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Topic: Is this molecule chiral or achiral?  (Read 1004 times)

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Offline bigfranz1

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Is this molecule chiral or achiral?
« 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)
« Last Edit: December 01, 2021, 10:19:32 AM by sjb »

Offline mjc123

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Re: Is this molecule chiral or achiral?
« Reply #1 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.)

Offline mjc123

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Re: Is this molecule chiral or achiral?
« Reply #2 on: December 01, 2021, 09:58:19 AM »
Not here anyway. Don't cross-post. Read forum rules.

Offline rolnor

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Offline billnotgatez

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Re: Is this molecule chiral or achiral?
« Reply #4 on: December 01, 2021, 10:44:56 AM »
I moved and merged both topics
please do not cross post

Offline Orcio_87

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Re: Is this molecule chiral or achiral?
« Reply #5 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.

Offline bigfranz1

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Re: Is this molecule chiral or achiral?
« Reply #6 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.

Offline Babcock_Hall

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Re: Is this molecule chiral or achiral?
« Reply #7 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?

Offline rolnor

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Re: Is this molecule chiral or achiral?
« Reply #8 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
« Last Edit: December 02, 2021, 04:50:26 AM by rolnor »

Offline Orcio_87

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Re: Is this molecule chiral or achiral?
« Reply #9 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.

Offline rolnor

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Re: Is this molecule chiral or achiral?
« Reply #10 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

Offline Babcock_Hall

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Re: Is this molecule chiral or achiral?
« Reply #11 on: December 02, 2021, 09:33:14 AM »

Offline bigfranz1

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Re: Is this molecule chiral or achiral?
« Reply #12 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.

Offline Babcock_Hall

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Re: Is this molecule chiral or achiral?
« Reply #13 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.

Offline rolnor

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Re: Is this molecule chiral or achiral?
« Reply #14 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.

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