Natural Science Forum / Chemistry / Organic Synthesis / October 2003

 1) The CC triple bond is rigid - LCAO model with two orthogonal
pairs of overlapping pi orbitals.
 2) The CC triple bond has unhindered rotation - MO theory with a
symmetric cylinder of bonding pi-electron density.

Perhaps somebody ought to do a quadrupole moment measurement of
acetylene.  Being a synthetic organiker, my heart is with (1) but my
head is with (2).

Uncle Al <UncleAl0@hate.spam.net> wrote in message news:<bkvh6b$86d@panther.Gsu.EDU>...

Symmetry says it.  It is linear.  There is no way to tell whether you
have "rotate" that bond or not let alone the energy for such process.
so (3) there is no such kind of rotation.

bch

Uncle Al <UncleAl0@hate.spam.net> wrote in message news:<bkvh6b$86d@panther.Gsu.EDU>...

Seriously though ... Rotation about a bond connotes variation of
dihedral angles. Think of ethane. How do you define a dihedral angle
in a linear system? Solve that problem and then you can talk about the
rotational energy barrier in a CC triple bond.

Uncle Al <UncleAl0@hate.spam.net> wrote in message news:<bkvh6b$86d@panther.Gsu.EDU>...

Symmetry says it.  It is linear.  There is no way to tell whether you
have "rotate" that bond or not let alone the energy for such process.
so (3) there is no such kind of rotation.

bch

Not possible in acetylenic system since the two external bonds are sigma
bonds with very good rotation abilities!
CH3-C#C-CH3 will represent the problem like a car weel system...central
untwistable axis with two rolling rings.
Now you can play with a different molecule that would give you an
approximation of an intermediate barrier between C#C and C=C....
ketene familly is that intermediary.
H2C=C=CH2 <==> CH3-C#C-H

PhZ

Bun Chau Han wrote:

The symmetry might not be quite as high as it appears, in the manner
that the two free electron pairs in H2O are not identical and
degenerate specifically because of higher order symmetry effects.  A
very good quadrupole moment measurment would be interesting (unless a
skilled theoretical chemist can show otherwise).

Thanks to all who replied- much appreciated.  To give a bit more
background: this question started out as a discussion in the coffee
room after a seminar.   I should have given more details: the focus of
the discussion was on the bonding situation as opposed to the
stereochemical consequences (or absence thereof) in such a rotation.

The opinions were pretty much along the lines of the comments made by
Uncle Al in his reply, and so were the owners of these opinions, i.e.
org. chemists vs. theoreticians.   My  first guess above probably
reflects my background.  Unfortunately, nobody in the coffee room had
data.  Only opinions.

I remember seeing conformational data on substituted C5 or C6 molecules
with a central (or near central) triple bond, which gave two or more
conformers.   The energy barrier was low, but then again the
conformers could arise, IMO, from a rotation around the sigma bond(s),
i.e. outside of the triple bond.

Any additional comments, preferably with data, experimental or
calculations, are still much appreciated.   I'll eventually have a
look at the library, but at the moment I cannot travel.

Roger    

Thanks to all who replied- much appreciated.  To give a bit more
background: this question started out as a discussion in the coffee
room after a seminar.   I should have given more details: the focus of
the discussion was on the bonding situation as opposed to the
stereochemical consequences (or absence thereof) in such a rotation.

The opinions were pretty much along the lines of the comments made by
Uncle Al in his reply, and so were the owners of these opinions, i.e.
org. chemists vs. theoreticians.   My  first guess above probably
reflects my background.  Unfortunately, nobody in the coffee room had
data.  Only opinions.

I remember seeing conformational data on substituted C5 or C6 molecules
with a central (or near central) triple bond, which gave two or more
conformers.   The energy barrier was low, but then again the
conformers could arise, IMO, from a rotation around the sigma bond(s),
i.e. outside of the triple bond.

Any additional comments, preferably with data, experimental or
calculations, are still much appreciated.   I'll eventually have a
look at the library, but at the moment I cannot travel.

Roger