Halogenoalkanes
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What is the general formula for a haloalkane?
C𝑛H2𝑛+1X
Explain why carbon-halogen bonds are polar?
• because halogens are more electronegative than carbon. (as you go down the group the halogens get less electronegative and therefore the C-X bond gets less polar)
Why aren’t haloalkanes soluble in water?
The C-X bonds are not polar enough
What factors impact the boiling point of halogenoalkanes?
• chain length (increases with increased chain length) • the halogen in the C-X bond (increases as go down the group) these both increase VDWs
Why does reactivity increase as you go down the halogenoalkanes group?
• C-X bond gets longer and therefore weaker • easier to break This is despite the fact that the bond gets less polar, showing bond enthalpies to be of more importance than bond polarity
What is a nucleophile?
A species with a negative charge and a lone pair of electrons, which it can donate to an electron deficient carbon atom, forming a covalent bond
What conditions are required for the hydrolysis of haloalkanes (by nucleophilic substitution)?
• Aqueous solution containing hydroxide ions (e.g NaOH) • ethanol as a solvent (haloalkanes are not soluble in water) • Heat
Why does the nucleophile approach from the opposite side to the halogen atom?
To minimise repulsion from the electronegative halogen atom
Describe the steps of an elimination reaction
1) OH- ions bond to the H (on the C-H bond adjacent to the C-X bond) 2) Electrons move from the C-H bond to form a C=C double bond 3) This halogen takes the pair of electrons in the C-X bond and leaves as a halide ion
What are CFCs?
Halogenoalkanes containing chlorine, flourine and carbon, but no hydrogen
How can short chain CFCs be used?
• refrigerants • aerosol propellants • blowing agents for foams like expanded polystyrene
How can long chain CFCs be used?
• dry cleaning • as de-greasing solvents
How do CFCs lead to the depletion of Ozone in the atmosphere?
• the C-Cl bond is broken due to the presence of UV light • this produces a chlorine free radical • this chlorine free radical acts as a catalyst in the decomposition of Ozone
Give 2 equations that show chlorine as a catalyst in the decomposition of Ozone?
• Cl. + O3 → ClO. + O2 • ClO. + O3 → 2O2 + Cl.
Give 3 examples of a nucleophile?
:NH3 :OH CN:-
Describe the reaction conditions when reacting a halogenoalkane with KCN. What products are formed?
• ethanol and heat • R-CN, nitrile formed
Describe the reaction conditions when reacting a halogenoalkane with NH3. What products are formed?
• excess of NH3 • heat in a sealed tube • R-NH2, amine
Why is the Ozone layer important?
protects from harmful UV rays which can cause skin cancer
Why cant nucleophilic substitution happen with tertiary haloalkanes?
nowhere for the nucleophile to attack
What does a nucleophile act as in elimination reactions?
base
What conditions enable an elimination reaction to occur?
• heat under reflux • ethanol as a solvent • concentrated
What reagent is needed for an elimination reaction? What does the OH- ion act as?
KOH, OH acts as a base
What reaction condition are needed for the reaction of haloalkanes with CN- ions?
• warm, ethanolic KCN • reflux
What reaction condition are needed for the reaction of haloalkanes with OH-?
• warm aqueous NaOH • reflux