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 C_nH_2n+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 alkanes names end by -ANE. The 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.

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

and

2-methylbutane

Physical properties

• Alkanes are insoluble into water.

• Alkanes's density is inferior to 1.

• Melting point and boiling point increase with molecular weight.

• At standard conditions from CH₄ to C₄H₁₀, alkanes are gaseous; From C₅H₁₂ to C₁₇H₃₆, they are liquids; And after C₁₈H₃₈, they are solids.

Chemical properties

• Alkanes tend to be generally unreactive because the C-H and C-C single bonds are stable and hard to break.

Cracking properties

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

• 1st possibility (48%): breaking is done on the CH₃-CH₂ bond.

CH₃* / *CH₂-CH₂-CH₃

after a certain number of steps, we will obtain an alkane and a alkene : CH₄ + CH₂=CH-CH₃

• 2nd possibility (38%): breaking is done on the CH₂-CH₂ bond.

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

• 3rd possibility (14%): breaking of an C-H bond

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

• Cl* and Cl*, or
• R* and Cl*, or
• CH₃* and CH₃*.

Combustion

R + O₂ → CO₂ + H₂O + H₂

It is very exothermic reaction. But, if the quantity of O₂ is insufficient, it can 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 lesser O₂, there is a lighting flame :

CH₄ + O₂ → C + 2 H₂0

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