Chapter 15
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What is a nucleophile? Required.
• An electron pair donor
What do nucleophiles usually have? Give 3 examples of nucleophiles.
• A lone pair or a negative charge • Hydroxide ions • Water molecules • Ammonia molecules
Why do haloalkanes react with nucleophiles?
• Halogens are more electronegative than carbon atoms, so the bond between them is polar • Carbon would therefore have a positive dipole, and would attract a nucleophile
What happens when a haloalkane reacts with a nucleophile?
• The nucleophile replaces the halogen in a substitution reaction • This mechanism is known as nucleophilic substitution
What is hydrolysis? What is the hydrolysis of haloalkanes an example of?
• A reaction involving water or the aqueous solution of a hydroxide that causes the breaking of a bond in a molecule • Nucleophilic substitution
How does the hydrolysis of haloalkanes work?
• The C atom bonded to the halogen has a positive dipole, and the halogen has a negative dipole • A curly arrow goes from the lone pair on the hydroxide ion (you don’t draw the sodium ion as it’s a spectator ion) • Another curly arrow goes from the C-X bond to X • You get a primary alcohol and a sodium halide
What are the conditions for the hydrolysis of haloalkanes by NaOH?
• Heat under reflux
What does the rate of hydrolysis depend on?
• The bond enthalpy (strength) of the C-X bond, seeing as that bond is broken inside the haloalkane
How does the strength of the C-X bond change down the group, and in what 2 ways does this affect the hydrolysis of haloalkanes?
• The strength of the bond decreases down the group • This means that iodoalkanes react the fastest • It also means that fluoroalkanes are unreactive due to the amount of energy needed to break the C-F bond
How can you test the bond enthalpies of different carbon-halogen bonds in primary haloalkanes?
• Using aqueous AgNO3 and ethanol • Halides (from the haloalkane) react with silver ions to form a precipitate when they are heated together, and you can time how long it takes for the precipitate to form (chlorine takes the longest)
What is the other product that is formed when a haloalkane is reacted with silver nitrate?
• Alcohol
Why is aqueous silver nitrate used?
• The water in the solution acts as a nucleophile
Why is ethanol used?
• It acts as a solvent so that the haloalkane and water can mix, instead of being 2 separate layers
What are organohalogen compounds?
• Molecules that contain at least one halogen atom joined to a carbon chain
Why are organohalogen compounds a cause for concern?
• They cannot be naturally broken down by the environment
What is the ozone layer, and its purpose?
• It is the outer edge of the stratosphere • It contains O3 (ozone), which blocks out UV-B radiation • UV-B radiation causes genetic damage, and possibly skin cancer
How is ozone formed?
• O2 -UV-> 2O (2 oxygen radicals, although they aren’t shown with a dot) • O2 + O <-> O3 (reversible reaction)
What are CFCs, and what were they used for?
• Chlorofluorocarbons • Refrigerants, etc
Why are CFCs bad for the environment?
• They are very stable due to the strength of the C-X bond, but they only remain stable until they reach the stratosphere • There they break down and form chlorine radicals (the C-Cl bond has lower bond enthalpy, so it is the one that breaks) • These chlorine radicals catalyse the breakdown of the ozone layer
Show how CFCs deplete the ozone layer using the example of CF2Cl2.
• Initiation: • CF2Cl2 -> CF2Cl· + Cl· • Propagation: • Cl· + O3 -> ClO· + O2 • ClO· + O -> Cl· + O2 • Overall equation: • O3 + O -> 2O2 • The chlorine radical that is made in the second propagation step would cause a chain reaction
What is the initiation step called in this scenario, and why?
• Photodissociation, since it is caused by UV radiation
What is another radical that can catalyse the breakdown of ozone, and how does it do this?
• ·NO Propagation: • ·NO + O3 -> NO2· + O2 • NO2· + O -> NO· + O2 Overall equation: • O3 + O -> 2O2
List 2 sources of nitrogen oxides.
• Lightning • Aircraft