31: Typical Reactions of Selected Functional Groups Flashcards
In the reaction of methane with halogens, what is the difference in reaction conditions with chlorine and bromine?
Chlorine is more reactive than bromine, so it requires less energy to activate the reaction. The reaction with chlorine can take place in diffused sunlight while that with bromine can only take place in direct sunlight.
What is the reaction condition of the hydrogenation of alkenes?
Room temperature in the presence of a platinum catalyst,
or on heating at 150ºC in the presence of a nickel catalyst.
What is the reaction condition of the chlorination of alkenes?
The reaction proceeds at room temperature and pressure.
What is the reaction condition of the bromination of alkenes?
The reaction proceeds at room temperature and pressure.
What is the chemical equation of the reaction when chlorine is mixed with water?
Cl2 + H2O <-> HCl + HOCl
What is the chemical equation of the reaction when bromine is mixed with water?
Br2 + H2O <-> HBr + HOBr
What is the word equation of the bromination of ethene?
Ethene + bromine (in 1,1,1-trichloroethane) -> 1,2-dibromoethane
What is the word equation of the reaction when bromine water is added to ethene?
Ethene + bromine + water -> 2-bromoethan-1-ol + hydrogen bromide
What is the word equation of the reaction when hydrogen bromide is added to ethene?
Ethene + hydrogen bromide -> bromoethane
What is the reaction condition of the reaction between alkenes and gaseous hydrogen halides?
The reaction proceeds at room temperature and pressure.
What is the condition for different isomers to be produced when an alkene reacts with a hydrogen halide?
Both the alkene and hydrogen halide are unsymmetrical molecules.
What is the Markonikov’s Rule?
Markonikov’s rule states that when a molecule HX adds to an unsymmetrical alkene, the hydrogen atom attaches mainly to the carbon atom that already carries the greater number of hydrogen atoms.
What are primary haloalkanes?
Primary haloalkanes are haloalkanes which have only one carbon atom bonded to the carbon atom bearing the halogen atom.
What are secondary haloalkanes?
Secondary haloalkanes are haloalkanes which have two carbon atoms bonded to the carbon atom bearing the halogen atom.
What are tertiary haloalkanes?
Tertiary haloalkanes are haloalkanes which have three carbon atoms bonded to the carbon atom bearing the halogen atom.
What are the reaction conditions of the reaction of haloalkanes to form alcohols?
Sodium hydroxide solution is added and the reaction mixture is heated under reflux.
What is an alternative name for the reaction of haloalkanes to form alcohols?
Hydrolysis
What is heating under reflux?
Heating under reflux means heating with a condenser fitted vertically in the reaction flask.
What is the importance of heating under reflux?
Heating under reflux ensures that the volatile reactants are condensed back into the reaction mixture, which prevents them from evaporating before they have a chance to react.
What is the importance of using the correct direction of water flow in the condenser when heating under reflux?
The place where water enters should be lower than the place where water exits on the condenser column. This ensures that water can build up in the condenser column before flowing out, which provides a better cooling effect.
What is the importance of adding anti-bumping granules when heating under reflux?
Anti-bumping granules provide a larger surface area for the formation of bubbles, so a lot of small bubbles will be formed instead of a big one. This prevents a big bubble from pushing the liquid up the condenser column, which may lead to spilling.
What are primary alcohols?
Primary alcohols are alcohols which have only one carbon atom bonded to the carbon atom bearing the hydroxyl group.
What are secondary alcohols?
Secondary alcohols are alcohols which have two carbon atoms bonded to the carbon atom bearing the hydroxyl group.
What are tertiary alcohols?
Tertiary alcohols are haloalkanes which have three carbon atoms bonded to the carbon atom bearing the hydroxyl group.
What are the reaction conditions of the reaction of tertiary alcohols with concentrated hydrochloric acid?
The reaction proceeds on shaking at room temperature.
What are the reaction conditions of the reaction of secondary alcohols with concentrated hydrochloric acid?
Anhydrous zinc chloride is added as a catalyst, and the reaction mixture is heated under reflux.
What are the reaction conditions of the reaction of secondary alcohols with dry hydrogen chloride gas?
Anhydrous zinc chloride is added as a catalyst.
What are the reaction conditions of the reaction of alcohols with phosphorus pentachloride?
The reaction proceeds on shaking at room temperature.
What are the reaction conditions of the reaction of alcohols with phosphorus trichloride?
The reaction mixture is heated under reflux.
Explain the difference in reactivity of phosphorus pentachloride and phosphorus trichloride.
Phosphorus pentachloride is more reactive than phosphorus trichloride. This is because the bond angle of phosphorus pentachloride is smaller and the chlorine atoms are brought together to as close as 90º surrounding the central phosphorus atom. After reacting with the alcohol, phosphorus trichloride oxide is formed, which is more stable since the bond angles are larger than that in phosphorus pentachloride. Meanwhile, the bond angle of the phosphorus molecule is not changed after reaction of alcohol with phosphorus trichloride, so the reaction proceeds less readily.
What are the products and chemical formulas of the reaction of alcohols with phosphorus pentachloride?
Chloroalkane (RCl), phosphorus trichloride oxide (POCl3), and hydrogen chloride gas (HCl).
What are the products and chemical formulas of the reaction of alcohols with phosphorus trichloride?
Chloroalkane (RCl) and phosphorous acid (H3PO3).
What are the reaction conditions of the reaction of alcohols with sulphur dichloride oxide?
The reaction mixture is heated under reflux.
What are the products and chemical formulas of the reaction of alcohols with sulphur dichloride oxide?
Chloroalkane (RCl), sulphur dioxide gas (SO2), and hydrogen chloride gas (HCl).
Explain the choice of chlorinating method which is widely used in industry.
SOCl2 substitution, since the side products are sulphur dioxide gas and hydrogen chloride gas, which will escape freely and leave behind the chloroalkane of high purity.
What are the ways to brominate an alcohol using hydrogen bromide?
- Add the alcohol to hydrogen bromide and heat under reflux
- Add a mixture of sodium bromide and concentrated sulphuric acid to the alcohol and heat under reflux.
Why does orange colour appear when brominating an alcohol with sodium bromide and concentrated sulphuric acid?
Hydrogen bromide is produced in the reaction between sodium bromide and concentrated sulphuric acid. It is oxidised by the concentrated sulphuric acid to form bromine liquid, causing orange colour to appear in the reaction mixture.
What are the ways to iodinate an alcohol using hydrogen iodide?
- Add the alcohol to hydrogen iodide and heat under reflux
- Add a mixture of sodium iodide and concentrated phosphoric acid to the alcohol and heat under reflux.
Explain the choice of acid when producing hydrogen iodide for the iodination of alcohols.
Concentrated phosphoric acid is used.
If oxidising acids like concentrated sulphuric acid are used, the hydrogen iodide produced will quickly be oxidised to iodine by the acid, since hydrogen iodide is a strong reducing agent. This leaves almost no hydrogen iodide left for reaction.
If volatile acids like ethanoic acid are used, the acid evaporates easily since a large amount of heat is produced when hydrogen iodide reduces the acid.
What are the ways to brominate an alcohol using phosphorus tribromide?
- Add the alcohol to phosphorus tribromide and heat under reflux
- Add a mixture of red phosphorus and bromine liquid to the alcohol and heat under reflux.
What are the products and chemical formulas of the reaction of alcohols with phosphorus tribromide?
Bromoalkane (RBr) and phosphorous acid (H3PO3)
What are the ways to iodinate an alcohol using phosphorus triiodide?
- Add the alcohol to phosphorus triiodide and heat under reflux
- Add a mixture of red phosphorus and iodine solid to the alcohol and heat under reflux.
What are the products and chemical formulas of the reaction of alcohols with phosphorus triiodide?
Iodoalkane (RI) and phosphorous acid (H3PO3)
Explain whether PBr5 and PI5 substitutions are possible with alcohols.
No, because PBr5 and PI5 are unstable. They have a large atomic size and experience large forces of repulsion, so they cannot be held close to each other that the bond angles between some pairs are 90º.
List all chlorinating agents and their chemical formulas for alcohols.
Hydrogen chloride / hydrochloric acid (HCl) liquid or gas, phosphorus pentachloride (PCl5) or phosphorus trichloride (PCl3), sulphur dichloride oxide (SOCl2).
List all brominating agents and their chemical formulas for alcohols.
Hydrogen bromide gas (HBr) or in situ production of HBr by sodium bromide and concentrated sulphuric acid (NaBr + H2SO4), phosphorus tribromide (PBr3) or in situ production of PBr3 by red phosphorus and bromine liquid (red P + Br2).
List all iodinating agents and their chemical formulas for alcohols.
Hydrogen iodide gas (HI) or in situ production of HI by sodium iodide and concentrated phosphoric acid (NaI + H3PO4), phosphorus triiodide (PI3) or in situ production of PI3 by red phosphorus and iodine solid (red P + I2).
What is dehydration?
Dehydration is a reaction in which a water molecule is removed from a reactant molecule.
Explain whether all alcohols can undergo dehydration.
No, dehydration removes the hydroxyl group and the hydrogen atom attached to the adjacent carbon atom. If the adjacent carbon atom(s) have no hydrogen atoms, then they cannot undergo dehydration.
What is the systematic name of the alcohol with smallest molar mass that cannot be dehydrated?
2,2-dimethylpropan-1-ol / 2,2-dimethylpropanol
What is an elimination reaction?
An elimination reaction is a reaction where a small molecule is removed from a reactant molecule.
What are the reaction conditions of the dehydration of primary alcohols?
- Pass the alcohol over a catalyst of aluminium oxide at 300ºC.
- Heat the alcohol with concentrated sulphuric acid catalyst at 180ºC.
What are the reaction conditions of the dehydration of secondary alcohols?
Heat the alcohol with 85% phosphoric acid catalyst at 165-170ºC.
What are the reaction conditions of the dehydration of tertiary alcohols?
Heat the alcohol with 20% sulphuric acid catalyst at 85ºC.
What is the relative ease of dehydration of different alcohols?
3º ROH > 2º ROH > 1º ROH
For dehydration reactions producing more than one alkene, which one is the major product?
The more branched alkene / the alkene with more alkyl groups attached to the carbon-carbon double bond is a more stable one, hence it is the major product.
What are the products when primary alcohols are oxidised?
Aldehydes are first produced, which can be further oxidised to carboxylic acids.
What are the products when secondary alcohols are oxidised?
Ketones are produced, which do not undergo further oxidation.
Explain whether tertiary alcohols can be easily oxidised.
No, because they do not have any hydrogen atoms directly attached to the hydroxyl carbon.
Explain the reasons of choice of a common oxidising agent to oxidise alcohols.
Acidified potassium dichromate solution is usually used. It has an obvious colour change when orange dichromate ions are reduced to green chromium(III) ions. The production of aldehydes from primary alcohols can also be indicated easily when the colour changes slightly.
What are the reaction conditions of the reaction of primary alcohols to form aldehydes?
The alcohol should be in excess, with the product distilled off immediately.
Why is the distillation of aldehydes possible when oxidising primary alcohols?
The boiling point of the aldehyde (product) is lower than that of the primary alcohol (reactant), so the aldehyde can be distilled off.
What is the chemical equation of the oxidation of ethanol to ethanal?
CH3CH2OH + [O] -> CH3CHO + H2O
What is the half equation for the reduction of dichromate ions to chromium(III) ions?
Cr2O72- + 14H+ + 6e- -> 2Cr3+ + 14H2O
Explain the reaction conditions of the reaction of primary alcohols to form carboxylic acids.
Acidified potassium dichromate solution is added in excess to oxidise the alcohol completely. The mixture is heated under reflux to ensure that any aldehyde formed is further oxidised into carboxylic acid.
What is the chemical equation of the oxidation of ethanol to ethanoic acid?
CH3CH2OH + 2[O] -> CH3COOH + H2O
Explain the reasons of seldomly using acidified potassium permanganate solution to oxidise alcohols.
- It is a very strong oxidising agent, which directly oxidises primary alcohols to carboxylic acids instead of aldehydes.
- It has a strong purple colour, and its colour changes to colourless / very pale pink after its reduction to manganese(II) ions. The colour change is not as obvious as it remains purple at the end of the reaction, and we cannot easily determine whether the oxidation is complete.
What are the reaction conditions of the reaction of secondary alcohols to form ketones?
Acidified potassium dichromate solution is added and the reaction mixture is heated under reflux.
Explain the difference in the expected and actual colour of the product when warming propan-2-ol with acidified potassium dichromate solution.
The expected colour is green; however, there may be some traces of orange colour remaining in the mixture since the acidified potassium dichromate solution is added in excess to fully oxidise the alcohol. Therefore, the actual green colour will be darker than the expected colour.
How can we distinguish aldehydes and ketones using a common oxidising agent?
Using acidified potassium dichromate solution and warm it with the compound. If the colour changes from orange to green, this indicates that the compound has been oxidised, hence it is an aldehyde. If the colour remains orange, this indicates that the compound cannot undergo further oxidation, hence it is a ketone.
