Haloalkanes Flashcards
General formula of haloalkanes
Cn H2n+1 X
or R-X
Primary haloalkane
The carbon that the halogen is bonded to is itself only bonded to one other carbon
Secondary haloalkane
The carbon that the halogen is bonded to is itself bonded to two other carbons
Tertiary haloalkane
The carbon that the halogen is bonded to is itself bonded to three other carbons
Why are haloalkanes not soluble in water?
Because the C-X bond is not polar enough.
What mechanism forms haloalkanes?
Free radical substitution
What are the three steps in free radical substitution?
Initiation, propagation, termination.
Describe how free radical substitution happens
Initiation- UV light breaks the bond in the halogen molecule creating two halogen atoms each with a lone electron (free radical). This is homolytic fission. Propagation- Halogen radical reacts with hydrogen atom from an alkane making a methyl free radical and hydrogen halide. The methyl free radical reacts with a halogen molecule creating a haloalkane and another halogen free radical. Termination. Free radicals are removed by reacting with other free radicals.
Where is ozone naturally formed?
In the stratosphere and troposphere
upper atmosphere
Why is ozone beneficial?
It blocks most of the UV radiation from the sun which is harmful to life.
How do chlorine atoms form in the upper atmosphere?
CFCs from refrigerators end up in the stratosphere. The UV radiation is absorbed by them and it breaks the C-Cl bond. One of the shared electrons goes to the released chlorine atom forming a chlorine free radical.
Chlorine and ozone equations
CCl3F -> *CCl2F + Cl *Cl + O3 -> *ClO + O2 *ClO + O3 -> *Cl + 2O2 2O3 -> 3O2 Equation 3 shows how chlorine free radicals can be regenerated so continue to catalyse the decomposition of the ozone layer.
What alternatives to CFCs have been made?
Alkanes and hydrofluorocarbons (chlorine free)
What is the role of the chlorine atom in the decomposition of ozone?
To find an alternative route or lower activation energy.
Define nucleophile
A negative ion or molecule that is able to donate a pair of electrons (its lone pair) to an electron deficient area of another reactant.
Mechanism for nucleophilic substitution with hydroxide ions
Arrow from lone pair on oxygen atom to the delta positive carbon atom. Arrow from C-X bond to the delta negative halogen. OH is where X was and X is the leaving group with a lone pair and full negative charge. 50:50 ethanol:water. Can happen at room temperature but slowly. Forms an alcohol.
Mechanism for nucleophilic substitution with cyanide ions
Arrow from lone pair on the carbon atom (cyanide) to the delta positive carbon atom (haloalkane). Arrow from C-X bond to the delta negative halogen. CN is where X was and X is the leaving group with a lone pair and full negative charge. The solution is aqueous and alcoholic and warmed. The product is a nitrile with one more carbon than the original haloalkane.
Mechanism for nucleophilic substitution with ammonia
Arrow from lone pair of nitrogen atom to the delta positive carbon. Arrow from C-X bond to the delta negative halogen. NH3 is where X was and X is leaving group with a lone pair and full negative charge. Arrow from N+-H bond to N. Arrow from lone pair of nitrogen atom from other ammonia molecule to same H. Forms amine (with lone pair on N) and ammonium (+ on N). Needs excess concentrated ammonia solution in ethanol carried out under pressure.
Define hydrolysis
Splitting molecules using water
Experiment to compare the rates of hydrolysis of haloalkanes
Add 1cm^3 of each to separate test tubes. Pipette 1cm^3 of silver nitrate solution into one and time how long it takes until a precipitate is first seen. Do the same with the other test tubes under the same temperature. Shorter time means a higher rate of reaction,
Mechanism for elimination with hydroxide ions
Arrow from lone pair of oxygen atom to hydrogen bonded to carbon next to the carbon the halogen is bonded to. Arrow from C-H bond (same H) to C-C bond. Arrow from C-X bond to halogen. X is leaving group with a lone pair and full negative charge. Forms an alkene and water. Needs ethanol as solvent with no water and is heated.
Which reaction will each type of haloalkane tend to do?
Primary - substitution
Secondary - both
Tertiary- elimination
