Key Terminology
Term Definition
Halogenoalkane Saturated organic molecule with C, H and X atoms
Nucleophile Negatively charged ions or neutral molecules with a partial negative charge
that donate a pair of electrons to form a covalent bond
Base Source of hydroxide ions; a proton acceptor
Halogens are electronegative atoms, causing polar bonds when bound to carbon. C-X bonds provide a
δ+ atom for attack by nucleophiles.
Rate of nucleophilic substitution depends on the halogen present as C-X bonds have different strengths:
C-Cl > C-Br > C-I. The C-X bond breaks during nucleophilic substitutions. The strength of C-F makes
fluoroalkanes unreactive and of little use in organic chemistry.
Rate of nucleophilic substitution can be investigated by comparing rate of precipitate formation when
1-halobutanes reacts with ethanol. When silver nitrate is present, the halide ions will react with the
silver to form a precipitate. The iodoalkane should form a precipitate first, then Br, then Cl, as C-I is the
weakest bond so can be overcome easiest.
Nucleophilic Substitution
Nucleophiles are negatively charged ions or neutral molecules with a partial negative charge that
donate a pair of electrons to form a covalent bond. Nucleophiles are strongly attracted to areas of
positive charge. Examples include OH-
, NH3, CNand H2O. The result in all cases is the substitution of a
halide ion by the incoming nucleophile.
The lone pair of the nucleophile is strongly attracted to the δ
+ C atom bonded to the halogen and begins
to form a coordinate bond. In addition, the approaching nucleophile will repel electron density towards
the polarisable halogen atom, inducing a larger dipole in the bond until the carbon-halogen bond
breaks. releasing a halide ion, while the incoming nucleophile takes its place.
• Using an OH- nucleophile, an alcohol is produced
• Using cyanide produces a nitrile with an extra carbon added to the chain; the nitrile can be
hydrolysed to form a carboxylic acid by heating under reflect with a strong acid.
• Using ammonia will produce an amine. In the first step, ammonia acts as a nucleophile and
substitutes for the halogen. In the second step, ammonia acts as a base, removing a proton from
the intermediate. The amine produced can also acts as a nucleophile to produce secondary or
tertiary amines or even quaternary ammonium salts. Yield can be maximised using excess ammonia
to ensure the main product is the primary amine
Nucleophile Conditions Mechanism
OH- Heat under reflux
with dilute NaOH or
KOH
CN- Warm with ethanolic
solution of
potassium cyanide
NH3 Heat in a sealed tube
with excess
ammonia dissolved
in ethanol
Elimination
When haloalkanes react with hydroxide ions, either nucleophilic substitution or elimination occurs –
this depends on the conditions.
OH-
is a Temperature Concentration Solvent Type Product
Substitution Nucleophile Warm Dilute Aqueous Primary Alcohol
Elimination Base Hot Concentrated Ethanolic Tertiary Alkene
In elimination reactions, the hydroxide acts as a base, removing a proton from the C atom neighbouring
the C-X bond, producing an alkene. OHremoves a proton from a β carbon. The pair of electrons from
the C-H bond move to form a double bond between the two C atoms. This causes the electron pair in
the C-X bind to be repelled onto the halogen until the bond breaks, forming a halide and an alkene
Since the β carbon could be on either side of the C-X bond there are again multiple potential products
in unsymmetrical haloalkanes.
Ozone Depletion
Ozone, O3, formed naturally in the upper atmosphere, absorbs UV radiation, making the Earth’s surface
habitable. A decrease in the ozone layer could lead to increased skin cancer rates, weakened immunity,
reduced plant growth and increased levels of toxic gases like hydrogen peroxide and nitrogen oxides.
CFCs were used widely in the 20th Century before full appreciation of the global effects; they caused a
hole in the Ozone layer above Antarctica. The natural decomposition of ozone is catalysed by Cl∙ in the
stratosphere. CCl2F2 → ∙CClF2 + Cl∙. This chlorine radical can then react in the following way:
Cl∙ + O3 → ∙ClO + O2 ∙ClO + O3 → Cl∙ + 2O2
Research has provided evidence for legislation to ban the use of CFCs around the world, agreed to in
the Montreal Protocol. Alternative compounds include HCFCs, containing hydrogen, which break down
lower in the atmosphere, posing no risk to the ozone layer, whilst HFCs contain no carbon so no radicals
are produced – however, these compounds are greenhouse gases so have different impacts.
