AS - Halogenoalkanes Flashcards
What is a halogenoalkane?
An alkane with at least on halogen atom in place of a hydrogen atom.
Why is the C-H bond in halogenoalkanes polar?
Halogen atoms are much more electronegative compared to carbon atoms.
What is a nucleophile?
An electron-pair donor.
Give three examples of nucleophiles. Name these.
- OH- / hydroxide ion
- CN- / cyanide ion
- NH3 / ammonia
Describe what happens in a nucleophilic substitution reaction with a halogenoalkane.
The lone pair of electrons of a nucleophile is attracted towards a partially positively charged carbon atom, attacking the carbon atom.
The C-X bond breaks (X being the halogen atom).
The halogen leaves the molecule becoming a halide ion with the lone pair of electrons. The nucleophile takes the place of the halogen molecule in the alkane.
What are curly arrows used to show in organic reactions?
How electron pairs move.
Halogenoalkanes react with hydroxides to form what?
What type of reaction is this?
Alcohols.
Nucleophilic substitution.
Halogenoalkanes reacts with cyanide to form what?
What type of reaction is this?
Nitriles.
Nucleophilic substitution.
What cyanide reactant must be used in the nucleophilic substitution reaction with a halogenoalkane?
What conditions are necessary for the reaction?
Ethanolic potassium cyanide (potassium cyanide dissolved in ethanol).
Needs to be warmed.
What is the functional group in nitriles?
-C-N where the - between C and N is a triple bond.
In a nitrile, how is the -CN group counted?
Counted as part of the group so the product has one extra carbon in the chain than the starting halogenoalkane.
A reaction between a halogenoalkane and ammonia forms what?
What type of reaction is this?
An amine.
Nucleophilic substitution.
What reagent is used in the reaction between a halogenoalkane and ammonia?
What conditions are necessary for the reaction?
Ethanolic ammonia (ammonia dissolved in ethanol).
Ethanolic ammonia needs to be concentrated, warmed, in excess and under pressure.
What happens in the second step in the nucleophilic substitution reaction between a halogenoalkane and ammonia?
What products are formed?
An ammonia molecule takes a hydrogen from the NH3 group of the product formed from the first step to form an ammonium ion and an anime.
How can the amine act as a nucleophile from the product of the nucleophilic substitution reaction between a halogenoalkane and ammonia?
Because the amine group in the product still has a lone pair of electrons so can react with the halogenoalkane molecules itself.
What determines the reactivity of halogenoalkanes in nucleophilic substitution reactions?
Which halogenoalkanes react faster and why?
The C-X bond strength.
The C-F bond reacts more slowly than other halogenoalkanes because it’s bond is the strongest.
The C-I bond has the lowest bond enthalpy so it’s easier to break and reacts more quickly than other halogenoalkanes.
If different conditions are used, what other reaction may occur with a halogenoalkane other than nucleophilic substitution?
What conditions are necessary for this to occur?
Elimination reactions occur in anhydrous conditions (no water present).
What is produced in an elimination reaction with a halogenoalkane?
An alkane, water and a halogen molecule or ion.
How does the hydroxide ion act in an elimination reaction with a halogenoalkane?
As a base, removing an H+ ion from the halogenoalkane.
Explain what happens during the elimination reaction mechanism between 2-bromopropane and potassium hydroxide.
- The OH- nucleophile from potassium hydroxide uses its lone pair of electrons to form a bond with one of the hydrogen atoms on the carbon next to the C-Br bond.
- The electron pair from the C-H bond now becomes part of the carbon-carbon double bond.
- The bromine takes the pair of the electrons in the C-Br bond and leaves as a bromide ion to eventually form potassium bromide.
What conditions are necessary for an elimination reaction of a halogenoalkane?
Warm the hydroxide ions in ethanol.
Anhydrous conditions.
Heat the mixture under reflux to prevent losing volatile substances.
What two ways might a hydroxide ion react with a halogenoalkane?
As a nucleophile or a base.
Halogenoalkanes are classified as primary, secondary and tertiary. How do you determine this?
Primary - halogen atom is at the end of the hydrocarbon chain.
Secondary - the halogen atom is in the body of the chain.
Tertiary - the halogen atom is on a branch of the chain.
How can you influence a substitution or elimination reaction?
What conditions favour each reaction?
By changing the conditions.
Substitution favoured - aqueous conditions with OH-, using water as the solvent and warmed under reflux.
Elimination favoured - anhydrous conditions with OH-, using ethanol as the solvent and heating under reflux.
What happens if a mixture of water and ethanol are used as solvents in reactions with OH- and a halogenoalkane?
Both elimination and nucleophilic substitution will occur forming a mixture of the two products.
What are CFCs?
Chlorofluorocarbons- halogenoalkanes containing chlorine, fluorine and carbon atoms but no hydrogen.
Ozone is formed naturally in the upper atmosphere. Why is this beneficial?
It absorbs UV radiation.
What is the formulae for ozone?
O3.
What are the dangers of the ozone layer not absorbing UV radiation?
Ultraviolet radiation can cause sunburn and skin cancer as it is ionising.
How is ozone formed naturally?
Give the two equations for this process.
An oxygen molecule is broken down into two free radicals by UV radiation.
O2 —(UV)—> O• + O•
The free radicals attack other oxygen molecules forming ozone.
O2 + O• —> O3
What were CFCs used for in the past?
Used as coolant gas in fridges, as solvents and as propellants in aerosols.
How were CFCs contributing to the hole in the ozone layer?
UV radiation causes C-Cl bonds in CFCs to break, forming chlorine free radicals.
Chlorine free radicals catalyse the decomposition of ozone, contributing to the hole in the ozone layer.
Give the equa bc tion for the reaction of a chlorine free radical with ozone, and the reaction between the intermediate of this reaction and ozone.
Cl• + O3 —> O2 + ClO•
ClO• + O3 —> 2O2 + Cl•
