Question

Is there any organic compound that can rebond?

Answer 1

When you say "rebond", I can think of these ways from inorganic and organic chemistry:

Inorganic/Organic Rearrangements

• Bond hapticity being adjusted so that the total valence electron count on a transition metal complex is kept the same.

Organic Rearrangements

• Ring Expansion/Contraction in the presence of a carbocation.
• Alkene rearrangements due to light- or heat-induced catalysis, or olefin metathesis.

Since you should be taught about olefin metathesis in class eventually, I'll leave that to your professor.

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DISCLAIMER: LONG ANSWER!

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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.