Obtaining alkenes equation. Obtaining alcohols, application, properties

In organic chemistry, one can find hydrocarbon substances with different amounts of carbon in the chain and a C=C bond. They are homologues and are called alkenes. Because of their structure, they are chemically more reactive than alkanes. But what exactly are their reactions? Consider their distribution in nature, different methods of obtaining and application.

What are they?

Alkenes, which are also called olefins (oily), get their name from ethene chloride, a derivative of the first member of this group. All alkenes have at least one C=C double bond. C n H 2n is the formula of all olefins, and the name is formed from an alkane with the same number of carbons in the molecule, only the suffix -an changes to -ene. The Arabic numeral at the end of the name through a hyphen indicates the carbon number from which the double bond begins. Consider the main alkenes, the table will help you remember them:

If the molecules have a simple unbranched structure, then the suffix -ylene is added, this is also reflected in the table.

Where can they be found?

Since the reactivity of alkenes is very high, their representatives in nature are extremely rare. The principle of life of the olefin molecule is "let's be friends." There are no other substances around - it does not matter, we will be friends with each other, forming polymers.

But they exist, and a small number of representatives are included in the accompanying petroleum gas, and higher ones are in oil produced in Canada.

The very first representative of alkenes, ethene, is a hormone that stimulates the ripening of fruits; therefore, representatives of the flora synthesize it in small quantities. There is an alkene cis-9-tricosene, which in female houseflies plays the role of a sexual attractant. It is also called Muscalur. (Attractant - a substance of natural or synthetic origin, which causes attraction to the source of the smell in another organism). From the point of view of chemistry, this alkene looks like this:

Since all alkenes are very valuable raw materials, the methods for obtaining them artificially are very diverse. Let's consider the most common.

What if you need a lot?

In industry, the class of alkenes is mainly obtained by cracking, i.e. splitting of the molecule under the influence of high temperatures, higher alkanes. The reaction requires heating in the range from 400 to 700 °C. The alkane splits as it wants, forming alkenes, the methods for obtaining which we are considering, with a large number of variants of the structure of molecules:

C 7 H 16 -> CH 3 -CH \u003d CH 2 + C 4 H 10.

Another common method is called dehydrogenation, in which a hydrogen molecule is separated from a representative of the alkane series in the presence of a catalyst.

Under laboratory conditions, alkenes and methods of preparation are different, they are based on elimination reactions (elimination of a group of atoms without replacing them). Most often, water atoms are eliminated from alcohols, halogens, hydrogen or hydrogen halide. The most common way to obtain alkenes is from alcohols in the presence of an acid as a catalyst. It is possible to use other catalysts

All elimination reactions are subject to the Zaitsev rule, which says:

The hydrogen atom is split off from the carbon adjacent to the carbon bearing the -OH group, which has fewer hydrogens.

Applying the rule, answer which reaction product will prevail? Later you will know if you answered correctly.

Chemical properties

Alkenes actively react with substances, breaking their pi-bond (another name for the C=C bond). After all, it is not as strong as a single (sigma bond). An unsaturated hydrocarbon turns into a saturated one without forming other substances after the reaction (addition).

• addition of hydrogen (hydrogenation). The presence of a catalyst and heating is needed for its passage;
• addition of halogen molecules (halogenation). It is one of the qualitative reactions to a pi bond. After all, when alkenes react with bromine water, it becomes transparent from brown;
• reaction with hydrogen halides (hydrohalogenation);
• addition of water (hydration). The reaction conditions are heating and the presence of a catalyst (acid);

The reactions of unsymmetrical olefins with hydrogen halides and water follow the Markovnikov rule. This means that hydrogen will join that carbon from the carbon-carbon double bond, which already has more hydrogen atoms.

• combustion;
• partial oxidation catalytic. The product is cyclic oxides;
• Wagner reaction (oxidation with permanganate in a neutral medium). This alkene reaction is another high quality C=C bond. When flowing, the pink solution of potassium permanganate discolors. If the same reaction is carried out in a combined acidic medium, the products will be different (carboxylic acids, ketones, carbon dioxide);
• isomerization. All types are characteristic: cis- and trans-, double bond movement, cyclization, skeletal isomerization;
• polymerization is the main property of olefins for industry.

Application in medicine

The reaction products of alkenes are of great practical importance. Many of them are used in medicine. Glycerin is obtained from propene. This polyhydric alcohol is an excellent solvent, and if used instead of water, the solutions will be more concentrated. For medical purposes, alkaloids, thymol, iodine, bromine, etc. are dissolved in it. Glycerin is also used in the preparation of ointments, pastes and creams. It prevents them from drying out. By itself, glycerin is an antiseptic.

When reacting with hydrogen chloride, derivatives are obtained that are used as local anesthesia when applied to the skin, as well as for short-term anesthesia with minor surgical interventions, using inhalations.

Alkadienes are alkenes with two double bonds in one molecule. Their main use is the production of synthetic rubber, from which various heating pads and syringes, probes and catheters, gloves, nipples and much more are then made, which is simply indispensable when caring for the sick.

Application in industry

Alkenes and their derivatives have found wider application in industry. (Where and how alkenes are used, table above).

This is only a small part of the use of alkenes and their derivatives. Every year the need for olefins only increases, which means that the need for their production also increases.

1. From alkanes. Methane can be selectively oxidized on a heterogeneous catalyst - silver with a calculated amount of oxygen to methanol:

Alkanes with a large number of carbon atoms, such as propane and butane, are oxidized to a mixture of primary and secondary alcohols calculated amount of oxygen in the presence of catalysts - manganese salts. The reaction is not selective - it turns out quite a large amount of impurities: aldehydes and ketones with the same number of carbon atoms, aldehydes and alcohols - degradation products

2. From alkenes. Water can be added to any alkene in the presence of acids.

The addition follows the Markovnikov rule.

3. From alkynes. Acetylene and terminal alkynes react with formaldehyde, other aldehydes, and ketones to give primary, secondary, and tertiary alcohols, respectively.

4. From alkadienes. Alkadienes similar to alkenes join in the presence of acids water.

The addition of the first mole of water goes mainly to positions 1 - 4. When

addition of the second mole of water are formed diols. Below are examples of both

5. From alkyl halides. Halogen alkyls enter into the reaction of nucleophilic substitution of halogen for hydroxyl with aqueous solutions of alkalis:

6. From dihaloid derivatives. Under the action of alkalis on dihalogen derivatives of alkanes, dihydric alcohols (or diols) are obtained:

As shown above, 1,2-ethanediol (ethylene glycol) is obtained from 1,2-dibromoethane. This diol is very widely used in the production of antifreezes. For example, in antifreeze liquid for cooling internal combustion engines - "Tosol-A 40" its 40%.

7. From trihaloid derivatives. From 1,2,3-trichloropropane, for example, the widely used glycerol (1,2,3-propanetriol) is obtained.

8. From amines. When heated with water vapor in the presence of a catalyst, a reversible reaction occurs, in which the final products are alcohol with the same carbon skeleton structure and ammonia.

Primary amines can also be converted into alcohols by the action of sodium nitrite in hydrochloric acid when cooled to 2 - 5 ° C:

9. From aldehydes and ketones according to the reaction of Meerwein - Ponndorf - Verley. A ketone or aldehyde is reacted with an alcohol in the presence of a catalyst, aluminum alcoholate. As alkoxy groups take the remains of the same alcohol, which is taken as a reagent. For example, in the reaction below, aluminum tributoxide is taken along with normal butyl alcohol. The reaction is reversible and the equilibrium in it is shifted according to the Le Chatelier principle by an excess of the reagent alcohol.

The first publications about this reaction appeared almost simultaneously in two different German and one French chemical journals in 1925-1926. The reaction is of great importance, since it allows you to restore the carbonyl group to an alcohol without restoring double bonds, nitro and nitroso groups, which are converted by hydrogen and other reducing agents, respectively, into simple bonds and amino groups, for example:

As seen double bond, present in the ketone, preserved and in the resulting alcohol. It is shown below that when the keto group is hydrogenated, the double bond is simultaneously hydrogenated.

A similar picture is also observed in the presence of a nitro group in the ketone: in the Meerwein-Ponndorf-Werley reaction, it is preserved, and when hydrogenated on a catalyst, it is reduced to an amino group:

10. From aldehydes and ketones by hydrogenation on catalysts - platinum group metals: Ni, Pd, Pt:

11. Preparation of alcohols from aldehydes and ketones by Grignard syntheses.

The reactions discovered by François Auguste Victor Grignard in 1900-1920 are of great importance for the synthesis of many classes of organic substances. So, for example, with their help it is possible to obtain primary alcohol from any halide alkyl and formaldehyde in three stages:

To obtain secondary alcohol, it is necessary to take any other aldehyde instead of formaldehyde:

Upon hydrolysis of such a salt, an alcohol is obtained with the number of carbon atoms equal to the sum of them in an organomagnesium compound and in an aldehyde:

To obtain a tertiary alcohol, a ketone is used instead of an aldehyde in the synthesis:

12. From carboxylic acids alcohols can be obtained only in two stages: in the first, from a carboxylic acid, by the action of phosphorus pentachloride or by the action of sulfur (IV) oxide dichloride, its acid chloride is obtained:

At the second stage, the resulting acid chloride is hydrogenated on palladium to alcohol:

13. From alcoholates alcohols are very easily obtained by hydrolysis at room temperature:

Boric esters are more difficult to hydrolyze - only when heated:

Precipitates if it is more than 4g/100g H 2 O

14. From esters, alcohols along with carboxylic acids can be obtained by autocatalytic, acidic or alkaline hydrolysis. In the autocatalytic process, as a result of very slow hydrolysis with water, a weak carboxylic acid appears, which in the further course of the reaction plays the role of a catalyst, significantly accelerating the consumption of the ester and the appearance of alcohol over time. For example, for the reaction second-butyl ester of 2-methylpropanoic acid kinetic curves, that is, the dependence of the change in molar concentrations over time are sigmoids or S-shaped curves (see graph below the reaction).

15. If you add to an ester a strong acid, which is a catalyst, then

the reaction will not have an induction period when hydrolysis almost does not occur (from 0 to 1 time).

The kinetic curves in this case will be exponents: descending

for ester and ascending for alcohol. The process is called acid hydrolysis:

16. If you add alkali to ester(mole per mole or excess), then the reaction is also described by exponential kinetic curves, but unlike acid hydrolysis, where the concentrations of substances tend to equilibrium values, here the final concentration of alcohol is almost equal to the initial concentration of ether. Below is the reaction alkaline hydrolysis of the same ester and a graph with kinetic curves. As you can see, the alkali here is not a catalyst, but a reagent, and the reaction is irreversible:

17. From esters, alcohols you can also get according to Bouveau and Blanc. This method was first published by the authors in two different French chemical journals in 1903 and 1906 and consists in the reduction of esters with sodium in alcohol, for example:

As you can see, two alcohols are obtained in the reaction: one from the acid part of the ester and it is always primary, the second from the alcohol part and it can be anything - primary, secondary or tertiary.

18. A more modern way to get alcohols from esters lies in their reduction with complex hydrides to alcoholates (reaction (1)), which are then easily converted into alcohols by hydrolysis (reactions (2a) and (2b)), for example.

Lesson topic: Alkenes. Obtaining, chemical properties and application of alkenes.

Goals and objectives of the lesson:

• consider the specific chemical properties of ethylene and the general properties of alkenes;
• deepen and concretize the concepts of ?-bonds, the mechanisms of chemical reactions;
• give initial ideas about polymerization reactions and the structure of polymers;
• analyze laboratory and general industrial methods for obtaining alkenes;
• continue to develop the ability to work with a textbook.

Equipment: device for obtaining gases, KMnO 4 solution, ethyl alcohol, concentrated sulfuric acid, matches, spirit lamp, sand, tables "Structure of the molecule of ethylene", "Basic chemical properties of alkenes", demonstration samples "Polymers".

DURING THE CLASSES

I. Organizational moment

We continue to study the homologous series of alkenes. Today we have to consider the methods of obtaining, chemical properties and applications of alkenes. We must characterize the chemical properties due to the double bond, get an initial understanding of polymerization reactions, consider laboratory and industrial methods for obtaining alkenes.

II. Activation of students' knowledge

1. What hydrocarbons are called alkenes?

1. What are the features of their structure?

1. In what hybrid state are the carbon atoms that form a double bond in an alkene molecule?

Bottom line: alkenes differ from alkanes in the presence of one double bond in the molecules, which determines the features of the chemical properties of alkenes, methods for their preparation and use.

III. Learning new material

1. Methods for obtaining alkenes

Compose reaction equations confirming the methods for obtaining alkenes

– cracking of alkanes C 8 H 18 ––> C 4 H 8 + C 4 H 10 ; (thermal cracking at 400-700 o C)
octane butene butane
– dehydrogenation of alkanes C 4 H 10 ––> C 4 H 8 + H 2; (t, Ni)
butane butene hydrogen
– dehydrohalogenation of haloalkanes C 4 H 9 Cl + KOH ––> C 4 H 8 + KCl + H 2 O;
chlorobutane hydroxide butene chloride water
potassium potassium
– dehydrohalogenation of dihaloalkanes
- dehydration of alcohols C 2 H 5 OH -–> C 2 H 4 + H 2 O (when heated in the presence of concentrated sulfuric acid)
Remember! In the reactions of dehydrogenation, dehydration, dehydrohalogenation and dehalogenation, it must be remembered that hydrogen is predominantly detached from less hydrogenated carbon atoms (Zaitsev's rule, 1875)

2. Chemical properties of alkenes

The nature of the carbon - carbon bond determines the type of chemical reactions that organic substances enter into. The presence of a double carbon-carbon bond in the molecules of ethylene hydrocarbons determines the following features of these compounds:
- the presence of a double bond makes it possible to classify alkenes as unsaturated compounds. Their transformation into saturated ones is possible only as a result of addition reactions, which is the main feature of the chemical behavior of olefins;
- a double bond is a significant concentration of electron density, so the addition reactions are electrophilic in nature;
- a double bond consists of one - and one -bond, which is quite easily polarized.

Reaction equations characterizing the chemical properties of alkenes

a) Addition reactions

Remember! Substitution reactions are characteristic of alkanes and higher cycloalkanes having only single bonds, addition reactions are characteristic of alkenes, dienes and alkynes having double and triple bonds.

Remember! The following break-link mechanisms are possible:

a) if alkenes and the reagent are non-polar compounds, then the -bond breaks with the formation of a free radical:

H 2 C \u003d CH 2 + H: H -–> + +

b) if the alkene and the reagent are polar compounds, then breaking the bond leads to the formation of ions:

c) when connecting at the site of the break-bond of reagents containing hydrogen atoms in the molecule, hydrogen always attaches to a more hydrogenated carbon atom (Morkovnikov's rule, 1869).

- polymerization reaction nCH 2 = CH 2 ––> n – CH 2 – CH 2 ––> (– CH 2 – CH 2 –) n
ethene polyethylene

b) oxidation reaction

Laboratory experience. Obtain ethylene and study its properties (instruction on student desks)

Instructions for obtaining ethylene and experiments with it

1. Place 2 ml of concentrated sulfuric acid, 1 ml of alcohol and a small amount of sand into a test tube.
2. Close the test tube with a stopper with a gas outlet tube and heat it in the flame of an alcohol lamp.
3. Pass the escaping gas through a solution of potassium permanganate. Note the change in color of the solution.
4. Ignite the gas at the end of the gas tube. Pay attention to the color of the flame.

- Alkenes burn with a luminous flame. (Why?)

C 2 H 4 + 3O 2 -–> 2CO 2 + 2H 2 O (with complete oxidation, the reaction products are carbon dioxide and water)

Qualitative reaction: "mild oxidation (in aqueous solution)"

- alkenes decolorize a solution of potassium permanganate (Wagner reaction)

Under more severe conditions in an acidic environment, the reaction products can be carboxylic acids, for example (in the presence of acids):

CH 3 - CH \u003d CH 2 + 4 [O] -–> CH 3 COOH + HCOOH

– catalytic oxidation

Remember the main thing!

1. Unsaturated hydrocarbons actively enter into addition reactions.
2. The reactivity of alkenes is due to the fact that - the bond is easily broken under the action of reagents.
3. As a result of the addition, the transition of carbon atoms from sp 2 - to sp 3 - hybrid state occurs. The reaction product has a limiting character.
4. When ethylene, propylene and other alkenes are heated under pressure or in the presence of a catalyst, their individual molecules are combined into long chains - polymers. Polymers (polyethylene, polypropylene) are of great practical importance.

3. Use of alkenes(student's message according to the following plan).

1 - obtaining fuel with a high octane number;
2 - plastics;
3 - explosives;
4 - antifreeze;
5 - solvents;
6 - to accelerate the ripening of fruits;
7 - obtaining acetaldehyde;
8 - synthetic rubber.

III. Consolidation of the studied material

Homework:§§ 15, 16, ex. 1, 2, 3 p. 90, ex. 4, 5 p. 95.

The physical properties of alkenes are similar to those of alkanes, although they all have slightly lower melting and boiling points than the corresponding alkanes. For example, pentane has a boiling point of 36°C, while pentene-1 has a boiling point of 30°C. Under normal conditions, C 2 - C 4 alkenes are gases. C 5 - C 15 - liquids, starting with C 16 - solids. Alkenes are insoluble in water, soluble in organic solvents.

Alkenes are rare in nature. Since alkenes are valuable raw materials for industrial organic synthesis, many methods have been developed for their production.

1. The main industrial source of alkenes is the cracking of alkanes that make up oil:

3. Under laboratory conditions, alkenes are obtained by cleavage (elimination) reactions, in which two atoms or two groups of atoms are cleaved from neighboring carbon atoms, and an additional p-bond is formed. These reactions include the following.

1) Dehydration of alcohols occurs when they are heated with water-removing agents, for example, with sulfuric acid at temperatures above 150 ° C:

When H 2 O is cleaved from alcohols, HBr and HCl from alkyl halides, a hydrogen atom is predominantly cleaved off from that of the neighboring carbon atoms that is associated with the smallest number of hydrogen atoms (from the least hydrogenated carbon atom). This pattern is called Zaitsev's rule.

3) Dehalogenation occurs when dihalides having halogen atoms at neighboring carbon atoms are heated with active metals:

CH 2 Br -CHBr -CH 3 + Mg → CH 2 \u003d CH-CH 3 + Mg Br 2.

The chemical properties of alkenes are determined by the presence of a double bond in their molecules. The electron density of the p-bond is quite mobile and easily reacts with electrophilic particles. Therefore, many reactions of alkenes proceed according to the mechanism electrophilic addition, denoted by the symbol A E (from English, addition electrophilic). Electrophilic addition reactions are ionic processes that occur in several stages.

At the first stage, the electrophilic particle (most often it is the H + proton) interacts with the p-electrons of the double bond and forms a p-complex, which then turns into a carbocation by forming a covalent s-bond between the electrophilic particle and one of the carbon atoms:

alkene p-complex carbocation

At the second stage, the carbocation reacts with the anion X - , forming a second s-bond due to the electron pair of the anion:

The hydrogen ion in electrophilic addition reactions attaches to the carbon atom in the double bond, which has more negative charge. The charge distribution is determined by the displacement of the p-electron density under the influence of substituents: .

Electron-donor substituents that exhibit the +I effect shift the p-electron density to a more hydrogenated carbon atom and create a partial negative charge on it. This explains Markovnikov's rule: when polar molecules of the HX type (X = Hal, OH, CN, etc.) are attached to unsymmetrical alkenes, hydrogen preferentially attaches to the more hydrogenated carbon atom at the double bond.

Consider specific examples of addition reactions.

1) Hydrohalogenation. When alkenes interact with hydrogen halides (HCl, HBr), alkyl halides are formed:

CH 3 -CH \u003d CH 2 + HBr ® CH 3 -CHBr-CH 3.

The reaction products are determined by Markovnikov's rule.

However, it should be emphasized that in the presence of any organic peroxide, polar HX molecules do not react with alkenes according to the Markovnikov rule:

This is due to the fact that the presence of peroxide causes a radical rather than an ionic reaction mechanism.

2) Hydration. When alkenes interact with water in the presence of mineral acids (sulphuric, phosphoric), alcohols are formed. Mineral acids act as catalysts and are sources of protons. The addition of water also follows Markovnikov's rule:

CH 3 -CH \u003d CH 2 + HOH ® CH 3 -CH (OH) -CH 3.

3) Halogenation. Alkenes decolorize bromine water:

CH 2 \u003d CH 2 + Br 2 ® BrCH 2 -CH 2 Br.

This reaction is qualitative for a double bond.

4) Hydrogenation. Hydrogen addition occurs under the action of metal catalysts:

where R \u003d H, CH 3, Cl, C 6 H 5, etc. The CH 2 \u003d CHR molecule is called a monomer, the resulting compound is a polymer, the number n is the degree of polymerization.

Polymerization of various alkene derivatives gives valuable industrial products: polyethylene, polypropylene, polyvinyl chloride and others.

In addition to addition, alkenes are also characterized by oxidation reactions. With the mild oxidation of alkenes with an aqueous solution of potassium permanganate (Wagner reaction), dihydric alcohols are formed:

ZSN 2 \u003d CH 2 + 2KMn O 4 + 4H 2 O ® ZNOCH 2 -CH 2 OH + 2MnO 2 ↓ + 2KOH.

As a result of this reaction, the violet solution of potassium permanganate quickly becomes colorless and a brown precipitate of manganese oxide (IV) precipitates. This reaction, like the decolorization of bromine water, is qualitative for a double bond. During the hard oxidation of alkenes with a boiling solution of potassium permanganate in an acidic medium, a complete cleavage of the double bond occurs with the formation of ketones, carboxylic acids or CO 2, for example:

Oxidation products can be used to determine the position of the double bond in the starting alkene.

Like all other hydrocarbons, alkenes burn, and with abundant air they form carbon dioxide and water:

C n H 2 n + Zn / 2O 2 ® n CO 2 + n H 2 O.

With limited air access, the combustion of alkenes can lead to the formation of carbon monoxide and water:

C n H 2n + nO 2 ® nCO + nH 2 O.

If you mix alkene with oxygen and pass this mixture over a silver catalyst heated to 200 ° C, then alkene oxide (epoxyalkane) is formed, for example:

At any temperature, alkenes are oxidized by ozone (ozone is a stronger oxidizing agent than oxygen). If gaseous ozone is passed through a solution of an alkene in carbon tetrachloride at temperatures below room temperature, an addition reaction occurs and the corresponding ozonides (cyclic peroxides) are formed. Ozonides are very unstable and can explode easily. Therefore, they are usually not isolated, but immediately after they are decomposed with water - in this case, carbonyl compounds (aldehydes or ketones) are formed, the structure of which indicates the structure of the alkene subjected to ozonation.

Lower alkenes are important starting materials for industrial organic synthesis. From ethylene, ethyl alcohol, polyethylene, polystyrene are obtained. Propene is used for the synthesis of polypropylene, phenol, acetone, glycerin.

Alkenes- unsaturated hydrocarbons, which contain one double bond. Examples of alkenes:

Methods for obtaining alkenes.

1. Cracking of alkanes at 400-700°C. The reaction proceeds according to the free radical mechanism:

2. Dehydrogenation of alkanes:

3. Elimination reaction (cleavage): 2 atoms or 2 groups of atoms are cleaved off from neighboring carbon atoms, and a double bond is formed. These reactions include:

A) Dehydration of alcohols (heating above 150 ° C, with the participation of sulfuric acid as a water-removing reagent):

B) Cleavage of hydrogen halides when exposed to an alcoholic solution of alkali:

The hydrogen atom is split off mainly from the carbon atom that is associated with a smaller number of hydrogen atoms (the least hydrogenated atom) - Zaitsev's rule.

B) Dehalogenation:

Chemical properties of alkenes.

The properties of alkenes are determined by the presence of a multiple bond, therefore, alkenes enter into electrophilic addition reactions, which proceeds in several stages (H-X - reagent):

1st stage:

2nd stage:

.

The hydrogen ion in this type of reaction belongs to the carbon atom that has a more negative charge. The density distribution is:

If there is a donor as a substituent, which manifests itself as an +I- effect, then the electron density shifts towards the most hydrogenated carbon atom, creating a partially negative charge on it. The reactions go along Markovnikov's rule: when attaching polar molecules of the type HX (HCl, HCN, HOH etc.) for unsymmetrical alkenes, hydrogen is added preferentially to the more hydrogenated carbon atom at the double bond.

A) Addition reactions:
1) Hydrohalogenation:

The reaction proceeds according to Markovnikov's rule. But if peroxide is present in the reaction, then the rule is not taken into account:

2) Hydration. The reaction proceeds according to Markovnikov's rule in the presence of phosphoric or sulfuric acid:

3) Halogenation. As a result, bromine water becomes decolorized - this is a qualitative reaction to a multiple bond:

4) Hydrogenation. The reaction proceeds in the presence of catalysts.