Topic 6 Organic Chemistry Flashcards
Evidence that organic compounds do not only come from living things
Wöhler synthesised urea by heating inorganic ammonium cyanate.
Hydrocarbon
A compound that only contains carbon & hydrogen atoms.
Saturated
A compound containing only single bonds, i.e. as much hydrogen as possible.
Unsaturated
A compound containing one or more multiple bonds.
Multiple bond
Two or more covalent bonds between two atoms.
Displayed formula
Shows every atom & every bond.
Structural formula
Shows unambiguously how the atoms are joined together.
Skeletal formula
Shows all the bonds between carbon atoms.
Molecular formula
Shows the actual numbers of each atom in the molecule.
Empirical formula
Shows the numbers of each atom in the simplest whole number ratio.
Functional group
An atom or group of atoms in a molecule responsible for its chemical reactions.
Homologous series
A family of compounds with the same functional group, which differ in formulae by CH2 from the next member.
Predictable properties of compounds with the COOH functional group
Sour, acidic taste.
Chemical & physical properties of each homologous series
Similar chemical properties and physical properties that show a gradation– a gradual change from one compound to the next.
Alkanes: a homologous series
That don’t contain a functional group.
General formula
n = the number of carbon atoms, excluding those in the functional group.
General formula of alkanes
CnH2n+2
General formula of alkenes
CnH2n
General formula of halogenoalkanes
CnH2n+1X
General formula of alcohols
CnH2n+1OH
General formula of carboxylic acids
CnH2n+1COOH
How do alkanes (a homologous series) show the similarity in their chemical properties?
Upon complete combustion in air, they form the same products: CO2 + H2O.
How do alcohols show a gradation in physical properties, as a homologous series?
Boiling temperature increases as the number of carbon & hydrogen atoms increases.
Locant
A number used to indicate to which carbon in the chain an atom or group is attached.
Names for compounds use the locants that…
… add up to the smallest number.
What part of the IUPAC name indicates the number of carbon atoms in the longest chain?
A letter code, e.g. eth, or prop.
What part of the IUPAC name indicates the presence of atoms other than C & H?
Prefix, e.g., bromo.
What part of the IUPAC name indicates the functional group?
The suffix, e.g., -ol.
What part of the IUPAC name indicates the presence of two or more identical groups?
Multiplying prefixes, e.g., di, tri; tetra.
What part of the IUPAC name indicates where atoms and groups have different positions in a molecule?
numbers and hyphens/locants, e/g., 2-.
Structural isomerism
Compounds with the same molecular formula, but different structural formula.
Stereoisomerism
Compounds with the same structural & molecular formulae, but with the atoms or groups arranged differently in 3D, e.g., geometric isomerism.
Geometric isomerism
Compounds containing a C=C double bond with atoms or groups attached at different positions, e.g. cis-trans isomerism.
What is the significance of restricted rotation around the C=C double bond?
It fixes the position of the atoms or groups attached to the C=C atoms.
Types of structural isomerism
- Chain isomerism.
- Position isomerism.
Chain isomerism
Molecules with different carbon chains, e.g., butane & methylpropane.
Position isomerism
Molecules with the same functional group attached in different positions on the same carbon chain.
Trans isomer
Identical groups are further apart, across the double bond, at opposite ends of the molecule. One above & one below.
Cis isomer
Identical groups are both either above or below the double bond.
What is the problem with cis-trans notation?
It only works with compounds that have two identical groups. Not compounds where no two groups are the same.
E isomer
Atoms with higher priority are on opposite sides of the C=C double bond.
Z isomer
Atoms with higher priority are on the same side of the C=C double bond.
How is priority decided?
Whichever atom has the higher atomic number has a higher priority.
What is the typical number of fractions of crude oil?
6
Fractional distillation
The process used to separate a liquid mixture into fractions by boiling & condensing.
Cracking
The breakdown of molecules into shorter ones by heating with a catalyst.
Reforming
The conversion of straight-chain hydrocarbons into branched-chain and cyclic hydrocarbons.
The temperature gradient in the fractioning column
Hotter near the bottom & cooler near the top.
The first stage of fractional distillation of crude oil
Crude oil is heated in a furnace, which turns most of it into a vapour.
It is then passed into a column near the bottom.
The 2nd stage of fractional distillation
As the vapour passes up the column through a series of bubble caps, different fractions condense at different heights in the column, depending on the boiling temperature range of the molecules in the fraction.
What fractions condense near the bottom of the column?
Fractions containing larger molecules with longer chains & higher boiling points.
What fractions condense near the top of the column?
Fractions containing smaller molecules with shorter chains and lower boiling temperatures.
Dissolved gases in crude oil
Hydrocarbons that rise to the top of the column without condensing.
Why is the demand for shorter-chain hydrocarbons much higher?
They are better fuels, and can be made into other substances, e.g., polymers.
Zeolite
A compound used as a catalyst and made up of aluminium, silicon & oxygen.
Cracking: the process
Hydrocarbons in the heavier fractions are passed through a heated, usually zeolite, catalyst. This causes larger molecules to break up into smaller ones.
Which hydrocarbons burn more efficiently?
Cyclic and branched-chain hydrocarbons burn more efficiently than straight-chained hydrocarbons.
What is the useful byproduct of reforming?
Hydrogen
Complete combustion
All of the atoms in the fuel are fully oxidised. Produces CO2 + H2O.
Incomplete combustion
Some of the atoms in the fuel are not fully oxidised. Produces H2O + CO (g) or C(s).
Why would combustion of an alkane be incomplete?
- Insufficient oxygen.
- The combustion is very rapid.
How is solid carbon due to incomplete combustion observed?
Smoke in the air or soot on the burner.
Carbon monoxide
Toxic, colourless; odourless gas that prevents the transport of oxygen around the body.
What happens in complete combustion when the hydrocarbon does not burn at all?
A small % of hydrocarbons in the fuel are released into the atmosphere unchanged– unburned hydrocarbons.
Equations for when sulfur reacts during the combustion of alkanes:
S + O2 –> SO2
2SO2 + O2 –> 2SO3
Both of these gases are acidic oxides, so dissolve in water in the atmosphere to form sulfurous and sulfuric acids. This contributes to acid rain.
SO2 + H2O –> H2SO3
SO3 + H2O –> H2SO4
How do oxides of nitrogen NOx form?
At very high temperatures around the spark plugs in cars, nitrogen from the fuel reacts with oxygen in the air.
N2 + O2 –> 2NO
2NO + O2 –> 2NO2
How does nitrogen dioxide contribute to acid rain?
Nitrogen dioxide is acidic, so dissolves in water in the atmosphere to form nitrous & nitric acid.
2NO2+ H2O –> HNO2 + HNO3
How is low sulfur fuel or ultra-low sulfur fuel produced?
Sulfur compounds are removed from the fuel before the fuel is burnt, as it is not removed well by catalysts.
Even in incomplete combustion of an alkane, what happens to the hydrogen atoms?
They are completely oxidised to water.
Precious metals used in catalytic converters
Platinum, rhodium & palladium.
Why are the metal catalysts spread thinly over a honeycomb mesh in a catalytic converter?
To increase surface area & to save money.
Three-way catalyst
CO + unburned hydrocarbons + NOx are converted into N2 + H2O + CO2.
3 reactions that occur in a catalytic converter
Oxidation removes unburnt hydrocarbons & carbon monoxide:
2CO + O2 –> 2CO2
C8H18 + 12.5O2 –> 8CO2 + 9H2O
CO & nitrogen dioxide are reacted together to be removed:
2NO + 2CO –> N2 + 2CO2
Carbon neutral
The CO2 formed in combustion = the CO2 absorbed during a plant’s lifetime by photosynthesis.
Why are biofuels only close to carbon neutral?
The plants must be harvested, transported, processed in a factory and transported to be sold. These processes use energy that involves the formation of carbon dioxide.
Why are fossil fuels not carbon neutral?
They absorbed CO2 from the atmosphere millions of years ago when the amount in the atmosphere was much higher. Thus, when fossil fuels are burnt, they increase the amount of CO2 in the atmosphere.
Biofuels
Fuels obtained from living matter that has died recently.
Renewable
Energy sources that can be continuously replaced.
Non-renewable
Energy sources that are not replenished, except over large geological timescales.
Biodiesel
A fuel made from vegetable oils obtained from plants. Can be mixed with ordinary diesel.
Bioalcohols
Fuels made from plant matter, often using enzymes or bacteria.
Bioethanol
The most common bioalcohol. Traditionally, it’s produced by fermentation using yeast enzymes, but this limits the concentration that can be produced and requires energy to separate the bioethanol from the water. Now, it can be produced from more plants & plant waste than just sugar using bacteria. The upper limit to the yield of bioethanol from a given unit of starting material is increasing.
Biofuels vs. natural gas: land use
Natural gas is derived from underground sources, so requires no land.
Biofuels require lots of land, which might replace land to grow crops.
Biofuels vs. natural gas: yield
Biofuels have a low, but increasing yield. Natural gas has a high yield.
Biofuels vs. natural gas: manufacture/transport
Costs in growing, processing and transport of biofuels.
Natural gas transport costs are low by pipeline, but it has very high exploration & drilling costs.
Biofuels vs. natural gas: carbon neutrality
Biofuels are much closer to being carbon neutral.
Natural gas is not carbon neutral.
Why are alkanes fairly unreactive?
They only contain C & H, and single bonds. The bonds are non-polar, so they do not react with acids, alkalis or reactive metals.
Substitution reaction
One in which an atom or group is replaced by another atom or group.
Mechanism
The sequence of steps in an overall reaction. Each steps shows what happens to the electrons involved in bond making or bond breaking.
Homolytic fission
The breaking of a covalent bond where each bonding electron leaves with one species, forming a radical.
Radical
A species that contains an unpaired electron.
The initiation step
Involves the formation of radicals, usually as a result of bond breaking caused by UV radiation.
The propagation steps
The two steps that, when repeated many times, convert the starting materials into the products of the reaction.
The termination step
Involves the formation of a molecule from two radicals.
- Initiation
- Propagation
- Termination
- One molecule becomes 2 radicals.
- A molecule + a radical become a different molecule + a radical. 2 reactions in this step.
- 2 radicals become one molecule.
Why is the yield low in free radical substitution of alkanes?
Due to further reactions and the products needing to be separated from the mixture.
Initiation: homolytic fission of chlorine
Cl2 –> Cl dot + Cl dot
Species
Any substance that can represented by a formula, including atoms, ions, molecules & radicals.
The 2 propagation reactions
With methane:
The radical is always very reactive. The Cl dot radical takes an H atom to form HCl. A methyl radical forms.
Cl dot + CH4 –> HCl + CH3 dot
The methyl radical reacts by taking a Cl atom from a Cl2 molecule. A Cl dot radical forms.
CH3 dot + Cl2 –> CH3Cl + Cl dot
When two radicals collide, a molecule is formed:
Cl dot + Cl dot –> Cl2
Cl dot + CH3 dot –> CH3Cl
CH3 dot + CH3 dot –> C2H6
The sequence of reactions comes to an end because 2 reactive species are converted into an unreactive species.
Formation of dichloromethane (free radical substitution)
CH3Cl + Cl2 –> CH2Cl + HCl
Formation of trichloromethane (free radical substitution)
CH2Cl + Cl2 –> CHCl3 + HCl
Formation of tetrachloromethane (free radical substitution)
CHCl3 + Cl2 –> CCl4 + HCl
Bond angle in alkenes
120 degrees
Sigma bonds
Covalent bonds formed when electron orbitals overlap axially (end-on).
Pi bonds
Covalent bonds formed when electron orbitals overlap sideways. Electrons in pi bonds are further from the nuclei, so more available for reactions.
Test for C=C
Decolourisation of bromine water/Br2 (aq).
C2H4 + Br2 –> C2H4Br2
Addition reaction
Reaction in which two molecules combine to form one molecule.
Hydrogenation
Involves the addition of hydrogen.
Halogenation
Involves the addition of a halogen.
Hydration
Involves the addition of water or steam.
Diol
Compound containing two OH (alcohol) groups.
Equation for the hydrogenation of ethene
C2H4 + H2 –> C2H6
Conditions for hydrogenation
Nickel catalyst. Heat.
What is hydrogenation used for?
Margarine manufacture. Converts vegetable oil into a solid.
Chlorination
Type of halogenation using Cl2.
Product of halogenation
Dihalogenoalkane.
Equation for the halogenation of ethene
C2H4 + Br2 –> C2H4Br2 1,2 dibromoethane
Equation for hydration of ethene
C2H4 + H2O (g) –> C2H5OH ethanol
Conditions for hydration
Heating with steam. Phosphoric acid catalyst. H3PO4.
Electrophilic addition of hydrogen halides
Product: halogenoalkane.
CH2=CH2 +H-Br –> CH3-CH2Br
No colour change. Reactants & products are colourless.
Oxidation of alkenes to diols
Oxidation then addition.
Oxidising agent: potassium manganate (VII) in acidic conditions (usually dilute sulfuric acid).
The potassium manganate provides an O atom. The H2O provides another another O atom & 2 H atoms.
Colour change in the oxidation of alkenes to diols
Purple to colourless.
Equation for the oxidation of ethene to ethane-1,2-diol
CH2=CH2 + [O] + H2O –> CH2OH-CH2OH
Curly arrows
Represent the movement of electron pairs.
Electrophile
A species that is attracted to a region of high electron density. I.e., attracted to negative charge.
Electrophilic addition
Reaction in which two molecules form one molecule, and the attacking molecule is an electrophile.
Heterolytic fission
The breaking of a covalent bond, so that both electrons are taken by one atom.
Carbocation
A positive ion in which the charge is shown on a carbon atom.
Electron-releasing group
One that pushes electrons towards the atom to which it is joined.
Why are alkenes attractive to electron-deficient species?
They contain a region of high electron density, i.e. they are electron rich arounds the C=C double bond.
Electrophilic addition of HBr: step 1
Curly arrow from the C=C double bond to the partially positive H atom.
Curly arrow from the H-Br bond to the partially negative Br.
This forms a carbocation + a bromide ion. Draw the lone pair on the bromide ion!
Electrophilic addition of HBr: step 2
Curly arrow from the Br- to the C+ in the carbocation. A new covalent bond forms between them to form a halogenoalkane.
How does the Br-Br bond become polar during the electrophilic addition of halogens?
As it approaches the C=C bond, the electrons in the pi bond repel the electrons in the Br-Br bond and induce a dipole in the molecule.
Mechanism for the electrophilic addition of halogens
The same as for hydrogen halides.
Under what circumstances are there two possible products of electrophilic addition?
When both reactants are asymmetrical.
Why is a tertiary carbocation the most stable type of carbocation?
A carbocation in which positive charge can be spread over more atoms is more stable than one in which there are fewer atoms available to spread the charge. Alkyl groups are electron-releasing, so the more alkyl groups there are, the more the positive charge is spread.
How does electrophilic addition proceed?
Via a carbocation.
How does the major product form?
From the most stable carbocation.
Monomers
The small molecules that combine together to form a polymer.
Repeat unit
The set of atoms that are joined together in large numbers to produce the polymer structure.
How to name an addition polymer
Poly(alkene monomer); polymers do not have a fixed molecular formula.
General equation for polymerisation
n alkene monomer –> [repeat unit] with the n in the corner.
Advantages of polymers
- Large-scale, relatively cheap manufacture.
- Can have complex shapes & a variety of physical properties.
- Unreactive.
- Lighter in weight.
Uses of polymer waste.
- Chemical feedstock.
- Incineration.
- Recycling.
Recycling
Involves converting polymer waste into other materials.
Incinerator
Converts polymer waste into heat energy, which can be used to heat homes & generate electricity.
Polymer waste as a feedstock
Involves converting polymer waste into chemicals (gases) that can be used in new reactions, e.g., to make new polymers.
Biodegradable polymers
Can be broken down by microbes.
1st stage of recycling
Sorting.
Why are polymers sorted before recycling?
There are many types of polymer in use, and mixtures of these types cannot be processed together.
2nd stage of recycling
Processing. The waste is chopped into small pieces, and washed. Melting, moulding and fibre production are methods to make new materials.
Pollutants due to incineration
Chlorine & toxic heavy metals used in pigments to colour plastics.
Life cycle analysis
Raw material extraction.
Materials processing.
Manufacture of products.
Distribution & sales.
Product use.
Disposal or recycling.
Recycling allows materials to be reused in materials processing, manufacture of products or for other other products.
Primary halogenoalkane/alcohol
1 alkyl group attached to the C atom bonded to the halogen/OH group.
Secondary halogenoalkane/alcohol
2 alkyl groups attached to the C atom bonded to the halogen/OH group.
Tertiary halogenoalkane/alcohol
3 alkyl groups attached to the C atom bonded to the halogen/OH group.
Nucleophile
A species that donates a lone pair of electrons to form a covalent bond with an electron-deficient atom.
Hydrolysis reaction
One in which water or hydroxide ions replace an atom in a molecule with an -OH group.
What makes halogenoalkanes reactive?
The halogen atom has a higher electronegativity than carbon, so the C-X bond is polar (X is the halogen). Carbon has a partial +, and X has a partial -. The carbon is a nucleophile.
Hydrolysis of halogenoalkanes: general equation
RX + H2O –> ROH + HX, where X is the halogen.
Reagent: water.
Product: alcohol.
The partially negative O in H2O is attracted to the partially positive carbon.
How can we compare the rates of halogenoalkane hydrolysis?
Reagent: silver nitrate.
How long it takes for AgX to form– time the appearance of the precipitate.
Control temperature and concentration & quantity of the halogenoalkanes.
Ethanol is used as a solvent, as halogenoalkanes + aqueous silver nitrate do not mix.
Trend in the rates of hydrolysis of halogenoalkanes: same structure, different halogen.
1-iodobutane = fastest
1-bromobutane = medium
1-chlorobutane = slowest
Bond polarity is outweighed by bond enthalpy, in terms of the relative influence of different factors in explaining this trend.
C-I bond is the weakest, so requires less energy to be broken for hydrolysis to occur.
Why are fluoroalkanes not typically used in these hydrolysis experiments?
The C-F bond is much stronger than the others (+467 kJ mol-1).
Trend in the rates of hydrolysis of halogenoalkanes: same halogen, different structure.
Tertiary = fastest. Secondary = medium. Primary = slowest.
Tertiary reacts faster via an SN1 mechanism with a low activation energy. Primary reacts slower via a high-energy transition state in SN2.
Summary of halogenoalkane substitution reactions:
RX –> ROH Reagent: H2O, warm.
RX –> ROH Reagent: KOH, heat under reflux.
RX –> RCN Reagent: KCN, heat under reflux.
RX –> RNH2 Reagent: NH3, heat in a sealed tube.
RX –> ROH Reagent: H2O, warm.
Hydrolysis.
RX –> ROH Reagent: KOH, heat under reflux.
The nucleophile is the OH-/hydroxide ion. This is shown clearly in an ionic equation.
How to make nitriles from halogenoalkanes:
Heating a halogenoalkane with KCN dissolved in ethanol under reflux.
Nucleophile: CN-.
RX –> RCN Reagent: KCN, heat under reflux.
The carbon chain is extended by one carbon atom, which is useful in synthesising more complex compounds.
Nitriles
Organic compounds containing the C-CN group.
Primary amines
Compounds containing the C-NH2 group.
Nucleophilic substitution reaction
An attacking nucleophile replaces an existing atom or group in a molecule.
Ethanolic solution
A solution in which ethanol is the solvent.
Elimination reaction
A molecule loses atoms attached to the adjacent carbon atoms, forming a C=C double bond.
RX –> RNH2 Reagent: NH3, heat in a sealed tube.
This makes primary amines. The sealed tube is needed as NH3 is a gas. Nucleophile: NH3. This is slightly different to the other 3 halogenoalkane substitution reactions– an example equation:
Step 1: CH3CH2CH2CH2I + NH3 –> CH3CH2CH2CH2NH3+ + I-
The HI (acid) that initially forms reacts with the organic product (which is basic, like NH3) to form a salt, as shown above. To produce a high yield of amine, an excess of NH3 is used to react with the salt.
Step 2: CH3CH2CH2CH2NH3+I- + NH3 –> CH3CH2CH2CH2NH2 + NH4+I-
Overall:
CH3CH2CH2CH2I + 2NH3 –> CH3CH2CH2CH2NH2 + NH4+I-
Mechanism for RX –> ROH Reagent: KOH (aq) , heat under reflux.
Curly arrow from the lone pair on the hydroxide ion to the partially positive carbon of the halogenoalkane.
Curly arrow from the C-X bond to the partially negative X. Bond breaking by heterolytic fission.
This forms an alcohol + a X- ion.
Mechanism for RX –> RNH2 Reagent: NH3, heat in a sealed tube.
Step 1:
Curly arrow from the lone pair on the NH3 to the partially positive C.
Curly arrow from the C-X bond to the partially negative X.
This forms a C-N bond with a + on the N + a Cl- ion.
Step 2:
Another NH3 molecule then acts as a base, and removes a hydrogen ion from the ion formed in step 1.
Curly arrow from the lone pair on the NH3 to an H attached to the N+.
Curly arrow from the N-H bond to the N+.
This forms an amine + NH4+.
What happens when a halogenoalkane is heated with KOH in ethanolic instead of aqueous solution?
An elimination reaction. The OH- ion acts as a base, not a nucleophile. The OH- reacts with the H bonded to the carbon attached to the halogen.
Products: alkene + H2O + KX. The H & Br are removed from the halogenoalkane, but are not replaced by any other atoms.
Halogenation reaction
Results in the replacement of the hydroxyl group in an alcohol molecule by a halogen atom.
Dehydration reaction
Results in the removal of the hydroxyl group in an alcohol molecule , together with the hydrogen atom from the adjacent carbon atom, forming a C=C double bond.
Dehydration: alcohol –> alkene + water
Heat with concentrated phosphoric acid. The OH group and the H atom from the adjacent carbon atom are removed. A C=C double bond is formed. Remember to account for any isomers. Example equation:
CH3CH(OH)CH2CH3 –> CH3CH=CHCH3 + H2O
The water formed mixes with the concentrated H3PO4 to dilute it.
Combustion of alcohols
Complete combustion produces H2O + CO2. Remember that alcohols already contain one O when balancing combustion of alcohols equations.
Halogenation of alcohols
Converts alcohols to halogenoalkanes.
Chlorination of alcohols
Reagent: phosphorous (V) chloride/phosphorous pentachloride. Vigorous at room temperature. Products: a chloroalkane + phosphorous oxychloride + HCl. Example equation:
CH3CH2CH2OH + PCl5 –> CH3CH2CH2Cl + POCl3 + HCl
Chlorination of tertiary alcohols
The alcohol is mixed by shaking with concentrated HCl at room temperature. Products: a chloroalkane + H2O. Example equation:
(CH3)3COH + HCl –> (CH3)3CCl + H2O.
Bromination of alcohols
Reagent: a mixture of KBr + 50% concentrated H2SO4, warmed with the alcohol.
CH3CH2CH2CH2OH + HBr –> CH3CH2CH2CH2Br + H2O
Equation for the formation of the reagent for bromination (of alcohols)
KBr + H2SO4 –> KHSO4 + HBr
or
2KBr + H2SO4 –> K2SO4 + 2HBr
Iodination of alcohols
Reagent: mixture of red phosphorous + iodine. Heat the reaction mixture, incl. the alcohol, under reflux.
2P + 3I2 –> 2PI3
Example equation:
3C2H5OH + PI3 –> 3C2H5I + H3PO3 (phosphonic acid).
Oxidation of alcohols
Loss of hydrogen from the OH attached to a carbon and the H on the other side, in an alcohol molecule. The product contains a C=O group.
Why cannot tertiary alcohols be oxidised?
There is no hydrogen atom on the C of the C-OH group.
What is formed when a secondary alcohol is oxidised?
A ketone , RCOR.
What is formed when a primary alcohol is oxidised?
An aldehyde, RCHO.
What happens when an aldehyde is oxidised?
It gains an oxygen atom between the C and H of the CHO group. Product: carboxylic acid.
What is the reagent for the oxidation of alcohols?
Potassium dichromate (VI) and dilute sulphuric acid.
Equation of propan-1-ol oxidation to propanal
CH3CH2CH2OH + [O] —> CH3CH2CHO + H2O
How is the oxidising agent represented in an equation?
[O]
Equations for oxidation of alcohols to form aldehydes & ketones
Also produce a mole of water. When forming carboxylic acids, no water is formed.
Ketones
A homologous series of organic compounds formed by the oxidation of secondary alcohols.
Aldehydes
A homologous series of organic compounds formed by the partial oxidation of primary alcohols.
Carboxylic acids
A homologous series of organic compounds formed by the complete oxidation of primary alcohols.
Heating under reflux
Involves heating a reaction mixture with the condenser fitted vertically.
Distillation with addition
Involves heating a reaction mixture, but adding another liquid and distilling off the product as it forms.
What practical technique is used in oxidation to obtain a ketone or carboxylic acid (complete oxidation)?
Heating under reflux.
The products of oxidation stay in the reaction mixture because, if they boil off, they condense in the vertical condenser and return to the heating flask.
What apparatus is used when oxidation is intended to be incomplete to obtain an aldehyde?
Distillation with addition.
Only the oxidising agent is heated, and the alcohol is slowly added to the oxidising agent. The aldehyde distills off immediately, when formed, as it has a much lower boiling point than the alcohol.
What could contaminate the organic product?
- Unreacted starting material.
- Other organic products.
- The inorganic reagents used, or the inorganic products formed from them.
- Water.
4 techniques to purify an organic liquid
- Simple distillation.
- Fractional distillation.
- Solvent extraction.
- Drying.
Why do organic compounds pose more of a problem when setting up apparatus? What a can be done to mitigate this?
They can be flammable & toxic, and they may attack corks and bungs.
Use mostly glass apparatus, which can be fitted together using ground-glass joints.
How can simple distillation be used to purify an organic liquid?
Heat the liquid in a flask connected to a condenser. The liquid with the lowest boiling temperature evaporates/boils off first, and passes into the condenser. This means it can be collected in the receiver separately from any liquid that evaporates later.
What is the purpose of the thermometer in simple distillation?
To monitor the temperature of the vapour as it passes into the condenser,
If the temperature remains steady, 1 compound is distilling over.
If the temperature begins to rise after a while, a different compound is distilling over.
Pros of simple distillation
Easier to set up & quicker than fractional distillation.
Cons of simple distillation
It does not separate liquids as effectively as fractional distillation. It should only be used if the boiling temperature of the liquid being purified is very different from the other liquids in the mixture (ideally, at least a 25 degree difference).
Simple distillation
Used to separate liquids with very different boiling temperatures.
Fractional distillation
Used to separate liquids with similar boiling temperatures.
Solvent extraction
Used to separate a liquid from a mixture by causing it to move from the mixture into the solvent.
How can fractional distillation be used to purify an organic liquid?
There is a fractionating column between the heating flask & the still head. The column is filled with glass beads, which act as surfaces upon which vapour leaving the column can condense, then be evaporated again as more hot vapour passes up the column. Effectively, the vapour undergoes several repeated distillations as it passes up the column, which provides a better separation.
Pros/Cons of fractional distillation
Takes longer than simple distillation. Best used when there are several compounds to be separated from the mixture, and the difference in boiling temperatures is small.
Criteria for the solvent used in solvent extraction
- Immiscible with the solvent containing the desired organic product.
- The organic product should be much more soluble in the new solvent than in the reaction mixture.
How can solvent extraction be used to purify an organic liquid?
Place the reaction mixture in a separating funnel. Add the chosen solvent. A separate layer forms.
Place a stopper in the neck of the funnel, and gently shake the contents of the funnel.
Allow the contents to settle into 2 layers.
Remove the stopper, and open the tap to allow the lower layer to drain into a flask. Pour the upper layer into a separate flask. Simple/fractional distillation must then be used to separate the organic product from the new solvent.
Why is it better to use the solvent (involved in solvent extraction) in small portions rather than one large volume?
This is more efficient. More portions of solvent with the same total volume removes more of the desired organic product.
How can drying be used to purify an organic liquid?
- The drying agent is added to the organic liquid, and the mixture is swirled/shaken, then left for a period of time.
- Before use, a drying agent is powdery, but after absorbing water it looks more crystalline.
- If a bit more drying agent is added and the mixture remains powdery, the liquid is dry.
- The drying agent is removing by either decantation or filtration.
Why can anhydrous calcium chloride only be used as a drying agent for some organic compounds?
It reacts with other compounds, and is soluble in alcohols.
Common drying agents
Anhydrous metal salts: calcium sulfate, magnesium sulfate & sodium sulfate. They form hydrated salts by absorbing water as the water of crystallisation.
What impurities are removed by drying agents?
Water in which the organic liquid is partially/completely dissolved.
Inorganic reagents, used in the preparation of the organic compound in aqueous solution.
How can we test the purity of an organic liquid?
Measure the boiling temperature, and compare it to a data book value. Impurities raise the boiling temperature. However, you may not be able to measure the boiling temperature accurately enough. Different organic compounds may also have the same boiling temperature.
Test for the OH alcohol functional group?
Add PCl5. Observe misty/steamy/white fumes.
Or: warm with acidified K2Cr2O7. The solution changes from orange to green.
Apart from restricted rotation about the double bond, what else is required for E/Z isomerism?
Each carbon atom in the double bond must be attached to 2 different groups.
What reagent, condition and mechanism is required to convert ethane into chloroethane?
Cl2.
UV light.
Free radical substitution.
Why might the molecular ion peak be absent in a particular mass spectrum?
The molecular ion is unstable.
Why might data book/mean bond enthalpy values not be accurate for a particular enthalpy change of reaction?
Data book bond energies refer to the gaseous state. Some reactants/products may be given in other states of matter.
Characteristics of a homologous series
Same functional group, similar chemical properties, same general formula, each compound differs from the next by a -CH2 group; they show a gradual trend in physical properties.
Why are hydrocarbons cracked?
Alkenes, useful starting molecule in organic synthesis & used for making polymers, and shorter-chain alkanes, which are more in demand, are produced.
Why is incineration not suitable for disposal of poly(chloroethene)?
HCl/chlorinated molecules are produced, which are corrosive, toxic & cause acid rain.
Acidified potassium dichromate(VI).
Oxidising agent for alcohols, not alkenes.
Why can’t LiAlH4 be used to reduce alkenes?
It is a source of H- (hydride) ions, which would be repelled by regions of high electron density.
Conditions for the hydrolysis of a nitrile to form an acid.
Heat under reflux then add HCl
Halogenolakane –> alkene
Elimination reaction. Reflux. Concentrated KOH/NaOH in ethanol.
Why is ethanol often used as a solvent?
It increases the solubility of reactants, so reactants are miscible.
Experiment to compare the relative rates of hydrolysis of halogenoalkanes.
Using KOH is too fast to time precipitate formation, so use AgNO3(aq), acidified with HNO3, in ethanol. Use a water bath to control temperature. Add equal amounts of halogenoalkane. Measure the time taken for a precipitate to form. Shake to mix.
Halogenoalkanes can undergo elimination (to form alkenes) or substitution (to form alcohols) when reacting with OH- ions under reflux. How do you know which occurs?
To favour elimination: use a concentrated OH- ion solution, higher temperatures & a pure ethanol solvent.
To favour substitution: use more dilute OH- ion solutions, lower temperatures and more water in the solvent mixture.
Ethanol Encourages…
Elimination!
What type of reaction with OH- ions do primary halogenoalkanes undergo?
Mainly substitution. Whereas tertiary halogenoalkanes mainly undergo elimination.
When comparing SN1 & SN2 mechanisms what is it important to include?
What species are in the respective RDSs/rate equations. Both are nucleophilic substitution.
When counting isomers…
Don’t just count structural isomers, also include stereoisomers.
When comparing the rates of hydrolysis of tertiary, secondary & primary halogenoalkanes, what must be ensured?
Halogenoalkanes must be isomeric, so have the same number of carbon atoms.
Why might yield be less than 100%?
Transfer losses, side reactions & incomplete reaction.
How can KMnO4 be used to test for C=C bonds? Note: KMnO4 can also oxidise aldehydes & alcohols, so bromine water is a better test for unsaturation.
Neutral KMnO4(VII): purple to brown precipitate.
Alkaline KMnO4(VII): purple to green.
Acidified KMnO4(VII): purple to colourless.
Why add ice water to the collecting flask in distillation?
You collect as much product as possible.
What happens if the thermometer is not opposite the entrance to the condenser?
You would be collecting over the wrong temperature range.
The conical flask in distillation should be open.
Add a vent before the condenser.
Why is reflux advantageous?
Prevents the loss of any volatile substances.
Anti-bumping granules.
Prevent superheating, uncontrolled heating & localised boiling. Facilitate the formation of small bubbles for smooth boiling by providing nucleation sites. Distribute heat more evenly.
The honeycomb structure of a catalytic converter
Increases the surface area and allows gases to flow through the exhaust.
Incineration.
Releases energy and avoids landfill.
