alkene reaction very dufficult

👨🏻‍🏫 學術台化學

用戶25529

2011-07-03 8:42 pm

I have studied a reaction of alkene in the book :
alkene + acidified potassium permanganate ---> diol

The book used ethene as example had gave a formula as below :
H2C=CH2 + [O] + H2O ----> CH2OHCH2OH
and said
[O] is from acidified KMnO4 as it is a strong oxidizing agent & H2O is from the KMnO4 solution

However, the book doesn't state the machanism or further explain; so, I have asked a question about this before, at
http://hk.knowledge.yahoo.com/question/question?qid=7011060600871
I have had a set of explanation.

However, recently, I have read a reference book(John McMurry's Organic Chemistry), it said that :
alkene + peroxyacid ----> epoxide + acid

圖片參考：http://imgcld.yimg.com/8/n/HA00656648/o/701107030037913873466760.jpg

and epoxide can undergo acid-catalyzed reaction with water to give diol,
圖片參考：http://imgcld.yimg.com/8/n/HA00656648/o/701107030037913873466771.jpg

The net result of alkene become diol is called hydroxylation.

Then, the book said that the hydroxylation can be carried out by treating with OsO4, osmium tetroxide. The cyclic osmate intermediate will then cleaved by NaHSO3(aq).

As I have seen in the previous question I asked, OsO4 seems to be able to be replaced by MnO4(-).

So, now, I want to ask :

1. Should the reaction of alkene with MnO4- to yield diol, be in a basic, acidic or neutral condition to carry out reaction?
Or, is alkene able to react with MnO4(-) in any aqueous conition as H3O(+) and OH(-) is present in any solution?
2. What is the reaction that really exist between alkene and MnO4(-) to yield diol, syn addition? Hydroxylation? Cleavage? Or others?
3. What is the condition for the reaction to take place?

更新1:

May you also tell me the machanism of the reaction? THX

更新2:

Thx a lot, but would you mind telling me the whole mechanism? the thing that I really want to know is the ''several steps'' THX a lot!~

回答 (3)

wanszeto

2011-07-03 10:02 pm

✔ 最佳答案

1.
The reaction of alkene with MnO4⁻ to yield diol should be carried out in an alkaline medium. In an acidic medium, the oxidizing power of MnO4⁻ is strong enough to breakthe C=C double bonds in alkene molecules. In a neutral medium, the cleavage ofC=C double bonds still occurs but is to a small extent.

2.
This reaction is classified as sym addition. It canalso be specified as dihydroxylation, as dihydroxylation means addition of twohydroxyl groups to the substrate. From the view point of redox, the reaction isalso oxidation.

3.
The condition is that cool alkaline potassium permanganate solution is used to reactwith the alkene.

2011-07-04 00:22:22 補充：
A simplified mechanism is as follows:

http://i1096.photobucket.com/albums/g340/pingshek/Mechanism_syn_dihydroxylation.jpg

2011-07-07 23:06:17 補充：
The detailed mechanism is as follows:
http://i1096.photobucket.com/albums/g340/pingshek/Mechanism_syn_addition_KMnO4.png

參考: wanszeto, wanszeto, wanszeto

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用戶25529

2011-07-06 7:38 am

Re : 冷風
Yes, both 唯物主義者_魑魅魍魎 and 黑白灰_唯物主義者_魑魅魍魎 are my names

I've actually studied the detailed answer in that link.

2011-07-05 23:38:29 補充：
But as I mentioned in this Question, I've studied a reaction call hydroxylation in a reference, borrowed from university,
I'm not challenging your answer in my previous question, it is definitely correct and

2011-07-05 23:38:41 補充：
useful
But what I really want to know is that :
what reaction occurs to yield a diol from an alkene with addition of acidified KMnO4(aq), and the mechanism of the reaction

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冷風

2011-07-06 6:13 am

黑白灰_唯物主義者_魑魅魍魎 you can see the following Yahoo Knowledge

http://hk.knowledge.yahoo.com/question/question?qid=7011060600871

But I wann ask
Are 唯物主義者_魑魅魍魎 you, aren't you???

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