Alkenes tend to burn with sooty, luminous yellow flames. This is because the presence of the double bond reduces the amount of hydrogen in the molecule. This means that it will contain a greater proportion of carbon compared to a saturated molecule. It is difficult to get enough oxygen to the flame to ensure complete combustion, so carbon appears in the flame which is seen as black specks.
Alkyne names are formed by dropping the "ane" and replacing it with "yne". Fossil Fuels. Hydrocarbons compounds. Elmhurst College. Alkanes or Chime. Alkynes or Chime. Rings or Chime. Chemistry Department. Unsaturated hydrocarbons, like alkenes and alkynes, have double and triple bonds between neighbouring carbon atoms, for example, ethene and ethyne. Combustion reaction is the process of burning of a carbon compound in air to give carbon dioxide, water, heat, and light.
For example, when wood burns in air, the carbon present in the wood reacts with the oxygen present in the air, releasing carbon dioxide and water, along with heat and light. When saturated hydrocarbons burn, they produce a blue, non-sooty flame. Liquefied petroleum gas, also known as LPG, is a fuel used for cooking. LPG mainly contains butane, along with small amounts of propane. To make one of these soot particles, the first step is to construct a planar, graphene-like layer of carbon.
The similarities between this sp 2 -hybridised carbon sheet and a smaller polyaromatic structure such as that of naphthalene are clear figure 1. Structures of this type are so important to the study of soot that they have been given their own name: polycyclic aromatic hydrocarbons PAHs.
To make soot it is necessary to rapidly create and grow PAHs. The mechanism of the conversion of a small molecule like naphthalene to an extensive graphite-like structure is known as hydrogen-abstraction-C 2 H 2 -addition, or HACA for short.
The first step to forming a new PAH ring is the abstraction of a hydrogen atom from a benzene ring. An acetylene molecule then adds at this site, to make a two carbon long side chain. A further HACA reaction onto the end of this chain grows the unsaturated side group to four carbons long. This chain then wraps around to form another aromatic ring and build up the PAH see figure 2.
Two key components explain the success of this growth pathway: thermodynamically stable intermediates and a kinetic lever which pushes the reaction forward from each of these stable intermediates. The stable intermediates are themselves PAHs, each populated with relatively strong carbon-carbon sp 2 -hybridised covalent bonds with the extra stability afforded by aromatic numbers of p electrons.
Numerous intermediates have been identified and studied so far. The reactions proceed like ratchet mechanisms — ensuring that once formed, each intermediate is difficult to break down, preventing the reverse reaction taking place.
This stability is however somewhat balanced by the entropic disfavourability of producing one large molecule from many small ones. A swell of hydrogen atoms is necessary to overcome the thermodynamic break applied to the growth of the PAH. These hydrogens abstract other hydrogen atoms from the thermodynamically stable growing structure and push the reaction onwards.
You can think of complete combustion versus incomplete combustion as the outcome of a race between soot formation and the formation of CO 2 , rather than being due to an overall deficiency of oxygen. Looking at the mechanism of how PAHs grow it is clear how the presence of acetylene in a flame dramatically favours the formation of soot, by providing a competing reaction pathway to the traditional oxidation route to CO 2.
If you burn acetylene itself, the fates of the carbon atoms within it are all but sealed.
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