Organic chemistry

Explanation

• Organic compounds may be represented by empirical, molecular, general, structural, displayed or skeletal formulae, each showing different information.
• Members of a homologous series share a functional group and general formula, show similar chemical properties and differ successively by $\mathrm{CH_2}$.
• IUPAC naming selects the longest chain or ring containing the principal functional group, numbers it to give the required lowest locants, and identifies substituents alphabetically with multiplicative prefixes where needed.
• Students must name structures and draw structures from names for chains and rings containing up to six carbon atoms each.
• Every unlabelled skeletal line end and vertex represents carbon.

Explanation

• A reaction mechanism explains bond making and breaking through a sequence of species and intermediates. For electron-pair mechanisms, a curly arrow starts at the source of a pair: a lone pair or covalent bond.
• It ends at the atom or bond receiving that pair. Heterolytic bond breaking is shown by an arrow from the bond to the atom taking both electrons.
• Structures, charges, lone pairs and every required intermediate must be unambiguous.
• A radical contains an unpaired electron shown by a dot.
• Free-radical mechanisms use balanced step equations and radical dots; electron-pair curly arrows are not required and must not be mixed into that notation.

Explanation

• Structural isomers have the same molecular formula but different structural formulae. Chain isomers differ in carbon skeleton, position isomers move the same functional group or multiple bond, and functional-group isomers contain different functional groups.
• Stereoisomers share a structural formula but differ in spatial arrangement.
• E–Z isomerism arises from restricted rotation about planar C=C and requires two different groups on each double-bonded carbon.
• Apply Cahn–Ingold–Prelog priorities separately at each carbon by comparing atomic numbers, proceeding outward only after a tie.
• Higher-priority groups on the same side give Z; those on opposite sides give E.

Explanation

• Alkanes are saturated hydrocarbons containing only carbon–carbon single bonds. Petroleum is a mixture consisting mainly of alkane hydrocarbons and is separated by fractional distillation.
• Crude oil is vaporised and enters a fractionating column with a hot bottom and cooler top. Repeated condensation separates fractions by boiling-point range.
• Longer molecules contain more electrons and have larger contact surfaces, so stronger London forces give higher boiling points and condensation lower in the column.
• Shorter hydrocarbons remain gaseous to cooler, higher levels.
• The process is physical: intermolecular attractions are overcome and re-formed, while covalent bonds, molecular formulae and compounds remain unchanged.

Explanation

• Cracking breaks carbon–carbon bonds in long-chain alkanes to form smaller, more useful molecules.
• Thermal cracking uses high temperature and high pressure and produces a high proportion of alkenes; its mechanism is not required.
• Catalytic cracking uses high temperature, slight pressure and a zeolite catalyst, producing mainly motor fuels and aromatic hydrocarbons; its mechanism is also not required.
• Every cracking equation must conserve carbon and hydrogen and commonly contains an alkane plus one or more unsaturated products.
• The economic reason is to convert less-demanded heavy fractions into shorter fuels and alkene or aromatic feedstocks whose demand exceeds their direct supply from crude oil.

Explanation

• Alkanes are used as fuels. Complete combustion in excess oxygen forms carbon dioxide and water; incomplete combustion with limited oxygen forms carbon monoxide and/or carbon as well as water.
• Internal-combustion engines also produce $\mathrm{NO_x}$ and release unburned hydrocarbons. Catalytic converters remove gaseous pollutants by converting carbon monoxide, nitrogen oxides and hydrocarbons into less harmful products.
• Sulfur-containing hydrocarbons form sulfur dioxide, an acidic air pollutant.
• Flue-gas desulfurisation uses basic calcium oxide or calcium carbonate to neutralise and remove $\mathrm{SO_2}$.
• Combustion equations must conserve atoms using coefficients without altering the fuel formula.

Explanation

• Methane reacts with chlorine by free-radical substitution under ultraviolet radiation. Initiation is homolytic Cl–Cl fission: $\mathrm{Cl_2\xrightarrow{UV}2Cl\mathbin{\bullet}}$.
• Propagation sustains the chain because one radical is consumed and another formed: $\mathrm{Cl\mathbin{\bullet}+CH_4\rightarrow HCl+CH_3\mathbin{\bullet}}$, then $\mathrm{CH_3\mathbin{\bullet}+Cl_2\rightarrow CH_3Cl+Cl\mathbin{\bullet}}$.
• Termination removes radicals when any two combine; possible products include chlorine, chloromethane and ethane.
• All steps must balance atoms and show radical dots; electron-pair curly arrows are not required.
• Continued irradiation causes further substitution, so chlorination produces a mixture of increasingly chlorinated products rather than pure chloromethane.

Explanation

• Halogenoalkanes contain polar C–X bonds, leaving carbon electron-deficient. Nucleophiles donate an electron pair to this carbon.
• Required substitutions use $\mathrm{OH^-}$, $\mathrm{CN^-}$ and $\mathrm{NH_3}$ to form alcohols, nitriles and amines. For primary halogenoalkanes with hydroxide or cyanide, a curly arrow runs from the nucleophile's lone pair to carbon while another runs from C–X to X.
• Cyanide attacks through carbon.
• Ammonia first forms $\mathrm{RNH_3^+}$; another ammonia removes a proton.
• Hydrolysis rate follows C–X bond enthalpy: iodoalkanes react faster than bromoalkanes, then chloroalkanes, despite the opposite bond-polarity trend.

Explanation

• A halogenoalkane can undergo substitution and elimination concurrently with potassium hydroxide.
• In aqueous conditions, $\mathrm{OH^-}$ mainly acts as a nucleophile and replaces halide; hot ethanolic conditions favour its role as a base and alkene formation.
• The elimination mechanism has three simultaneous electron-pair movements: an oxygen lone pair accepts a hydrogen from the carbon adjacent to C–X, the C–H bond pair forms C=C, and the C–X bond pair moves to X.
• Unsymmetrical substrates may eliminate from either adjacent carbon and form more than one structural alkene; any E–Z forms are then considered separately.

Explanation

• Ozone in the upper atmosphere is beneficial because it absorbs ultraviolet radiation. UV also breaks C–Cl bonds in chlorofluorocarbons and releases chlorine radicals.
• These catalyse ozone decomposition through $\mathrm{Cl\mathbin{\bullet}+O_3\rightarrow ClO\mathbin{\bullet}+O_2}$ and $\mathrm{ClO\mathbin{\bullet}+O_3\rightarrow2O_2+Cl\mathbin{\bullet}}$.
• Adding the steps gives $\mathrm{2O_3\rightarrow3O_2}$; $\mathrm{Cl\mathbin{\bullet}}$ is regenerated and $\mathrm{ClO\mathbin{\bullet}}$ is an intermediate, so one chlorine radical destroys many ozone molecules.
• This catalytic amplification explains why small CFC concentrations matter.
• Consistent evidence from different research groups supported legislation banning CFC uses as solvents and refrigerants, while chemists developed chlorine-free replacements.

Explanation

• Alkenes are unsaturated hydrocarbons containing a carbon–carbon double covalent bond.
• The double covalent bond is a centre of high electron density.
• This electron-rich region attracts electrophiles, which accept an electron pair.
• During electrophilic addition, an electron pair from the double bond forms a new bond to the electrophile; further bond formation gives a saturated product in which C=C has become C–C.
• Reactivity explanations must connect electrophile attraction and electron-pair donation to this high electron density.

Explanation

• Alkenes undergo electrophilic addition with $\mathrm{HBr}$, $\mathrm{H_2SO_4}$ and $\mathrm{Br_2}$. Bromine water tests for unsaturation by changing orange to colourless as bromine adds across C=C.
• With HBr or sulfuric acid, the double-bond pair attacks $\mathrm{H^+}$ while the reagent bond breaks; the resulting anion attacks the carbocation.
• The electron-rich alkene induces a dipole in bromine before attack and bromide completes addition.
• Unsymmetrical alkenes can give major and minor products.
• Their proportions are explained by carbocation stability: tertiary is more stable than secondary, which is more stable than primary, so the pathway through the more stable intermediate dominates.

Explanation

• Addition polymerisation joins many alkene or substituted-alkene monomers after their C=C bonds open. To derive a repeating unit, replace C=C with C–C, retain every substituent on its original carbon, enclose the smallest repeated section in brackets and draw a continuation bond through each bracket.
• The reverse process identifies the monomer by restoring C=C between backbone carbons.
• IUPAC names use poly(monomer).
• Polyalkenes are unreactive because their backbones contain strong, non-polar C–C and C–H bonds; chains attract through London forces.
• Rigid PVC has structural uses, while a plasticiser separates chains and increases movement to make flexible PVC.

Explanation

• Alcohols are produced industrially by acid-catalysed hydration of alkenes; ethene and steam form ethanol through electrophilic addition, with the acid regenerated. Ethanol is also produced by yeast fermentation of glucose under warm, aqueous, anaerobic conditions chosen to maintain enzyme activity and prevent unwanted oxidation.
• Fractional distillation separates ethanol from the dilute fermentation mixture.
• A biofuel comes from recently living material.
• Fermentation appears carbon neutral because crop photosynthesis removes the carbon dioxide later released on combustion, but farming, fertiliser manufacture, processing and transport use energy.
• Land use, food competition and other environmental or ethical effects must therefore be included in decisions.

Explanation

• Alcohols are classified as primary, secondary or tertiary by the number of carbon groups attached to the carbon bearing $\mathrm{-OH}$.
• Acidified potassium dichromate(VI) oxidises a primary alcohol first to an aldehyde and then to a carboxylic acid, while a secondary alcohol forms a ketone and a tertiary alcohol is not easily oxidised.
• Distil the aldehyde as it forms to prevent further oxidation; heat under reflux with excess oxidant to obtain the acid.
• Equations may use $\mathrm{[O]}$.
• Tollens' reagent gives an aldehyde a silver mirror and Fehling's solution gives a brick-red precipitate; ketones give neither observation.

Explanation

• Alcohols form alkenes by acid-catalysed elimination of water, also called dehydration. Protonation converts the poor $\mathrm{-OH}$ leaving group into water.
• After water leaves, an electron pair from a C–H bond on an adjacent carbon forms C=C while $\mathrm{H^+}$ is lost and the acid catalyst is regenerated.
• The electron-pair arrow must start at the bond supplying electrons.
• An alcohol with suitable hydrogen atoms on both adjacent carbons can produce more than one structural alkene; any E–Z isomers are considered separately.
• Alkenes made this way can supply addition-polymer monomers without deriving them directly from crude oil.

Explanation

• Functional groups are identified using reactions specified elsewhere in the course. Bromine water changes orange to colourless with an alkene.
• A carboxylic acid effervesces with carbonate as $\mathrm{CO_2}$ forms. Warmed Tollens' reagent gives an aldehyde a silver mirror and Fehling's gives a brick-red precipitate, while ketones do not respond.
• Acidified dichromate(VI) changes orange to green with oxidisable primary or secondary alcohols, so it cannot alone distinguish a primary alcohol from an aldehyde.
• Required practical 6 tests alcohol, aldehyde, alkene and carboxylic acid.
• Fresh portions and precise colours, precipitates or gas tests make a valid identification sequence.

Explanation

• High-resolution mass spectrometry can determine molecular formula because isotopes have precise, non-integer masses.
• Candidate formulae may share a nominal $M_r$ but have different exact molecular masses.
• For each candidate, multiply each isotope's precise mass by its atom count, sum all contributions and compare with the measured molecular-ion mass at the stated precision.
• Carbon-12 contributes exactly $12.00000$, while hydrogen-1 and oxygen-16 require their precise data-book values.
• The molecular ion represents the intact molecule after electron loss, so its accurate $m/z$ for a singly charged ion supplies the molecular mass used in this comparison.

Explanation

• Molecular bonds absorb infrared radiation at characteristic wavenumbers when photon energy matches a vibrational change.
• Data-book ranges identify bonds and hence possible functional groups; the fingerprint region can identify a particular molecule by comparison with a reference spectrum.
• Structure deductions use both present and absent absorptions, and unexpected peaks can reveal impurities.
• For example, a strong C=O absorption near $1700\,\mathrm{cm^{-1}}$ plus a very broad O–H range around $2500$–$3000\,\mathrm{cm^{-1}}$ supports a carboxylic acid.
• Infrared absorption by bonds in carbon dioxide, methane and water vapour contributes to global warming because these gases absorb outgoing terrestrial infrared radiation.

Explanation

• Optical isomerism is stereoisomerism caused here by chirality at one carbon attached to four different groups.
• The two enantiomers are non-superimposable mirror images and rotate plane-polarised light by equal amounts in opposite directions.
• A racemic mixture contains equal amounts of both enantiomers and is optically inactive because their rotations cancel.
• A chiral centre is identified by tracing all four substituents far enough to find the first difference; four bonds alone are insufficient if two groups are identical.
• Enantiomer drawings must use a valid three-dimensional representation, and rotating a molecule in space does not create the other enantiomer.

Explanation

• Aldehydes readily oxidise to carboxylic acids and give positive Tollens' and Fehling's tests; ketones do not under those conditions.
• Aqueous $\mathrm{NaBH_4}$ reduces aldehydes to primary alcohols and ketones to secondary alcohols by nucleophilic addition: $\mathrm{H^-}$ attacks the $\delta+$ carbonyl carbon, the C=O pair moves to oxygen and the alkoxide is protonated.
• KCN followed by dilute acid adds HCN to form a hydroxynitrile and lengthens the carbon skeleton by one carbon.
• KCN is highly toxic.
• Attack on either face of a planar aldehyde or unsymmetrical ketone produces a racemate only if the new tetrahedral carbon has four different groups.

Explanation

• Carboxylic acids are weak acids but liberate $\mathrm{CO_2}$ from carbonates. With alcohols and an acid catalyst they form esters in reversible condensation; esters are used as solvents, plasticisers, perfumes and flavourings.
• Acid hydrolysis gives a carboxylic acid and alcohol, whereas alkaline hydrolysis gives a carboxylate salt and alcohol.
• Vegetable oils and animal fats are triesters of propane-1,2,3-triol.
• Their alkaline hydrolysis produces glycerol and soap, the salts of long-chain carboxylic acids.
• Biodiesel is a mixture of long-chain methyl esters made by reacting vegetable oil with methanol in the presence of a catalyst.

Explanation

• Acid anhydrides, acyl chlorides and amides contain acyl-derived structures.
• Acyl chlorides and acid anhydrides react with water, alcohols, ammonia and primary amines by nucleophilic addition–elimination, producing carboxylic acids, esters or amides as appropriate.
• The nucleophile attacks the $\delta+$ acyl carbon, the C=O pair moves to oxygen, then C=O reforms as the leaving group departs; proton transfer completes the product.
• Ethanoic anhydride is preferred to ethanoyl chloride for industrial aspirin manufacture because it is safer to handle and produces ethanoic acid rather than corrosive HCl.
• Required practical 10 prepares and purifies a solid and a liquid and tests purity.

Explanation

• Benzene is planar and all six C–C bonds have equal length intermediate between ordinary single and double bonds.
• Each carbon contributes a p electron; sideways overlap around the ring produces delocalised electron density above and below the plane.
• Thermochemical evidence confirms extra stability: a theoretical cyclohexa-1,3,5-triene with three isolated C=C bonds would release three times cyclohexene's hydrogenation enthalpy, but actual benzene hydrogenation is substantially less exothermic.
• The difference is delocalisation stability.
• Benzene therefore undergoes substitution in preference to addition because substitution restores the delocalised ring, whereas addition would permanently disrupt it.

Explanation

• Electrophilic attack on benzene gives monosubstitution. Ring electrons attack the electrophile to form a positive sigma complex; loss of $\mathrm{H^+}$ returns the C–H electron pair to the ring and restores delocalisation.
• Nitration uses concentrated nitric and sulfuric acids. Sulfuric acid generates the nitronium ion: $\mathrm{HNO_3+H_2SO_4\rightarrow NO_2^++HSO_4^-+H_2O}$.
• Nitration is important in explosive manufacture and routes to amines.
• Friedel–Crafts acylation uses an acyl chloride and anhydrous $\mathrm{AlCl_3}$ to generate an acylium electrophile, $\mathrm{RCO^+}$, forming an aromatic ketone.
• Both mechanisms must show electrophile generation and delocalisation restoration.