12.1 Properties of alkanes and radical substitution Flashcards
Sigma bond
The overlap of atomic orbitals directly between bonding atoms
2 characteristics of sigma bonďs
2 e- shared between the bonding atoms
sigma bond is positioned on a line directly between the atoms acting as an axis around which the atoms can rotate
Standard states of C1 -C4
gas
Standard states of C5-C16
colourless liquid
Standard states of C17 +
white waxy solids
Affect of chain length on b.p? 3
As C chain length increases:
Surface area inc
Surface contact inc
Greater london forces
More E req so b.p up
Affect of branching on b.p
As branching up:
Less surface points of contact= fewer LF
And molecules cant pack as closely together =weaker LF
Why are alkanes so stable 2
C-C and C- H are very strong
C-C and C-H are non polar
Why are alkanes used as fuels? 2
Readily available
Easily transportable
What condition is required for the reaction of alkanes with halogens
UV radiation to provide initial energy
What type of reaction is that between halogens and alkanes (mechanism?)
Radical Substitution (halogenation)
Observations of alkane+ halogen
Decolorisation of halogen
Moist litmus paper turns red
Products of halogen+ alkane and why does it make moist litmus red
Haloalkane and
hydrogen halide gas
Gas dissolves in moisture on litmus to give acidic sol
3 stages of radical substitution
Initiation
Propagation
Termination
What happens during initiation phase
Formation of radicals via homolytic fissions
Why does the halogen undergo homolytic fissions in initiation not the alkane
Bond in alkane are too strong to be overcome by the energy form UV
What happens in propagation
We see chain reaction of radicals reacting with molecules
What happens in termination
2 radicals collide forming a molecule with all e- paired
Further substitution?
Once the haloalkane is formed in propagation steps another halogen radical could attack to form a dihaloalkane
Could continue until all H atoms have been substituted with halogen
limitations of radical substitution
Mixture of products (several termination reactions possible)
Further substitution
Substitution at different points in the chain (produces different compounds)
Overall difficult to achieve 100% atom economy of a specified compound
Indication of a propagation reaction
Radicals on both sides
If dot for radicals not shown, count up e-
