- Bromine water added to hexane
When bromine water is added to hexane and shaken in a test tube no reaction occurs. This is because the bonds between the hexane are fairly strong. There is no sufficient energy provided in this reaction to break the bonds between the hexane so the bromine can bond onto the hexane.
- Reagent (bromine) added to hexane with light
Bromine is added to hexane and light is applied to this. A reaction takes place. The equation below shows this and also gives clues as to what type of reaction this is called.
C6 H14 (l) + Br2 (l) C6 H13 Br (l) + HBr (g)
We can see that the products made are bromohexane, which is a halogenoalkane, and hydrogen bromide. This type of reaction is a substitution reaction. This is a reaction in which one atom or group of atoms is replaced by another. Energy is required in this reaction to break bonds so a lamp is used as a light source to provide the energy. Here one hydrogen atom is being substituted for bromine to form bromohexane. And the hydrogen bonds with bromine to form hydrogen bromide.
- Concentrated ammonia added to hydrogen bromide
Concentrated ammonia when put near hydrogen bromide, which is a product given off when bromine reacts with hexane, produces white fumes. The following equation shows this.
NH3 (g) + HBr (g) NH4 Br (g)
The ammonia joins with the hydrogen bromide to form ammonium bromide.
Combustion with oxygen is the most important reaction of alkanes giving their immediate use as fuels in the home and industry. In this reaction a alkane is heated and burnt. The following equation shows this.
C6 H14 (l) + 9O2 (g) 7H2O (l) + 6CO2 (g)
In this reaction hexane is heated in the atmosphere and burnt, reacting with oxygen. This releases carbon dioxide and water. This reaction is very harmful to the atmosphere. That’s why there is so much trouble with pollution and carbon dioxide levels rising as a result of burning fossil fuels.
In this reaction longer chain hydrocarbons are broken down into shorter chain hydrocarbons and an alkene. This reaction is a type of thermal decomposition. The following equation shows this.
C8 H18 C6 H14 + C2 H4
This reaction requires heat as you probably already guessed and a catalyst. Typically a broken pot catalyst.
This implies that any alkane will react in a similar way to that of hexane because they all have the same structure. Some alkanes in longer chains such as decane are less reactive and need much more energy in order to react. This is explained below.
- Reactivity trend of alkanes
As the carbon chain gets longer on an alkane the boiling points increase. This is due to an increase in strength of intermolecular forces. The intermolecular forces involved are called Van der Waals’ forces. As the carbon chain length increases there is more surface of contact between molecules. This means greater surface area between molecules so greater Van der Waals’ forces result from this. If the alkane involved is branched such as a structural isomer of hexane. The Van der Waals’ forces decrease due to a smaller surface of contact between molecules. So the main trend is the longer the carbon chain in one direction the stronger the intermolecular forces so the reactivity of the molecule is less because bonds are harder to break.
Bibliography
OCR Chemistry 1 ISBN: 0-521-78778-5 Page 100-141
Revise AS Chemistry Heinemann ISBN: 0-453-58302-6 Page 46
AS Chemistry Revision Notes Letts ISBN: 1-84085-518-5 Page 71-74
Personal Notes
