ALKANES
ALKANES
Describe the properties of alkanes (exemplified by methane) as being…
● Generally unreactive, except in terms of burning
- They have the General Formula: CnH2n+2 with all C–C bonds. Example: C2H6 – Ethane.
- They can exist as LINEAR, BRANCHED or forming rings: CYCLOALKANES.
- They are non-polar .
- They have only weak intermolecular forces. (Van der Waals only), that makes alkanes insoluble in water but soluble in non polar liquids..
- They have low melting and boiling points; The values increase with the size (number of carbons in the molecule).
Describe the bonding in alkanes
● Contain no C=C double bonds, therefore the carbons are saturated, because they
each form 4 single bonds i.e. they form as many bonds as possible
● they have C-C and C-H single covalent bonds
● CnH2n+2 is the general formula e.g. ethane is C2H6
● Alkane molecules can be represented in the following forms:
SKELETAL FORMULA
CONDENSED FORMULA
DISPLAYED FORMULA
● The first 4 alkanes are methane, ethane, propane and butane (MEPB: Monkeys Eat Peanut Butter)
Substitution reactions (Chlorine or Bromine)
These reactions of alkanes with all halogens are very similar, the rate of reaction is the only difference. We will concentrate in one example but remember that once you know one of them, you know them all.
A mixture of chlorine and methane:
a) Does not react if kept in the dark at room temp.
b) Does not react if kept in the dark at 300oC
c) Reacts at room temperature if exposed to sunlight or U.V. light.
d) Explodes if exposed to bright sunlight or sparked.
So, energy is required to initiate the reaction. Four products are formed from this reaction
These products are explained in terms of a step by step reaction:
CH4(g) + Cl2(g)
CH3Cl(g) + HCl(g)CH3Cl(g) + Cl2(g)
CH2Cl2(g) + HCl(g)CH2Cl2(g) + Cl2(g)
CHCl3(g) + HCl(g)CHCl3(g) + Cl2(g)
CCl4(g) + HCl(g)
Each of these reaction is an example of a substitution reaction (a H atom is substituted by a Cl atom)