Is there any organic compound that can rebond?

INORGANIC REARRANGEMENTS

Bond Hapticity Aajustment -

Hapticity (#eta#) is the number of contiguous atoms to which a transition metal is being bonded. So, a dihapto (#eta^2#) ligand, for instance, bonds to a transition metal via two directly-connected atoms.

For this compound, #eta^1#-allylmanganesepentacarbonyl:

• Each neutral #"CO"# contributes two valence electrons, for a total of #\mathbf(10)#.
• The neutral #eta^1#-allyl ligand, which bonds via one atom, contributes #\mathbf(1)# valence electron.
• Manganese is therefore #"Mn"^(0)#, and has #\mathbf(7)# valence electrons.

  Thus, this compound is a #10+1+7 = \mathbf(18)#-electron complex. It happens to be stable with #18# valence electrons.

  Upon heating or subjecting this compound to UV-light, one #"CO"# ligand is lost, taking away #2# valence electrons.

  But, the #18# electrons provided stability, so the allyl ligand changes hapticity (rebonds) to donate #3# valence electrons as #eta^3# instead of #1# as #eta^1#, making up for the loss of two valence electrons.

  Now, the allyl (an organic delocalized #pi# system) is bonded via three contiguous atoms, having rebonded!

  There are plenty more examples in Transition Metal chemistry, but that's one I could think of off the top of my head.

  ORGANIC REARRANGEMENTS

  These are much more interesting to describe, and usually happen with conjugated #pi# systems, like 1,3-butadiene, 1,3,5-hexatriene, etc.

  There are quite a few variations, but as some examples, I'll look at:

  • a ring expansion/contraction (you should have seen this before)
  • a disrotatory ring closure (thermal catalysis)
  • a conrotatory electrocyclic ring-opening (thermal catalysis)

    Ring Expansion/Contraction -

    Expansion usually occurs when you have a formed cationic carbon adjacent to a small ring (4/5 members) that can be stabilized by expanding intramolecularly.

    (Ring contractions will occur for 7/8 membered rings.)

    The cationic carbon can appear when you add a strong acid (e.g. #"H"_2"SO"_4#, #"HCl"#, etc) to a double bond, for instance.

    The major product is the expanded ring.

    Disrotatory Electrocyclic Ring Closure

    This usually occurs in straight-chained conjugated #pi# systems. A disrotatory process occurs for a system with an odd number of #pi# bonds upon being subjected to thermal catalysis.

    A conrotatory process means the end-#\mathbf(pi)#-orbitals of the HOMO in the molecule rotate in the same direction (say, both CCW) for the bond migration.

    (The HOMO contains matching signs on orbital pairs, going #(+)(+)(-)(-)(+)(+)#.)

    So, a disrotatory process is when they rotate towards each other (say, CW vs. CCW). This example is of 2,4,6-octatriene.

    Because these orbitals rotated towards each other, the stereochemistry of the final product's methyl groups is cis. The arrow-pushing mechanism would look like this:

    Conrotatory Electrocyclic Ring-Opening

    A ring-opening tends to occur with small #pi#-system rings. It is conrotatory when an even number of #pi# bonds are in the straight-chained system, and the process is thermally-induced.

    Then, the end-#\mathbf(pi)#-orbitals rotate in the same direction and break the #sigma# bond, generating the HOMO of the #pi# system.

    In the image, both end-orbitals rotate CCW, generating the HOMO of the system, which has matching signs on orbital pairs, as in, #(+)(+)(-)(-)(+)(+)#.

    The conrotatory process resulted in the methyl groups facing in the same direction, generating the cis,trans-2,4-hexadiene isomer.