1. Boiling and melting points of alkanes
The reason for this rise is an increase in the intermolecular forces. As shown in the following figure, larger molecules can make close contact with each other over a much larger surface area than can smaller molecules.Alkanes are non-polar molecules, the intermolecular forces are van der Waals forces
The total force exerted between the two is thus greater. In the same way we can also explain why branched-chain hydrocarbons boil at lower temperatures than straight-chain compounds. The branched molecules are more compact and provide less area over which intermolecular forces can act.
Boiling point [°C]
Melting point [°C]
Density [g·cm3] (at 20°C)
2. Insoluble in water
Alkanes cannot dissolve in water. Alkanes are saturated nonpolar hydrocarbons. Water, H2O, is a polar molecule. Organic molecules only mix readily with other molecules of the same polarity. Therefore, alkanes will not dissolve in water. Moreover, polar molecules dissolves with polar molecule only and non-polar molecule dissolves with non-polar only.
The viscosity of liquid alkanes increases with the number of carbons. Increased intermolecular attractive forces, as well as an increase in the extent to which nearby molecules become entangled when they have an extended shape, cause unbranched alkanes to be more viscous than their branched-chain isomers.
In general, alkanes show a relatively low reactivity, because their C bonds are relatively stable and cannot be easily broken. Unlike most other organic compounds, they possess no functional groups.
All Alkanes burn in air to give carbon dioxide and water
All alkanes react with oxygen in a combustion reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases. The general equation for complete combustion is
- CnH2n+2 + (1.5n+0.5)O2 → (n+1)H2O + nCO2
- CnH(2n+2) + nO2 → (n+1)H2O + nCO
The standard enthalpy change of combustion, ΔcH
o, for alkanes increases by about 650 kJ/mol per CH2 group. Branched-chain alkanes have lower values of ΔcH o than straight-chain alkanes of the same number of carbon atoms, and so can be seen to be somewhat more stable..
Methane is a colorless, odorless gas. Methane has a boiling point of −161 °C (−257.8 °F) at a pressure of one atmosphere. As a gas it is flammable only over a narrow range of concentrations (5–15%) in air. Liquid methane does not burn unless subjected to high pressure (normally 4–5 atmospheres)
The combustion of methane needs oxygen to form water and carbon dioxide. The reaction is exothermic.
- CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) + 891 kJ/mol (at standard conditions.
Methane reacts with all halogens given appropriate conditions, as follows:
Properties of Alkenes
The physical properties of alkenes are comparable with those of alkanes. The physical state depends on molecular mass (gases from ethene to butene - liquids from pentene onwards). The simplest alkenes, ethene, propene and butene are gases. Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids.
Combustion of Alkenes
The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water,. For example, ethene burns as follows :
C2H4 + 3 O2 ==> 2 CO2 + 2 H2O
Addition Reactions across the Double Bond
Because the alkenes are unsaturated hydrocarbons, their most important reactions are addition reactions across the double bond.
The alkenes are readily oxidised by potassium permanganate to form glycols. For example, ethene is oxidised to ethylene glycol.
3 H2C=CH2 + 2 KMnO4 + 4 H2O
==> 2MnO2 + 2KOH + CH2OHCH2OH
During the oxidation of alkenes, the purple colour of the permanganate solution disappears and the reaction constitutes a test, known as Baeyer's Test, to detect unsaturation in any compound.
Addition of Hydrogen
The alkenes are readily reduced by the addition of hydrogen across the double bond to form alkanes (i.e. reduction of alkenes). For example, when an alkenes is passed over a nickel catalyst at 150 degC, the alkene is reduced to an alkane.
H2C=CH2 + H2 ==> CH3CH3
Addition of Halogen
Halogens readily add across the double bond of the alkenes to form dihalides
H2C=CH2 + Cl2 ==> CH2Cl CH2Cl
H2C=CH2 + Br2 ==> H2Br CH2Br
The decolourisation of bromine is a second test for an unsaturated organic compound.