alkanes
a saturated hydrocarbon that only contains single carbon-carbon bonds
what’s a sigma bonds
a bond formed by the direct overlap of orbitals directly between two bonding atoms
why are alkane un reactive
because all C-C and C-H are sigma bonds and can freely rotate, they are the strongest type of covalent bonds and have high bond enthalpy making alkanes unreactive
what shape do alkanes form
tetrahedral structure- 109.5 because there’s 4 bonding pairs which repel equally
how does the boiling point change with the number of carbon atoms present
as the number of carbon atoms increases boiling point increases because the molecule is bigger so more electrons so we have stronger london forces between the molecules and more energy is required to overcome the forces
what’s the trend in boiling point as the number of branches increases
decreases between there’s les surface contact between molecules so there’s weaker london forces and less energy is required to overcome them
write the general equation of incomplete combustion of an alkane
alkane + oxygen —-> carbon monoxide OR carbon + water
write the general equation of complete combustion of an alkane
alkane + oxygen —-> carbon dioxide and water
whats the issue with carbon monoxide
haemoglobin carries oxygen around the body in red blood cells, CO can bond to the haemoglobin which blocks normal cell respiration reducing blood oxygen concentration leading to unconsciousness and eventually death!
what 2 fissions can break a covalent bond
- homolytic fission
-heterolytic fission
what is homolytic fission
when a covalent bond is broken and each bonding atom receives one electron forming 2 radicals
what is a radical
a species with an unpaired electron
e.g AB—> A’ + B’
(‘ represents a dot)
how does an alkane react with a halogen
alkane reacts with a halogen in the presence of UV light forming a halogenoalkane and a hydrogen halide, this reaction involves homolytic fission (a hydrogen is replaced by a halogen and then joins the second part of the halogen and this can continue until all hydrogens are replaced)
whats the condition for free radical substitution and why is it needed
UV light- to provide energy to break the covalent bond and from 2 free radicals
what is the initiation step
-when you create free radicals
-the halogen is broken by homolytic fission, using UV light and it forms 2 free radicals
whats the 1st propagation step
the halogen radical reacts with the alkane, it removes a hydrogen from the alkane and creates a hydrogen halide molecule leaving the alkane as an alkyl radical
whats the second propagation step
the alkyl radical reacts with a halogen molecule and forms a halogenoalkane, the remaining halogen atom is left as a halogen radical
whats the termination step
when two radicals combine and form nuteral molecules which has 3 possible reactions ( 2 halogens or 2 alkyls or a halogen and alkyl radicals combining)
whats the issues with free radical substitution
the yield is often low due to:
-different termination steps can take place forming a mixture of products
-substitution can occur in different positions creating position isomers
how can you reduce the chance of further substitution
use excess of alaknes
