The ALKANE series of saturated hydrocarbons - naming, bonding, structure, physical and chemical properties

3a. The structure and names of alkanes

• The principal source of alkanes is crude oil and natural gas (see section 2. OIL)

• Alkanes are a group of hydrocarbon molecules in which all the carbon and hydrogen atoms are only joined by single covalent bonds (e.g. C–H or C–C).

  • Carbon has an electronic combining power (valency) of 4 and hydrogen a valency of 1. So in alkane molecules carbon forms four bonds to hydrogen atoms or other carbon atoms. Hydrogen can only form one single bond to a carbon atom.

  • Check this out with the structural/displayed formula above.

  • Note that the name ends in ...ane eg methane, ethane, propane, butane etc.

  • They are obtained from the fractional distillation of crude oil.

  • A hydrocarbon, e.g. an alkane, can only consist of carbon and hydrogen atoms.

    • If another type of atom (element) is present in the molecule it cannot be a hydrocarbon e.g. alcohols, esters and carboxylic acids contain oxygen atoms as well as carbon and hydrogen.

  • Alkanes very useful chemicals, particularly as fuels like natural gas, petrol, diesel etc.

• Alkanes are an example of a homologous series of organic compounds.

  • A homologous series is a family of compounds which have a lot in common, but their common features must be carefully defined.

  • Features of members of a homologous series as exemplified by alkanes:

    • They have a general formula, in this case C_nH_2n+2 for alkanes where n = number of carbon atoms in the alkane molecule (n = 1, 2, 3 etc.).

    • Alkanes differ by the addition of an extra -CH₂- unit from one member to the next e.g.

      • ==> ==> ==> for the first four alkanes

    • Members of a homologous series show a gradual variation in physical properties, e.g. with increase in size of the carbon chain the boiling point gradually rises from one alkane to the next (see table further down).

    • They have the same functional group. However, unlike all other homologous series of organic molecules, alkanes don't have a functional group of characteristic atoms displaying a particular set of chemical reactions like alkenes, alcohols or carboxylic acids, which I'm sure you will study later. BUT, they must have, and do have, a similar molecular structure, (lots of examples below) AND this similarity in molecular structure gives the alkanes a set of similar chemical properties e.g. similar chemical reactions.

    • This means you can not only predict the formula of an alkane, but you can predict the outcome of its chemical reactions.

  • See section 8. for more on homologous series and the variety of organic compounds

• Alkanes are a family saturated hydrocarbons with the general formula C_nH_2n+2 where n is the number of carbon atoms in the molecule, so ..

  • From the general alkane formula you can deduce the molecular formula for ANY alkane (that isn't a ring compound) given the number of carbon atoms e.g.

  • when n = 1 you get CH₄, n = 2 gives C₂H₆, n = 3 gives C₃H₈,

    • then C₄H₁₀, C₅H₁₂, C₆H₁₄, C₇H₁₆, C₈H₁₈ etc.

  • As with naming many organic molecule series, eth.. means 2 carbon atoms in the chain, prop... means 3 and but... means 4 etc. After that the name is directly derived from the number of carbon atoms in the chain eg pentane for 5 carbons, hexane for 6, heptane for 7, octane for 8 etc. to match with the formulae quoted above.

• Alkanes are known as saturated molecules because other atoms cannot add to them.

  • In other words, the carbon atoms are bonded to as many other atoms as they can.

  • Being saturated means the alkane molecules have no C=C double bonds, only carbon–carbon single bonds, and so alkanes combined with the maximum number of atoms i.e. no atoms can add to it.

  • Compare alkanes with unsaturated alkenes – unsaturated with a double C=C bond.

  • For example, unlike alkenes (with a double bond to which atoms can add across) they do NOT react with, and decolourise, bromine water.

  • For the same reason, alkanes cannot be converted into polymers like poly(ethene), made from ethene.

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3b. The physical properties of alkanes

• The relationship between molecular mass and physical properties like boiling points

  • Various physical properties of the first twenty in the alkane homologous series are shown in the table below.

  • The first four are all colourless smelly highly flammable gases.

  • The larger alkanes are colourless liquids and the bigger members of the series are white waxy solids (20th onwards).

• General formula C_nH_2n+2 where n = number of carbon atoms in the linear chain (na = not applicable)

• As the molecular mass of an alkane increases, quite clear trends in physical properties emerge ...

  • ... the melting points and boiling points of alkanes steadily increase

    • This is because the bigger the alkane molecule, the greater the attractive intermolecular bonding forces (intermolecular bonding) between the alkane molecules - this increases the kinetic energy the molecules need to overcome the these attractive forces to change from liquid to gas - see the boiling point graph below.

      • You need to distinguish this relatively intermolecular attractive force from the much stronger force of the covalent bonds between the carbon atoms (C-C) of the chain of the hydrocarbon.

    • 

    • The graph shows the boiling point of alkanes from methane CH₄ (boiling point -164^oC/109 K)  to tetradecane C₁₄H₃₀ (boiling point 254^oC/527 K). [Remember K = ^oC + 273]

    • For alkane liquids, this increase in intermolecular forces with increase in length of carbon chain, means they also become less volatile, less flammable and more sticky (more viscous, less runny).

  • ... the density of the alkane increases, but all the liquid and solid alkane hydrocarbons float on water (density 1.00 g/cm³).

  • ... alkanes become more flammable e.g. more easily vaporised and ignited with a spark

    • this is measured by the 'flash point', this is the lowest temperature at which the alkane liquid gives off sufficient vapour to ignite in air (you don't need to know this for GCSE).

    • The first four gaseous alkanes are very flammable and explosive in air!

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3c. The various ways of representing the molecular structure of alkanes

The principal source of alkane hydrocarbons is crude oil –

See section 2. Fractional distillation of crude oil & uses of fractions

The molecular structure of ALKANES, all the C–C and C–H bonds are single covalent bonds

(more details lower down)

(1) is the molecular formula: a summary of the totals of each atom of each element in one molecule e.g. of an alkane.

(2) is a 'shorthand' or 'condensed' version of the full alkane structural formula (3).

(3a) is called the structural formula or 2–D displayed formula: it shows how all the atoms are linked by covalent bonds in the alkane molecule (the dashes — represent bonds), but only in 2–D, not the real shape.

(3a) In a correct displayed formula for the alkane (or any other molecule), all the atoms are clearly and individually shown AND dashes to represent the covalent bonds between the atoms in the molecule. – for a single bond in alkanes (C–C, C–H), or = for a double bond in alkenes (C=C as well as C–C and C–H).

(3b) Sometimes the atoms are just portrayed as spheres, but NOT considered the proper displayed formula for a molecule and such diagrams do not show the covalent bonds clearly.

(4a) is either a 3D version (3–D model) of the 'displayed formula', it gives some idea of the way the bonds are directed spatially and a better impression of the shape of the molecule.

(4b) is a '3D' 'ball and stick' model representation of the structural formula (3) showing the spatial arrangement of the atoms in the alkane.

(5) Is called a 'space filling' model and gives an idea of all the space used by the electrons around the nucleus and the electrons between the nuclei forming the bond.

(1=2)   (3a)   (3b)

(4a)   (4b)

(main molecule in natural gas)

(1) (2) 

CH₃CH₃ (3a)  (3b)

(4a)       (4b)

(5) you can't see the 6th H atom!

(1)   (2) or

(3a) (3b)  (4a)

in bottled gas

(1)   (2) or

(3a)   (3b)

in bottled gas

The full displayed formula for the first five members of the homologous series of ALKANES

These diagrams show ALL the single covalent bonds (C-H and C-C) in alkane molecules

The formulae can also be written in an abbreviated way as:

CH₄,   CH₃CH₃,   CH₃CH₂CH₃,   CH₃CH₂CH₂CH₃   and   CH₃CH₂CH₂CH₂CH₃

pentane ball and stick model

C₅H₁₂

C₆H₁₄

C₇H₁₆

C₈H₁₈

C₉H₂₀

C₁₀H₂₂

  C₁₁H₂₄

C₁₂H₂₆

NOTE:

Although the longer alkanes are drawn above in a linear way, in reality, the molecule is very flexible and can adopt all sorts of 'wiggly' shapes.

See the diagram below as examples of the multitude of shapes the alkane molecules can adopt! The backbone of carbon atoms of the alkane molecules are quite flexible and the longer the chain the more flexible or 'wiggly' they are!

There are hundreds of different alkanes known and many do not have a 'straight' chain of carbon atoms, but have 'branches', some are shown below, but don't bother about their names (leave that for A level!)

           this last one with 8 carbon atoms is called 'isooctane' and is an ingredient of petrol to ensure smoother combustion in car engines.

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 3d. and 3e. MORE ON THE CHEMISTRY OF ALKANES

They are not very reactive unless they are burned! or brought into contact with very reactive molecules like chlorine

3d. The combustion reactions of Alkanes

The complete combustion of hydrocarbons e.g. an alkane in excess air

• The diagram shows how to detect the products of hydrocarbon combustion e.g. burning candle wax.

• Candle wax is a long chain alkane hydrocarbon molecule which should burn completely to carbon dioxide and water - however, the combustion is inefficient and soot particles are formed, which are so hot they are incandescent and give out light.

• When hydrocarbons are burned in air a fast exothermic reaction occurs releasing heat and forming carbon dioxide and water – their formation is an oxidation reaction.

• The water pump draws the combustion gases through the two chemical test systems in the big U tubes.

• It is an oxidation reaction because of oxygen atom gain by the carbon and hydrogen atoms of the hydrocarbon molecules.

• The carbon dioxide is chemically detected with limewater – with which it forms a white precipitate (milky appearance) of calcium carbonate.

• The water is chemically detected either by

  • (i) anhydrous white copper sulphate turning blue

  • or

  • (ii) dried blue cobalt chloride paper turning pink.

• A physical test for water is to measure its boiling point (should be 100^oC), you could test the colourless liquid, if enough of it is collected.

You can do some simple experiments to see how long a candle burns in various sized beakers.

You can then plot a graph of burning times versus volume of beaker.

The candle goes out when the oxygen falls to a low value.

The candle should burn longer in a larger beaker because more oxygen from the air is available.

Equations for the complete combustion of a hydrocarbon like an alkane

When a hydrocarbon molecule (reactant) burns in an excess of air–oxygen their are only two products of the reaction.

The carbon atoms are oxidised on combining with oxygen to form carbon dioxide molecules, and the hydrogen atoms are oxidised to water molecules ('hydrogen oxide').

Blue flames indicate complete combustion releasing lots of heat energy, but smokey yellow flames indicate incomplete combustion releasing less energy and producing dirty sooty carbon (and sometimes deadly carbon monoxide too).

See Pollution, carbon monoxide, nitrogen oxides, what makes a good fuel?, climate change–global warming

So complete oxidation = complete combustion

general word equation: alkane hydrocarbon + oxygen ===> carbon dioxide + water

word equations e.g. methane + oxygen ===> carbon dioxide + water

and the corresponding symbol equation is

CH_4(g) + 2O_2(g) ===> CO_2(g) + 2H₂O_(l)

Note that one CO₂ for every C, and one H₂O for every two H's in the hydrocarbon molecule.

In terms of displayed formula the equation would be written as ...

... in which every individual atom is shown and how it is bonded ('connected') with other atoms in the molecule. All the dashes represent the covalent bonds between the atoms in the molecules.

This is an example of an oxidation reaction - atoms (carbon/hydrogen) have gained oxygen (become combined with oxygen).

Another example is the complete combustion of another alkane, propane ...

propane + oxygen ===> carbon dioxide + water

C₃H_8(g) + 5O_2(g) ===> 3CO_2(g) + 4H₂O_(l)

and in terms of displayed formula and balancing numbers ...

and the above diagrams show how the atoms have rearranged themselves in the reaction after the reactant bonds are broken (C–H, O=O and C–C in ethane etc. below)) and the new bonds formed in the products (C=O and O–H). Note the number of atoms of each element must be the same on each side of the equation (1C, 4H's and 4 O's, Law of Conservation of mass) and the products are different substances with different properties compared to the reactants.

See Elements, Compounds and Mixtures page for more on writing and balancing equations

for the alkanes ethane and butane etc. the word and more awkward symbol equations are ...

 (note the use of 3½ and 6½ in balancing equations is perfectly legitimate)

ethane + oxygen ===> carbon dioxide + water

2C₂H_6(g) + 7O_2(g) ===> 4CO_2(g) + 6H₂O_(l)

avoiding the ¹/₂ molecule! or not avoiding it!

C₂H_6(g) + 3¹/₂O_2(g) ===> 2CO_2(g) + 3H₂O_(l)

butane + oxygen ==> carbon dioxide + water

2C₄H_10(g) + 13O_2(g) ===> 8CO_2(g) + 10H₂O_(l)

or not avoiding not the ¹/₂ molecule !

C₄H_10(g) + 6¹/₂O_2(g) ===> 4CO_2(g) + 5H₂O_(l)

and for pentane the equations are ...

pentane + oxygen ===> carbon dioxide + water

C₅H_12(l) + 8O_2(g) ===> 5CO_2(g) + 6H₂O_(l)

See also Calorimeter methods of determining energy changes - burning fuels

For notes and equations on incomplete combustion see ...

... Fossil fuel air pollution - incomplete combustion, carbon monoxide & soot particulates

Uses of propane gas C₃H₈

In the UK propane gas is used from red coloured cylinders for domestic heating and cooking.

BUT, it is also used in cutting equipment for the demolition and recycling of metal structures.

In the past oxy-acetylene cutters were used, but the hydrocarbon acetylene, (now called ethyne, C₂H₂), is a very dangerous explosive gas and so propane, another hydrocarbon, is used instead - as an oxy-propane blow torch.

The oxygen is supplied in a separate cylinder, well away from the propane cylinder for safety reasons!

The two gases are piped to the cutting tool where they are mixed and ignited to give a powerful cutting torch - illustrated below.

Quite a 'fireworks' display in the end from: C₃H_8(g) + 5O_2(g) ===> 3CO_2(g) + 4H₂O_(l)  - very exothermic !!!

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