Lewis Acid or Double Bond?

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Vivien Ho 3L
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Joined: Fri Sep 26, 2014 2:02 pm

Lewis Acid or Double Bond?

Postby Vivien Ho 3L » Mon Oct 27, 2014 9:41 pm

In the course reader, it says boron trifluoride has only 6 valence electrons. It completes its octet if it forms a coordinate covalent bond with another atom. Why can't it complete its octet by having 8 valence electrons then share its electrons to have one double bond? On the other hand, the nitrate ion has two extra electrons so it must form a double bond to achieve a complete octet. Why can't it just have 6 valence electrons like boron trifluoride? How do we know when to put double bonds so that it will have an octet or just leave it at less than 8 valence electrons and let it form a coordinate covalent bond?

HimaniMadnawat3L
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Joined: Fri Sep 26, 2014 2:02 pm

Re: Lewis Acid or Double Bond?

Postby HimaniMadnawat3L » Tue Oct 28, 2014 7:35 pm

In the course of this reaction, the NH3 acts as an electron-pair donor, or Lewis base. The electron-pair acceptor is the boron atom in BF3, which makes this the lewis acid. When the boron atom picks up a pair of electrons from the water molecule, it no longer needs to form double bonds. This is called a coordinate covalent bond. Boron is an exception to the rules we are used to. You can see that the fluorine atoms possess extra lone pairs that they can use to make additional bonds with boron, and you might think that all you have to do is make one lone pair into a bond and the structure will be correct. But if we add one double bond between boron and one of the fluorines, we get strange formal charges. The BF3 model without the double bond and only 6 valence electrons for the boron has 0 formal charge for each atom, while the double bond model has B=-1, F(with double bond)=+1, F(other)=0. Using formal charge we can see that the best lewis structure is the one where boron has an incomplete octet.

Chem_Mod
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Re: Lewis Acid or Double Bond?

Postby Chem_Mod » Tue Oct 28, 2014 7:38 pm

Your question can be more completely answered once we get to MO theory and VB theory. Most of the time the octet rule leads to the right answer. There are a few exceptions. Boron is usually electron poor making only 3 bonds. Sulfur and phosphorous are large atoms and can accommodate more bonds.

Memorize these for now. You can reason them out when we teach you more chemistry. The vast majority of molecules will follow the octet rule.

CHemistry is full of exceptions which is why chemist love it and students sometimes dont!

Vivien Ho 3L
Posts: 30
Joined: Fri Sep 26, 2014 2:02 pm

Re: Lewis Acid or Double Bond?

Postby Vivien Ho 3L » Fri Oct 31, 2014 2:40 pm

Oh I see. I tried using formal charge to see which is the best structure for the nitrate ion NO3 and it shows that for the double bond NO3, the single bond Os have an FC of -1 , the double bond oxygen has an FC of 0 and N has an FC of +1. However if N were to have 6 valence electrons it would result in weird formal charges where the three Os would have an FC of -1 while N would have an FC of +2. So is it safe to assume that when facing a problem like this, I have to check the formal charges to see which would be the best bond?


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