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Acetylide

Alkaline earth alkyne

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Acetylide: the hydrogen atom connected to the sp hybrid carbon atom in the alkyne molecule is affected by the three bond. It is relatively active, belongs to the range of active hydrogen, and has weak acidity (pK a =25), which can be replaced by alkali metal or heavy metal atoms to generate metal alkynes.

Chinese name: Acetylide
Foreign name: acetylide
Brief description: The hydrogen linked to the triple bond in alkynes is replaced by metal

Principle: Hydrogen linked by alkyne bond has weak acidity
Common: Sodium diyne, silver acetylene, etc
Application: Organic synthesis

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▪ Reduction of acetylides

brief introduction

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Acetylide, the substance obtained by the replacement of hydrogen in the alkyne molecule that is connected to the three bond carbon with metal, is the acetylide, such as HC ∨ CNa, AgC ∨ CAg, CuC ∨ CCu, etc., for example, acetylene reacts with molten metal sodium at 101 ℃ to produce sodium acetylene (CH ∨ CNa); If the temperature is 190~220 ℃, sodium acetylene (NaC ∨ CNa), alkali metal and alkaline-earth metal The alkynyl compounds of N, N, N, N, N and N are decomposed into alkynes and metals by interaction with water hydroxide Heavy metal alkynides explode and decompose when heated or vibrated in a dry state, and are not dangerous in a wet state.

Most can be used for organic synthesis. Calcium carbide, an important chemical raw material, is a kind of alkaline earth metal alkyne.

Formation of acetylides

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The hydrogen atoms connected by the acetylene bonded carbon atoms have weak acidity (pK a =25), which can be replaced by metal to form alkyne. If acetylene is introduced silver nitrate Ammonia solution or Cuprous chloride White silver acetylene precipitation or brownish red cuprous acetylene precipitation are precipitated in the ammonia solution of

HC≡CH +2AgNO three +2NH four OH —→Ag—C≡C—Ag +NH four NO three +2H two O (silver acetylene is white)

HC≡CH +Cu two Cl two +2NH four OH—→Cu—C≡C—Cu +2NH four CI +2H two O (cuprous acetylene is brownish red)

These two reactions are sensitive and obvious, and can be used to identify acetylene and R-C ∨ CH alkynes. But R-C ∨ C-R alkynes can not carry out these two reactions.

Dry silver or cuprous alkynides are easy to explode to generate metal and carbon when heated or vibrated.

Ag—C≡C—Ag —→2Ag +2C +364kJ/mol

Therefore, after the test, concentrated hydrochloric acid should be added immediately to decompose the alkyne to avoid danger.

Ag—C≡C—Ag +2HCI —→HC≡CH +2AgCl↓

Cu—C≡C—Cu +2HCI —→HC≡CH +Cu two Cl two ↓

Acetylene and R-C ∨ CH alkynes neutralize with sodium amino in liquid ammonia to produce sodium alkyne:

2HC≡CH +2NaNH two —→2HC≡CH—Na +NH three

R—C≡CH +NaNH two —→R—C≡C—Na +NH three

Why is the hydrogen atom of acetylene more active than that of ethylene and ethane? This is because the C-H bond of acetylene is sp - s. Ethylene and ethane are respectively sp two One s, sp three One s; The electronegativity of carbon atoms hybridized by sp is greater than that of sp two And sp three In comparison with the hybrid carbon atom, the hydrogen alkyne is easy to dissociate into hydrogen protons, which is acidic and easy to be replaced by metals.^[1]

Preparation of sodium alkyne

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In acetylene and 1-alkyne (R-C ∨ CH) molecules, the hydrogen atom connected to the sp hybrid carbon atom is affected by the three bond, which is more active, belongs to the range of active hydrogen, and has weak acidity. It can be replaced by alkali metal or heavy metal atoms to form metal alkynes. For example, 1-alkyne can react with sodium amino in liquid ammonia to form sodium alkyne.

R—C≡CH+NaNH two - → R-C ∨ CNa+NH three Reaction conditions: liquid ammonia

Sodium amino It is a white solid with alkaline ratio sodium hydroxide But also strong. In a small amount of Fe 3+ In the presence of, it is prepared by reacting metal sodium directly with liquid ammonia.

2Na+2NH three —→2NaNH two +H two ↑ Reaction condition: Fe 3+ As catalyst

Sodium amino can easily absorb moisture in the air and decompose. In the laboratory, it is usually suspended in an inert medium or made into its liquid ammonia solution. Acetylene reacts with sodium amino to form a sodium salt of acetylene. Acetylene or sodium salt of acetylene can react with stronger alkali metal sodium at higher temperature to form disodium salt of acetylene.

CH≡CH +NaNH two —→CH≡CNa +NH three

CH≡CH +2Na—→NaC≡CNa +H two ↑ Reaction conditions: high temperature 190~200 ℃

Sodium alkyne reacts with primary haloalkanes, which can be used for the synthesis of higher alkynes. It is also one of the synthesis methods for increasing carbon chain.

For example:

R—C≡CNa +X—Rˊ —→R—C≡C—Rˊ +NaX^[2]

Synthetic route of alkyne

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Synthesis of primary halogenated alcohols

use Heptane (or toluene, etc.) continuous extraction The multiple methyl glycols can be converted into ω halo-1-alcohols by the method of. In mass production, the purification step is to first cool the ether solution of the crude product to filter out the crystalline glycols, and then use the filtrate (after removing the ether) Triethyl borate Treatment, extraction of ethanol triethyl borate Azeotropic mixture 。 The dihalide is evaporated in vacuum, and the residue is treated with methanol (or NaOH brine mixture) to regenerate halohydrin. Another mass purification method is to partition and extract the primary brominated alcohol with hexane and hydrous methanol. Dibasic alcohol and 1:2 equivalent PBr three The mixture is obtained by reaction in benzene, and the ratio of dibromide: diol: bromohydrin is 10:40:50. Separation from mixture（ diatom The total yield of brominated alcohol from soil column chromatography is about 35%. The above triethyl borate method can also be used for separation. Acetyl chloride (or acetyl bromide) and zinc chloride catalyst can also be used to Tetrahydropyran The ring is opened, and then the intermediate chlorohydrin acetate is hydrolyzed to produce 5-halopentyl alcohol.^[3]

Selection of reaction solvent

In order to overcome the difficulty of low solubility of higher haloalkanes and metal alkynes (especially sodium salts) in liquid ammonia, the method of using other solvents alone or as the latent solvent of liquid ammonia was studied. Under high speed stirring, bromoalkane reacts with lithium alkynide in liquid ammonia, and tetrahydrofuran is used as the latent solvent to obtain high yield. Excess lithium amino is often used as a base. Although commercial lithium amino can be utilized, the newly prepared one is more suitable. Under these conditions, the yield of product obtained by using alkyl chloride as reagent is generally low. Solvents with strong dipolarity that are inert to protons, such as Dimethylformamide 、 Dimethyl sulfoxide and Hexamethylphosphoryl triamine (HMPT) is a good solvent. Dimethylformamide can be used as solvent alone, or it can be mixed with dimethylformamide or Tetrahydrofuran (THF) mixed use. A more suitable general laboratory synthesis method is to use alkyl chloride or alkyl bromide to carry out lithium alkynide with HMPT tetrahydrofuran (or HMPT alone) as the solvent Alkylation reaction 。 The reaction proceeds quickly at - 25 ℃ (usually below 25 ℃), and the yield is generally high. Lithium alkynide is prepared with 1.6N butyl lithium (hexane solution) in THF at 0 ℃, or with lithium amino in THF (reaction is slower at 65 ℃). A large number of laboratories can use 90% (by weight) butyl lithium (reagent in hydrocarbon) for synthesis, which has the advantage of small reaction volume and high yield of alkylation products. The alkylation of 1-alkyne sodium salt in HMPT-THF has some shortcomings, that is, the dehydrobromination of bromoalkanes often occurs. Many alkynols synthesized by this method can be crystallized and purified in pentane at low temperature after the protective group is removed.^[3]

Reduction of acetylides

Part of alkyne catalytic hydrogenation Use Lindlar catalyst Pd CaCO three -Pb（OAc） two , with synthesis quinoline There is, usually, the corresponding Z olefin with high yield is obtained. Generally speaking, the use of nonpolar solvent pentane or hexane for this hydrogenation is superior to that of alcohol. When the reaction is carried out at room temperature, it usually contains only 1.5%~3.5% of the corresponding E isomer. If the reaction temperature is high (especially with alcohol as solvent), the percentage of E isomer will increase (usually to 5%~10%). Gutmann and Lindlar proposed that the selective catalytic hydrogenation of three bond compounds into z double bond compounds should be carried out at a lower temperature (10~20 ℃), sometimes in an ice bath. It is generally believed that it is in use Platinum catalyst The stereo configuration change of z-alkenes during hydrogenation usually occurs only in the presence of hydrogen, and does not occur until all alkynides are hydrogenated.

Usually, the method of preparing pure E isomer is to use metal sodium (or lithium) to reduce the alkyne in liquid ammonia. Generally, the ether liquid of alkynide is added to the mixture of sodium and liquid ammonia. Reversing the feeding order will make the three keys restore incompletely. When reducing high molecular weight compounds (carbon chain is greater than 13 carbon atoms), the amount of liquid ammonia should be increased (compared with the raw material alkyne). To avoid incomplete reduction due to low solubility. Generally, olefin products are very pure E isomers, containing only trace or undetectable Z isomers.^[3]

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