General Organic Chemistry

Organic chemistry becomes manageable when you stop memorizing molecules one at a time and start reading their architecture. This chapter, worth about 3 questions, supplies that grammar: how carbon builds frameworks, how functional groups create families, how IUPAC names encode structure, how the same formula can represent different substances, and how electrons move when bonds break. These ideas decide nearly every mechanism in Chapters 17–20.

16.1 Tetravalency and catenation; classification; alkyl and aryl groups; functional groups and homologous series

Carbon has four valence electrons. In ordinary neutral organic compounds it shares four pairs of electrons, acquiring an octet while retaining a formal charge of zero. The four bonds may be arranged as four single bonds (tetrahedral, sp³), a double and two single bonds (trigonal planar, sp²), or a triple and one single bond/two double bonds (linear, sp). Charged carbon is the deliberate exception encountered later in the chapter: a carbocation has three bonds and an empty p orbital; a carbanion has three bonds and a lone pair.

Catenation is self-linking. It is exceptionally strong in carbon because the small atom forms strong C–C and C–H bonds and can also use p orbitals effectively in multiple bonds. Chains can branch, close into rings and support many functional groups. Silicon sits below carbon but forms weaker Si–Si bonds and much stronger Si–O bonds, so silicon chemistry flows toward silicate networks rather than an equally rich family of discrete Si-chain molecules.

Classification by carbon skeleton

Class	Meaning	Examples
Acyclic/aliphatic	Open chain, straight or branched	butane, 2-methylpropane, but-2-ene
Alicyclic	Ring with broadly aliphatic behaviour	cyclopropane, cyclohexane
Aromatic	Cyclic conjugated system with special stability; benzene is the prototype	benzene, phenol, aniline
Heterocyclic	Ring contains an atom other than carbon	furan (O in ring), pyridine (N in ring), thiophene (S in ring)

Compounds are also called saturated when the carbon framework has only single bonds and unsaturated when it contains a double bond, triple bond or ring equivalent. A ring creates the same hydrogen deficiency as one π bond even though cycloalkanes do not behave like alkenes in addition reactions.

Alkyl, aryl and related groups

A group obtained by removing one hydrogen from an alkane is an alkyl group, general formula CₙH₂ₙ₊₁–: methyl CH₃– (from methane), ethyl CH₃CH₂–, propyl CH₃CH₂CH₂– and propan-2-yl/isopropyl (CH₃)₂CH–. The atom bearing the free valence dictates names such as butan-2-yl. Carbon atoms are described as 1°, 2°, 3° or 4° according to how many other carbons are attached to them; alcohols and alkyl halides are similarly classified by the carbon carrying –OH or halogen.

An aryl group is formed by removing hydrogen directly from an aromatic ring. Phenyl is C₆H₅–; a methylphenyl group is tolyl. By contrast, C₆H₅CH₂– is benzyl: its free valence is on a saturated carbon next to the ring, making it an aralkyl group. This distinction matters in reactivity — a benzylic carbocation or radical is resonance-stabilized through the benzylic carbon, whereas a phenyl cation has its empty orbital in the ring plane and is very unstable. Two other pattern names recur: vinyl, CH₂=CH–, and allyl, CH₂=CH–CH₂–.

Functional groups and homologous series

The carbon skeleton is the stage; the functional group is usually where the action occurs. Members of one family therefore have similar chemistry, while physical properties change gradually as the hydrocarbon part grows.

Family	Functional pattern	Typical suffix/prefix	Example
Alkane	C–C, C–H only	-ane	propane
Alkene	C=C	-ene	but-2-ene
Alkyne	C≡C	-yne	pent-1-yne
Alcohol	–OH on carbon	-ol / hydroxy-	propan-2-ol
Ether	C–O–C	alkoxy- (or ether name)	methoxyethane
Aldehyde	–CHO, terminal carbonyl	-al / oxo-	butanal
Ketone	C–CO–C	-one / oxo-	pentan-3-one
Carboxylic acid	–CO₂H	-oic acid	ethanoic acid
Ester	–CO₂R	alkyl …-oate	methyl ethanoate
Acid halide	–COX	-oyl halide	ethanoyl chloride
Amide	–CONH₂ or substituted	-amide	ethanamide
Nitrile	–C≡N	-nitrile / cyano-	propanenitrile
Amine	–NH₂, –NHR, –NR₂	-amine / amino-	ethanamine
Haloalkane	C–F/Cl/Br/I	halo- prefix	2-bromobutane
Nitro compound	–NO₂	nitro- prefix	nitroethane

A homologous series shares a general formula and functional group, has a common method of preparation, shows similar chemical reactions and displays gradual changes in melting point, boiling point, density and solubility. Adjacent members differ by –CH₂–. Useful general formulae for acyclic compounds are:

Series	General formula	Also fits
Alkanes	CₙH₂ₙ₊₂	—
Alkenes	CₙH₂ₙ	cycloalkanes
Alkynes	CₙH₂ₙ₋₂	dienes, cycloalkenes
Monohydric alcohols/ethers	CₙH₂ₙ₊₂O	each other (functional isomers)
Aldehydes/ketones	CₙH₂ₙO	each other
Carboxylic acids/esters	CₙH₂ₙO₂	each other

The degree of unsaturation (DU, also index of hydrogen deficiency) packs that comparison into one calculation:

Halogen X is counted as hydrogen because both are monovalent; nitrogen adds one available hydrogen equivalent; divalent oxygen or sulfur does not alter the count. Benzene, C₆H₆, has DU = 4 — one ring plus three π bonds. DU never tells you which rings or π bonds are present, but it prevents impossible structures when enumerating isomers.

16.2 IUPAC nomenclature of aliphatic compounds and isomerism

A reliable four-pass naming routine

1. Seniority pass. Find the principal functional group. It decides the suffix and should be included in the parent chain. The carbon of –COOH, –CHO or –CN is counted in the chain.
2. Parent pass. Choose the longest continuous carbon chain containing the principal group; if there is a tie, prefer the chain with more multiple bonds, then more substituents.
3. Numbering pass. Give the principal group the lowest possible locant. Next consider multiple bonds (for an enyne, the direction giving the lower locant to the first cited multiple bond, normally the alkene), then substituents as a set. If a set is tied, give the lower locant to the substituent cited first alphabetically.
4. Assembly pass. Write substituents alphabetically with locants, ignoring multiplicative prefixes such as di- and tri- in alphabetization. Separate numbers with commas and numbers from words with hyphens: 3-ethyl-2,4-dimethylhexane.

When a lower-priority group is present it becomes a prefix: –OH is hydroxy- in the presence of a ketone, C=O is oxo- in the presence of a carboxylic acid, –NH₂ is amino-, –OR is alkoxy-, and halogens and –NO₂ are always prefixes in substitutive names. The parent chain name drops its terminal “e” before a vowel-starting suffix (pentan-2-one), while locants for unsaturation are placed immediately before -ene or -yne in modern format (but-2-ene, pent-1-yne).

Isomerism: same formula, different substance

Isomers have the same molecular formula but different structures and therefore at least some different properties. First ask whether atom-to-atom connections differ.

Structural (constitutional) isomerism

• Chain isomerism: different carbon skeleton. C₄H₁₀ is CH₃CH₂CH₂CH₃ (butane) or (CH₃)₃CH (2-methylpropane).
• Position isomerism: same skeleton and functional group at a different position. Butan-1-ol and butan-2-ol; but-1-ene and but-2-ene.
• Functional-group isomerism: different functional family. C₂H₆O is ethanol or methoxymethane; C₃H₆O is propanal or propanone.
• Metamerism: different alkyl groups flank a polyvalent functional atom/group, as in ethoxyethane versus 1-methoxypropane (both C₄H₁₀O), or different substitution patterns around a ketone/secondary amine.
• Tautomerism: rapidly interconverting structural isomers differing chiefly in position of a proton and a π bond, classically keto ⇌ enol. For simple ketones the keto form dominates overwhelmingly. Tautomers are distinct chemical species in equilibrium; resonance forms are not.

Useful alkane counts are C₄H₁₀ = 2, C₅H₁₂ = 3, C₆H₁₄ = 5 and C₇H₁₆ = 9 structural isomers. When counting substituted compounds, work skeleton by skeleton and mark symmetry-equivalent positions only once.

Stereoisomerism

Geometrical isomerism occurs when rotation is restricted — most often about C=C, but also in some rings. Each alkene carbon must carry two different substituents. In but-2-ene, cis has the two CH₃ groups on the same side and trans has them opposite. When all four substituents are different, cis/trans language has no clear reference pair; the E/Z system assigns priority to the two groups on each carbon by atomic number (Cahn–Ingold–Prelog rules). High-priority groups on the same side give Z (zusammen, together); opposite sides give E (entgegen, opposite).

Optical isomerism is the school label for chirality at stereogenic centres. A tetrahedral carbon attached to four different groups lacks a symmetry plane through that arrangement and exists as two enantiomers. Enantiomers rotate plane-polarized light by equal magnitudes in opposite directions and have identical melting points, boiling points and spectra in achiral environments. In a chiral environment — an enzyme pocket, receptor or chiral reagent — they can behave very differently, which is why drug stereochemistry matters. An equal mixture is a racemate and shows no net rotation (external compensation).

Stereoisomers that are not mirror images are diastereomers; they generally have different physical properties and can be separated by ordinary crystallization or chromatography. A meso compound contains stereocentres but also an internal symmetry element, so the molecule is superposable on its mirror image and optically inactive (internal compensation). Thus “has two stereocentres” does not automatically mean “has four optically active forms”. Likewise, R/S is a configuration label assigned from viewing priorities; it does not by itself predict whether rotation is (+) or (−).

16.3 Bond fission, reactive intermediates and electronic effects

Homolytic and heterolytic fission

A covalent bond contains two electrons, so there are only two ways to divide them.

Feature	Homolysis	Heterolysis
Electron division	One electron to each fragment	Both electrons to one fragment
Products	Two free radicals	A cation and an anion
Arrow	Single-barbed “fishhook” arrow	Double-barbed curly arrow
Favoured by	Heat, UV light, peroxides; nonpolar/gas-phase conditions	Polar solvents and polarized bonds; stable ions
Typical chemistry	Radical chain substitution, combustion	Ionic substitution, elimination and addition mechanisms

For A–B heterolysis, the electron pair normally follows the more electronegative atom. In (CH₃)₃C–Br, heterolysis gives (CH₃)₃C⁺ + Br⁻; in aqueous solvent the ions are solvated and the tertiary carbocation is relatively stable. Homolysis of the same bond would give (CH₃)₃C• + Br•.

Carbocations, carbanions and free radicals

Intermediate	Carbon electrons/geometry	Stability pattern	Usual reactivity
Carbocation, C⁺	6 valence electrons; usually planar sp² with empty p orbital	3° > 2° > 1° > methyl; allyl/benzyl resonance strongly stabilizing	Electrophile; rearranges by hydride/alkyl shift if a better cation can form
Free radical, C•	7 valence electrons; near-planar at carbon; one unpaired electron	Benzyl/allyl and 3° > 2° > 1° > methyl (context dependent)	Abstracts atoms, adds to π bonds, combines with another radical
Carbanion, C⁻	8 valence electrons; lone pair, often pyramidal unless resonance flattens it	Methyl > 1° > 2° > 3° for alkyl substitution; resonance and −I/−R groups stabilize	Nucleophile and strong base

Why are tertiary carbocations and radicals relatively stable? Neighbouring C–H/C–C σ bonds can donate electron density into the empty or half-filled p orbital (hyperconjugation), and alkyl groups exert a weak electron-releasing inductive effect. A positive charge or unpaired electron is spread over a larger region. In a carbanion, alkyl donation pushes still more electron density onto an already negative carbon and destabilizes it; electron-withdrawing and resonance-accepting groups (carbonyl, nitrile, nitro) stabilize carbanions by dispersal.

Electrophiles and nucleophiles

An electrophile (“electron-pair loving”, despite acting as an acceptor) is electron-poor: H⁺, NO₂⁺, Br⁺ equivalents, BF₃, AlCl₃, carbocations and the δ+ carbon of a carbonyl or alkyl halide. A nucleophile has an electron pair available: OH⁻, CN⁻, halide ions, NH₃, H₂O, thiolates, carbanions and the π electrons of an alkene/arene. The curly arrow always starts at the nucleophile's electron pair (or a bond) and points to the electrophilic atom/bond — never the reverse.

Basicity and nucleophilicity are not synonyms. Basicity is an equilibrium affinity for a proton; nucleophilicity is a rate of attack at an electrophilic atom, often carbon. Solvent, size and polarizability matter. In protic solvents, larger and less strongly solvated I⁻ is generally more nucleophilic than F⁻ even though F⁻ is more basic. A bulky base such as tert-butoxide can remove a proton yet be a poor nucleophile at crowded carbon.

Inductive effect: a σ-bond tug that fades with distance

If X is more electronegative than carbon in C–X, the bonding electrons sit closer to X. That polarity is relayed weakly through adjacent σ bonds. Electron-withdrawing groups have a −I effect: –NO₂, –CN, –COOH, carbonyl groups and halogens, with halogen order F > Cl > Br > I. Electron-releasing alkyl groups have a modest +I effect relative to hydrogen, increasing roughly tert-butyl > isopropyl > ethyl > methyl. The effect is permanent but becomes small after about three bonds.

This explains carboxylic-acid strength. A −I substituent stabilizes the negatively charged carboxylate ion and pulls the acid equilibrium toward ionization. One chlorine near the acid increases strength, more chlorines increase it further, and moving chlorine farther away weakens the effect. Conversely, +I alkyl groups destabilize a nearby carboxylate slightly and make a carbocation slightly more stable.

Resonance: delocalization through a conjugated π system

Resonance appears when π electrons or a lone pair can overlap through adjacent p orbitals: π–π conjugation in butadiene/benzene, lone pair–π conjugation in phenoxide or an amide, and π–empty-p overlap in an allyl cation. Only electrons move in resonance drawings; atomic nuclei and σ frameworks stay fixed. Each drawing is a contributing structure, and the actual molecule is a lower-energy hybrid with fractional bond orders. Equivalent contributors contribute equally — acetate has two equivalent C–O bonds, benzene has six equal C–C bonds.

A group that donates π/lone-pair density into a conjugated system has a +R (or +M) effect: –NH₂, –OH, –OR and halogens (although halogens are simultaneously −I). A group that withdraws by π conjugation has a −R effect: –NO₂, –CN, –CHO, –COR and –CO₂H when conjugated with the system. Resonance requires orbital alignment; twisting a π system out of coplanarity reduces it.

High-yield consequences are best read from the ion or molecule left after reaction:

• Phenol is more acidic than ethanol because phenoxide is resonance-stabilized by the ring, whereas ethoxide localizes negative charge on oxygen.
• Aniline is a weaker base than an alkylamine because nitrogen's lone pair is delocalized into the benzene ring and less available to bind H⁺.
• Carboxylate is stabilized by two equivalent resonance forms, one reason carboxylic acids are far more acidic than alcohols.
• Amide nitrogen is weakly basic and the C–N bond has partial double-bond character because the nitrogen lone pair conjugates with C=O; this is the electronic basis of the planar peptide bond.
• Allyl and benzyl cations/radicals disperse charge or spin over several carbons and form more readily than ordinary primary systems.

Effect	Path	Distance behaviour	Needs π system?	Classic readout
Inductive	Through σ bonds	Falls rapidly with each bond	No	Chloroacetic-acid strengths
Resonance	Through overlapping p orbitals	Can extend across the conjugated system	Yes (or adjacent p/lone pair)	Phenol acidity, aniline basicity
Hyperconjugation	Neighbouring σ(C–H/C–C) → p/π*	Immediate neighbour	Needs adjacent p/π system	Carbocation and alkene stability

Formula and fact sheet

Topic	Must-know
Carbon framework	Tetravalent, catenating; chains/branches/rings; single/multiple bonds
Group names	Alkyl CₙH₂ₙ₊₁–; phenyl C₆H₅– is aryl; benzyl C₆H₅CH₂– is aralkyl
Homologues	Same functional group/general formula; differ by CH₂ (14 u)
Unsaturation	DU = (2C + 2 + N − H − X)/2; O ignored
IUPAC	Senior group → parent chain → lowest locants → alphabetical substituents
Isomer map	Structural: chain/position/functional/metamer/tautomer. Stereo: geometrical/optical
Chirality	Enantiomers = mirror pair; racemate inactive; meso has internal symmetry
Fission	Homolysis → radicals; heterolysis → ions
Stability	Cation/radical helped by alkyl substitution and resonance; anion helped by withdrawal/resonance
Electronic effects	−I/+I through σ, fades fast; +R/−R through conjugation, needs orbital overlap

Rapid-revision tables and high-yield facts

Item	Rule or value
Alkyl / phenyl	CₙH₂ₙ₊₁ / C₆H₅
Series step	CH₂ (14 u)
Alkane/ene/yne	CₙH₂₊₂ / CH₂ₙ / CH₂ₙ₋₂
Lowest locants	before alphabetical courtesy
Cation stability	3° > 2° > 1° > CH₃⁺
Anion stability	CH₃⁻ > 1° > 2° > 3°
Electrophiles	H⁺, NO₂⁺, BF, SO₃
Nucleophiles	OH⁻, CN⁻, NH₃, H₂O
+M / −M	–OH, –NH₂ / –NO₂, –CHO
Chiral centre	four different groups

Chapter summary

• Carbon's tetravalency, strong self-linking and multiple bonding create an enormous set of stable molecular architectures.
• Functional groups and homologous series convert that diversity into learnable families; alkyl, aryl and benzyl must be distinguished by where the free valence lies.
• Degree of unsaturation counts rings plus π bonds and is the fastest check before drawing or naming structures.
• IUPAC names are built by seniority, parent selection, low-locant numbering and alphabetical assembly.
• Structural isomers differ in connectivity; stereoisomers keep connectivity but differ in arrangement. E/Z, enantiomers, diastereomers, racemates and meso forms each answer a different question.
• Homolysis makes radicals, heterolysis makes ions. Curly arrows track electron pairs from nucleophile to electrophile.
• Alkyl donation and hyperconjugation stabilize carbocations/radicals but destabilize carbanions; resonance can dominate all three.
• Inductive effects fade through σ bonds, while resonance operates only where p orbitals overlap; acid/base and intermediate-strength orders usually reveal which effect controls the system.

Practice questions

Set A — Recall (25 questions)

1. Tetravalency of carbon means that a neutral carbon atom normally formsatwo covalent bondsbone ionic bondcsix covalent bondsdfour covalent bonds
2. Catenation is the ability of an element to formachains and rings by self-linkingbonly ionic crystalschydrogen bonds onlydisotopes
3. The phenyl group has the formulaaC₆H₅CH₂–bCH₃–cC₆H₅–dC₆H₁₁–
4. The group C₆H₅CH₂– is calledaphenylbbenzylctolyldvinyl
5. The general formula of an acyclic alkyne with one triple bond isaCₙH₂ₙ₋₂bCₙH₂ₙ₊₂cCₙH₂ₙdCₙHₙ
6. Successive members of a homologous series differ byaCHbC₂H₂cCH₃dCH₂
7. The suffix for an alcohol as the principal functional group isa-alb-olc-oned-oate
8. The suffix for a carboxylic acid isa-amideb-nitrilec-oic acidd-oyl chloride
9. Ethanol and methoxymethane (C₂H₆O) areachain isomersbfunctional-group isomerscenantiomersdtautomers
10. Butane and 2-methylpropane areachain isomersbposition isomerscgeometrical isomersdmetamers
11. Butan-1-ol and butan-2-ol areachain isomersboptical isomers onlycfunctional isomersdposition isomers
12. Metamers differ in the alkyl groups attached around aacarbon–carbon triple bondbbenzene ring onlycpolyvalent functional atom or groupdchiral centre only
13. Keto–enol tautomerism involves migration of a proton together with relocation of aaπ bondbringchalogen atomdmethyl group
14. Geometrical isomerism in an alkene requires restricted rotation and, at each alkene carbon,atwo identical groupsbone hydrogen only totalctwo different substituentsda triple bond
15. A common chiral centre is a tetrahedral carbon attached toathree identical groupsbfour different groupsctwo double bondsdonly hydrogen atoms
16. A racemic mixture contains equal amounts of twoastructural isomersbhomologuescdiastereomersdenantiomers
17. Homolytic fission of a covalent bond producesatwo ionsbone proton onlyctwo free radicalsda coordinate bond
18. Heterolytic fission of a covalent bond producesatwo radicalsba cation and an anionctwo neutral atoms alwaysdan isotope pair
19. An electrophile is a species thataaccepts an electron pairbdonates an electron pairchas an unpaired electron onlydis always negatively charged
20. A nucleophile is a species thataaccepts a proton onlybis always a metal cationclacks electronsddonates an electron pair to an electrophilic centre
21. A simple carbocation is usually planar because its carbon isasp³ with a lone pairbsp² with an empty p orbitalcsp with two lone pairsdbonded to five groups
22. A carbanion is a carbon species carryingapositive charge and no lone pairbfour bonds and positive chargecone unpaired electron onlydnegative charge and a lone pair
23. The inductive effect is transmitted mainly throughaπ bonds onlybhydrogen bonds in solventcσ bonds and decreases with distancedcrystal lattice energy
24. A group with a strong −I effect isa–NO₂b–CH₃c–C(CH₃)₃d–H
25. Resonance contributing structures differ only in the distribution ofaatomsbisotopescelectronsdσ-bond connectivity

Answers and explanations — Set A

1. (d) — four shared electron pairs complete carbon's octet in a neutral molecule.

2. (a) — strong C–C bonds allow chains, branches and rings.

3. (c) — phenyl is benzene minus one ring hydrogen.

4. (b) — benzyl's free valence is on the CH₂ next to the ring, so it is aralkyl rather than aryl.

5. (a) — one triple bond gives DU = 2 relative to an alkane.

6. (d) — the constant homologous increment is CH₂, 14 u.

7. (b) — replace the final -e of the parent hydride pattern appropriately: propan-2-ol.

8. (c) — for example CH₃CO₂H is ethanoic acid.

9. (b) — an alcohol and an ether share C₂H₆O but have different functional groups.

10. (a) — both are C₄H₁₀ alkanes, but the carbon skeleton changes from straight to branched.

11. (d) — only the –OH locant changes on the same four-carbon skeleton.

12. (c) — classic metamers are ethers/ketones/amines with different alkyl distribution around O, C=O or N.

13. (a) — proton shift is coupled to movement of unsaturation between C=O/C–C and C=C/C–O forms.

14. (c) — if either alkene carbon carries two identical substituents, exchanging sides gives the same molecule.

15. (b) — four different substituents normally remove symmetry at that tetrahedral centre.

16. (d) — equal enantiomers cancel optical rotation by external compensation.

17. (c) — each fragment receives one electron from the bond.

18. (b) — the atom taking both bonding electrons becomes anionic; the other becomes cationic.

19. (a) — electrophiles are Lewis-acidic electron-pair acceptors.

20. (d) — nucleophiles supply the pair used to form the new bond.

21. (b) — the empty p orbital is perpendicular to the plane of the three σ bonds.

22. (d) — a carbanion has an octet with a lone pair and behaves as a base/nucleophile.

23. (c) — σ polarization is relayed weakly and fades after a few bonds.

24. (a) — the nitro group is strongly electron-withdrawing by both induction and resonance when conjugated.

25. (c) — nuclei and σ connectivity stay in place; only π/lone-pair placement changes among contributors.

Set B — Understanding (15 questions)

1. Carbon forms far more discrete compounds than silicon mainly because carbon combines strong self-linking with stableaionic bonds onlybmetallic bondingcmultiple bonds and strong C–H/C–C bondsda complete octet without sharing
2. The benzyl carbocation is unusually stable for a primary cation because its positive charge isaneutralized by the ringbconverted to a lone pairchidden inside the nucleusddelocalized into the benzene π system
3. The order of simple alkyl carbocation stability 3° > 2° > 1° is explained chiefly byahyperconjugation and weak +I donationbionic size alonecstronger C–H bonds in 3° ionsdabsence of electrons
4. Simple alkyl carbanion stability decreases with more alkyl substitution because alkyl groupsawithdraw electronsbdonate electron density to an already negative centrecform hydrogen bonds to carbondremove the lone pair
5. An allyl radical is more stable than a simple primary alkyl radical mainly because the unpaired electron isapaired by oxygenblost as radiationclocated in a σ bondddelocalized over the π system
6. Trichloroethanoic acid is much stronger than ethanoic acid because three Cl atomsamake the molecule heavierbstabilize carboxylate by a strong −I effectcdonate protons themselvesdprevent hydration
7. Phenol is much more acidic than ethanol because phenoxide ion isaa stronger basebunable to hold chargecresonance-stabilized by the ringdprotonated on carbon
8. Aniline is a weaker base than ethylamine because the nitrogen lone pair in aniline isadelocalized into the benzene ringbremoved by oxidationctied in an N–H π bonddshielded by a methyl group
9. Chlorine attached to benzene is overall deactivating in electrophilic substitution because itsa+I effect dominatesb−I withdrawal outweighs lone-pair +R donationcmass slows collisionsdπ bond breaks permanently
10. The E/Z system becomes necessary when an alkene hasafour substituents with no obvious common cis/trans pairbany double bondca ringdonly hydrogen substituents
11. A meso compound can contain stereocentres yet be optically inactive because the molecule hasano carbon atomsba triple bondcequal amounts of two compoundsdinternal symmetry making it superposable on its mirror image
12. Two enantiomers may have identical boiling points yet interact differently with an enzyme because the enzyme binding site isaachiralbmade only of carboncchiraldat absolute zero
13. Iodide can be a better nucleophile than fluoride in a protic solvent although fluoride is more basic, mainly because iodide isalarger, polarizable and less strongly solvatedbsmaller and less polarizablecpositively chargedda free radical
14. A chlorine substituent affects acidity less when it is farther from –CO₂H because inductionatravels only through spacebreverses after one carboncrequires a π bonddweakens rapidly through σ bonds
15. Resonance forms of benzene do not represent two rapidly interconverting cyclohexatrienes because the actual molecule isaone of the drawings at a timeba single delocalized hybrid with equal C–C bondsca mixture of isomersdnonplanar

Answers and explanations — Set B

1. (c) — small carbon atoms form strong σ frameworks and effective pπ–pπ bonds; silicon prefers Si–O networks.

2. (d) — the empty benzylic p orbital overlaps the ring π system, spreading charge to ortho/para positions.

3. (a) — neighbouring C–H σ donation into the empty p orbital becomes more effective with substitution.

4. (b) — electron donation intensifies, rather than relieves, negative charge at carbon.

5. (d) — resonance structures place the radical at either allylic terminus; the hybrid shares it.

6. (b) — the anion's charge is stabilized inductively by three electronegative chlorine atoms.

7. (c) — phenoxide charge is dispersed into the ring; ethoxide has no comparable π reservoir.

8. (a) — conjugation lowers lone-pair availability for protonation; anilinium formation also sacrifices that resonance.

9. (b) — halogens are a useful tug-of-war example: −I controls overall rate, while +R influences orientation.

10. (a) — CIP priority supplies an unambiguous high-priority group on each alkene carbon.

11. (d) — internal compensation within one achiral molecule, unlike a 50:50 racemic mixture.

12. (c) — a chiral receptor forms diastereomeric complexes with two enantiomers, so binding energies can differ.

13. (a) — nucleophilicity is kinetic and solvent-dependent; protic solvation particularly suppresses small F⁻.

14. (d) — each additional σ bond attenuates relayed polarization.

15. (b) — electron delocalization is simultaneous in the hybrid; no nuclei or σ bonds oscillate between Kekulé forms.

Set C — Application (10 questions)

1. The IUPAC name of CH₃–CH(Cl)–CH(CH₃)–CH₂–CH₃ isa4-chloro-3-methylpentaneb2-chloro-3-methylpentanec2-methyl-3-chloropentaned1-chloro-2-methylpentane
2. The IUPAC name of CH₃–CO–CH₂–CH(OH)–CH₃ isa2-hydroxypentan-4-onebpentane-2,4-diolc4-hydroxypentan-2-oned4-oxopentan-2-ol
3. The degree of unsaturation of C₆H₁₀ isa2b1c3d4
4. How many saturated acyclic aldehyde/ketone structural isomers have formula C₄H₈O?a2b5c4d3
5. The number of structural isomers of hexane, C₆H₁₄, isa3b6c5d9
6. Which compound is chiral?abutan-2-olbbutan-1-olcpropan-2-old2-methylpropan-2-ol
7. Which alkene can exist as E/Z isomers?apropeneb2-methylpropenecethenedbut-2-ene
8. Which acid is the strongest?aCH₃CO₂HbFCH₂CO₂HcClCH₂CO₂HdBrCH₂CO₂H
9. Which carbocation is most stable?a(CH₃)₃C⁺bCH₃⁺cCH₃CH₂⁺d(CH₃)₂CH⁺
10. Which carbanion is most stabilized by resonance?atert-butyl anionbbenzyl anioncethyl aniondmethyl anion

Answers and explanations — Set C

1. (b) — the five-carbon chain gives locant set 2,3 from the left; alphabetically chloro is cited before methyl.

2. (c) — ketone outranks alcohol, so the ketone takes locant 2 and –OH becomes the hydroxy prefix at C4.

3. (a) — DU = (2×6 + 2 − 10)/2 = 2: two rings/π bonds in any combination, or one triple bond.

4. (d) — butanal, 2-methylpropanal and butan-2-one.

5. (c) — hexane; 2- and 3-methylpentane; 2,2- and 2,3-dimethylbutane.

6. (a) — C-2 of butan-2-ol carries –OH, –H, –CH₃ and –CH₂CH₃.

7. (d) — each C of but-2-ene carries H and CH₃; the other alkenes have an alkene carbon with identical substituents.

8. (b) — fluorine has the strongest halogen −I effect, stabilizing FCH₂CO₂⁻ most effectively.

9. (a) — the tertiary cation benefits most from hyperconjugation/inductive donation by three methyl groups.

10. (b) — benzyl anion delocalizes its lone pair/negative charge into the aromatic π system; alkyl anions cannot.