Chemical properties: Due to the strong electron-withdrawing property of oxygen, nucleophilic addition reaction is easy to occur on carbon atoms. Other common chemical reactions include nucleophilic reduction and aldol condensation.
Reaction: the reaction of α -hydrogen
(1) aldol condensation
(1) aldol condensation
Under the action of dilute alkali or dilute acid, two molecules of aldehydes or ketones can interact, in which α-hydrogen in one aldehyde (or ketone) molecule is added to carbonyl oxygen atom of another aldehyde (or ketone) molecule, and the rest is added to carbonyl carbon atom to generate one molecule of β-hydroxyaldehyde or one molecule of β-hydroxyketone. This reaction is called aldol condensation or aldol condensation. Through aldol condensation, new carbon-carbon bonds can be formed in molecules and carbon chains can grow.
Taking acetaldehyde as an example, the process of aldol condensation is as follows:
Step one, alkali combines with alpha-hydrogen in ether to form enol anion or negative carbon ion;
In the second step, this negative ion, as a nucleophile, immediately attacks the carbonyl carbon atom in another acetaldehyde molecule to generate an intermediate negative ion (alkoxy negative ion) after the addition reaction.
Thirdly, alkoxy anions react with water to obtain hydroxyaldehyde and OH.
Dilute acid can also change aldehyde into hydroxyaldehyde, but the reaction process is different. In the process of acid catalysis, firstly, the polarization of carbon-oxygen double bond is enhanced by protons, making it into enol form, and then an addition reaction takes place to obtain hydroxyaldehyde.
The α-hydrogen atom in the product molecule is activated by carbonyl and hydroxyl groups on β-carbon at the same time, so only a small amount of heat or acid is needed to dehydrate the molecule to produce α, β-unsaturated aldehyde.
All β -hydroxyaldehydes and ketones with hydrogen atoms on α -carbon are easy to lose a water molecule. This is because α -hydrogen is relatively active, and the dehydrated product has conjugated double bonds, so it is relatively stable.
Except acetaldehyde, aldol condensation products obtained from other aldehydes are all aldol or alkenal with branched chain on α-carbon atom. Aldehyde condensation reaction plays an important role in organic synthesis, which can be used to grow carbon chains and produce branched chains.
Ketones containing α-hydrogen can also carry out this kind of condensation reaction under the action of dilute alkali, but it is difficult to carry out the reaction due to the electronic effect and spatial effect. If you operate in the ordinary way, basically you can't get the product. Generally speaking, the reaction needs to be carried out under special conditions. For example, acetone can be converted into diacetone alcohol in the presence of alkali, but the yield in equilibrium system is very low. If the product can be separated from the alkali catalyst immediately after production, it can be separated from the equilibrium system, and finally more acetone can be converted into diacetone alcohol, and the yield can reach 70% ~ 80%. Catalyzed by iodine, diacetone alcohol can be dehydrated by heating to produce α, β-unsaturated ketone.
The condensation reaction between different aldehyde and ketone molecules is called cross aldol condensation. If the aldehydes and ketones used have α-hydrogen atoms, four products can be generated after the reaction, and the actual mixture is always complicated and has no practical value. Some aldehydes and ketones without α -hydrogen atoms do not undergo aldol condensation reaction (such as HCHO, RCCHO, ArCHO, RCCOCR, ArCOAr, ArCOCR, etc. ), but they can cross aldol condensation reaction with aldehydes and ketones containing α-hydrogen atoms, mainly the reaction between benzaldehyde and formaldehyde. The types of products are reduced, and condensation products are mainly obtained with high yield. In the product after the reaction, the aldehyde group with α -hydrogen atom must be retained. A single product can be obtained by maintaining excess formaldehyde without α -hydrogen atoms during the reaction. The aldol condensation reaction of aromatic aldehydes with aldehydes and ketones containing α-hydrogen atoms under the catalysis of alkali yields α, β-unsaturated aldehydes and ketones with high yield by dehydration, which is called Claessen-Schmidt condensation reaction. Under the catalysis of alkali, benzaldehyde can also be condensed with aliphatic ketones or aromatic ketones containing α -hydrogen atoms. In addition, some compounds containing active methylene, such as malonic acid, dimethyl malonate, ethyl α-nitroacetate, etc., can react with aldehydes and ketones similar to aldol condensation.
(2) Halogenation reaction on hydrocarbon group
(2) Halogenation reaction on hydrocarbon group
Due to the strong electron-withdrawing effect of carbonyl group, the α -hydrogen atom of aldehydes and ketones is easily replaced by halogen, resulting in α -halogenated aldehydes and ketones.
This reaction can be catalyzed by acids or bases. When acid is used as a catalyst, the product can be mainly mono-,di-or trihalogenated by controlling the reaction conditions (such as the amount of acid and halogen, reaction temperature, etc.). ).
The step that determines the whole reaction speed is the step of producing enol, that is, it depends on the concentration of acetone and acid and has nothing to do with the concentration of halogen.
The rate at which the generated monohalide continues to react with halogen decreases. This is because the electronegativity of halogen atoms is very large, which reduces the electron cloud density on the double bond of monohalide enol, so it is difficult to electrophilicly add with halogen. Therefore, the acid-catalyzed halogenation reaction often stops at the monohalide product.
In the alkali-catalyzed halogenation reaction, the step that determines the whole reaction speed is the step of generating negative carbon ions (enol anions), that is, the reaction speed is related to the concentration of acetone and alkali, and has nothing to do with the concentration of halogen.
When catalyzed by alkali, the reaction speed is very fast, so it is generally impossible to control the reaction at the stage of generating monohalide or dihalide. This is because when a halogen atom introduces α-carbon atom, because halogen is electron-withdrawing, α-hydrogen atom is more active, and it is easier to form new negative carbon ions, and the formed negative carbon ions are more stable, so the reaction of formula (1) is faster, which is also the reason why alkali catalysis in halogenated compounds is difficult to control.
When aldehydes or ketones (acetaldehyde and methyl ketone) with the structural formula CH3-C==O react with hypohalous acid or alkali halide solution, all three α -hydrogen atoms on the methyl group are replaced by halogen atoms to generate trihalogenated derivatives. However, due to the strong electron-withdrawing effect of halogen, this trihalogenated derivative greatly enhances the positive electricity of carbon. In the presence of alkali, the carbon-carbon bond breaks and decomposes to form trihalomethanes (commonly known as halomimetics) and carboxylates. Therefore, when the alkaline solution of sodium hypohalide reacts with acetaldehyde or ketone, all three hydrogen atoms of α-methyl are replaced by halogen atoms, and the carbon-carbon bond of trihalogen derivative generated during heating is broken, and the reaction of generating haloform and carboxylate is called haloform reaction. Because sodium hypohalide is an oxidant, it can oxidize alcohols with -CHOH-CH3 structure into aldehydes or ketones with -COCH3 structure. Therefore, any alcohol containing -CHOH-CH3 structure can also undergo halogen imitation reaction.
If sodium iodate (iodine plus sodium hydroxide) is used as a reagent, the reaction of forming yellow crystalline iodoform (CHI) which is insoluble in water and has a special smell is called iodoform reaction.
Therefore, this reaction is often used to identify aldehydes, ketones and alcohols with -coch-CH3 structure. People's Republic of China (PRC) (PRC) Pharmacopoeia uses this reaction to distinguish methanol from ethanol.
Halogenation of methyl ketone is a method to prepare carboxylic acid. In addition, because hypohalite does not interfere with double bonds, some unsaturated methyl ketones can also be converted into corresponding carboxylic acids through halogen imitation reaction.
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Reaction: nucleophilic addition reaction of carbonyl group
abstract
π bond in carbonyl group is similar to π bond in carbon-carbon double bond and easy to break, so similar to carbon-carbon double bond, carbonyl group can also undergo addition reaction by breaking π bond. Different from the carbon-carbon double bond, because the electronegativity of carbonyl oxygen atom is greater than that of carbon atom, the easily flowing π electrons are strongly pulled to the oxygen atom, and the oxygen atom of carbonyl group is rich in electrons, which makes the oxygen atom partially negatively charged, while the carbon atom of carbonyl group lacks electrons, which makes the carbon atom partially positively charged (). So carbonyl group is a polar group with a certain dipole moment, and the direction of dipole moment is from carbon to oxygen, so carbonyl group has two reaction centers, which appear on carbon atoms. Generally speaking, carbon atoms with partial positive charges are more chemically reactive than oxygen atoms with negative charges. Therefore, unlike the electrophilic addition reaction of carbon-carbon double bonds, the nucleophilic addition reaction of carbon-oxygen double bonds attacked by nucleophiles is most likely to occur. Generally, the nucleophilic moiety (Nu) of nucleophilic reagent (NuA) first attacks carbonyl carbon atoms, and then the positively charged electrophilic moiety (A) is added to carbonyl oxygen atoms. Therefore, the typical reaction of carbonyl group is nucleophilic addition reaction.
Add hydrocyanic acid (1)
Add hydrocyanic acid (1)
Aldehydes and ketones react with hydrocyanic acid to produce α -hydroxynitrile (also known as cyanohydrin).
The addition reaction between carbonyl group and hydrocyanic acid is very useful in organic synthesis and is one of the methods to grow carbon chains. Hydroxynitrile is an active compound which can be easily converted into other compounds, so it is an organic synthesis intermediate. For example, α-hydroxynitrile can be hydrolyzed into α-hydroxy acid and further dehydrated into α, β-unsaturated acid.
Acetone reacts with hydrocyanic acid in sodium hydroxide solution to generate acetone cyanohydrin, and acetone cyanohydrin reacts with methanol in the presence of sulfuric acid, that is, hydrolysis, esterification and dehydration, and the cyano group becomes methoxyacyl group, and finally methyl methacrylate is generated. Methyl methacrylate is polymerized to produce polymethylmethacrylate, that is, plexiglass.
Although hydrocyanic acid can be directly used as the reaction reagent when aldehydes and ketones are added with hydrocyanic acid, it is very volatile and toxic, so the operation should be particularly careful and it needs to be carried out in a fume hood. In order to avoid the direct use of hydrocyanic acid, aldehydes and ketones are often mixed with aqueous solution of potassium cyanide or sodium cyanide, and then sulfuric acid is slowly added to prepare cyanohydrin, so that HCN can be generated and reacted at the same time; Alternatively, aldehydes and ketones can react with sodium bisulfite first and then with sodium cyanide to prepare cyanohydrin.
(2) Grignard reagent addition
(2) Grignard reagent addition
In Grignard reagent, R can be regarded as negative carbon ion (R), which is similar to CN, OH and RO. Due to the strong nucleophilicity of negative carbon ions, Grignard reagents can react with most aldehydes and ketones to produce alcohols with more carbon atoms and new carbon frames.
Grignard reagent reacts with formaldehyde to generate primary alcohol, with other aldehydes to generate secondary alcohol, and Grignard reagent reacts with ketone to generate tertiary alcohol. However, when the volumes of the two alkyl groups in the ketone molecule and the alkyl groups in the Grignard reagent are both large, the addition of the Grignard reagent to carbonyl groups can be greatly slowed down due to the increase of steric hindrance, but on the contrary, the side reaction becomes important. For example, when diisopropyl ketone with large steric hindrance is added to tert-butyl magnesium bromide, there are two side reactions. One is that diisopropyl ketone alcoholizes to obtain magnesium compounds of enols. Another side reaction is that the carbonyl group is reduced to secondary alcohol, and the hydrocarbyl group in Grignard reagent loses hydrogen and becomes olefin. In this case, the addition product can still be obtained by replacing Grignard reagent with organic lithium compound with stronger activity, and the yield is high and it is easy to separate. The reaction of organolithium compounds with aldehydes and ketones is similar to that of Grignard reagent. For example, by reacting with aldehydes and ketones, secondary alcohols or tertiary alcohols are obtained respectively. Different from Grignard reagent, the addition products of organolithium compounds with sterically hindered ketones are still the main ones. Because Grignard reagent is a very active reagent, the first step of the reaction, that is, the addition of Grignard reagent and carbonyl group, must be carried out under absolutely anhydrous conditions. Generally, dry ether is used as solvent, and the existence of a very small amount of water will lead to the failure of the reaction.
(3) Add alcohol
(3) Add alcohol
At room temperature, carbonyl groups can react reversibly with hydroxyl groups to form hemiacetals and hemiketal;
C = O+HOR = = = C (or) (Oh)
In the presence of Lewis acids, the reaction can further take place to form acetals and ketals:
C(OR)(OH)+HOR ====C(OR)2
This reaction can be used to protect carbonyl groups.