3.3- mechanisms of halogenoalkanes Flashcards
what are free radicals (2)
-a reaction intermediate (made during a reaction then are destroyed)
-highly reactive species with one or more unpaired electrons
how do free radicals form (2)
-through homolytic fission of a covalent bond
-the bond is broken through a source of high energy eg. UV light/ high temps
what is the initiation step in free radical substitution
-creates 2 free radicals
X₂ ->2X⋅
what is the dot next to an element/compound in free radical susbtitution
a single, unpaired electron
what are the propagation steps in free radical substitution (2)
show the 2 propagation steps from a chlorine radical and methane
-when a free radical has formed, the unpaired electron is transferred to other molecules to create a series of new products
-halogenoalkanes are produced in the second equation/part
1. CH4 + Cl⋅ -> HCl + ⋅CH3
(starting molecule- CH4- reacts with one free chlorine radical to form methyl free radical)
2. ⋅CH3 + Cl₂ -> CH3Cl + Cl⋅
(methyl free radical reacts with diatomic halogen- Cl2- to produce a halogenoalkane and a free radical)
what is the termination step in free radical substitution
show 2 possible termination steps
-2 free radicals combine and the reaction ends
-goes from 2 free radicals to no free radicals
1. ⋅CH3 + ⋅CH3 -> C2H6
2. ⋅CH3 + Cl⋅ -> CH3Cl
what is the overall equation for the reaction between methane and chlorine
CH4 + Cl₂ -> CH3Cl +HCl
what is ozone and why is it bad (3)
protects us from UV radiation (can cause skin cancer)
-when they rise to the atmosphere, the initiation step occurs
-UV light causes the C-Cl bond to break
what are chloroflurorocarbons (2)
-hydrocarbons where all H atoms are substituted by Cl and F
-very unreactive due to C-F and C-Cl bonds
2 equations to show how ozone is formed
O2 –> 2O⋅
⋅O(g) + O2(g) –> 03(g)
why is it beneficial that ozone reduces the amount of UV reaching earth
reduces risk of skin cancer as UV damages DNA
what is a nucleophile (2)
an electron pair donor- they have a lone pair
-they attack electron-defficient species
what are the 3 nucleophiles
hydroxide ions, cyanide ions, ammonia
reagent for hydroxide ion in nucleophilic substitution
aqueous sodium/potassium hydroxide
(NaOH or KOH)
conditions for hydroxide ion in nucleophilic substitution(2)
heated under reflux
aqueous reagent
reagent for cyanide ion in nucelophilic substitution
potassium/sodium cyanide dissolved in ethanol
(NaCN or KCN)
conditions for cyanide ion in nucleophilic subsitution
heated under reflux
reagent for ammonia in nucleophilic substitution
excess concentrated solution of ammonia in ethanol (ethanolic ammonia solution)
conditions for ammonia in nucleophilic substitution
heated under pressure
what is the new functional group formed in hydroxide ions nucleophilic substitution
alcohol
what is the new functional group formed in cyanide ions nucleophilic substitution
nitrile
what is the new functional group formed in ammnia nucleophilic substitution
amine
what happens in nucleophilic substitution (3 stages)
- nucleophile donates a pair of electrons to the partially positively charged carbon in the halogenoalkane
- the C-H bond breaks heterolitically, with both electrons going to the halide ion
- a new bond forms between the carbon atom and the nucleophile- the halogen has been replaced by another functional group
why must the reaction cyanide ions in nucleophilic substitution have to take place under reflux conditions and in ethanol, not water
Water in an aqueous solution would act as a nucleophile and undergo nucleophilic substitution with the halogenoalkane, forming an alcohol instead.
how does ammonia nucleophilic substitution happen (3 stages )
- lone pair on NH3 attacks the partially charged carbon so that the C-halogen bond is replaced by a C-N bond
2.NH3 displaces the halogen and gains a positive charge
3.the leaving halide ion/another ammonia molecule in solution picks up a H+ from the NH3 by heterolytic fission and leaves an amine group
why must the reaction ammonia in nucleophilic substitution have to take place in ethanol and in excess ammonia (2)
-in aqueous solution, water would act as a nucleophile and an alcohol would form instead
-amines are strong nucleophiles so in the absence of excess ammonia, they would undergo nucleophilic substitution with the halogenoalkane instead
how are ammonium salts formed as a by product in ammonia nucleophilic substitution (2)
-the extra H+ is picked up by another NH3 molecule
-ammonium ion (NH4+) forms, which along with the halide ions in solution can form ammonium salts
why does the carbon–halogen bond enthalpy influences the rate of reaction (2)
The carbon-halogen bond has to be broken during the reaction.
The harder the bond is to break (the higher the bond enthalpy) the slower the haloalkanes react.
what is a base
a proton acceptor
what happens if ethanol is used in a reaction between halogenoalkanes and OH- ions (4)
elimination reaction
-alkene is produced
- -OH ion acts as a base
-condition: reflux
elimination reaction 4 stages
- :OH- bonds to H atom
- electrons move to form double bond
- the newly formed double bond repulses the electrons in the carbon-halogen bond onto the halogen
- halogen is eliminated as a negative halide ion
what always has to happen in elimination reactions
:OH- always attaacks the H atom bonded to the C which is ADJACENT to the C-halogen bond
why were CFCs not restricted until research found they were harmful (3)
lack of evidence that ozone was being depleted
lack of alternatives to CFCs
commercial interest to continue using CFCs
how do CFCs contribute to global warming
absorbs infrared radiation as the molecule has polar bonds
