A JC2 H2 Chemistry tutor can prepare a clearer curly-arrow question by checking where each electron pair starts, where it ends and which bonds and charges result. The arrows represent electron movement in a mechanistic model. A correct product alone does not establish that the route shown is chemically consistent.
For Hougang students, this audit can identify a smaller difficulty within an apparently large organic-Chemistry problem. A student may recognise nucleophilic substitution but start the arrow at a positive centre, omit the leaving-group bond arrow or lose a formal charge in a proton-transfer step.
This guide uses original paper-based mechanism audits for bromoethane substitution and related supplied examples. The 2027 H2 syllabus is the course reference. Match the mechanisms and conditions to what has been taught; the descriptions and endpoint table support drawing practice rather than provide practical reaction instructions.
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Full chapter index
Chapters 1–3: purpose, definitions and first example
Chapters 4–7: interpretation, comparisons and limits
Chapters 8–10: feedback, transfer and continuation
CHAPTER 1 OF 10 · Define the teaching decision
1. Define the electron-movement decision
Write the task outcome as a sequence of checks: identify an available electron pair, identify an accepting site, show the relevant bond changes and verify the resulting structure and total charge. Each arrow should have a purpose within the stated step.
Start with a familiar supplied reaction and a small number of arrows. A complete multistep synthesis may hide whether the student’s first problem is electron donation, leaving-group departure or proton accounting.
Ask the student to explain an arrow in words before correcting its shape. It should describe electrons moving from a specified lone pair or bond to a specified destination. Saying that the atom attacks is not enough to locate the arrow tail.
Prepare the teacher key with explicit lone pairs where an arrow begins. Include formal charges and relevant bond dipoles. The student copy should provide enough structural detail to infer the intended electron source.
Keep overall reaction arrows and curly arrows distinct. The overall arrow summarises a chemical transformation; a curly arrow tracks an electron pair or, with the appropriate half-headed convention, a single electron within the proposed mechanism.
CHAPTER 2 OF 10 · Define the teaching decision
2. Check the nucleophile and electrophilic site
For the main task, supply bromoethane, CH₃CH₂Br, and hydroxide ion, OH⁻, in a stated aqueous nucleophilic-substitution context. The oxygen lone pair is the donating pair, and the carbon bonded to bromine is the accepting site.
The C–Br bond is polarised with the carbon relatively electron-deficient and bromine relatively electron-rich. Marking Cδ⁺–Brδ⁻ can help the student select the correct carbon without turning partial charges into full ionic charges.
Hydroxide is a nucleophile because it can donate an electron pair to form a bond. Its negative charge is helpful evidence here, but nucleophiles need not all be negatively charged. Neutral ammonia will provide a later comparison.
Use a full-headed curly arrow for an electron pair. A half-headed arrow describes one electron and belongs to an appropriate radical account. Do not mix the two conventions simply because a leaving group is present.
Ask the student to point to the actual pair on oxygen and the actual C–Br bond before drawing. This checks whether the mechanism is being read from electrons and bonding rather than copied as an arrangement of arrows near familiar letters.
CHAPTER 3 OF 10 · Follow the worked reasoning
3. Audit the two arrows in the concerted substitution
In the supplied SN2 model, draw one electron-pair arrow from an oxygen lone pair on OH⁻ to the carbon bonded to bromine. This describes formation of the new C–O bond.
Draw the second electron-pair arrow from the C–Br bond to Br. The leaving bromine receives that bonding pair and becomes Br⁻. The arrow begins in the bond, not at the carbon nucleus.
These bond formation and bond breaking changes occur in one concerted step in this model. They are not two successive stable intermediates in which the carbon first carries a completed extra bond and bromine only leaves later.
The product account is CH₃CH₂OH + Br⁻. The original O–H bond remains, while the new bond connects oxygen to the ethyl carbon. Hydroxide does not need to lose its hydrogen in order to replace bromine in this worked step.
Use a transition-state representation only if it has been taught or supplied. Partial bonds and bracketed transition states should not be confused with an isolable species having a normal fifth bond around carbon.
CHAPTER 4 OF 10 · Follow the worked reasoning
4. Check atoms, valency and charge after the step
The overall supplied change is CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻. Carbon, hydrogen, oxygen and bromine atoms balance. The total charge is −1 on each side.
In ethanol, oxygen has two ordinary covalent bonds and two lone pairs in the usual Lewis account, with no formal charge. Bromide carries the negative charge after receiving the C–Br bonding pair.
If the product is drawn as neutral bromine while ethanol is also neutral, the overall negative charge has disappeared. Ask which electron pair became associated with the leaving group and restore the appropriate product charge.
If the ethyl carbon is shown with both a completed C–O bond and an unchanged C–Br bond as a stable product, inspect its valency. The endpoint of the supplied step requires replacement, not permanent addition to a saturated carbon.
Ask the student to check the product independently from the arrows. Electron movement, the proposed structure and the charge account should agree. A memorised product with inconsistent arrows still needs a mechanism repair.
Marking partial charges does not change the net charge of bromoethane. Its Cδ⁺ and Brδ⁻ labels describe unequal sharing within a neutral molecule. Treating δ⁺ as a full carbocation charge would alter both the species inventory and the mechanism being evaluated.
The table lists the source and destination for each electron-pair arrow in the main substitution and the later ammonia example. It supports the student’s drawn mechanism by making each arrow’s meaning explicit.
An origin should be an occupied lone pair or bond in the supplied structure. An arrow starting at a positive charge without an electron source does not describe donation of that missing pair.
A destination may be an atom where a bond forms, a bond region in an appropriate representation, or an atom receiving a breaking bond’s electrons. The key should specify which interpretation applies.
Do not treat arrows as trajectories showing nuclei flying between positions. Atoms and groups may change bonding or arrangement during the reaction, but the notation records the electron account of those changes.
Ask the student to cover the destination column and reconstruct it from the mechanism, then cover the source column. This separates an electron-source difficulty from an incorrect choice of electrophilic site.
| Supplied step | Electron-pair origin | Destination | Result described |
|---|---|---|---|
| OH⁻ substitution | O lone pair | Carbon bonded to Br | New C–O bond |
| Leaving-group departure in same step | C–Br bond | Br atom | Br⁻ forms |
| NH₃ substitution | N lone pair | Carbon bonded to Br | Ethylammonium product initially forms |
| Ammonia accepts a proton | Second NH₃ lone pair | H on ethylammonium N | New N–H bond in NH₄⁺ |
| Proton loss from ethylammonium | Original N–H bond | Ethyl-bound N | Neutral ethylamine forms |
Supply an analogous initial substitution of bromoethane by ammonia. A nitrogen lone pair on NH₃ donates to the carbon bonded to bromine, while the C–Br bonding pair moves to Br in the supplied substitution model.
The immediate product is CH₃CH₂NH₃⁺ with Br⁻. Nitrogen now has four bonds and a positive formal charge. Writing neutral ethylamine at this first stage loses the extra proton and obscures the subsequent acid–base step.
A second ammonia molecule can accept a proton from the ethylammonium ion in the supplied next step. Draw an arrow from the second nitrogen’s lone pair to that proton and an arrow from the original N–H bond back to the ethyl-bound nitrogen.
The products of this proton transfer are CH₃CH₂NH₂ and NH₄⁺, with bromide remaining a spectator in the overall account. Total charge and all hydrogen atoms are conserved across the stated sequence.
The sequence illustrates electron and proton bookkeeping. It does not establish that every real mixture gives only one alkylation product. Further reactions and product distributions require their appropriate conditions and evidence.
The net bookkeeping for the ammonia sequence is CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄⁺ + Br⁻. Both sides have the same atoms and total charge zero. This combined equation does not replace the two-step electron account, but it is a useful final conservation check.
CHAPTER 7 OF 10 · Repair and transfer
7. Distinguish an intermediate from a transition state
An intermediate is a species in a multistep mechanism with a finite lifetime between steps, while a transition state corresponds to the barrier region of a step. The drawing conventions should not imply that they are interchangeable.
In the supplied concerted bromoethane substitution, do not insert a free primary ethyl carbocation simply to make the arrows look sequential. That would change the proposed mechanism and needs a separate chemical justification.
For a separate supplied carbocation-capture example, give a carbocation already present and a nucleophile with an available lone pair. An arrow from that pair to the positive carbon describes bond formation. The species inventory differs from the concerted starting point.
The same electron-donation principle can apply across different mechanisms, but the order of steps and available species must match the stated model. Correct arrow direction does not by itself validate an invented intermediate.
Keep mechanism labels proportionate to the task. If the student has not yet been taught kinetic distinctions, explain the supplied step sequence without demanding an untaught rate-law derivation.
If the arrow points from the electrophilic carbon toward hydroxide, ask where the donated electrons are located before the step. Move the tail to the oxygen lone pair and the head to the accepting carbon.
If the C–Br departure arrow is missing, ask how the carbon avoids retaining both the incoming and leaving bonds as a stable product. The repair completes the bond-breaking account in the same supplied concerted step.
If the departure arrow begins at Br and points into the C–Br bond, ask where that bonding pair ends up when bromide leaves. The required arrow starts at the bond and ends on Br.
If ammonia gives an uncharged four-bond nitrogen, count nitrogen’s bonds and electron ownership in the immediate adduct. Restore the positive charge before considering proton transfer.
If a proton-transfer arrow begins at H⁺ rather than the accepting lone pair, ask which species donates the electron pair that forms the new bond. Then show where the old X–H bond electrons go when that bond breaks.
When a student has one correct arrow and one incorrect charge, preserve the correct electron-source decision in the feedback. Ask them to reconstruct the resulting structure from their own arrows before showing the key. This distinguishes a drawing-memory gap from a failure to track electron ownership.
CHAPTER 9 OF 10 · Plan the next lesson
9. Check independent transfer with a changed substrate
Give 1-bromopropane and hydroxide in the same supplied substitution context. Ask for the electron-pair origins, accepting carbon, product and charge account without the earlier endpoint table.
The oxygen lone pair goes to the carbon bonded to bromine, and the C–Br bond pair goes to Br. The product is propan-1-ol with bromide, retaining the same overall charge of −1.
The longer chain should not move the accepting site to the middle carbon. Ask the student to identify the carbon attached to the leaving group before drawing, rather than choose a position because it looks central in the formula.
Add a fresh proton-transfer prompt with a supplied ammonium-type ion and a neutral base. Require both the bond-forming and bond-breaking arrows, then check the resulting charges and hydrogen count.
For a parent update, describe whether the student can now name each electron source and verify the product charge. Mention if step sequencing or lone-pair recognition still needs prompting, so that the next mechanism task has a clear target.
Check the substrate, reagent, intended reaction context and step sequence. Verify that the student copy provides the lone pairs, charges or structural information needed to reconstruct the key.
Inspect every arrow tail and head separately, then reconstruct the resulting bonds and formal charges. The endpoint table is a tutor checking aid; it should agree with the full drawn mechanism used in teaching.
Choose repair when electron origins are insecure, stabilise with a changed primary halogenoalkane and extend through a taught neutral nucleophile or multistep mechanism when the basic electron account is independent.
Keep a stable product, a supplied intermediate and a transition-state representation distinct. Do not add a carbocation or a separate departure step merely to match a memorised picture from another reaction family.
Retain one original drawing, its corrected electron-source explanation and a fresh independent mechanism. These show whether the student can use curly arrows as a connected chemical account rather than decorative marks around the right product.
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Frequently asked questions
Where should a curly arrow start?
At the electron pair being donated, usually a shown lone pair or a bond in the supplied structure.
Does a full-headed arrow move one electron?
It represents an electron pair. A half-headed convention represents one electron in an appropriate radical mechanism.
Why is the C–Br bond arrow needed?
It accounts for the bond pair moving to bromine as the leaving group departs and the new C–O bond forms in the supplied concerted step.
Can a neutral molecule be a nucleophile?
Yes. Ammonia can donate its nitrogen lone pair, giving a positively charged four-bond nitrogen in the immediate substitution product.
Is a transition state the same as an intermediate?
No. A transition state belongs to a step’s barrier region; an intermediate is a species between steps in a multistep mechanism.
Does the correct product prove the mechanism is correct?
No. Check the electron sources, destinations, sequence, valencies and charges as well as the product identity.
Tutor preparation checklist
| Check | Before the lesson |
|---|---|
| Context | State the substrate, reagent and intended mechanism model. |
| Origins | Locate each occupied lone pair or bond before drawing. |
| Destinations | Check the accepting atom and leaving-group electron account. |
| Products | Reconstruct bonds, hydrogen counts and formal charges. |
| Transfer | Use a changed substrate or a supplied proton-transfer step. |
Continue with connected teaching guides
- JC2 H2 Chemistry Tutor Hougang: My Child Memorises Organic Reactions but Cannot Draw Mechanisms
- JC1 H1 Chemistry Tutor | Preparing a Polymer Repeat-Unit Task with Checked Chain Connections
- JC2 H2 Chemistry Tutor | Preparing an Electrode-Potential Task with a Stated Reaction Direction
- Hougang Chemistry Tuition and Tutor Library | Secondary 1–4 and JC Learning Routes
For a discussion about support, bring the student’s H1, H2 or H3 course details, examination year and a recent piece of schoolwork showing the difficulty. Confirm current subject provision, grouping and teaching location directly with the provider. This guide describes preparation practice and does not establish class availability.
Sources and scope
SEAB: 2027 H2 Chemistry syllabus, 9476 · SEAB: 2027 A-Level syllabus listings.
The worked examples, fictional datasets and tutor planning tools are original teaching illustrations. Use the applicable official examination-year documents to establish assessed scope and permitted resources. Supplied extensions and model assumptions are identified in the teaching discussion. These paper-based examples do not replace supervised practical instruction or constitute evidence of student outcomes.