Haloalkanes Flashcards
What is the general formula for haloalkanes?
CnH2n+1X
Definition of nucleophilic substitution
Replacement (of a halogen) by a nucleophile.
Definition of a nucleophile
An electron pair donor (lone pair).
List 4 examples of nucleophiles.
:OH- :NH3 :CN- H2O:
Explain the mechanism of nucleophilic substitution (3 points).
- C-halogen bond is polar as the halogen has a greater electronegativity than the carbon atom.
- This means the C atom will be attacked by a species with a lone pair of electrons to donate, these species are nucleophiles.
- The nucleophile replcaes the halogen atom.
Definition of hydrolysis
A reaction with water that breaks a chemical compound into two chemical compounds.
What is the reagent used for thew hydrolysis of haloalkanes and why is it used?
NaOH(aq). Using a metal hydroxide speeds up the rate of hydrolysis.
What is the condition for the hydrolysis of haloalkanes?
Reflux, in water solvent
Draw the mechanism for the hydrolysis of of C2H5Br
*Refer to hand-written notes
What do curly arrows represent in mechanisms?
The movement of a pair of electrons.
Explain the mechanism of the hydrolysis of C2H5Br (3 points).
- The nucleophile is the OH- ion and it attacks the delta-positive carbon.
- The C-halogen bond breaks and the halogen atom becomes a halide ion.
- A new covalent bond forms between the C atom and the nucleophile. The bromine is substituted by the OH.
Write the genreral reaction equ for how the halide ion can be identified by using silver nitrate (AgNO3(aq)) in ethanol.
Ag+(aq) + X-(aq) …. AgX(s)
Describe the method for finding the difference in the rate of hydrolysis (4 points).
- Place a measured sample of each halogenalkane, (equal molar amounts- concentration), into separate test tubes and place in to a water bath at 50 degrees (speeds up reaction).
- Into another test tube, place a solution of ethanol, water and aqueous silver nitrate and place in the same water bath.
- Once at equal temperatures, add equal volumes of the ethanol mixture and halolalkane solutions together.
- Timehow long it takes for each precipitation to form.
Results of hydrolysis rates reaction
Chloro- white, AgCl(s)
Bromo- cream, AgBr(s)
Iodo- yellow, AgI(s)
Describe how the rate of the hydrolysis reaction can be calculated
1 / time taken for the precipiate to form
Explain the reason for the different rates of the hydrolysis of haloalkanes (3 points).
- As the bond enthalpy strength decreases down the group, it is easier to break the C-X bond.
- The stronger the bond, the harder it is to break and the slower the rate of reaction will be.
- The weaker the bond, the faster the rate will be.
Rank the C-X bonds in terms of hydrolysis rates
C-I: Weakest bond, so fastest reaction as less energy to break bond.
C-Br: Reaction rate between C-I and C-Cl as bond enthalpy is higher than C-I but lower than C-Cl.
C-Cl: Strongest bond so slowest reaction.
C-X bond becomes weaker down group 7.
List 3 useful properties of CFC’s (chlorofluorocarbons).
- Non-toxic
- Inert
- Volatile
What are the 2 uses of trichlorofluoromethane?
- Refrigeration
- Air conditioning
State 2 dichlorodifluoromethane uses.
- Aerosal propellants
- Blowing agent for polystyrene
What are the 3 uses of 1,1,1-trichloro-2,2,2-trifluoroethane?
- Dry cleaning solvents
- Degreasing agents
- Circuits boards
Write the reaction for how the ozone is maintained in a balance with oxygen and explain how this is established
O2 + O …. O3
Reversible reaction
From the breakdown of O2 by UV-radiation
Draw the mechanism for the ozone depletion
*Refer to hand-written notes
Advantages, disadvantages and uses of HFC’s (hydrofluorocarbons)
ADV- Non-flammable, non-toxic, ozone friendly.
DIS- Can cause greenhouse effect.
Use- Aerosal propellant, refrigeration
Advantages, disadvantages and uses of HCFC’s (hydrochlorofluorocarbons)
ADV- Non-flammable, non-toxic.
DIS- Can damage ozone layer but at a slower rate than CFC’s.
Use- Refrigerant
Advantages, disadvantages and uses of alkanes
ADV- Ozone friendly
DIS- Flammable
Use- Aerosal propellant
