The Alkanes

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The Alkanes

Alkanes are saturated hydrocarbons; they only contain single bonds. Theyare called saturated because they contain the maximum amount ofhydrogen atoms possible.

The general formula for an alkane is CnH2n+2 where n is a positive integer.

Above is a table with the first ten alkanes. Note that at the end of eachname is -ane this is the same for all alkanes. At the beginning of theword is the stem meth-, eth-.. this relates to how many carbonsthere are in the alkane.

A series of compounds that are all related, like alkanes, are called ahomologous series.

All the series have the same general formula, but each member differsfrom the next by a CH2 unit.

All the compounds in the series have similar chemical properties, butdifferent physical properties such as boiling point and density. Howeverthe physical properties change gradually in the series as the number ofcarbons increase.

Alkane structural Isomers

As well has just having a straight line chained carbons, it is often possiblefor branches of chains to branch off the primary chain.

There are often many different structural formulas for a given molecularformula.

For Example:

C4H10 has two possible structural formulae:


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These two compounds are isomers, because they have the same chemical

formula, but different structural formulae. Notice that the branched alkane has a different name; it is considered to

have been formed from a propane group with an added methyl group. Itis therefore called methyl propane.

To name branched alkanes:

1) Find the longest chain of carbons. This doesn't have to be a straight line - it cango around corners, as long as it follows the carbon chain. This is the constituentchain and goes on the end of the name. In the above example the longest possiblechain is 3 (either a horizontal line or the one at the top, bottom middle then eitherbottom left or right). This number then names the isomer: e.g. three carbons meansthat the name of this ends with propane.2) Using the longest chain, identify any branching out from it. In the above isomerthere is just one branch - this branches off the second carbon from either end (if thenumbers from either end were different, the lowest would be taken). The branchconsists of just one carbon, or a methyl group.

3) The table below contains the names of the groups with more carbons in them:

As well open chained alkanes, it is also possible for alkane molecules to form rings. These rings are called cyclo-alkanes, and have the same general formula as an alkene CnH2n.

They have two less hydrogens, because there are no CH3 groups at the end of the chains.


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Every corner on the skeletal formula represents a CH2 group. Skeletal diagrams are very

useful when drawing cyclic hydrocarbons.

Skeletal diagrams can also be used to draw chained and branched isomers.

The above represents pentane, every kink in the line represents a CH2 group, and the endsrepresent a CH3 group.

The above represents 4 methyl heptane.Shapes of Alkanes

Representing three dimensional shapes in 2D on paper can give a misleading image of whatthe molecule looks like.

The pairs of electrons in covalent bonds repeal each other and so arrange themselvesaround the carbon atom as far apart as possible.

The C-H bonds in methane are directed so that they form a tetrahedron shape, with bondangles of109

o

The structure of ethane:


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The structure of butane:

Hydrocarbon chains are not really straight, but are "Zig-Zaggged". All the bond angles are109

o

Alkane characteristics

What an alkane looks like at room temperature depends upon its size.

Alkanes contain non-polar molecules and mix well with each other; however they do not mixwell with non-polar substances such as water.

Alkanes are unreactive towards many laboratory reagents; they are unaffected by acids,alkalis, metals and oxidising agents.

When they do react, it is usually in the gas phase, and energy is needed to get the reactionstarted.

Melting and Boiling points of Alkanes

As the molecular number increases, the boiling and melting point of the alkane increases.


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C1 to C4 are gases, C5 to C17 are liquids, C18 and higher are solids. The increase in the boiling point is due to the stronger intermolecular forces.

In hydrocarbons the only intermolecular forces are instantaneous dipole, induced dipoleinteractions.

In a molecule, electrons are mobile, and at one point they may all find themselves at oneend of the molecule. This will form a temporary dipole, affecting neighbouring molecules,

pulling the molecules together.

At the next instant the electrons could end up at the other side reversing the polarity of themolecule.

This permanent moving around of the electrons causes rapidly fluctuating dipoles whichhold together the hydrocarbons.

The higher the molecular weight of the compound, the more electrons there are in themolecules, meaning the stronger these Van der Waahl forces. This is why the melting points

and boiling points of the larger molecules are higher.

Branched alkanes have a lower Van der Waahl force than chains, due to their shape, whichdoesnt allow the molecules get close enough to each other.

Oxidation of Alkanes

o Alkanes do not react readily with air, but if they are heated they can combust to form carbondioxide and water.o The reaction has a high activation energy; the energy must be supplied in order for the

reaction to begin.

o Alkanes must be vaporised before they can combust, thus more volatile hydrocarbons igniteeasier.

o Once ignited, the reaction is very exothermic, which is why alkanes are often used for fuels.o If the air supply is limited, the combustion may be incomplete, producing carbon dioxide and

soot, along with partially combusted hydrocarbons.

Molecular and Empirical Formula of Alkanes

o A molecular formula shows how many atoms of each type are present in a given compound.


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o The empirical formula shows the smallest ratio of the different types of atom present.o The Molecular Formula = M x Empirical Formula (where M= a positive integer).o The empirical formula of a compound can be worked out from its percentage composition.

For a hydrocarbon this can be found out by burning a known mass in oxygen and working

out the amounts of carbon dioxide and water formed. This is called combustion analysis.

Example:

0.1g of a hydrocarbon x gave 0.309g CO2 and 0.142g H2O on

combustion. What is the empirical formula of x?

Calculate the mass of C and H in 0.1g of the compound (all of the

compound goes to H2O and CO2, so the mass of carbon and hydrogen

in the compound can be worked out from the mass's of carbon

dioxide and water produced).

44g CO2 contains 12g of C (Mr of CO2 is 44, and Ar of C is 12).

so 0.309g contains 0.309 x 12/44 = 0.0843g

18g H2O contains 2g of H

so 0.142g contains 0.142 x 2/18 = 0.0158g

So the compound contains 0.0843g of Carbon and 0.0158g of

Hydrogen.

So the empirical formula of x is C4H9.

The Alkanes

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