Halogenoalkanes Flashcards
What is the general formula for halogenoalkanes?
Halogenoalkanes have the general formula CnH2n+1X (x = halogen)
What do halogenoalkanes contain?
Halogenoalkanes contain a polar carbon-halogen bond. The polarity arises from the different electro-negativities of the carbon and halogen atoms.
What are more electronegative, halogen atoms or carbon atoms?
Halogen atoms are more electronegative.
What does the electron-deficient carbon atom in halogenoalkanes attract?
They attract nucleophiles such as H2O, OH- and NH3.
What sort of reaction occurs when halgenoalkanes react with nucleophiles?
Halogenoalkanes react with nucleophiles in substitution reactions.
In the reaction between halogenoalkanes and nucleophiles what is replaced?
The nucleophile replaces the halogen atom in the halogenoalkane, forming a compound containing a different functional group.
What type of reaction occurs when a halogenoalkane reacts with an aqueous solution of hot hydroxide ions? and what is the product from the reaction?
A nucleophilic substitution reaction occurs. The product is an alcohol.
What is the reaction called between a halogenoalkane and any aqueous hydroxide?
Hydrolysis.
What apparatus is used between a halogenoalkane and aqueous hydroxide?
The reaction is carried out by heating under a reflux.
Write out the step by step of what happens when a halogenoalkane reacts with an OH-.
1) OH- has a lone pair attracted and which is donated to the electron-deficient carbon atom in the halogenoalkane. This is known as nucleophilic attack.
2) The donation of the electron pair leads to a new covalent bond.
3) The carbon-halogen bond breaks by heterolytic fission. Both electron from the bond move to the halogen, forming a halide ion.
Draw the mechanism for a halogenoalkane reacting with an hydroxide ion.
Explain the rates of hydrolysis for different halogenoalkanes.
The halogenoalkane is heated with aqueous silver nitrate, with ethanol added.
- Water in the mixture behaves as the nucleophile.
- The ethanol acts as a common solvert, ensuring that halogenalkane and aqueous silver nitrate mix together and react.
- The aqueous silver nitrate, AgNO3(aq) reacts with any halide ions present forming a precipitate of the silver halide.
How can the rate of hydrolysis between a halogenoalkane and OH- be calculated?
The rate of hydrolysis can be found by calculating 1/time taken for the precipitate to occur.
What is needed to kept constant in a hydrolysis reaction?
1) Use equal amounts,in mol, of each halogenoalkane.
2) halogenoalkanes with the same chain length
3) use a water bath to ensure a constant temperature
What two factors about the reactants can affect the rate of hydrolysis?
Polarity - The C-F bond is the most polar bond so the super positive C should attract the nucleophile the most readily and give the fastest reaction.
Bond enthalpy - The C-I bond is the weakest of the halogenoalkanes and so the C-I bond should be be broken the most readily and give the fastest reaction.
