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RXN MECHANISM

RXN MECHANISM

by Danish Ishtiaq Khan -
Number of replies: 5
RESPECTED SIR,
MY DOUBT IS AS FOLLOWS:
SUPPOSE A RXN PROCEEDS THROUGH THE FORMATION OF A INTERMEDIATE.NOW IF IN A PARTICULAR CASE THERE ARE TWO PATHS OF SAME RXN.ONE PROCEEDS THROUGH A
STABLE FORM OF INTERMIDIATE BUT DOESNT GIVE A STABLER PRODUCT.AND OTHER PATH PROCEEDS THROUGH A NOT SO STABLE INTERMEDIATE BUT GIVES A RELATIVELY STABLER PRODUCT.IN SUCH CASE WHAT PATH WILL RXN ADOPT.FOR e.g WHAT WILL BE PRODUCT IN THE CASE OF DEHYDRATION OF 3-hydroxy-4-methylpentanal
In reply to Danish Ishtiaq Khan

Re: RXN MECHANISM

by kanakesh gupta -
HEY DANISH ,
I THINK U HAVE SKIPPED ONE MAJOR PORTION OF THE REACTION MECHANISM i.e THE PORTION REGARDING THE HYDRIDE SHIFT OR THE PROTON SHIFT.
FOR EVERY CHEMICAL REACTION A COMPOUND OR A MOLECULE ALWAYS TRIES TO ATTAIN THE MOST STABLE FORM WHETHER IT BE AN INTERMEDIATE OR IT BE THE PRODUCT ITSELF.
EVEN IF AN INTEMEDIATE IS LESS STABLE IT UNDERGOES HYDRIDE SHIFT TO ATTAIN A STABLE STRUCTURE AND FORM THE MAJOR PRODUCT.AND THE LESS STABLE STRUCTURE FORMS A VERY MINOR AMOUNT.AS IN THE CASE OF DEHYDRATION OF 3-HYDROXY-4-PENTANAL :
IT GIVES TWO PRODUCTS:
1)CH3-C(CH3)=CH-CH2-CHO(MAJOR)- AS THE INTERMEDIATE IS A 3 DEGREE CARBOCATION AND HENCE MORE STABLE.

2)CH3-CH(CH3)-CH=CH-CHO(MINOR) - AS THE INTERMEDIATE IS A 2 DEGREE CARBOCATION AND HENCE LESS STABLE.

FOR THE FORMATION OF THE MOST STABLE PRODUCT THE INTERMEDIATE 2 DEGREE CARBOCATION UNDERGOES 2,4 HYDRIDE SHIFT TO ATTAIN THE MORE STABLE 3 DEGREE CARBOCATION.

THIS MECHANISM IS FOLLOWED ACCORDING TO A RULE KNOWN AS THE SAYTZEFF RULE N IS THE MOST IMP. RULE IN RXN MECHANISM.

HOPE U GOT UR ANSWER

BEST OF LUCK TO U TOO FOR UR JEE.
N I'M NOT APPEARING FOR IIT I WILL BE APPEARING FOR MY STATE LEVEL ENGG ENTRANCE EXAMINATION.

BYE
In reply to kanakesh gupta

Re: RXN MECHANISM

by Danish Ishtiaq Khan -

HEY KANKESH!

THANX A LOT FOR SHOWING SINCERITY IN REPLYING.BUT I GUESS U DIDINT GET WHAT I WAS ASKING.WHAT I WAS ASKING WAS THIS:

LET US SUPPOSE THE DEHYDRATION OF  3-HYDROXY-4-METHYLPENTANAL:

 H+  FROM MEDIA ATTACKS ON OH GROUP AND AFTER -H2O FORMS A  20 CARBOCATION. NOW AFTER THIS THERE ARE TWO WAYS OF PROCEEDING. EITHER THE REACTION PROCEEDS THROUGH 20 CARBOCATION AND FORMS

4-METHYLPENT-2-EN-AL(say P1). AND OTHER PATH IS THAT IT FIRST  UNDERGO HYDRIDE SHIFT AND FORMS 30 CARBOCATION AND THEN FORMS  4-METHYLPENT-3-EN-AL(say P2)...... NOW P1 FORMED THROUGH 2 DEGREE CARBOCATION  BUT IT IS VERY SATBLE DUE TO CONJUGATION OF DOUBLE BOND WITH CARBONYL GROUP.ON THE OTHER HAND P2 FOMED THROUGH STABLE 3 DEGREE CARBOCATION BUT  P2 IS NOT AS STABLE AS P1....NOW IN SUCH CASE WHICH PRODUCT (P1 OR P2)  WOULD BE MAJOR.?.

In reply to Danish Ishtiaq Khan

Re: RXN MECHANISM

by vaibhav mathur -
Hey Danish,
You ever heard about Saytzeff's rule?
In reply to vaibhav mathur

Re: RXN MECHANISM

by Danish Ishtiaq Khan -

hey vaibhav!

i indeed have heard about saytzeff rule but my friend answer me one thing that if one compound is stabilised  by resonanace and other by  hypercojugation+steric factor(saytzeff rule) then which one has greater stability?

In reply to Danish Ishtiaq Khan

Re: RXN MECHANISM

by sushanth gs -

the reaction intermediate if unstable will decompose faster into the product .most reactions proceed through unstable intermediates.saytzeff rule is a direct consequence of this

major or minor product depends only on the stability of the product formed.