HELP NEEDED: Lewis structure for a) CH3CNO and b) CH3N3
R. Wong
life of me, I can't figure out the simple Lewis structures for
a) CH3CNO
and
b) CH3N3
Can anyone out there spend a few minutes and educate me on this one?
George Huber
>I'm sorry to bother everyone about this elementary problem, but for the
>life of me, I can't figure out the simple Lewis structures for
Alright here we go. Note the symbol -= should be read as a triple bond.
>a) CH3CNO
There are several lewis structures for a nitrile oxide:
Lets see: each hydrogen has one valence electron : 3 x 1 = 3
each carbon has four valence electrons : 2 x 4 = 8
each nitrogen has five valence electrons: 1 x 5 = 5
each oxygen has six valence electrons : 1 x 6 = 6
total valence electrons : 22
H H H
| _ | _ _ | _ _
H-C-C-=N-O| <------> H-C-C=N=O| <------> H-C-C=N-O|
| - | | -
H H H
nitrogen is +1 nitrogen is +1 second carbon is +1
oxygen is -1 second carbon is -1 oxygen is -1
note: the last form shows why nitrile oxides are considered 1,3-dipoles
note: the first form is the major contributer to the resonance hybrid since
all atomshave an octet and the negative charge is located on the most
electronegative element.
the second form is the next most prevalent contributer to the resonance
hybrid since atom has an octet, but is less stable then the first since
an electopositive element (C) is bearing a negative charage
the final form cotributes the smallest amount to the hybrid since not
all atoms have octets.
>and
>b) CH3N3
each carbon contributes four valence electrons : 1 x 4 = 4
each hydrogen contributes one valence electron : 3 x 1 = 3
each nitrogen contributes five valence electrons : 3 x 5 = 15
total valence electrons 22
H H H
| _ _ | _ | _
H-C-N=N=N| <---> H-C-N-N-=N| <---> H-C-N-N=N|
| | - | - -
H H H
formal charges:
N1 (nitrogen attached to carbon) 0 -1 -1
N2 (middle nitrogen) +1 +1 0
N3 (terminal nitrogen) -1 0 +1
Note: The first two forms are fairly equall in energy since each atom has an
octet and in both form an electo-negative atom has a positive charge (and a
negative charage). However the third form is a minor contributer since one
nitrogen (the terminal one) does not have a complete octet. Again it is this
third form which shows the 1,3-dipolar nature of azides.
>Can anyone out there spend a few minutes and educate me on this one?
Hope this helped,
George Huber
Jeff E. Janes
: I'm sorry to bother everyone about this elementary problem, but for the
That's because there is no simple Lewis structure for these.
Lewis structure's are models, and (un)fortunately the real world
doesn't always fit into our models. These molecules need both
formal charges and resonance to represent with Lewis structures.
To them justice would require Molecular orbital theory and
quantum mechanics, and I'm not about to attempt that, at
least for a few more semesters, anyways. And definitely not
on a weekend :)
:
: a) CH3CNO
H H
" " "
H:C:C:::N:O: <===> H:C:C::N::O:
" " " " "
H H
N +1 N +1 <formal charges
O -1 C -1
The one on the left is the probably the most accurate
:
: and
: b) CH3N3
H H
" " "
H:C:N::N::N: <==> H:C:N:N:::N:
" " " " "
H H
charges on N's of 0,+1, and -1, respectively for left model
-1,+1, and 0, for the right model
The left is probably slightly more accurate.
:
: Can anyone out there spend a few minutes and educate me on this one?
:
Hope this helped.
jeff
--
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ Jeff Janes at Michigan Tech,no logical University +
+ jej...@mtu.edu +
+===========================================================+
+ Everyone's paranoid, they all think I'm out to get them. +
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
R. Wong
Your help is greatly appreciated.
Rob Wong
David M Johnson
>I'm sorry to bother everyone about this elementary problem, but for the
>life of me, I can't figure out the simple Lewis structures for
No problem, it makes us feel important and highly educated...:)
>a) CH3CNO
H
| + -
H-C-C#N-O Methyl Isocyanate
|
H
Where # is a triple bond. (sorry, couldn't figure out a cute trick to
draw it nicely.)
>and
>b) CH3N3
H
| + -
H-C-N=N=N Methyl Azide
|
H
>Can anyone out there spend a few minutes and educate me on this one?
There ya go. All atoms have 8 electrons, except the Hydrogens.
-David M. Johnson
David M Johnson
>Alright here we go. Note the symbol -= should be read as a triple bond.
>>a) CH3CNO
> H H H
> | _ | _ _ | _ _
> H-C-C-=N-O| <------> H-C-C=N=O| <------> H-C-C=N-O|
> | - | | -
> H H H
I made a mistake and called this methyl isocyanante. This is actually
wrong, a proper name for it is methyl cyanate, or to use the terminology
in Mr. Huber's well written article, Acetonitrile oxide.
However this does cause me to remeber the other forms CNO can take.
Such as Isocyanate, or in this case methyl Isocyanate (which I improperly
named):
H
|
H-C-N=C=O
|
H
Or Methyl Fulminate
H
| + -
H-C-O-N-=C (Note the triple bond)
|
H
Fulminates are not very common, and have a tendency to rearrange to
cyanates or isocyanates readily (and often explosively) From a QM
perspective, there are *NO* resonance forms for the Fulminate (perhaps a
good explanation of it's instability), but the Isocyanate has a number as
do the cyanate diagrams provided by Mr. Huber. I leave the resonance
structures for Isocayante to you as an exercise, follow Mr. Huber's
example, and you'll knock 'em down well. Sorry for my earlier mistake.
-David M. Johnson
