Alkanes - Definition


Alkanes - Definition

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 general formula for acyclic/linear alkanes, also known as aliphatic hydrocarbons is C_nH_2n+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.

Contents

1 Properties

1.1 Arrangements
1.2 Physical properties
1.3 Chemical properties

2 Reactions

2.1 Cracking reactions
2.2 Halogenation reaction
2.3 Combustion

3 See also

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, difficult to break and non-polar. 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. Initiation 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
energy provided by UV.

2. Propagation (two steps): a hydrogen atom is pulled off from methane then the methyl pulls a Cl from Cl₂

CH₄ + Cl* → CH₃* + HCl

CH₃* + Cl₂ → CH₃Cl + Cl*

This results in the desired product plus another Chlorine radical. This radical will then go on to take part in another propagation reaction causing a chain reaction. If there is an excess of Chlorine, other products like CH₂Cl₂ may be formed.

3. Termination step: recombination of two free radicals

Cl* + Cl* → Cl₂, or
CH₃* + Cl* → CH₃Cl, or
CH₃* + CH₃* → C₂H₆.

The last possibilty in the termination step will result in an impurity in the final mixture; notably this results in an organic molecule with a longer carbon chain than the reactants.

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