alkanes Flashcards
waht is free radical substitiution
a hydrogen atom gets substituted by a halogen
what is needed for reaction to occur and why
UV light as alkanes are very unreactive
three steps of free radical substitution
initiation - halogen bond is broken by uv to form 2 free radicals
propagation - these radiacalls make firther radicals in a chain reaction
termination - two radicals collide to stop
hexane and bromine raction to prove that UV is needed
mix two chemicals
leave one in sunlight one in dark
no reaction (dark) stays orange
reaction (light) bromine colour dissapears - becomes colourless
initiation step
halogen bond is broken by UV
produces 2 radicals in a homolytic fission reaction
propagation step
free radicals are very reactive
C-H bonds are broken by free radical with each atom getting one half of electron pair from covelant bond
an alkyl free radical produced
which attacks another halogen to form a new free radical
this radical can then repeat the cycle
does formation of a free radical use up all the a halogens
no
termination
chain reaction stops
two free radicals come together to form a single unreactive molecule
Impurities
Remember that termination involves any free radical bonding with another free radical
If we have two ⋅CH3 radicals they can bond to form ethane,
CH3CH3
⋅CH3 + ⋅CH3 → CH3CH3
If we are trying to form a chloroalkane, then ethane is an impurity
further substitution
excess chlorine present when reacted with methane under UV promotes further substitution
could occur like
CH3Cl + ⋅Cl → HCl + ⋅CH2Cl
substitutions of different carbon atoms
alkanes with a middle carbon like propane can cause substitution there
propogation for substitution of propane with excess bromine in UV occurs like:
CH3CH2CH3 + ⋅Br → CH3⋅CHCH3 + HBr
note the radical on the middle carbon
what is a free radical
an atom which has an unpaired electron on it
