Alkane Arrangements Physical Properties Chemical Properties

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.

1 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.

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 heteroatomIn the nomenclature of organic chemistry, a heteroatom is any atom that is not carbon or hydrogen, typically, but not exclusively, nitrogen, oxygen, sulfur, phosphorus or boron. In the description of protein structure, particularly in the now-deprecated Ps 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:

2 Physical properties

Alkanes are virtually insoluble into water.

The density of an alkane is less than water density.

Melting point and boiling point increase with molecular weight and with 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.

3 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 paraffinParaffin is a common name for a group of high molecular weight alkane hydrocarbons with the general formula CH, where n is greater than about 20. Paraffin is also a technical name for an alkane in general, but in most cases it refers specifically to a lins (Latin para+affinis with the meaning here of "lacking affinity").

4 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 carbo cation 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 an C-H bond

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

5 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*.

6 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

7 See also

Hydrocarbons

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methane	CH₄
ethane	C₂H₆
propane	C₃H₈
\| \| \| H C C C C H \| \| \| \| H H H H It is a flammable, colorless, easily liquefied gas that is used extensively as a fuel for cigarette lighters	C₄H₁₀
n- pentanePentane (also known as amyl hydride or skellysolve is an alkane hydrocarbon with the chemical formula CH(CH)CH. External links http://www. gov/cgi-bin/duke/chemical. pl?PENTANE Alkanes.	C₅H₁₂
n- hexaneHexane is an alkane hydrocarbon with the chemical formula CH(CH)CH. The "hex" prefix refers to its six carbons, while the "ane" ending indicates that its carbons are connected by single bonds. Hexane isomers are largely unreactive, and are common constitu	C₆H₁₄
n- heptaneHeptane (also known as dipropyl methane gettysolve-C or heptyl hydride is an alkane hydrocarbon with the chemical formula CH(CH)CH. Heptane has nine isomers; n-heptane (straight-chain heptane) has an octane rating of 0 by definition; it is undesirable in	C₇H₁₆
n- octaneOctane is an alkane hydrocarbon with the chemical formula CH(CH)CH. It has many isomers; one of these, isooctane (2,2,4-trimetylpentane), is especially important, because its octane rating is by definition 100. It is an important constituent of petrol.	C₈H₁₈

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