carbocation shift

[joonkimdds]

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I usually make mistake of not shifting carbocation to the more stable carbocation.

I was wondering when I need to focus on this from happening.

Does it happen to all the reactions that start with alkene and we break it into alkane?


For example, alkene + H3O, X2, HX....etc?

 

[iwannabadentist]

shifts would only happen with Sn1 and E1.

 

[nze82]

Shift usually occurs when you have branching next to the "C" atom that's bearing the leaving group and the reaction is proceeding via SN1 and/or E1 mechanism.

 

[joonkimdds]

Is adding HBr to C=C also Sn or E? because I know that it casues carbocation shift.

 

[mitshi]

Is adding HBr to C=C also Sn or E? because I know that it casues carbocation shift.

They all entail similar rearrangement, except for the fact that hydride shift is what you'll usually experience on addition whereas Sn or E may encounter methyl (alkyl) or hydride shift.

 

[nze82]

Is adding HBr to C=C also Sn or E? because I know that it casues carbocation shift.

Adding HBr across the double bond can occur via two different mechanisms:
Under normal conditions-->Proceeds via carbocation intermediate.
In presence of peroxide-->Proceeds via free radical intemediate.

In either case, rearrangement could still occur to provide a more stable carbocation or free radical intermediate.

 

[iwannabadentist]

Is adding HBr to C=C also Sn or E? because I know that it casues carbocation shift.

this is neither Sn nor E. Its addition. In such cases, normal circumstances will give Markovnikov products (Br will add to the most substituted carbocation), and if a hydride or methyl shift is necessary to make a more subsituted carbocation, then it will happen. With peroxide, you will have anti-markovnikov producs, meaning that Br will add to the least substituted radical. One exception is with HCl, which will always add Markovnikov, peroxide or not.

 

[TexasDDS]

Can someone give a break down of SN1, E1, SN2, E2 better please I am having lots of trouble with the concepts.. LIke I know SN1, E1, E2 all prefer bulky , but E2 work better with strong bases.. but i'm not great with these concepts and how to apply them.. I would really appreciate the help.

Thank you

 

[elowdds]

I found this website on SDN that was pretty good for SN1,2 and E1,2.
http://www.brynmawr.edu/Acads/Chem/mnerzsto/SN2vsE2_flowchart.htm

 

[BiomajorPreDent]

If you are given a list of carbocations some secondary and some primary and you need to figure out which is most stable, can you have re arrangement from a 2ndary to a tertiary?

Or should you only worry about re arrangement with reactions?

 

[Klumzy]

Can someone give a break down of SN1, E1, SN2, E2 better please I am having lots of trouble with the concepts.. LIke I know SN1, E1, E2 all prefer bulky , but E2 work better with strong bases.. but i'm not great with these concepts and how to apply them.. I would really appreciate the help.

Thank you

Here we go:

Whenever you see KOH, NaOH, methoxide in methanol (CH3O- in CH3OH) or ethoxide in ethanol (CH3CH2O- in CH3CH2OH) and you have a 2-dary or 3-tiary halide you go for elimination and you get a more substituted alkene. If you have a 1-ary halide and these reagents you perform SN2 i.e. substitution. 1-ary will undergo E2 in the presence of a big bulky base like lithium diisopropyl amine (LDA) OR T-BUTOXIDE ((CH3)3CO-) to give less substituted product coz the bulky base can't access the other proton. Using the bulky base and 3ary or 2-dary halide will also give you less substituted alkene for the same reason.
SN1 will hapen with 3-ary halide in the presence of polar protic solvent (i.e. the one that has hydrogen bonding ) SN2 will hapen with 1-ary in the presence of polar aprotic solvent (e.g. acetone).
Also know the leaving groups, for the SN2 to happen we need a good leaving group so the group that is being "replaced" can leave immediately. Leaving groups for halogens are I>Br>Cl>F. Iodine being the best and Florine being absolutely horrible.
I suggest you go over destroyer problems because they tran you very well for eliminations and substitutions.