organic chemistry- haloalkanes Flashcards
(33 cards)
why is the carbon-halogen bond in a haloalkane polar?
because halogen atoms are more electronegative as the electron pair is closer than to the carbon atom. The slightly positive charge on the carbon attracts species containing a lone pair of electron.
what are nucleophiles?
an atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond.
what happens when a nucleophile reacts with a haloalkane?
nucleophile replaces halogen atom in a substitution reaction. A new compound is formed containing a different functional group. The reaction mechanism is nucleophilic substitution.
what is hydrolysis?
a chemical reaction involving water or an aqueous solution of hydroxide which causes the breaking of a bond in a molecule.
how does a hydrolysis reaction occur in a haloalkane?
halogen atom is replaced by -OH group which is an example of nucleophilic substitution.
Describe the steps involved in the hydrolysis of haloalkanes?
1)the nucleophile, OH-, approaches delta positive carbon on the opposite side of the halogen atom which minimises repulsion.
2)lone pair of electron is attracted and donated to the carbon atom. A new bond is formed.
3)carbon-halogen bond breaks in heterolytic fission. Halide ion and new alcohol compound is formed.
Can haloalkanes be converted into alcohols using aqueous sodium hydroxide.
Yes, but the reaction is very slow at room temperature so it is heated under reflux to obtain a good yield of product
what does the rate of hydrolysis on haloalkanes depend on mainly?
the strength of the carbon-halogen bond. Compounds with the slower rate of reaction have stronger carbon-halogen bonds.
which halogen has the strongest to least strong halogen bond? (Cl,Br,I)
1)most strong: carbon-chlorine bond (slowest rate)
2) carbon-bromine bond
3)least strong: carbon-iodine bond (fastest rate)
what is the other factor of the rate of hydrolysis of haloalkanes?
it structure depending if it is tertiary, secondary or primary. Primary structures are the slowest and tertiary structures are the fastest. This is because the teriary structure has a greater stability of carbocation and it has a two step mechanism.
what are organohalogen compounds?
compounds that contain at least one halogen atom joined to a carbon chain. They are rarely found naturally and are not broken down naturally. They are used in pesticides, refrigerants and solvents.
where is the ozone layer found?
outer edge of stratosphere.
How does the ozone layer protect earth?
by absorbing ultraviolet radiation from the sun which can cause genetic damage and skin cancer in humans.
what happens in the ozone layer usually?
-ozone is continually formed and broken by uv radiation. Oxygen molecules break down into oxygen radicals.
-a steady state is then set up involving O2 and oxygen radicals which forms O3 in a reversible reaction where formation and breaking down is at the same rate.
where are CFCs found?
aerosol propellants, refrigerants and air conditioning units. They are very stable due to the strong carbon-halogen bond.
how do CFCs damage the ozone layer?
CFCs begin to breakdown into chlorine radicals which catalyses the breakdown of the ozone layer when reaching the atmosphere.
how do CFCs form chlorine radicals?
UV radiation in the stratosphere provides enough energy to break the C-Cl bonds through homolytic fission. Since radiation initiates the process is called photodissociation of CF2Cl2.
how does a chlorine radical breakdown the ozone layer into oxygen?(propagation steps)
propagation step 1: Cl. + O3 forms ClO. + O2
propagation step 2: ClO. + O forms Cl. + O2
overall equation: O3 + O forms 2O2
chain reaction
what other radicals catalyse breakdown of ozone layer?
nitrogen oxide radicals from lightning strikes and aircraft travel in stratosphere.
How do nitrogen oxide radicals breakdown the ozone layer?(propagation steps)
propagation step 1: NO. + O3 forms NO2. + O2
propagation step 2: NO2. + O forms NO. + O2
overall equation is the same as CFCs
what are the properties of CFCs?
-very volatile
-chemically stable
-chemically stable
-non-flammable
Describe how to monitor the hydrolysis of haloalkanes
-add 1cm cubed of ethanol and two drops of haloalkane to test tube
-stand test tube in water bath at 60 degrees
-place test tube containing 0.1mol/dm cubed of silver nitrate in water bath to reach a constant temperature.
-ad. 1cm cubed of silver nitrate quickly to the test tube and immediately start stop watch.
-observe test tubes and record time taken for precipitate to form
what observations are made when observing chloro, bromo and iodo haloalkanes?
chloroalkane- white precipitate forms
bromobutane- cream precipitate
iodobutane- yellow precipitate
why are people still worried about ozone depletion despite CFCs being illegal? (2 marks)
-CFCs are still being used
-other substances still cause ozone depletion
