International nomenclature of alkanes. Alkans: structure, properties

It will be useful to begin by defining the concept of alkanes. These are saturated or saturated hydrocarbons, paraffins. It can also be said that these are carbons in which the combination of C atoms is carried out through simple bonds. The general formula has the form: CnH₂n + 2.

It is known that the ratio of the number of H and C atoms in their molecules is maximized if compared with other classes. In view of the fact that all valencies are occupied by either C or H, the chemical properties of the alkanes are not clearly expressed, therefore the second term is the term saturated or saturated hydrocarbons.

There is also an older name, which best reflects their relative chemistry - paraffins, which in translation means "devoid of affinity".

So, the theme of our today's conversation is: "Alkans: homology series, nomenclature, structure, isomerism". Data on their physical properties will also be presented.

Alkans: building, nomenclature

In them, the C atoms are in such a state as sp3-hybridization. In connection with this, the alkane molecule can be demonstrated as a set of tetrahedral C structures that are not only related to one another, but also to H.

Between the atoms C and H there are strong, very low polarity s-bonds. Atoms around simple bonds always rotate, so that the molecules of alkanes take a variety of forms, with the length of the connection, the angle between them - constant values. Forms that are transformed into each other due to the rotation of the molecule that occurs around the σ-bonds, it is customary to call it conformations.

During the detachment of the H atom from the molecule in question, 1-valent particles, called hydrocarbon radicals, are formed. They appear as a result of compounds of not only organic substances, but also inorganic compounds. If we take 2 hydrogen atoms from the molecule of the limiting hydrocarbon, then we obtain 2-valent radicals.

Thus, the nomenclature of alkanes can be:

• Radial (old version);
• Substitute (international, systematic). It is proposed by IUPAC.

Features of the radial nomenclature

In the first case, the nomenclature of alkanes is characterized by the following:

1. Consideration of hydrocarbons as derivatives of methane, in which one or several H atoms are substituted with radicals.
2. High degree of convenience in the case of not very complex connections.

Features of the Replacement Nomenclature

The alkane nomenclature has the following features:

1. The basis for the name is 1 carbon chain, while the remaining molecular fragments are considered as substituents.
2. In the presence of several identical radicals, before their name is indicated the number (strictly in words), and the radical numbers are separated by commas.

Chemistry: nomenclature of alkanes

For convenience, the information is presented in the form of a table.

The above nomenclature of alkanes includes names that have developed historically (the first 4 members of a number of terminal hydrocarbons).

The names of non-deployed alkanes with 5 or more C atoms are formed from Greek numerals, which reflect the given number of C atoms. Thus, the suffix -an indicates that the substance is from a number of saturated compounds.

When the names of deployed alkanes are drawn up, the main chain is chosen to contain the maximum number of C atoms. It is numbered so that the substituents are the lowest numbered. In the case of two or more chains of the same length, the main one is the one that contains the largest number of substituents.

Isomerism of alkanes

As a hydrocarbon, the parent of their series is methane CH₄. With each successive representative of the methane series, there is a difference from the previous one to the methylene group - CH. This regularity can be traced in the whole range of alkanes.

German scientist Schiel put forward a proposal to call this series homologous. In Greek, it means "similar, similar."

Thus, the homologous series is a set of related organic compounds having the same structure with similar chemical properties. Homologues are members of this series. The homological difference is a methylene group, to which 2 neighboring homologues differ.

As mentioned earlier, the composition of any saturated hydrocarbon can be expressed by the general formula CnH₂n + 2. So, the next member of the homologous series is methane-C₂H следующим. To deduce its structure from methane, it is necessary to replace 1 atom H with CH₃ (figure below).

The structure of each subsequent homologue can be deduced from the previous one in the same way. As a result, ethane produces propane - C₃H₈.

What is isomers?

These are substances that have identical qualitative and quantitative molecular composition (identical molecular formula), but different chemical structure, as well as possessing different chemical properties.

The above hydrocarbons differ in such a parameter as the boiling point: -0.5 ° - butane, -10 ° - isobutane. This kind of isomerism is referred to as the isomerism of the carbonaceous skeleton, it refers to the structural type.

The number of structural isomers grows rapidly with an increase in the number of carbon atoms. Thus, C₁₀H₂₂ will correspond to 75 isomers (not including spatial ones), and for C₁₅H₃₂ 4347 isomers are already known, for C₂₀H₄₂ it is 366 319.

So, it became clear what alkanes, homologous series, isomerism, nomenclature. Now it's time to move on to the rules for compiling names for IUPAC.

Nomenclature IUPAC: rules for the formation of names

First, it is necessary to find the carbon chain in the hydrocarbon structure, which is the longest and contains the maximum number of substituents. Then it is required to number the C atoms of the chain, beginning at the end to which the substituent is closest.

Secondly, the basis is the name of an unbranched saturated hydrocarbon, to which the most important chain corresponds to the number of C atoms.

Thirdly, before the basis it is necessary to indicate the numbers of the locks near which the substituents are located. They are followed by a hyphen with the names of deputies.

Fourthly, in the case of the presence of identical substituents for different C atoms, the locks are combined, with the multiplier prefix appearing before the name: di - for two identical substituents, three for three, tetra-four, penta for five, etc. Figures Should be separated from each other by a comma, and from words - by a hyphen.

If the same C atom contains two substituents at once, lokant is also recorded twice.

According to these rules, the international nomenclature of alkanes is formed.

Projections of Newman

This American scientist proposed special projection formulas-the Newman projections-for graphical demonstration of conformations. They correspond to forms A and B and are shown in the figure below.

In the first case, this is an A-screened conformation, while in the second case it is B-inhibited. In position A, the H atoms are located at a minimum distance from each other. This form corresponds to the largest value of energy, because the repulsion between them is greatest. This is an energetically unfavorable state, as a result of which the molecule tends to leave it and proceed to a more stable position B. Here, the H atoms are at a maximum distance from each other. Thus, the energy difference of these positions is 12 kJ / mol, due to which the free rotation around the axis in the ethane molecule, which connects the methyl groups, is obtained unevenly. After falling into an energetically favorable position, the molecule is delayed there, in other words, "slowed down". That is why it is called inhibited. The result - 10 thousand ethane molecules are in a hindered form of conformation under the condition of room temperature. Only one has a different form - a screened one.

Obtaining of limit hydrocarbons

From the article it already became known that these are alkanes (the structure, the nomenclature of them are described in detail earlier). It will be useful to consider ways to obtain them. They are allocated from such natural sources as oil, natural gas, associated gas, coal. Synthetic methods are also used. For example, H₂ 2H₂:

1. The process of hydrogenation of unsaturated hydrocarbons: CnH₂n (alkenes) → CnH₂n + 2 (alkanes) ← CnH₂n-2 (alkynes).
2. From the mixture of monoxide C and H - synthesis gas: nCO + (2n + 1) H₂ → CnH₂n + 2 + nH₂O.
3. Of the carboxylic acids (their salts): electrolysis on the anode, on the cathode:

• Kolbe electrolysis: 2RCOONa + 2H₂O → R-R + 2CO₂ + H₂ + 2NaOH;
• Dumas reaction (alloy with alkali): CH₃COONa + NaOH (t) → CH₄ + Na₂CO.

1. Oil cracking: CnH₂n + 2 (450-700 °) → CmH₂m + 2 + Cn-mH₂ (nm).
2. Gasification of fuel (solid): C + 2H₂ → CH.
3. Synthesis of complex alkanes (halogenated derivatives) that have fewer C atoms: 2CH₃Cl (chloromethane) + 2Na → CH₃- CH₃ (ethane) + 2NaCl.
4. Decomposition with water of methanides (carbides of metals): Al₄C₃ + 12H₂O → 4Al (OH₃) ↓ + 3CH₄ ↑.

Physical Properties of Ultimate Hydrocarbons

For convenience, the data are grouped into a table.

Conclusion

The article considered such a concept as alkanes (structure, nomenclature, isomerism, homology series, etc.). A little is told about the features of the radial and replacement nomenclatures. Methods for the preparation of alkanes are described.

In addition, the article lists in detail all the nomenclature of alkanes (the test can help to assimilate the information obtained).