A comparison of methane to propane is appropriate as a gauge for the heat of combustion of natural gas compared to that of propane; natural gas is generally around 95% methane, according to
this.
It's worth mentioning that methane, ethane, propane, butane, and pentane are all essentially similar- they are a chain of carbon atoms linked one to the next by single bonds, with the outer bonds all linked to single hydrogen atoms. These are all called "alkanes." Their generic structural formula is CnH(2n+2), where n is the number of carbon atoms. Methane CH4; ethane is C2H6, propane C3H8; butane C4H10; and pentane C5H12. Clearly, the ratio of hydrogen to carbon decreases as the inverse of the complexity of the molecules. As these molecules get heavier and heavier, they eventually stop being gasses at room temperature; heptane and octane are liquids. In fact, octane (actually, isomers of octane) is gasoline.
The "iso-" forms indicate that rather than being in a straight line, at least one carbon atom is connected "off to the side." These are called "isomers," thus the "iso-" form. Obviously, this is impossible for methane and ethane, and there is only one topologically distinguishable way to do it with propane; butane and pentane have two distinguishable forms, one with the "bend" at the end, and the other with it in the middle. Hexane (C6H14) has three distinguishable iso- forms. You will all be familiar with the equivalent percentage of isooctane as the "octane number" of gasoline; in fact, the precise isomer of octane used is 2,2,4-Trimethylpentane; that is, pentane with two methyl radicals (CH3s, that is) tacked onto the second, second, and fourth carbon atoms in the chain. Isooctane is mixed with neo-heptane (or n-heptane), which is the "straight-chain" version of heptane, and the percentage of isooctane that duplicates the anti-knock qualities of gasoline is assigned as the "octane number" of the gasoline. Because the chain is not straight, isooctane burns more slowly than n-heptane, and therefore it is less likely to explode or "knock" inside the cylinder of an internal combustion piston engine during the compression stroke. This explosion is bad, because it does not distribute the pressure well across the piston's stroke, or may even occur before the piston has reached the top of the stroke, thus robbing the engine of power or even counteracting it, and because it can damage the engine.
It will be obvious that petroleum is made up of very high concentrations of alkanes, and this is why we always find natural gas associated with petroleum; and extracting the "octane" fractions is the operation undertaken to manufacture gasoline. Kerosene (mostly used for jet fuel today, but also useful for heaters and lights before the advent of electricity) is from heavier fractions, in the twelve- to fifteen-carbon range. Diesel is composed of heavier fractions yet, along with some isomers that are circular ("cyclo-") rather than straight or branched, and some aromatic hydrocarbons, which are six-carbon rings arranged in various ways; we'll come back to that. Diesel is compounded such that it is capable of detonating in a controlled manner under high pressure and heat as found in a diesel engine, without the use of a spark plug to control the detonation. The alkane components of diesel can also be distilled to make paraffin, composed of 20- or more carbon alkanes, which is solid at room temperature, yet gives off volatile fractions under relatively mild heating, making it useful for candle waxes. Heavier fractions yet, but with different isomers included to make them liquid rather than solid, are used to make lubricating oils; these will burn, but not readily, as their molecules are sufficiently heavy that they are not volatile at room temperature, or even more intense heating than paraffin. Some heavy fractions, not useful for any of these purposes, are used as heating oil; this was quite common in the former Soviet Union, where these fractions were called "mazut." The heaviest fractions can be used to make plastics, after appropriate treatment.
These alkanes are all related to alcohols and ethers. An alcohol has the same carbon chain, but instead of capping the end with a CH3, they cap it with an OH radical, which is just water with one hydrogen atom taken off. It's generic formula is CnH(2n+1)OH. So methyl alcohol, also called methanol, is CH3OH; ethyl alcohol (which is the stuff you can drink, though pure ethanol is pretty poisonous and not very tasty- you usually get it mixed with water and various impurities; this is called "whiskey," "rum," "brandy," or whatever) is C2H5OH; and so forth. Beginning with propanol, iso- forms are also possible for alcohols.
Ethers replace
both of the hydrogen atoms in a water molecule. Their generic structural formulae go CnH(2n+1)OCmH(2m+1); they have two alkane radicals, which can be the same or different. Thus, we have dimethyl ether, CH3OCH3 (or C2H6O, if you prefer), methyl ethyl ether, diethyl ether, and so forth, and again, iso- forms begin with one or both of the alkane radicals being propyl- or greater, with the obvious topological implications.
I mentioned aromatic hydrocarbons earlier, and their six-carbon rings. The simplest of these is benzene, which has six carbon atoms arranged in a ring, with alternating single- and double-bonds between them; this leaves one unused valence bond for each carbon atom, which is taken up by a hydrogen atom. The chemical formula for benzene is thus C6H6. Two such rings can share a pair of the carbon atoms, making napthalene, C10H8. More complex interlockings of the rings, and the replacement of the hydrogen atoms with different radicals, makes a whole bunch of interesting chemistry, most of which yields useful solvents and fuels.
The replacement of the hydrogen atoms in alkanes, alcohols, ethers, and aromatic hydrocarbons with nitro- radicals, NO2, allows some types of explosives to be made; for example, toluene is an aromatic hydrocarbon made by replacing one hydrogen atom in a benzene ring with a methyl radical: C6H5CH3; if three other hydrogen atoms are replaced with nitro- radicals, one obtains trinitrotoluene, TNT, a high explosive.
A whole bunch of interesting chemistry devolves from these simple ingredients. The connection with carbohydrates, particularly sugars, implicit in the cyclo- forms of the alkanes, is particularly interesting. Have fun.
