Nucleophilic Substitution

01 / UNDERSTAND & APPLY

1. The same net replacement can occur by different pathways

In a nucleophilic substitution, an electron-pair donor replaces a leaving group at an electrophilic centre. For an alkyl halide, the elementary net pattern is Nu⁻+R–X→R–Nu+X⁻. This equation alone does not tell us whether bond formation and cleavage occur together or in separate steps. Kinetics, stereochemistry, rearrangement and solvent response supply mechanistic evidence.

Classify the carbon directly bonded to halogen. Methyl has no carbon neighbours; primary, secondary and tertiary saturated carbons have one, two or three. The classification does not count the halogen, hydrogen atoms or all carbons in the molecule. A molecule with many carbons can still have a primary CH₂X group.

Allylic halides have X on a saturated carbon adjacent to C=C; benzylic halides have X on a saturated carbon adjacent to an aromatic ring. Vinylic halides have X directly on an alkene carbon; aryl halides have X directly on an aromatic carbon. Moving the halogen by one bond can change the available mechanism drastically.

A substrate, nucleophile, solvent, leaving group and temperature form a reaction system. Statements such as “secondary means SN1” or “strong nucleophile means SN2 for every substrate” ignore competition and constraints. The aim is to assemble compatible evidence, not choose a mechanism from one isolated label.

02 / UNDERSTAND & APPLY

2. Naming haloalkanes: parent, locants and substituent order

For a simple saturated haloalkane, choose the appropriate longest carbon chain and treat fluoro, chloro, bromo and iodo as substituent prefixes. Number the chain to give the lowest substituent locant sequence at the first point of difference, considering alkyl and halo substituents together. A halogen does not automatically get the lowest number simply because the chapter is about halides.

List different substituent prefixes alphabetically. Multipliers such as di and tri do not control that alphabetical order. If two numberings give exactly the same locant set in a simple acyclic case, the substituent cited first alphabetically receives the lower locant at the relevant tie. Write commas between numbers and hyphens between numbers and words.

CH₃CH(Br)CH₂CH(CH₃)CH₂CH₃ is 2-bromo-4-methylhexane. Numbering from the opposite end would give 5-bromo-3-methylhexane, with the less favourable locant sequence 3,5 rather than 2,4. The carbon bearing bromine is secondary because it bonds to two carbon atoms, regardless of the methyl branch elsewhere.

For CH₃CH(Cl)CH₂CH(Br)CH₃, both directions produce locants 2,4. Bromo alphabetically precedes chloro, giving 2-bromo-4-chloropentane. When an alcohol or another senior characteristic group is present, its suffix and numbering requirements take precedence: HOCH₂CH(Br)CH₂CH₃ is 2-bromobutan-1-ol, not a halide name that ignores the OH function.

Common functional-class names such as ethyl bromide and isopropyl chloride correspond to bromoethane and 2-chloropropane. Benzyl chloride means C₆H₅CH₂Cl, whereas chlorobenzene means C₆H₅Cl. A geminal dihalide has two halogens on the same carbon; a vicinal dihalide has them on adjacent carbons. Thus 1,1-dibromoethane and 1,2-dibromoethane are different constitutional isomers.

For a haloalkene, retain the unsaturated parent and the required double-bond locant. CH₂=CHCH₂Br is 3-bromoprop-1-ene, an allylic bromide, while CHBr=CH₂ is bromoethene, a vinylic bromide. Stereodescriptors such as R/S or E/Z are added only when the structure actually defines that stereochemistry; an unspecified sketch does not justify inventing a descriptor.

03 / UNDERSTAND & APPLY

3. C–X polarity, length and strength are different properties

Halogens are more electronegative than carbon in the common C–X bonds, so the carbon is relatively electron-poor and the halogen relatively electron-rich. This polarity helps make the carbon susceptible to nucleophilic attack. A bond dipole is a vector and a molecular dipole is the sum of all contributions; even bond dipole magnitude depends on charge separation and bond length, not electronegativity alone.

For comparable alkyl halides, C–F is shortest and strongest, followed by progressively longer and generally weaker C–Cl, C–Br and C–I bonds. The very strong C–F bond and poor leaving-group behaviour of F⁻ make ordinary alkyl fluorides relatively resistant to common substitution. The most polar bond is therefore not automatically the most readily displaced.

For similar saturated substrates under comparable ordinary substitution conditions, leaving-group ability commonly follows I⁻>Br⁻>Cl⁻≫F⁻. The departing group must accommodate the bond electron pair. Basicity, polarizability, solvation and the breaking bond all matter. Hydroxide is a poor leaving group unless activated, for example by protonation so that water leaves.

Aryl C–X bonds differ from ordinary alkyl C–X bonds. The carbon has sp² character and can interact conjugatively with halogen lone pairs, contributing to a shorter, stronger bond in familiar comparisons. Ordinary aryl-cation formation is highly unfavourable, and the usual saturated-carbon backside substitution geometry is unavailable. These features explain resistance to normal SN1/SN2 without implying that all aryl substitution is impossible.

Halogen substitution often raises molecular mass and polarizability, influencing boiling points, but branching and molecular shape also matter. Haloalkanes can be polar yet only sparingly soluble in water because they do not replace water’s hydrogen-bond network favourably enough. Physical trends need matched structures; they do not independently predict the dominant reaction mechanism.

04 / UNDERSTAND & APPLY

4. SN2 forms and breaks bonds in one elementary step

In the ordinary SN2 mechanism, a nucleophile approaches opposite the leaving group, along the backside of the C–X bond. Its electron pair begins to form a C–Nu bond as the C–X bond electron pair moves onto X. Draw one curved arrow from the nucleophile’s electron pair to carbon and another from the C–X bond to X. An arrow from positive carbon toward the nucleophile would point in the wrong electron-flow direction.

The reaction has a transition state with partial Nu–C and C–X bonds rather than a stable five-coordinate carbon intermediate. The other three substituents pass through an approximately planar arrangement as the centre inverts. There is no free carbocation in this elementary pathway and therefore no ordinary carbocation rearrangement stage.

At a stereogenic reacting carbon, backside substitution gives inversion of spatial configuration. It does not universally guarantee that the written descriptor changes R to S, because the incoming group can change CIP priorities. In cases where the relative priority ordering is retained, R/S does switch. Track geometry first and assign the product descriptor again.

The elementary rate law is rate=k[RX][Nu]. Doubling either concentration doubles the rate while holding the other conditions fixed; doubling both quadruples it. If nucleophile is maintained in a large effectively constant excess, the observed substrate decay can be pseudo-first-order even though the mechanism remains bimolecular. Apparent first-order kinetics alone therefore do not prove SN1.

05 / UNDERSTAND & APPLY

5. SN2 competition depends on steric access and the nucleophile

Backside access is easiest for methyl and many primary substrates, more hindered for secondary substrates, and strongly blocked for ordinary tertiary saturated substrates. A typical matched order is methyl>primary>secondary≫tertiary. Branching near a primary CH₂X centre can still slow reaction greatly; neopentyl halides are a familiar warning against using “primary” as the whole analysis.

An unhindered benzylic or allylic substrate can react rapidly by SN2 because the transition state benefits from conjugative stabilization. This is different from a vinylic or aryl halide with X directly on an sp² carbon. Do not treat benzyl bromide and bromobenzene as equivalent just because both contain a benzene ring.

Nucleophilicity describes rate of attack at an electrophile; basicity describes an equilibrium tendency to accept a proton. Strong bases are often good nucleophiles, but steric bulk and polarizability can separate the two properties. A bulky alkoxide can favour beta-hydrogen abstraction and E2 elimination rather than approach a crowded carbon for substitution.

Polar aprotic solvents often increase the reactivity of anionic nucleophiles compared with strongly hydrogen-bonding protic media because the anion is less tightly solvated. The cation, ion pairing and actual solubility still matter. In protic solvents, small hard anions can be strongly hydrogen-bond solvated; nucleophilicity orders down a group can therefore differ from those in aprotic solvents.

A solvent label does not override a forbidden substrate geometry. Moving a tertiary alkyl halide into an aprotic solvent does not automatically create an ordinary SN2 pathway. Likewise, a nucleophile must actually be available in solution; precipitation, protonation or strong association can change its effective concentration.

06 / UNDERSTAND & APPLY

6. SN1 begins with ionization and may require a final proton transfer

In a classical SN1 solvolysis, the C–X bond ionizes to a carbocation-containing ion pair and leaving-group anion. The nucleophile then captures the electron-poor carbon. If a neutral nucleophile such as water attacks, the immediate product is protonated and a subsequent proton transfer gives the neutral alcohol. Omitting this final deprotonation leaves an incorrect product charge.

When ionization is rate-limiting and the usual model applies, rate=k[RX]. Nucleophile concentration does not appear in that simple rate law because the slow step precedes capture. Altering the solvent or adding salts can nevertheless alter k, ion-pair behaviour and competing pathways, so “nucleophile never affects anything in SN1” is too strong.

Substrates that can form stabilized carbocations favour ionization: tertiary centres and resonance-stabilized benzylic/allylic centres are familiar examples. Ordinary methyl and unstabilized primary carbocations are very unfavourable. A polar ionizing solvent stabilizes charged species and can support solvolysis with a weak neutral nucleophile.

A planar carbocation centre can be attacked from either face, producing loss of stereochemical purity. Ideal equal access gives racemization for a suitable single-centre substrate, but ion pairs and a departing group near one face can favour inversion or otherwise give unequal amounts. SN1 is therefore associated with racemization tendencies rather than a universal exact 50:50 product ratio.

The energy profile has an intermediate valley between ionization and capture barriers; if proton transfer is shown as another elementary step, additional features can occur. Do not represent this multistep route as the same one-peak profile as elementary SN2. Relative peak heights in a sketch must be referenced to the relevant preceding states when discussing individual activation barriers.

07 / UNDERSTAND & APPLY

7. Carbocations can rearrange, and bases can eliminate

Once a carbocation exists, a neighbouring hydride or alkyl group may migrate with its bonding electron pair if a more stable cation or favourable ring arrangement results. The migrating group comes from an adjacent carbon; it is not a free hydride floating in solution. The positive charge moves to the carbon that lost the migrating bond.

For example, ionization at C2 of CH₃CH(Br)CH(CH₃)CH₃ would initially give a secondary cation adjacent to a carbon bearing H and two methyl groups. A1,2-hydride shift from that adjacent carbon can generate a tertiary cation. Capture can then give the rearranged tertiary alcohol connectivity, rather than only replacement of Br at its original carbon.

This example illustrates an available pathway, not a promise of one pure product under every solvent and temperature. Direct capture, rearrangement and elimination compete. A clean concerted SN2 pathway has no corresponding free-carbocation interval and ordinarily retains the original carbon skeleton.

E1 can share the ionization step of SN1 and then lose a beta proton to form an alkene. E2 instead removes a beta proton while the leaving group departs in a concerted event, commonly favoured by a strong base with an appropriate substrate and geometry. Higher temperature often increases the relative importance of elimination, but the outcome still depends on the full system.

Use the substrate first: methyl halides cannot undergo ordinary beta elimination because no beta carbon exists. Tertiary substrates strongly resist ordinary SN2, and strong base commonly directs them toward E2. Secondary substrates require the most careful comparison of nucleophile/basicity, solvent and steric effects; a single universal product rule is inadequate.

08 / UNDERSTAND & APPLY

8. Identify which atom of the nucleophile forms the bond

Hydroxide substitution gives an alcohol; an alkoxide gives an ether; ammonia can give an amine after deprotonation, with further alkylation possible. Cyanide substitution through carbon gives a nitrile R–C≡N, adding one carbon to the organic skeleton. Hydrolysis of that nitrile retains the added carbon as a carboxyl carbon.

Ambident nucleophiles have more than one potential bonding atom. Nitrite can bond through oxygen to give an alkyl nitrite R–O–N=O or through nitrogen to give a nitro compound R–NO₂. Cyanide can give nitrile or isocyanide connectivity depending on reagent form and conditions. Elementary reagent comparisons involving alkali-metal versus silver salts illustrate that counterion and bonding mode can affect the product, but a universal solvent-independent rule would be too strong.

A halide-exchange reaction such as the familiar iodide displacement on a suitable alkyl chloride or bromide can be driven by the medium and precipitation of another salt. The reaction is not favourable solely because the new C–I bond is strongest; it is not. Solvation and phase equilibria contribute to the overall driving force.

When predicting a product, write the nucleophile’s attacking atom explicitly and count carbons. “Replace X by CN” can hide whether the bond is C–C≡N or C–N≡C. “Replace X by OH” must also specify whether the product is initially protonated after neutral-water attack. Correct connectivity and charge come before naming.

09 / UNDERSTAND & APPLY

9. Aryl substitution has its own mechanisms

Unactivated chlorobenzene does not undergo the ordinary SN1/SN2 chemistry of a primary alkyl chloride. An aryl cation is highly unstable in the simple ionization model, and the normal backside trajectory at a saturated tetrahedral carbon is not available. Resistance under ordinary conditions is not the same as absolute chemical inertness.

In the classic addition–elimination SNAr model, a nucleophile adds to the carbon bearing the leaving group, forming a nonaromatic anionic sigma intermediate. An electron-withdrawing group such as nitro at an appropriate ortho or para position can stabilize that intermediate by resonance. Loss of halide restores aromaticity. The order of events is addition then elimination, rather than initial formation of a carbocation.

A meta nitro group does not provide the same direct resonance stabilization to the usual intermediate. Multiple electron-withdrawing groups can increase activation further. These are trends for the specified classic mechanism; other substrates and modern pathways can fall outside the elementary rule that every aryl substitution needs an ortho/para nitro group.

The leaving-group rate trend can differ from ordinary alkyl SN2. In addition-controlled activated aryl substitution, aryl fluorides can be particularly reactive because their electronic effect promotes the addition step. Do not transplant I>Br>Cl>F mechanically into every aromatic mechanism.

A different elimination–addition pathway can form a benzyne intermediate under suitable strong-base conditions when an appropriate ortho hydrogen is available. Nucleophile addition to either end of the strained aryne unit, followed by protonation, can give positional outcomes unlike simple direct replacement. The formal triple bond in a six-membered ring is a strained bonding description, not an ordinary linear alkyne segment.

10 / UNDERSTAND & APPLY

10. Prepare haloarenes by ring substitution or diazonium replacement

Electrophilic aromatic halogenation replaces a ring hydrogen with halogen. Benzene bromination commonly uses Br₂ with a suitable Lewis-acid catalyst such as FeBr₃, and chlorination has an analogous Cl₂/Lewis-acid pattern. The catalyst polarizes the halogen reagent; the ring attacks an electrophilic halogen species, forms a sigma complex and loses H⁺ to restore aromaticity.

For an already substituted ring, directing and activating effects control the likely positions. Methyl directs mainly ortho/para, while nitro directs meta and deactivates. A halogen already on the ring is deactivating overall but ortho/para directing. This combination is not contradictory: inductive withdrawal slows reaction while lone-pair donation stabilizes selected sigma-complex positions.

Benzylic radical halogenation is a different operation. In a toluene-type substrate, radical conditions can replace a side-chain hydrogen and produce a benzyl halide. Electrophilic ring-halogenation conditions instead give a haloarene with X attached directly to the ring. Specifying only “toluene plus chlorine” leaves a crucial condition-dependent distinction unresolved.

Diazotization of a primary arylamine gives an arenediazonium intermediate under appropriate controlled aqueous conditions. Replacement using CuCl or CuBr gives aryl chloride or bromide in the Sandmeyer transformation. Iodide can replace the diazonium group without the same copper requirement. The aryl carbon that originally bore NH₂ becomes the carbon bearing halogen, preserving positional information.

The Balz–Schiemann transformation uses the corresponding arenediazonium tetrafluoroborate relationship to form an aryl fluoride with loss of nitrogen and BF₃ in the conventional net scheme. Direct fluorination with elemental fluorine is not the routine mild analogue of bromination. Diazonium salts and fluorinating systems can be hazardous; these are conceptual reaction maps, not isolation or operating instructions.

Diazonium replacement is not ordinary saturated-carbon SN2, and copper-mediated Sandmeyer chemistry involves electron-transfer/radical pathways rather than a simple freely solvated aryl carbocation. Preserve this distinction when drawing mechanisms. A diazonium route can access substitution patterns difficult to obtain selectively by direct ring halogenation.

11 / UNDERSTAND & APPLY

11. Integrate evidence rather than memorize isolated rankings

For an unhindered primary halide with a good anionic nucleophile and a suitable aprotic medium, ordinary SN2 is a strong starting hypothesis. For a tertiary substrate in an ionizing protic solvent with a weak nucleophile, SN1/E1 competition is plausible. A tertiary substrate with strong base commonly favours E2 instead. An activated aryl halide requires an aromatic substitution analysis.

Benzylic and allylic substrates can favour more than one pathway: both carbocation and SN2 transition-state stabilization can be relevant. Determine whether the reaction conditions favour ionization or direct attack. Resonance stabilization does not force a universal single mechanism.

If a problem supplies inversion with no rearrangement and a rate dependent on both substrate and nucleophile, the combined evidence supports SN2. If it supplies ionization-controlled kinetics, rearranged products and substantial loss of optical purity, SN1 becomes more plausible. Each observation has qualifications, but together they constrain the explanation.

Finally check products against conservation: carbon count, charge, proton transfers and leaving-group fate. A mechanism arrow begins at an electron pair or bond, never at empty space. Transition states have partial bonds and are not isolable intermediates. A plausible-looking organic product with impossible valence is still wrong.

12 / UNDERSTAND & APPLY

12. Solved practice

All cases use the ordinary mechanisms and scope stated in the question. No predicted product is an instruction to mix hazardous reagents.