asdf1999
Badges: 0
Rep:
?
#1
Report Thread starter 5 years ago
#1
Hi,
I'm currently writing an essay on electron deficient compounds and whether they contain an appropriate number of electrons for their structures or not and feel like there are a few points that I'm missing out on.

So far what I've got on either side is:
In terms of not having an appropriate number of electrons
-they do not comply with the Lewis model (do not fit with the octet rule and contain covalent bonds in which pairs of electrons are shared between more than two atoms)
-their structures are still subject to debate. Techniques used to extract the structures of simpler boron hydrides, such as diborane, do not work on the higher boron hydrides. In the past there were clashes amongst chemists over the bonding that took place in 3-centre-2 bonds
In terms of having an appropriate number of electrons
-one could argue that the Lewis model is flawed rather than the electron deficient compounds.
-they often react in a similar way to more conventional compounds, such as in displacing metals. They are used in multiple useful reactions such as hydroboration which is very important in certain organic syntheses.

This is basically all I've got so far. The essay is supposed to be 3000-4000 words long so any pointers on what to add to it would be invaluable.

TIA
0
reply
Infraspecies
Badges: 16
Rep:
?
#2
Report 5 years ago
#2
Have a little look at molecular orbital theory. It should be in all of your textbooks.

Understanding reactivity of molecules is very dependent upon the geometry of compounds, and the occupancy and multiplicity of the HOMO.
1
reply
asdf1999
Badges: 0
Rep:
?
#3
Report Thread starter 5 years ago
#3
(Original post by Infraspecies)
Have a little look at molecular orbital theory. It should be in all of your textbooks.

Understanding reactivity of molecules is very dependent upon the geometry of compounds, and the occupancy and multiplicity of the HOMO.
Thanks a lot for the reply. I have included some stuff about molecular orbital theory but should should some more about it. I've explained how the two electrons in the 3c-2e bonds are in the bonding orbital and the non bonding and anti bonding orbitals are empty. I'm sure there are many more things I could include about it though.

I'm still in lower sixth and my knowledge chemistry is relatively limited (most of this essay has involved doing extra research), so could you explain how the geometry of the compounds affects their bonding?

Thanks
0
reply
Peroxidation
Badges: 16
Rep:
?
#4
Report 5 years ago
#4
The main thing I can think of right now is steric hindrance. There needs to be enough space for the groups to occupy without the lone/bonding electrons in the structure repelling each other too strongly.

As for the Lewis structures, the electron deficient compounds are sound proof of the need for a new model. Chemists tend to stick to what's familiar. It's why the biggest breakthroughs in studies of the atomic and molecular structures and orbitals have come from physicists
0
reply
KombatWombat
Badges: 7
Rep:
?
#5
Report 5 years ago
#5
(Original post by asdf1999)
Thanks a lot for the reply. I have included some stuff about molecular orbital theory but should should some more about it. I've explained how the two electrons in the 3c-2e bonds are in the bonding orbital and the non bonding and anti bonding orbitals are empty. I'm sure there are many more things I could include about it though.

I'm still in lower sixth and my knowledge chemistry is relatively limited (most of this essay has involved doing extra research), so could you explain how the geometry of the compounds affects their bonding?

Thanks
You have to write a 4000 word essay on electron deficient compounds and you're in year 12? I'm definitely not jealous! To be honest I'm not sure what you'd write without learning good chunks of a 1st/2nd year undergrad course. You'd definitely need a simple understanding of MO theory - the point is that a molecule adopts the most favoured geometry. You can imagine what the orbitals would be like for different geometries and work out which would be the most stable. Electron deficient compounds are often clusters - sort of because they can share the few electrons about better.

I'm also not sure what you mean by not having an 'appropriate' number of electrons - they must do if we can isolate them and look at their structures! Its a chemists job to explain why they're stable. You're right that Lewis structures don't do this for boron hydrides, but there's actually a pretty good model that predicts their structures called Wade's rules - but you need a bit of MO theory to have a chance at understanding them.
0
reply
langlitz
Badges: 17
Rep:
?
#6
Report 5 years ago
#6
(Original post by asdf1999)
Thanks a lot for the reply. I have included some stuff about molecular orbital theory but should should some more about it. I've explained how the two electrons in the 3c-2e bonds are in the bonding orbital and the non bonding and anti bonding orbitals are empty. I'm sure there are many more things I could include about it though.

I'm still in lower sixth and my knowledge chemistry is relatively limited (most of this essay has involved doing extra research), so could you explain how the geometry of the compounds affects their bonding?

Thanks
The symmetry of compounds is very important for the type of bonding interactions that take place. Although that is far beyond the level you are at. May I ask why you labelled this undergraduate when you're in sixth form?

https://en.wikipedia.org/wiki/Molecular_symmetry
0
reply
Infraspecies
Badges: 16
Rep:
?
#7
Report 5 years ago
#7
(Original post by langlitz)
The symmetry of compounds is very important for the type of bonding interactions that take place. Although that is far beyond the level you are at. May I ask why you labelled this undergraduate when you're in sixth form?

https://en.wikipedia.org/wiki/Molecular_symmetry
It is an undergraduate topic.
0
reply
asdf1999
Badges: 0
Rep:
?
#8
Report Thread starter 5 years ago
#8
(Original post by KombatWombat)
You have to write a 4000 word essay on electron deficient compounds and you're in year 12? I'm definitely not jealous! To be honest I'm not sure what you'd write without learning good chunks of a 1st/2nd year undergrad course. You'd definitely need a simple understanding of MO theory - the point is that a molecule adopts the most favoured geometry. You can imagine what the orbitals would be like for different geometries and work out which would be the most stable. Electron deficient compounds are often clusters - sort of because they can share the few electrons about better.

I'm also not sure what you mean by not having an 'appropriate' number of electrons - they must do if we can isolate them and look at their structures! Its a chemists job to explain why they're stable. You're right that Lewis structures don't do this for boron hydrides, but there's actually a pretty good model that predicts their structures called Wade's rules - but you need a bit of MO theory to have a chance at understanding them.
The essay is for a competition (the Kelvin Prize) and the point of it is that you get to explore concepts outside of the a level curriculum. It's proving to be a pretty challenging topic though! By 'appropriate numbers of electrons' I mean that they are not deficient of electrons and that they contain the right amount for their structure, unlike what the name 'electron deficient' would suggest.

Thanks for the heads up about Wade's rules. Think that could be very useful to include
0
reply
asdf1999
Badges: 0
Rep:
?
#9
Report Thread starter 5 years ago
#9
(Original post by Peroxidation)
The main thing I can think of right now is steric hindrance. There needs to be enough space for the groups to occupy without the lone/bonding electrons in the structure repelling each other too strongly.

As for the Lewis structures, the electron deficient compounds are sound proof of the need for a new model. Chemists tend to stick to what's familiar. It's why the biggest breakthroughs in studies of the atomic and molecular structures and orbitals have come from physicists
Thanks for the suggestion; something that I hadn't come across before. It definitely does seem from what I've gathered so far that the Lewis model is now a bit dated and is not a totally reliable tool.
0
reply
asdf1999
Badges: 0
Rep:
?
#10
Report Thread starter 5 years ago
#10
(Original post by langlitz)
The symmetry of compounds is very important for the type of bonding interactions that take place. Although that is far beyond the level you are at. May I ask why you labelled this undergraduate when you're in sixth form?

https://en.wikipedia.org/wiki/Molecular_symmetry
Thanks, will look into that. I put it in the undergraduate section because the subject is one which isn't covered at a level. I'm doing the essay as part of an essay competition.
0
reply
SANTR
Badges: 8
Rep:
?
#11
Report 5 years ago
#11
(Original post by langlitz)
The symmetry of compounds is very important for the type of bonding interactions that take place. Although that is far beyond the level you are at. May I ask why you labelled this undergraduate when you're in sixth form?

https://en.wikipedia.org/wiki/Molecular_symmetry
Can you explain why symmetrical shaped molecules do not have permanent dipole-dipole?
Does the pull on the electron density from the various directions cancel out?
0
reply
langlitz
Badges: 17
Rep:
?
#12
Report 5 years ago
#12
(Original post by SANTR)
Can you explain why symmetrical shaped molecules do not have permanent dipole-dipole?
Does the pull on the electron density from the various directions cancel out?
You need to be careful when you say "symmetrical shaped compounds". There are many symmetry operations (identity, rotation, mirror planes, inversion and improper rotation) but the ones you're referring to are molecules which posses a centre of inversion. The distribution of electron density around the centre is balanced so it can't have a permanent dipole
3
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Are you tempted to change your firm university choice on A-level results day?

Yes, I'll try and go to a uni higher up the league tables (26)
27.96%
Yes, there is a uni that I prefer and I'll fit in better (9)
9.68%
No I am happy with my choice (52)
55.91%
I'm using Clearing when I have my exam results (6)
6.45%

Watched Threads

View All
Latest
My Feed