4.1.6/7 : Properties and reactions of alkanes Flashcards
What is a homologous series?
A homologous series is a family of organic compounds with the same functional group, but each successive member differs by CH2.
What does saturated mean?
Only contains single bonds
What is the general formula of an alkane?
CnH2n+2
What is a sigma bond?
An overlap of molecular orbitals/electron clouds between bonding atoms.
What is the shape and bond angles of alkanes?
Every C atom has a tetrahedral shape with a bond angle of 109.5 degrees.
Why is the bond angle 109.5 degrees?
Carbon forms 4 covalent bonds and the pairs of electrons in the bonds all repel each other, causing them to be as far apart as possible.
Why are alkane molecules considered to be non-polar?
The electronegativity values of C and H are very similar (C 2.55 and H 2.20) so there are no significant dipoles.
What are the type of intermolecular forces between alkane molecules and why do they occur?
As the electrons are always moving around the shells, there may occasionally be a lack of balance in charge distribution. This causes an instantaneous dipole, which will induce dipoles in neighbouring molecules, causing weak London forces.
Why does the boiling point of ab alkane increase as its carbon-chain length increases?
- As an alkane chain gets longer, it’s relative molecular mass increases
- Larger molecules have more surface area so more points of contact with adjacent molecules
- This increases the number of London forces
- so more energy is needed to overcome them ( so that the alkane can change state)
Why do compounds with more branches have lower boiling points?
- Manot fit together as closely and neatly
- so there are fewer surface area interactions between molecules
- This means that there are fewer London forces than in straight chain isomers
- so less energy is needed to overcome them
What are the two reasons for which alkanes have low reactivity?
- All of the covalent bonds have high bond enthalpies - a large amount of energy is required to break them
- The C-H sigma bonds have very low polarity due to the electronegativity values of C and H being very similar. The lack of dipoles means that the molecules are less likely to attract other molecules to react with.
Why are alkanes good fuels?
Alkanes are good fuels as they release a large amount of energy per gram of fuel.
What type of reaction is combustion?
Combustion is a highly exothermic oxidation reaction.
What is complete combustion?
Complete combustion is oxidising a fuel in a plentiful supply or air/oxygen
What kind of flame is produced by the complete and incomplete combustion of alkanes?
Complete - makes a clean blue flame and transfers the max. amount of thermal energy because it fully oxidises carbon and hydrogen
Incomplete - makes a cooler dirty yellow flame
What is the balanced equation for the complete combustion of methane?
CH4 (g) + 2O2 (g) -> CO2 (g) + 2H2O (l)
What is incomplete combustion?
Incomplete combustion is oxidising a fuel in a limited supply of air/oxygen
What are the products of complete and incomplete combustion?
Complete - carbon dioxide and water only
Incomplete - carbon particles (soot), water, carbon dioxide, carbon monoxide (toxic gas!!)
What are the 3 stages of the radical substitution mechanism?
- Initiation - the formation of the halogen radicals
- Propagation - two steps that build up the desired product in a side reaction
- Termination - two radicals collide and make a stable product
What is a disadvantage of the radical substitution mechanism?
It is very unpredictable and difficult to control as the reactions are randomly caused by a very reactive radical colliding with another species. A mixture of products is formed, which need to be separated by processes such as fractional distillation or chromatography before they can be used.
Show the chlorenation of methane to form chloromethane using the radical substitution mechanism
- Initiation
Cl2 -> 2Cl. (UV light/300 degrees celcius needed)
- Propagation
CH4 + Cl. -> .CH3 + HCl
.CH3 + Cl2 -> CH3Cl + Cl.
- Termination
2Cl. -> Cl2
2.CH3 -> C2H6
.CH3 + Cl. -> CH3Cl
A mixture of products is produced by random collisions between radicals. Only CH3Cl is desirable so this reaction has a low atom economy.
What is the general formula for balancing the complete combustion of alkanes?
CnH2n+2 + 3n+1/2 oxygen -> n carbon dioxide + n+1 water
