Alkanes

alkane formula

Alkanes are acyclic hydrocarbons of linear or branched structure containing only simple connections and forming a homological series with the general formula C_nH_2n+2 (CH₄, C₂C₆, ...). Alkanes are also called paraffins. Each carbon atom in an alkane molecule has the maximum number of other atoms associated with it, that is, four, therefore such hydrocarbons and they call them saturated.

Connections

The electronic configuration of a carbon atom with atomic number 6, 1s²2s²2p², cannot to form four bonds, but only two, so there was sp³-hybridization, that is, redistribution four electrons from two different energy levels to one. Formed carbon electron bonds (with the orbital sp³) and hydrogen (orbital s) form a very strong bond. Due to the bond strength, saturated hydrocarbons have a low reactivity.

Geometry

The presence of four orbitals at the carbon atom creates the shape of a regular tetrahedron and all angles between the orbitals are equal to 109°28'. The bond length between carbon and hydrogen atoms is 0.109 nm, between two carbon atoms - 0.154 nm.

Reactions

The atoms in the alkane molecules are connected by a strong σbond. In the reaction, the C-C and C-H bonds have equal the probability of breaking down to form a new compound, therefore, the result of the reaction is always is a complex mixture of products. Under normal conditions, alkanes do not react with acids, with bases, nor with strong oxidizing agents.

When the bond breaks in alkanes, two scenarios are possible: the bond breaks with the formation of two radicals, A:B→ A^• + _• B. Such a gap is called homolytic (homo - identical). In another case, a rupture occurs with the formation of ions when a common pair of electrons departs to one of the atoms: A:B → A + :B, such a gap is called heterolytic. The species are named accordingly alkane reactions: homolytic and heterolytic reactions.

At the moment, two types of alkane reactions are known in which C-C bonds are not broken - these are halogenation and nitriding. Examples of methane reactions are given below.

Halogenation

The halogenation reaction takes place at a temperature of 300-400 °C or under the influence of ultraviolet rays. During the reaction, haloalkanes are formed. Reactions with chromium and bromine are most common, reactions with fluorine are dangerous due to the possibility of an explosion, with iodine the reaction does not take place.

The halogenation process consists of three stages: initiation, chain growth and chain breakage.

1. Initiation - homolytic splitting of a halogen into two radicals:
Cl₂ → 2Cl• (exposure to light energy, hν)

2. Chain development - free radicals interact with molecules and two reactions are possible:
(1) Cl• + CH₄ → HCl + •CH₃
(2) Cl• + CH₄ → CH₃Cl + H•
The energy of atomic hydrogen is significantly higher than that of the methyl radical CH₃, so the reaction (2) does not proceed.

3. Chain breakage - radicals react with each other and form products:
Cl• + Cl• → Cl₂
CH₃• + CH₃• → 2CH₃
CH₃• + Cl• → CH₃Cl

Gorenje

The main use of alkanes is fuel, so the gorenje reaction can be called the most popular for limit hydrocarbons. In the gorenje reaction, alkanes are converted into water and carbon dioxide. The gorenje reaction is exothermic and requires a large amount of energy, for example, a spark or fire. The general reaction of gorenje alkanes:

R + O₂ → CO₂+H₂O + heat
2C_nH_2n+2 + (3n+1)O₂ → 2nCO₂ + (2n+2)H₂O + heat

Methane gorenje reaction

CH₄ + 2O₂ → CO₂ + 2H₂O + 212 kcal/mol

Nitriding

At a temperature of 140°C, with increasing pressure, alkanes react with nitric acid, a hydrogen atom it is replaced by a nitric acid residue NO₂, the reaction products are called nitro compounds:

CH₄ + HO-NO₂ → CH₃-NO₂ + H₂O (140°C, p)

Synthesis

Wurtz synthesis

In 1855, Adolf Wurtz discovered that the reaction of metallic sodium with haloalkan forms sodium salt:

2CH₃I + 2Na• → 2Na⁺I^- + CH₃CH₃

Haloalkane free radicals react with each other to form longer compounds. The general reaction equation has the form:

2R-X + 2Na → 2NaX + R-R

Haloalkyl recovery

Most haloalkyl in reaction with zinc and hydrogen cations (or Brensted-Lowry acid) form alkanes. In such a reaction, zinc is a reducing agent and allows you to replace halogen with hydrogen:

2C₄H₉Br (2-brombutane) + H⁺ (acid) + Zn→ 2C₄H₁₀ (Bhutan) + ZnBr₂

Grignard reagents

Grignard reagents are organic compounds in which a metal-carbon bond is present. Such reagents formed as a result of the reaction of haloalkyl with magnesium in a solution of diethyl ether:

R-X + Mg → RMgX (in diethyl ether solution)

The reaction also takes place with chlorides, bromides and iodides of alkyls. In the process of hydrolysis, Grignard reagents converted to alkanes:

CH₃MgI + H₂O → CH₄ + HO-Mg-I
C₂H₅MgBr + H₂O → C₂H₆ + HO-Mg-Br

Getting and applying

Alkanes are obtained either by synthesis or from natural sources (natural gas, oil, coal). Application saturated hydrocarbons are very extensive, alkanes are used as gas, gasoline, diesel and rocket fuel. Vaseline, solvents and paraffin are also the merit of alkanes.

Properties of alkanes

Alkanes containing from one to four carbon atoms are in a gaseous state, from five to 17 atoms carbon - liquids with more than 18 carbon atoms are in a solid state. Colorless, tasteless and odorless, usually impurities are added to alkanes so that they have a smell, for example methane gas or gasoline.

sp³-hybridization makes alkanes the least polar of all organic compounds, which means that they are poorly soluble in polar solutions, so the boiling and melting points will mainly depend only on from the molecular weight, on average, the boiling point of marginal hydrocarbons increases by 25-30 degrees for each carbon atom after the pentane. Branched alkanes have a lower boiling point because more branched molecules they have a smaller surface area, so intermolecular bonds are weaker and boil earlier.

The viscosity of a substance depends on the size of the molecule, so the more carbon atoms in the molecule, the larger it is and the greater the probability of interaction of molecules and, as a result, the greater the viscosity. Alkanes with a number of carbons from 20 to 35 are the main component for lubricants.

Nomenclature

The name of alkanes consists of two parts: the prefix denotes the number of carbon atoms to it the suffix -an is attached, which means the type of compound, i.e. alkane.