Alkanes from a chemical point of view are hydrocarbons, that is, the general formula of alkanes includes exclusively carbon and hydrogen atoms. In addition to the fact that these compounds do not contain any functional groups, they are formed only due to single bonds. Such hydrocarbons are called saturated.
Types of alkanes
All alkanes can be distinguished into two large groups:
- Aliphatic compounds. Their structure has the form of a linear chain, the general formula of alkanes of the aliphatic type is C n H 2n + 2 , where n is the number of carbon atoms in the chain.
- Cycloalkanes. These compounds have a cyclic structure, which leads to a significant difference in their chemical properties from linear compounds. In particular, the structural formula of alkanes of this type determines the similarity of their properties with alkynes, i.e. hydrocarbons with a triple bond between carbon atoms.
Electronic structure of aliphatic compounds
This group of alkanes can have either a linear or branched hydrocarbon chain. Their chemical activity is low compared to other organic compounds, since all bonds within the molecule are saturated.
The molecular formula of aliphatic alkanes indicates that their chemical bonding has sp 3 hybridization. This means that all four covalent bonds around a carbon atom in terms of their characteristics (geometric and energetic) are absolutely equal. With this type of hybridization, the electron shells of levels s and p of carbon atoms have the same shape of an elongated dumbbell.
The bond between the carbon atoms in the chain is covalent, and between the carbon and hydrogen atoms it is partially polarized, while the electron density is drawn to carbon, as to a more electronegative element.
It follows from the general formula of alkanes that only CC and CH bonds exist in their molecules. The former are formed as a result of the overlap of two electronic hybridized sp 3 orbitals of two carbon atoms, and the latter are formed when the hydrogen orbitals s and the sp 3 carbon orbital overlap. The CC bond is 1.54 angstroms, and the CH bond is 1.09 angstroms.
The geometry of the methane molecule
Methane is the simplest alkane, which consists of only one carbon atom and four hydrogen atoms.
Due to the energy equivalence of its three 2p and one 2s orbitals obtained as a result of sp 3 hybridization, all orbitals in space are located at the same angle to each other. It is equal to 109.47 °. As a result of such a molecular structure in space, a semblance of a triangular equilateral pyramid is formed.
Simple alkanes
The simplest alkane is methane, which consists of one carbon atom and four hydrogen atoms. The next propane, ethane, and butane in the alkanes after methane are formed by three, two, and four carbon atoms, respectively. Starting with five carbon atoms in the chain, the compounds are named according to the IUPAC nomenclature.
A table with alkane formulas and their names is given below:
| Title | methane | ethane | propane | butane | pentane | hexane | heptane | octane | nonan | dean |
| Formula | CH 4 | C 2 H 6 | C 3 H 8 | C 4 H 10 | C 5 H 12 | C 6 H 14 | C 7 H 16 | C 8 H 18 | C 9 H 20 | C 10 H 22 |
With the loss of one hydrogen atom, an alkane forms in the alkane molecule, the end of which changes from an to silt, for example, ethane C 2 H 6 - ethyl C 2 H 5 . The structural formula of ethane alkane is shown in the photo.
Nomenclature of Organic Compounds
The rules for determining the names of alkanes and compounds based on them are established by the IUPAC international nomenclature. For organic compounds, the following rules apply:
- The name of the chemical compound is based on the name of its longest chain of carbon atoms.
- The numbering of carbon atoms should begin from the end, closer to which branching of the chain begins.
- If two or more carbon chains of the same length are present in the compound, then the one that has the least radicals is selected as the main one, and they have a simpler structure.
- If there are two or more identical groups of radicals in a molecule, then the corresponding prefixes are used in the name of the compound, which double, triple, and so on the names of these radicals. For example, instead of the expression “3-methyl-5-methyl”, “3,5-dimethyl” is used.
- All radicals are written in alphabetical order in the general name of the compound, while prefixes are not taken into account. The last radical is written together with the name of the chain itself.
- The numbers reflecting the numbers of radicals in the chain are separated from the names by a hyphen, and the numbers themselves are written with a comma.
Compliance with the rules of the IUPAC nomenclature makes it easy to determine the molecular formula of an alkane by the name of the substance, for example, 2,3-dimethylbutane has the following form.
Physical properties
The physical properties of alkanes are largely dependent on the length of the carbon chain forming the particular compound. The main properties are as follows:
- The first four representatives, according to the general formula of alkanes, under normal conditions are in a gaseous state, that is, butane, methane, propane and ethane. As for pentane and hexane, they already exist in the form of liquids, and starting from seven carbon atoms, alkanes are solids.
- With an increase in the length of the carbon chain, the density of the compound also increases, as well as its temperature of first-order phase transitions, i.e., the melting and boiling points.
- Since the polarity of the chemical bond in the formula of the substance of alkanes is negligible, they do not dissolve in polar liquids, for example, in water.
- Accordingly, they can be used as good solvents for compounds such as non-polar fats, oils and waxes.
- The home gas stove uses a mixture of alkanes, rich in the third member of the chemical series - propane.
- During oxygen combustion of alkanes, a large amount of energy is released in the form of heat, therefore these compounds are used as combustible fuel.
Chemical properties
Due to the presence of stable bonds in alkane molecules, their reactivity is low compared to other organic compounds.
Alkanes practically do not react with ionic and polar chemical compounds. They behave inertly in solutions of acids and bases. Alkanes react only with oxygen and halogens: in the first case we are talking about oxidation processes, in the second - about substitution processes. They exhibit some chemical activity in reactions with transition metals.
Branches of the carbon chain of alkanes, that is, the presence of radical groups in them, play an important role in all these chemical reactions. The more of them, the stronger the ideal angle between bonds of 109.47 ° in the spatial structure of the molecule changes, which leads to the creation of stresses inside it and, as a result, increases the chemical activity of such a compound.
The reaction of simple alkanes with oxygen occurs as follows: C n H 2n + 2 + (1.5n + 0.5) O 2 → (n + 1) H 2 O + nCO 2 .
An example of a reaction with chlorine is shown in the photo below.
The danger of alkanes to nature and man
When the methane content in the air in the concentration range of 1-8%, an explosive mixture is formed. The danger to humans also lies in the fact that this gas has no color or smell. In addition, methane has a strong greenhouse effect. The remaining alkanes, having several carbon atoms in their composition, also form explosive mixtures with air.
Heptane, pentane and hexane are flammable liquids and are hazardous both to the environment and to human health because they are toxic.