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Topic: Reaction schemes and the sequences of the steps  (Read 4537 times)

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Offline katastrophickid

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Reaction schemes and the sequences of the steps
« on: February 20, 2015, 01:31:20 AM »



Those are the problems I'm working on now as practice for my organic chemistry exam. The answers are already given in blue. I can figure out the reagents fine, the problem is the sequence of the steps. Looking at the first picture's 1st example, how do you know those are the steps for sure? I'm quite confused. I'm almost certain it has something to do with Ortho/Para vs Meta directing. Also in the 2nd picture, I mixed up the 2nd and 3rd step's order (I switched them around). If anyone could clear this up for me, that would be amazing!

Offline kriggy

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Re: Reaction schemes and the sequences of the steps
« Reply #1 on: February 20, 2015, 06:18:40 AM »
The order of the steps in your case has also something to do with orto/meta directing but also with the reactivity of your intermediate. For example, nitro group strongly deactivates the system for electrophilic substitution so you need to keep that in mind.
So you need to think about the reactivity of the intermediate for later reactins you want to do and in those cases also about the position in which it directs the reaction. Usualy there are multiple possible orders but untill you do the synthesis in a lab, you cant know which one is better (for example you want to make 1-methyl-4-isobutylbenzene, so you start with methyl or isobutyl? etc..)

Offline Dan

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Re: Reaction schemes and the sequences of the steps
« Reply #2 on: February 20, 2015, 09:01:28 AM »
Looking at the first picture's 1st example, how do you know those are the steps for sure? I'm quite confused. I'm almost certain it has something to do with Ortho/Para vs Meta directing.

Do you think you would get the same product if you reversed the order of steps? Why?

Quote
Also in the 2nd picture, I mixed up the 2nd and 3rd step's order (I switched them around). If anyone could clear this up for me, that would be amazing!

Can you post your proposed sequence for clarity?

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Offline katastrophickid

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Re: Reaction schemes and the sequences of the steps
« Reply #3 on: February 20, 2015, 09:14:57 AM »
The order of the steps in your case has also something to do with orto/meta directing but also with the reactivity of your intermediate. For example, nitro group strongly deactivates the system for electrophilic substitution so you need to keep that in mind.
So you need to think about the reactivity of the intermediate for later reactins you want to do and in those cases also about the position in which it directs the reaction...

So you are saying that because NO2 is a deactivating molecule, it is not going to be added first, yes? Am I able to generalize that activating Ortho/Para directors are normally added to Benzene first? What if you have a case where you're adding a halogen, a deactivator but also an Ortho/Para director, and an NO2 (a  stong deactivator, Meta directing) to Benzene? What is a generalized rule for a case like that?

Offline katastrophickid

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Re: Reaction schemes and the sequences of the steps
« Reply #4 on: February 20, 2015, 09:40:01 AM »
Do you think you would get the same product if you reversed the order of steps? Why?

One sequence that crossed my mind was the addition of NO2 first, followed by Br, and then CH3. Obviously, it's wrong and I was thinking is it because NO2 is a deactivator?

Can you post your proposed sequence for clarity?


That was my proposed sequence.

Offline sjb

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Re: Reaction schemes and the sequences of the steps
« Reply #5 on: February 20, 2015, 04:08:37 PM »
A number of things stand out with your proposed sequence. Check the regiochemistry of the first Friedel Crafts, and your reagent for the last one

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