Chapter 13- Haloalkanes Flashcards
What is the general formula for a haloalkane with one halogen atom?
CnH2n+1X
Often shortened to R — X
How soluble are haloalkanes?
The polar C — X bonds are not polar enough for them to be soluble in water.
The longer the C chain the less soluble in water.
Dissolve in ethanol.
How does boiling point vary with different haloalkanes?
Increases with increased chain length.
Increases going down halogen group (larger molecules, VdWs).
Branching reduces melting point.
Higher boiling points than alkanes of similar length due to higher RMM and polarity.
How does bond polarity vary with different halogens?
The bond is more polar the higher up the halogen is in the group. Electronegativity of halogen increases going up the group.
How does C—X bond enthalpy vary with different halogens?
Decreasing bond strength (bond enthalpy) going down the group as the halogen atom radius increases.
Bond length increases meaning bond is weaker.
Therefore, iodo-compounds are most reactive due to lowest bond enthalpy.
How does reactivity of a haloalkane change going down group 7?
Reactivity increases as bond enthalpy decreases which is a more important factor than bond polarity.
What is a nucleophile?
A species with a lone pair of electrons to donate
What are some example of nucleophiles?
Hydroxide ion -:OH
Ammonia :NH3
Cyanide ion -:CN
What is nucleophilic substitution?
A reaction in which a nucleophile attacks a (partial) positive charge to replace an atom or group of atoms
What are three examples of nucleophilic substitution reactions?
Haloalkane —> alcohol
Haloalkane —>nitrile
Haloalkane —> amine
What does a double headed curly arrow show?
How a pair of electrons move in a reaction
What are the conditions needed for a haloalkane to alcohol reaction?
-:OH ions (nucleophile) from aqueous NaOH or KOH (heated to split ions).
Alcohol to act as universal solvent
What are the steps of a haloalkane to alcohol reaction?
-:OH ion attacks partial positive C atom bonded to halogen.
Electron pair from covalent bond moves to the halogen, making it a halide ion (which reacts with Na or K from metal hydroxide).
The halide ion is the leaving group.
What are the conditions needed for a haloalkane to nitrile reaction?
-:CN ions (nucleophiles) from aqueous NaCN or KCN with alcoholic solution as universal solvent.
What are the steps of the haloalkane to nitrile reaction?
Cyanide ion attacks partial positive C atom bonded to halogen.
Electron pair from covalent bond moves to halogen atom, making it a halide ion which is the leaving group (reacting with Na or K).
Produces a nitrile with an extra carbon atom in the chain than haloalkane.
What are the conditions needed for a haloalkane to amine reaction?
:NH3 from excess concentrated solution of ammonia in ethanol
Requires pressure
What are the steps of the haloalkane to amine reaction?
Ammonia attacks partial positive C atom and electron pair moves to halogen atom, making it a halide ion (leaving group).
Electron pair from one N—H covalent bond moves to the positive N atom and a second NH3 molecule reacts with H+ ion to form an ammonium ion producing an amine and ammonium ion.
What is an elimination reaction?
A reaction in which an atom or group of atoms are removed from a molecule, eg a hydrogen halide
What are the conditions needed for elimination of a haloalkane?
-:OH ions from NaOH or KOH dissolved in ethanol. These act as a base to remove an H+ ion rather than a nucleophile.
NO WATER present
What are the steps in the elimination of a haloalkane?
-:OH ion forms bond with slightly positive H atom on carbon next to C—X bond.
Electron pair from old C—H bond then becomes part of C=C double bond.
Halogen takes pair of electrons from the
C—X bond and leaves as halide ion (leaving group).
What is ozone and what does it do?
O3, it is formed naturally in the upper atmosphere and it is beneficial as it absorbs UV radiation
How is ozone depleted by chlorine free radicals?
UV radiation in upper atmosphere causes C-Cl bonds to break in CFCs (chlorofluorocarbons), producing chlorine free radicals which catalyse the decomposition of ozone and contribute to the hole in the ozone layer
How have CFCs been restricted?
Research by scientific groups led to legislation to ban CFCs in solvents and refrigerants and chlorine-free compounds are now used instead
What are the equations for the depletion of ozone by chlorine free radicals?
Cl. + O3 -> ClO. + O2
ClO. + O3 -> 2O2 + Cl.
overall: 2O3 -> 3O2