A cycloaddition that destroys bonds: ozonolysis

Our last type of cycloaddition is most unusual. It starts as a 1,3-dipolar cycloaddition but eventually becomes a method of cleaving π bonds in an oxidative fashion so that they end up as two carbonyl groups. The reagent is ozone, O₃. Again, you met this reaction in Chapter 19, but we can now show you the full, remarkable details of the reaction mechanism. Ozone is a symmetrical bent molecule with a central positively charged oxygen atom and two terminal oxygen atoms that share a negative charge. It is a 1,3-dipole and does typical 1,3-dipolar cycloadditions with alkenes. The product is a very unstable compound. The O–O single bond (bond energy 140 kJ mol−1) is a very weak bond—much weaker than the N–O bond (180 kJ mol−1) we have been describing as weak in previous examples—and this heterocycle has two of them. It immediately decomposes—by a reverse 1,3-dipolar cycloaddition.

The products are a simple aldehyde on the left and a new, rather unstable looking molecule—a 1,3-dipole known as a carbonyl oxide—on the right. At least it no longer has any true O–O single bonds (the one that looks like a single bond is part of a delocalized system like the one in ozone). Being a 1,3-dipole, it now adds to the aldehyde in a third cycloaddition step. It might just add back the way it came, but it much prefers to add in the other way round, with the nucleophilic oxyanion attacking the carbon atom of the carbonyl group like this.

This compound—known as an ozonide—is the first stable product of the reaction with ozone. It is the culmination of two 1,3-dipolar cycloadditions and one reverse 1,3-dipolar cycloaddition. It is still not that stable and is quite explosive, so for the reaction to be of any use it needs decomposing. The way this is usually done is with dimethylsulfide or Ph3P, which attacks the ozonide to give DMSO and two molecules of aldehyde.

The ozonide will also react with oxidizing agents such as H₂O₂ to give carboxylic acids, or with more powerful reducing agents such as NaBH4 to give alcohols. Here are the overall transformations—each cleaves a double bond—it is called an ozonolysis.

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قسم الإعلام ينظم دورة تخصصية في تقنيات الذكاء الاصطناعي لعددٍ من منتسبيه

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المزيد

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علامات فارقة بين النوبة أو السكتة القلبية.. ماهي؟

الفلفل الأحمر.. سعرات حرارية منخفضة وفوائد صحية عديدة

8 أعراض إذا شعرت بها.. اذهب للطوارئ فورا !

تعرف على أكثر 8 أماكن اتساخا في المنزل

المزيد

الآخبار التكنلوجية

إنتاج الهيدروجين الأخضر في البحر.. هذه أحدث الدراسات

"ناسا" تخطط لبناء مفاعل نووي على سطح القمر !

ابتكار ومواهب وأموال.. مارك زوكربيرغ يعلن الحرب على أبل

عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

المزيد

مواضيع ذات صلة

Non-linear Hammett plots

A complete picture of the transition state from Hammett plots

Using the Hammett ρ values to uncover mechanisms

Reactions with small Hammett ρ values

Reactions with negative Hammett ρ values

Reactions with positive Hammett ρ values

Equilibria with positive Hammett ρ values

The Hammett substituent constant σσ A

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms