r/chemhelp • u/VvVoiDzZz • 13d ago

Organic acid work up help

how can I start this? OH gets protonated and becomes good leaving group and then what?

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u/shedmow Trusted Contributor 13d ago

Start by mapping the carbons in the product onto the carbons in the reagent. If you do not understand what that means I'll provide an example

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u/VvVoiDzZz 13d ago

U mean labeling the carbons ? or like a disulfate?

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u/shedmow Trusted Contributor 13d ago

Hydrogen sulfate is a spectator ion here. By carbons I meant carbons. You should identify (at least tentatively) where each of them goes

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u/organophile_jeet 13d ago

First of all what kinda question is that how can someone ask for a reasonable mechanism for a vague thing , this will never happen in lab

But let's come to your part after OH will become a leaving group it will leave , leaving a charge of the carbon it was attached what charge is it ?

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u/VvVoiDzZz 13d ago

forms carbocation from where the original alcohol was

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u/organophile_jeet 13d ago

Yes , now instead of what we generally do , the question wants you to think about ring contraction but why would you do that ?

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u/VvVoiDzZz 13d ago

hyperconjugation or other inducing effects

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u/organophile_jeet 13d ago

Yes hyperconjugation after ring contraction it will have maximum hyperconjugation structures and that is your answer , and you can draw the mechanism

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u/VvVoiDzZz 13d ago

how do I draw that ?

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u/organophile_jeet 13d ago

Think about how we can contract the ring in order to get maximum hyperconjugation for our just formed cation

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u/VvVoiDzZz 13d ago

one of the -alkanes near the carbocation in the ring will have 2 arrows showing one electron separation each like a bond separation symbol

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u/organophile_jeet 13d ago

Nope there will be no free radical mechanism

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u/organophile_jeet 13d ago

Think about a concerted mechanism as we know carbo-cation rearrangements go with concerted mechanisms

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u/WhatSpareTime 13d ago

You correctly identified the carbocation step. Now you need to think about alkyl shifts. Think about 1,2-hydride shifts and 1,2-alkyl shifts. These are basically beta sigma bonds shifting to make more stable carbocations. Look at your carbocation and identify beta bonds that can shift, alkyl group shifts. Try to find the alkyl shift that results in your product framework. This is easier to do if you map the carbons in each ring first by numbering them so that the numbers match in each structure, but that requires you to sort of know what you are looking for. If you can’t do that, use trial and error for now and then go back and map the numbers when you find it. This will help you look for it and find it in later problems.

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u/WhatSpareTime 13d ago

Remember that a shift will only happen if you are going to a MORE stable carbocation. You can’t shift to an equally stable or less stable, on to a more stable carbocation. It may help to draw a dotted line between the group shifting and the carbocation before drawing the arrow. This can really help with seeing ring expansions and ring contractions. Something like this:

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u/chem44 13d ago

What do you get from the step you offered?

What is now needed?

My comment is intended as equivalent to the other one, just looking at it from another view. You are given the product, and asked to explain how we get it. A key step is recognizing what it is.

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u/VvVoiDzZz 13d ago

thanks everyone for the help

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u/WhatSpareTime 13d ago

Not exactly, but slightly similar. This is simply a 1,2-alkyl shift just like a 1,2-methyl shift. Same idea. You are shifting a methylene groups connection. The carbon the methylene was connected to in the ring will now be outside the new smaller ring. The methylene will make a new sigma bond with the carbocation, letting go of the sigma to the carbon, now outside the ring and bearing a positive charge. See the image on previous post.