Online Encyclopedia
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). The next in the series is ethane (C2H6) and the series continues with larger and larger molecules. Each C atom is hybridized sp3. The series of alkanes is often called the paraffin series.
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Properties
Arrangements
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. For a more complete list, see List of alkanes.
| methane | CH4 |
| ethane | C2H6 |
| propane | C3H8 |
| n-butane | C4H10 |
| n-pentane | C5H12 |
| n-hexane | C6H14 |
| n-heptane | C7H16 |
| n-octane | C8H18 |
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")
Physical properties
- Alkanes are virtually insoluble in water.
- Alkanes are less dense than water.
- Melting points and boiling points of alkanes generally increase with molecular weight and with the length of the main carbon chain.
- 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 have a low reactivity because the C-H and C-C single bonds are relatively stable and difficult to break. They are also known as paraffins (Latin para+affinis with the meaning here of "lacking affinity").
Reactions
Cracking reactions
"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
- 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 an 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 an alkene from different types: CH3-CH3 + CH2=CH2
- 3rd possibility (14%): breaking of a C-H bond
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: splitting of a chlorine molecule to form two chlorine atoms. A chlorine atom has an unpaired electron and acts as a free radical.
Cl2 → Cl* / *Cl
catalysed with UV.
2. Initiation step (slow step): a hydrogen atom is pulled off from methane
CH4 + Cl* → CH3+ + HCl
3. Propagation step:
CH3+ + Cl2 → CH3Cl + Cl*
4. Breaking step: recombination of two free radicals
- Cl* and Cl*, or
- R* and Cl*, or
- CH3* and CH3*.
Combustion
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 H2O
with less O2:
2 CH4 + 3 O2 → 2 CO + 4 H2O
with even less O2:
CH4 + O2 → C + 2 H2O