Basic concepts of organic chemistry. Alkanes

Table 12.1.

Alkanes of the acyclopic series C n H2 n +2 .

Table 12.2.

Names of branched radicals.

Carbon, of course, cannot be attributed to the number of the most common chemical elements. In the earth's crust, it is only 0.12%. But it differs from all other elements in an exceptional variety of chemical compounds. The number of currently known compounds of carbon is more than twice the number of compounds of all other elements combined.

This peculiarity of carbon is explained by the special abilities of its atoms to form chemical bonds. As a rule, carbon is tetravalent. Its atoms can attach to each other to form more or less long chains, as well as rings.

The remaining free units of valence are easily saturated with hydrogen. The result is hydrocarbons. With the simplest of them - methane - we have already met. The next, more complex hydrocarbon is called ethane. Its molecule contains two carbon atoms and six hydrogen atoms. The addition of a third carbon atom and the saturation of free valences with hydrogen leads to the formation of propane with the formula C 3 H 8.

The next hydrocarbon with four carbon atoms is called butane and has the composition C 4 H 10 . It is also possible to make ever longer carbon chains. Members of the series with more than 100 carbon atoms are now known. Hydrocarbons from methane to butane are gaseous under normal conditions. Starting with pentane, which has five carbon atoms, they are liquids. Compounds containing 17 or more carbon atoms are solids at room temperature.

Hydrocarbons methane, ethane, propane, butane, etc. form a number of compounds that are very close to each other in structure and chemical properties. The table "Series of alkanes" shows the names and formulas of the most important members of this series. Obviously, each subsequent substance differs in composition from the previous one by the presence of an additional CH 2 group. Therefore, the general formula for hydrocarbons with n carbon atoms is СnН 2 n+2. Thus, the number of hydrogen atoms in a molecule is 2 more than twice the number of carbon atoms.

A number of alkanes

These two extra hydrogen atoms are at the ends of the carbon chain. Such a series of compounds is called a homologous series. The names of individual members of the given series of hydrocarbons end with the suffix "an", and together they are called alkanes.

Liquid and solid alkanes are found mainly in oil, as well as in tar obtained from brown coal. Alkanes with predominantly six to ten carbon atoms, such as octane, are part of gasoline. Liquid alkanes following them in the series are the main component of diesel fuel and lubricating oils. A mixture of solid hydrocarbons of this series is called paraffin.

Alkanes are known not only with a straight chain, but also with a branched carbon chain. For example, for a hydrocarbon C 4 H 10, two variants of the structure are possible:

For the following hydrocarbon C 5 H 12, three structures are already possible:

In the higher members of the series, the number of such forms increases rapidly. The calculation shows that for C 10 H 12 it is possible already 75, for C 13 H 28 - 802, and for C 20 H 42 - 366 319 different structures! Such compounds, which differ from each other only in structure, but not in composition, are called isomers. However, both in nature and in technology, hydrocarbons with a straight carbon chain are more common.

Lecture scheme

1. Nomenclature and isomerism.

2. Nomenclature.

3. Methods of obtaining.

4. Physical properties and structure.

5. Chemical properties.

6. Properties of a covalent bond.

7. Theory of molecular orbitals.

8. Hybridization.

9. Classification of organic reactions.

10. Classification of organic compounds.

1. Nomenclature and isomerism.

Definition : Organic compounds made up of carbon and hydrogen atoms are called hydrocarbons.

Limit saturated hydrocarbons, alkanes - are called organic compounds built from carbon and hydrogen atoms, in the molecules of which each carbon atom is connected to the neighboring carbon atom by no more than one bond (one valency). The valences not spent on the connection with carbon atoms are saturated with hydrogen. All carbon atoms are in the state sp 3 -hybridization.

Limit hydrocarbons form a homologous series with the general formula C n H 2n+2. The ancestor of the homologous series of alkanes is methane. The first ten members of the homologous series of alkanes are:

Methane, ethane, propane, butane, pentane, hexane. heptane, octane, nonane, decane .

A carbon atom in an alkane molecule, connected to no more than one neighboring carbon atom, is called primary. A carbon atom in an alkane molecule, connected to no more than two neighboring carbon atoms, is called secondary. A carbon atom in an alkane molecule connected to no more than three neighboring carbon atoms is called tertiary. The carbon atom in the alkane molecule, connected to four neighboring carbon atoms, is called quaternary.

Alkanes are characterized by structural isomerism. Starting with butane, the fourth member of the homologous series, several structures can correspond to one molecular formula:

Butane can have two isomers, pentane can have three, hexane can have five, and so on. The number of isomers for any homologue can be calculated from the formula if the number of isomers of the preceding member of the homologous series is known.

The structure of molecules of isomeric compounds can be represented as a carbon backbone to which monovalent groups or residues of alkane molecules are attached. Such remains have a special name.

Definition : The rest of the alkane molecule after the removal of a hydrogen atom is called a radical. In this case, an alkyl or alkyl radical.

Univalent radicals are named after the parent alkane molecule. In this case, the suffix "an" of saturated hydrocarbons - is replaced by "silt". For example:

Depending on which atom carries an unpaired electron, primary, secondary and tertiary radicals are distinguished. An unbranched primary radical is called normal and is denoted by the lowercase letter " n-».

The variety of structures of organic compounds is reflected in the nomenclature - a system of names in which each name corresponds to only one compound.

2. Nomenclature.

There are three nomenclatures in organic chemistry. But each name must match.

1. Trivial nomenclature is a system of historically established names, but still used today. These names are given in the early period of the development of organic chemistry and do not reflect the structure of molecules. Examples of trivial names are the names of the first four members of the homologous series of alkanes.

2. The structure of molecules is taken into account by rational nomenclature. The name of the first member of the homologous series is taken as the basis for the name of an organic compound. The remaining compounds are considered as its derivatives, in which hydrogen atoms are replaced by alkyl radicals. For example:

Alkyl radicals are listed in ascending order of mass. The normal radical is older than the isomeric radical. If there is more than one substituting radical, then their number is indicated by prefixes di-three-tetra- .

As the number of compounds grew, the use of rational nomenclature became inconvenient, and chemists switched to considering an atom or a group of atoms, but a structure formed by a chain of carbon atoms.

3. Consideration of the largest chain is carried out in a systematic nomenclature. The foundations of systematic nomenclature were laid at the Congress of Chemists in 1892 in Geneva. What gave the basis for the name of the nomenclature - Geneva. The systematic nomenclature was improved at the congress in 1930 in Liege. Liege nomenclature appeared. The modern version of the systematic nomenclature was adopted by the International Union for Pure and Applied Chemistry (IUPAC) in 1957. and improved in 1965.

In order to name an organic compound according to the systematic nomenclature, it is necessary:

Choose the longest (main) chain;

Decide on the seniority of groups;

Number the main chain, giving the highest group the smallest of the locant numbers;

List prefixes;

Compile the full name of the connection.

Limit, hydrocarbons of the methane series (alkanes)

Alkanes, or paraffins, are aliphatic saturated hydrocarbons, in the molecules of which carbon atoms are linked by a simple s -communication. The remaining valences of the carbon atom, not spent on bonding with other carbon atoms, are completely saturated with hydrogen. Therefore, saturated hydrocarbons contain the maximum number of hydrogen atoms in the molecule.

Hydrocarbons of a number of alkanes have the general formula C n H 2n+2. The table shows some representatives of a number of alkanes and some of their physical properties.

The table shows that these hydrocarbons differ from each other in the number of groups - CH 2 -. Such a series of similar in structure, having similar chemical properties and differing from each other in the number of these groups is called a homologous series. And the substances that make it up are called homologues .

Simulator No. 1 - Homologs and isomers

Trainer number 2. - Homologous series of saturated hydrocarbons

Physical properties

The first four members of the homologous series of methane are gaseous substances, starting with pentane are liquids, and hydrocarbons with a number of carbon atoms of 16 and above are solids (at ordinary temperature). Alkanes are non-polar compounds and are difficult to polarize. They are lighter than water and practically insoluble in it. They also do not dissolve in other solvents with high polarity. Liquid alkanes are good solvents for many organic substances. Methane and ethane, as well as higher alkanes, are odorless. Alkanes are combustible substances. Methane burns with a colorless flame.

Obtaining alkanes

To obtain alkanes, mainly natural sources are used.

Gaseous alkanes are obtained from natural and associated petroleum gases, and solid alkanes from oil. A natural mixture of solid high molecular weight alkanes is mountain wax - natural bitumen.

1. From simple substances:

n C+2 n H 2 500 °С, cat → FROM n H 2 n + 2

2. The action of metallic sodium on halogen derivatives of alkanes A.Wurtz reaction:

2CH 3 -Cl + 2Na → CH 3 -CH 3 + 2NaCl

Chemical properties of alkanes

1. Substitution reactions - Halogenation (staged)

CH 4 + Cl 2 hν → CH 3 Cl (chloromethane) + HCl (1 stage);

methane

CH 3 Cl + Cl 2 hν→ CH 2 Cl 2 (dichloromethane) + HCl (stage 2);

C H 2 Cl 2 + Cl 2 hν → CHCl 3 (trichloromethane) + HCl (stage 3);

CHCl 3 + Cl 2 hν → CCl 4 (chloromethane) + HCl (stage 4).

2. Combustion reactions (burn with a light non-smoking flame)

C n H 2n+2 + O 2 t → nCO 2 + (n+1)H 2 O

3. Decomposition reactions

a) Cracking at a temperature of 700-1000 ° C, (-С-С-) bonds are broken:

C 10 H 22 → C 5 H 12 + C 5 H 10

b) Pyrolysis at a temperature of 1000 ° C, all bonds are broken, the products are C (soot) and H 2:

C H 4 1000°С → C + 2 H 2

Application

· Limit hydrocarbons are widely used in a wide variety of areas of human life and activity.

· Use as a fuel - in boiler plants, gasoline, diesel fuel, aviation fuel, propane-butane mixture cylinders for household stoves

· Vaseline is used in medicine, perfumery, cosmetics, higher alkanes are part of lubricating oils, alkane compounds are used as refrigerants in home refrigerators

· A mixture of isomeric pentanes and hexanes is called petroleum ether and is used as a solvent. Cyclohexane is also widely used as a solvent and for polymer synthesis.

· Methane is used to make tires and paint

· The importance of alkanes in the modern world is enormous. In the petrochemical industry, saturated hydrocarbons are the basis for obtaining a variety of organic compounds, an important raw material in the processes for obtaining intermediate products for the production of plastics, rubbers, synthetic fibers, detergents and many other substances. Great value in medicine, perfumery and cosmetics.

Tasks for fixing

No. 1. Write the equations for the combustion reactions of ethane and butane.

№2. Write the reaction equations for the production of butane from the following haloalkanes:

CH 3 - Cl (chloromethane) and C 2 H 5 - I (iodoethane).

Number 3. Carry out the transformations according to the scheme, name the products:

C → CH 4 → CH 3 Cl → C 2 H 6 → CO 2

No. 4. Solve the crossword

Horizontally:

1. An alkane having the molecular formula C 3 H 8 .
2. The simplest representative of saturated hydrocarbons.
3. French chemist, whose name bears the reaction of obtaining hydrocarbons with a longer carbon chain by the interaction of halogen derivatives of saturated hydrocarbons with metallic sodium.
4. A geometric figure that resembles the spatial structure of the methane molecule.
5. Trichloromethane.
6. The name of the radical C 2 H 5 -.
7. The most characteristic type of reactions for alkanes.
8. Aggregate state of the first four representatives of alkanes under normal conditions.

If you answered the questions correctly, then in the highlighted column vertically get one of the names of saturated hydrocarbons. Name this word?

.
Alkanes - the name of saturated hydrocarbons according to the international nomenclature. Paraffins are the historically preserved name for saturated hydrocarbons.

In the molecules of these compounds, all valence bonds of carbon and hydrogen are completely saturated. That is why these hydrocarbons are not capable of addition reactions. In this regard, this class of hydrocarbons can be defined as follows:
Hydrocarbons with the general formula C n H 2n+2 that do not add hydrogen and other elements are called saturated hydrocarbons or alkanes (paraffins).

The simplest representative of saturated hydrocarbons is methane.

The structure of the methane molecule.

The molecular formula of methane is CH 4 .
Since hybridization is involved s- electron and three p- electron, then this type of it is called sp 3 - hybridization.
Valence angle: 109 degrees.

Methane homologues.

There are many hydrocarbons similar to methane, i.e. methane homologues (Greek "homolog" - similar). Molecules contain two, three, four or more carbon atoms. Each subsequent hydrocarbon differs from the previous one by a group of atoms CH 2. For example, if you mentally add a CH 2 group to a CH 4 methane molecule (the CH 2 group is called a homological difference), then the next hydrocarbon of the methane series is obtained - ethane C 2 H 6, etc.

The homologous rad of methane.

CH 4 - Methane

C 2 H 6 - Ethane

C 3 H 8 - Propane

C 4 H 10 - Butane

C 5 H 12 - Pentane

C 6 H 14 - Hexane

C 7 H 16 - Heptane

C 9 H 20 - Nonan

Isomerism and nomenclature.

To compile the names of saturated branched chain hydrocarbons, it is assumed that in all molecules the hydrogen atoms are replaced by various radicals. To determine the names of a given hydrocarbon, a certain order is followed:

1. The longest carbon chain is chosen in the formula and the symbols of carbon atoms are numbered, starting from the end of the chain, to which the branching is closer.
2. They name the radicals (starting with the simplest) and use numbers to indicate the place at the numbered carbon atoms. If the same carbon atom has two identical radicals, then the number is repeated twice. The number of identical radicals is indicated using numbers in Greek ("di" - two, "three" - three, "tetra" - four, etc.)
3. The full name of this hydrocarbon is given by the number of carbon atoms in the numbered chain.

Finding in nature.

The simplest representative of saturated hydrocarbons is methane- is formed in nature as a result of the decomposition of the remains of plant and animal organisms without air access. This explains the appearance of gas bubbles in swampy water bodies. Sometimes methane is released from coal seams and accumulates in mines. Methane makes up the bulk of natural gas ( 80 -97% ). It is also found in gases released during oil production. The composition of natural gas and petroleum gases also includes ethane C 2 H 6 , propane C 3 H 8 , butane C 4 H 10 and some others. Gaseous, liquid and solid saturated hydrocarbons are contained in oil.

physical properties.

Methane is a colorless and odorless gas, almost 2 times lighter than air, slightly soluble in water. Ethane, propane, butane under normal conditions are gases, from pentane to pentadecane are liquids, and the following homologues are solids.
With an increase in the relative molecular masses of saturated hydrocarbons, their boiling and melting points naturally increase.