Alkanes
The Saturated hydrocarbon. A compound of hydrogen and carbon only, with no C=C bonds. form a A 'family' of organic compounds that have the same functional group and similar chemical properties. . Like all homologous series, the alkanes:
- have the same An algebraic formula that sets out a rule or trend which is followed by all members of a homologous series. For example, all non-cyclic alkanes have the general formula CnH2n+2.
- differ by CH2 in Chemical formula showing the actual number of atoms of each element in a molecule. from neighbouring A substance formed by the chemical union of two or more elements.
- show a gradual variation in A description of the appearance of a substance or how it acts without involving chemical reactions. For example, state, melting point, conductivity, etc. for example The temperature at which a substance rapidly changes from a liquid to a gas. which increase as the alkanes get longer
- have similar A description of how a substance reacts with other substances. For example, flammability, pH, reaction with acid, etc. (flammability decreases as the alkanes get longer)
General formula
The general formula for the alkanes is CnH2n+2 (where n is the number of carbon atoms in the molecule).
Worked example
Decane is an alkane. Its A collection of two or more atoms held together by chemical bonds. contain 10 carbon The smallest part of an element that can exist.. Predict the molecular formula of decane and explain your answer.
The formula will be C10H22.
This is because n = 10.
So, 2n + 2 = (2 Ă— 10) + 2 = 20 + 2 = 22.
Question
Hexane is an alkane. Its molecules contain six carbon atoms. Predict the molecular formula of hexane.
The formula will be C6H14.
Structures
The table shows four alkanes, their molecular formulae, displayed formulae and structural formulae as well as a drawing of their ball-and-stick constructional models.
Unlike the displayed formulae, the ball-and-stick models show that the A bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons. are not really at angles of 90°.
Question
The alkane pentane has the molecular formula C5H12.
Write down the displayed and structural formula of pentane.
CH3CH2CH2CH2CH3
Note that this can also be written as CH3(CH2)3CH3
The alkanes are saturated hydrocarbons:
- A compound that contains hydrogen and carbon only., because they are compounds containing hydrogen and carbon only
- A saturated hydrocarbon contains no carbon-to-carbon double bonds, only single bonds., because their carbon atoms are joined by C-C single bonds