Chapter 15 Flashcards
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 primary 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 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, and you can time how long it takes for the precipitate to form (chlorine takes the longest)
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 (they rarely exist in nature)
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)
