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Halogenoalkanes
13.1 Halogenoalkanes - introduction: For this reaction to occur you must add heat, water and
KOH.
Halogenoalkanes: an alkane with at least one halogen
atom attached to the chain, R-X. They’re polar bonds
because there’s a large difference in electronegativity
between carbon and halogens.
• insoluble in water
• Have dipole-dipole forces and van der waals 1. The nucleophile is attracted to the slightly positive
• Boiling point will increase with the length of the carbon atom.
hydrocarbon chain and the size of the halogen 2. It donates the lone pair of electrons to form a dative
atom attached, more van der waal forces so there’s bond.
more electrostatic attraction to overcome. 3. Carbon can only form 4 bonds so the weakest bond, C-
• Can be attacked by nucleophiles due to polar C-X X breaks, creating the leaving group
bond, makes C electron deficient 4. Produces a nitrile
• C-X bonds gets weaker going down the group as For this reaction you must add heat, water and KCN.
the C-X bond gets further from the halogen
nucleus so the electrostatic attraction with the
shared electron decreases.
• Reactivity increases going down the group, so
bond enthalpy more important than the bond
polarity.
Naming-
• prefixes fluoro, chloro, broom, iodo to show which
halogen is present
• Numbered, to show which carbon atom they’re 1. the ammonium ion is attracted to the slightly positive
bonded to carbon atom
• If there’s more than one of the same type of halogen 2. It donates the lone pair of electrons to form a dative
di-, tri-, tetra- will be used. bond
• When different halogens are present they’re listed 3. Carbon has too many bonds so the weakest bond C-X
alphabetically breaks
4. the N atom has formed too many bonds so has a
positive charge, it breaks a N-H bond to it has a lone
pair of electrons again, the H atom bonds with he
13.2 Nucleophilic Substitution:
halide ion
5. Produce an amine
Nucleophilic: a negatively charged ion, or an atom that
Must have water, heat and excess ammonia
is electron rich and has a lone pair of electrons on the
atom it can use to form a covalent bond.
Eg. OH- , NH3 , - CN
In Nucleophilic substitution the Nucleophilic will 13.3 Elimination reactions:
replace the halogen atom, making the halide ion the
leaving group. Instead of the OH- ion acting as a nucleophile it can also
The rate of Nucleophilic substitution changes with the work as a base, accepting a H+ ion from the compound.
halogen, the weaker the C-X bond is the faster the rate of This results in an elimination reaction instead of a
reaction. substitution.
1. The C-X bond makes carbon electron deficient so 1. OH- ion attracted to the H atom on the adjacent C
has a slight positive charge. atom to the C with the C-X bond, donates its lone pair
2. This attracts the OH- ion as a nucleophile of electrons to form a dative bond.
3. The nucleophile donates the lone pair electrons, 2. The electrons from the C-H bond is used to form a
forms a coordinate bond. double bond between C atoms
4. Carbon breaks it’s weakest bond, the C-X bond, 3. The C atom has too many bonds and so breaks the C-X
producing an alcohol bond. Produces an alkene.
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