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20 Jun 2012 12:54:58 IST

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SN2 V/S E2

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3 alkyl halide always gives elimination product over subs ( SN2). but if i am to use a solvent which doesnot stabilize the carbocation and with a strong nucleophile and weak base what will i get?

i think with a strong base e2 elimination will happen.

also, why do we take aqueous solutions, when we want an alkoxide ion to exist. even though it has na+( from NaOH) water should happily give its proton and thus we can have hydroxide ion reacting thus giving an alcohol ( of the substrate) and not an ether. also there is a considerable conc of hydroxide ion and thus the probability should increase to get an alcohol.

Comments (17)

Janardhan K

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22 Jun 2012 03:45:09 IST

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yes u r right that E2 elimination will happen.....but understand this why....1. SN1 can't happen as u have taken solvent that doesnot stabalize C+ like benzene or chloroform and also u have taken strong nucleophile so no chances of SN1....2.3 degree substitution reaction is not the characteristics of SN2, they never give reactions with tertiary halides due to steric reasons......

Janardhan K

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22 Jun 2012 03:46:55 IST

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3.E1 is not possible becoz C+ is not stabalised....no solvolysis and hence no E1.

Janardhan K

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22 Jun 2012 03:51:49 IST

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4. E2 might happen because u have taken strong nucleophile and they are "generally" good base so with a transition state E2 is quite possible....

Janardhan K

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22 Jun 2012 03:54:04 IST

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5. there are also chances that reaction proceeds through E1CB because of very good base.....

Janardhan K

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22 Jun 2012 04:02:56 IST

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i'm not able to understand ur third statement......r u talking about williamson ether synthesis?????

Janardhan K

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22 Jun 2012 04:09:10 IST

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well if u consider williamson ether synthesis then see alkoxide are very good nucleophiles rather than being good base because it has been derived from poor acid i.e. alcohol......being a good nucleophile it attacks alkyl halide from backside giving ether......

Janardhan K

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22 Jun 2012 04:11:20 IST

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and u see water does not easily protonate considering the common ion effect of OH-(from NaOH)......so H+ is far less in conc. to give alcohol......

Janardhan K

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22 Jun 2012 04:11:37 IST

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I hope u understood it nicely......

noel

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22 Jun 2012 11:32:21 IST

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yes i do janardhan thanks

noel

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22 Jun 2012 11:43:23 IST

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thanks for ur time.

also, this is regarding the 1st question. i was asking that hypothetically considering a strong nucleophile which is not a strong base :

now, as i have taken a non polar solvent, probalitiy of SN1 is approaching 0 and very few amount Of E1.

but having a 3 degree alkyl halide, with a strong nucleophile and say a low temp, can i still get a SN2 reaction even though it is a tert alkyl halide?

Janardhan K

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22 Jun 2012 23:53:11 IST

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first thing is u don't take non-polar solvent in substitution reaction....it is either polar protic or polar aprotic.....in case for SN1 polar protic is preferred and in SN2 polar aprotic is preferred....

Janardhan K

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22 Jun 2012 23:53:35 IST

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let me first tell u the reason for that

Janardhan K

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22 Jun 2012 23:55:04 IST

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in SN1 RDS is leaving of leaving group so in RDS the as the solvent is protic it helps to solvate the leaving group increasing the rate hence helping the reaction.......

Janardhan K

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22 Jun 2012 23:58:28 IST

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while in SN2 there is transition state but attack of nucleophile is the most imp. step hence quality of nucleophile very much matters.....also if u take polar protic solvent here it will solvate the nucleophile retarding the rate of reaction and if u take non polar solvent then leaving group will not be solvated and again it will retard the reaction......hence polar aprotic is used in SN2....

Janardhan K

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23 Jun 2012 00:03:01 IST

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so u see in ur ques. u have taken non-polar solvent which-1. can't result in substitution reaction....2.in any condition no chances of SN2 in tertiary alkylhalide due to steric hinderence...

Janardhan K

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23 Jun 2012 00:05:38 IST

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now i think u must be very clear about reaction mech. .....but still if u have any doubt regarding any thing feel free to post in this thread only......or u may nudge me.....

noel

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23 Jun 2012 13:02:29 IST

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nudged u enoufcheers

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