Learning Arc 03 · Choose elimination and carbon-construction routes

Demonstrate elimination and carbon-construction mastery

  • Board sequences mix several arrows.
  • The safest method treats every arrow as a small proof with a current substrate, one structural job and independent checks.
Foundation for this Concept8 symbols · 0 recovery lessons · 3 entry checksOpen

Start here: words and symbols used in this Concept

Open only the recovery material you need before the Mystery.

Read these words and symbols first

immediate substrate
The actual structure present immediately before the arrow being solved.
The starting compound from two arrows earlier is not automatically the current substrate.
target bond change
The exact bond that must form, break or change order in the next step.
A functional-group name alone is not a complete bond ledger.
route
An ordered sequence of structures and conditions from starting substance to target.
Every arrow must produce the substrate needed by the next arrow.
route intermediate
Read this as route intermediate.
It is the complete compound formed by one reaction step and used as the starting compound in the next step.
For CH3CH2Br → CH3CH2MgBr → CH3CH3, CH3CH2MgBr is the route intermediate.
It is a compound in the route, not a transition state.
carbon ledger
A count and connection check for every carbon fragment across an arrow.
A carbon change is allowed only when the reagent or operation supplies or joins carbon.
condition ledger
The reagent, solvent and temperature carried with the exact arrow they control.
A condition from one arrow cannot be moved silently to another arrow.
feasibility gate
A structural check that must pass before a route rule can be used.
For elimination, an actual beta hydrogen is required.
forward verification
Replay the proposed route from start to target and check every intermediate.
Matching only the final molecular formula is insufficient.

Entry check 1

What must be generated before the Saytzeff preference is applied?

Show the recovery

Every feasible distinct alkene.

Return to Concepts 3.1 and 3.3.

Four-carbon haloalkane with Br on C2. Every carbon-hydrogen bond is explicit.

Entry check 2

What happens when R-MgX meets water?

Show the recovery

R-H forms and the reagent is consumed.

Return to Concept 3.4.

The two-carbon fragment is bonded from C2 to Mg and Br in the bounded Board model. Every carbon-hydrogen bond is explicit.

Entry check 3

What pairings are possible for two different Wurtz fragments A and B?

Show the recovery

A-A, A-B and B-B.

Return to Concept 3.5.

1 · ObjectiveOpen

What you will understand

Plan and verify Board and unfamiliar multi-step routes using carbon, bond, condition and feasibility ledgers at every arrow.

Problems this Concept unlocks

  • choose elimination, Grignard or coupling
  • solve multi-step reaction sequences
  • write concise Board answers
  • diagnose missing route intermediates
  • complete atom-balanced equations
  • solve unfamiliar multi-select transfer
2 · MysteryOpen

The problem to explain

Read these words and symbols first

immediate substrate
The actual structure present immediately before the arrow being solved.
The starting compound from two arrows earlier is not automatically the current substrate.
target bond change
The exact bond that must form, break or change order in the next step.
A functional-group name alone is not a complete bond ledger.
route
An ordered sequence of structures and conditions from starting substance to target.
Every arrow must produce the substrate needed by the next arrow.
route intermediate
Read this as route intermediate.
It is the complete compound formed by one reaction step and used as the starting compound in the next step.
For CH3CH2Br → CH3CH2MgBr → CH3CH3, CH3CH2MgBr is the route intermediate.
It is a compound in the route, not a transition state.
carbon ledger
A count and connection check for every carbon fragment across an arrow.
A carbon change is allowed only when the reagent or operation supplies or joins carbon.
condition ledger
The reagent, solvent and temperature carried with the exact arrow they control.
A condition from one arrow cannot be moved silently to another arrow.
feasibility gate
A structural check that must pass before a route rule can be used.
For elimination, an actual beta hydrogen is required.
forward verification
Replay the proposed route from start to target and check every intermediate.
Matching only the final molecular formula is insufficient.
  • The same bromoethane can give ethene, ethane or butane.
  • The first two routes retain two carbons.
  • The coupling route joins two bromoethane molecules and gives four carbons.
Two-carbon haloalkane with Br on C2. Every carbon-hydrogen bond is explicit.
Two-carbon alkene with C1=C2. Every carbon-hydrogen bond is explicit.
Two-carbon saturated hydrocarbon. Every carbon-hydrogen bond is explicit.
Four-carbon chain with one central bond joining two ethyl fragments. Every carbon-hydrogen bond is explicit.

How can one starting haloalkane lead to three different targets without guessing from a reaction-name list?

Your decision

  • Mark the required bond change.
  • Choose the route family.
  • Write each route intermediate.
  • Then run the carbon, bond, condition and feasibility ledgers.
3 · InvestigationOpen

Investigation

Investigation 1

Classify the required bond change

Read these words and symbols first

immediate substrate
The actual structure present immediately before the arrow being solved.
The starting compound from two arrows earlier is not automatically the current substrate.
target bond change
The exact bond that must form, break or change order in the next step.
A functional-group name alone is not a complete bond ledger.
route
An ordered sequence of structures and conditions from starting substance to target.
Every arrow must produce the substrate needed by the next arrow.
route intermediate
Read this as route intermediate.
It is the complete compound formed by one reaction step and used as the starting compound in the next step.
For CH3CH2Br → CH3CH2MgBr → CH3CH3, CH3CH2MgBr is the route intermediate.
It is a compound in the route, not a transition state.
carbon ledger
A count and connection check for every carbon fragment across an arrow.
A carbon change is allowed only when the reagent or operation supplies or joins carbon.
condition ledger
The reagent, solvent and temperature carried with the exact arrow they control.
A condition from one arrow cannot be moved silently to another arrow.
feasibility gate
A structural check that must pass before a route rule can be used.
For elimination, an actual beta hydrogen is required.
forward verification
Replay the proposed route from start to target and check every intermediate.
Matching only the final molecular formula is insufficient.

Do

Circle the one bond change required by the next arrow.

Two-carbon haloalkane with Br on C2. Every carbon-hydrogen bond is explicit.
Two-carbon alkene with C1=C2. Every carbon-hydrogen bond is explicit.
The two-carbon fragment is bonded from C2 to Mg and Br in the bounded Board model. Every carbon-hydrogen bond is explicit.
Four-carbon chain with one central bond joining two ethyl fragments. Every carbon-hydrogen bond is explicit.

Check

The selected family can create the marked bond change.

Investigation 2

Write the immediate substrate

Read these words and symbols first

immediate substrate
The actual structure present immediately before the arrow being solved.
The starting compound from two arrows earlier is not automatically the current substrate.
target bond change
The exact bond that must form, break or change order in the next step.
A functional-group name alone is not a complete bond ledger.
route
An ordered sequence of structures and conditions from starting substance to target.
Every arrow must produce the substrate needed by the next arrow.
route intermediate
Read this as route intermediate.
It is the complete compound formed by one reaction step and used as the starting compound in the next step.
For CH3CH2Br → CH3CH2MgBr → CH3CH3, CH3CH2MgBr is the route intermediate.
It is a compound in the route, not a transition state.
carbon ledger
A count and connection check for every carbon fragment across an arrow.
A carbon change is allowed only when the reagent or operation supplies or joins carbon.
condition ledger
The reagent, solvent and temperature carried with the exact arrow they control.
A condition from one arrow cannot be moved silently to another arrow.
feasibility gate
A structural check that must pass before a route rule can be used.
For elimination, an actual beta hydrogen is required.
forward verification
Replay the proposed route from start to target and check every intermediate.
Matching only the final molecular formula is insufficient.

Do

Create a start, state 1 and final-state row.

Two-carbon haloalkane with Br on C2. Every carbon-hydrogen bond is explicit.
The two-carbon fragment is bonded from C2 to Mg and Br in the bounded Board model. Every carbon-hydrogen bond is explicit.
Two-carbon saturated hydrocarbon. Every carbon-hydrogen bond is explicit.

Check

State 1 is both the first product and the second substrate.

Investigation 3

Run four ledgers

Read these words and symbols first

immediate substrate
The actual structure present immediately before the arrow being solved.
The starting compound from two arrows earlier is not automatically the current substrate.
target bond change
The exact bond that must form, break or change order in the next step.
A functional-group name alone is not a complete bond ledger.
route
An ordered sequence of structures and conditions from starting substance to target.
Every arrow must produce the substrate needed by the next arrow.
route intermediate
Read this as route intermediate.
It is the complete compound formed by one reaction step and used as the starting compound in the next step.
For CH3CH2Br → CH3CH2MgBr → CH3CH3, CH3CH2MgBr is the route intermediate.
It is a compound in the route, not a transition state.
carbon ledger
A count and connection check for every carbon fragment across an arrow.
A carbon change is allowed only when the reagent or operation supplies or joins carbon.
condition ledger
The reagent, solvent and temperature carried with the exact arrow they control.
A condition from one arrow cannot be moved silently to another arrow.
feasibility gate
A structural check that must pass before a route rule can be used.
For elimination, an actual beta hydrogen is required.
forward verification
Replay the proposed route from start to target and check every intermediate.
Matching only the final molecular formula is insufficient.

Do

  • Use the displayed 2-bromobutane and but-2-ene structures.
  • Use this supplied condition: alcoholic KOH with heat.
  • Carbon row: compare the carbon atoms before and after.
  • Bond row: identify the bond broken and the bond-order change.
  • Condition row: copy the complete supplied condition.
  • Feasibility row: identify the adjacent carbon that supplies H.
Four-carbon haloalkane with Br on C2. Every carbon-hydrogen bond is explicit.
Four-carbon alkene with C2=C3. Every carbon-hydrogen bond is explicit.

Check

  • Carbon count stays four.
  • C-Br and the adjacent C-H bond are broken.
  • C2-C3 changes from single to double.
  • Alcoholic KOH with heat is recorded.
  • C3 supplies the adjacent H.

Investigation 4

Plan backward, verify forward

Read these words and symbols first

immediate substrate
The actual structure present immediately before the arrow being solved.
The starting compound from two arrows earlier is not automatically the current substrate.
target bond change
The exact bond that must form, break or change order in the next step.
A functional-group name alone is not a complete bond ledger.
route
An ordered sequence of structures and conditions from starting substance to target.
Every arrow must produce the substrate needed by the next arrow.
route intermediate
Read this as route intermediate.
It is the complete compound formed by one reaction step and used as the starting compound in the next step.
For CH3CH2Br → CH3CH2MgBr → CH3CH3, CH3CH2MgBr is the route intermediate.
It is a compound in the route, not a transition state.
carbon ledger
A count and connection check for every carbon fragment across an arrow.
A carbon change is allowed only when the reagent or operation supplies or joins carbon.
condition ledger
The reagent, solvent and temperature carried with the exact arrow they control.
A condition from one arrow cannot be moved silently to another arrow.
feasibility gate
A structural check that must pass before a route rule can be used.
For elimination, an actual beta hydrogen is required.
forward verification
Replay the proposed route from start to target and check every intermediate.
Matching only the final molecular formula is insufficient.

Do

  • Inspect the displayed butane target.
  • Mark the central C-C bond.
  • Draw the two fragments produced by cutting only that bond.
  • Attach Br to each open carbon endpoint.
  • Rejoin the fragments and compare with the original target.
Four-carbon chain with one central bond joining two ethyl fragments. Every carbon-hydrogen bond is explicit.

Check

The replay produces the exact target connectivity.

Investigation 5

Complete and balance every reaction step

Read these words and symbols first

immediate substrate
The actual structure present immediately before the arrow being solved.
The starting compound from two arrows earlier is not automatically the current substrate.
target bond change
The exact bond that must form, break or change order in the next step.
A functional-group name alone is not a complete bond ledger.
route
An ordered sequence of structures and conditions from starting substance to target.
Every arrow must produce the substrate needed by the next arrow.
route intermediate
Read this as route intermediate.
It is the complete compound formed by one reaction step and used as the starting compound in the next step.
For CH3CH2Br → CH3CH2MgBr → CH3CH3, CH3CH2MgBr is the route intermediate.
It is a compound in the route, not a transition state.
carbon ledger
A count and connection check for every carbon fragment across an arrow.
A carbon change is allowed only when the reagent or operation supplies or joins carbon.
condition ledger
The reagent, solvent and temperature carried with the exact arrow they control.
A condition from one arrow cannot be moved silently to another arrow.
feasibility gate
A structural check that must pass before a route rule can be used.
For elimination, an actual beta hydrogen is required.
forward verification
Replay the proposed route from start to target and check every intermediate.
Matching only the final molecular formula is insufficient.

Do

  • Use these four complete cards.
  • Card 1: KOH in alcoholic solvent with heat gives KBr and H2O.
  • Card 2: Mg in dry ether forms the carbon-magnesium intermediate.
  • Card 3: H2O gives Mg(OH)Br after hydrogen transfer.
  • Card 4: 2 Na in dry ether gives 2 NaBr after coupling.
  • Match each card to the displayed starting structure and target.
  • Count every element on both sides.
Two-carbon haloalkane with Br on C2. Every carbon-hydrogen bond is explicit.
Two-carbon alkene with C1=C2. Every carbon-hydrogen bond is explicit.
The two-carbon fragment is bonded from C2 to Mg and Br in the bounded Board model. Every carbon-hydrogen bond is explicit.
Two-carbon saturated hydrocarbon. Every carbon-hydrogen bond is explicit.
Four-carbon chain with one central bond joining two ethyl fragments. Every carbon-hydrogen bond is explicit.

Check

  • Elimination accounts for KBr and H2O.
  • Grignard formation accounts for Mg.
  • Hydrolysis accounts for Mg(OH)Br.
  • Wurtz coupling accounts for 2 NaBr.
4 · MeaningOpen

Build the meaning

Meaning 1

Classify the required bond change

  • C=C formation points to elimination.
  • C-Mg formation points to Grignard.
  • A new C-C bond between fragments points to coupling.

Check

The selected family can create the marked bond change.

Meaning 2

Write the immediate substrate

  • Solve one arrow at a time.
  • Write the complete product before reading the next reagent.

Check

State 1 is both the first product and the second substrate.

Meaning 3

Run four ledgers

For each arrow, record carbon, bond, condition and feasibility evidence.

Check

  • Carbon count stays four.
  • C-Br and the adjacent C-H bond are broken.
  • C2-C3 changes from single to double.
  • Alcoholic KOH with heat is recorded.
  • C3 supplies the adjacent H.

Meaning 4

Plan backward, verify forward

  • Backward planning proposes the previous structure.
  • Forward replay tests whether every proposal is lawful.

Check

The replay produces the exact target connectivity.

Meaning 5

Complete and balance every reaction step

  • Write the starting compound and every reagent.
  • Write the required solvent or heat condition.
  • Write the organic product and the Board-level inorganic by-products.
  • Then count every element on both sides.

Check

  • Elimination accounts for KBr and H2O.
  • Grignard formation accounts for Mg.
  • Hydrolysis accounts for Mg(OH)Br.
  • Wurtz coupling accounts for 2 NaBr.
5 · ExamplesOpen

Worked examples

Example 1 of 4

Two different outcomes from bromoethane

Compare bromoethane to ethene with bromoethane to ethane.

Two-carbon haloalkane with Br on C2. Every carbon-hydrogen bond is explicit.
Two-carbon alkene with C1=C2. Every carbon-hydrogen bond is explicit.
The two-carbon fragment is bonded from C2 to Mg and Br in the bounded Board model. Every carbon-hydrogen bond is explicit.
Two-carbon saturated hydrocarbon. Every carbon-hydrogen bond is explicit.

Key evidence

  • immediate substrate
  • target bond
  • condition
  • carbon count

One lawful move at a time

One move at a time

Step 1 of 4
Action
Mark the changed bond
Why
Ethene contains C=C.
Result
C-C becomes C=C for ethene.

Answer

  • Alcoholic KOH with heat gives ethene.
  • Mg in dry ether followed by water gives ethane.

Independent check

Each route preserves C1-C2 and balances every displayed atom.

Counter-pattern

The same molecular start does not imply the same condition or product.

6 · QuestionsOpen

Questions

Question 1 of 12

Distinguish elimination, Grignard and coupling by the bond target.

Match each required bond change to the reaction family that can produce it.

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