- Contain a halogen; F, Cl, Br, I, etc.
- One or more H's have been replaced by a halogen.
- General formula: CnH2n+1X (X = halogen).
- Halo-part of the name comes first (i.e. 2-bromobutane).
1 of 7
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^
2 of 7
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.
3 of 7
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).
- Marks: Curly arrows, equation, dipoles, lone pairs.
- 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.
4 of 7
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)
-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.
5 of 7
Uses Of Haloalkanes
- CFC's are useful because they are non-toxic, inert gases, and volatile.
- 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.
6 of 7
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.
Initiation - CFCl3 -> Cl. + .CFCl2
Propogation - Cl. + O3 -> ClO. + O2
Termination - Cl. + Cl. -> Cl2
- Overall = O3 + O -> 2O2
- 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.
7 of 7