Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 80
Alkanes
ОглавлениеAlkanes (sometimes referred to as paraffins or paraffin hydrocarbons) are aliphatic hydrocarbons (non-aromatic hydrocarbon derivatives) that contain carbon and hydrogen only in which all of the binding orbitals of the carbon atoms are satisfied by bonding to another carbon atom or to a hydrogen atom.
Normal alkanes (straight-chain paraffins) consist of a chain of carbon atoms. Each carbon atom is linked to four atoms, which can be either carbon or hydrogen, their general formula CnH2n+2 (Table A-16). The carbon skeleton can be structured as straight chains as are the normal paraffin. CH3(CH2)nCH3. The boiling points increase with the number of carbon atoms. With the low carbon numbers, the addition of a carbon increases the boiling point by approximately 25°C (77°F). Further additions give smaller increase. At the same time, the density increases with the molecular weight 0.626 kg/L for pentane, and 0.791 kg/L for pentacosane; on the other hand, the density is always much lower than 1. The normal alkanes from C1 to C4 are colorless gases; C5 to C17 colorless liquids; and from C18 onwards, colorless solids. Other physical properties, such as melting point, density, and viscosity, also increase in the same way as boiling point. There is a relationship between physical properties and chemical composition. The variation in the boiling point of compounds is due to different intermolecular forces such as hydrogen bonding. The alkanes are insoluble in water.
Table A-16 Physical properties of n-paraffins.
| Alkane | Melting point, °C | Boiling point°C | Density, g/ml @20°C |
|---|---|---|---|
| Methane | -183 | -162 | |
| Ethane | -172 | -88.5 | |
| Propane | -167 | -42 | |
| Butane | -138 | 0 | |
| Pentane | -130 | 36 | 0.626 |
| Hexane | -95 | 69 | 0.659 |
| Heptane | -90 | 98 | 0.684 |
| Octane | -57 | 126 | 0.703 |
| Nonane | -54 | 151 | 0.718 |
| Decane | -30 | 174 | 0.730 |
| Undecane | -26 | 196 | 0.740 |
| Dodecane | -10 | 216 | 0.749 |
| Tridecane | -6 | 234 | 0.757 |
| Tetradecane | 5.5 | 252 | 0.764 |
| Pentadecane | 10 | 266 | 0.769 |
| Hexadecane | 18 | 280 | 0.775 |
| Heptadecane | 22 | 292 | |
| Octadecane | 28 | 308 | |
| Nonadecane | 32 | 320 |
Isoparaffins are paraffins in which branching is present, usually at the number 2 carbon atom, although branching can take place at a different position in the chain, although such molecules are not strictly isoparaffins. Isoparaffins have a boiling point lower than normal paraffin with same number of carbon atoms, and generally, the greater branching has the lower boiling point (Table A-17).
Table A-17 Physical properties of selected branched paraffins.
| Paraffins | Melting point, °C | Boiling point, °C | Density, g/ml @20 °C |
|---|---|---|---|
| Isobutane | -159 | -12 | |
| Isopentane | -160 | 28 | 0.620 |
| Neopentane | -17 | 9.5 | |
| Isohexane | -154 | 60 | 0.654 |
| 3-Methylbutane | -118 | 63 | 0.676 |
| 2,2-Dimethylbutane | -98 | 50 | 0.649 |
| 2,3-Dimethylbutane | -129 | 58 | 0.668 |
The octane number is a measure of the ability of a fuel (gasoline) to avoid knocking. The test engine is adjusted to give knock from the fuel rated. Then, various mixtures of isooctane (2,2,4-trimethylpentane) and n-heptane are used to find the ratio of the two reference fuels that will give the same intensity knock as that from unknown fuel. Defining isooctane as 100 octane number and n-heptane as 0, the octane number is the volumetric percentage of isooctane in heptane that matches knock from the unknown fuel is reported as the octane number of the fuel.
