Naming Haloalkanes: If two+ halogen present, listed in alphabetical order. Like alcohols,
haloalkanes classed as primary, secondary and tertiary.
• Nucleophiles: An atom or group of atoms that is attracted to an electron- deficient carbon atom,
where it donates a pair of electrons to form a new covalent bond. Examples are hydroxide ions
(OH-
), water and ammonia (NH3). Molecule acts a nucleophile as contains a lone pair of electrons
that can be donated.
Polar Bond: Halogen atoms more electronegative than carbon atoms. The carbon atom has
slightly positive charge which attract nucleophile.
• When reacted with CH3O
–
, make sure lone pair of electrons is on O and O is the one attached to
carbon bond in reacting molecule.
• Nucleophilic Substitution: When a primary haloalkane reacts with a nucleophile, the
nucleophile replaces the halogen. So can have other nucleophiles.
• Hydrolysis: A reaction with water that breaks compound, water molecule becomes incorporated
into the compounds. Instead of water can be an aqueous solution of hydroxide.
• Hydrolysis of Haloalkane: Can use aqueous sodium hydroxide or potassium hydroxide KOHhydroxide ions. Mixture is heated under reflux.
• CH3CH2CH2CH2Br + NaOH (aq) -> CH3CH2CH2CH2OH + NaBr.
• Reaction Mechanism: An example of nucleophilic substitution. Remember dipoles. Shorten
everything but the CH2 group and halogen. Omit charge from nucleophile if ever unsure. Leave
the Br- by itself, only put NaBr in equations.
– The nucleophile, OH-
, approaches the carbon atom from opposite side of halogen. This
minimises repulsion between the nucleophile and the δ- halogen atom.
– A lone pair of electrons on hydroxide ion donated to δ+ carbon atom. New bond formed
between oxygen of hydroxide ion and carbon atom.
– Carbon- halogen bond breaks by hetrolytic fission.
– The halogen is replaced by an –OH. Haloalkane to alcohol.
– A halide ion is formed. If NaOH used to provide for ions, produces product NaBr.
• Name reaction mechanism if question does not.
• Rate of Hydrolysis:
– Important to compare even if question doesn’t say to compare.
– The rate of hydrolysis, depends on strength of carbon- halogen bond.
– C- F has highest bond enthalpy and C- I bond enthalpy lowest.
– So C-I reacts faster (in nucleophilic sub.)
– As C-I has weaker bond.
– Therefore C-I requires less energy to break than C-Br bond- which reacts slower.
– Fluoroalkanes are unreactive as large quantity of energy required to break C-F bond.
– To find rate of hydrolysis, measure how long it takes ppt to form.
• Factors Affecting Hydrolysis:
– At higher temperatures, rate of hydrolysis increases as more molecules exceed activation
energy.
– Branching affects rate of hydrolysis. C- Hal bond is weaker in tertiary haloalkane. So tertiary
hydrolyses faster than secondary.
• Therefore conditions would need to be changed- heat for longer or reflux.
• Do not refer to them halogens, but haloalkane.
• Measuring Rate of Hydrolysis: Carry out reaction in presence of aqueous silver nitrate. Halide ions
X
–
react with Ag+ ions to form a precipitate of the silver halide. Add ethanol and heat.
– CH3CH2X + H2O -> CH3CH2OH + H+ + X-
.
Ag+
(aq) + X-
(aq) -> AgX (s) precipitate of silver halide.
– The nucleophile in the reaction is water- present in the aqueous silver nitrate.
– Haloalkanes are insoluble in water, so ethanol used to allow water and the haloalkane to mix
rather than two layers- used in other experiments as well.
– Precipitate colours in alphabetical order gets darker- chlorine is white, bromine is cream and
iodine is yellow.
• The Method: Add ethanol and haloalkane. Stand test tubes in water bath- heating makes
experiment faster. Place a test tube containing silver nitrate in water bath. Allow all tubes to reach
constant temperature. Add this silver nitrate (AgNO3) to each test tube and immediately start
stop- clock. Record time taken for precipitate to form.
• Catalyst: A catalyst is anything that is used in first step but made in last step.