Fischer esterification trans 4-methoxycinnamic acid

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Once the proton adds to one of the carbon, is there free rotation around the C-C bond?

When the proton adds at one carbon of C=C bond I'll get a single bond....and there could be a rotation

SO I could have a 180 degree rotation ...and after that rotation if the C=C is reformed  I'll have the cis isomer instead the trans one?
Do you mean this?

Thanks.

Once the proton adds to one of the carbon, is there free rotation around the C-C bond?

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Can you think of any other chemistry that might happen besides esterification?  I would focus on the catalyst.

the "catalyst" is the acid environment

The only thing  that comes to mind is the electrophilic addition of H⁺ to C=C....

Yes, but there will be some fraction of of the cis-form.

You are on the right track, but think about chemical equilibrium.

Can you think of any other chemistry that might happen besides esterification?  I would focus on the catalyst (as you did), but I am not quite sure what you mean by "attack."

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Yes, but there will be some fraction of of the cis-form.

Thankss!

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You are on the right track, but think about chemical equilibrium.

If I thought at the equilibrium between cis and trans ....the trans isomer should be more stable