4.1.2 - Alkanes Flashcards
What is the general formula for alkanes?
CnH2n+2
What are alkanes?
Saturated hydrocarbons containing single C-C and C-H bonds as sigma bonds.
What are sigma bonds?
Overlap of orbitals directly between the bonding atoms.
What is there around the sigma bond?
Free rotation.
What is the shape of alkanes around each carbon?
Tetrahedral
What happens and why to the boiling point of straight chain alkanes as you increase chain length?
Boiling point increases as the number of electrons in molecule increases and there is more surface contact resulting in an increase in strength of london forces.
Why do branched alkanes have lower boiling points?
They can’t pack as closely together and they have smaller molecular surface areas so induced dipole-dipole interactions are reduced.
What is produced in the complete combustion of alkanes?
Carbon dioxide and water.
What is produced in the incomplete combustion of alkanes?
Carbon monoxide and water.
What is a tip that’s good to know when balancing combustion reactions?
Balance carbons first, then hydrogens and then oxygens.
Why is carbon monoxide poisonous?
It is better at binding to the haemoglobin than oxygen, which means less oxygen will be carried around your body leading to oxygen deprivation.
Name the type of mechanism which leads to the formation of bromo/chloromethane?
Free radical substitution.
What is reaction and what happens in initiation?
Cl2 -> (UV) 2Cl. - This is homolytic fission.
What happens in propagation 1?
CH4 + Cl. -> HCl + CH3.
What happens in propagation 2?
CH3. + Cl2 -> CH3Cl + Cl.
What are the terminal reactions?
CH3. + Cl. -> CH3Cl
Cl. + Cl. -> Cl2
CH3. + CH3. -> C2H6
Why is ethane produced as trace?
As two methyl radicals have bonded together to form it.
What happens when two radicals join together?
They make stable molecules.
What is the major product formed when a large excess of chlorine reacts with methane in the presence of UV light?
CCl4 - tetrachloromethane
What are the limitations of radical substitution in synthesis?
A mixture of products is produced, therefore you have to separate the desired product from the others. Moreover it can take place at any point along the carbon chain, so a mixture of isomers can be formed.
