With α -H aldehyde, generated in the dilute alkali catalysis Carbon anion , and then carbon anion as Dear nucleophiles Of aldehydes and ketones Dear nucleophilic addition , generating beta hydroxy aldehyde, beta hydroxy aldehyde heating dehydration in unsaturated aldehyde.
In dilute alkali or acid under the action of two molecules, Aldehyde Or Ketone Can interact with each other, wherein an aldehyde (or ketone) molecules in the alpha hydrogen is added to another aldehyde (or ketone) carbonyl oxygen atoms and molecules, the remaining part is added to the carbonyl carbon atom, generating a molecular beta hydroxy aldehyde or a molecular beta hydroxy ketone. This reaction is called the aldol condensation or Aldol condensation (aldol condensation). Through the aldol condensation, forming carbon carbon bond in the molecule and new growth, carbon chain.
Aldol condensation reaction process, to Acetaldehyde As an example, the following:
The first step, alkali and acetaldehyde in alpha hydrogen bonding, form an enolate anion or negative carbon ion:
The second step is the negative ion as dear nucleophiles, immediately attack the carbonyl carbon atom of another aldehyde molecules, occurred after addition reaction to generate a negative ion (alkoxy anion).
The third step, alkoxy anion and water by aldol and OH.
Dilute acid can also make aldehyde aldol reaction course, but different. Acid Catalysis Because, first of all Proton The enhancement of carbon and oxygen Double bond Polarization, make it become Enol Type, then Addition reaction Get the aldol.
The resultant Molecules of the alpha hydrogen atom is also Carbonyl And the beta carbon Hydroxyl The activation, so just a little heat or acid that intramolecular dehydration and generation, alpha, beta unsaturated aldehyde.
Where alpha carbon with hydrogen atoms beta hydroxy aldehydes, ketones are easy to lose a molecule of water. This is because the alpha hydrogen is more lively, and dehydration after The resultant Have The conjugated double bond Therefore, relatively stable.
Except for acetaldehyde, aldol condensation products produced by other aldehyde, all is in Alpha carbon atom A branched hydroxyl aldehyde or acrolein. Aldol Condensation reaction In the Organic synthesis There are important applications, it can be used to increase the carbon chain, and can produce branched chain.
With α - hydrogen ketone in dilute alkali role, although also can have this kind of Condensation reaction However, due to Electronic effect , Spatial effect The influence of reaction, difficult, as with the ordinary methods of operation, basically not products. The general need for reaction in special conditions. For example: Acetone In the presence of alkali, can Mr. into two acetone alcohol, but in equilibrium system, the production rate is very low. If you can make the product in the generation, be out of Alkali catalyst The product, is out of balance system, finally can make more acetone into two acetone alcohol, the yield is 70- ~ 80-. Two acetone alcohol in iodine Catalysis After dehydration, heat can be generated alpha, beta unsaturated ketones.
Cross aldol condensation reaction was: in different aldehyde, ketone molecules called cross aldol condensation. If the aldehyde, ketone with alpha The hydrogen atom After the reaction, it can generate four kinds of products, the actual always get the complex Mixture , no practical value. Some without alpha The hydrogen atom Aldehyde, ketone is not taking place aldol Condensation reaction (such as HCHO, RCCHO, ArCHO, RCCOCR, ArCOAr, ArCOCR), but they can be the same as with alpha The hydrogen atom Aldehyde, ketone cross aldol condensation, which is mainly benzene Formaldehyde And Formaldehyde Reaction. And the product variety is reduced, can get a condensation product, yield is also higher. After the completion of the reaction products, must be original with alpha hydrogen atom Aldehyde Be retained. Always keep not containing alpha in reaction time The hydrogen atom The excessive formaldehyde, can be a single product.
Claisen Schmidt condensation: (Claisen-Schmidt) Fang Xiangquan And containing an alpha hydrogen atom aldehyde, ketone in alkali Catalysis Aldol under the Condensation reaction Dehydrated, high yield of alpha, beta unsaturated aldehydes, ketones, this type of reaction, called Claisen Smit condensation reaction (Claisen-Schmidt). In the alkali Catalysis Next, benzaldehyde and can also contain an alpha hydrogen atom Aliphatic ketone Or Aromatic ketone Condensation. In addition, some containing active Ya Jiaji A compound, such as Malonic acid , Two methyl malonic acid , alpha Nitro Ethyl acetate, can be similar to the aldol condensation reaction with aldehydes, ketones. The main reason is the strong electron withdrawing group for the activation of α -H, make it easy to become Hydrogen ion And out of, Ethyl acetoacetate And Malonic acid ethyl ester two In the Organic synthesis On the application of relating.
Aldol condensation from Mechanism Speaking, is carbon negative ions on the carbonyl carbon dear nucleophilic addition. Carbonyl aldehyde or ketone molecules in the hydrogen alpha carbon atom has higher activity, in the presence of an acid catalyst, the carbonyl oxygen atom protonated, enhance the inducing effect of carbonyl alpha hydrogen dissociation into enol promotion. Loss of hydrogen atoms in the alpha carbon atom alkaline catalyst formed carbon negative ion resonance hybrids, and reached equilibrium after generating enolate. Enolate, followed by carbonyl and another molecule of aldehydes or ketones were dear nucleophilic addition, formation of carbon new, β hydroxy aldehyde or ketone. Due to a hydrogen atom is lively, containing an alpha hydrogen atoms of the beta hydroxy aldehyde or ketone is easy to lose water molecules form a more stable conjugated double bond structure of alpha, beta unsaturated aldehyde or ketone.
For the cross aldol condensation of two kinds of aldehyde or ketone between non catalytic process, because of many side reactions, resulting in selective is not high. The catalytic process so that the selectivity of the objective product meets the requirements of industrial application. For the catalyst using the reaction pH activity center, according to its, can be divided into acidic catalyst, alkaline catalyst, acid and base catalysts.
1, the acid catalyst
Acidic catalyst are commonly used (VO) 2P207, α VOHPO4, niobic acid and MFI zeolite. In the cationic active center of acidic catalysts (Brnsted or Lewis Center), the aldehyde carbonyl formation of enol carbonium ions resulting in condensation reaction. Pregnenolone balance acid catalysis can be expressed that the existing research, type, number and distribution of the surface acidity of the catalyst activity center will affect its catalytic performance, appropriate acidity can effectively promote the formation of carbenium ion vapor phase aldol reaction process, improve reaction activity.
Tanner (VO) using phosphoric acid vanadium oxide 2P2P7 and α VOHPO4 as catalyst, the self condensation of acetone and acetone, formaldehyde cross aldol condensation was studied, indicate that the acid active center for vanadium phosphate catalysts for the reaction with the catalyst, carbonyl groups on the surface of the rapid completion of the reaction process of protonation and nucleus plus.
Niobic acid using Paulis  (Nb2o5 · nH2O) as catalyst, acetone vapor phase aldol condensation reaction, reaction products that closely related species and catalyst acidity center of the acid strength and acid. Research shows that, niobic acid catalyst surface acid center bR nsted Acid strength is strong, the catalytic acetal and ketal reaction, and has excellent catalytic activity, selectivity and stability.
Dumitriu will have a MFI zeolite with different acidity for low-carbon aldehyde vapor phase aldol condensation reaction, by changing the proportion of Fe3+ Si in the catalyst, the acid strength and acidity regulation Br nsted Acid center, found that the enhanced surface acid strength, can promote low-carbon aldehyde vapor phase aldol condensation reaction, improve the conversion rate of reaction.
2, the alkaline catalyst
Alkaline catalyst used in the aldol condensation, which comprises a basic compound (alkali metal or alkaline earth metal oxides, hydroxides, bicarbonates, carbonates and carboxylates), organic amine compound and an anion exchange resin. Practical industrial applications, alkaline catalyst by aldol condensation reaction, can be a weak base (such as sodium carbonate, sodium bicarbonate, sodium acetate), can also be a strong base (such as sodium hydroxide, calcium hydroxide, sodium hydride, Sodium alkoxide Etc.). The former is usually used between more active aldehyde condensation, product for beta hydroxy compounds; the latter is used between the less active, large steric hindrance aldehyde or ketone condensation, and the reaction in polar aprotic solvents.
Alkali metal compound catalyst used in aldol condensation reaction in the preparation of hydroxy aldehyde, the product obtained by hydrogenation purification can get diol and polyols, such as 3 hydroxybutanal acetaldehyde condensation obtained response, choice of caustic soda solution as catalyst, the crude product can be obtained by catalytic hydrogenation of 1,3 butanediol. Similarly, formaldehyde and butyraldehyde cross aldol condensation to generate 2,2 dimethylol butyric, selection of sodium carbonate and sodium hydroxide solution as catalyst can reduce the side reaction, raise the reaction selectivity.
Lopez, NaBEA, KF/ respectively with bauxite and La2O3 solid catalyst, aldol condensation reaction of benzaldehyde and acetophenone catalyst deactivation mechanism was studied. Experiments show that, generated in the reaction process of benzoic acid can greatly reduce the rate of proton transfer reaction process, and the amine addition has little influence on the rate, so that the basic active sites of the catalyst can effectively catalyze the reaction, and the catalyst deactivation and loss associated with alkaline activity center. Organic amine is a kind of alkaline catalysts are widely used in the aldol condensation. For example, the condensation of formaldehyde and isobutyraldehyde generating hydroxypivalaldehyde reaction, using three triethylamine as the condensation catalyst, the condensation product after hydrogenation to obtain new Neopentyl glycol . Formaldehyde and butyraldehyde in three triethylamine catalyzed condensation and hydrogenation, produce high purity trimethylolpropane. Patent  reported an organic amine salt condensation catalyst, process for aldol condensation of 1,3 propylene glycol.
Anion exchange resin is a new alkaline catalyst. Alkali metal hydroxide solution traditional (such as nAOH, KOH) as catalyst, the presence of a catalyst to recovery, easy corrosion of equipment, production liquid and reaction separation, disadvantages of complex process, long production cycle, and anion exchange resin overcomes the defects in the foundation to maintain catalytic activity, get more and more attention of researchers.
Industrial 2,2 dimethylol propionic acid production mainly formaldehyde, propionaldehyde, in Inorganic alkali Catalysis or organic base, aldol condensation reaction of 2,2 dimethylol propionic aldehyde, obtained with H2O2 oxidation. New research shows that, in a fixed bed reactor, using silanized basic anion exchange resin catalyst, the reaction results depend on the size of spherical surface area of catalyst, activity groups, adsorption and desorption rate. [. The presence of a catalyst in solid form, to avoid a series of problems with alkali as the catalyst, but also ensures the conversion and selectivity of the reaction.
The industrial synthesis of 2 methyl 2 pentene aldehyde CAS also widely used in NaOH aqueous solution as catalyst, the yield was about 80-, but the NaOH water solution corrosion test equipment and the difficulty in separation of products. Tang Siping studied with anion exchange resin as catalyst, propionaldehyde bimolecular condensation preparation 2 methyl pentene aldehyde CAS 2 new processes, the target product of 2 methyl 2 pentene aldehyde CAS yield could reach 9354-.
There are many reports on the anion exchange resin used in aldol condensation reaction. Ou Zhize selected by experiments as the phase transfer catalyst three amine aminating strongly basic anion exchange resin, catalytic synthesis of benzylidene acetone, under optimized reaction conditions, benzylidene acetone yield up to 98-, and the catalyst can be used repeatedly. Strongly acidic styrene quaternary ammonium ion exchange resin as the catalyst and Hu Wei choice, acetone and formaldehyde condensation of acetyl ethanol, then dehydrated in the presence of oxalic acid methyl vinyl ketone. Shi Xiumin  the selected model anionic catalyst for catalytic distillation two acetone alcohol, i.e. macroporous strongly basic styrene anion resin, the catalytic activity and the selectivity is very high.
Aldol condensation is refers in dilute alkali or acid catalyzed condensation reaction, two aldehydes and ketones containing α -H molecules, resulting in beta hydroxy aldehydes and ketones, the reaction is its name. Containing α -H aldehyde, in dilute alkali conditions, can play the aldol reaction, condensation products after heating dehydration to acrolein, acrolein can be further polymerized resin like material color. Some details of the reaction can be found in University Organic Chemistry textbooks.
Aldol condensation is an important organic chemical reaction, it has extensive application in organic synthesis. Aldol condensation reaction is a compound containing active alpha hydrogen atoms such as aldehyde, ketone, carboxylic acid and ester, dear nucleophilic addition occurred under the action of catalyst and carbonyl compounds, the alpha hydroxy aldehydes and ketones or acid, or further dehydration to obtain alpha, beta unsaturated aldehyde ketone or ester reaction. Aldol condensation of molecules are often used to synthesize some beta hydroxy compounds, such as 1, 3- propanediol, l, 3- butanediol and neopentyl glycol. It can be used for further production of spices, drugs and other polymer or polyethylene terephthalate (PET), polyethylene terephthalate (PBT Ding Erzhi) and polyethylene terephthalate (PTT) monomer polymer; condensation dehydrated products of alpha, beta unsaturated aldehyde oxidation to obtain corresponding can be widely used as raw material of fine chemical production of carboxylic acid, such as 2, 2- dimethylol propionic acid can be used as waterborne polyurethane chain extender and preparation of polyester, photosensitive resin and liquid crystal, 2- methyl -2- pentenoic acid is a fruity flavor, can be widely used in food processing and the fragrance industry; in addition, alpha, beta unsaturated aldehyde hydrogenation was completely saturated primary aldehyde, used as a solvent or detergent, plasticizer.