# Is there any organic compound that can rebond?

Jul 13, 2016

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.

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 $\text{CO}$ contributes two valence electrons, for a total of $\setminus m a t h b f \left(10\right)$.
• The neutral ${\eta}^{1}$-allyl ligand, which bonds via one atom, contributes $\setminus m a t h b f \left(1\right)$ valence electron.
• Manganese is therefore ${\text{Mn}}^{0}$, and has $\setminus m a t h b f \left(7\right)$ valence electrons.

Thus, this compound is a $10 + 1 + 7 = \setminus m a t h b f \left(18\right)$-electron complex. It happens to be stable with $18$ valence electrons.

Upon heating or subjecting this compound to UV-light, one $\text{CO}$ 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. ${\text{H"_2"SO}}_{4}$, $\text{HCl}$, 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-$\setminus m a t h b f \left(\pi\right)$-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 $\left(+\right) \left(+\right) \left(-\right) \left(-\right) \left(+\right) \left(+\right)$.)

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-$\setminus m a t h b f \left(\pi\right)$-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, $\left(+\right) \left(+\right) \left(-\right) \left(-\right) \left(+\right) \left(+\right)$.

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