Org Q

[Mstoothlady2012]

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guys is this right? I thought HBr add markov and in this reaction its adding to the primary carbon whereas you have the other possible tertiary carbon???? what's going on????

 

[flycd05]

guys is this right? I thought HBr add markov and in this reaction its adding to the primary carbon whereas you have the other possible tertiary carbon???? what's going on????

hmm thats really wierd! are u sure theres no HOOR or hv involved. i always thought the same - hbr adds mark unless theres light or peroxide involved - where is this question from?

 

[harrygt]

guys is this right? I thought HBr add markov and in this reaction its adding to the primary carbon whereas you have the other possible tertiary carbon???? what's going on????

Nice question. It is because once H+ adds to the tertiary carbon, the + charge gets on the carbon connected to Oxygen. Then oxygen donates one of its lone pair electrons to the single bond next to it, to stablize the carbocation [this results in oxygen having one single and one double bond and a positive formal charge on it, which is called the oxonium ion]. The oxonium ion is a very stable intermediate, even more than a tertiary carbocat. Then Br- attacks the carbon connected to oxygen, and the donated lone pair of oxygen gets back to oxygen.

 

[flycd05]

Nice question. It is because once H+ adds to the tertiary carbon, the + charge gets on the carbon connected to Oxygen. Then oxygen donates one of its lone pair electrons to the single bond next to it, to stablize the carbocation [this results in oxygen having one single and one double bond and a positive formal charge on it, which is called the oxonium ion]. The oxonium ion is a very stable intermediate, even more than a tertiary carbocat. Then Br- attacks the carbon connected to oxygen, and the donated lone pair of oxygen gets back to oxygen.

ok so then what forces the carbocation to be on the 2ndary carbon versus the more stable tertiary - is it b/c it will eventually become stabilized by the oxygen so having the carbocation on the 2ndary carbon is ok? also, does this rule hold true for ALL instances when you have 2 single cyclic ether joined together and are adding HBr? how about when you have just 1 cyclic ether and are adding HBr?

 

[Mstoothlady2012]

Nice question. It is because once H+ adds to the tertiary carbon, the + charge gets on the carbon connected to Oxygen. Then oxygen donates one of its lone pair electrons to the single bond next to it, to stablize the carbocation [this results in oxygen having one single and one double bond and a positive formal charge on it, which is called the oxonium ion]. The oxonium ion is a very stable intermediate, even more than a tertiary carbocat. Then Br- attacks the carbon connected to oxygen, and the donated lone pair of oxygen gets back to oxygen.

I thought oxonium ion is not stable, that is why we turn R-OH group into R-OH2 (which is also oxonium ion) so that -OH2 can become a better leaving group since it is not stable???

 

[harrygt]

ok so then what forces the carbocation to be on the 2ndary carbon versus the more stable tertiary - is it b/c it will eventually become stabilized by the oxygen so having the carbocation on the 2ndary carbon is ok? also, does this rule hold true for ALL instances when you have 2 single cyclic ether joined together and are adding HBr? how about when you have just 1 cyclic ether and are adding HBr?

Yeah, the oxygen stablises the positive charge on the carabon next to it by donating a lone pair. THis results in the formation of oxonium ion which is much more stable than a tertiary carbocat.
This is true for all ethers that have a double bond next to them./

 

[harrygt]

I thought oxonium ion is not stable, that is why we turn R-OH group into R-OH2 (which is also oxonium ion) so that -OH2 can become a better leaving group since it is not stable???

Sorry, but this example was not relevant😀
oxonium ion is when you have an oxygen double bonded to a carbon, with one lone pair electron on oxygen and a + formal charge on Oxygen.

 

[Mstoothlady2012]

Sorry, but this example was not relevant😀
oxonium ion is when you have an oxygen double bonded to a carbon, with one lone pair electron on oxygen and a + formal charge on Oxygen.

okay! thanks!

 

[TheGreenWave]

okay! thanks!

That will definitely be one of the products. When there are two double bonds seperated by a single bond, the HBR can add 1,2 or 1,4. Youre showing the 1,4 addition.

1,2 addition is favored at LOW temperatures (-40 C) = kinetic control
1,4 addition is favored at regular temp (40 C) = thermodynamic control

 

[harrygt]

That will definitely be one of the products. When there are two double bonds seperated by a single bond, the HBR can add 1,2 or 1,4. Youre showing the 1,4 addition.

1,2 addition is favored at LOW temperatures (-40 C) = kinetic control
1,4 addition is favored at regular temp (40 C) = thermodynamic control

This is a different situation. We have the oxygens there which are stablizing the carbocations of this product strongely.

 

[doc3232]

This is a different situation. We have the oxygens there which are stablizing the carbocations of this product strongely.

Ya, this isn't a 1,4 or 1,2 reasoning.
Nice explanation harry.

 

[Maygyver]

Very nice reasoning Harry. Attaboi!