Haloalkanes Flashcards
What is the reaction mechanism of methane with chlorine as a free-radical substitution.
- Intiation; (photodissociation; U.V. breaks the Cl-Cl bond; bond split equally, each keeps one unpaired electron; reactive)
- Propagation (free radicals are used up and created in a chain reaction)
- Termination (free radicals are mopped up)
- ) Cl2 → 2Cl•
- ) Cl• + CH4 → •CH3 + HCl
Cl2 + •CH3 → CH3Cl + Cl•
3.) Cl• + •CH3 → CH3Cl
Cl• + Cl• → Cl2
•CH3 + •CH3 → C2H6
What are chloroalkanes and chlorofluroalkanes used for?
Can be used as solvents; dry cleaning/degreasing.
Why is ozone beneficial?
Ozone (O3) exists in the upper atmosphere to absorb UV radiation (or sun burn, skin cancer arises through damage to DNA).
It protects us from U.V.
What effects do chlorine have on ozone?
Chlorine atoms catalyse the decomposition of ozone
and contribute to the formation of a hole in the ozone layer.
Cl• + O3 → ClO• + O2
ClO• + O3 → 2O2 + Cl•
Cl• is recycled; acts as a catalyst, being reformed to undergo the reaction again. The chlorine free radicals attack ozone and regenerates again.
Overall: 2O2 → 3O2
(can see that the chlorine free radical is not destroyed in this process; regenerated; catalyst)
(the role of the Cl• / chlorine atom is to find an
alternative route OR lower Ea / activation energy)
How are chlorine atoms formed in the upper atmosphere?
Energy from U.V. radiation causes the C-Cl bonds in chloroflurocarbons (CFCs) to break homolytically (dissociation into two neutral fragments). The chlorine free radicals attack ozone to form an intermediate (ClO•) which goes on to attack ozone again, reforming the chlorine free radical. (Cl•)
Explain how CFCs came to be banned.
- Legislation to ban the use of CFCs was supported by chemists and they have now developed alternative chlorine-free compounds, to reduce the damage to the ozone.
- CFCs were used in fire extinguishers, aerosols, coolant gas in fridges and to foam plastics to make insulation and packaging materials, due to their uncreactive, non-flammable, non-toxic nature.
What are the properties of haloalkanes?
Haloalkanes contain polar bonds, and are susceptible to nucleophlic attack (from OH-, CN- and NH3).
Describe the hydrolysis of a haloalkane (bromoethane) to make alcohol.
(include conditions + mechanism)
It is a nucleophilic substitution (nucleophile; electron pair donor, attacks slightly positive exposed C atom; halogen pulls away bonding pair of electrons to expose nucleus)
- C - Br bond is polar; Cδ+ attracts a lone pair of electrons from the OH- ion.
- C - Br bond breaks; both electrons taken by Br, new bond forms between C and OH.
- :Br- product also produced.
Conditions:
- Aqueous NaOH/KOH
- Warm; room temperature
What is the reaction with a haloalkane to form a nitrile?
(bromoethane, conditions)
Reaction with :CN-.
The product is a nitrile.
It has one more carbon atom than the starting material.
Conditions:
- ethanolic KCN (potassium cyanide in ethanol)
- heat under reflux
Reaction often useful if we want to make a product that has one carbon more than the starting material; lengthening the carbon chain.
Describe the reaction between a haloalkane (bromoethane) and ammonia.
:NH₃
Two-stage reaction to making a primary amine.
As ammonia is a neutral nucleophile, a proton, H+, must be lost.
Conditions:
- excess concentrated NH3
- under pressure
- in ethanol
Ethylamine + ammonium bromide (NH4Br) formed.
How does carbon–halogen bond enthalpy influence the rate of hydrolysis?
Two factors determine how readily the C-X bonds reacts:
- bond polarity
- bond enthalpy
Bond polarity
Halogen are more electronegative than carbon; carbon becomes electron deficient; polarity of the C-X bond predicts C-F bond would be most reactive; it is the most polar, with the Cδ+ being most positive.
Bond enthalpy
However, the bond enthalpies are lesser going down the group; the bond is weaker as the shared electron in the C-X covalent bond get further and further away from the halogen nucleus (Fluorine is the smallest atom and the shared electrons are strongly attached to the F nucleus).
Predicts that reactivity increases down the group; iodo-compounds having weakest bonds.
Experiments reinforce this; bond enthalpy is more important a factor than bond polarity.
Appreciate how useful nucleophilic substitution reactions are. Appreciate.
Great way of introducing new functional groups into organic compounds. Haloalkanes can be converted into alcohols, amines and nitriles. These in turn can be converted to other functional groups.
- The substitution reaction allows you to produce any alcohol molecule ou need; alcoholds can be the starting point for synthesis reactions that produce aldehydes, ketones, esters and carboxylic acids.
Elimination reaction of hydrogen bromide from bromoethane.
(Conditions, mechanism)
If you warm a haloalkane with OH- ions dissolved in ethanol instead of water, elimination occurs, and an alkene is produced.
Conditions:
- hot ethanolic NaOH/KOH (anhydrous)
- Under reflux
- OH ions uses its lone pair to form a bond with one of the hydrogen atoms on the carbon next to the C-Br bond. (these hydrogens are very slightly δ+)
- Electron pair from C-H bond bcomes part of a C=C double bond.
- Bromine takes the pair of electrons in the C-Br bond, leaves as a bromide ion (leaving group).
What factors influence if the reaction of OH- ions with haloalkanes will be an elimination or a substituion?
- Reaction conditions (aqueous vs. ethanolic)
- Type of haloalkane (primary, secondary, tertiary)
Reaction conditions:
- OH- ions at room temperature dissolved in water (aqueous) favour substitution.
- OH- ions at high temperature dissolved in ethanol favour elimination.
Typr of haloalkane:
- Primary haloalkanes tend to react by substitution.
- Tertiary haloalkanes tend to react by elimination.
- Secondary do both.
In general, a mixture of the two occur.
What is the usefulness in elimination reactions in organic synthesis?
- Elimination reaction is a good way of introducing a double bond into a molecule; a lot of organic synthesis reactions use alkenes, so it makes a good starting point.
