tính thơm và khả năng PƯ của dị vòng
 

Theo trong sách thì tính thơm của dị vòng giảm dần từ thiophen , pirol tới fural nhưng sao mình thấy có PƯ sau :
[IMG]http://img295.imageshack.us/img295/7876/sevi7.png[/IMG]

ai có thể giải thích dùm mình với

ủa vậy bạn hỏi gì vậy nhỉ? ko hiểu câu hỏi.
phản ứng trên là pứ thế chất thân điện tử vào vòng thơm thôi mà. chất thơm đóng vai trò là chất thân hạch tác kích vào chất thân điện tử, sau đó để giữ lại tính thơm của nó nó sẽ bị 1 base như pyridin lấy H+ để trả lại tính thơm cho hợp chất.
còn tại sau nó thế bên furan theo mình nghỉ là thế này do C,O và N đều thuộc cùng chu kỳ nên mức năng lượng tương đồng nhau, do đó nó sẽ xen phủ hiệu quả hơn so với chu kỳ khác nhau. và nếu trên đó có thêm mạch nhánh thì sẽ ưu tiên xảy ra ở vị trí theo động học ( ít chướng ngại lập thể ).
có gì thảo luận nha !
thân

[QUOTE=phianh]Theo trong sách thì [B]tính thơm của dị vòng giảm dần từ thiophen , pirol tới fural[/B] nhưng sao mình thấy có PƯ sau :
[IMG]http://img295.imageshack.us/img295/7876/sevi7.png[/IMG]

ai có thể giải thích dùm mình với[/QUOTE]

I know very little about organic chemistry but my book at home contradicts what you said earlier. Could you please check your reference again? According to my book, the nucleophilic reactivity increases in the order benzene << thiophene < furan < pyrrole.
(K. Peter, C. Vollhardt and Neil E. Schore, [I]Organic Chemistry[/I] 2nd ed., p. 998.)
This may explain why NO2+ attacks the furan side of the initial compound during the electrophilic aromatic substitution.

Về khả năng phản ứng của dị vòng thơm thì lúc trước mình và admin BM có trao đổi với nhau về chuyện này. Và ở đây mình cũng khẳng định luôn hai việc.
1) Xét về tính thơm hay độ bền của vòng thì thứ tự như sau: benzen > thiophen > pirole > furan
2) Xét về khả năng thế SE thì thứ tự lại như sau: benzen < thiophen < furan < pirole
Lý do được giải thích cho khả năng thế Se kém của thiophen là do khả năng tham gia của cặp electron vào cấu tạo giải tỏa điện tích dương ở vị trí 2 kém hơn do khả năng liên hợp của  3p kém hơn nhiều so với 2p

[QUOTE=LessThanPerfect]I know very little about organic chemistry but my book at home contradicts what you said earlier. Could you please check your reference again? According to my book, the nucleophilic reactivity increases in the order benzene << thiophene < furan < pyrrole.
(K. Peter, C. Vollhardt and Neil E. Schore, [I]Organic Chemistry[/I] 2nd ed., p. 998.)
This may explain why NO2+ attacks the furan side of the initial compound during the electrophilic aromatic substitution.[/QUOTE]

Hi !
Maybe you have misunderstood ! The nucleophilic reactivity and the aromatic stability are separate, they are different in both agurments and practical results.

As zero said earlier, it's very difficult to estimate the aromatic stability order, and normally, base on the practical reports, compare the endothermal energy of breaking aromatic system (e.g by hydrogenation reaction) to accord which systems are more stablility. Of course, if you want to agurment with theory aspect, you should makd exception for thiophene. Don't estimate the aromatic stability order base on the ability of conjugating of atoms.

But, the nucleophilic reactivity can estimate by the ability of conjugating of heteroatom, exception for benzene system since it's prefect aromatic system.

It's is my opinion !

It was all my bad. I thought that "tính thơm" of an aromatic compound was equivalent to its ability to have electrophilic substitutions. I didn't know that the term was meant for the stability of the aromatic ring. [B]phianh[/B] seems to have the same confusion, and I am glad that you have clarified the differences. Thanks for the info.
See, I told you that I know very little about o. chem. Isn't this a perfect excuse? :D

[QUOTE=LessThanPerfect]It was all my bad. I thought that "tính thơm" of an aromatic compound was equivalent to its ability to have electrophilic substitutions. I didn't know that the term was meant for the stability of the aromatic ring. [B]phianh[/B] seems to have the same confusion, and I am glad that you have clarified the differences. Thanks for the info.
See, I told you that I know very little about o. chem. Isn't this a perfect excuse? :D[/QUOTE]

Yah, It wasn't a problem, don't worry about it !
I know u have a strong academic background in some speciality, e.g material analysis ( I just predict). I expect u will write more than, deeper about ur major in some day, to contribute to make this forum become a best academic forum. Are you happy with my proposal !
Good luck for u !
:sep ( :chaomung

Không có gì đâu bác [B]bluemonster[/B]. Chắc tại bác thấy bài trả lời của mình trong chủ đề "Đo Kích Thước Hạt" nên đoán ngành học của mình hả? Mình không phải chuyên về materials characterization đâu. Tại vì trong chương trình học có XRD, SEM, TEM và NMR nên mình học vậy thôi. Có một số cái (như NMR) mình không cần dùng trong nghiên cứu nên mình đã trả lại cho thầy cô hết rồi! Trong diễn đàn mình thấy có hứng thú nhiều nhất với các rooms về:
-Solid-state chemistry
-Inorganic materials chemistry
-Chemistry of transition metals (nhưng mà là chemistry of transition metals ở trong... solid materials như spinels, perovskites, pseudobrookites... chứ không phải ở trong metal complexes. :D

Mình học về một số khía cạnh của spark plasma sintering (SPS) như:
-Effects of electric field / electrical current during SPS
-Oxygen fugacity and phase stability of materials during SPS
-Reaction kinetics during SPS of ceramic oxides
Đấy, nó fundamental vậy đó. Không thấy immediate applications gì nhiều. Mà viết về mấy cái này trong một forum về hóa học chắc không ai thèm đọc!

[QUOTE=LessThanPerfect]
Mình học về một số khía cạnh của spark plasma sintering (SPS) như:
-Effects of electric field / electrical current during SPS
-Oxygen fugacity and phase stability of materials during SPS
-Reaction kinetics during SPS of ceramic oxides
Đấy, nó fundamental vậy đó. Không thấy immediate applications gì nhiều. Mà viết về mấy cái này trong một forum về hóa học chắc không ai thèm đọc![/QUOTE]

:nhau ( Không sao cả, nếu anh có thời gian cứ viết một vài bài tutorial về những vấn đề này, tuy là forum hóa, nhưng về Khoa học nói chung thì chắc hẳn member trong chemvn đều đón chào ! Đặc biệt mảng của anh cũng liên quan đến hóa đấy !

Anh cứ post trong category Hóa lý, hi vọng sẽ có bạn thảo luận để cùng lên level !

Thân !

Aromaticity and nucleophilicity of five-membered hetrocycles
 

Pyrrole, furan, and thiophene are five-membered-ring heterocycles. Each has three
pairs of delocalized π electrons: Two of the pairs are shown as π bonds, and one pair
is shown as a lone pair on the heteroatom. Pyrrole, furan, and thiophene are aromatic because they are cyclic and planar,
every carbon in the ring has 3 sp2 orbitals and a p orbital perpendicular to the ring plane and parallel to one another, and the π cloud contains three pairs of π
electrons filling three bonding π MO in accordance with Huckel's rule of aromaticity.<!--cut and paste--><a href="http://my.opera.com/nhathavosong/albums/showpic.dml?album=820234&picture=11167123"><img src="http://files.myopera.com/nhathavosong/albums/820234/resonance%20contributors%20of%20pyrrole.JPG" alt="" /></a>
We knew that the more stable and more nearly equivalent the resonance
contributors, the greater is the resonance energy. The resonance energies of
pyrrole, furan, and thiophene are not as great as the resonance energies of benzene which has equivalent resonance contributors. Thiophene, with the least electronegative heteroatom, has the greatest
resonance energy of these five-membered heterocycles; and furan, with the most electronegative
heteroatom, has the smallest resonance energy.
[IMG]http://files.myopera.com/nhathavosong/albums/820234/rellative%20resonace%20energies%20of%20aromatic%20compounds.JPG[/IMG]
Because these heterocycles are aromatic, they can undergo electrophilic substitution reaction. However, you should distinguish between these two concepts: aromaticity and reactivity to electrophilic reagents.Pyrrole, furan, and thiophene are all more reactive than benzene toward electrophilic
substitution because they are better able to stabilize the positive charge on the
carbocation intermediate, since the lone pair on the hetereoatom can donate electrons
into the ring by resonance.
Furan is not as reactive as pyrrole in electrophilic aromatic substitution reactions.
The oxygen of furan is more electronegative than the nitrogen of pyrrole, so the oxygen
is not as effective as nitrogen in stabilizing the carbocation. Thiophene is less
reactive than furan toward electrophilic substitution because sulfur’s electrons are in
a 3p orbital, which overlaps less effectively than the 2p orbital of nitrogen or oxygen
with the 2p orbital of carbon (because large atomic radius of sulfur results in large distance between overlapping orbitals although energy level of sulfur's 3p orbital is as comparably high as that of carbon's 2p orbitals and this leads to great resonance energy of thiophene) . The electrostatic potential maps illustrate the different
electron densities of the three rings.(see the attachment below)
What I mentioned above is based on out-of-date theories of molecular structure. We could also explain the problem in the view point of modern MO theory.
I used Hyperchem software to compute and display HOMOs of these three heterocycles by semi-empirical AM1 method. You should realize that the HOMO of thiophene differs from that of pyrrole and furan. It consists of two separate partitions and you can see that the bigger one has form of 3p orbital of Sulfur. It mean that this 3p orbital overlaps ineffectively with π electron system of carbon atoms and therefore donates electrons less effectively, too. This results in less reactivity to electrophile reagents of thiophene than furan and pyrrole.

<!--cut and paste--><a href="http://my.opera.com/nhathavosong/albums/showpic.dml?album=820234&picture=11166887"><img src="http://files.myopera.com/nhathavosong/albums/820234/thumbs/pirrole%27s%20HOMO.JPG_thumb.jpg" alt="" /></a>

<!--cut and paste--><a href="http://my.opera.com/nhathavosong/albums/showpic.dml?album=820234&picture=11166883"><img src="http://files.myopera.com/nhathavosong/albums/820234/thumbs/furan%27%20HOMO.JPG_thumb.jpg" alt="" /></a>

<!--cut and paste--><a href="http://my.opera.com/nhathavosong/albums/showpic.dml?album=820234&picture=11166884"><img src="http://files.myopera.com/nhathavosong/albums/820234/thumbs/Thiophene%27s%20HOMO.JPG_thumb.jpg" alt="" /></a>
I also used Hyperchem to display electrostatic potential maps of these three rings. Do you agree with me that pyrrole is the most reactive because of greatest π electron density in the ring system?Furan is less reactive because of the presence of the electron withdrawing oxygen heteroatom. And thiophene with sulfur's lone pair electrons partially localized in 3p orbital is also illustrated.

<!--cut and paste--><a href="http://my.opera.com/nhathavosong/albums/showpic.dml?album=820234&picture=11168556"><img src="http://files.myopera.com/nhathavosong/albums/820234/thumbs/2pirrole.JPG_thumb.jpg" alt="" /></a>

<!--cut and paste--><a href="http://my.opera.com/nhathavosong/albums/showpic.dml?album=820234&picture=11168551"><img src="http://files.myopera.com/nhathavosong/albums/820234/thumbs/1furan.JPG_thumb.jpg" alt="" /></a>

<!--cut and paste--><a href="http://my.opera.com/nhathavosong/albums/showpic.dml?album=820234&picture=11168549"><img src="http://files.myopera.com/nhathavosong/albums/820234/thumbs/1Thiophene.JPG_thumb.jpg" alt="" /></a>

Do you find it all somewhat interesting?