Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Double bond

  1. Feb 9, 2005 #1
    Double bond....

    We always talk about the double bond in an alkene being an area of particularly high electron density, which induces charges on other molecules, which is what usually causes it to react. But why is this only the case for double not single? I mean, single bonds are not overly attractive since the electrons attract as much as the nuclei repel. Is the double bond different because the pi bond is that little bit further from the nucleus, due to the p orbitals orientation, and thus attract more than the nuclei repel in-coming molecule's electrons?

    Thanks in advance. :wink:
  2. jcsd
  3. Feb 9, 2005 #2
    Although single bonds polarize compounds such as bromine when in close proximity, I believe that the electron density is not high enough to polarize to the point of breaking. Therefore, the single bonds will not prompt a reaction while the double bonds will. An example of this would be benzene and bromine, which yields dibromobenzene.
  4. Feb 10, 2005 #3
    A double bond is made of three orbital bonds, known as Pi (two fo them) and Sigma ([tex]\pi[/tex] and [tex]\sigma[/tex] respectively).

    You probably know how these are formed so I will get to the point. The electrons that can be used to form other compounds will be in the two [tex]\pi[/tex] orbitals as these are were the free electrons are. A [tex]\pi[/tex] and a [tex]\sigma[/tex] are, together, not as strong as one [tex]\sigma[/tex] bond. Why?? I don't know yet but I intent to find out.

    I think the best way to explain why single bonds are stronger is that, because of the position the orbitals are, a [tex]\sigma[/tex] bond is more direct in attraction than a [tex]\pi[/tex] bond. This means that the attraction through the orbitals is stronger. It is the old saying: 'The shortest point to A and B is a stright line'. This is the same for a [tex]\sigma[/tex] bond.

    If any of this is wrong then I ask someone to correct it but this is my understand of it after reading on the subject for 10 minutes in my chemistry lesson today.

    The Bob (2004 ©)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook