3.4. Characteristic chemical properties of hydrocarbons: alkanes, cycloalkanes, alkenes, dienes, alkynes, aromatic hydrocarbons (benzene and toluene).

The oxidation of alkanes can be carried out at both high and low temperatures. The vigorous oxidation of these substances by atmospheric oxygen at high temperatures, which leads to the complete destruction of alkane molecules and the formation of $CO_2$ and $H_2O$, is called combustion. This process is important because it occurs in all internal combustion engines and is the basis for extracting energy from fuel.

The oxidation of saturated hydrocarbons goes through the stage of formation of hydroperoxides with the -$OOH$ group. In internal combustion engines, hydrocarbons of normal structure form peroxides, which cause premature combustion of gasoline vapors without the participation of a spark plug. This phenomenon is called detonation. It causes great harm, as it leads to premature wear of engines, and also does not allow full use of its power. Saturated hydrocarbons with a branched structure do not detonate. Isooctane is especially valuable as a motor fuel:

Are you an expert in this subject area? We invite you to become the author of the Directory Working Conditions

2, 2, 4-trimethylpentane (isooctane)

Isooctane was used as the basis for a conventional scale for assessing motor fuels by “octane number”. Isooctane in this scale was assigned an octane number of 100, and n-heptane - 0. Gasolines are compared with this scale, that is, with a mixture of isooctane and n-heptane. For example, gasoline with an octane number of 72 burns as a mixture that consists of 72% isooctane and 28% n-heptane.

Decarboxylation of carboxylic acid salts (Dumas reaction)

The Dumas reaction is the interaction of salts of carboxylic acids with alkalis during fusion.

R–COONa + NaOH → R–H + Na 2 CO 3

Decarboxylation is the removal (elimination) of a carbon dioxide molecule from a carboxyl group (-COOH) or an organic acid or carboxylate group (-COOMe) of a salt of an organic acid.

When sodium pentanoate reacts with sodium hydroxide during fusion, butane and sodium carbonate are formed:

CH3–CH2–CH2– CH2 –COONa + NaOH → CH3–CH2 – CH2 – CH3 + Na 2 CO 3

Chemical properties of alkynes

Alkynes are unsaturated (unsaturated) hydrocarbons and therefore are capable of undergoing addition reactions. Among the addition reactions for alkynes, electrophilic addition is the most common.

Halogenation

Since the triple bond of alkyne molecules consists of one stronger sigma bond and two weaker pi bonds, they are capable of attaching either one or two halogen molecules. The addition of two halogen molecules by one alkyne molecule proceeds through an electrophilic mechanism sequentially in two stages:

Hydrohalogenation

The addition of hydrogen halide molecules also occurs via an electrophilic mechanism and in two stages. In both stages, the accession proceeds in accordance with Markovnikov’s rule:

Hydration

The addition of water to alkynes occurs in the presence of ruti salts in an acidic medium and is called the Kucherov reaction.

As a result of hydration, the addition of water to acetylene produces acetaldehyde (acetic aldehyde):

For acetylene homologues, the addition of water leads to the formation of ketones:

Hydrogenation of alkynes

Alkynes react with hydrogen in two steps. Metals such as platinum, palladium, and nickel are used as catalysts:

Trimerization of alkynes

When acetylene is passed over activated carbon at high temperature, a mixture of various products is formed from it, the main of which is benzene, a product of acetylene trimerization:

Dimerization of alkynes

Acetylene also undergoes a dimerization reaction. The process takes place in the presence of copper salts as catalysts:

Alkyne oxidation

Alkynes burn in oxygen:

СnH2n-2 + (3n-1)/2 O2 → nCO2 + (n-1)H2O

Reaction of alkynes with bases

Alkynes with a triple C≡C at the end of the molecule, unlike other alkynes, are able to enter into reactions in which the hydrogen atom at the triple bond is replaced by a metal. For example, acetylene reacts with sodium amide in liquid ammonia:

HC≡CH + 2NaNH2 → NaC≡CNa + 2NH3,

and also with an ammonia solution of silver oxide, forming insoluble salt-like substances called acetylenides:

Thanks to this reaction, it is possible to recognize alkynes with a terminal triple bond, as well as to isolate such an alkyne from a mixture with other alkynes.

It should be noted that all silver and copper acetylenides are explosive substances.

Acetylenides are capable of reacting with halogen derivatives, which is used in the synthesis of more complex organic compounds with a triple bond:

CH3-C≡CH + NaNH2 → CH3-C≡CNa + NH3

CH3-C≡CNa + CH3Br → CH3-C≡C-CH3 + NaBr

Butane plus water catalyst

Members of the homologous series of alkanes share common chemical properties. Under normal conditions, alkanes are chemically inert. They are resistant to the action of many reagents: they do not interact with concentrated sulfuric and nitric acids, with concentrated and molten alkalis, and are not oxidized by strong oxidizing agents - KMnO4, etc.

Video experiment “The ratio of methane to a solution of potassium permanganate and bromine water”

The chemical stability of alkanes is explained by the high strength of the σ bonds C ─ C and C ─ H, as well as their nonpolarity. Non-polar bonds C ─ C and C ─ H in alkanes are not prone to ionic cleavage, but are capable of being homolytically cleaved under the action of active free radicals. Therefore, alkanes are characterized by radical reactions ( substitution reactions ), which result in compounds where hydrogen atoms are replaced by other atoms or groups of atoms.

Alkanes enter into reactions that occur via a free radical (chain) mechanism and usually occur in the light or when heated. According to this mechanism, hydrogen atoms are most easily replaced at tertiary, then secondary and primary carbon atoms. During chlorination, this pattern is not observed at T>400˚C.

Alkanes are hydrocarbons that do not contain multiple bonds. Due to the extreme nature of alkanes, addition reactions are not typical for them.

Substitution reactions (breaking C ─ H bonds)

1. Halogenation (replacement of a hydrogen atom with a halogen atom - F, Cl, Br with the formation of a haloalkane).

The halogenation reaction of alkanes proceeds by a radical chain mechanism, i.e. as a chain of successive transformations involving free radical particles.

The theory of chain reactions was developed by the Soviet scientist, one of the founders of chemical physics, Academician N.N. Semenov (1896-1986), for which he was awarded the Nobel Prize in 1956.

The reaction rate depends on the activity of the halogens, which decreases with increasing atomic radius.

Alkanes react very actively with fluorine; the reaction of alkanes with its participation is accompanied by an explosion and oxidation to CF4.

Chlorination and bromination reactions occur under the influence of light (photochemical chain reaction) or at 300–400 o C.

Iodination is reversible, so an oxidizing agent is required to remove HI from the reaction.

Lower alkanes (CH4, C2H6, C3H8) can be completely chlorinated. In a methane molecule, chlorine atoms can replace from one to four hydrogen atoms, depending on the ratio of the reagents

(hv - light quantum formula)

The trivial name for trichloromethane is chloroform .

Reaction mechanism

The reaction occurs in three stages:

A) Initiation (chain nucleation) – homolytic cleavage of the Cl2 molecule with the formation of free chlorine radicals:

Free radicals are atoms or groups of atoms with unpaired electrons (•Cl, •H, •CH3...)

B) Chain development ( interaction of a chlorine radical with an alkane molecule, a methyl radical with a new chlorine molecule, etc.):

C) Chain termination (occurs when two radicals combine with each other):

The rate of the reaction of replacing hydrogen with a halogen atom in haloalkanes is higher than that of the corresponding alkane, this is due to the mutual influence of the atoms in the molecule:

The electron density of the C–Cl bond is shifted to the more electronegative chlorine atom, as a result of which a partial negative charge is formed on it, and a partial positive charge is formed on the carbon atom.

A lack of electron density is created on the carbon atom in the methyl group (-CH3), so it absorbs electron density from neighboring hydrogen atoms, as a result of which the C–H bonds become less strong and hydrogen atoms are more easily replaced by chlorine atoms.

As the hydrocarbon radical increases, the most mobile hydrogen atoms remain at the carbon atom closest to the substituent:

Alkanes discolor bromine solution when heated, entering into a radical substitution reaction.

Video experiment “Interaction of alkanes with bromine when heated”

2. Nitration (replacement of a hydrogen atom with a nitro group – NO2 with the formation of nitroalkanes R-NO2). The nitrating reagent is dilute nitric acid HNO3 (HO─NO2).

Nitration with dilute nitric acid at t = 140 0 C and at elevated or normal pressure - reaction of M.I. Konovalov.

As a result of the reaction, a mixture of isomeric nitro compounds is formed. Hydrogen atoms are most easily replaced at the tertiary carbon atom, more difficult at the secondary, and most difficult at the primary:

3. Sulfonation (replacement of hydrogen atoms with the sulfonic group SO3H to form alkanesulfonic acids RSO3H). The sulfonating reagent is sulfuric acid H2SO4 (HO─SO3H). Sulfonation of alkanes occurs under the action of very concentrated H2SO4 with slight heating.

The hydrogen atom at the tertiary carbon atom is most easily replaced:

Oxidation reactions

Alkanes are compounds with low oxidation states of carbon and, depending on the reaction conditions, they can be oxidized to form various compounds.

Under normal conditions, alkanes are resistant to strong oxidizing agents (KMnO4, K2Cr2O7).

1. Combustion (oxidation by air oxygen at high temperatures)

A) Complete oxidation (excess O2)

With an excess of oxygen, complete oxidation of alkanes occurs to CO2, where carbon has the highest oxidation state of +4, and water. The combustion of hydrocarbons leads to the rupture of all C–C and C–H bonds and is accompanied by the release of a large amount of heat (exothermic reaction).

Lower homologues (methane, ethane, propane, butane) form explosive mixtures with air, which must be taken into account when using them. As the molecular weight increases, alkanes are more difficult to ignite.

Video experiment “Explosion of a mixture of methane and oxygen”

Video experiment “Combustion of liquid alkanes”

Video experiment “Combustion of solid hydrocarbons (using the example of paraffin)”

The combustion process of hydrocarbons is widely used to produce energy (in internal combustion engines, thermal power plants, etc.).

General view of the combustion reaction of alkanes:

B) Incomplete oxidation (lack of O2)

When higher alkanes ((n >>1)) burn with a lack of oxygen, partial oxidation products are formed: carbon monoxide CO (carbon oxidation state +2), soot (fine carbon, with oxidation state 0).

Therefore, higher alkanes burn in air with a smoky flame, releasing toxic carbon monoxide, which is dangerous to humans.

Combustion of methane with a lack of oxygen occurs according to the equations:

The latter reaction is used industrially to produce soot from natural gas containing 80-97% methane.

2. Catalytic oxidation

Partial oxidation of alkanes at a relatively low temperature and with the use of catalysts is accompanied by the rupture of only part of the C–C and C–H bonds and is used to obtain valuable products: carboxylic acids, ketones, aldehydes, and alcohols.

For example, with incomplete oxidation of butane, the bond (C2–C3) is broken and two molecules of acetic acid are obtained:

Acetic acid is produced industrially using this method.

By mild oxidation of methane with atmospheric oxygen in the presence of catalysts, methyl alcohol, formaldehyde and formic acid can be obtained.

Higher alkanes (n>25) under the influence of atmospheric oxygen in the liquid phase in the presence of manganese salts are converted into a mixture of carboxylic acids with a medium chain length of C12–C18, which are used to produce detergents and surfactants.

Educational film “Catalytic oxidation of higher paraffin hydrocarbons”

Thermal transformations of alkanes (decomposition reactions)

1. Cracking (eng. cracking - splitting) of alkanes is the basis of oil refining in order to obtain products of lower molecular weight, which are used as motor fuels, lubricating oils, as well as raw materials for the chemical and petrochemical industries.

To carry out this process, two methods are used: thermal cracking (heating without air access) and catalytic cracking (more moderate heating in the presence of a catalyst).

Thermal cracking is the breaking of C ─ C bonds in alkanes with long carbon chains, resulting in the formation of alkanes and alkenes with fewer carbon atoms.

Thermal cracking (pyrolysis) is carried out at a temperature of 450 – 700 0 C:

Cracking n-hexane (work by Alexey Litvishko, Samara)

Catalytic cracking is carried out in the presence of catalysts (usually aluminum and silicon oxides) at a temperature of 500 0 C and atmospheric pressure. In this case, with the rupture of molecules, an isomerization and dehydrogenation reaction occurs.

Octane cracking (work by Alexey Litvishko, Samara)

2. When methane or ethane is heated to a temperature of 1000 0 C, pyrolysis - decomposition into simple substances:

The carbon obtained in this way is quite pure, in technology it is called soot and is used, for example, in the production of car tires.

3. Conversion of methane with the formation of synthesis gas (CO + H2)

The reaction of interaction of methane with water vapor is important, as a result of which a mixture of carbon monoxide (II) with hydrogen is formed - “synthesis gas” (water gas, generator gas):

This reaction is used to produce hydrogen. Synthesis gas serves as a raw material for the production of various hydrocarbons.

Elimination reactions

1. Dehydrogenation (elimination of hydrogen; occurs as a result of the rupture of C ─ H bonds; carried out in the presence of a catalyst at elevated temperatures).

When alkanes are passed over a catalyst (Pt, Pd, Ni, Al23, Cr23) at high temperatures (400 - 600°C), a hydrogen molecule is eliminated and an alkene is formed:

2. If methane is heated to a higher temperature (1500 0 C) and quickly cooled, then intermolecular dehydrogenation and ethylene (acetylene) is formed:

3. Dehydrocyclization (aromatization) is a dehydrogenation reaction that leads to the closure of the chain into a stable cycle.

Alkanes containing more than 4 carbon atoms in the main chain are used to prepare cyclic compounds.

If the main chain of an alkane molecule contains 5 (but not more) carbon atoms (n-pentane and its alkyl derivatives), then at a temperature of 300 0 C above the Pt catalyst, hydrogen atoms are split off from the terminal atoms of the carbon chain and a five-membered cycle is formed (cyclopentane or its derivatives):

Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives:

Chemical properties of alkadienes

Addition reactions

For example, the addition of halogens:

Bromine water becomes discolored.

Under normal conditions, the addition of halogen atoms occurs at the ends of the 1,3-butadiene molecule, while the π-bonds are broken, bromine atoms are added to the extreme carbon atoms, and the free valences form a new π-bond. Thus, a “movement” of the double bond occurs. If there is an excess of bromine, another molecule can be added at the site of the formed double bond.

Polymerization reactions

Combustion of alkanes

The combustion of alkanes serves as the main source of heat (energy) for vehicles (aviation, motor vehicles, marine transport), power plants, and household heating devices. High calorific value, absence of ash (see Table 9-2), ease of transportation and supply of fuel to the combustion site place alkanes so far beyond competition among energy sources. Let us show the amount of energy released during the combustion of alkanes using methane as an example. It has been experimentally established that when methane is burned at 250 °C, about 900 kJ/mol is released:

CH4 (gas) + O2 (gas) → CO2 (gas) + 2H2O (liquid)

ΔН = -890.3 kJ/mol

For the case when water is in a gaseous state, it is necessary to make a correction for the heat of evaporation of water (43.5 kJ/mol at 25 °C):

The magnitude of the thermal effect of the reaction, calculated from the values ​​of the bond breaking energy (see Table 1-9), is equal to

The calculated and experimentally found values ​​of the thermal effect are quite close, that is, for comparative evaluation purposes, the data in Tables 1-9 can be used when calculating ΔH .

However, in fact, at 25 °C, the combustion of alkanes cannot begin spontaneously, since the C-H , C-C , 0=0 bonds are too strong to be broken by thermal vibrations. Initiation is carried out by heating a mixture of alkane and oxygen to 300 °C .

In the gas phase, at a high concentration of vapors, the chain reaction occurs with an explosion, since breaking the chain under these conditions is difficult. This circumstance must always be kept in mind from a safety point of view, therefore the use of open fire in enclosed spaces saturated with alkane vapors is strictly prohibited.

The high exothermicity of the combustion process further supports oxidation-combustion with self-acceleration. The very high reaction rate makes the combustion process difficult to control. The theory and practice of combustion processes were developed by N.N. Semyonov and his students.

Possible transitions between organic compounds

Black gold deposits often contain cycloalkanes or naphthenes. Processing of raw materials produces saturated cyclic hydrocarbons containing 5–7 C atoms in the ring; they are of greatest practical importance. How to obtain a cycloalkane from an alkane if naphthene reserves are depleted? To obtain saturated cyclic hydrocarbons from saturated acyclic compounds, the dehydrocyclization method is used. Chains of 4 or more C atoms close, and a stable cycle arises. Other typical transformations of organic substances can be reflected in simple diagrams:

• Oil → hydrocarbons → alkanes → carboxylic acids.
• Natural gas → saturated hydrocarbons → carboxylic acids.
• Coal → hydrocarbons → alkanes → unsaturated hydrocarbons → polymers.
• Oil → hydrocarbons → arenes → benzene → isopropyl benzene → acetone, phenol.
• Natural gas → unsaturated hydrocarbons → ethanol.
• Hard coal → methanol.
• Oil → hydrocarbons → alkenes → butadiene and isoprene.

Let us take a closer look at what chemical compounds can be obtained due to the genetic relatedness of organic substances.

Some features of oxidation in the Unified State Examination, with which we do not entirely agree.

We consider those “rules”, principles and reactions that will be discussed in this section to be not entirely correct. They contradict not only the real state of affairs (chemistry as a science), but also the internal logic of the school curriculum and the Unified State Exam in particular.

But nevertheless, we are forced to provide this material exactly in the form required by the Unified State Exam.

We are talking specifically about HARD oxidation.

Remember how benzene homologues and their derivatives are oxidized under harsh conditions? The radicals are all terminated and carboxyl groups are formed. The scraps undergo oxidation “on their own”:

So, if suddenly a hydroxyl group or multiple bond appears in the radical, you need to forget that there is a benzene ring there. The reaction will proceed ONLY through this functional group (or multiple bond).

The functional group and multiple bond are more important than the benzene ring.

Let's look at the oxidation of each substance:

First substance:

You need to ignore the fact that there is a benzene ring. From the point of view of the Unified State Examination, this is just secondary alcohol

Secondary alcohols are oxidized to ketones, but ketones are not oxidized further:

Let this substance be oxidized with dichromate:

Second substance:

This substance is oxidized simply as a compound with a double bond (we do not pay attention to the benzene ring):

Let it oxidize in neutral permanganate when heated:

The resulting alkali is enough to completely neutralize carbon dioxide:

2KOH + CO2 → K2CO3 + H2O

Final equation:

Oxidation of the third substance:

Let oxidation proceed with potassium permanganate in an acidic environment:

Oxidation of the fourth substance:

Let it oxidize in a highly alkaline environment. The reaction equation will be:

And finally, this is how vinylbenzene is oxidized:

And it oxidizes to benzoic acid, you need to keep in mind that according to the logic of the Unified State Examination, it oxidizes this way not because it is a benzene derivative. But because it contains a double bond.

Option 2

Exercise 1 . Which of the substances shown are isomers? Write down their formulas and name them. Are there other isomers of this composition?

a) CH3-CH2-CH2-CH2-CH3

b) CH3-CH=CH-CH2-CH3

c) CH3-C(CH3)=CH-CH3

d) CH3-CH=C(CH3)-CH3

e) CH3-C(CH3)H-CH=CH2

e) CH3-C≡C-CH2-CH3

Task 2. Make up formulas for all possible isomers of butene. Name them.

When preparing for work, you must repeat:

What is homology? What substances are called homologues? What is a homological difference? General formulas of homologous series of hydrocarbons. What is isomerism? What substances are isomers?

Numerals that are roots in the formation of the names of hydrocarbon molecules. Suffixes showing the presence of simple, double, triple bonds between carbon atoms and their location in the hydrocarbon molecule. What is a radical and how is it shown in the name of a substance?

Liquid phase oxidation

Liquid-phase oxidation of alkanes is carried out industrially at 200 °C in the presence of manganese catalysts (A. N. Bashkirov, 1959). The main purpose of this process is the oxidation of alkanes to higher alcohols of the limiting series, which are then converted by the action of H2SO4 into alkyl sulfates - synthetic detergents.

In this case, the C-C bonds are simultaneously broken with the formation of alcohols with a shorter carbon chain length compared to the original alkane. The theoretical foundations of liquid-phase oxidation of alkanes were developed by N. M. Emanuel and E. T. Denisov, students of N. N. Semenov [61; 62].

Reactions of phenol on the benzene ring

2.1. Halogenation

Ammonia, preparation, properties, chemical reactions

2.2. Nitration

Under the influence of 20% nitric acid HNO3, phenol is easily converted into a mixture of ortho- and para-nitrophenols.