Halogenoalkanes Flashcards
(13 cards)
Properties of halogenoalkanes
- much more reactive than alkanes=polar C-X
- used as solvents, pharmaceuticals, to make PVC, Teflon, etc
Use has been restricted due to CFCs
Types of halogenoalkanes
Primary=halogen attached to a carbon w 1 C attached to it
Secondary= halogen attached to a carbon w 2 C attached to it
Tertiary= halogen attached to a carbon w 3 C attached to it
Polarity of C-X bond
- decreases down grp 7 as atom decreases in electronegativity
Physical properties of halogenoalkanes
- insoluble in water=not polar enough+mixed w hydrocarbons
- up bp as up chain length due to stronger vdW forces
- more reactive down the grp=down bond enthalpy=easier to break C-X bond
What is a nucleophile?
Species that donates a lone pair of electrons to an electron deficient atom to form a covalent bond
Nucleophillic substitution: OH-
- halogenoalkanes —> alcohols
- type of reaction=:hydrolysis
- reagents: aqueous NaOH/KOH=provides OH-, ethanol used as a solvent=ensures halogenoalkane+metal hydroxide mix
- condition: heat under reflux
Nucleophillic substitution: CN-
- lone pair must be on C
- halogenoalkanes —> nitriles
- reagents: aqueous ethanolic KCN
- conditions: heat under reflux
- important for lengthening the C-chain
Nucleophillic substitution: NH3
- halogenoalkanes —> (primary) amines
- reagents: excess conc sol of ethanolic ammonia
- conditions: heated under pressure
Elimination reaction
- halogenoalkane —> halogen+H removed=alkene
- reagents: ethanolic NaOH/KO=no water present (anhydrous conditions favour elimination)
- condition: heat under reflux
- OH- removes H+ from the halogenoalkane=form water
- leaves the adjacent carbon with a spare electron to form a π bond
- movement of electrons breaks the carbon-halogen bond heterolytically=eliminates halide ion
- OH- acts as a base in elimination reactions by removing a proton
Why is ozone important?
- stops too much UV light reaching Earth
- absorbs UV radiation
- UV radiation will lead to various skin probs
Properties of CFCs
- chemically inert
- down b.p
- insoluble in water
- non-toxic
- non-flammable
How do CFCs damage the ozone layer?
CFCs are stable at ground lvl=survive long enough to be carried up in the atmosphere
- UV radiation breaks the C-X bond=releases Cl free radicals=react w ozone (CF2Cl2 -UV light—> .CF2Cl+Cl.)
Cl.+O3 —>ClO.+O2
ClO.+O3 —>Cl.+2O2 (Cl free radical regenerated)
2O3 —> 3O2
Cl free radical acts as a catalyst in ozone’s breakdown
Alternatives to CFCs
HFCs=x harm ozone layer but powerful greenhouse gases
Hydrocarbons=decomposes rapidly but flammable+contribute to greenhouse gases emissions
