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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Problem of the Week Archive => Topic started by: Borek on April 30, 2012, 10:22:49 AM

Title: Problem of the week - 30/04/2012
Post by: Borek on April 30, 2012, 10:22:49 AM
Simple one, and an open one - that is, I think I know the answer, but I am ready for better solutions.

What is the lightest (lowest molar mass) chiral compound made of C and H only?

What is the lightest chiral compound (no restriction to CH)?

Title: Re: Problem of the week - 30/04/2012
Post by: cth on April 30, 2012, 01:12:11 PM
What is the lightest (lowest molar mass) chiral compound made of C and H only?

I think it's C5H8:

H3C          H
   \          /
    C=C=C
   /          \
  H           CH3



What is the lightest chiral compound (no restriction to CH)?
Does the compound have to be known and stable, or something more "exotic" would do?

For example, a cyclopropane with 2 trans-substituted lithium atoms C3H4Li2 (I haven't check if this particular compound exists):

     CH2   
     /\
Li-C-C-H
  /      \
 H       Li
Title: Re: Problem of the week - 30/04/2012
Post by: fledarmus on April 30, 2012, 01:44:41 PM
Personally I was thinking of the ammonia analog NHDT for the smallest chiral compound -

      N
    / | \
   H D T

Of course, at a temperature cold enough to freeze out the umbrella transition...

MW ~ 20

But the methyllithium analog Li-CHDT (MW~21) might be a better choice, being more stably chiral.
Title: Re: Problem of the week - 30/04/2012
Post by: sjb on April 30, 2012, 01:51:41 PM
Personally I was thinking of the ammonia analog NHDT for the smallest chiral compound -

      N
    / | \
   H D T

Of course, at a temperature cold enough to freeze out the umbrella transition...

MW ~ 20

perhaps you can go one further and utilise the 12N isotopologue (or the 10N, if you want really exotic)?

But the methyllithium analog Li-CHDT (MW~21) might be a better choice, being more stably chiral.

Similarly, there are lighter isotopes of Li and C???
Title: Re: Problem of the week - 30/04/2012
Post by: AWK on May 01, 2012, 05:51:46 PM
Quote
But the methyllithium analog Li-CHDT (MW~21) might be a better choice, being more stably chiral.
CH3Li is an ionic compounds, hence  chiral stability of Li-CHDT is questionable.
Title: Re: Problem of the week - 30/04/2012
Post by: fledarmus on May 01, 2012, 08:08:33 PM
Quote
But the methyllithium analog Li-CHDT (MW~21) might be a better choice, being more stably chiral.
CH3Li is an ionic compounds, hence  chiral stability of Li-CHDT is questionable.

Not really ionic - carbon-lithium bonds actually have a surprising amount of covalent character. Methyllithium in particular does not tend to dissociate; in both solutions and solids it tends to form aggregate structures with the lithiums in the center and the methyl groups on the outside. This would tend to keep the methyl groups from flipping. At least, I would predict them to be more stable than ammonia...
Title: Re: Problem of the week - 30/04/2012
Post by: Ann1234 on May 05, 2012, 12:37:57 AM
Simple one, and an open one - that is, I think I know the answer, but I am ready for better solutions.

What is the lightest (lowest molar mass) chiral compound made of C and H only?

for this one so far I have:

                     T
                     /
                D -C*-H
                   /
                  C*H3

* Being C-8 (lowest-mass C isotope I found) both carbons in the molecule. Molar mas is around 25 g/mol.
Title: Re: Problem of the week - 30/04/2012
Post by: AWK on May 07, 2012, 02:06:27 AM
But C-8 has half life of 2.0 x 10−21 s.
If we are talking about chirality we should measure it. in this case and in the case of CHDTLi the measurement is impossible because of stability.
Chirality of CHDTCOOH is known - I think this is the lowest weight molecule for which chirality is proved.
We can expect a stable chirality  for example from CHDTBH2, CHDTCH3 CHDTNH2.
From theoretical point of view CHDT4H should be chiral - 4H shows half life 1.39  × 10−22 s - in this case  8C can also be used.
Title: Re: Problem of the week - 30/04/2012
Post by: Borek on May 07, 2012, 09:32:27 AM
H3C          H
   \          /
    C=C=C
   /          \
  H           CH3

Nice one. I was thinking about 1,3-dimethylcyclopropene, but it has one hydrogen more.

I guess to say which of the other answers is best I should define the question much more precisely first. I was thinking about adding some remarks about stability and "isolability" to the question, but they are not obvious, besides, I have a feeling as long as we don't have a sample of H,D,T substituted compound it is hard to predict if we would be able to separate chiral isomers.
Title: Re: Problem of the week - 30/04/2012
Post by: AWK on May 07, 2012, 10:03:53 AM
Chiral acetate
http://pubs.acs.org/doi/abs/10.1021/ed080p333
http://www.ch.ic.ac.uk/local/organic/pericyclic/ja992498e.pdf