Module 4: Chapter 12 - Alkanes Flashcards
What are alkanes?
Saturated hydrocarbons, containing only carbon and hydrogen atoms joined together by single covalent bonds
What is a sigma bond(σ bond)?
The result of the overlap of 2 orbitals, one from each bonding atom, positioned on a line directly between bonding atoms
What is the shape of alkanes?
Due to electron pair repulsion, there is a tetrahedral arrangement around each carbon atom with a bond angle of 109.5*
Are alkanes rigid?
No, sigma bonds act as axes around which the atoms can rotate freely
What is the trend in boiling point in the alkanes?
Increases with chain length
How does alkane chain length affect boiling point?
As the chain length increases, the molecules have a larger surface area so more surface contact is possible between the molecules. Therefore the London forces between the molecules will be greater and so more energy is required to overcome the forces
How does alkane branching affect boiling point?
There are fewer surface points of contact between molecules of the branched alkanes, giving fewer London forces. The branches also get in the way and prevent the branched molecules packing as closely as
Why are alkanes unreactive?
- The sigma bonds are strong (high bond enthalpy)
- C-C bonds are non-polar
- The C-H bonds have such a low difference in electronegativity it is considered to be non-polar
What is the complete combustion of alkanes?
In a plentiful supply of oxygen, alkanes combust completely to prdouce carbon dioxide and water
What is the incomplete combustion of alkanes?
When there is insufficient oxygen, carbon may combust to produce carbon monoxide or solid carbon (soot)
What is the balanced chemical equation for the complete combusion of heptane?
What is the balanced chemical equation for the incomplete combusion of heptane producing carbon monoxide?
What is the balanced chemical equation for the incomplete combusion of heptane producing carbon?
What is the mechanism for the reaction of alkanes with halogens?
Radical Substitution
What are the 3 stages of radical substitution?
- Initiation
- Propagation
- Termination
What is the initiation stage of radical substitution in the halogenation of alkanes?
The reaction is started when the covalent bond in the halogen molecule is broken by homolytic fission. Each halogen atom takes 1 electron from the pair, forming 2 highly reactive bromine radicals. The energy for this bond fission is provided by UV radiation
What is the propagation stage of radical substitution in the halogenation of alkanes?
In the propagation stage, the reaction propagates through 2 propagation steps, a chain reaction:
* In the first propagation step, a halogen radical reacts with a C-H bond in the alkane, forming an alkyl radical and a molecule of hydrogen halide
* In the second propagation step, Each alkyl radical reacts with another halogen molecule, forming the haloalkane and a halogen radical.
The new halogen radical then reacts in the first propagation step causing a cycle (chain reaction)
What is the termination stage of radical substitution in the halogenation of alkanes?
In the termination stage, 2 radicals collide, forming a molecule with all electrons paired. There are a number of possible termination steps with different radicals in the reaction mixture. When 2 radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction
What are the limitations of using radical substitution to synthesise haloalkanes?
It produces a mixture of organic products due to:
* Further substitutions
* Reactions at different positions in a carbon chain
How does further substitution cause radical substitution to produce a mixture of organic products?
Another halogen radical can collide with the haloalkane, forming a dihaloalkane, which can then collide with another halogen radical to form a trihaloalkane etc etc. The result is a mixture of haloalkanes with different number of halogen atoms
How does reactions at different points in the carbon chain cause radical substitution to produce a mixture of organic products?
There may be a mixture of monosubstituted isomers by substitution at different positions in the carbon chain
