The simplest enolizable aldehyde is acetaldehyde (ethanal, CH₃CHO). What happens if we add a small amount of base, say NaOH, to this aldehyde? Some of it will form the enolate ion.

Only a small amount of the nucleophilic enolate ion is formed: hydroxide is not basic enough to enolize an aldehyde completely. Each molecule of enolate is surrounded by molecules of the aldehyde that are not enolized and so still have the electro philic carbonyl group intact. The enolate ion will attack one of these aldehydes to form an alkoxide ion, which will be protonated by the water molecule formed in the fi rst step.

The product is an aldehyde with a hydroxy (ol) group and it has the trivial name aldol. The name aldol is given to the whole class of reactions between enolates (or enols) and carbonyl compounds even if in most cases the product is not a hydroxy-aldehyde at all. Notice that the base catalyst (hydroxide ion) is regenerated in the last step, so it is truly a catalyst. This reaction is so important because of the carbon–carbon bond formed when the nucleophilic enolate attacks the electrophilic aldehyde. This bond is shown as a black bond in this version of the key step.

The reaction occurs with ketones as well. Acetone is a good example for us to use at the start of this chapter because it gives an important product, and as it is a symmetrical ketone, there can be no argument over which way it enolizes. Each step is the same as the aldol sequence with acetaldehyde, and the product is again a hydroxy-carbonyl compound, but this time a hydroxy-ketone.

The acetaldehyde reaction works well when one drop of dilute sodium hydroxide is added to acetaldehyde. The acetone reaction is best done with insoluble barium hydroxide, Ba(OH)₂. Both approaches keep the base concentration low. Without this precaution, the aldol products are not the compounds isolated from the reaction. With more base, further reactions occur because the aldol products dehydrate rather easily under the reaction conditions to give stable conjugated unsaturated carbonyl compounds.

These are elimination reactions, where the possible mechanisms are discussed. You cannot normally eliminate water from an alcohol in basic solution as hydroxide is a bad leaving group. It is the carbonyl group that allows elimination here: these are E1cB reactions, with a second enolization allowing the loss of OH⁻.

In the examples that follow you will see that the base-catalysed aldol reaction sometimes gives the aldol and sometimes the elimination product. The choice is partly based on conditions—the more vigorous the conditions (stronger base, higher temperature, longer time) the more likely elimination is to occur—and partly on the structure of the reagents. The elimination is even easier in acid solution and acid-catalysed aldol reactions commonly give unsaturated products instead of aldols. In this simple example with a symmetrical cyclic ketone, the enone is formed in good yield in acid or base. We shall use the acid-catalysed reaction to illustrate the mechanism. First the ketone is enolized under acid catalysis as.

Then the aldol reaction takes place. Enols are less nucleophilic than enolates, and the reaction occurs because the electrophilic carbonyl component is protonated: the addition is acid catalysed. An acid-catalysed aldol reaction takes place.

The aldol is a tertiary alcohol and would be likely to eliminate by an E1 mechanism in acid even without the carbonyl group. But the carbonyl ensures that only the stable conjugated enone is formed. Notice too that the dehydration is genuinely acid-catalysed as the acid re appears in the very last step.

None of these intermediates is detected or isolated in practice—simple treatment of the ketone with acid gives the enone in good yield. A base-catalysed reaction gives the same product via the aldol–E1cB elimination mechanism.

• Base-catalysed aldol reactions may give the aldol product, or may give the dehydrated enone or enal by an E1cB mechanism. • Acid-catalysed aldol reactions may give the aldol product, but usually give the dehydrated enone or enal by an E1 mechanism.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الفكرية ينظم مجلسًا عزائيًّا في دولة بوروندي الإفريقية

العتبة العباسية المقدسة تواصل إقامة مجالسها العزائية لإحياء شهر المحرَّم

سماحة السيد الصافي يدعو إلى التدبر في القرآن والاستفادة منه في مواجهة تحديات الحياة

ضمن برنامج التطوير الحسيني التاسع.. جمعية كشافة الكفيل تقيم مهرجان الكشاف الحسيني

المزيد

الآخبار الصحية

دراسة تكشف علاقة خطيرة بين عمر الأم وصحة مولودها

دراسة تحذر.. اضطراب نوم بسيط قد يسبب الموت المبكر ؟

الاستحمام بالماء الساخن.. راحة مؤقتة وأضرار دائمة للبشرة

ماذا يحدث للجسم عند تخطي وجبة الإفطار؟

المزيد

الآخبار التكنلوجية

كيف تساعد الصيانة الوقائية في الحفاظ على كفاءة التكييف في سيارتك ؟

في ظاهرة غريبة حيرت العلماء.. كوكبنا قد يسجل أقصر يوم في التاريخ هذا الصيف!

السد الصيني العملاق يعيق دوران الأرض ويغيّر شكلها ؟

السعودية تطلق 10 تجارب علمية إلى محطة الفضاء الدولية

المزيد

مواضيع ذات صلة

The Claisen ester condensation and other self-condensations

Problems with acylation at carbon

Cross-condensations

Nitroalkanes are superb nucleophiles for conjugate addition

A variety of electrophilic alkenes will accept enol(ate) nucleophiles

Conjugate addition of silyl enol ethers leads to the silyl enol ether of the product

Enamines are convenient stable enol equivalents for conjugate addition

Alkali metal (especially Na, K) enolates can undergo conjugate addition

1,3-Dicarbonyl compounds undergo conjugate addition

Using Michael acceptors as electrophiles

Conjugate addition to enones gives enolates regiospecifically

Enones provide a solution to regioselectivity problems