These are detailed Revision Notes for Topic 3.2 of AQA A-Level Chemistry (Alkanes). They were written by me using a combination of the textbook and class notes. I will also be uploading the other topics and creating bundles.
Topics Included:
- Halogenoalkanes - introduction
- Nucleophilic subst...
Halogenoalkanes
13.1 Halogenoalkanes - Introduction 13.1 Halogenoalkanes – introduction
- The general formula of a halogenoalkane with a single
13.2 nucleophilic substitution in halogenoalkanes
halogen atom is CnH2n+1X (shortened to R-X).
- Halogenoalkanes have a polar Cd+-Xd-, because 13.3 elimination reaction in halogenoalkanes
halogens are more electronegative than carbon.
- However, this bond is not polar enough to make them soluble in water.
- The main forces of attraction are dipole-dipole attractions and van der Waals forces.
- The boiling point increases with chain length and decreases down the halogen group.
- The bonds get weaker going down the group, F is the smallest halogen atom, and the electrons are
strongly attracted to the fluorine nucleus, this makes a strong bond. Going down the group, the shared
electrons in the C-X bond get further from the halogen nucleus, so the bond becomes weaker.
13.2 Nucleophilic Substitution in Halogenoalkanes
- Nucleophile – an atom or group of atoms which is negatively charged or has a region of negative
charge. It is attracted to an electron deficient centre or atom, where is donates a pair of electrons to
form a covalent bond.
- They can be halide ions, hydroxide ions, ammonia, cyanide ion.
- They will replace the halogen in a halogenoalkane in a nucleophilic substitution reaction.
- This reaction is shown by a reaction mechanism, with curly arrows to show the movement of electrons
(always starting with the nucleophile)
- The C-Cl bond is polarised due to a difference in electronegativity
- The nucleophile (-OH) attacks the carbon on the opposite side to the chlorine. This minimises repulsion
between the 𝛿 - chlorine and the nucleophile.
- The lone pair of electrons on the hydroxide is attracted to the 𝛿 + carbon.
- A new bond is formed between the oxygen atom and the carbon atom.
- The carbon halogen bond breaks by
homolytic fission.
Aqueous NaOH or KOH conditions:
- Haloalkanes with aqueous Na/KOH the
reaction is slow so needs to be warmed,
ethanol is used as a solvent.
- An alcohol is formed, a hydrolysis
reaction.
Cyanide Ions:
- Haloalkane warmed with aqueous
alcoholic solution of potassium cyanide.
- This is a way to extend the carbon
chain as more carbon is added.
- Nitriles are formed
Ammonia:
- Haloalkanes with
ammonia, reacts with
excess concentrated
solution of ammonia
in ethanol under
pressure.
- Amine is produced.
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