18:Haloalkanes Flashcards
What is the functional group in haloalkanes
The halogen atom (F, Cl, Br or I)
Why are haloalkanes insoluble in water
They cannot form hydrogen bonds
Why does the boiling point of a haloalkane increase with the number of carbons
Increase in surface contact area between molecules means an increase in induced dipole-dipole forces, so more energy is needed to break them.
Equation for reaction of ethane with bromine
CH₃CH₃ + Br₂ → CH₃CH₂Br + HBr
Why is the reaction of an alkane and a halogen a bad way to synthesise haloalkanes
because more than one hydrogen atom can be substituted and (in alkanes bigger than ethane) substitution can occur at different positions. Consequently a mixture of products is likely to be formed, with only a low percentage yield of the desired product
What is the name of the mechanism for alkane + halogen
Radical substitution
Equation for reaction of ethene with HBr
CH₂=CH₂ + HBr → CH₃-CH₂Br
What are the products for alkanes + halogens
haloalkane + hydrogen halide
What are the products for alkene + hydrogenhalide
haloalkane
What is the name of the mechanism for alkene + hydrogen halide
Electrophillic addition
Why is the reaction of an alkene + hydrogenhalide quite an efficient way to synthesise haloalkanes
There’s 100% atom economy
Only one hydrogen halide molecule can react with each C=C double bond so in principle there is only 1 or 2 possible products.
How is the major or minor product decided
The major product is formed via a more stable carbocation intermediate
Equation for reaction of methanol + HBr
CH₃-OH + HBr → CH₃-Br + H₂O
What are the products for the reaction of alcohol + hydrogen halide
Haloalkane + H₂O
What are the conditions for the reaction of an alcohol + hydrogen halide to form a haloalkane
Warm the alcohol with a mixture of NaBr or NaCl and concentrated H₂SO₄. This mixture generates the HBr in the reaction.
What are the conditions needed for substitution of a halogen atom by -OH to form an alcohol from a haloalkane
Heating with aqueous sodium hydroxide
What is the name for the mechanism for the substitution of a halogen atom by -OH to form an alcohol from a haloalkane
Nucleophilic substitution
What is a nucleophile
An electron pair donor
Equation for substitution of bromine in bromoethane by -OH
CH₃CH₂Br + OH¯ → CH₃CH₂OH + Br¯
What type of bond fission is in nucleophilic substitution
heterolytic fission
What is hydrolysis
The substitution of a halogen atom by -OH to form an alcohol from a haloalkane
How does the rate of hydrolysis change with the different halogens in the haloalkane
The rate of reaction increases going down the group.
eg. R-I has a higher rate of reaction than R-F
R represents the rest of the chain.
What is added to the haloalkane in the test for rate of hydrolysis
Ethanol- as a solvent
Aqueous silver nitrate- to find which halogen is in there
What is the method for the test for rate of hydrolysis of haloalkanes
To each of the haloalkanes:
- Add ethanol and silver nitrate.
- Warm the tubes in the same hot water bath- so all at the same temp.
Observe the rate at which the precipitate appears
What reactions occur in test for rate of hydrolysis of haloalkanes
-Haloalkane + water
C₂H₅X + H₂O → C₂H₅OH + H† + X¯
-Halide ion + silver ion
Ag†(aq) + X¯(aq) → AgX(s)
What are some uses for Chlorofluorocarbons (CFCs)
Refrigerants, solvents and aerosol propellants
What is the formula for ozone
O₃
Why is ozone useful
it absorbs UV radiation from the sun
What is the initiation step for the reaction between CFCs and ozone
Forms a chlorine radical
CF₂Cl₂ → CF₂Cl• + Cl•
in the presence of UV light
What are the 2 equations to show a chlorine radical reacting with ozone
Cl• + O₃ → ClO• + O₂
ClO• + O → Cl• + O₂
Overall: O₃ + O → 2O₂
How do the chlorine radicals act as a catalyst in the reaction between chlorine radicals and ozone
They are used up and then regenerated, so technically aren’t used up
What are the equations to show nitrogen oxide reacting with ozone
- NO + O₃ → •NO₂ + O₂
- NO₂ + O → •NO + O₂
Overall: O₃ + O → 2O₂
How are nitrogen oxides formed at high altitudes.
Lightning acting on nitrogen and oxygen in the air.
More NO is formed from the intense heat from jet engines
