Chapter 15 Haloalkanes Flashcards
What are haloalkanes?
An alkane where at least one hydrogen has been replaced by one halogen atom.
What are the three levels of classification for haloalkanes?
Primary
Secondary
Tertiary
What is a primary haloalkane?
One Carbon atom is directly attached to the Carbon bonded with the halogen atom
What is a secondary haloalkane?
Two Carbon atoms are directly attached to the Carbon bonded with the halogen atom
What is a tertiary haloalkanes?
Three Carbon atoms are directly attached to the Carbon bonded with the halogen atom
Why are haloalkanes polar?
The Carbon to halogen bond in the molecule is polar because the halogen has a greater electronegativity compared to the Carbon atom. Therefore, the Halogen becomes slightly negative while the Carbon atom becomes slightly positive.
What are nucleophiles?
Electron pair donors
What do haloalkanes react with?
Nucleophiles because they are attracted to the slightly positive carbon atom
What mechanism does haloalkanes undertake?
Nucleophilic substitution
What nucleophiles can haloalkanes react with?
OH-
H2O
NH3
Explain the mechanism of nucleophilic substitution
The nucleophile approaches the Carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom
This direction of attack by the nucleophile minimises repulsion between the nucleophile and the slightly negative halogen atom
The lone pair of electrons of the nucleophile is attracted and donated to the slightly positive Carbon atom
A bond forms between the nucleophile and the carbon atom
The carbon- halogen bond breaks by heterolytic fission
An alcohol is formed and a halide ion
Why does the nucleophile approach from the opposite end of the haloalkanes?
Minimised repulsion between the nucleophile and the slightly negative halogen atom.
What is hydrolysis?
A chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule. Resulting in the molecule being split into two products.
How can haloalkanes be converted into alcohols?
Using aqueous sodium hydroxide heating under reflux as the reaction is slow under room temperature. This ensures that a good yield of product is obtained.
How would you compare rates of hydrolysis in haloalkanes?
-Use haloalkanes of the same chain length
-In a test tube add ethanol and the haloalkane (the ethanol is a solvent which allows reactants to mix more efficiently)
-In another test tube add aqueous silver nitrate solution and place both test tubes into a water bath at 50 degrees
-Once at the same temperature mix the contents of the test tubes
-Measure the time taken for the precipitate to form
-Repeat using the same volume of other haloalkanes
-Calculate rate using 1/time taken
-The strength of the Carbon to halogen bond determines the rate. C-Cl has the highest bond enthalpy so has the lowest rate of reaction while C-I has the weakest bond so has the greatest rate of reaction.
How can the different halide ions be identified following nucleophilic substitution?
Silver Nitrate is added and the precipitate will change colour:
AgCl- White precipitate
AgBr- Cream precipitate
AgI- Yellow precipitate
Explain the boiling point of haloalkanes
The boiling point increases at the chain length of haloalkanes increases because there are more London Forces (induced dipole dipole interactions).
There is also an increase in boiling point going from chloride to bromide to iodide for a given number of carbon atoms because there is an increase in the number of electrons leading to stronger London forces so more energy is required to overcome the intermolecular forces.
However, as the number of branches increases the boiling point decreases as the molecules have less surface contact.
Explain the solubility of haloalkanes
Soluble in organic solvents but insoluble in water because they are unable to form hydrogen bonds with the water molecules.
How does the rate of hydrolysis change going down the group?
The rate of hydrolysis depends on the strength of the carbon to halogen bond. Therefore, bond enthalpy gets weaker going down the group as less energy is required to break the C-X bond so rate of hydrolysis increases going down the group.
What are Organohalogen compounds?
Molecules that contain at least one halogen atom joined to a carbon chain
What are the uses of Organohalogen?
General solvents, flame retardants, refrigerants, aerosol propellants, pesticides
What are the properties of haloalkanes?
Volatile
Non-flammable
Unreactive (long residence time)
Non-toxic
What is the function of the ozone layer?
Absorbs most of the biologically damaging ultraviolet radiation (UV-B)- linked to sunburn
What is the equation for the formation and breakdown of the ozone layer?
O2 + O ⇌ O3
How has the use of CFC impacted the ozone layer?
In the stratosphere, UV radiation provides enough energy to break the Carbon- halogen bond in the CFC by homolytic fission forming Chlorine radicals which catalyse the breakdown of the ozone layer.
The Chlorine radical will then react with an ozone molecule breaking it down to O2.
What is the initiation equation for the depletion of the ozone layer via CFC?
CF2Cl2 —> CF2Cl⚫️ + Cl⚫️
What is the propagation equation for the depletion of the ozone via CFC?
1) Cl⚫️ + O3 —> ClO⚫️ + O2
2) ClO⚫️ + O —> Cl⚫️ + O2
What is the overall equation for the depletion of the ozone via the use of CFC?
O3 + O —> 2O2
What is photodissociation?
Radiation initiating the breakdown of the CFC
Are CFC responsible for all ozone depleting reactions?
No, Nitrogen oxide radicals are formed naturally during lightning and released by aeroplane fumes
NO⚫️ + O3 —> NO2⚫️ + O2
NO2⚫️ + O —> NO⚫️ + O2
Overall- O3 + O —> 2O2
Why is ethanol added when measuring the rate of hydrolysis?
Acts as a solvent so the water and haloalkanes can mix to produce a single solution
OH⚫️ radicals are known to deplete the ozone layer. Complete the two propagation steps and write the overall equation for the reaction
OH⚫️ + O3 —> O2H⚫️ + O2
O2H⚫️ + O3 —> OH⚫️ + 2O2
Overall- 2O3 —> 3O2
Why is the rate of ozone depletion constant?
Because the Chlorine radical acts as a catalyst because its is used and regenerated so it can continue to react with O3.
The research chemist would like to improve the atom economy of preparing propan-1-ol. Suggest the name of a different haloalkane that could be used to improve atom economy
1-chloropropane or 1-fluropropane
Because the total relative molecular mass of the products will be less so the atom economy will be greater
