حجرة مدرسى الكيمياء

[أ. الدادمونى]

هذه المشاركة كانت للدكتور / عاطف
06-11-2007, 06:38 AM
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الي الاستاذ الحبيب سيد تاكيدا ان الرابطة الثنائية انشط من الثلاثية وعفوا اخي مع كل الحب والتقدير
When the addition reactions of electrophilic reagents, such as strong Brønsted acids

and halogens, to alkynes are studied we find a curious paradox. The reactions are even more exothermic than the additions to alkenes, and yet the rate of addition to alkynes is slower by a factor of 100 to 1000 than addition to *****alently substituted alkenes. The reaction of one *****alent of bromine with 1-penten-4-yne, for example, gave
the chief product as
4,5-dibromo-1-pentyne


HC≡C-CH2-CH=CH2 + Br2 ——> HC≡C-CH2-CHBrCH2Br
هذا التقاعل يثبت ذلك
Although these electrophilic additions to alkynes are sluggish, they do take place and generally display Markovnikov Rule regioselectivity and anti-stereoselectivity. One problem, of course, is that the products of these additions are themselves substituted alkenes and can therefore undergo further addition. Because of their high electronegativity, halogen substituents on a double bond act to reduce its nucleophilicity, and thereby decrease the rate of electrophilic addition reactions. Consequently, there is a delicate balance as to whether the product of an initial addition to an alkyne will suffer further addition to a saturated product. Although the initial alkene products can often be isolated and identified, they are commonly present in mixtures of products and may not be obtained in high yield. The following reactions illustrate many of these features. In the last example, 1,2-diodoethene does not suffer further addition inasmuch as vicinal-diiodoalkanes are relatively unstable.
[attachmentid=5524]

Why are the reactions of alkynes with electrophilic reagents more sluggish than the
corresponding reactions of alkenes? After all, addition reactions to alkynes are generally more exothermic than additions to alkenes, and there would seem to be a higher π-electron density about the triple bond ( two π-bonds versus one ). Two factors are significant in explaining this apparent paradox. First, although there are more π-electrons associated with the triple bond, the sp-hybridized carbons exert a strong attraction for these π-electrons, which are consequently bound more tightly to the functional group than are the π-electrons of a double bond. This is seen in the ionization potentials of
Acetylene HC≡CH + Energy ——> [HC≡CH •(+) + e(–) ΔH = +264 kcal/mole

Ethylene H2C=CH2 + Energy ——> [H2C=CH2] •(+) + e(–) ΔH = +244 kcal/mole
Ethane H3C–CH3 + Energy ——> [H3C–CH3] •(+) + e(–) ΔH = +296 kcal/mole
As defined by the preceding equations, an ionization potential is the minimum energy required to remove an electron from a molecule of a compound. Since pi-electrons are less tightly held than sigma-electrons, we expect the ionization potentials of ethylene and acetylene to be lower than that of ethane, as is the case. Gas-phase proton affinities show the same order, with ethylene being more basic than acetylene, and ethane being less basic than either. Since the initial interaction between an electrophile and an alkene or alkyne is the formation of a pi-complex, in which the electrophile accepts electrons from and becomes weakly bonded to the multiple bond, the relatively slower reactions of alkynes becomes understandable.
The second factor is the stability of the carbocation intermediate generated by sigma-bonding of a proton or other electrophile to one of the triple bond carbon atoms. This intermediate has its positive charge localized on an unsaturated carbon, and such vinyl cations are less stable than their saturated analogs. Indeed, we can modify our earlier ordering of carbocation stability to include these vinyl cations.
Carbocation
Stability CH3(+) ≈ RCH=CH(+) < RCH2(+) ≈ RCH=CR(+) < R2CH(+) ≈ CH2=CH-CH2(+) < C6H5CH2(+) ≈ R3C(+)
Methyl 1°-Vinyl 1° 2°-Vinyl 2° 1°-Allyl 1°-Benzyl 3°

Application of the Hammond postulate then suggests that the activation energy for the generation of such an intermediate would be higher than that for a lower energy

intermediate. This is illustrated by the following energy diagrams

[attachmentid=5525]

Despite these impediments, electrophilic additions to alkynes have emerged as exceptionally useful synthetic transforms. For example, addition of HCl, acetic acid and hydrocyanic acid to acetylene give respectively the useful monomers vinyl chloride, vinyl acetate and acrylonitrile, as shown in the following equations. Note that in these and many other similar reactions transition metals, such as copper and mercury salts, are effective catalysts.

HC≡CH + HCl ——> H2C=CHCl Vinyl Chloride
HC≡CH + CH3CO2H ——> H2C=CHOCOCH3 Vinyl Acetate
HC≡CH + HCN ——> H2C=CHCN Acrylonitrile.

[أ. الدادمونى]

هذه المشاركة كانت للأستاذ / سيد عثمان
06-11-2007, 09:26 AM
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أخي العزيز الدكتور عاطف أشكرك كثيرا جدا على أهتمامك بهذا الموضوع لأنه بجد موضوع مهم وأرجو من حضرتك أن تتقبل عذري في هذا الموضوع لأنه موضوع يحتاج بحث وليس بالبساطة
وطبعا أنت معي أكيد ان هذا الموضوع شائك ففي بعض التفاعلات تجد الألكاينات أنشط من الألكينات وفي البعض الآخر تجد العكس تماما
هل انت متفق معي في ذلك
1- electrophilic reagents تتفاعل بسرعة مع الألكين وأقل سرعة مع الألكاين والعكس مع neocleophilic
وانا وبكل حب واعتزاز أتقبل الراي والرأي الآخر حتى نصل للحقيقة كاملة
وإذا ظهر عنك الجديد لاتبخل علينا بعلمك

When the addition reactions of electrophilic reagents, such as strong Br&oslash;nsted acids

and halogens, to alkynes are studied we find a curious paradox. The reactions are even more exothermic than the additions to alkenes, and yet the rate of addition to alkynes is slower by a factor of 100 to 1000 than addition to *****alently substituted alkenes. The reaction of one *****alent of bromine with 1-penten-4-yne, for example, gave
the chief product as
4,5-dibromo-1-pentyne


HC≡C-CH2-CH=CH2 + Br2 ——> HC≡C-CH2-CHBrCH2Br
هذا التقاعل يثبت ذلك
Although these electrophilic additions to alkynes are sluggish, they do take place and generally display Markovnikov Rule regioselectivity and anti-stereoselectivity. One problem, of course, is that the products of these additions are themselves substituted alkenes and can therefore undergo further addition. Because of their high electronegativity, halogen substituents on a double bond act to reduce its nucleophilicity, and thereby decrease the rate of electrophilic addition reactions. Consequently, there is a delicate balance as to whether the product of an initial addition to an alkyne will suffer further addition to a saturated product. Although the initial alkene products can often be isolated and identified, they are commonly present in mixtures of products and may not be obtained in high yield. The following reactions illustrate many of these features. In the last example, 1,2-diodoethene does not suffer further addition inasmuch as vicinal-diiodoalkanes are relatively unstable.


Why are the reactions of alkynes with electrophilic reagents more sluggish than the
corresponding reactions of alkenes? After all, addition reactions to alkynes are generally more exothermic than additions to alkenes, and there would seem to be a higher π-electron density about the triple bond ( two π-bonds versus one ). Two factors are significant in explaining this apparent paradox. First, although there are more π-electrons associated with the triple bond, the sp-hybridized carbons exert a strong attraction for these π-electrons, which are consequently bound more tightly to the functional group than are the π-electrons of a double bond. This is seen in the ionization potentials of
Acetylene HC≡CH + Energy ——> [HC≡CH •(+) + e(–) ΔH = +264 kcal/mole

Ethylene H2C=CH2 + Energy ——> [H2C=CH2] •(+) + e(–) ΔH = +244 kcal/mole
Ethane H3C–CH3 + Energy ——> [H3C–CH3] •(+) + e(–) ΔH = +296 kcal/mole
As defined by the preceding equations, an ionization potential is the minimum energy required to remove an electron from a molecule of a compound. Since pi-electrons are less tightly held than sigma-electrons, we expect the ionization potentials of ethylene and acetylene to be lower than that of ethane, as is the case. Gas-phase proton affinities show the same order, with ethylene being more basic than acetylene, and ethane being less basic than either. Since the initial interaction between an electrophile and an alkene or alkyne is the formation of a pi-complex, in which the electrophile accepts electrons from and becomes weakly bonded to the multiple bond, the relatively slower reactions of alkynes becomes understandable.
The second factor is the stability of the carbocation intermediate generated by sigma-bonding of a proton or other electrophile to one of the triple bond carbon atoms. This intermediate has its positive charge localized on an unsaturated carbon, and such vinyl cations are less stable than their saturated analogs. Indeed, we can modify our earlier ordering of carbocation stability to include these vinyl cations.
Carbocation
Stability CH3(+) ≈ RCH=CH(+) < RCH2(+) ≈ RCH=CR(+) < R2CH(+) ≈ CH2=CH-CH2(+) < C6H5CH2(+) ≈ R3C(+)
Methyl 1°-Vinyl 1° 2°-Vinyl 2° 1°-Allyl 1°-Benzyl 3°

Application of the Hammond postulate then suggests that the activation energy for the generation of such an intermediate would be higher than that for a lower energy

intermediate. This is illustrated by the following energy diagrams



Despite these impediments, electrophilic additions to alkynes have emerged as exceptionally useful synthetic transforms. For example, addition of HCl, acetic acid and hydrocyanic acid to acetylene give respectively the useful monomers vinyl chloride, vinyl acetate and acrylonitrile, as shown in the following equations. Note that in these and many other similar reactions transition metals, such as copper and mercury salts, are effective catalysts.

HC≡CH + HCl ——> H2C=CHCl Vinyl Chloride
HC≡CH + CH3CO2H ——> H2C=CHOCOCH3 Vinyl Acetate
HC≡CH + HCN ——> H2C=CHCN Acrylonitrile.

[أ. الدادمونى]

هذه المشاركة كانت للأستاذ / سيد عثمان
06-11-2007, 01:13 PM
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ملحوظة هامة جدا
هذه المناقشات العلمية ليست في المنهج المقرر على الطلبة حتى لا يحدث بلبلة عندهم هى مجرد مناقشة علمية بين المدرسين فقط

[أ. الدادمونى]

هذه المشاركة كانت للدكتور / عاطف خليفة
06-12-2007, 03:06 AM
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استاذي سيد انا معك والله الموفق
ودعوة عامة لكل مدرسي الكيمياء الي ذلك

[أ. الدادمونى]

هذه المشاركة كانت للأستاذ / جابر شكرى
06-17-2007, 12:29 AM
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Ethane H3C–CH3 + Energy ——> [H3C–CH3] •(+) + e(–) ΔH = +296 kcal/mole
وتحياتي لكم جميعا ولكم الشكر والتقدير على مجهودكم العظيم وبارك الله فيكم

أخوكم / جابر شكري

[أ. الدادمونى]

هذه المشاركة كانت للدكتور / عاطف خليفة
06-17-2007, 05:29 AM
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استاذنا الحبيب
جابر شكري
لك كل الحب والتقدير
وحشتنا جدا
الله يبارك فيك ويزيدك
عاطف

[أ. الدادمونى]

هذه كانت مشاركتى بتارخ 06-19-2007, 04:14 PM
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أساتذتى الأعزاء

تحية طيبة، معذرة فلم تقع عينى على هذه المناقشة العلمية الأكثر من رائعة إلا الآن

ونظراً لانشغالى الآن .. فبعد انتهاء الامتحانات (خلال أجازة الصيف) أعدكم بالبحث فى المراجع العلمية عندى أو عند غيرى

ولكن كل ما يهمنى الآن الإجابة بالنسبة للطلبة

** تفاعل الكلور مع البنزين فى ضوء الشمس تفاعل إضافة بينما فى غياب ضوء الشمس تفاعل احلال (استبدال)

لأن طاقة ضوء الشمس تعمل على كسر الروابط المزدوجة فى حلقة البنزين فتضاف ذرات الكلور على هذه الروابط

C6H6Cl6 <-----ضوء شمس مباشر-----C6H6 + 3 Cl2
سداسى كلوريد البنزين
(الجامكسان)

C6H5Cl + HCl <-----غياب ضوء الشمس-----C6H6 + Cl2
أحادى كلورو بنزين

بالنسبة للسادة الزملاء هناك أسباب تتعلق بطول وقوة الرابطة - وظاهرة الرنين فى حلقة البنزين والتى يمكن توضحيها فى هذين الملفين:

[أ. الدادمونى]

هذه المشاركة كانت لـ mr.Chemistry
06-20-2007, 01:43 AM
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السلام علي الجميع
الحقيقة أنا لسه جديد في المنتدي وأول مرة أعرفه بالصدفة
المهم ماأروع المناقشة في مجال نشاط الألكانات،الألكينات والألكاينات وطبعا مش هأقدر أضيف جديد بعض الدلائل والصور والمرفقات الموجودة.
لكني أضم صوتي مع صوت الأخ الديداموني وهو تبسيط المعلومة للطالب مع سرد الموضوع في مكان آخر علي هيئة مناقشة بين المعلمين.
وجزاكم الله خيرا

[أ. الدادمونى]

هذه كانت مشاركتى بتارخ بتاريخ 06-20-2007, 01:03 PM

وعليكم السلام أخى الكريم : مستر / علاء

جزاكم الله كل خير، وما أجمل هذه الصدفة التى تجعلك إضافة

قوية لهذاالمنتدى لتفيدنا وتثرى هذا المنتدى بخبرتك،

لك منى تحية خاصة، ومن كل أعضاء المنتدى جزيل

الشكر والتقدير.