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Alkanes - Organic Chemistry Tutor

This is the correct chemical term for compounds known as paraffins. They are considered the simplest organic compounds and are a family of chain hydrocarbons having the general formula C2H2n+2. All of the bonds are single bonds (-C-H-, and -C-C-). The chains can be straight or branched. The smaller members (less than 4 carbons) are gases, while larger ones (five to seventeen carbons) are liquids. Beyond seventeen carbons the alkanes are waxy solids.

Structure

The simplest saturated acyclic hydrocarbon is methane, CH4.  The other hydrocarbons belonging to this class can be taken as descendents of methane, resulted from the substitution of one or more atoms of hydrogen with hydrocarbon radicals. Therefore, if one hydrogen atom from methane is replaced by a methyl radical, -CH3, the hydrocarbon (superior to methane) will have the composition C2H6, named ethane.

If one hydrogen atom is replaced by a methyl radical -CH3, the saturated hydrocarbon obtained (-C3H8) will be named propane: CH3-CH2-CH3. Propane is also considered as the derivation of methane resulted by the replacement of two hydrogen atoms with two methyl radicals or with one ethyl radical (-C2H5).

If you continue the successive substitution of one hydrogen atom with from the hydrocarbon with one radical –CH3you will obtain a series of hydrocarbons, each different from one another with one group of CH2:

Methane           CH4
Ethane             CH3-CH3
Propane           CH3-CH2-CH3
Butane             CH3-CH2-CH2-CH3
Pentane           CH3-CH2-CH2-CH2-CH3
Hexane             CH3-CH2-CH2-CH2-CH2-CH3
Heptane           CH3-CH2-CH2-CH2-CH2-CH2-CH3
Octane             CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3
Nonane           CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
Decan             CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3

Such a series, where every last term of the line differs from the former through a CH2 group is named homologous series. Each term of this series is the superior homologue of the previous one. The chemical structure of the chain of the homologous series terms, meaning of the homologues, is identical. Therefore, the chemical properties of the homologues are very similar.

The alkanes can be: with a continuous chain, if all the carbon atoms are bound with at most two other carbon atoms, and with a branched chain, if one or more carbon atoms from the molecule are bound with at least two other carbon atoms. Such an isomerism of chain is possible starting with butane. The number of isomers of saturated acyclic hydrocarbons is very big; he grows with each number of carbon atoms from the molecule. For example, the carbide C10H22 has 75 isomers; the hydrocarbon C20H42 can have approximately 366.319 isomers.

Nomenclature

The first four alkanes with a unbranched chain, meaning normal (n-alkanes) are called: methane, ethane, propane, butane. The superior alkanes names are compound with a numeric prefix (which shows the number of carbon atoms present in the molecule) and the ending “-ane”. For example: pentane, hexane, heptane, and octane. For the hydrocarbons with two groups of methyl (-CH3) at the end of a linear chain, you use the prefix “iso-” at the name of the hydrocarbon, and for those containing three -CH3 groups at the end of the chain, you use the prefix “neo-” at the name of the hydrocarbon.

Mono valence radicals derived from normal alkanes by removing one hydrogen atom from a terminal carbon atom are called normal alkyls. Their name is formed by the replacement of the ending “-ane” from the hydrocarbon’s name with the ending “-yl”. For example, CH3 is called methyl; CH3-CH2 is called ethyl and CH3-CH2-CH2 is called propyl.

The branched chain alkanes use the name of the longest sidewise chain as a prefix in their denomination. The position of the sidewise chain is indicated by a number. For this, the longest chain is numbered from left to right or vice verse, in the direction in which the sidewise chain will have the position with the smallest number. The presence of more identical sidewise chains is indicated by the prefixes: di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-.

Bivalence and tri-valence radicals derived from alkyls by removing one or two hydrogen atoms from the mono-valence carbon atom, have their names formed by adding the ending “-iden” or “-idin”. For example, CH3-CH= is called ethyliden. The radical CH2= is named methylene, CH is name ethylidin.

General formula

In a alkane molecule at n carbon atoms you fin 2n+2 hydrogen atoms. Therefore the general formula of the alkanes is CnH2n+2. Relying on the general formula you can obtain the molecular formula of each and every alkane if the number of carbon atoms is known. For example, for a molecule which contains 2 carbon atoms (n=2), the number of hydrogen atoms is 2*2+2=6 so the hydrocarbon’s molecular formula is C2H6 (ethane).

Alkanes in nature

Some alkanes form through the slow decomposition of organic substances, especially wood, lignite and mineral coal. Therefore, they can be found in the distillery tars of these substances. Big quantities of alkanes can be found in mineral oil and natural gases. Hence, paraffin in mineral oil is a mixture of superior alkanes. Alkanes with a bigger number of carbon atoms are found in some plants. The extraction of the alkanes in pure state out of natural product is difficult, their properties being very much alike and their boiling point very close.

Obtaining methods


1. From aluminium chloride:

Al4C3 + 12H20 3CH4 + 4Al(OH)3
Al4C3 + 12HCl 3CH4 + 4AlCl3

2. Unsaturated hydrocarbons (which contain a double or triple bound)

alkenes + H2 alkanes (addition reaction) – nickel is used as a catalyser

3. From halogenated derivates:

The Wurtz reaction: R-X + R-X + 2Na 2NaX + R-R (the obtained alkane is a symmetrical alkane)
X can be: Cl, Br, I

R-X + Mg RMgX

4. Carboxylic acids:

R-COOH + NaOH H2O + R-COONa (sodium carboxyl) Na2CO3 + R-H (alkaline melt)

5. The Kolbe electrolyze Kolbe of sodium carboxyls:

R-COONa RCOO + Na
H20 H + OH

cathode(-) H + 1e H (reduction); H20 H + OH

anode(+) RCOO -1e RCOO R + CO2; R + R R-R (symmetrical alkane)

2RCOONa + 2H20 R-R + 2CO2 + H2 + 2NaOH


Chemical reactions


1. Halogenated (Cl2, Br2)

R-X + X2 R-X + HX (the obtained product is a halogenated derivate)

Example: CH4 + Cl2 CH3-Cl + HCl (methyl chloride or methane chlorine)

2. Thermic decomposition (< 650 grade - cracking; > 650 grade – pyrolize)

CnH2n+2 CmH2m+2 + CpH2p where n = m + p and p >= 2

Pyrolize methane: 2CH4 CH CH + 3H2

3. Alkanes isomerism

It takes place in the presence of AlCl3 at temperature within 50 and 100 Celsius degrees. The isomerism reaction is a reversible.

4. Oxidation

a) Combustion: CnH2n+2 + O2 CO2 + H2O + Q – exothermic reaction dH < 0

b) Incomplete oxidation: R-CH3 + O2 R-CH2-OH + R-CH=O + RCOOH

c) CH4 Oxidation: CH4 + O2 CH3-O-H (methylic alcohol); CH4 + 3/2 O2 CO + 2H20 (incomplete combustion)

5. Nitrating reaction

R-H + HNO3 R-NO2 + H2O (nitro derivate)

Examples:
CH4 + HNO3 CH3-NO2 + H2O (nitro methane)
CH3-CH2-CH3 + HNO3 CH3-CH2-CH2-NO2 + H20 (1-nitropropan)


Use

» alkanes can be taken as fuel
» alkanes are used to obtain chemical compounds for different uses
» methyl chloride is a frigorific agent
» prussic acid is used to obtain synthetic fiber

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