Bored, was thinking cis to trans

[sizzeR]

Would it be possible to somehow force a cis oriented unsaturated compound to a trans oriented version? Has this ever been done? I mean without synthesis but rather some sort of immense force applied to the molecule to shift its orientation? Is it even possible...or would the bond be too strong and resist it since double bonds do not allow for free rotation...

 

[Char. Limit]

It's possible, all right: the changing of a double bond from cis to trans is an essential process in vision.

In the retina, the molecule 11-cis-retinal is isomerized by a photon of sufficient strength (which just happens to be in the frequency range of visible light) into 11-trans-retinal, which is the fundamental chemical reaction of vision.

There are other examples, I'm sure, but I don't feel like looking them up.

 

[sjb-2812]

Isn't the retinal example more a synthesis approach, though, which the OP was trying to avoid? There are also things like "push-pull" alkenes, with for instance an amine at one end of the double bond and a nitro at the other, such that you can draw resonance structures with charges separated and the former double bond having some single character and so can readily rotate.

 

[Char. Limit]

I wouldn't call it synthesis, as I know synthesis as the combination of two molecules. This molecule simply absorbs energy and switches.

 

[flatmaster]

The retinal is a good example. If you look at the molecule, there are some alternating single and double bonds down the length of the molecule. There's a resonance structure that allows these bonds to take some extra electron density. There's sort of a half pi bond among these carbons similar to the resonance structure in benzene.

Also, you can have a thermal cis to trans transition. If there's enough excess thermal energy, the pi bond can go into an excited anti-bond state and rotate around the sigma bond.

My chemistry is rusty, but I think that's mostly correct.

 

[sjb-2812]

True, similar to the example I mentioned with push-pull alkenes. I'd still count these as synthesis of a sort though, as you are adding a reactant, even if it is a photon and not a fully fledged molecule.

 

[Char. Limit]

Er... a photon is pure energy. It cannot be considered a "reactant", I think.

 

[sjb-2812]

Er... a photon is pure energy. It cannot be considered a "reactant", I think.

Semantics, perhaps. It's clear to me that the cis retinal (in this case) exists at an energy minima, otherwise it could not be isolated at room temperature, now you're doing something to force it over an energy barrier that it would not necessarily jump over by itself, hence you are adding a reactant to it. Perhaps the photon is like a catalyst for the reaction, would that be a better description in your view?

 

[Char. Limit]

A catalyst is a better term... if you accept that the reaction is "energy-catalyzed".

 

[sigmavirus]

i think energy can be considered a reactant. or at least it is treated like one in thermodynamics.

 

[sizzeR]

very interesting stuff guys. thanks.

 

[Scratchem]

A catalyst is a better term... if you accept that the reaction is "energy-catalyzed".

I believe it is better used as a reactant. I have never heard the term "energy catalyzed" before. See, the reason I think it is a catalyst is that reactants are consumed or in other words used up. It's kind of different that, for example, a chemical catalyst. Have you seen this used in text and if so was it organic?

 

[Char. Limit]

Organic Chemistry by John McMurry, Fifth Edition

if you need page numbers, I'll try to provide them.