Halogenoalkanes And Nucleophilic Substitution Flashcards
What is a halogenoalkane
Compound in which one or more of the hydrogen atoms is replaced by a halogen
Why are halogenoalkanes reactive
As the carbon in the carbon-halogen bond is partially positive meaning it is easily attacked by nucleophiles
What type of bond breaking is it with the carbon halogen bond
Heterolytic fission with the halogen taking both bonding electrons to form a negative ion
What is a nucleophile
An electron pair donor so has a negative charge
Four examples of nucleophiles
OH-, H2O, NH3, CN-
What is hydrolysis
The breaking of a bond in a reaction involving water
How do the carbon-halogen bond reactivity change down the group and why
Get more reactive as you go down group so react faster as bond becomes weaker(bond enthalpy decreases) so requires less energy to break due to increase in number of inner electron shells(increasing shielding) and increase in atomic radius which decreases force of electrostatic attraction between bond pair and halogen nuclei
Why does reactivity increase from primary to secondary to tertiary carbocation
As more alkyl groups are added there is more of a positive inductive effect which stabilises the positive charge on the carbon atom so tertiary carbocation most stable and reactive
Why do we test for carbon-halogen bond using OH- instead of H2O
As hydrolysis with water is slow so use OH- as nucleophile in form sodium hydroxide
Steps for test for halogenoalkanes
-add drop of ethanol to test tube(so halogenoalkane dissolves) and add halogenoalkane
-add sodium hydroxide and heat under reflux
-acidify with dilute nitric acid
-add silver nitrate solution and begin stopclock, stop when precipitate forms and observe its colour
What is heating under reflux
Heating with a reflux condenser placed vertically in the flask to prevent vapour escaping while liquid is being heated so the vapour from the boiling reaction mixture condenses and flows back into the flask
What does substitution reaction with cyanide ions produce
A nitrile compound(eg butanenitrile) and halogen ion
Conditions for substitution reaction by cyanide ions
Heat halogenoalkane under reflux with potassium cyanide in ethanol
Why use reflux condenser in substitution reaction by cyanide ions
Ensures that the volatile substances (halogenoalkane and ethanol) condense and drip back into the reaction mixture and prevents the release of toxic vapours from these
Why is substitution reaction by cyanide ions a useful one
Reaction useful way of increasing the length of the carbon chain in a molecule
What are the two steps of substitution reaction with ammonia
1st: produces a primary amine(nitrogen of amine group is attached to one alkyl group)
2nd: in presence of excess ammonia it reacts with two ammonia molecules to form an ammonium salt with the halogen ion that is released eg ammonium bromide
Conditions for substitution by reaction with ammonia
Warm halogenoalkane with a concentrated solution of ammonia in ethanol in a sealed tube
Why can there be mixing of products in substitution reaction with ammonia
As there is not only a lone pair on the ammonia but also a lone pair on the nitrogen in the primary amine which has an increased reactivity due to the inductive effect of the alkyl group it is attached to meaning the primary amine can react with the halogenoalkane and lead to further mixing of products
How to limit further reactions in substitution by reaction with ammonia
Use excess ammonia to increase the chance of ammonia(not amine as there is greater proportion of ammonia than primary amine) acting as nucleophile with the halogenoalkanes
Conditions for hydroxide ions as nucleophiles
Add aqueous sodium hydroxide in aqueous ethanol solvent and heat under reflux
What is produced when halogenoalkanes nucleophilic substitution with hydroxide ions
Alcohol and halide ions
Why use a sealed tube in the reaction with ammonia
To prevent the escape of toxic ammonia
Why is primary amine a stronger nucleophile than ammonia
Due to positive inductive effect of alkyl group which release electrons so the electron density on the nitrogen is higher compared to ammonia
What is a base
A proton acceptor
