Specific acid catalysis

Specific acid catalysis (SAC) involves a rapid protonation of the compound followed by the slow step, which is accelerated in comparison with the uncatalysed reaction because of the greater reactivity of the protonated compound. You have just seen an example with an epoxide; ester hydrolysis (or formation) is another.

A more interesting reaction is the dienone–phenol rearrangement. Rearrangement in the absence of acid is very slow but once the ketone oxygen is protonated, it occurs very rapidly.

Again we have fast equilibrium protonation, followed by a rate-determining step involving a reaction of the protonated species: this is SAC.

This catalysis depends only on the protonating power of the solution. The compound must be protonated to react, so the catalyst must be a strong enough acid to do the job. It is not necessary that every molecule is protonated—just enough to set the reaction going because the catalytic acid is regenerated at the end. In a specifi c acid catalysed reaction, the rate of the reaction depends on the pH of the reaction mixture. SAC works only if the pH is similar to, or below, the pKa of the conjugate acid of the substrate, and the log of the rate of the reaction is proportional to the pH of the solution. There is one rather remarkable experimental indication of this mechanism. If the reaction is carried out in a deuterated solvent (D₂O instead of H₂O) the rate of the reaction increases. This is a solvent isotope effect rather than a kinetic isotope effect and needs some explanation. If you examine the three examples of SAC in the previous pages you will see that they share these characteristics: a fast proton exchange is followed by a rate-determining step that does not involve the making or breaking of any bonds to hydrogen. In general terms:

The rate of the reaction is the rate of the rate-determining step: rate = k[XH⁺]

The concentration of the intermediate [XH⁺] is related to the pH and to the concentration of the substrate by the equilibrium constant, K, of the protonation. This gives us:

 rate = kK[H⁺][X]

 In the acid-catalysed reaction, the bond to H (or D) is not broken in the rate-determining step, so k cannot change when hydrogen is replaced by deuterium. That means that if a reaction goes faster in D₂O than in H₂O then it must be K that is different (i.e. larger) in D₂O. SAC is more effective with D₃O+ in D₂O than with H₃O+ in H₂O because more of the substrate is protonated at any one time.

● An inverse solvent isotope effect (k[D₂O] > k[H₂O]) is indicative of specifi c acid catalysis.

This is sometimes explained by saying that D3O+ is a stronger acid than H3O+. This is partly true. The full truth is that D3O+ in D2O is a stronger acid than H3O+ in H2O. Water (H2O) is a better solvating agent for H3O+ than D2O is for D3O+ because O–H bonds are longer than O–D bonds. Look again at the potential energy curve we showed you on p. 1050 and reproduced below, this time representing the energies of O–H and O–D bonds. The average length of a bond is the mid-point of the line in the potential energy well represent ing its energy level. You can easily see that the mid-point for the O–H is further out than the mid-point for the O–D bond because of the asymmetry of the well. O–H bonds are longer than O–D bonds, and can therefore make stronger hydrogen bonds. These hydrogen bonds are better at allowing solvation of H₃O⁺, making H₃O⁺ in H₂O less willing to proton ate a substrate than D₃O⁺ in D₂O.

Let’s illustrate all this with an example. The Z allylic alcohol below dehydrates in acid solution to the E diene. We have lots of data on this mechanism, all summarized in the diagrams. You may like to note as well that the product contains no deuterium after de hydration in D₂O.

The Hammett ρ value of –6.0 suggests a carbocation intermediate and the positive entropy of activation suggests a rate-determining step in which disorder increases, perhaps one molecule breaking into two. The inverse solvent deuterium isotope effect (faster reaction in D₂O than in H₂O) strongly suggests SAC. Putting all this together we have a mechanism—a simple example of SAC. There is no protonation at carbon.

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شعبة متابعة الأداء تنظم محاضرة تربوية لمتطوعات مجمع العلقمي استعدادًا لزيارة الأربعين

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العتبة العباسية المقدسة تعقد ملتقى خاصًا لمنتسبيها العاملين في مراكز إرشاد التائهين خلال زيارة الأربعين

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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

Bonding and reactions in transition metal complexes

Ligands can be attached in many different ways

The 18-electron rule

Transition metals extend the range of organic reactions

The Ritter reaction and the Beckmann fragmentation

Specific base catalysis

Entropy of activation

The kinetic isotope effect

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