dont remember the name? Amines with 3unique substituents on nitrogen belongs to this class I believe.
Title: Re: Is this molecule chiral or achiral?
Post by: Borek on December 06, 2021, 06:03:51 PM
There is an name for chiral compounds that have fast interconversion of the enantiomers, I dont remember the name? Amines with 3unique substituents on nitrogen belongs to this class I believe.

No idea about the name, but the process they undergo is called https://en.wikipedia.org/wiki/Pyramidal_inversion