Halo-part of the name comes first (i.e. 2-bromobutane).
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Classification Of Haloalkanes
Can be classed as Primary, Secondary or Tertiary.
Primary = The halogen is attached to a C, which is only attached to one other C.
Secondary = The halogen is attached to a C, which is attached to two other C's. Tertiary = The halogen is attachd to a C, which is attached to three other C's.
PRIMARY^ SECONDARY^ TERTIARY^
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Reactions: Nucleophilic Substitution
"Replacement of a halogen by a nucleophile (a lone pair donor)". (i.e. H2O, NH3, OH-)
The C-X bond is polar as the halogen has greater electronegativity.
The C atom will be attacked by a species with a lone pair of electrons to donate, and then the nucleophile repleaces the halogen atom.
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Reactions: Hydrolysis (Nuc.Sub.)
"Reaction with water that breaks a chemical compound into two chemical compounds".
Reagent = NaOH (aq)
Conditions = Reflux, in water solvent.
Equation = C2H5Cl + NaOH -> C2H5OH + NaCl (Produces an alcohol).
The nucleophile is the OH- ion, which attacks the C+ dipole, the C-X bond breaks and the halogen atom becomes a halide ion. (i.e. Cl-); a new covalent bond forms between the C atom and the nucleophile, the Br is SUBSTITUTED by the OH.
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Rate Of Hydrolysis (Primary)
Hydrolysis does not occur at the same rate for all the haloalkanes.
The lost X- ion can be identified by using AgNO3(aq) in ethanol.
Ag+ + X- -> AgX (precipitate)
Method:
-Place a measured sample of each haloalkane into test tubes in water baths of 50 degrees.
-In another test tube place a solution of ethanol, water and AgNO3; into the same water bath.
-Once at equal temperature, add equal volumes of ethanol mixture and haloalkane solution.
-Time how long it takes for the precipitate to form.
Chloro- White (AgCl) 2.5 mins
Bromo- Cream (AgBr) 1.5 mins
Iodo- Yellow (AgI) 1 min
So Iodo- is the fastest and Chloro- the slowest.
This is because the bond enthalpy affects the rate of hydrolysis; as the bond enthalpy decreases it is easier to break the C-X bond; the stronger the bond the slower and harder it is to break the C-X bond. (So C-I bond is weakest).
SO bond enthalpy takes precidence over polarity of the halogen.
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Uses Of Haloalkanes
CFC's are useful because they are non-toxic, inert gases, and volatile.
E.g. Trichlorofluoromethane
These are used for refrigeration, aerosols, air conditioning and solvents.
Haloalkanes can also be used in to the production of plastics.
E.g. Chlorethane is used to produce PVC. E.g. Tetrfluoroethane makes polyPTFE; used to produce non-stick on pans.
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Problems With CFC's
CFC's are very stable and break down the ozone layer (O3); the CFC's affect the layer of the stratosphere and stops it working as a barrier against UV-B radiation (skin cancer).
They are inert but broken down by UV light at the stratosphere.
Alternatives are more commonly used; such as HFC's, HCFC's and alkanes.
These "ozone friendly" alternative break down rapidly before the ozone layer. HFC's = refrigent, but still cause the greenhouse affect.
HCFC's = refrigent, still break down ozone layer at 1/10 rate.
HC's = aerosol propellent, but flammable.
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