The four substitution and elimination pathways (SN1, SN2, E1, E2) each favour different substrates, and knowing which structural features speed up or shut down each pathway is the single most tested skill in this unit. This problem set drills that by asking you to rank real compounds by reactivity under each mechanism, then separately rank nucleophile strength for SN2.
Difficulty: Intermediate | Prerequisites: Lewis structures, electronegativity, basic alkyl halide nomenclature (methyl, primary, secondary, tertiary), familiarity with carbocation stability.
Organic chemistry's substitution and elimination reactions are the gateway to understanding how molecules transform. Every time you see an alkyl halide and a reagent, you need to decide: does the nucleophile attack the carbon (substitution), or does a base strip a proton and form a double bond (elimination)? And within each category, is the mechanism one-step (SN2 / E2) or two-step via a carbocation intermediate (SN1 / E1)?
This problem set asks you to predict which compound in a set reacts fastest, and which reacts slowest or not at all, for each of the four pathways. It also tests whether you can rank nucleophiles for SN2. If you can nail these comparisons, you have the conceptual toolkit for the rest of the substitution/elimination unit.
SN1 (Substitution, Nucleophilic, Unimolecular)
A two-step substitution in which the leaving group departs first to form a carbocation intermediate, and then the nucleophile attacks. The rate depends only on the substrate concentration.
In simple terms: the leaving group leaves on its own, making a positively charged carbon, and the nucleophile swoops in afterwards.
SN2 (Substitution, Nucleophilic, Bimolecular)
A one-step substitution in which the nucleophile attacks the electrophilic carbon at the same time the leaving group departs. The rate depends on both substrate and nucleophile concentrations.
Think of it as a backside hit: the nucleophile comes in from the opposite side of the leaving group in one concerted motion.
E1 (Elimination, Unimolecular)
A two-step elimination in which the leaving group departs first to form a carbocation, and then a base removes a neighbouring proton to form a double bond. Rate depends only on substrate.
In simple terms: same first step as SN1, but instead of a nucleophile attacking, a base pulls off a proton.
E2 (Elimination, Bimolecular)
A one-step elimination in which a strong base removes a proton while the leaving group departs simultaneously, forming a double bond. Rate depends on both substrate and base.
Think of it as a coordinated exit: proton out, leaving group out, double bond in, all at once.
Carbocation
A positively charged carbon atom with only six electrons in its valence shell. Stability order: tertiary > secondary > primary > methyl.
In simple terms: a carbon that has lost its leaving group and is electron-hungry. The more alkyl groups around it, the more stable it is (hyperconjugation and induction donate electron density).
Leaving Group
An atom or group that departs with the bonding electrons during substitution or elimination. Good leaving groups are weak bases (e.g. I⁻, Br⁻, Cl⁻, tosylate).
Think of it as: the better a group can stabilise a negative charge on its own, the more readily it leaves.
Nucleophile
An electron-rich species that donates a pair of electrons to an electrophilic carbon. Strength depends on charge, polarisability, basicity, and solvent.
In simple terms: the "attacker" in a substitution reaction.
Steric Hindrance
The slowing or blocking of a reaction due to bulky groups around the reactive centre. Critical in SN2, where the nucleophile must access the carbon directly.
Think of it as: more stuff around the carbon means less room for the nucleophile to get in.
The rate-determining step is formation of the carbocation, so the only structural factor that matters is how stable that carbocation will be.
Substrate class: Tertiary > secondary >> primary ≈ methyl (primary and methyl essentially do not undergo SN1)
Leaving group quality: Better leaving group = faster departure. I⁻ > Br⁻ > Cl⁻ (bond strength and base stability)
Nucleophile strength: Irrelevant to rate (nucleophile attacks after the rate-determining step)
Solvent: Polar protic solvents stabilise the carbocation intermediate and the departing leaving group, favouring SN1
The three substrates shown are: isopropyl iodide (secondary), a benzylic/allylic iodide on a ring system (secondary, with resonance stabilisation of the carbocation), and a phenyl iodide (vinyl-type, cannot form carbocation).
Fastest: The benzylic substrate, because resonance delocalises the resulting carbocation across the ring, making it far more stable than an ordinary secondary carbocation.
Slowest / will not react: The aryl (phenyl) halide. The carbon bearing the leaving group is sp² hybridised and bonded tightly; a carbocation at an sp² aromatic carbon is extremely unstable. SN1 does not occur here.
Both the nucleophile and the substrate appear in the rate-determining (and only) step, so both matter.
Substrate class: Methyl > primary > secondary >> tertiary (tertiary is essentially blocked by steric hindrance)
Steric environment: The nucleophile must approach the carbon from the backside. Branching at or near the reactive carbon slows or prevents this.
Leaving group quality: Still matters: better leaving group = faster reaction
Nucleophile strength: Strong, good nucleophiles accelerate SN2. Charged nucleophiles are generally stronger than their neutral counterparts.
The substrates shown include a secondary iodide (with a cyclohexane ring) and methyl iodide (CH₃I).
Fastest: CH₃I. Methyl substrates have zero steric hindrance at the reactive carbon, so the nucleophile has unobstructed backside access.
Slowest: The more sterically hindered substrate. A secondary carbon flanked by a bulky ring makes backside attack difficult.
The three nucleophiles are: a carboxylate anion (benzoate, PhCOO⁻), methanethiolate (CH₃S⁻), and methoxide (CH₃O⁻).
Best SN2 nucleophile: CH₃S⁻ (methanethiolate). Sulphur is large and highly polarisable, making it an excellent nucleophile in SN2, particularly in polar protic solvents.
Middle: CH₃O⁻ (methoxide). Strong base and good nucleophile, but smaller and less polarisable than sulphur.
Weakest of the three: The carboxylate (PhCOO⁻). The negative charge is delocalised across two oxygens, reducing its nucleophilic character.
E1 shares the same rate-determining step as SN1 (carbocation formation), so the factors are nearly identical.
Substrate class: Tertiary > secondary >> primary (same carbocation-stability logic as SN1)
Leaving group quality: Better leaving group = faster first step
Base strength: Irrelevant to rate (base acts after carbocation forms)
Temperature: Heat generally favours elimination over substitution
The substrates shown are: a secondary alkyl chloride, a tertiary substrate bearing a CN group, and a secondary/tertiary alkyl iodide.
Fastest: The substrate that forms the most stable carbocation. A tertiary carbon with a good leaving group wins here.
Slowest / will not react: The substrate with CN as the "leaving group" deserves scrutiny. CN⁻ (cyanide) is a poor leaving group because it is a strong base and holds its bond tightly. If the leaving group cannot depart, E1 cannot proceed.
Key point: in E1, the leaving group must actually be able to leave. A poor leaving group shuts down the reaction entirely, regardless of how good the rest of the substrate looks.
E2 is a concerted, one-step mechanism. A strong base removes a proton that is anti-periplanar to the leaving group, and the leaving group departs simultaneously.
Base strength: Must be a strong base (unlike E1). Bulky strong bases (e.g. t-BuOK) especially favour E2 over SN2.
Substrate class: Works across methyl, primary, secondary, and tertiary, but E2 is the dominant pathway for tertiary substrates treated with strong base.
Anti-periplanar geometry: The proton being removed and the leaving group must be anti-periplanar (180° dihedral). In rigid ring systems like norbornane, this geometry may be unavailable.
Leaving group quality: Still matters, same trend
The substrates shown are: an open-chain secondary bromide, a cyclohexane-based bromide, and a norbornyl (bicyclic) bromide.
Fastest: The open-chain secondary bromide, where free rotation easily gives an anti-periplanar arrangement of H and Br.
Slowest / will not react: The norbornyl bromide. In bicyclo[2.2.1]heptane (norbornane) frameworks, the rigid geometry often prevents any beta-hydrogen from achieving the required 180° anti-periplanar alignment with the leaving group. Without that geometry, E2 cannot proceed.
Charge: A negatively charged species is almost always a better nucleophile than its neutral conjugate acid. CH₃O⁻ >> CH₃OH.
Polarisability: Larger atoms with more diffuse electron clouds are more nucleophilic in polar protic solvents. Sulphur beats oxygen, iodide beats fluoride.
Basicity (within the same row of the periodic table): Stronger base = stronger nucleophile when comparing atoms of similar size. CH₃O⁻ > CH₃COO⁻ (methoxide vs. acetate).
Resonance stabilisation: If the lone pair is delocalised (as in a carboxylate), the species is a weaker nucleophile because its electron density is spread out.
Steric bulk: A bulkier nucleophile has more trouble reaching the carbon. t-BuO⁻ is a strong base but a poor nucleophile for SN2.
CH₃S⁻ (best): large, polarisable sulphur atom
CH₃O⁻ (middle): strong base, good nucleophile, but smaller than sulphur
PhCOO⁻ (weakest): negative charge delocalised across two oxygens by resonance
Students often think a "good nucleophile" and a "strong base" are the same thing. They overlap, but polarisability matters for nucleophilicity and does not matter for basicity. CH₃S⁻ is a better nucleophile than CH₃O⁻ but a weaker base.
Students often assume tertiary substrates react fastest in every mechanism. They do for SN1 and E1 (carbocation stability), but tertiary substrates are the worst for SN2 (steric hindrance blocks backside attack).
Students confuse the leaving group with the nucleophile. The leaving group is the atom or group already bonded to the carbon that departs. The nucleophile is the incoming species.
Students forget that E2 requires anti-periplanar geometry. Simply having a beta-hydrogen is not enough; it must be positioned at 180° to the leaving group. This is why rigid ring systems like norbornane can block E2 entirely.
⚠️ Ranking substrates by reactivity for each of the four mechanisms is a staple exam question. You will almost certainly see a version of "which reacts fastest / slowest" on the midterm.
⚠️ The SN1 vs. SN2 decision tree (substrate class + nucleophile + solvent) is tested in multiple formats: multiple choice, short answer, and mechanism drawing.
⚠️ Nucleophile ranking questions for SN2 show up frequently. Remember the polarisability vs. basicity distinction.
⚠️ Anti-periplanar geometry in E2 is a favourite "trick question" topic. If the geometry is locked (bridged bicyclics, certain chair conformations), E2 cannot occur regardless of how strong the base is.
True or false: SN1 reactions proceed fastest with methyl substrates. Answer: False. SN1 requires a stable carbocation. Methyl carbocations are the least stable. Tertiary substrates are fastest.
Fill in the blank: In SN2, the nucleophile attacks from the ______ side of the leaving group. Answer: Back (opposite) side.
True or false: E1 and SN1 share the same rate-determining step. Answer: True. Both begin with departure of the leaving group to form a carbocation.
Fill in the blank: E2 requires the hydrogen and leaving group to be in a(n) ______ arrangement. Answer: Anti-periplanar (180° dihedral).
True or false: CH₃O⁻ is a better SN2 nucleophile than CH₃S⁻ in polar protic solvent. Answer: False. Sulphur is larger and more polarisable, making CH₃S⁻ the better nucleophile in protic solvents.
Q: You are given tert-butyl bromide, isopropyl bromide, and ethyl bromide. Rank them from fastest to slowest for an SN1 reaction in water. Explain.
A: Tert-butyl bromide > isopropyl bromide > ethyl bromide. SN1 rate depends on carbocation stability: tertiary > secondary > primary. Water is a polar protic solvent that stabilises the carbocation and leaving group, further supporting SN1.
Q: Why does methyl iodide react faster than cyclohexyl iodide in an SN2 reaction with NaCN?
A: Methyl iodide has no substituents on the carbon bearing the leaving group, so the nucleophile (CN⁻) has unimpeded backside access. Cyclohexyl iodide is a secondary substrate with significant steric hindrance from the ring hydrogens, slowing backside attack.
Q: A student claims that norbornyl bromide will readily undergo E2 with potassium tert-butoxide. Is this correct?
A: No. In the norbornane (bicyclo[2.2.1]heptane) framework, the rigid bridged structure prevents any beta-hydrogen from achieving the required anti-periplanar geometry with the bromine leaving group. E2 cannot proceed regardless of base strength.
Q: Rank these nucleophiles for SN2 reactivity in methanol: I⁻, Cl⁻, F⁻. Explain your reasoning.
A: I⁻ > Cl⁻ > F⁻. In a polar protic solvent, smaller anions (F⁻) are more strongly solvated, reducing their effective nucleophilicity. Iodide is large and polarisable, weakly solvated, and therefore the strongest nucleophile of the three.
Q: An alkyl halide undergoes both SN1 and E1 when dissolved in hot ethanol. What structural class is the substrate most likely?
A: Tertiary. SN1 and E1 both require a stable carbocation intermediate, which points to a tertiary substrate. Heat favours elimination (E1) over substitution (SN1), and ethanol is a weak nucleophile/base consistent with unimolecular pathways.
This material connects directly to reaction mechanisms and energy diagrams (covered in the companion study notes for Q2 of this problem set), where you draw the arrow-pushing steps and energy profiles for specific SN1 and SN2 reactions.
It also feeds forward into alcohol and ether synthesis, since many of those reactions rely on SN2 displacement of halides by alkoxide nucleophiles, and into alkene synthesis via E2 and E1 pathways. Understanding which mechanism dominates for a given substrate/reagent combination is the foundation for predicting products in those later chapters.
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