International nomenclature of alkanes. Alkans: structure, properties

It is useful to begin with the definition of the conceptalkanes. It is 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 atomsin their molecules as much as possible, 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 sufficiently pronounced, 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 assp3 hybridization. In this regard, 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 C and H atoms are strong, verylow polarity s-bonds. Atoms around simple bonds always rotate, so that the molecules of alkanes assume a variety of forms, with the length of the bond, the angle between them being constant. 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.

In the process of detachment of the atom H from the consideredmolecules are formed 1-valent particles, called hydrocarbon radicals. 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 ultimate 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 alkaline 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.

Name of substance	The basis of the name (root)	Molecular formula	The name of the carbon substituent	The carbon substituent formula
Methane	Met-	CH₄	Methyl	CH₃
Ethane	Et-	C₂H₆	Ethyl	C₂H₅
Propane	Prop-	C₃H₈	Has drunk	C₃H₇
Butane	But-	C₄H₁₀	Butyl	C₄H₉
Pentane	Penthouse-	C₅H₁₂	Pentyl	C₅H₁₁
Hexane	Hex-	C₆H₁₄	Hexil	C₆H₁₃
Heptane	Hept-	C₇H₁₆	Heptyl	C₇H₁₅
Octane	Octo-	C₈H₁₈	Octyl	C₈H₁₇
Nonan	Non-	C₉H₂₀	Nonil	C₉H₁₉
Dean	Dec-	C₁₀H₂₂	Decil	C₁₀H₂₁

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

Names of non-deployed alkanes with 5 or moreC atoms are formed from Greek numerals that reflect a given number of C atoms. So, the suffix -an says that the substance is from a series of saturated compounds.

When drawing up the names of deployed alkanes inthe role of the main chain is chosen to be one that contains the maximum number of C atoms. It is numbered so that the substituents are of the lowest number. 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 the hydrocarbon-progenitor of their seriesacts methane CH₄. With each successive representative of the methane series, there is a difference from the previous one to the methylene group - CH₂. This regularity can be traced in the whole range of alkanes.

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

Thus, the homology series is a setrelated 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 two neighboring homologues differ.

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

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

What is isomers?

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

The above hydrocarbons differ in thesuch 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 rapidlyAt a rate 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 we need to move on to the rules for the compilation of names for IUPAC.

Nomenclature IUPAC: rules for the formation of names

First, it is necessary to find in the hydrocarbonstructure of the carbon chain, 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.

Fourth, in the case of identicalsubstitutions for different C atoms are combined, the prefix is ​​followed by a multiplier: 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 the words - a hyphen.

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

According to these rules, an 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, andin the second, B is retarded. 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 getting 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 another form - a screened one.

Obtaining of limit hydrocarbons

From the article it became already known that these are alkanes(the structure, the nomenclature of them are described in detail earlier). It will not be out of place to consider ways of obtaining 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 a mixture of monoxide C and H - synthesis gas: nCO + (2n + 1) H₂ → CnH₂n + 2 + nH₂O.
3. Of the carboxylic acids (their salts): electrolysis at the anode, at the cathode:

• Kolbe electrolysis: 2RCOONa + 2H₂O → R-R + 2CO₂ + H₂ + 2NaOH;
• Dumas reaction (alloy with alkali): CH₃COONa + NaOH (t) → CH₄ + Na₂CO₃.

1. Oil cracking: CnH₂n + 2 (450-700 °) → CmH₂m + 2 + Cn-mH₂ (n-m).
2. Gasification of fuel (solid): C + 2H₂ → CH₄.
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 the ultimate hydrocarbons

For convenience, the data are grouped into a table.

Formula	Alcan	Melting point in ° C	Boiling point in ° C	Density, g / ml
CH₄	Methane	-183	-162	0.415 at t = -165 ° C
C₂H₆	Ethane	-183	-88	0.561 at t = -100 ° C
C₃H₈	Propane	-188	-42	0.583 at t = -45 ° C
n-C₄H₁₀	n-Butane	-139	-0,5	0.579 at t = 0 ° C
	2-Methylpropane	- 160	- 12	0.557 at t = -25 ° C
	2,2-Dimethyl-propane	- 16	9,5	0,613
n-C₅H₁₂	n-pentane	-130	36	0,626
	2-Methylbutane	- 160	28	0,620
n-C₆H₁₄	n-Hexane	- 95	69	0,660
	2-Methylpentane	- 153	62	0,683
n-C₇H₁₆	n-Heptane	- 91	98	0,683
n-C₈H₁₈	n-Octane	- 57	126	0,702
	2,2,3,3-Tetra-methylbutane	- 100	106	0,656
	2,2,4-Trimethylpentane	- 107	99	0,692
n-C₉H₂₀	n-Nonan	- 53	151	0,718
n-C₁₀H₂₂	n-Decan	- 30	174	0,730
n-C₁₁H₂₄	n-Undekan	- 26	196	0,740
n-C₁₂H₂₆	n-Dodecane	- 10	216	0,748
n-C₁₃H₂₈	n-Tridecan	- 5	235	0,756
n-C₁₄H₃₀	n-tetradecane	6	254	0,762
n-C₁₅H₃₂	n-Pentadecane	10	271	0,768
n-C₁₆H₃₄	n-Hexadecane	18	287	0,776
n-C₂₀H₄₂	n-Eicosane	37	343	0,788
n-C₃₀H₆₂	n-Triacultan	66	235 with 1 mm Hg. art.	0,779
n-C₄₀H₈₂	n-Tetracontan	81	260 at 3 mm of mercury. Art.
n-C₅₀H₁₀₂	n-Pentacontan	92	420 at 15 mm of mercury. Art.
n-C₆₀H₁₂₂	n-Hexacontane	99
n-C₇₀H₁₄₂	n-HeptaContan	105
n-C₁₀₀H₂₀₂	n-Hexane	115

Conclusion

The article considered such a concept asalkanes (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).