Alkyne

Hydrocarbons with carbon carbon triple bonds in their molecules

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Alkyne, which contains carbon carbon triple bond in the molecule hydrocarbon It is a general term for unsaturated aliphatic hydrocarbons. The general formula of linear alkynes is C n H two n -2 (where n is a non-1 positive integer), simple alkyne compounds include acetylene （C two H two ）， Propargyl （C three H four ）Etc. Among them, acetylene is the most important alkyne, which can be used for lighting, welding and cutting off metals in industry（ Oxyacetylene flame ）It is also the basic raw material for manufacturing acetaldehyde, acetic acid, benzene, synthetic rubber, synthetic fiber, etc.

Chinese name: Alkyne
Foreign name: alkyne
Category: hydrocarbon

functional group: triple bond
General molecular formula: C n H two n -2 (of which n Is a non 1 positive integer)
solubility: Insoluble in water, soluble in organic solvents
Melting boiling point: Alkanes and alkenes higher than the same carbon atom number

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Introduction to alkynes

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Alkynes are a class Organic compound , of Unsaturated hydrocarbon 。 Its functional group is carbon carbon triple bond (- C ∨ C -). General formula C of straight chain alkynes n H two n -2 , where n It is a positive integer ≥ 2. Simple alkyne compounds include acetylene （C two H two ）、 Propargyl （C three H four ）Etc. Alkynes are also called calcium carbide, which is usually used to refer to the simplest acetylene among alkynes.

The word "acetylene" is a new word, and the word "fire" on the left is taken from the word "carbon", which means it can burn; The 夬夬夬夬夬夬 on the right is taken from the word "lack", which indicates the number of hydrogen atoms and valence ratio olefin More scarce means that alkynes are unsaturated derivatives of alkanes (intact) and alkenes (rare).

The melting point, boiling point and density of simple alkynes are higher than those with the same carbon atom number alkane or olefin Higher. Insoluble in water, soluble in Ether 、 benzene 、 carbon tetrachloride And other organic solvents. Alkynes can occur with halogen, hydrogen, hydrogen halide and water Addition reaction , can also occur polymerization 。^[1] Because acetylene gives off a lot of heat when burning, it is often used as raw material for welding.

Structure of alkynes

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The 2s orbital of the carbon atom of alkyne hybridizes with the same 2p orbital, forming two identical sp hybrid orbitals. The pile is distributed on both sides of the carbon atom, and the included angle between the two is 180 degrees.

One sp hybrid orbital of acetylene carbon atom forms hydrocarbon with 1s orbital of hydrogen atom σ Bond, another sp hybrid orbital forms carbon carbon with the sp hybrid orbital of the connected carbon atom σ Bond, an acetylene molecule that forms a linear structure. The two unhybridized p orbitals and the two p orbitals of another carbon are parallel to each other and overlap "side by side" to form two mutually perpendicular pi bond 。^[2]

Structure determination

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Organic Key length available electron diffraction , microwave, infrared or Raman spectra Determine. ethane 、 ethylene and acetylene The carbon carbon bond length and hydrocarbon bond length in are as follows:

Bond length in ethylene molecule

Bond length in ethane molecule

Bond length in acetylene molecule

Because of the appearance of π bond, the distance between carbon and carbon is shortened, and the triple bond is shorter than the double bond. This is because with Unsaturation The electron cloud density between the two carbon atoms also increases with the increase of, so the carbon atoms are getting closer and closer. The bond length of hydrocarbon bond in hydrocarbon is also not a constant. This shows that bond length is not only related to the unsaturation of bonding atoms, but also related to the hybridization mode of carbon atoms involved in bonding. That is, with the increase of s component in the hybrid orbital, the bond length of carbon carbon bond shortens. The s component of carbon atom in ethane, ethylene and acetylene is 25%, 33% and 50% respectively three To sp, the s component of carbon atom doubled, so the bond length of carbon carbon bond became shorter and shorter.

Because the s composition of hybrid carbon atoms is different, propane 、 propylene 、 Propargyl The bond length of the carbon carbon single bond in is unequal. The more the s component is, the shorter the bond length of the carbon carbon single bond is. With the shortening of the bond length, the bond energy between atoms will increase.

physical property

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Alkyne It has low melting and boiling point, low density, is insoluble in water and soluble in organic solvents, and generally changes with the increase of carbon atoms in the molecule. Alkynes are slightly more soluble in water than alkanes and alkenes. Acetylene, acetylene, and 1-butylene are weakly polar, slightly soluble in water, and easily soluble in non-polar solutions in alkynes with the same carbon frame. The three bonds have low polarity at the end of the chain. Alkynes have dipole moments, and alkynes with more alkyl branches are more stable.

Solubility: slightly soluble in ethanol, soluble in acetone, chloroform and benzene.

The boiling point, melting point and density of simple alkynes are generally higher than those of alkanes and alkenes with the same carbon atom number. This is because alkyne molecules are short and slender. In liquid and solid state, molecules can be very close to each other Van der Waals Very strong.

Names and physical properties of some common alkynes
chemical compound	Melting point/℃	Boiling point/℃	relative density
acetylene	-82 (under pressure)	-82 (sublimation)	—
Propargyl	-102.5	-23	—
1-Butyne	-122	eight	—
1-Pentyne	-98	forty	zero point six nine five
1-hexyne	-124	seventy-one	zero point seven one nine
1-heptyne	-80	one hundred	zero point seven three three
1-Octyne	-70	one hundred and twenty-six	zero point seven four seven
2-Butyne	-24	twenty-seven	zero point six nine four
2-Pentyne	-101	fifty-six	zero point seven one four
2-hexyne	-88	eighty-four	zero point seven three zero
3-hexyne	-105	eighty-one	zero point seven two five

chemical property

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Hazard characteristics

Extremely flammable and explosive. It can form explosive mixture when mixed with air, and can cause combustion and explosion when encountering open fire and high heat. React violently in contact with oxidants. Violent chemical reaction will occur in contact with fluorine, chlorine, etc. It can generate explosive substances with copper, silver, mercury and other compounds.

Addition reaction

Electrophilic addition

Alkynes can occur Electrophilic addition reaction However, due to the electronegativity ratio of sp carbon atom two The strong electronegativity of the carbon atom makes the electron and the sp carbon atom combine more closely. Although the triple bond has more electrons than the double bond, it is not easy to give the electron and Electrophilic reagent Thus, the electrophilic addition reaction of triple bond is slower than that of double bond.

Alkynes can react with two molecular electrophilic reagents. First, it reacts with one molecule reagent to generate olefin derivatives, and then reacts with another molecule reagent to generate saturated compounds. The addition of asymmetric reagents to alkynes also follows the Markov rule, and most of the addition is trans addition.

Addition with halogen

The addition of halogen and alkyne is trans addition. The reaction mechanism is similar to the addition of halogen and olefin, but the reaction is generally more difficult than that of olefin. For example, olefins can immediately fade the carbon tetrachloride solution of bromine, while alkynes can take several minutes to fade it. Therefore, there are non conjugated double bonds and triple bonds in the molecule. When it reacts with bromine, the addition of double bonds is the first step.

For another example, the addition reaction of acetylene and chlorine must be carried out under the catalysis of light or ferric chloride or stannous chloride, the intermediate product is anti dichloroethylene, and the final product is 1,1,2,2-tetrachloroethane (CHCl two CHCl two ）。

Addition with halogen acid

The addition reaction of alkynes and hydrohalides is carried out in two steps. The reaction can be controlled in the first step by selecting appropriate reaction conditions. This is also a method for preparing halogenated alkenes.

The addition reaction of monosubstituted acetylene with hydrohalide acid follows Markov rule 。

When there are substituents on both sides of the alkyne bond, it is necessary to compare the Conjugate effect and Induced effect , to determine the regioselectivity of the reaction, but generally we get a mixture of two isomers.

Addition with water

Mercury salts are commonly used as catalysts for addition of alkynes to water. For example, the addition of acetylene and water is between 10% sulfuric acid and 5% Mercurous sulfate Occurring in aqueous solution.

Water is first added to the triple bond to form a very unstable adduct - enol. Enols are quickly isomerized to form stable Carbonyl compound 。

The addition of alkynes and water follows the Markov rule, so all the adducts of substituted acetylene and water are ketones except acetylene, but the adduct of monosubstituted acetylene and water is methyl ketone, and the water product of binary substituted acetylene is usually a mixture of two ketones.

Free radical addition

In the presence of peroxide, alkynes and hydrogen bromide undergo radical addition reaction to obtain Anti Markov rule The product of.

Addition with hydrocyanic acid

Hydrocyanic acid can undergo nucleophilic addition reaction with acetylene. CN in reaction - Nucleophilic addition formation restricted by three bonds Carbonium anion And then react with the proton to complete the reaction of forming acrylonitrile. Due to the high cost of acetylene, the ammoxidation of propylene to acrylonitrile is almost adopted in the world. The reaction process is that the mixture of propylene and ammonia is oxidized with air under the catalysis at 400~500 ℃.

Hydrogenation

Alkynes can be mixed with organic compounds with the following "active hydrogen", such as - OH, - SH, - NH two ，=NH，—CONH two Or - COOH is subject to addition reaction to produce products containing double bonds. For example, ethanol reacts with acetylene at 150~180 ℃ and 0.1~1.5MPa under alkali catalysis to generate Vinyl ether 。

The reaction conditions (i.e. temperature, pressure, catalyst, etc.) can be different depending on the raw materials. The reaction mechanism of this kind of reaction is the nucleophilic addition of alkoxy anion and triple bond to produce a carbonic anion intermediate, which gets protons from alcohol molecules to obtain products.

reduction

Catalytic hydrogenation: under the action of palladium, platinum or nickel, alkynes and 2mol H two Add to form alkanes. It is difficult to separate the intermediate products.

If Lindlar catalyst is used (palladium is attached to calcium carbonate and a small amount of lead oxide, reducing the activity of the catalyst) catalytic hydrogenation , then alkyne only adds 1 mol H two Z alkene is obtained. For example, a natural stearic acid containing three bonds will generate natural cis type oleic acid Identical product.

Palladium catalyst supported by barium sulfate can also make carbon carbon triple bond compound add only 1 mol H in pyridine two To produce cis olefin derivatives. This shows that the activity of the catalyst has a decisive influence on the product of catalytic hydrogenation. Catalytic hydrogenation of alkynes is an important method to prepare Z-type alkenes, which has a wide range of applications in synthesis.

Borohydride alkynes and Ethane borane The reaction produces alkenyl borane, which reacts with acetic acid React to generate Z-type alkenes. The first step is alkyne Borohydride reaction The second step is the reduction reaction of alkenyl boron, which is generally called borohydride reduction reaction.

Alkali metal reducing alkyne compounds are reduced with metal sodium in liquid ammonia, mainly producing E-type olefin derivatives.

Reduction of alkynes with lithium aluminum hydride: E-type alkenes can also be obtained by reduction of alkynes with lithium aluminum hydride.

oxidation

When alkynes are oxidized by ozone or potassium permanganate, the carbon carbon triple bond can be broken to form two carboxylic acid 。

In the presence of water and potassium permanganate, under mild conditions: PH=7.5, RC ∨ CR '→ RCO-OCR'

Severe condition: at 100 ° C, RC ∨ CR '→ RCOOH+R'COOH

CH≡CR →CO two + RCOOH

Alkynes react with ozone to form ozone oxide, which is hydrolyzed to form α— Diketone and peroxide, then peroxide will α－ Diketone is oxidized to carboxylic acid.

terminal alkyne

C of C ∨ C in alkynes is sp hybrid, which makes Csp-H σ Keyed Electronic cloud Closer to the carbon atom, the polarity of the C-H bond is enhanced, making the hydrogen atom easy to dissociate, showing "acidity".

Electronegativity: sp>sp two >sp three , the order of acidity is acetylene>ethylene>ethane.

The hydrogen atom connected to the carbon atom of C ∨ C is quite active and easy to be replaced by metals, so the alkyne metal derivatives are called alkynides.

CH≡CH + Na → CH≡CNa + 1/2H two ↑ (condition NH three )

CH≡CH + 2Na → CNa≡CNa + H two ↑ (condition NH three ，190℃~220℃）CH≡CH + NaNH two → CH≡CNa + NH three ↑

CH≡CH + Cu two Cl two （2AgCl） → CCu≡CCu（CAg≡CAg）↓ + 2NH four Cl +2NH three (Note: it only occurs when there is a hydrogen atom on the triple bond, which is used to identify the terminal alkyne RH ∨ CH).

polymerization

Acetylene will polymerize: 2CH ∨ CH → CH two =CH-C ∨ CH (vinyl acetylene)+CH ∨ CH → CH two =CH-C≡C-CH=CH two (divinyl acetylene)^[1]

Under the action of different catalysts, acetylene can be selectively polymerized to form chain or Cyclic compound 。 For example, under the action of cuprous chloride or ammonium chloride, dimerization or trimerization can occur to generate benzene. However, the yield of benzene in this reaction is very low, and many other aromatic by-products are also produced, so it has no preparation value, but it provides a powerful clue for the study of the structure of benzene.

In addition to the trimer ring, acetylene can be polymerized in tetrahydrofuran catalyzed by nickel cyanide at 1.5~2MPa and 50 ℃ to produce Cyclooctetraene 。 This compound is used to understand Aromatic compound In the process, it plays a great role. In the past, it was thought that acetylene could not react under pressure, because it was easy to explode after pressure. Later, it was found that acetylene can be safely reacted under pressure by diluting it with nitrogen, thus opening up many new reactions of acetylene and preparing many important compounds. Cyclooctetraene is one of them.

Structural identification

When acetylene is introduced into silver ammonia solution or cuprous ammonia solution, white and reddish brown acetylides precipitate respectively.

Other terminal alkynes can also undergo the above reaction, so we can identify the - C ∨ CH contained in the molecule through the above reaction Group 。

Like the oxidation of alkynes, alkynes can be identified according to the color change of potassium permanganate solution, and the structure of protoalkynes can be inferred from the structure of the product obtained.

preparation

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Alkynes are generally prepared by o-dihalogenated alkanes Dehalogenated hydrogen It can also be prepared by the reaction of metal alkyne compounds with primary haloalkanes. In Fritsch Buttenberg Wiechell rearrangement, alkynes are initially prepared by brominating vinyl.

Alkynes can also be made from aldehydes by Corey Fuchs reaction, or by Seyferth Gilbert assimilation.

Acetylene production uses coal or oil as raw materials, which are two main ways to produce acetylene. With the development of natural gas chemical industry, natural gas will soon become the main source of acetylene.

Arc method

Methane is heated in an arc at 1500 ℃ for a very short time (0.1~0.01s) and cracked into acetylene, namely:

2CH four →C two H two +3H two ， Δ H =397.4KJ/mol

Because acetylene decomposes into carbon quickly at high temperature, the reaction gas must be cooled quickly with water, and the acetylene yield is about 15%. If the reaction gas is cooled by air flow, the acetylene yield can be increased by 25%~30%. The cracking gas also contains ethylene, hydrogen and carbon dust. The general feature of this method is that the raw materials are very cheap, and it is economical to use this method in areas rich in natural gas. Naphtha can also be used to produce acetylene in this way.^[1]

Carbide method

Calcium carbide (CaC) is prepared by heating coke and calcium oxide to 2200 ℃ through electric arc two ）, it reacts with water to generate acetylene and calcium hydroxide:

CaO+3C<—2200℃—>CaC two +CO， Δ H =460kJ/mol

CaC two +2H two O——>C two H two +Ca(OH) two

The cost of this method is high. Except for a few countries, this method is not used.^[1]

Plasma method

Acetylene is prepared by hot cracking of petroleum and extremely hot hydrogen, that is, hydrogen is heated in an arc at 3500~4000 ℃, and then part of the plasma hydrogen (positive and negative ions are equal) reacts with gaseous or gasified petroleum gas in the separation reaction chamber at the outlet of the arc heater. The products generated are acetylene and ethylene (the total yield of both is more than 70%) And methane and hydrogen.

Acetylene used to be a very important raw material for organic synthesis. Due to the high production cost of acetylene, the route of using acetylene as raw material to produce chemicals has gradually been replaced by the route of using other compounds (especially ethylene and propylene) as raw materials.

Pure acetylene is a gas with ether smell, which has anesthetic effect. When burning, the flame is bright and can be used for lighting. The unpleasant smell of industrial acetylene is caused by impurities such as hydrogen sulfide, phosphine, organic phosphorus and sulfur compounds. Unlike ethylene and ethane, acetylene has a certain solubility in water, but is easily soluble in acetone. Liquefied acetylene can explode violently after collision and heating. When acetylene is mixed with air, it will explode violently when its content reaches 3~70%. In order to handle acetylene safely in commerce, it is put into a steel cylinder with porous materials, such as diatomite, pumice or charcoal, and then into acetone. Acetone can dissolve about 25 times its volume of acetylene under normal pressure, and 300 times its volume under 1.2 MPa. The combustion of acetylene and oxygen can produce a high temperature of 2800 ℃, which is used for welding or cutting steel and other metals.^[1]

matters needing attention

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1. Acetylene is different from alkanes. Alkynes are unstable and very active. Acetylene combustion generates a lot of heat. Acetylene flame is often used for welding.

2. After drying, the alkynide will explode strongly after impact, and generate metal and carbon. Therefore, at the end of the reaction, dilute nitric acid should be added to decompose it.

4. Acetylene is unstable and very active. Acetylene shall be stored away from heat.

5. It is forbidden to prepare strong oxidant, strong acid and halogen for acetylene.

6. Acetylene can form explosive mixture when mixed with air, and can cause combustion and explosion when encountering open fire and high heat.

7. Acetylene can form explosive substances with copper, silver, mercury and other compounds.

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