It would be useful to start with the definition of alkanes. These are saturated or saturated hydrocarbons, paraffins. You can also say that these are carbons in which the connection of C atoms is carried out through simple bonds. The general formula is: CnH₂n+ 2.
It is known that the ratio of the number of H and C atoms in their molecules is maximum when compared with other classes. Due to the fact that all valences are occupied by either C or H, the chemical properties of alkanes are not expressed clearly enough, so the phrase saturated or saturated hydrocarbons is their second name.
There is also an older name that best reflects their relative chemical inertness - paraffins, which means "no affinity" in translation.
So, the topic of our today's conversation: "Alkanes: homologous series, nomenclature, structure, isomerism." Data regarding their physical properties will also be presented.
Alkanes: structure, nomenclature
In them, the C atoms are in such a state as sp3-hybridization. Concerningan alkane molecule can be demonstrated as a set of C tetrahedral structures that are linked not only to each other, but also to H.
There are strong, very low-polarity s-bonds between C and H atoms. Atoms, on the other hand, always rotate around simple bonds, which is why alkane molecules take on various forms, and the bond length and the angle between them are constant values. Forms that transform into each other due to the rotation of the molecule around the σ-bonds are called its conformations.
In the process of detachment of the H atom from the molecule under consideration, 1-valent particles are formed, called hydrocarbon radicals. They appear as a result of compounds of not only organic substances, but also inorganic ones. If you subtract 2 hydrogen atoms from a saturated hydrocarbon molecule, you get 2-valent radicals.
Thus, the nomenclature of alkanes can be:
- radial (old version);
- substitutive (international, systematic). It was proposed by IUPAC.
Features of the radial nomenclature
In the first case, the nomenclature of alkanes is characterized by the following:
- Consideration of hydrocarbons as derivatives of methane, in which 1 or more H atoms are replaced by radicals.
- High degree of convenience in the case of not very complex connections.
Features of replacement nomenclature
The substitutional nomenclature of alkanes hasthe following features:
- The basis for the name is 1 carbon chain, the rest of the molecular fragments are considered as substituents.
- If there are several identical radicals, a number is indicated before their name (strictly in words), and radical numbers are separated by commas.
Chemistry: alkane nomenclature
For convenience, the information is presented in the form of a table.
| Name of substance | Basic name (root) | Molecular formula | Name of carbon substituent | Carbon substituent formula |
| Methane | Met- | CH₄ | Methyl | CH₃ |
| Ethan | T- | C₂H₆ | Ethyl | C₂H₅ |
| Propane | Prop- | C₃H₈ | Drill | C₃H₇ |
| Bhutan | But- | C₄H₁₀ | Butyl | C₄H₉ |
| Pentane | Pent- | C₅H₁₂ | Pentyl | C₅H₁₁ |
| Hexane | Hex- | C₆H₁₄ | Gexyl | C₆H₁₃ |
| Heptane | Hept- | C₇H₁₆ | Heptyl | C₇H₁₅ |
| Octane | Oct- | 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 the series of saturated hydrocarbons).
The names of unfolded alkanes with 5 or more C atoms are derived from Greek numerals that reflect the given number of C atoms. Thus, the suffix -an indicates that the substance is from a series of saturated compounds.
When naming unfolded alkanes, the one that contains the maximum number of C atoms is chosen as the main chain. It is numbered so that the substituents are with the smallest number. In the case of two or more chains of the same length, the one that contains the largest number of substituents becomes the main one.
Alkanes isomerism
Methane CH₄ acts as the hydrocarbon-ancestor of their series. With each subsequent representative of the methane series, there is a difference from the previous one in the methylene group - CH₂. This patterntraceable throughout the alkane series.
The German scientist Schiel put forward a proposal to call this series homological. Translated from Greek means "similar, similar."
Thus, a homologous series is a set of related organic compounds that have the same type of structure with similar chemical properties. Homologues are members of a given series. The homologous difference is the methylene group by which 2 adjacent homologues differ.
As mentioned earlier, the composition of any saturated hydrocarbon can be expressed using the general formula CnH₂n + 2. Thus, the next member of the homologous series after methane is ethane - C₂H₆. To deduce its structure from methane, it is necessary to replace 1 H atom with CH₃ (figure below).
The structure of each successive homologue can be derived from the previous one in the same way. As a result, propane is formed from ethane - C₃H₈.
What are isomers?
These are substances that have identical qualitative and quantitative molecular composition (identical molecular formula), but different chemical structure, and also have different chemical properties.
The above hydrocarbons differ in such a parameter as the boiling point: -0.5° - butane, -10° - isobutane. This type of isomerism is referred to as carbon skeleton isomerism, it belongs to the structural type.
The number of structural isomers grows rapidly with the increase in the number of carbon atoms. Thus, C₁₀H₂₂ will correspond to 75 isomers (not includingspatial), and for C₁₅H₃₂ 4347 isomers are already known, for C₂₀H₄₂ - 366 319.
So, it has already become clear what alkanes, homologous series, isomerism, nomenclature are. Now it's time to move on to the IUPAC naming conventions.
IUPAC Nomenclature: Naming Rules
Firstly, it is necessary to find in the hydrocarbon structure the carbon chain that is the longest and contains the maximum number of substituents. Then you need to number the C atoms of the chain, starting from the end to which the substituent is closest.
Secondly, the base is the name of a straight-chain saturated hydrocarbon, which, by the number of C atoms, corresponds to the main chain.
Thirdly, it is necessary to indicate the numbers of the locants near which the substituents are located before the stem. They are followed by the names of the substitutes with a hyphen.
Fourth, if there are identical substituents at different C atoms, the locants are combined, and a multiplying prefix appears before the name: di - for two identical substituents, three - for three, tetra - four, penta - for five and etc. Numbers must be separated from each other by a comma, and from words by a hyphen.
If the same C atom contains two substituents at once, the locant is also written twice.
According to these rules, the international nomenclature of alkanes is formed.
Newman projections
This American scientistproposed for the graphical demonstration of conformations special projection formulas - Newman projections. They correspond to forms A and B and are shown in the figure below.
In the first case, this is an A-shielded conformation, and in the second, it is B-inhibited. In the A position, the H atoms are located at the minimum distance from each other. This form corresponds to the largest value of energy, due to the fact that the repulsion between them is the largest. This is an energetically unfavorable state, as a result of which the molecule tends to leave it and move to a more stable position B. Here, the H atoms are as far apart as possible. So, the energy difference between 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 uneven. After getting into an energetically favorable position, the molecule lingers there, in other words, “slows down”. That is why it is called inhibited. The result - 10 thousand molecules of ethane are in a hindered form of conformation at room temperature. Only one has a different shape - obscured.
Obtaining saturated hydrocarbons
It has already become known from the article that these are alkanes (their structure, nomenclature are described in detail earlier). It would be useful to consider how to obtain them. They are emitted from such natural sources as oil, natural gas, associated gas, and coal. Synthetic methods are also used. For example, H₂ 2H₂:
- The process of hydrogenation of unsaturated hydrocarbons:CnH₂n (alkenes)→ CnH₂n+2 (alkanes)← CnH₂n-2 (alkynes).
- From a mixture of monoxide C and H - synthesis gas: nCO+(2n+1)H₂→ CnH₂n+2+nH₂O.
- From carboxylic acids (their s alts): electrolysis at the anode, at the cathode:
- Kolbe electrolysis: 2RCOONa+2H₂O→R-R+2CO₂+H₂+2NaOH;
- Dumas reaction (alkali alloy): CH₃COONa+NaOH (t)→CH₄+Na₂CO₃.
- Oil cracking: CnH₂n+2 (450-700°)→ CmH₂m+2+ Cn-mH₂(n-m).
- Gasification of fuel (solid): C+2H₂→CH₄.
- Synthesis of complex alkanes (halogen derivatives) that have fewer C atoms: 2CH₃Cl (chloromethane) +2Na →CH₃- CH₃ (ethane) +2NaCl.
- Water decomposition of methanides (metal carbides): Al₄C₃+12H₂O→4Al(OH₃)↓+3CH₄↑.
Physical properties of saturated hydrocarbons
For convenience, the data is grouped in a table.
| Formula | Alkane | Melting point in °С | Boiling point in °С | Density, g/ml |
| CH₄ | Methane | -183 | -162 | 0, 415 at t=-165°С |
| C₂H₆ | Ethan | -183 | -88 | 0, 561 at t=-100°C |
| C₃H₈ | Propane | -188 | -42 | 0, 583 at t=-45°C |
| n-C₄H₁₀ | n-Bhutan | -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-Trimethyl-pentane | - 107 | 99 | 0, 692 | |
| n-C₉H₂₀ | n-Nonan | - 53 | 151 | 0, 718 |
| n-C₁₀H₂₂ | n-Dean | - 30 | 174 | 0, 730 |
| n-C₁₁H₂₄ | n-Undecane | - 26 | 196 | 0, 740 |
| n-C₁₂H₂₆ | n-Dodecane | - 10 | 216 | 0, 748 |
| n-C₁₃H₂₈ | n-Tridecane | - 5 | 235 | 0, 756 |
| n-C₁₄H₃₀ | n-Tetradecane | 6 | 254 | 0, 762 |
| n-C₁₅H₃₂ | n-Pentadecane | 10 | 271 | 0, 768 |
| H-C₁₆H₃₄ | n-Hexadecane | 18 | 287 | 0, 776 |
| n-C₂₀H₄₂ | n-Eicosan | 37 | 343 | 0, 788 |
| n-C₃₀H₆₂ | n-Triacontan | 66 |
235 at 1 mmHg st |
0, 779 |
| n-C₄₀H₈₂ | n-Tetracontan | 81 |
260 at 3 mmHg st. |
|
| n-C₅₀H₁₀₂ | n-Pentacontan | 92 |
420 at 15 mmHg st. |
|
| n-C₆₀H₁₂₂ | n-Hexacontane | 99 | ||
| n-C₇₀H₁₄₂ | n-Heptacontane | 105 | ||
| n-C₁₀₀H₂₀₂ | n-Hectane | 115 |
Conclusion
The article considered such a concept as alkanes (structure, nomenclature, isomerism, homologous series, etc.). A little is told about the features of the radial and substitution nomenclature. Methods for obtaining alkanes are described.
In addition, the entire nomenclature of alkanes is listed in detail in the article (the test can help to assimilate the information received).
