查看完整版本 : Suggest a reasonable mechanism

sam2003116 2012-10-30 07:51 PM

Suggest a reasonable mechanism

Suggest a reasonable mechanism for the following transformations.
由於題目無注明溶劑及其他情況例如溫度等,所以我用Free radial的想法去猜想這個反應。
但是問過PROFESSOR後,佢用一句你相信我的權威去解釋,令我很不爽。
所以我想問下,有無人做過相關的研究,到底Free radial有無可能成為這個反應的[font=arial, MingLiU, PMingLiU][size=15px]途徑。[/size][/font]
[font=arial, MingLiU, PMingLiU][size=15px]其實大家可以用中文或英文回應,其實就算我的[/size][/font][font=arial, MingLiU, PMingLiU][size=15px]途徑有問題,希望大家指教一下小弟,為何不能?[/size][/font]
[font=arial, MingLiU, PMingLiU][size=15px]其實我只是想知道自己的猜想有什麼不合理,而並非一句權威去解釋。[/size][/font]

Zzlaz 2012-10-30 11:04 PM

double bond + Br2, 走electrophilic addition reaction ,合理哦

sam2003116 2012-10-30 11:53 PM

Model answer 當然合理
我是想問,我的如何不合理?

Zzlaz 2012-10-31 12:25 AM

先唔講乜野先,你第一步畫fish curl, e-無跳到DB另一方,餘下的Br. radical where?

其實你畫free radical 的東西,要有齊initiation, propagation, termination.

點解唔合理
1. Br2 要分開成Br. 一係有能量,一係有其他野可以製造radical.
2. 點解會走去同O成鍵,而唔去同H成鍵?any special reason?
3. 連鎖反應來的,你一定要提供番Br. 出來,唔係好快玩完。(你係出H.) 唔合理,不然你每個Br2都先自行分開成2個Br.

chungkin81 2012-10-31 12:27 AM

個 alkene 係 electron rich,Br radical 有單一粒電子,這如何可以克服電子之間的排斥而收到由 Br2 breaking 所形成的 Br radical呢﹖

Zzlaz 2012-10-31 12:32 AM

[quote]原帖由 [i]chungkin81[/i] 於 2012-10-31 12:27 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=346244053&ptid=21065930][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
個 alkene 係 electron rich,Br radical 有單一粒電子,這如何可以克服電子之間的排斥而收到由 Br2 breaking 所形成的 Br radical呢﹖ [/quote]

Br. is electron deficit

jmlo 2012-10-31 02:27 AM

回覆 1# 的帖子

First of all, please bear in mind that no one can ever prove that [b]ONE MECHANISM IS RIGHT[/b] but it is easy to prove that [b]ONE MECHANISM IS WRONG[/b]. Therefore, you can remain skeptical although your professor told you one mechanism is right. When you propose a mechanism for a reaction, you have to be careful if all required conditions are present.

In your case, as other have pointed out, radical reaction does require a number of prerequisites. A major point would be how radical is formed in the first place. While heat, UV radiation or special reagents may trigger a radical reaction, usual electrophilic attack in most cases does not. There exists no driving force that favors radical additon over electrophilic addition.

Also, assuming the initial radical addition of Br is successful, the reaction leads to a secondary radical C. Is there any stabilization effect (e.g. electronic, steric, etc) that makes this intermediate long-lived enough for the OH group to come in? In addition, the second step in your mechanism, a concerted addition of OH on C radical to form C-O bond plus H radical, is highly unlikely. (To my knowledge, I have never seen such type of transformation.) FYI: NEVER propose a step which yields H radical or H- ion unless you are VERY CERTAIN it does occur! If you end up writing a mechanism with H or H-, you'd better think again!

If both conditions are not present, then you should ask yourself whether the radical mechanism is reasonable. To experimentally prove its validity, one may add a radical terminator (or scavenger) that captures existing radical species in the reaction mixture. If Br-scavenger species is detected, then one may argue that the radical reaction does occur. In addition, dimerization may take place for radical reactions; the detection of reactant dimer is a sign that proves the existence of radical reaction.

On the other hand, the usual electrophilic attack of C=C bond to form bromonium intermediate that undergoes intramolecular cyclization is a more reasonable mechanism as it agrees with experimental evidences that we see for electrophilic additions in general. C=C bond being electron-rich is able to attack Br-Br, capturing one Br to from bromonium ion intermediate. Because of its electron-deficient property, the bromonium is susceptible to nucleophilic attack by OH group from the bottom side, yielding the cyclic species that leads to the product.

Proposing reaction mechanisms for organic reactions is seemingly easy but indeed very difficult. Every single detail has to be making real chemical sense. Of course, there are no guidelines that are always true; mechanisms vary case by case. But at the end of the day, we can't make up mechanism out of just imagination. Read and think more about different types of mechanisms (those well-proposed and analyzed), and you will get some ideas how chemists propose a mechanism for a reaction. :loveliness:

[[i] 本帖最後由 jmlo 於 2012-10-31 02:28 AM 編輯 [/i]]

Zzlaz 2012-10-31 11:52 PM

而家dse 無rxn mechanism



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jmlo 2012-11-1 08:09 AM

But the OP's question apparently comes from a university course assignment ...

sam2003116 2012-11-1 04:16 PM

多謝各位的指導。
1。由於題目沒有指明環境及溶劑等,所以到底Br2會否產生Radical,我先假設可以。
2。點解會走去同O成鍵,而唔去同H成鍵?any special reason?
  以下是我自己的猜想,如果與H成鍵,RADICAL會產生在O身上,由於H,O太過  接近,以上過程只會不停重覆,沒有任何變化。所以我才選擇直接與O成鍵。
3。連鎖反應來的,你一定要提供番Br. 出來,唔係好快玩完。(你係出H.) 唔合理,不然你每個Br2都先自行分開成2個Br。
H。 攻擊BR2,產生Br。

以我所知,1。C-C 要在有極性的溶劑中才會比較穩定,假設我們不是用極性溶劑,而是用非極性溶劑,BR+真的可以保持穩定?
       \ /
        BR+      
     2。H。到底為什麼特別不穩定?

Zzlaz 2012-11-1 10:36 PM

你唔明chain rxn 道理
如果每一個Br2 都分得開 那你玩哂
not rational mechanism

再講一次free radical rxn needs initation propagation and elimination

this is addition rxn u wont expect HBr and other side products formed in this rxn
so imposs to be free radical addition rxn



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sam2003116 2012-11-1 10:57 PM

[quote]原帖由 [i]Zzlaz[/i] 於 2012-11-1 10:36 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=346402419&ptid=21065930][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
你唔明chain rxn 道理
如果每一個Br2 都分得開 那你玩哂
not rational mechanism

再講一次free radical rxn needs initation propagation and elimination

this is addition rxn u wont expect HBr and other side ... [/quote]
H。 攻擊BR2,產生Br。,之後就重覆?到底有什麼問題?

Zzlaz 2012-11-1 11:17 PM

你出H. 仲有一個Br. 唔知做乜唔見左



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jmlo 2012-11-2 05:32 AM

[quote]原帖由 [i]sam2003116[/i] 於 2012-11-1 10:57 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=346404629&ptid=21065930][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

H。 攻擊BR2,產生Br。,之後就重覆?到底有什麼問題? [/quote]
OK. You may propose that the reaction yields the cyclic product plus HBr. HBr is expected to form for this cyclization reaction, and this may tempt you to think about the radical mechanism. However, you still have to resolve the problem of the initial generation of Br radical, which cannot be accomplished by simple nucleophilic attack of C=C bond. Also, how about the propagation step? Just H + Br2 ---> HBr + Br? Any justification in terms of energy and stability? Besides these, you also have to consider the questions I have mentioned in previous posts.

Back to the problem of H radical. This species is just hydrogen atom, which is energetically highly unstable. It can be formed only if large energy input (e.g. UV radiation) is present, or if its formation is the only possible route. This, however, is not the case in the given reaction. Similarly, H-, called hydride, is very unstable, and cannot exist in free form.

↗亢龍無悔↙ 2012-11-2 04:48 PM

Jmlo師兄好西利,介唔介意講下你major咩呀?



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Zzlaz 2012-11-2 09:24 PM

surely chem la:smile_34:



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Zzlaz 2012-11-3 12:36 AM

[quote]原帖由 [i]sam2003116[/i] 於 2012-11-1 10:57 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=346404629&ptid=21065930][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

H。 攻擊BR2,產生Br。,之後就重覆?到底有什麼問題? [/quote]
我捻JMLO CHING 都講哂所有原因, 我用考試角度分析
1. 估野唔合理, 俗D 叫斷9估, 神說要有光就有光。(類似homogeneous catalysis)

2. 無任何條件下, 係高考化學/DSE 化學會理解成"自然發生,spontaneous", 而double bond + Br2 , decolourized 都係test for presence of unsaturated hydrocarbons/alkenes的一種方法, 無需質疑。

3. 大學ORGANIC CHEM mechanism, 我當一條15分先算, 你又無INITIATION, 又無TERMINATION, 你的得分已經所餘無幾。

regioselective, stereoselective? 這些問題你又有無捻過?
SIDE PRODUCTS, 一定只會按你捻去去反應?

如果你BR 可以隨意分開? 點解打完REACTANT 出的RADICAL 唔可以兩個合成一個?
簡單TAKE EXAMPLE,CH4+Cl2 (under UV light)
initation
Cl2 -->2Cl.
propagation
Cl. +CH4 --->CH3. +HCl
CH3.+Cl2 -->CH3Cl+Cl.
Cl.+CH3Cl -->CH2Cl.+Cl.
.....Cl in excess
CH2Cl.+Cl2 -->CH2Cl2 +Cl.
termination (very important)
{if you have Cl., CH3. only three combinations... if more than that...very funny}

Cl.+Cl. -->Cl2 (fine)
CH3.+Cl. -->CH3Cl (also fine)

CH3. +CH3. -->CH3CH3 (bad)
CH2Cl.+CH3. -->CH3CH2Cl (bad)

and the rest is statistics.

sam2003116 2012-11-3 11:58 PM

[quote]原帖由 [i]Zzlaz[/i] 於 2012-11-3 12:36 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=346492848&ptid=21065930][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

我捻JMLO CHING 都講哂所有原因, 我用考試角度分析
1. 估野唔合理, 俗D 叫斷9估, 神說要有光就有光。(類似homogeneous catalysis)

2. 無任何條件下, 係高考化學/DSE 化學會理解成"自然發生,spontaneous", 而do ... [/quote]
2.我想你理解錯了我的問題,我並非質疑反應是否發生,而是用什麼途徑發生,題中並不注明溶劑等等,這對自己去推測途徑造成了一個很大的障礙,溶劑對判斷一個途徑的合理性擔當了一個重要的角色.

Zzlaz 2012-11-4 01:10 AM

[quote]原帖由 [i]sam2003116[/i] 於 2012-11-3 11:58 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=346565940&ptid=21065930][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

2.我想你理解錯了我的問題,我並非質疑反應是否發生,而是用什麼途徑發生,題中並不注明溶劑等等,這對自己去推測途徑造成了一個很大的障礙,溶劑對判斷一個途徑的合理性擔當了一個重要的角色. [/quote]

如果溶係水, 反應會點, 可唔可以估計下?

Br2 itself is a liquid in state.
The use of organic solvent in electrophilic addition reaction is to facilitate interaction between reactants.

I just doubt whether u learn this mechanism in high school? I personally learnt the mechanism in AL.

for Bromine, for bromine water, for sulphuric acid, conc. sulphuric acid....etc
So, no need to doubt too much.

w/o solvent, how Br. is formed ??
w/ solvent, how Br. is formed.
This is crucial.

[[i] 本帖最後由 Zzlaz 於 2012-11-4 01:16 AM 編輯 [/i]]

sam2003116 2012-11-6 07:28 PM

[quote]原帖由 [i]Zzlaz[/i] 於 2012-11-4 01:10 AM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=346572904&ptid=21065930][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]


如果溶係水, 反應會點, 可唔可以估計下?

Br2 itself is a liquid in state.
The use of organic solvent in electrophilic addition reaction is to facilitate interaction between reactants.

I just  ... [/quote]
我只知道溴不一定要溶於水中.如果是非極性有機溶劑,那又會怎樣?我質疑的原因是,他可以跟我說,CONDITION 可以自己估到,那麼,我猜測有UV或加熱那也不足為奇.重點並不是我的答案是否正確,是到底FREE RADICAL MECHANISM是否可行,又或者有其他MECHANISM同樣地可以完成這個反應,所以我一開始就問,有沒有人有做過相關的研究?如果有其他途徑是一樣可行,那證明溶劑影響反應的途徑,舉個例,如取代反應,SN1,SN2.我質疑的是,在沒有寫出任何溶劑時,為何各位如何肯定MODEL ANSWER會是唯一的正確答案?我猜測就是各位有做過相關的實驗去反證?

jmlo 2012-11-6 11:08 PM

回覆 20# 的帖子

[quote] 我只知道溴不一定要溶於水中.如果是非極性有機溶劑,那又會怎樣? [/quote]
The same bromination process can take place in non-polar solvents such as CCl4.

[quote] 我質疑的原因是,他可以跟我說,CONDITION 可以自己估到,那麼,我猜測有UV或加熱那也不足為奇. [/quote]
You may want to argue that you can assume whatever reaction conditions if they are not stated explicitly in the question. But this is not quite possible in general. Since the reagent is already given (i.e., Br2) and the product is unknown, you can't assume additional reagents/reaction conditions. If your professor were to expect the products under photochemical reaction conditions, he should have mentioned "UV light" in the question. Otherwise, the question will become open-ended. Do you understand what I mean?

[quote] 重點並不是我的答案是否正確,是到底FREE RADICAL MECHANISM是否可行,又或者有其他MECHANISM同樣地可以完成這個反應,所以我一開始就問,有沒有人有做過相關的研究?如果有其他途徑是一樣可行,那證明溶劑影響反應的途徑,舉個例,如取代反應,SN1,SN2. [/quote]
The free radical mechanism is obvious possible and probable if required conditions are provided. The differences between these two channels (i.e., conventional bromination over C=C bond versus free radical bromination) have been experimentally studied long time ago, and people understand the respective mechanisms fairly thoroughly.

[quote] 我質疑的是,在沒有寫出任何溶劑時,為何各位如何肯定MODEL ANSWER會是唯一的正確答案?我猜測就是各位有做過相關的實驗去反證? [/quote]
We kind of agreed the proposed mechanism simply because it makes complete sense subjected to the reaction conditions given in the questions. In this reaction, the choice of solvents is not a very big deal as long as no UV light or sunlight is given.

FYI: For the courses of organic chemistry you take in universities, you may see a lot of questions regarding organic synthesis or transformations. There are in majority two types of such questions. You are either given (1) reagents; or (2) products. (Depending on your level, stereochemical constraints of the reagents/products are sometimes required.) Because of a vast number of possible reaction pathways under various conditions, the necessary reagents are listed clearly and specified. You can't add any additional reagents or omit any from the list.

But honestly I really admire your attitude of learning. Keep it up! :loveliness:

sam2003116 2012-11-18 01:00 AM

Thank for all of you.I know i am quite a stubborn person.
Yeah the professor had told me the reaction can happen in non-palar soln.he said Br+ can still be stable in non-polar soln and someone had did it.
When i think in exam is base on Maxwell distribution ,some Br2 may split into radical ,that radical will initiated the reaction in radical mechanism.
But i don't understand why not providing soln in the question is possible unless the reaction can really complete without soln.

Zzlaz 2012-11-18 01:09 AM

其實果隻野係intermediate, not transition state, can be detected theoretically by experiment.

and 你係唔係考DSE? HKAL 生應該明白這個反應機理, 因為是頗普通的RXN

jmlo 2012-11-18 01:14 AM

回覆 22# 的帖子

[quote] When i think in exam is base on Maxwell distribution ,some Br2 may split into radical ,that radical will initiated the reaction in radical mechanism. [/quote]
In order for a photochemical process to occur, enough energy has to be provided initially so that sufficient amount of radicals can be generated. This process cannot be simply accounted for by Maxwell distribution which is basically used for describing the variation of thermal energy (or velocity, or momentum, etc) within a whole bunch of molecule. The energy required to produce radicals is much higher than the range of thermal energy fluctuation, and that's why the source of energy has to be mentioned clearly if a photochemical reaction is expected.

[quote] But i don't understand why not providing soln in the question is possible unless the reaction can really complete without soln. [/quote]
It is pretty much like a convention in chemistry. Taking this bromination as example, if no UV source is provided, the reaction is expected to proceed through normal addition reaction mechanism, and the choice of solvent is thus inferred since it is already known which solvent is needed for such process.
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