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.
Those alkanes, and their derivatives, with four or fewer carbons have non-systematic common names, established by long precedence.
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.
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:
and
Physical properties
Chemical properties
Cracking propertie
This operation break huge molecules into smaller ones. This can be done by a thermic or a catalytic way. The cracking's mechanism is a homolytic breaking and so, there is formation of free radicals. This mechanism is relatively complex but we can say that there it form in great proportions a light alkane and a heavy alkene, and sometimes a deshydrogenation.
Here is an example of cracking with butane CH3-CH2-CH2-CH3
CH3* / *CH2-CH2-CH3
after a certain number of steps, we will obtain an alkane and a alkene : CH4 + CH2=CH-CH3
CH3-CH2* / *CH2-CH3
after a certain number of steps, we will obtain an alkane and a alkene from different types : CH3-CH3 + CH2=CH2
after a certain number of steps, we will obtain an alkene and a dihydrogen gaz : CH2=CH-CH2-CH3 + H2
Halogenation reaction
R + X2 → RX + HX
This is the example of chloration of methane. This a really dangerous reaction that can leads to explosion.
1. Activation step : formation of two free radicals of Cl
Cl2 → Cl* / *Cl
catalysed with UV.
2. Initiation step (slow step) : an 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
Combustion
R + O2 → CO2 + H2O
It is very exothermic reaction. But, if the quantity of O2 is insufficient, it can form a poison called carbon's monoxyde CO. Here is an example with methane :
CH4 + 2 O2 → CO2 + 2 H20
with less O2 :
CH4 + 3/2 O2 → CO + 2 H20
with lesser O2, there is a lighting flame :
CH4 + O2 → C + 2 H20
links
See also: cycloalkane, functional group
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