Haloalkanes Flashcards
Describe the atom economy of radical substitution
Radical substitution gives a relatively poor atom economy as it produces a large proportion of unpredictable waste products.
Describe the atom economy of electrophilic addition
100%
Compare the reactivity of haloalkanes and alkanes
Haloalkanes are more reactive as halogens are electronegative so form polar bonds which can be attacked by nucleophiles.
Why are fluoroalkanes not reactive?
C-F bonds have an extremely high bond enthalpy so are not easily broken.
Define hydrolysis.
A chemical reaction involving water or aqueous OH- ions which cause the breaking of a bond in a molecule
What is a haloalkane hydrolysis reaction?
A reaction in which the halogen atom on an alkane is substituted for an OH group
What are the conditions for the hydrolysis of a haloalkane?
Heat under reflux
Define a nucleophile.
An electron rich species which is attracted to an electron deficient centre where it can provide a lone electron pair for the formation of a new covalent bond.
What is the name of the reaction mechanism for the hydrolysis of haloalkanes?
Nucleophilic substitution
Describe the mechanism of nucleophilic substitution in haloalkanes.
A nucleophile (eg. OH-) is electrostatically attracted to a delta + charge on a carbon atom which is involved in a polar bond with a halogen atom. The C-X bond breaks heterolytically to leave an X- ion and a new C-OH bond.
What details must be included when drawing the reaction mechanism of nucleophilic substitution.
Delta charges on the atoms in the polar C-X bond
A curly arrow from the C-X bond to the X atom
A lone electron pair and negative charge on the nucleophile
A curly arrow from the lone electron pair to the delta + charge on the carbon atom
What type of bond breaking occurs in nucleophilic substitution?
Heterolytic
Give the order of the rate of hydrolysis of the haloalkanes.
iodo > bromo > chloro
Explain the reactivity of chloroalkanes.
Despite C-Cl bonds being most electronegative and so the most susceptible to nucleophiles, they also have the highest bond enthalpy and so require more energy to break apart in a reaction.
Explain the reactivity of iodoalkanes.
Although C-I bonds are the least electronegative, they are the weakest of C-X bonds and so are most easily broken in reaction.
Describe a method to determine the relative hydrolysis rates of haloalkanes.
Three drops of three different haloalkanes (Cl, I and Br) are added to separate boiling tubes, each with the same volume of ethanol. The boiling tubes are placed in the same hot water bath. Silver nitrate solution is added to each test tube in equal quantities at the same time as a stopwatch is started. Time is taken for each reaction to produce a precipitate.
Describe the purpose of ethanol in the hydrolysis of haloalkanes.
It acts as a co-solvent between the haloalkane and the OH- ions.
Describe the purpose of silver nitrate solution in the hydrolysis of haloalkanes
It provides the OH- ions for the reaction to take place, and Ag+ ions, which react with the halide ions that are released in the reaction to form a AgX precipitate which makes the reaction visible.
Why is a hot water bath used in the hydrolysis of haloalkanes?
To increase reaction rate.
Where in the atmosphere is ozone found?
The stratosphere
Give two equations for the formation of ozone.
O2 –(uv)–> 2O•
2O• + 2O2 —> 2O3 (+ heat)
What is ozone?
O3
Give an equation to show the breakdown of ozone by UV radiation
O3 —(uv)—> O2 + O•
Describe the concentration of ozone in the atmosphere.
It is constant, as an equilibrium has formed between ozone and O2 + O•. Ozone is constantly being created and broken down.
