Alkane
An alkane in organic chemistry is a type of hydrocarbon, in which the molecule has the maximum possible number of hydrogen atoms, and so has no double bonds (they are saturated).The generic formula for acyclic alkanes, also known as aliphatic hydrocarbons is CnH2n+2; the simplest possible alkane is methane (CH4). Each C atom is hybridized sp3.
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2 Physical properties 3 Chemical properties 4 Cracking reactions 5 Halogenation reaction 6 Combustion 7 See also |
The atoms in alkanes with more than three carbon atoms can be arranged in multiple ways, forming different isomers. "Normal" alkanes have the most linear, unbranched configuration, and are denoted with an n. The number of isomers increases rapidly with the number of carbon atoms; for acyclic alkanes with n = 1..12 carbon atoms, the number of isomers equals 1, 1, 1, 2, 3, 5, 9, 18, 35, 75, 159, 355 (sequence A000602 in OEIS).
The names of all alkanes end with -ane. The alkanes, and their derivatives, with four or fewer carbons have non-systematic common names, established by long precedence.
Arrangements
| methane | CH4 |
| ethane |  |
| propane |  |
| n-butane |  |
| n-pentane |  |
| n-hexane |  |
| n-heptane |  |
| n-octane |  |
and so on . . . .
Branched alkanes have some non-systematic (or "trivial") names in common use, but there is also a systematic way of naming most such compounds, which starts from identifying the longest non-branched parent alkane in the molecule, counting up from one sequentially starting from the carbon involved in the most prominent functional group (or, more formally, attached to the collection of heteroatoms with highest priority according to some rules), and then numbering the side chains according to this sequence.
i-butane 
(or "isobutane")
is the only other C4 alkane isomer possible, aside from n-butane. Its formal name is 2-methylpropane.
Pentane, however, has two branched isomers, in addition to its strictly linear, normal form:
2,2-dimethylpropane (or "neopentane")
and
2-methylbutane (or "isopentane")
"Cracking" breaks larger molecules into smaller ones. This can be done with a thermic or catalytic method. The thermal cracking process follows a homolytic mechanism, that is, bonds break symmetrically and thus pairs of free radicals are formed. The catalytic cracking process involves the presence of acid catalysts (usually solid acids such as silica-alumina and zeolites) which promote a heterolytic (asymmetric) breakage of bonds yielding pairs of ions of opposite charges, usually a carbocation and the very unstable hydride anion. Carbon-localized free radicals and cations are both highly unstable and undergo processes of chain rearrangement, C-C scission in position beta (i.e., cracking) and intra- and intermolecular hydrogen transfer or hydride transfer. In both types of processes, the corresponding reactive intermediates (radicals, ions) are permanently regenerated and thus they proceed by a self-propagating chain mechanism. The chain of reactions is eventually terminated by radical or ion recombination.
Here is an example of cracking with butane CH3-CH2-CH2-CH3
after a certain number of steps, we will obtain an alkane and an alkene:
CH4 + CH2=CH-CH3
after a certain number of steps, we will obtain an alkane and an alkene
from different types: CH3-CH3 + CH2=CH2
Physical properties
Chemical properties
Cracking reactions
CH3* / *CH2-CH2-CH3
CH3-CH2* / *CH2-CH3
after a certain number of steps, we will obtain an alkene and hydrogen gas: CH2=CH-CH2-CH3 + H2
R + X2 → RX + HX
These are the steps when methane is chlorinated. This a highly exothermic reaction that can lead to an explosion.
1. Activation step: formation of two free radicals of Cl
Cl2 → Cl* / *Cl
2. Initiation step (slow step): a H atom is pulled off from methane
CH4 + Cl* → CH3+ + HCl
3. Propagation step:
CH3+ + Cl2 → CH3Cl + Cl*
4. Breaking step: recombinaison of two free radicals
R + O2 → CO2 + H2O + H2
Is a very exothermic reaction. If the quantity of O2 is insufficient, it will form a poison called carbon monoxide (CO). Here is an example with methane:
CH4 + 2 O2 → CO2 + 2 H20
with less O2:
CH4 + 3/2 O2 → CO + 2 H20
with even less O2:
CH4 + O2 → C + 2 H20Halogenation reaction
catalysed with UV.Combustion