Organic Chemistry: It is completely optional to know by heart, to cram the properties of organic substances.
As you study, you will understand that the properties of a substance are determined by its structure and learn how to easily predict the class of reactions.
In this regard, of particular interest is the theory of chemical structure, which was created by A.M. Butlerov in 1861. It includes several key points:
- Atoms in a molecule are connected in a certain sequence, in accordance with their valency. The bond order of atoms reflects the chemical structure.
- Knowing the properties of substances, one can establish their chemical structure, and vice versa, knowing the structure of a substance, one can draw a conclusion about its properties.
- Atoms or groups of atoms mutually influence each other directly or through other atoms
- The properties of a substance depend on the quantitative and qualitative composition, as well as on the chemical structure of the molecule.
Alkanes (paraffin) are saturated hydrocarbons having a linear or branched structure, containing only simple bonds. Relate to aliphatic hydrocarbons, as they do not contain aromatic bonds.
Alkanes are saturated compounds – contain the maximum possible number of hydrogen atoms. The general formula for their homologous series is CnH2n+2.
Nomenclature of Alkanes
Nomenclature (from lat. Nomen – name + calare – to convene) – a set of names of individual chemicals, as well as the rules for the compilation of these names. Alkane names are formed by adding the suffix “an”: methane, ethane, propane, butane, etc.
Substances are similar in structure and properties but differing in one or more groups of CH2.
The alkanes listed above are homologous to each other, that is, they form one homological series (Greek homólogos – corresponding).
The names of alkanes are formed according to several rules. If you know them, you can skip this point, however, I must introduce the reader to them. So, the algorithm for compiling the names is as follows:
In the structural formula of a substance, it is necessary to choose the longest (albeit curved in the figure!) Chain of carbon atoms
The atoms of the selected chain are numbered starting from the end to which the branching (radical) is closer.
In the beginning, the name lists radicals and other substituents indicating the numbers of carbon atoms with which they are associated. If there are several identical radicals in a molecule, then the numbers indicate the presence of each of them in the main chain and particles of di-, tri-, tetra-, etc.
The name is based on the name of a saturated hydrocarbon with the same number of carbon atoms as in the main chain
Carefully study the names compiled for various substances below.
Several types of carbon atoms are distinguished in a hydrocarbon chain, depending on how many other carbon atoms the given atom is connected to. There are primary, secondary, tertiary and quaternary carbon atoms.
Isomers (Greek isomeros – made up of equal parts) are substances that have one molecular formula but differ in structure (structural isomerism) or in the arrangement of atoms in space (spatial isomerism).
Isomerism is structural (interclass, carbon skeleton, the position of a functional group or bond) and spatial (geometric, optical). As you study the classes of organic substances, you will learn about all of these species.
In alkane molecules, there are no functional groups, multiple bonds. For alkanes, only the carbon skeleton isomerism is possible. So, pentane C5H12 has 3 structural isomers.
Some data regarding alkanes must be learned:
- Alkane molecules contain single sigma bonds (σ bonds), the length of which is 0.154 nm
- Type of hybridization of carbon atoms – sp3
- The valence angle (between chemical bonds) is 109°28′
Natural gas and oil
Alkanes are part of natural gas: methane 80-97%, ethane 0.5-4%, propane 0.2-1.5%, butane 0.1-1%, pentane 0-1%. The oil composition cannot be expressed in one formula, it is variable and depends on the field.
The composition of the oil includes alkanes with long carbon chains, for example C8H18, C12H26. Alkanes are obtained from oil by cracking.
In industry, alkanes are obtained by:
Oil cracking results in one alkane and one alkene.
C8H18 → C4H8 + C4H10
C12H26 → C6H12 + C6H14
Hydrogenation of coal (peat, shale)
C + H2 → (t, p) CH4
Hydrogenation of Carbon Monoxide II
CO + H2 → (t, p, cat.) CH4 + H2O
In the laboratory, alkanes are prepared in the following ways:
This synthesis consists of the fusion of a salt of a carboxylic acid with an alkali, and alkane is formed as a result.
This reaction involves the interaction of a haloalkane with metallic sodium, potassium or lithium. As a result, there is a doubling of the hydrocarbon radical, the chain growth is mirror-like: in the place where the halogen atom was located.
The electrolysis of carboxylic acid salts can result in the formation of alkanes.
Decomposition of aluminum carbide
The decomposition of aluminum carbide produces methane and aluminum hydroxide.
Al4C3 + 12H2O → 3CH4 + 4Al (OH)3
Hydrogenation of unsaturated hydrocarbons
CH3-CH = CH2 + H2 → (t, p, Ni) CH3-CH2-CH3
CH2 = CH2 + H2 → (t, p, Ni) CH3-CH3
Chemical properties of alkanes
Alkanes are saturated hydrocarbons, they do not enter into the reactions of hydrogenation (hydrogen addition), hydration (water addition). Alkanes are characterized by substitution, rather than addition, reactions.
A halogen atom replaces a hydrogen atom in an alkane molecule. Remember that substitution at the tertiary carbon atom is easiest, slightly harder at the secondary and much harder at the primary.
Reactions with chlorine in the light occur according to the free radical mechanism. In the light, a chlorine molecule breaks down into free radicals, which carry out an attack on a hydrocarbon molecule.
Nitration (Konovalov reaction)
Konovalov’s reaction consists of nitration of aliphatic (as well as aromatic) compounds with dilute nitric acid. The reaction proceeds at elevated pressure, according to the free radical mechanism.
CH3-CH3 + HNO3 (decomp.) → CH3-CH2-NO2 + H2O
For convenience and a deeper understanding, nitric acid – HNO3 – can be represented as HO-NO2.
All organic substances, including alkanes, burn with the formation of carbon dioxide and water.
C3H8 + O2 → CO2 + H2O
During catalytic, controlled oxidation, it is possible to stop at the stage of alcohol, aldehyde, acid.
CH4 + O2 → CH3-OH (methanol)
Pyrolysis – thermal decomposition of inorganic and organic compounds. The fundamental difference between pyrolysis and combustion is in the absence of oxygen.
CH4 → (t> 1000°C) C + H2
CH4 → (t = 1500-1600°С) CH≡CH + H2 ↑
CH4 → (t = 1200°С, cat., P) CH2 = CH2 + H2 ↑
C2H6 → (t = 1200°С, cat., P) CH2 = CH2 + 2H2 ↑
In the reactions resulting in the formation of isomers, the characteristic AlCl3 catalyst is used.
You already know that as a result of cracking, one alkane and one alkene are formed. This is not only a method for producing alkanes but also their chemical property.
C8H18 → (t) C4H10 + C4H8
C14H30 → (t) C7H14 + C7H16
Take the test to consolidate knowledge
1. Alkanes react
2. According to the free radical mechanism occurs
- Würz reaction
- The reaction of methane with chlorine in the light
- Kolbe synthesis
- Dumas synthesis
3. The reaction of Konovalov consists in the interaction of alkanes with
- Hydrochloric acid
- Sulfuric acid
- Sulfurous acid
- Nitric acid
4. Methane dimerization (pyrolysis) can be obtained
5. A characteristic catalyst for the reaction of isomerization
- C (active)