structure and properties

Doc Brown's GCSE/IGCSE/O Level KS4 science CHEMISTRY Revision Notes

Oil, useful products, environmental problems, introduction to organic chemistry

3. ALKANES – saturated hydrocarbons - their physical and chemical properties

The alkanes are a homologous series of hydrocarbon molecules (made of carbon and hydrogen atoms). Alkanes are referred to as 'saturated' hydrocarbons because they do NOT contain a carbon C=C carbon double bond and other atoms cannot add to alkane molecules. The molecular structure and naming of alkanes is described and explained. The simple physical properties of alkanes e.g. melting point, boiling point and density are quoted. The chemical reactions of alkanes with oxygen (combustion, burning) and chlorine (substitution reaction to form a chloroalkane) are fully described with word and balanced symbol equations for the alkane in question. These notes on alkanes are designed to meet the highest standards of knowledge and understanding required for students/pupils doing GCSE chemistry, IGCSE chemistry, O Level chemistry and KS4 science courses. These revision notes on physical properties and molecular structure of alkanes, and chemical reactions of alkanes should prove useful for the NEW AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1), (9-5) & (5-1) science courses.

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3a. The ALKANE series of saturated hydrocarbons

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

  • doc b oil notesAlkanes 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 bond to a carbon atom.

    • 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.

    • 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:

      1. They have a general formula, in this case CnH2n+2 for alkanes where n = number of carbon atoms in the alkane molecule (n = 1, 2, 3 etc.) and from the general alkane formula you can deduce the number of hydrogen atoms, hence the complete molecular formula for ANY alkane (that isn't a ring compound).

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

        • doc b oil notes ==> doc b oil notes ==> doc b oil notes ==> doc b oil notes for the first four alkanes

      3. 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).

      4. 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.

      5. 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

  • Carbon forms four bonds (C–C or C–H) and hydrogen just one bond (C–H) in alkane hydrocarbon molecules.

    • Check this out with the structural/displayed formula in the table of alkane molecule structures below.

  • Alkanes are a family saturated hydrocarbons with the general formula CnH2n+2 where n is the number of carbon atoms in the molecule, so ..

    • 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, hexane, heptane, octane etc. – 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.

    • 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) 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.

  • Physical properties of alkanes:

    • The first twenty in the alkane series are shown in the table below and are all colourless smelly highly flammable gases.

    • The larger alkanes are colourless liquids and the biggest members of the series are white waxy solids.

  • Physical Properties of linear ALKANES (n–alkanes), see the table below for the first 20 members of the series

  • General formula CnH2n+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 forces (intermolecular bonding) between the alkane molecules.

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

      • This is exemplified by the fact that at room temperature and pressure, the first four alkanes are gases, then the alkanes are all liquids until the 18th alkane onwards, when the alkanes become white waxy solids.

      • 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!

n molecular formula (state at RTP) abbreviated structural formula name of alkane relative molecular mass Mr melting point temperature oC/K boiling point temperature oC/K flash point oC density g/cm3
1 CH4 (g) CH4 methane 16.0 –182/91 –164/109 na 0.466(l)
2 C2H6 (g) CH3CH3 ethane 30.1 –183/90 –88/185 na 0.572(l)
3 C3H8 (g) CH3CH2CH3 propane 44.1 –190/83 –42/231 na 0.585(l)
4 C4H10 (g) CH3CH2CH2CH3 butane 58.1 –138/135 0/273 na 0.601(l)
5 C5H12 (l) CH3(CH2)4CH3 pentane 72.2 –130/143 36/309 –49 0.626
6 C6H14 (l) CH3(CH2)4CH3 hexane 86.2 –95/178 69/342 –22 0.660
7 C7H16 (l) CH3(CH2)5CH3 heptane 100.2 –90/183 99/372 –4 0.684
8 C8H18 (l) CH3(CH2)6CH3 octane 114.2 –57/216 126/399 13 0.703
9 C9H20 (l) CH3(CH2)7CH3 nonane 128.3 –51/222 151/424 31 0.718
10 C10H22 (l) CH3(CH2)8CH3 decane 142.3 –30/243 174/447 46 0.730
11 C11H24 (l) CH3(CH2)9CH3 undecane 156.3 –25/248 196/469 60 0.740
12 C12H26 (l) CH3(CH2)10CH3 dodecane 170.3 –9/264 216/489 71 0.749
13 C13H28 (l) CH3(CH2)11CH3 tridecane 184.4 –5/268 234/507 102 0.756
14 C14H30 (l) CH3(CH2)12CH3 tetradecane 198.4 4/279 250/523 99 0.763
15 C15H32 (l) CH3(CH2)13CH3 pentadecane 212.4 10/283 267/540 132 0.769
16 C16H34 (l) CH3(CH2)14CH3 hexadecane 226.4 18/291 281/554 135 0.773
17 C17H36 (l) CH3(CH2)15CH3 heptadecane 240.5 22/295 302/575 148 0.777
18 C18H38 (s) CH3(CH2)16CH3 octadecane 254.5 28/301 326/599 165 0.777
19 C19H40 (s) CH3(CH2)17CH3 nonadecane 268.5 31/304 330/603 168 0.786
20 C20H42 (s) CH3(CH2)18CH3 eicosane 282.5 37/310 343/616 na 0.789
n molecular formula abbreviated structural formula name of alkane relative molecular mass Mr melting point temperature oC/K boiling point temperature oC/K flash point oC density g/cm3
*** ***************** ************************ ******************* ************** ******************* ****************** ********* ***********
Note: (i) RTP = room temperature and pressure, (ii) na = not applicable to that alkane, (III) the use of parentheses (brackets) to give, for long molecules like the higher alkanes, a more convenient abbreviated formula.

e.g. CH3CH2CH2CH2CH2CH2CH3 can be expressed as CH3(CH2)5CH3


They are not very reactive unless burned!

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' 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.

Table showing the structure of the first twelve ALKANES name of alkane
(1=2)doc b oil notes   (3a)doc b oil notes   (3b)

(4a)doc b oil notes   (4b)


(main molecule in natural gas)

(1)doc b oil notes (2)doc b oil notes (3a)doc b oil notes  (3b)

(4a)doc b oil notes       (4b)

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

(1)doc b oil notes   (2)doc b oil notes

(3a)doc b oil notes (3b)  (4a)alkanes structure and naming (c) doc b


in bottled gas

(1)doc b oil notes   (2)doc b oil notes

(3a)doc b oil notes   (3b)


in bottled gas

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

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

The formulae can also be written as: CH4, CH3CH3, CH3CH2CH3, CH3CH2CH2CH3 and CH3CH2CH2CH2CH3

The final examples are shown as the displayed formula and molecular formula of the alkane




pentane, hexane and heptane in petrol


octane and nonane



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!

decane, undecane and dodecane

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!)

alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b  alkanes structure and naming (c) doc b  this last one is called 'isooctane' and is an ingredient of petrol to ensure smoother combustion in car engines.


3b. The chemical reactions of Alkanes

doc b oil notesThe complete combustion of hydrocarbons e.g. an alkane in excess air

doc b oil notes

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

  • 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 100oC), you could test the colourless liquid, if enough of it is collected.

Equations for the complete combustion of a hydrocarbon like an alkane

doc b oil notesWhen 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'). This section ignores the combustion of the pollutant sulphur. 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

CH4(g) + 2O2(g) ==> CO2(g) + 2H2O(l)

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

doc b oil notes

topIn 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

C3H8(g) + 5O2(g) ==> 3CO2(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 more awkward symbol equations are ...


ethane + oxygen ==> carbon dioxide + water

2C2H6(g) + 7O2(g) ==> 4CO2(g) + 6H2O(l)

or avoiding the 1/2 molecule!

C2H6(g) + 31/2O2(g) ==> 2CO2(g) + 3H2O(l)


butane + oxygen ==> carbon dioxide + water

2C4H10(g) + 13O2(g) ==> 8CO2(g) + 10H2O(l)

or avoiding the 1/2 molecule !

C4H10(g) + 61/2O2(g) ==> 4CO2(g) + 5H2O(l)


and for pentane the symbol equations is ...


pentane + oxygen ==> carbon dioxide + water

C5H12(l) + 8O2(g) ==> 5CO2(g) + 6H2O(l)


More on incomplete combustion and its consequences are described on the fossil fuel pollution page

See also Calorimeter methods of determining energy changes - burning fuels


3c. More on ALKANESsaturated hydrocarbons

  • Alkanes are obtained directly from crude oil by fractional distillation (see oil notes section 2).
  • The saturated hydrocarbons form an homologous series called alkanes with a general formula CnH2n+2
  • Saturated means the molecule has 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.
    • The alkanes don't really have a functional group like many other series of organic molecules, and have quite a limited chemistry BUT they are still a clearly defined homologous series.
  • Alkane examples: The gases (names and molecular formula): methane CH4, ethane C2H6, propane C3H8, butane C4H10, liquids: pentane C5H12, hexane C6H14 etc.
  • The first four alkane structures are shown above and the names end in ...ane
  • Carbon always forms 4 bonds with other atoms and hydrogen 1 bond with other atoms e.g. Propane: molecular formula C3H8, structural and displayed formula styles include ...
    • or or
  • Isomerism occurs when two or more compounds have the same chemical formula but have different structures. e.g. for the molecular formula C4H10 there are two possibilities – one 'linear' and one with carbon chain 'branching', both isomeric structures are shown in three ways ...
    • butane:
    • , ,
    • and its isomer is methylpropane (shown below)
    • , ,
    • The variations in molecular structure for the same molecular formula are called isomers.
    • What you should realise is that isomers can exist because the atoms can be arranged in different ways as long as the valency (numerical combining power) of each atom is obeyed i.e. carbon forms four bonds and hydrogen forms one bond.
    • So, in the two isomers above, each molecule has three carbon – carbon single bonds and eight carbon – hydrogen bonds.
  • topCan you work out the structures of the 3 isomers of C5H12 ? (you will find enough to work out the answers on the Advanced Level page on ALKANES)
  • Isomers show variation in physical properties which depend upon the strength of the intermolecular forces. Intermolecular forces are due to weak electrical attractive forces that exist between all molecules.
    • e.g. 'linear' butane has a higher boiling point than the 'branched' methylpropane (diagrams above).
  • Molecular structure of alkanes and physical properties

  • (a) For a homologous series the strength of intermolecular forces (intermolecular bonding) increases as the carbon chain length increases, exemplified by the alkanes illustrated above.
    • This leads to a steady increase in melting point, boiling point (see the alkanes data table above), density and viscosity (if a liquid alkane).
  • (b) For isomers (same C number, molecular formula), the forces decrease as the amount of chain branching increases.
  • This is because the attractive forces are a function of the potential surface–surface contact i.e. the compactness or size of the molecules.
    • (a) as the chain length increases the surface–surface contact must increase per molecule,
    • (b) for isomers, with more branching, the chain length decreases and the molecule is more 'compact' reducing the surface–surface contact per molecule.
  • For example in the series ...
    • From methane ... ethane ... propane ... petrol ... oils ... grease ... waxes etc. the melting point/boiling points rise and so does the viscosity (stickiness! less runny, more sticky) as the carbon chain length of the alkane increases.
    • This trend also indicated by the change in alkanes from gases to liquids to solids ...
      • ... illustrated above by the boiling points of alkane hydrocarbons obtained from crude oil.
      • See 2. Uses of Oil Products page for more details – the use of alkanes is very strongly linked to their physical properties.
  • Alkanes and alkenes undergo combustion reactions (see above).
    • Alkanes are NOT very reactive because ....
      • (i) All the C–C and C–H bonds are very strong and not easily broken to allow the alkane molecule to undergo a chemical change.
        • However alkanes do react with very reactive molecules like chlorine BUT the reaction must still be initiated by heating the mixture to a high temperature or shining uv light into the alkane- chlorine mixture.
      • (ii) Other hydrocarbons like alkenes, have a reactive group of atoms, e.g. alkenes have carbon – carbon double bond that easily breaks open to allow new bonds to form. i.e. new molecules can readily form.
  • CHLOROALKANES (halogenoalkanes): Alkanes are usually not very reactive unless burned! BUT they will react with reactive chemicals like chlorine when heated or subjected to uv light to form chlorinated hydrocarbons.
    • Despite the reactivity of chlorine you still need something extra to initiate the reaction.
    • A substitution reaction occurs and a chloro–alkane is formed e.g.
    • a hydrogen is swapped for a chlorine and the hydrogen combines with a chlorine atom e.g.
      • methane + chlorine ==> chloromethane + hydrogen chloride
        • CH4 + Cl2 ==> CH3Cl + HCl
        • alkanes structure and naming (c) doc b + Cl2 ==> (c) doc b + HCl
      • ethane + chlorine ==> chloroethane + hydrogen chloride
        • C2H6 + Cl2 ==> C2H5Cl + HCl
        • + Cl2 ==> + HCl
    • Chloromethane and chloroethane are gases at room temperature, but bigger chloro–alkane molecules are useful solvents in the laboratory or industry but they are still quite volatile and chlorohydrocarbon vapours can be harmful if breathed in.
  • The chemical bonding in alkane molecules
    • Alkanes are relatively small molecules in which all the chemical bonds are covalent bonds.
    • All the bonds in alkane molecules are single bonds i.e. C–C carbon – carbon or single C–H carbon – hydrogen bonds.
    • Each carbon atom forms four single bonds and hydrogen atoms form one single bond.
    • All single covalent bonds are formed by sharing a pair of electrons e.g. one from each of a carbon atom and a hydrogen atom, or two carbon atoms contributing (sharing) an electron each to the covalent bond.
    • Four (c) doc b hydrogen atoms (1 outer electron) and one  (c) doc b carbon atom (four outer electrons) combine to form methane so that the hydrogen atoms are electronically like helium (full outer shell of 2 electrons) and the carbon atom becomes like neon (with a full outer shell of 8 electrons, the two inner electrons of carbon are not shown).
    • (c) doc b or the alkane methane
    • Similarly six hydrogen atoms combine with two carbon atoms to form the ethane molecule.
    • or the alkane ethane
    • More notes on molecules and covalent bonding

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ALL my Advanced A Level Organic Chemistry revision notes

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equation keywords: O2 CH4 + 2O2 ==> CO2 + 2H2O H2O * C3H8 + 5O2 ==> 3CO2 + 4H2O * 2C2H6 + 7O2 ==> 4CO2 + 6H2O * C2H6 + Cl2 ==> C2H5Cl + HCl * CH4 + 2O2 ==> CO2 + 2H2O * C3H8 + 5O2 ==> 3CO2 + 4H2O * 2C2H6 + 7O2 ==> 4CO2 + 6H2O * C2H6 + Cl2 ==> C2H5Cl + HCl *

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