Sir,
would you please tell me about relative shift of different groups e.g. Hydride, methide, C6h5- etc in carbocationic rearrangements. I missed out 1 chem question in screening due to this misunderstanding. also please tell me how can I clearly tell whether a reaction is going SN1,SN2,E2,E1.eg 'sometimes'
aq.KOH can be used in E2,SN1. please clarify this 'sometimes'.
dear friend a 12 ion shift happens when the c+ formed is less stable and a more stable carbocation is possible with a rearrangement. Usualyy when a i degree c+ is formed, then a rearrangement takes place to give a 2 degree or a 3 degree c+. If the beta C+ has a h-atom then a hydride shift takes place and when a h-atom is not present on the beta C+ then a methyde or the shift any other group takes place as accd. to which grp. is present on the beta C+. so friend this is what i could say on rearrangements. for further help refer rxns and rearrangements in organic che. by S N SANYAL(bharathi bhavan publications). U could get useful information on the other part of ur doubt from this. NOW PLEASE DONT MIND TO GIVE SOME TIPS TO PREPARE FOR THE IITJEE-06.THANK U.
There is a thumb rule about the relative shifts of the groups you have mentioned. However, it may be a better policy to check carbocation stability.
Consider the question that appeared in screening:
Here, after Cl- is abstracted, one alcohol would be formed by the direct replacement of Cl- by OH-.
However, there can be 1,2 H- shift from the carbon to the left. There can also be 1,2 CH3- shift from the carbon to the right.
The first possibility is favoured due to the electron release effect of the CH3O group and the second possibility is very adverely affected due to the electron withdrawing effect of NO2 group.
Hence, 1,2 CH3- shift should not be considered in this case.
(Note: In this particular question, since acetone is also present, there would be SN2 reaction as well. However, the answer remains unchanged. See the note below in the heading SN2 Mechanism.)
Coming to the second part of your query regarding elimination and substitution reactions. Here are some thumb rules:
E1 mechanism
This is most common with:
- good leaving groups
- stable carbocations
- weak bases.
E2 mechanism
This is most common with:
- high concentration of a strong base
- poorer leaving groups
- R-LG that would not lead to stable carbocations (when the E1 mechanism will occur). LG stands for leaving group.
SN1 mechanism
This is most common for systems with good leaving groups, stable carbocations and weaker nucleophiles. Polar solvents which can stabilise carbocations which can favour the SN1 reaction (e.g. H2O, ROH).
SN2 mechanism
This is most common for systems with poorer leaving groups, 1o or 2o substrates and stronger nucleophiles. Polar aprotic solvents can be used to enhance the reactivity of the nucleophile and help promote an SN2 reaction. (Aprotic solvents cannot have hydrogen bond to the nucleophile. (e.g. acetone).
One should also note that substitution and elimination reactions are competing reactions. They are not mutually exclusive. In many cases you have both substitution and elimination reactions and you get a mixture of the products. In such cases all the products should be shown.
the hydride or methide shifts take place to form a more stable carbocation,preferrable formation of carbocations is teriary>secondary>primary>methyl but care has to be taken of the mechanism ,only SN1 and E1 allow this to happen as only the involve an intermediate carbocation. the shift is such tht the hydrogen [in hydride shift]along with its pair of electrons shifts to the nearby carbon.
both e1 and e2 mechanisms increase from primary to tertiary the former due to formation of a more stable carbocation and the latter due to formation of a more stable alkene.out of sn2 and e2 which takes place is easily decided as they decrease and increase from primary to tertiary respectively.but your problem is in differentiating between sn1 and e1 both of which increase from primary to tertiary. this poses a problem.but this to can be solved .more nucleophilic the reagent or solvent sn2 follows and if the reagent is more basic than nucleophilic then E1 folows.now you might argue tht all nucleophiles are bases and so whats the difference between in the 2 and what am i trying to say, but thats where you go wrong,nucleophilic capability depends on the rate,on how fast the attack takes place while basic power depends not on rate but on EQUILIBRIUM ,that is at equilibrium how much portion of the acid does the reagent cling to the more it does this [at equilibrium] the more basic it is and e1 follows
FOR MORE ON THIS REFER MORRISON BOYD ITS THE BEST FOR ORGANIC AND THERE NOTHING BETTER IT EXPALINS THE MINUTEST OF DETAILS AND IT MAKES YOU FEEL YOU ARE PREPARING FOR SOMETHING AS BIG AS IIT
both e1 and e2 mechanisms increase from primary to tertiary the former due to formation of a more stable carbocation and the latter due to formation of a more stable alkene.out of sn2 and e2 which takes place is easily decided as they decrease and increase from primary to tertiary respectively.but your problem is in differentiating between sn1 and e1 both of which increase from primary to tertiary. this poses a problem.but this to can be solved .more nucleophilic the reagent or solvent sn2 follows and if the reagent is more basic than nucleophilic then E1 folows.now you might argue tht all nucleophiles are bases and so whats the difference between in the 2 and what am i trying to say, but thats where you go wrong,nucleophilic capability depends on the rate,on how fast the attack takes place while basic power depends not on rate but on EQUILIBRIUM ,that is at equilibrium how much portion of the acid does the reagent cling to the more it does this [at equilibrium] the more basic it is and e1 follows
FOR MORE ON THIS REFER MORRISON BOYD ITS THE BEST FOR ORGANIC AND THERE NOTHING BETTER IT EXPALINS THE MINUTEST OF DETAILS AND IT MAKES YOU FEEL YOU ARE PREPARING FOR SOMETHING AS BIG AS IIT