Benzene, C6H6, Consists of a Sigma-Bonded Framework of Carbon and Hydrogen Atoms
- Benzene, C6H6, consists of a sigma-bonded framework of carbon and hydrogen atoms.
- Above and below the plane of atoms is a bond, which consists of a delocalised electron cloud.
- The Kekule structure of benzene assumes that all the bonds are localised i.e. cannot move. However, evidence to support the delocalised form of benzene comes from bond lengths, enthalpy change of hydrogenation and resistance to reaction.
- Electrophiles (accept lone pair of electrons) can attack a benzene ring – usually resulting in an electrophilic substitution process as a mechanism for a reaction.
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2 / Reactions of benzene
- Benzene’s chemical reactions tend to have a high activation energy due to the delocalised -electron system.
- Benzene can react with the following:
b)a halogen in the presence of a halogen carrier to form a mono-halogenated benzene compound e.g. chlorobenzene or bromobenzene. This reaction is slower than with cyclohexene due to the enhanced thermodynamic stability provided by the delocalised electron system in benzene.
3 / Phenols
- Phenol has the molecular formula C6H5OH. It is used in the production of plastics, antiseptics, disinfectants and resins for paints.
- Phenol reacts with:
b)sodium to form sodium phenoxide and hydrogen.
- Phenol reacts rapidly with bromine to form 2,4,6-tribromophenol and hydrogen bromide. This reaction is faster than with benzene because the oxygen lone pair of electrons in phenol overlaps with the system on the ring, increasing its electron density and facilitating electrophilic attack.
/ 4 / Carbonyl compounds
- Carbonyl compounds include aldehydes and ketones.
- Aldehydes can be oxidised in the presence of acidified potassium dichromate(VI) to form carboxylic acids. The orange dichromate(VI) ion is reduced to the green chromium(III) ion.
- Ketones cannot be oxidised under these conditions.
- Aldehydes can be reduced using sodium borohydride to form primaryalcohols.
- Ketones can be reduced using sodium borohydride to form secondary alcohols.
5 / Tests for carbonyl compounds
- 2,4-dinitrophenylhydrazine (2,4-DNP/2,4-DNPH/ Brady’s reagent) is used to detect the presence of the >C=O group in aldehydesand ketones – if present, an orange precipitate is formed.
- The precipitate may be recrystallised in ethanol and its melting point measured – this information may then be used to determine the identity of the original molecule.
- Tollens’reagent (ammoniacal silver(I) nitrate) is used to detect the presence of an aldehyde group. A silver mirror is formed on warming the aldehyde with Tollens’reagent and a carboxylic acid is also formed. There is no reaction with ketones.
/ 6 / Carboxylic acid and esters
- Carboxylic acids contain the –COOH group. This group is highly polar, and that iswhy carboxylic acids:
b)arewater-soluble– hydrogen bonding between carboxylic acid and water molecules.
- Carboxylic acids are weak acids and will react with reactive metals, metal oxides (and hydroxides) and metal carbonates to form carboxylate salts containing the –COO– ion.
7 / Reactions of carboxylic acids
- Carboxylic acids react reversibly with alcohols, in the presence of an acid catalyst, to form esters – used in perfumes and flavourings.
- Esters may be hydrolysed in the presence of:
b)hot acids to form the corresponding alcohol and the carboxylic acid.
- Fats and oils are naturallyoccurring esters. The fatty acid part of the molecule may be either:
b)unsaturated –can be either cis or trans.
/ 8 / Amines
- Amines are molecules containing a nitrogen atom –e.g. the primary amine functional group –NH2.
- Amines are bases since they can accept a proton by using the lone pair of electrons on the nitrogen atom.
- Amines react with acids to form salts.
- Aliphatic amines may be prepared by the substitution of halogenoalkanes with excess ammonia.
- Aromatic amines can be prepared by reducing nitroarenes with tin and concentrated hydrochloric acid.
- Azo dyes are made by reacting an aromatic amine with nitrous acid and then phenol in alkali.
9 / Amino acids
- The general formula for an -amino acid is RCH(NH2)COOH.
- As an amino acid has both an acidic group (–COOH) and a basic group (–NH2), it can act as both an acid (proton donor) and a base (proton acceptor).
- At a certain pH known as the isoelectric point, a zwitterion forms. This contains the –NH3+ group and the –COO– group in the same molecule.
- Amino acids polymerise to form proteins while proteins can undergo hydrolysis to form -amino acids.
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10 / Optical isomerism
- Optical isomers are non-superimposible mirror images about an organic chiral centre – four different groups attached to a carbon atom.
- When a chiral molecule is synthesised in the laboratory,usually many optical isomers can form.
- However, if an enzyme is involved in the synthesis, normally only one optical isomer forms.
- Ifpharmaceutical products contain onlyone optical isomer the possibility of side effects is reduced and the pharmacological activity is improved.
- Separating optical isomers is expensive, so chiral synthesis producing just one isomer is better.
11 / Condensation polymers
- Examples of condensation polymers are polyesters (e.g.Terylene)and polyamides(e.g. nylon-6,6).
- When a condensation polymer is formed, a small molecule such as water or hydrogen chloride is also formed.
- Terylene is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.
- Nylon-6,6 is made from 1,6-diaminohexane and hexane-1,6-dicarboxylic acid.
- Kevlar is a special type of nylon that is made from benzene-1,4-diamine and benzene-1,4-dicarboxylic acid.
/ 12 / Hydrolysis and degradable polymers
- Condensation polymers have chemical groups that are vulnerable to chemical attack from either acids or alkalis – polyesters (ester group) and polyamides (amide group). This process is known as hydrolysis and results in the breakdown of the polymer.
- Disposing of polymers is an environmental problem. Scientists are working to develop degradable polymers similar in structure to poly(lactic acid).
- Condensation polymers may photodegrade as the C=O bond absorbs radiation.
13 / Synthetic routes
- Novel, useful molecules can be synthesised using organic chemistry.
- A chiral molecule is more difficult to synthesise since many other optical isomers may also form – costly in money and time to separate or resolve the isomers.
- Enzymes, bacteria, chiral catalysts and chiral-starting points (e.g.l-amino acids) are used to promote the formation of one chiral product.
- The chemical reactions you studyduring your A level chemistry course may be used to suggest how a target molecule can be synthesised, starting with a particular organic molecule.
/ 14 / Types of chromatography
- Chromatography is an analytical technique that separates components in a mixture between a mobile phase and a stationary phase.
- The mobile phase may be a liquid or a gas.
- The solid phase may be:
b)a liquid or a solid on a solid support (as in gas chromatography (GC)).
- The Rf value measures the ratio of the distance moved by the solute to the distance moved by the solvent.
- GCdoes have limitations –e.g. similar compounds have similar retention times.
15 / Combining mass spectrometry with chromatography
- Mass spectrometry (MS) can be combined with chromatography:
b)to generate mass spectra that can be analysed or compared with a spectral database by a computer for positive identification of a component.
- GC–MS can be used in analysis (e.g. in forensics), environmental analysis, airport security and space probes.
/ 16 / Proton NMR spectroscopy
- In NMR, protons (hydrogen atoms) in a sample absorb and emit low-energy radiowave radiation in the presence of a powerful magnetic field.
- The number of peaks gives information about the number of proton environments.
- The area under each peak gives information about the number of hydrogen atoms in each environment.
- Their horizontal position in a spectrum gives the chemical nature of each proton region.
- Protons that are adjacent to unequivalent protons may be split into more peaks, where n+1 gives the number of splits (if adjacent to n hydrogen atoms).
17 / Carbon-13 NMR spectroscopy
- The number of peaks in a carbon-13 spectrum is the same as the number of different carbon environments.
- The relative position of each peak on the horizontal axis (the chemical shift) suggests the chemical environment of a particular carbon atom.
- This means it is possible from the carbon-13 spectrum alone to suggest the likely structure of a molecule – by counting the number of peaks and deducing the chemical nature of the bonded atoms to each carbon.
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18 / Rate graphs and orders
- Rate of reaction is defined as the rate of change of concentration with time and has units of mol dm–3 s–1.
- When a graph is plotted of concentration against time, a characteristicshapeis formed depending on the order of the reactant:
b)First order –the rate is directly proportional to the concentration (exponential decay curve).
c)Second order –the rate is proportional to square of the concentration (half-life increases with time).
- When rate is plotted against concentration:
b)First order – a sloping straight line through the origin.
c)Second order – the rate vs. concentration graph is not a straight line.
19 / Rate equations
- The rate equation summarises the link between the rate of reaction and the concentrations of the reactants raised to their appropriate powers (orders).
- The constant of proportionality in the equation is called the rate constant – its units depend on the overall order of the process.
- The rate determining step in a mechanism is the slowest step – it will always consist of chemicals that are also involved in the rate equation (as non-zero powers).
- Zero-order species is not involved in the slowest step of the mechanism.
/ 20 / Chemical equilibrium
- Chemical equilibrium is a special state that is achieved when the forward and reverse processes in a closed system are occurring at an equal rate.
- An equilibrium constant, Kc, may be written:
for aA + bB ⇌ cC + dDKc =[C]c [D]d
[A]a [B]b - Kc therefore has units that depend on the reaction being considered.
- Changing the temperature will affect Kc – but how it changes depends on the enthalpy change of the forward reaction.
- For example, if the forward reaction is exothermic the value of Kcdecreases with increasing temperature.
21 / Brønsted–Lowry acids and bases
- Brønsted–Lowry acids are proton donors.
- Brønsted–Lowry bases are proton acceptors.
- A weak acid e.g. ethanoic acid dissociates in water to form a strong conjugate base:
- A strong acid forms a weak conjugate base e.g. HCl forms the weak conjugate base, Cl–.
- The acid dissociation constant, Ka, is an equilibrium constant that can be written for any weak acid.
- The term pKa = –log10Ka.
/ 22 / pH
- pH is defined as –log10[H+(aq)] (or –log10[H3O+]).
- Rearranging this equation gives[H+] = 10–pH.
- The ionic product of water, Kw, is equal to
[H+(aq)] [OH–(aq)] and its value at 298 K is
10–14 mol2 dm–6. - The pH of a strong acid may be calculated by using pH = –log10[H+] where complete dissociation is assumed.
- The pH of a strong base is calculated by first finding [H+] from Kw = [H+] [OH–] knowing [OH–], from the concentration of the solution, and Kw (if at 298 K).
23 / Buffers
- A buffer is defined as a system that minimises pH changes when small amounts of an acid or a base are added.
- An acid buffer (pH less than 7) is usually a mixture of a weak acid and the sodium salt of that weak acid – e.g.ethanoic acidand sodium ethanoate.
- When acid is added to a buffer, the strong conjugate base reacts with the excess acid – thereby reducing its impact.
- Both carbonic acid and hydrogencarbonate are found in blood and help control its pH.
/ 24 / Born–Haber cycles
- Lattice enthalpy is defined as the enthalpy change when one mole of an ionic compound is formed from its constituent gaseous ions under standard conditions.
- The lattice enthalpy for a compound is determined by constructing a Born–Haber cycle.
- The enthalpy of solution of a compound depends on:
b)hydration enthalpy of the ions formed (exothermic).
- Lattice and hydration enthalpies both depend on the ionic charges and radii.
25 / Entropy
- Entropy, S, is a measure of the disorder of a system.
- A system becomes more energetically stable whenit becomes more disordered.
- Solids have a relatively low entropy compared with liquids and especially gases. This is because solids have more order to their structure.
- When a solid dissolves, entropy increases– the system is becoming more disordered.
- In a reaction in which more gas molecules are formed than are consumed, the entropy will increase – the system is becoming more disordered.
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26 / Free energy change
- Free energy change, ΔG, is a measure of the energy available during a process that can be useful.
- The more free energy, the more spontaneous the process.
- ΔG must be negative for a reaction to be spontaneous.
- If ΔG is positive, the reaction will not be spontaneous – i.e. it will not happen on its own.
- The equation ΔG = ΔH – TΔS relates free energy change to the reaction enthalpy and the entropy change of the system.
27 / Redox reactions
- A redox reaction is one involving oxidation (loss of electrons) and reduction (gaining electrons).
- Halfequations may be written that summarise the two processes taking place. For example, in the reaction between iron and chlorine gas:
b)Cl2 + 2e– 2Cl– a reduction.
- Oxidation numbers are the formal charges on a bonded atom and can be assigned whether the bonding is covalent or ionic.In the reaction above:
b)Chlorine’s oxidation number, however, changes from 0 to –1.
/ 28 / Electrode potentials
- An electrode potential indicates the observed potential when the appropriate halfcell is connected to the standard hydrogen electrode, and is measured in volts.
- The standard hydrogen electrode is arbitrarily assigned the value zero volts.
- Electrode potentials for two halfequations combine to give the cell potential for the overall process.
- The half cell with the more positive electrode potential will be reduced in the overall cell reaction.
- However, cell potentials do not indicate whether a reaction takes place at an appreciable rate, and standard electrode potentials apply only to standard concentrations.
29 / Storage and fuel cells
- A fuel cell uses the energy from the reaction between a fueland oxygen to create a voltage.
- The hydrogen–oxygen fuel cell is important:
b)Oxygen is reduced– it gains electrons.
The two halfcells are then connected so that an electrical current may flow.
- Fuel cell vehicles (FCVs) are being developed to be fuelled by hydrogen or hydrogen-rich fuels.
- FCVs have advantages over petrol- or diesel-powered vehicles – less pollution and greater efficiency.
/ 30 / Transition metals
- Transition metals are found in the d-block of the periodic table.
- They form one or more ions that have a partiallyfilled d-orbital.
- They also:
b)have variableoxidationstates on their ions
c)form complexes
d)often have catalytic properties.
- Ions can react with sodium hydroxide solution to form metal hydroxide precipitates of certain colours: Fe2+ – green; Fe3+ –brown; Cu2+– light blue; and Co2+–blue.
31 / Complexes
- Transition metal ions may bond with ligands to form complexes.
- Ligands are species that may donate a lone pair to form a dative bond with a transition metal.
- Ligands may be able to donate:
b)two lone pairs per ligand – bidentate ligands.
- Complexes are able to exhibit: structural; geometrical; and optical isomerism.
- Cis-platin is a complex used in the treatment of cancer
/ 32 / Transition metal complexes
- A ligand substitution is a process in which one ligand bonded to the complex is swapped or substituted for another.
- A stability constant, Kstab, may be written for a complex. The greater its value, the more energeticallystable the complex relative to its constituent ions, and vice versa.
- Manganate(VII) ions in acidic conditions may be used to determine the concentration of solutions containing iron(II) ions.