Posted by: Mark Foreman | January 16, 2011

Resonance forms and boron.

I like valence shell electron pair repulsion theory (VSEPR), I think it is one of the best things ever in inorganic chemistry. It is a quick and simple method of predicting the structure of a main group molecule. But before you use it you must consider if another resonance form can exist for your molecule.

Recently in my professional life I have been thinking about boron chemistry, so I will show you a couple of examples of things which contain boron.

Well here is an example of a neutral complex which can be viewed as being a complex of pinacol and (HO)2BOB(OH)2, it was reported by V.Montiel-Palma, M.Lumbierres, B.Donnadieu, S.Sabo-Etienne and B.Chaudret in the Journal of the American Chemical Society, 2002, 124, 5624. In this complex the borons are close to having perfect trigonal planar geometries.

 

The first boron compound to look at

In the case of the anionic bis 1,2-diphenyl ethane-1,2-diol complex you can see that the boron has a tetrahedral environment. This structure was reported by N.D.Coombs, S.Aldridge, G.Wiltshire, D.L.Kays, C.Bresner and Li-ling Ooi, Journal of Organometallic Chemistry, 2005, 690, 2725.

 

The second boron compound

While I have chosen compounds which do not have any alternative resonance forms which involve the boron, it is important to check for alternative resonance forms before you try to use VSEPR on a molecule. When you draw two resonance forms which are interconverting the convention in organic chemistry is to use a double headed arrow.

 

Double headed arrow

For example the nitrogen in pyrrole might appear at first to be an “ordinary” amine nitrogen. For those of you who know what pyrrole is well done (pat yourselves on the back) for the rest of you here is a picture of pyrrole.

Pyrrole

 

But the nitrogen lone pair is moved around the molecule by resonance through the aromatic ring. Below are some of the different resonance forms of pyrrole. It is clear that the lone pair on the nitrogen does not stay there, instead it is moved around (delocalised) by resonance. This means that the lone pair electrons enter the molecular orbitals associated with all five atoms in the ring.

 

Resonance forms of pyrrole

Now looking at the middle resonance form it should be clear that in this resonance form the nitrogen has three sigma bonds, and no lone pairs. I always say that the lone pair is the sigma bond to nowhere. The general rule is that any main group atom which has three sigma bonds (including these ‘bonds to nowhere’ called lone pairs) will be a sp2 centre.

 

Now some of you might be inclined to repeat this process on the left hand resonance form and based on the fact that there are three sigma bonds plus the lone pair, you might say that it is a sp3 centre. Oh please no !

 

For goodness sake do not do this !

 Dr Foreman’s health warning. This change of hybridizations does not happen.

One of the key basic rules of resonance structures is that when you change from one resonance form to another you can only move electrons (formal charges on atoms and bonds) and never the atomic nuclei.

For example do not ever claim that a ketone changes into an enol through resonance. This is an isomerisation which has the special name of tautomerism. Below I am showing you acetone. I am showing you in the centre and the left the two resonance forms of acetone and on the right the enol form of acetone. Please note that I use different types of arrows to indicate the different interconversions.

When changing from one resonance form to another it is impossible for an atom to change from one hybridization to another, it is the chemical version of a cat changing into a giant spider. This is because to change the hybridization would require you to move some atoms from one place to another. Please just do not entertain this idea, and for goodness sake do not write or draw anything to suggest that it can happen in the exam. I will not be lenient in the future with this.

The realitiy is some combination of all the possible resonance forms, some forms (with higher energies) will make a smaller contribution to the final reality while the lower energy forms will make a greater contribution to the final form. For instance below are two low energy and two high energy resonance forms for benzene. You should try to spot which is which.

 

Resonance forms for benzene, two reasonable and two 'silly' (high energy)

Well, if you have not worked it out think for a moment…..The two forms on the right have a greater degree of charge separation than the two on the left. The general rule might be that the more charges a resonance form has then the higher its energy will be, but you also need to consider the electronegativities of the atoms.

 Consider for a moment methyl formate (ester) and dimethyl formamide (amide). In dimethyl formamide (A.K.A. DMF) the carbonyl carbon is bonded to a nitrogen, the nitrogen is less electronegative than the oxygen and is very able to transfer electron density through resonance to the carbonyl oxygen.

 

Two resonance forms of DMF

 

While the oxygen of the alkoxy group on methyl formate is less able to donate electron density through resonance to the carbonyl oxygen.

Two resonance forms of methyl formate

In the case of the amide the difference in the energies of the two resonance forms shown is smaller than the difference between the energies of the two resonance forms of the amide which are shown. So in the case of the amide the resonance form with the separated charges makes a greater contribution to the final reality than in the case of the ester.

 

We can have even more fun with a molecule where both an oxygen and a nitrogen can donate electron density through resonance to a carbonyl oxygen. This is an example of a urethane. The urethane is an important building block in polymer chemistry, typically polyurethane is used in chairs and sofa to provide a soft thing to sit your bottom on. Here it is important to note that two different atoms (a nitrogen and an oxygen) can donate electron density to the carbonyl oxygen by resonance. Here is our molecule.

 

Trimethyl urethane

Now when you draw the diagram of the different resonance forms the convention in organic chemistry is to use a big arrow for the resonance form which makes a bigger contribution to the overall reality and smaller one for conversion into a high energy resonance form which makes only a very small contribution to the overall reality.

 

Resonance forms of trimethyl urethane

I want you to remember that when thinking about resonance forms be careful of those with lots of charges.

 

For example when considering the electrophilic bromination of benzene, the following two resonance forms will make a contribution to the structure of the cationic intermediate.

 

Two resonance forms of the cationic intermidate in the bromination of benzene

The one on the left is a nice low energy resonance form while the one on the right is a nasty high energy resonance form which will make little or no contribution to the overall reality. Now I have finished on resonance, for the mean time at least.

Back to boron chemistry I have shot a film showing what happens when sodium borate is added to PVA (school glue or indoor woodworker’s water based glue).

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Categories

%d bloggers like this: