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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.

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 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.


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:




Physical properties

Chemical properties

Cracking properties

"Cracking" breaks larger molecules into smaller ones. This can be done with a thermic or catalytic method. The cracking mechanism is a homolytic process, thus, free radicals are formed.

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 an alkene: CH4 + CH2=CH-CH3

CH3-CH2* / *CH2-CH3

after a certain number of steps, we will obtain an alkane and an alkene from different types: CH3-CH3 + CH2=CH2

after a certain number of steps, we will obtain an alkene and hydrogen gas: CH2=CH-CH2-CH3 + H2

Halogenation reaction

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
catalysed with UV.

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 H20

See also: cycloalkane, alkene, alkyl, functional group .\n