Stereochemical Environments in a Molecule

Stereochemical Environments in a Molecule


Let’s return to the question
the same or not the same, but now let’s ask
that question for groups of atoms
within a molecule. Before, we were comparing two
different molecular structures trying to decide if they
were homomers or isomers. Now we want to look at groups
of atoms within a molecule to decide whether they’re
stereochemically equivalent or not, whether the stereochemical
environment in one place is the same as the
stereochemical environment in another place
within one molecule. These are known as
topic relationships. The word “topic” means place, and we’ve actually already
addressed this question. For example, at the
beginning of the webcast on stereochemistry, we
talked about 3-methylvaline. The three different methyl
groups in 3-methylvalene we found to be
stereochemically equivalent. We said they were. And we also concluded that
in the case of valine, the two different methyl
groups, we concluded, for reasons we’ll learn
about in the next webcast that they were stereochemically
not equivalent. So, let’s actually
look at this question a little bit more deeply because it’s not
quite as simple as the same or not the same. It turns out that how we
interrogate a molecule is going to make the difference
as to whether or not groups of atoms are the
same or not the same. Can they be
distinguished or not depends on how we
ask that question. Let’s take a look at
this achiral molecule pentanoic acid. Those two hydrogen atoms
have the same connectivity to that carbon atom, and
we could ask the question – are the two hydrogen atoms
the same or not the same? Well that’s going to depend on whether we ask that question
by using a chiral probe, such as an enzyme
active site. An enzyme is made of
chiral amino acids and an enzymes active
site is chiral. And a chiral probe would be
able to distinguish those hydrogen atoms as
being different. Or in fact, if you took your
left hand and wrapped that, your left hand
around that molecule, say with your thumb on
the carboxylic acid, your fingers in
the your left hand would see those hydrogen
atoms as different than the fingers
in your right hand if you prob- put the molecule
in the same orientation with the the thumb next to
the carboxylic acid. So those are chiral probes and they would distinguish
those two hydrogen atoms. If we used an achiral probe
like an aqueous solution, which is made up of
achiral water molecules, or if we used an
NMR spectrometer, which is another
achiral probe, those two hydrogen
atoms would not be able to be distinguished. So, to summarize
this, basically, a stereochemically
equivalent set of, ah, atoms are going to able, are, are never going to be
able to be distinguished, whether we’re probing
it with a chiral or an achiral object, or an achiral or,
chiral or achiral probe; whereas, stereochemically
non-equivalent groups of atoms may be able to be
distinguished and the word “may” needs a little
bit more clarification, which we can find
in the next slide. So what we’re going to be
able to do is to classify stereochemical environments, not just as whether they’re
equivalent or not equivalent, but just like in the case of comparing two
different molecules, we could go beyond saying whether they were
homomers or isomers. If we knew they were isomers, we could decide whether
they were enantiomers, diastereomers or constitutionally- or constitutional
isomers, and in the case of
stereochemical environments, we can make many of
the same distinctions. Are those stereochemical
environments homotopic, enantiotopic, diastereotopic or constitutionally
heterotopic. And the reason this matters
is because we can then know whether we could distinguish
stereochemical environments and it depends on
whether we’re probing, interrogating that
structure with a chiral or an achiral probe. So for example,
homotopic environments – they can never be distinguished, whether we’re using a
chiral or an achiral probe. However, enantiotopic
environments can’t be distinguished if we’re using
an achiral probe like an aqueous solution
or an NMR spectrometer. But they could be
distinguished if that, if those, ah, groups were being
probed by a chiral source, say putting a molecule
in an enzyme active site. In the case of diastereotopic and constitutionally
heterotopic groups, those stereochemical
environments are always distinguishable whether we’re using a
chiral or an achiral probe. So, let’s just take a look at
an example, a few examples. First, let’s imagine that
we’re trying to distinguish groups of atoms according to
whether, according to, a, or interrogating that molecule
using a chiral object. In the case of this diacid, those two hydrogens
are homotopic, we’ll learn that in
the next webcast, and so we would not be able
to distinguish under, um, this set of conditions, actually under any
set of conditions, ah, a distinction between
those two hydrogen atoms. We’d never be able to
distinguish them in any way. However, the two hydrogen
atoms in pentanoic acid are enantiotopic and so with a chiral probe, we would be able to
distinguish those. However, in that
pentanoic acid, those enantiotopic
hydrogens would not be able to be distinguished if we’re using an
achiral probe to in- interrogate the molecule. Here’s a pair of
hydrogen atoms that we’ll learn as
being diastereotopic and those hydrogens would
be able to be distinguished with a chiral or
an achiral probe, and here’s a, a group
of hydrogen atoms, a pair of hydrogen atoms that we can see are
distinguishable because they have
different constitution. The one on the right is
close to this methyl group. The one on the left is close
to this carboxylic acid. They’re constitutionally
different and so they’re
distinguishable by a chiral or an
achiral probe.

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