Haloalkanes

• Like alcohols, we can have primary, secondary and tertiary haloalkanes.
• Depending on the classification of haloalkane you hydrolyse, you would produce the same classification of alcohol.
• The C-X bond in haloalkanes is polar and this effects the properties and reactivity of the haloalkane.
• When naming haloalkanes with multiple different halogens, we list them in order of the alphabet.
• Haloalkanes are less volatile than their alkane equivalents as their boiling point is higher due to being able to form induced dipole-dipole forces that require more energy to break than purely London forces.
• Going down group 7, the boiling point of the haloalkane formed increases as by descending the group, the number of electrons increases, thus increasing the size of the London forces made which require more energy to break.
• Haloalkanes cannot form hydrogen bonds and are therefore insoluble in water. However they are soluble in non-polar solvents such as cyclohexane.
• Haloalkanes react with chemicals called nucleophiles in hydrolysis by the process of nucleophilic substitution.
• A nucleophile is an electron pair donor and always have a lone pair of electrons to do so.
• The lone pair of electrons on a nucleophile is attracted to the electron deficient carbon atoms which have a complete or partial positive charge.
• The nucleophile donates the lone pair of electrons to form a covalent bond between the nucleophile and the carbon atom.
• Due to the dipole in a C-X bond of a haloalkane, the carbon atom in this bond has a slight positive charge and is the electron deficient carbon atom.
• The lone pair of the nucleophile attacks the electron deficient carbon from the side opposite to that bonded to the halogen atom. By doing this, there is less repulsion of the negative hydroxide ion by the negative halogen atom.
• At the same time that the nucleophile attacks the carbon atom, the C-X bond breaks by heterolytic fission with a pair of electrons proving to the halogen atom, producing an alcohol and a halide ion.
• Because the halide atom leaves the haloalkane, it is called the leaving group.
• For the hydrolysis reaction via nucleophilic substitution, we use an aqueous solution of a hydroxide ion as well as ethanol, which solubilises the haloalkane so it can react.
• To measure the rate of hydrolysis of haloalkanes, we measured how long it takes for the halide ion to be formed, and thus react with aqueous silver nitrate to produce a silver halide precipitate.
• To test the rate o hydrolysis, we use water molecules as our nucleophile as if we used hydroxide ions, a precipitate of silver hydroxide could form which may effect the results of our silver halide precipitates.
• During hydrolysis, we must break the C-X bond, so the bond enthalpy would determine how fast the rate of hydrolysis is. The greater the enthalpy, the slower the rate.