IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

kislay

note: it is ncert example
which is more reactive towards sn1 reaction..

CH₃CH₂CH₂CH₂Br

or (CH₃)₂CHCH₂Br

ans given is

(CH₃)₂CHCH₂Br

with reasoning give (CH₃)₂CH- has more +I effect than CH₃CH₂CH_2-

....
but
if we see the no. of H ATOMS AVAILABLE FOR HYPERCONJUGATION DURING TRTANSITION STATE
(CH₃)₂CHCH₂Br HAS GOT ONLY ONE........

CH₃CH₂CH₂CH₂Br HAS GOT TWO.........

AND AS WE KNOW HYPERCONJUGATION IS STRONGER THAN I EFFECT THEREFORE...............

THE ANSWERE GIVEN IS WRONG........

PLZ HHELP........

punnima

c basically the reactivity order 4 SN1 reaction is:

3^o>2^o>1^o

secondary haloalkanes have a tendency 2 follow either SN1 or SN2 nd this is decidedby d additional conditions specified

in this case its given one primary nd 1 secondary haloalkane

hence ther neednt b any dbt as 2 which follows SN1 reaction easily

donn complicate matters by takin hyperconjugative aspect wen u cud find out d ans easily

varshavallig

i think the 2nd cpd will easily lead to the formation of tert. carbocation by rearrangement.so its more reactive towards SN1.
@punnima, here both are primary carbocations.plz check.
!!!!!!!!!!!!!!!

kislay

@varshavallig

rate of sn1 reaction deepends on the ease of formation of carbocation...........
in both cases 1 degree carbocation is formed in rate determing step.........
shift........ is secondary thing , it will determine the major product............

madhusudan.chavan

In both the compounds the halogen is attached to CH2 group.In first compound this CH2 is attached to another CH2 while in second it is attached to (CH3)2CH. You can compare them with n-propyl and isopropyl. The SN1 mechanism involves ionisation as the first step, then it is expected that as the electron density increases on the central carbon atom, the bonding pair in C-X becomes more and more displaced towards the atom X, and consequently ionisation of X as a negative ion will become easier. Thus in case of the second compound the two methyl groups are pumping electrons to CH while in first there is only one. Another reason is since the SN1 proceeds through the carbonium ion, the more stable this ion, the lower is the activation energy and more favoured will be this mechanism. In second compound the carbonium ion is likely to easily change to tertiary, hence this will have less activation energy.

kislay

sir ur first part seems to be correct.............
but in ur second pat
"Another reason is since the SN1 proceeds through the carbonium ion, the more stable this ion, the lower is the activation energy and more favoured will be this mechanism. In second compound the carbonium ion is likely to easily change to tertiary, hence this will have less activation energy."

activation energy is required first to convert it to 1 degree carbocatrion....the first TS
next TS(3 degree carbocation) though will have lower energy........
but activation energy will be
TS 1 - REACTANT ENERGY....................

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