IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

champ1

dear frenz please help me.............

what will the product when an alkene is reacted with cold dilute KMnO4..........

CH3 -CH2 = CH2 + COLD KMNO4 --------------------->> PRODUCT

PLEASE TELL THE PRODUCTS FRENZ....

what makes the difference with reaction with a hot kmno4 ?? please tell....

hot kmno4 should also function as the cold one....only difference is that it goes on adding OH- until the sigma bonds are also broken...
so..where comes the difference pls ?

champ1

PLEASE TELL .......... !!

ramyadiamond

With KMnO₄ (alkaline-1%, and cold), hydroxylation takes place,

so product is

CH₃-CH₂-CH₂

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OH OH

_{{Plz rate me if its right, and plz correct me if its wrong, thanks}}

champ1

YA...i think u r right...... then what makes the difference with reaction with a hot kmno4 ?? please tell....

hot kmno4 should also function as the cold one....only difference is that it goes on adding OH- until the sigma bonds are also broken...
so..where comes the difference pls ?

ramyadiamond

It brings out oxidative cleavage.

In acidic or basic medium, with hot KMnO₄, it brings our oxidation, and -COOH group obtained in the formula,

For eg.:

CH₃-CH=CH₂ ---------------------(with hot KMnO₄/H⁺) CH₃COOH +CO₂+H₂O

Hope this helps.

Also remember, formic acid, tartaric acid and oxalic acid can be oxidised to CO₂ and H₂O.

champ1

ok..... though u did not tell the reason...... still gud yaar.... thanks !

iitkgp_bipin

Cold KMnO₄ is a mild oxidizing agent and it will cause the hydroxylation of of alkene forming CH₃-CH₂(OH)-CH₂(OH) whereas hot KMnO₄ will cause oxidative cleavage of alkene thus forming CH₃COOH , CO₂ and H₂O.

ramyadiamond

Ok, see this explanation, maybe this ill help, but i'm not able to post diagrams, so tell me wherever its not clear.

When KMnO₄ reacts wit an alkene in cold, alkaline comditions, it is possible to obtain cis-diol products, jst as one would from oxidation using OsO₄. However, this reaction is not easy to control and yields are typically low bcoz permanganate is such a strong oxidizing agent that it readily oxidises the diol product further.

When the reaction is carried out in acidic solutions, the diol is not formed. Instead, the alkene is cleaved. The reaction proceeds by the same mechanism at the start, forming a cyclic manganate ester (although since the reaction is under acidic conditions the structure is protonated). Under these conditions, the manganate ester is not very stable and it undergoes a cyclic fragmentation process, which results in breaking the C-C single bond between the two oxygens(sorry, u'll have to get the structures from elsewhere). Initially, there was a hydrogen atom attached to each of the alkene carbons in the starting material, there is a hydrogen attached to the carbonyl carbon in the product and therefore the product is an aldehyde.

Aldehydes are very easily oxidised to carboxylic acids. and thus the aldehydes formed in the cleavage reaction do not survive. They are rapidly transformed into carboxylic acid groups by a complex reaction (dont orry abt the mechanism).

I think this would be enuf to clear ur doubts.

_{{Plz do rate me for typing this out, thanks}}

khinart

Cold KMnO4 is a mild oxidising agent and hence forms glycols

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