Light allows the interconversion of the two isomers of an alkene by promoting a π electron into the π* orbital and transiently breaking the π bond, but the way light favours formation of the Z isomer is rather subtle. One difference between cis and trans alkenes is that the trans alkenes usually absorb light better than the cis alkenes—they absorb light of a higher wave length and they absorb more of it, particularly when conjugated with carbonyl groups. Steric hindrance forces the cis alkene to twist about the σ bond joining the alkene to the carbonyl group and conjugation is then less efficient. In a mixture of E and Z alkenes, the E alkene is more prone to isomerization by light, so the Z isomer builds up in the mixture. Here is an example. Aldol condensation of cyclohexanone and benzaldehyde gives pure E alkene for the reasons explained above. Irradiation with longer-wavelength UV light equilibrates this to the Z alkene in excellent yield.

It is not possible for the benzene ring and the enone system to be planar in the Z-enone and so they twist, making conjugation not as good as in the E-enone. Longer-wavelength light is absorbed only by the E-enone, which is continually equilibrated back to the excited state. Eventually, all the E-enone is converted to the Z-enone, which is not as efficiently excited by the light. The final mixture of E- and Z-enone is known as a ‘photostationary state’.

The chemistry of vision

 The human eye uses a cis alkene, 11-cis-retinal, to detect light, and a cis–trans isomerism reaction is at the heart of the chemical mechanism by which we see. The light-sensitive pigment in the cells of the retina is an imine, formed by reaction of 11-cis-retinal with a lysine residue of a protein, opsin. Absorption of light by the opsin–retinal compound, known as rhodopsin, promotes one of the electrons in the conjugated polyene system to an antibonding orbital. Free rotation in this excited state allows the cis double bond to isomerize to trans, and the conformational changes in the protein molecule that result trigger a cascade of reactions that ultimately leads to a nerve signal being sent to the brain.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تواصل تنظيم مجالس العزاء الصباحية بذكرى زيارة عاشوراء

مواكب العزاء تستذكر في التاسع من شهر المحرم شهادة عبدالله الرضيع بن الحسين (عليهما السلام)

العتبة العباسية المقدسة تستذكر تضحيات القاسم بن الحسن (عليهما السلام)

شعبة الحرم الشريف تواصل تنفيذ خطتها الخاصة بشهر المحرّم داخل مرقد أبي الفضل العباس (عليه السلام)

المزيد

الآخبار الصحية

لهذا السبب.. إغلاق 25 مركزًا لعلاج الإدمان في مصر ؟

خلافا للاعتقاد الشائع.. علماء يكشفون أسرار بذور البطيخ ؟!

هذا ما يحدث لجسمك عند تناولك الطماطم بانتظام !

تعرف على أطعمة تسرّع "تعافي الجسم" من حروق الشمس

المزيد

الآخبار التكنلوجية

الثلوج تغطي الصحراء الأكثر جفافا في العالم!

بتلسكوب "ألما".. التقاط صورا غير مسبوقة لبدايات الكون !

هل هاتفك مراقب؟.. 5 علامات خطيرة تكشف التجسس عليك

دبي تطلق أول رحلة تجريبية للتاكسي الجوي

المزيد

مواضيع ذات صلة

Predominantly E alkenes can be formed by stereoselective elimination reactions

Addition of nucleophiles to alkynes

E-selective reduction of alkynes using sodium in liquid ammonia

Equilibration of alkenes

Exploiting cyclic compounds

Lewis acids promote couplings via oxonium ions

Allyl silanes are more reactive than vinyl silanes but also react through β-silyl cations

Allyl silanes as nucleophiles

Vinyl silanes offer a regio- and stereoselective route to alkenes

Aryl silanes undergo ipso substitution with electrophiles

Silicon stabilizes a positive charge on the β carbon

Alkynyl silanes are used for protection and activation