Basic Concept of Organic Chemistry

More than twenty million compounds are known to chemistry, and over ninety per cent of them are compounds of a single element — carbon. This unit is the door into that enormous subject. It explains what makes carbon special, how its compounds are sorted into families, how chemists draw and name them, and how the crude oil pumped out of the ground is turned into the petrol and diesel that reach the pumps in Nepal. Nothing here is difficult, but everything here is used again in every organic chapter that follows.

12.1 Introduction to organic chemistry and organic compounds

Until the early nineteenth century, chemists divided substances into mineral compounds, obtained from rocks, and organic compounds, obtained only from living things. Berzelius argued in 1807 that organic compounds could be made only inside a plant or animal, under the influence of a mysterious vital force.

That idea was destroyed in 1828 by Friedrich Wöhler, who heated the purely inorganic salt ammonium cyanate and obtained urea — a substance until then found only in urine:

NH₄CNO --Δ--> NH₂CONH₂ (urea)

Kolbe synthesised acetic acid in 1845 and Berthelot made methane in 1856, and the vital force theory was finished.

Note that the modern definition says nothing about living organisms. Petrol, polythene, aspirin, DDT and nylon are all organic and none of them comes from a living cell.

12.2 Reasons for separate study of organic compounds

Organic chemistry is studied as a separate branch for practical reasons, not because carbon obeys different laws.

1. Sheer number. Over 20 million organic compounds are known against roughly half a million inorganic ones, and the list grows every day.
2. Isomerism. One molecular formula may stand for many different compounds. C₄H₁₀O alone represents seven substances. Nothing comparable happens in inorganic chemistry, so a structural formula, not a molecular formula, is needed to identify an organic compound.
3. Distinct physical properties. Organic compounds are almost all covalent, so they melt and boil at low temperatures, are usually insoluble in water but soluble in organic solvents, and do not conduct electricity.
4. Distinct chemical behaviour. Their reactions are molecular rather than ionic: they are slow, often reversible, need heat or a catalyst, and give side products. Inorganic ionic reactions are practically instantaneous.
5. Reactions are governed by the functional group, so millions of compounds can be handled in a few dozen families with a common set of reaction mechanisms.
6. A separate nomenclature (the IUPAC system) had to be built for them.
7. Vast importance: food, fuels, medicines, plastics, dyes, soaps, pesticides and the molecules of life itself are all organic.

Property	Organic compounds	Inorganic compounds
Bonding	mostly covalent	mostly ionic
Melting and boiling point	low	generally high
Solubility	in organic solvents	usually in water
Electrical conductivity	non-conductors	conduct when fused or in solution
Rate of reaction	slow, needs catalyst or heat	fast, often instantaneous
Combustibility	usually inflammable	usually non-inflammable
Isomerism	very common	rare
Thermal stability	low; char on strong heating	generally high

12.3 Tetra-covalency and catenation properties of carbon

Tetra-covalency

Carbon is element number 6, with the ground-state configuration 1s² 2s² 2p_x¹ 2p_y¹. That gives only two unpaired electrons, so carbon ought to be divalent — yet methane is CH₄, not CH₂.

The explanation is promotion. One of the paired 2s electrons is promoted to the empty 2p_z orbital:

ground state 1s² 2s² 2p_x¹ 2p_y¹ → excited state 1s² 2s¹ 2p_x¹ 2p_y¹ 2p_z¹

This costs about 406 kJ mol⁻¹, but the two extra C–H bonds formed release far more (about 2 × 413 kJ mol⁻¹), so the excited state is worth reaching. The four singly occupied orbitals then mix to give four equivalent sp³ hybrid orbitals pointing to the corners of a regular tetrahedron, with a bond angle of 109°28′ (109.5°).

Why covalency rather than ionic bonding? Carbon would have to lose four electrons to become C⁴⁺ (the sum of the first four ionisation energies is huge) or gain four to become C⁴⁻ (impossible to force four electrons onto so small an atom). Sharing is the only option, so carbon is always tetracovalent in its stable compounds.

Catenation

Carbon catenates better than any other element. The reason is the strength of the carbon–carbon bond, which in turn comes from carbon's small size: the two nuclei can approach closely and overlap their orbitals effectively.

Bond	C–C	Si–Si	N–N	O–O	S–S
Bond energy / kJ mol⁻¹	348	222	163	146	266

Two further points complete the picture. First, the C–H bond (413 kJ mol⁻¹) is strong and non-polar, so a hydrocarbon chain is wrapped in a protective, chemically inert skin — silicon chains, by contrast, are attacked at once by water. Second, carbon also forms strong multiple bonds (C=C 614 kJ mol⁻¹, C≡C 839 kJ mol⁻¹), which no other catenating element does well. Together these give open chains, branched chains, rings of any size, and double and triple bonds — hence the millions of compounds.

12.4 Classification of organic compounds

Organic compounds are classified twice over: once by the shape of the carbon skeleton, and once by the functional group they carry.

• Acyclic (open-chain or aliphatic) compounds have carbon chains with two free ends. The chain may be straight (n-pentane) or branched (2-methylbutane).
• Cyclic (closed-chain or ring) compounds have at least one ring.
• Homocyclic (carbocyclic) rings are built of carbon only. If the ring behaves like an open chain it is alicyclic (cyclohexane); if it has the special stability and substitution chemistry of benzene it is aromatic. Aromatic compounds that contain a benzene ring are benzenoid (benzene, naphthalene, phenol); the few that are aromatic without one are non-benzenoid (azulene, tropolone).
• Heterocyclic rings contain at least one atom other than carbon — a hetero atom, usually O, N or S (furan, pyrrole, thiophene, pyridine).

12.5 Alkyl groups, functional groups and homologous series

Alkyl groups

Remove one hydrogen atom from an alkane CₙH₂ₙ₊₂ and the fragment that is left, CₙH₂ₙ₊₁, is an alkyl group, written R–. Alkyl groups never exist on their own; they are the building blocks from which names are made.

Alkane	Alkyl group	Formula	Symbol
Methane CH₄	methyl	CH₃–	Me
Ethane C₂H₆	ethyl	CH₃CH₂–	Et
Propane C₃H₈	n-propyl	CH₃CH₂CH₂–	n-Pr
Propane C₃H₈	isopropyl (propan-2-yl)	(CH₃)₂CH–	i-Pr
Butane C₄H₁₀	n-butyl	CH₃CH₂CH₂CH₂–	n-Bu
Butane C₄H₁₀	sec-butyl	CH₃CH₂CH(CH₃)–	s-Bu
2-Methylpropane	isobutyl	(CH₃)₂CHCH₂–	i-Bu
2-Methylpropane	tert-butyl	(CH₃)₃C–	t-Bu

A carbon atom is called primary (1°), secondary (2°) or tertiary (3°) according to whether it is attached to one, two or three other carbon atoms. This classification runs right through organic chemistry, so learn it now.

Functional groups

Class	Functional group	General formula	IUPAC prefix	IUPAC suffix
Alkene	>C=C<	CₙH₂ₙ	—	-ene
Alkyne	–C≡C–	CₙH₂ₙ₋₂	—	-yne
Haloalkane	–X (F, Cl, Br, I)	R–X	halo-	—
Alcohol	–OH	R–OH	hydroxy-	-ol
Ether	–O–	R–O–R′	alkoxy-	—
Aldehyde	–CHO	R–CHO	oxo- / formyl-	-al
Ketone	>C=O	R–CO–R′	oxo-	-one
Carboxylic acid	–COOH	R–COOH	carboxy-	-oic acid
Ester	–COO–	R–COOR′	alkoxycarbonyl-	-oate
Acid chloride	–COCl	R–COCl	—	-oyl chloride
Amide	–CONH₂	R–CONH₂	carbamoyl-	-amide
Amine	–NH₂	R–NH₂	amino-	-amine
Nitrile (cyanide)	–C≡N	R–CN	cyano-	-nitrile
Nitro	–NO₂	R–NO₂	nitro-	—

Homologous series

Characteristics of a homologous series:

1. All members have the same general formula (alkanes CₙH₂ₙ₊₂, alcohols CₙH₂ₙ₊₁OH).
2. Successive members differ by a –CH₂– group, that is by 14 atomic mass units.
3. All members contain the same functional group and therefore show essentially the same chemical properties.
4. They can all be made by the same general methods.
5. Physical properties (melting point, boiling point, density) change gradually and regularly with molecular mass, so the properties of an unknown member can be predicted by interpolation.

n	Alkane (CₙH₂ₙ₊₂)	Molar mass	Boiling point / °C
1	CH₄ methane	16	−162
2	C₂H₆ ethane	30	−89
3	C₃H₈ propane	44	−42
4	C₄H₁₀ butane	58	−0.5
5	C₅H₁₂ pentane	72	36
6	C₆H₁₄ hexane	86	69

The molar mass rises by exactly 14 each time and the boiling point rises steadily — that regularity is the whole point of a homologous series.

IUPAC nomenclature

The International Union of Pure and Applied Chemistry name of a compound is built from five parts, always written in this order and joined into a single word:

Secondary prefix + Primary prefix + Word root + Primary suffix + Secondary suffix

Part	What it gives	Example
Word root	number of carbons in the parent chain	meth (1), eth (2), prop (3), but (4), pent (5), hex (6)
Primary suffix	saturation	-ane, -ene, -yne
Secondary suffix	the principal functional group	-ol, -al, -one, -oic acid
Primary prefix	cyclo-, for a ring	cyclohexane
Secondary prefix	substituents	methyl-, chloro-, nitro-

The rules, in the order you apply them.

1. Find the parent chain — the longest continuous chain of carbon atoms that contains the principal functional group. If two chains are equally long, choose the one with more substituents.
2. Number the chain from the end that gives the lowest locant to the principal functional group. If there is none, give the lowest locant to the multiple bond; failing that, to the substituents (the lowest set of locants rule).
3. Name the substituents as prefixes with their locants, and list them in alphabetical order. The multiplying prefixes di-, tri-, tetra- are not counted when alphabetising (dimethyl files under "m").
4. Punctuate: numbers are separated from words by hyphens and from each other by commas. There are no spaces inside the name, except before "acid".
5. When the same substituent occurs more than once, use di-, tri-, tetra- and repeat the locant for each: 2,2-dimethylpropane, not 2-dimethylpropane.

When a molecule has more than one functional group, only the senior one becomes the suffix; the rest become prefixes. The order of seniority is

–COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂ >

C=C< > –C≡C–

while –OR (alkoxy), –X (halo) and –NO₂ (nitro) are always prefixes, never suffixes.

12.6 Structural formula, contracted formula and bond line structural formula

A molecular formula (C₄H₁₀O) tells you only how many atoms there are. Because of isomerism that is not enough, so organic chemists use three ways of showing how the atoms are joined.

Representation	What is shown	Example: butan-2-ol
Complete (displayed) structural formula	every atom and every bond drawn as a line	see the figure below
Condensed (contracted) formula	atoms written in order, bonds mostly omitted; branches in brackets	CH₃CH(OH)CH₂CH₃
Bond-line (skeletal) structural formula	only the carbon skeleton as a zig-zag; carbons and their hydrogens are not written	see the figure below

Rules for a bond-line structure. Each line is a bond. Every vertex and every free line end is a carbon atom. The hydrogens attached to carbon are not drawn — you work out how many there are from the fact that carbon is tetravalent. All atoms other than carbon and hydrogen (O, N, Cl, and any H attached to them) are written in.

Isomerism

Isomerism splits into two branches:

• Structural (constitutional) isomerism — the atoms are joined in a different order. Sub-types: chain, position, functional, metamerism and tautomerism.
• Stereoisomerism — the atoms are joined in the same order but arranged differently in space. Sub-types: geometrical (cis–trans) and optical (d and l).

Type	Difference between the isomers	Example (same molecular formula)
Chain	length or branching of the carbon skeleton	C₄H₁₀: butane and 2-methylpropane
Position	position of the functional group or multiple bond	C₃H₇OH: propan-1-ol and propan-2-ol
Functional	a different functional group altogether	C₂H₆O: ethanol and methoxymethane
Metamerism	different alkyl groups on either side of a divalent atom	C₄H₁₀O: ethoxyethane and methoxypropane
Tautomerism	a hydrogen atom migrates, giving two forms in equilibrium	CH₃COCH₃ ⇌ CH₃C(OH)=CH₂ (keto–enol)
Geometrical	arrangement about a C=C that cannot rotate	cis- and trans-but-2-ene
Optical	mirror-image arrangements about a chiral carbon	d- and l-lactic acid

Geometrical isomerism appears whenever a double bond carries two different groups on each of its carbons. The C=C bond cannot rotate — rotation would break the π bond — so the two arrangements are separate, isolable compounds.

12.7 Cracking and reforming; quality of gasoline; octane number; cetane number; gasoline additive

Crude petroleum is separated by fractional distillation into fractions, of which gasoline (petrol), boiling between about 40 °C and 200 °C and containing C₅ to C₁₂ hydrocarbons, is the most valuable. Crude oil yields only about 20% of this fraction directly, while the market wants far more — and the straight-run petrol that does come off is of poor quality. Two processes fix both problems.

Cracking

C₁₆H₃₄ --Δ--> C₈H₁₈ + C₈H₁₆

C₁₂H₂₆ --Δ--> C₆H₁₄ + C₆H₁₂

	Thermal cracking	Catalytic cracking
Conditions	750–900 K, 10–70 atm, no catalyst	700–800 K, 1–2 atm
Catalyst	none	zeolite or silica–alumina
Intermediate	free radicals	carbocations
Main products	straight-chain alkanes and alkenes	branched alkanes, cycloalkanes, aromatics
Quality of petrol	moderate	high, because branched products resist knocking

Reforming

Three kinds of change occur together over a platinum-on-alumina catalyst at about 770 K and 10–20 atm — the process is called platforming:

• Isomerisation: n-hexane → 2,2-dimethylbutane
• Cyclisation: C₆H₁₄ → C₆H₁₂ (cyclohexane) + H₂
• Aromatisation: C₆H₁₄ → C₆H₆ (benzene) + 4H₂, and C₇H₁₆ → C₇H₈ (toluene) + 4H₂

Cracking makes more petrol; reforming makes better petrol.

Knocking, octane number and cetane number

In a petrol engine the fuel–air mixture should burn smoothly only when the spark plug fires. If part of the mixture ignites on its own beforehand, the engine produces a sharp metallic rattle, loses power and eventually damages itself. This premature explosion is called knocking or pinking.

Knocking depends on structure. Straight-chain alkanes knock badly; branched alkanes, cycloalkanes and aromatics knock very little:

n-alkane > cycloalkane > alkene > branched alkane > aromatic hydrocarbon

So a petrol of octane number 91 knocks exactly like a blend of 91 parts iso-octane with 9 parts n-heptane. The higher the octane number, the better the antiknock quality and the higher the compression ratio the engine may use.

Gasoline additives

A gasoline additive is a substance added in small quantity to petrol to improve its performance, chiefly its antiknock quality.

Additive	Formula / nature	Purpose
Tetraethyl lead, TEL (now banned)	Pb(C₂H₅)₄	raised the octane number by about 10 units at 0.5 mL per litre
Ethylene dibromide (scavenger, used with TEL)	C₂H₄Br₂	converted lead residues to volatile PbBr₂ so they left with the exhaust
MTBE	methyl tert-butyl ether	oxygenate; raises octane number and helps complete combustion
Ethanol	C₂H₅OH	renewable oxygenate, octane number about 108
Aromatics	benzene, toluene, xylene	high octane blending components
Detergents and antioxidants	organic amines, phenols	keep injectors clean, prevent gum formation

Tetraethyl lead was outstandingly effective and outstandingly poisonous: it filled city air with lead compounds and destroyed the catalytic converters fitted to modern exhausts. Nepal, like the rest of South Asia, phased out leaded petrol around the turn of the century, and the fuel sold today by Nepal Oil Corporation is unleaded, its octane number raised by reforming and by oxygenates instead.

12.8 Bond fission and electronic effects — a bridge to Unit 13

Everything after this unit is about how organic reactions happen. Two ideas underlie all of it: how a bond breaks, and how electron density is pushed about inside a molecule that has not yet reacted.

Homolytic and heterolytic fission

	Homolytic fission	Heterolytic fission
How the pair splits	one electron to each atom	both electrons to one atom
Products	two free radicals, A• and B•	two ions, A⁺ and B:⁻
Arrow used	half-headed "fish-hook" arrow	full curved arrow
Favoured by	high temperature, UV light, peroxides, non-polar solvents	polar bonds and polar solvents
Example	Cl₂ --hν--> 2Cl•	CH₃–Br → CH₃⁺ + Br⁻

Heterolysis produces the two reactive carbon species you will meet constantly:

• A carbocation has a positively charged carbon with only six electrons. It is sp² hybridised and planar. Stability: 3° > 2° > 1° > CH₃⁺.
• A carbanion has a negatively charged carbon carrying a lone pair. It is sp³ hybridised and pyramidal. Stability runs the other way: CH₃⁻ > 1° > 2° > 3°.

Electrophiles and nucleophiles

	Electrophile	Nucleophile
Meaning	"electron loving"	"nucleus loving"
Electronically	electron deficient, accepts an electron pair (a Lewis acid)	electron rich, donates an electron pair (a Lewis base)
Charge	positive or neutral	negative or neutral
Examples	H⁺, Cl⁺, Br⁺, NO₂⁺, R⁺, BF₃, AlCl₃, SO₃, the carbon of >C=O	OH⁻, CN⁻, Cl⁻, RO⁻, NH₃, H₂O, ROH, RNH₂, alkenes

A free radical is the third species: neutral, with one unpaired electron, paramagnetic and extremely reactive. Stability: 3° > 2° > 1° > CH₃•.

Inductive, resonance and hyperconjugative effects

Inductive effect (I). When an atom more electronegative than carbon is attached to a chain, the σ electrons of the C–X bond are pulled towards it, and the polarisation is relayed along the chain with rapidly decreasing strength — it is negligible beyond the third carbon. It is a permanent effect.

• –I groups withdraw electrons: –NO₂ > –CN > –COOH > –F > –Cl > –Br > –I > –OCH₃ > –C₆H₅
• +I groups release electrons (taking H as the standard): –C(CH₃)₃ > –CH(CH₃)₂ > –CH₂CH₃ > –CH₃

The classic demonstration is acid strength. Chlorine is –I, so it drains electron density from the –COO⁻ ion and stabilises it, making the acid stronger:

Acid	CH₃COOH	ClCH₂COOH	Cl₂CHCOOH	Cl₃CCOOH
pKa	4.76	2.86	1.29	0.66
Strength	weakest	→	→	strongest

Resonance (mesomeric) effect (R or M). In a conjugated system, π electrons or a lone pair are delocalised over several atoms. Unlike the inductive effect it is transmitted through the whole conjugated system without dying away, and it is usually the stronger of the two.

• +R groups push a lone pair into the system: –OH, –OR, –NH₂, –NHR, –X
• –R groups pull π electrons out of it: –NO₂, –CN, –CHO, –COR, –COOH, –SO₃H

Hyperconjugation. The σ electrons of a C–H bond on the carbon next to an sp² centre can also delocalise into the empty or π orbital. It is sometimes called "no-bond resonance" or the Baker–Nathan effect. Its strength depends simply on the number of α-hydrogens available, which is why

(CH₃)₃C⁺ (9 α-H) > (CH₃)₂CH⁺ (6 α-H) > CH₃CH₂⁺ (3 α-H) > CH₃⁺ (0 α-H)

and why the same order of stability appears again for free radicals and for substituted alkenes.

Chapter summary

• Organic chemistry is the chemistry of hydrocarbons and their derivatives. The vital force theory died when Wöhler made urea from ammonium cyanate in 1828, NH₄CNO --Δ--> NH₂CONH₂.
• Organic compounds are studied separately because of their number, their isomerism, their covalent character, their slow molecular reactions and their functional-group chemistry.
• Carbon is tetracovalent: promotion of a 2s electron gives four unpaired electrons, four sp³ hybrid orbitals and a tetrahedral angle of 109°28′. Catenation is possible because the C–C bond (348 kJ mol⁻¹) is far stronger than Si–Si, N–N or O–O.
• Compounds are classified as acyclic, or cyclic — homocyclic (alicyclic or aromatic, benzenoid or non-benzenoid) and heterocyclic.
• An alkyl group is CₙH₂ₙ₊₁; a functional group decides chemical behaviour; a homologous series has one general formula, a constant difference of –CH₂– (14 u), identical chemical properties and a regular gradation of physical properties.
• An IUPAC name is built as secondary prefix + primary prefix + word root + primary suffix + secondary suffix, numbering the parent chain so that the principal functional group gets the lowest locant.
• Structures are written in three ways: complete structural, condensed and bond-line, where every vertex and line end is a carbon and hydrogens on carbon are omitted.
• Isomers share a molecular formula but differ in structure: chain, position, functional, metamerism and tautomerism (structural); geometrical and optical (stereo). C₅H₁₂ has 3 isomers, C₄H₁₀O has 7.
• Cracking breaks big molecules into smaller ones and makes more petrol; reforming rearranges molecules into branched and aromatic forms and makes better petrol.
• Octane number = % by volume of iso-octane in the iso-octane/n-heptane blend that knocks equally; cetane number = % by volume of cetane in the cetane/α-methylnaphthalene blend with the same ignition delay. High octane and high cetane are opposite requirements.
• A bond breaks homolytically (fish-hook arrows, free radicals) or heterolytically (full curved arrow, ions). Electrophiles accept an electron pair, nucleophiles donate one. The inductive effect dies out along a chain, the resonance effect travels through a conjugated system, and hyperconjugation grows with the number of α-hydrogens.

Practice questions

Group A — Multiple choice (1 mark each)

1. The bond angle in methane is [1]a90°b104°31′c109°28′d120°
2. The number of structural isomers of C₅H₁₂ is [1]a2b3c4d5
3. The IUPAC name of (CH₃)₃C–OH is [1]abutan-1-olbbutan-2-olc2-methylpropan-1-old2-methylpropan-2-ol
4. Octane number 100 is assigned to [1]an-heptanebn-octanec2,2,4-trimethylpentanedcetane
5. Homolytic fission of a covalent bond produces [1]aa carbocationba carbanioncfree radicalsdan electrophile and a nucleophile
6. Which of the following shows the +I effect? [1]a–NO₂b–CNc–COOHd–C(CH₃)₃
7. Ethanol and methoxymethane are an example of [1]achain isomerismbposition isomerismcfunctional isomerismdtautomerism

Answers to Group A

1. (c) — four equivalent sp³ orbitals directed to the corners of a regular tetrahedron.

2. (b) — pentane, 2-methylbutane and 2,2-dimethylpropane.

3. (d) — the longest chain is three carbons with a methyl and the OH both on C2.

4. (c) — iso-octane, 2,2,4-trimethylpentane, is the 100 mark; n-heptane is 0.

5. (c) — each atom keeps one electron of the shared pair.

6. (d) — alkyl groups release electrons; –NO₂, –CN and –COOH are all –I.

7. (c) — same formula C₂H₆O but different functional groups, alcohol and ether.

Group B — Short answer (5 marks each)

1. Why is organic chemistry studied as a separate branch of chemistry? Give any five reasons. [5]
2. Explain the tetra-covalency of carbon on the basis of its electronic configuration, and state why carbon catenates better than silicon. [5]
3. Define a homologous series and give its five characteristics, illustrating with the first four alkanes. [5]
4. Write the IUPAC names of: (i) (CH₃)₂CHCH₂CH₃ (ii) CH₃CH₂CH(OH)CH₃ (iii) CH₃CH₂CHO (iv) CH₃CH(CH₃)COOH (v) CH₃CH₂C≡CH [5]
5. Draw the structures of all the isomers of C₄H₉Cl and name them. [5]
6. Distinguish between cracking and reforming. Write one equation for each. [5]
7. Define octane number and cetane number. Why does a high octane number make a poor diesel? [5]
8. Distinguish between homolytic and heterolytic fission with one example of each, and define an electrophile and a nucleophile. [5]

Answers to Group B

4. (i) 2-methylbutane (ii) butan-2-ol (iii) propanal (iv) 2-methylpropanoic acid (v) but-1-yne.

5. Four isomers: 1-chlorobutane CH₃CH₂CH₂CH₂Cl; 2-chlorobutane CH₃CH₂CHClCH₃; 1-chloro-2-methylpropane (CH₃)₂CHCH₂Cl; 2-chloro-2-methylpropane (CH₃)₃CCl. The first pair and the second pair are position isomers of each other, and the two skeletons are chain isomers.

6. Cracking breaks large molecules into smaller ones and so increases the quantity of petrol: C₁₆H₃₄ → C₈H₁₈ + C₈H₁₆. Reforming rearranges molecules of about the same size into branched or aromatic forms and so increases the quality: C₆H₁₄ → C₆H₆ + 4H₂. Cracking uses 750–900 K, or a zeolite catalyst at 700–800 K; reforming uses Pt/Al₂O₃ at about 770 K and 10–20 atm.

7. Definitions as in Section 12.7. A petrol engine needs a fuel that will not ignite before the spark, so it needs branched and aromatic molecules; a diesel engine ignites its fuel by compression and needs one that ignites as readily as possible, so it needs straight-chain alkanes. The two requirements are opposite, so a high-octane fuel necessarily has a low cetane number.

Group C — Long answer (8 marks each)

1. (a) Classify organic compounds on the basis of the carbon skeleton, giving one example of each class. [4](b) Define isomerism and classify it. Draw and name all seven isomers of C₄H₁₀O, stating the type of isomerism in each case. [4]
2. (a) State the rules of IUPAC nomenclature and use them to name CH₃–CH(CH₃)–CH₂–CH(OH)–CH₃ and CH₃–CH=CH–COOH. [4](b) An organic compound contains 92.3% carbon and 7.7% hydrogen and has a vapour density of 13. Find its molecular formula. A petrol made largely from this kind of compound is blended, 75 L of octane number 96 with 25 L of octane number 84; find the octane number of the blend. [4]

Answers to Group C

1. (a) See the classification figure in Section 12.4 — acyclic (propane); cyclic → homocyclic → alicyclic (cyclohexane); homocyclic → aromatic → benzenoid (benzene) and non-benzenoid (azulene); heterocyclic (pyridine). (b) See Worked example 12.2: four alcohols (butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol) and three ethers (methoxypropane, 2-methoxypropane, ethoxyethane). Alcohol against alcohol is chain or position isomerism; ether against ether is metamerism; alcohol against ether is functional isomerism.

2. (a) Rules as in Section 12.5. For CH₃–CH(CH₃)–CH₂–CH(OH)–CH₃ the parent chain is five carbons and the OH must get the lowest locant, so number from the right: the OH is on C2 and the methyl on C4 — 4-methylpentan-2-ol. For CH₃–CH=CH–COOH the –COOH carbon is C1, the chain has four carbons and the double bond lies between C2 and C3 — but-2-enoic acid.

(b) C: 92.3 ÷ 12 = 7.69; H: 7.7 ÷ 1 = 7.7; ratio 1 : 1, so the empirical formula is CH with empirical mass 13. Molecular mass = 2 × 13 = 26, so n = 26/13 = 2 and the molecular formula is C₂H₂ (ethyne). Blend: \text{ON} = \dfrac{75(96) + 25(84)}{100} = \dfrac{7200 + 2100}{100} = 93.