Learning Arc 03 · Choose elimination and carbon-construction routes

Build a target by joining two carbon fragments

  • Reaction names are easy to confuse.
  • A target-bond cut shows which carbon fragments are required and which coupling family applies.
Foundation for this Concept15 symbols · 0 recovery lessons · 1 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

alkyl fragment
An alkyl fragment is formed on paper when one H is removed from an alkane.
Ethane gives ethyl, CH3-CH2-.
The terminal dash is the attachment bond, not a minus sign or an extra atom.
aryl fragment
An aryl fragment has its attachment point directly on an aromatic ring carbon.
Benzene gives phenyl, C6H5-.
The benzene ring remains connected in the coupling equations used here.
R
Read R as one alkyl fragment.
In C2H5-Br, R is C2H5-.
R is not an element, a free radical or a complete isolated molecule.
R′
Read R′ as R prime, the label for a second alkyl fragment.
The prime is not a charge or carbon locant.
In the mixed-Wurtz case, R′ is different from R.
Ar
Read Ar as an aryl fragment.
In bromobenzene, Ar is C6H5-.
Ar is a placeholder here, not the element symbol Ar for argon.
C6H5-
Read C6H5- as the phenyl fragment.
It contains a six-carbon benzene ring and five H atoms.
The terminal dash marks the ring-carbon attachment bond.
It is not a negative charge or an isolated molecule.
X
Read X as one halogen atom, such as Br in C2H5-Br.
Replace X by the halogen shown in each concrete equation.
Na
The element symbol for sodium.
Two sodium atoms are needed in the balanced coupling equations used here.
NaX
Read NaX as sodium halide.
NaBr is the concrete salt when X is Br.
The coefficient 2 in 2 NaX counts two formula units
it is not part of one formula.
coupling
Formation of a new carbon-carbon bond between two carbon fragments.
A formal equation does not by itself prove that one pure product is obtained.
Wurtz reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Wurtz-Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
dry ether
Ether solvent from which water has been excluded.
It is carried with the equation as a condition.
product mixture
More than one distinct organic product can form from the supplied fragments.
Carbon-count arithmetic alone cannot establish a clean synthesis.

Entry check 1

Can you preserve a carbon skeleton and distinguish an alkyl endpoint from a ring endpoint?

Show the recovery
  • A conserved fragment keeps its internal carbon bonds.
  • An aryl endpoint lies directly on a benzene ring.
  • An alkyl endpoint does not.

Return to the Arc 1 carbon-ledger and benzene-recognition cards.

Every atom and bond in bromoethane is shown. The ethyl attachment carbon C2 is bonded to Br.
Every atom and bond in bromobenzene is shown. Br is bonded directly to ring carbon C1.
1 · ObjectiveOpen

What you will understand

Cut one target carbon-carbon bond, rebuild the two starting halides and then learn which sodium-coupling name matches the fragment types.

Problems this Concept unlocks

  • identify a coupling bond
  • balance Wurtz and Fittig equations
  • distinguish three coupling families
  • reverse-plan a target hydrocarbon
  • explain a mixed-Wurtz product mixture
  • write Maharashtra HSC coupling answers
2 · MysteryOpen

The problem to explain

Read these words and symbols first

alkyl fragment
An alkyl fragment is formed on paper when one H is removed from an alkane.
Ethane gives ethyl, CH3-CH2-.
The terminal dash is the attachment bond, not a minus sign or an extra atom.
aryl fragment
An aryl fragment has its attachment point directly on an aromatic ring carbon.
Benzene gives phenyl, C6H5-.
The benzene ring remains connected in the coupling equations used here.
R
Read R as one alkyl fragment.
In C2H5-Br, R is C2H5-.
R is not an element, a free radical or a complete isolated molecule.
R′
Read R′ as R prime, the label for a second alkyl fragment.
The prime is not a charge or carbon locant.
In the mixed-Wurtz case, R′ is different from R.
Ar
Read Ar as an aryl fragment.
In bromobenzene, Ar is C6H5-.
Ar is a placeholder here, not the element symbol Ar for argon.
C6H5-
Read C6H5- as the phenyl fragment.
It contains a six-carbon benzene ring and five H atoms.
The terminal dash marks the ring-carbon attachment bond.
It is not a negative charge or an isolated molecule.
X
Read X as one halogen atom, such as Br in C2H5-Br.
Replace X by the halogen shown in each concrete equation.
Na
The element symbol for sodium.
Two sodium atoms are needed in the balanced coupling equations used here.
NaX
Read NaX as sodium halide.
NaBr is the concrete salt when X is Br.
The coefficient 2 in 2 NaX counts two formula units
it is not part of one formula.
coupling
Formation of a new carbon-carbon bond between two carbon fragments.
A formal equation does not by itself prove that one pure product is obtained.
Wurtz reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Wurtz-Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
dry ether
Ether solvent from which water has been excluded.
It is carried with the equation as a condition.
product mixture
More than one distinct organic product can form from the supplied fragments.
Carbon-count arithmetic alone cannot establish a clean synthesis.

Butane, biphenyl and toluene can all be written as two carbon fragments joined by one new C-C bond.

Every atom and bond in butane is shown. C2-C3 joins two ethyl fragments.
Every atom and bond in biphenyl is shown. C1-C7 is the single bond joining the two rings.
Every atom and bond in toluene is shown. Ring carbon C1 is bonded to methyl carbon C7.

How can the same sodium condition lead to three different reaction-family names?

Your decision

  • Cut the target bond.
  • Classify both exposed carbon fragments.
  • Then select the family.
3 · InvestigationOpen

Investigation

Investigation 1

Find the new C-C bond

Read these words and symbols first

alkyl fragment
An alkyl fragment is formed on paper when one H is removed from an alkane.
Ethane gives ethyl, CH3-CH2-.
The terminal dash is the attachment bond, not a minus sign or an extra atom.
aryl fragment
An aryl fragment has its attachment point directly on an aromatic ring carbon.
Benzene gives phenyl, C6H5-.
The benzene ring remains connected in the coupling equations used here.
R
Read R as one alkyl fragment.
In C2H5-Br, R is C2H5-.
R is not an element, a free radical or a complete isolated molecule.
R′
Read R′ as R prime, the label for a second alkyl fragment.
The prime is not a charge or carbon locant.
In the mixed-Wurtz case, R′ is different from R.
Ar
Read Ar as an aryl fragment.
In bromobenzene, Ar is C6H5-.
Ar is a placeholder here, not the element symbol Ar for argon.
C6H5-
Read C6H5- as the phenyl fragment.
It contains a six-carbon benzene ring and five H atoms.
The terminal dash marks the ring-carbon attachment bond.
It is not a negative charge or an isolated molecule.
X
Read X as one halogen atom, such as Br in C2H5-Br.
Replace X by the halogen shown in each concrete equation.
Na
The element symbol for sodium.
Two sodium atoms are needed in the balanced coupling equations used here.
NaX
Read NaX as sodium halide.
NaBr is the concrete salt when X is Br.
The coefficient 2 in 2 NaX counts two formula units
it is not part of one formula.
coupling
Formation of a new carbon-carbon bond between two carbon fragments.
A formal equation does not by itself prove that one pure product is obtained.
Wurtz reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Wurtz-Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
dry ether
Ether solvent from which water has been excluded.
It is carried with the equation as a condition.
product mixture
More than one distinct organic product can form from the supplied fragments.
Carbon-count arithmetic alone cannot establish a clean synthesis.

Do

Mark one target C-C bond and cut it once.

Every atom and bond in butane is shown. C2-C3 joins two ethyl fragments.
Every atom and bond in biphenyl is shown. C1-C7 is the single bond joining the two rings.
Every atom and bond in toluene is shown. Ring carbon C1 is bonded to methyl carbon C7.

Check

Rejoining the two cut endpoints recreates the target.

Investigation 2

Rebuild the halides

Read these words and symbols first

alkyl fragment
An alkyl fragment is formed on paper when one H is removed from an alkane.
Ethane gives ethyl, CH3-CH2-.
The terminal dash is the attachment bond, not a minus sign or an extra atom.
aryl fragment
An aryl fragment has its attachment point directly on an aromatic ring carbon.
Benzene gives phenyl, C6H5-.
The benzene ring remains connected in the coupling equations used here.
R
Read R as one alkyl fragment.
In C2H5-Br, R is C2H5-.
R is not an element, a free radical or a complete isolated molecule.
R′
Read R′ as R prime, the label for a second alkyl fragment.
The prime is not a charge or carbon locant.
In the mixed-Wurtz case, R′ is different from R.
Ar
Read Ar as an aryl fragment.
In bromobenzene, Ar is C6H5-.
Ar is a placeholder here, not the element symbol Ar for argon.
C6H5-
Read C6H5- as the phenyl fragment.
It contains a six-carbon benzene ring and five H atoms.
The terminal dash marks the ring-carbon attachment bond.
It is not a negative charge or an isolated molecule.
X
Read X as one halogen atom, such as Br in C2H5-Br.
Replace X by the halogen shown in each concrete equation.
Na
The element symbol for sodium.
Two sodium atoms are needed in the balanced coupling equations used here.
NaX
Read NaX as sodium halide.
NaBr is the concrete salt when X is Br.
The coefficient 2 in 2 NaX counts two formula units
it is not part of one formula.
coupling
Formation of a new carbon-carbon bond between two carbon fragments.
A formal equation does not by itself prove that one pure product is obtained.
Wurtz reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Wurtz-Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
dry ether
Ether solvent from which water has been excluded.
It is carried with the equation as a condition.
product mixture
More than one distinct organic product can form from the supplied fragments.
Carbon-count arithmetic alone cannot establish a clean synthesis.

Do

  • On the displayed butane, mark the central C2-C3 bond.
  • Cut only C2-C3.
  • Compare each open endpoint with the carbon bonded to Br in displayed bromoethane.
Every atom and bond in bromoethane is shown. The ethyl attachment carbon C2 is bonded to Br.
Every atom and bond in butane is shown. C2-C3 joins two ethyl fragments.

Check

The starting fragments contain the same total carbons as the target.

Investigation 3

Classify the two fragments

Read these words and symbols first

alkyl fragment
An alkyl fragment is formed on paper when one H is removed from an alkane.
Ethane gives ethyl, CH3-CH2-.
The terminal dash is the attachment bond, not a minus sign or an extra atom.
aryl fragment
An aryl fragment has its attachment point directly on an aromatic ring carbon.
Benzene gives phenyl, C6H5-.
The benzene ring remains connected in the coupling equations used here.
R
Read R as one alkyl fragment.
In C2H5-Br, R is C2H5-.
R is not an element, a free radical or a complete isolated molecule.
R′
Read R′ as R prime, the label for a second alkyl fragment.
The prime is not a charge or carbon locant.
In the mixed-Wurtz case, R′ is different from R.
Ar
Read Ar as an aryl fragment.
In bromobenzene, Ar is C6H5-.
Ar is a placeholder here, not the element symbol Ar for argon.
C6H5-
Read C6H5- as the phenyl fragment.
It contains a six-carbon benzene ring and five H atoms.
The terminal dash marks the ring-carbon attachment bond.
It is not a negative charge or an isolated molecule.
X
Read X as one halogen atom, such as Br in C2H5-Br.
Replace X by the halogen shown in each concrete equation.
Na
The element symbol for sodium.
Two sodium atoms are needed in the balanced coupling equations used here.
NaX
Read NaX as sodium halide.
NaBr is the concrete salt when X is Br.
The coefficient 2 in 2 NaX counts two formula units
it is not part of one formula.
coupling
Formation of a new carbon-carbon bond between two carbon fragments.
A formal equation does not by itself prove that one pure product is obtained.
Wurtz reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Wurtz-Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
dry ether
Ether solvent from which water has been excluded.
It is carried with the equation as a condition.
product mixture
More than one distinct organic product can form from the supplied fragments.
Carbon-count arithmetic alone cannot establish a clean synthesis.

Do

Label each endpoint alkyl or aryl before naming the reaction.

Every atom and bond in bromoethane is shown. The ethyl attachment carbon C2 is bonded to Br.
Every atom and bond in bromobenzene is shown. Br is bonded directly to ring carbon C1.
Every atom and bond in biphenyl is shown. C1-C7 is the single bond joining the two rings.
Every atom and bond in toluene is shown. Ring carbon C1 is bonded to methyl carbon C7.

Check

The family follows from the pair type, not from target size.

Investigation 4

Balance the equation

Read these words and symbols first

alkyl fragment
An alkyl fragment is formed on paper when one H is removed from an alkane.
Ethane gives ethyl, CH3-CH2-.
The terminal dash is the attachment bond, not a minus sign or an extra atom.
aryl fragment
An aryl fragment has its attachment point directly on an aromatic ring carbon.
Benzene gives phenyl, C6H5-.
The benzene ring remains connected in the coupling equations used here.
R
Read R as one alkyl fragment.
In C2H5-Br, R is C2H5-.
R is not an element, a free radical or a complete isolated molecule.
R′
Read R′ as R prime, the label for a second alkyl fragment.
The prime is not a charge or carbon locant.
In the mixed-Wurtz case, R′ is different from R.
Ar
Read Ar as an aryl fragment.
In bromobenzene, Ar is C6H5-.
Ar is a placeholder here, not the element symbol Ar for argon.
C6H5-
Read C6H5- as the phenyl fragment.
It contains a six-carbon benzene ring and five H atoms.
The terminal dash marks the ring-carbon attachment bond.
It is not a negative charge or an isolated molecule.
X
Read X as one halogen atom, such as Br in C2H5-Br.
Replace X by the halogen shown in each concrete equation.
Na
The element symbol for sodium.
Two sodium atoms are needed in the balanced coupling equations used here.
NaX
Read NaX as sodium halide.
NaBr is the concrete salt when X is Br.
The coefficient 2 in 2 NaX counts two formula units
it is not part of one formula.
coupling
Formation of a new carbon-carbon bond between two carbon fragments.
A formal equation does not by itself prove that one pure product is obtained.
Wurtz reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Wurtz-Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
dry ether
Ether solvent from which water has been excluded.
It is carried with the equation as a condition.
product mixture
More than one distinct organic product can form from the supplied fragments.
Carbon-count arithmetic alone cannot establish a clean synthesis.

Do

Count the two C-Br bonds in two bromoethane molecules and the new C-C bond in butane.

Every atom and bond in bromoethane is shown. The ethyl attachment carbon C2 is bonded to Br.
Every atom and bond in butane is shown. C2-C3 joins two ethyl fragments.

Check

Each equation contains two Na atoms and two X atoms on both sides.

Investigation 5

Test the mixed-Wurtz trap

Read these words and symbols first

alkyl fragment
An alkyl fragment is formed on paper when one H is removed from an alkane.
Ethane gives ethyl, CH3-CH2-.
The terminal dash is the attachment bond, not a minus sign or an extra atom.
aryl fragment
An aryl fragment has its attachment point directly on an aromatic ring carbon.
Benzene gives phenyl, C6H5-.
The benzene ring remains connected in the coupling equations used here.
R
Read R as one alkyl fragment.
In C2H5-Br, R is C2H5-.
R is not an element, a free radical or a complete isolated molecule.
R′
Read R′ as R prime, the label for a second alkyl fragment.
The prime is not a charge or carbon locant.
In the mixed-Wurtz case, R′ is different from R.
Ar
Read Ar as an aryl fragment.
In bromobenzene, Ar is C6H5-.
Ar is a placeholder here, not the element symbol Ar for argon.
C6H5-
Read C6H5- as the phenyl fragment.
It contains a six-carbon benzene ring and five H atoms.
The terminal dash marks the ring-carbon attachment bond.
It is not a negative charge or an isolated molecule.
X
Read X as one halogen atom, such as Br in C2H5-Br.
Replace X by the halogen shown in each concrete equation.
Na
The element symbol for sodium.
Two sodium atoms are needed in the balanced coupling equations used here.
NaX
Read NaX as sodium halide.
NaBr is the concrete salt when X is Br.
The coefficient 2 in 2 NaX counts two formula units
it is not part of one formula.
coupling
Formation of a new carbon-carbon bond between two carbon fragments.
A formal equation does not by itself prove that one pure product is obtained.
Wurtz reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
Wurtz-Fittig reaction
One named sodium-coupling family.
Its exact fragment pair will be derived in this Concept.
The name alone does not identify the required starting fragments.
dry ether
Ether solvent from which water has been excluded.
It is carried with the equation as a condition.
product mixture
More than one distinct organic product can form from the supplied fragments.
Carbon-count arithmetic alone cannot establish a clean synthesis.

Do

  • Let A be the ethyl fragment CH3-CH2-.
  • Let B be the propyl fragment CH3-CH2-CH2-.
  • Write the three unordered pairs A-A, A-B and B-B.
Every atom and bond in bromoethane is shown. The ethyl attachment carbon C2 is bonded to Br.

Check

Three distinct pairings are possible when R and R′ differ.

4 · MeaningOpen

Build the meaning

Meaning 1

Find the new C-C bond

The target bond joins exactly two carbon endpoints.

Check

Rejoining the two cut endpoints recreates the target.

Meaning 2

Rebuild the halides

Attach X to each exposed carbon endpoint after the target bond is cut.

Check

The starting fragments contain the same total carbons as the target.

Meaning 3

Classify the two fragments

  • Alkyl plus alkyl is Wurtz.
  • Aryl plus aryl is Fittig.
  • Alkyl plus aryl is Wurtz-Fittig.

Check

The family follows from the pair type, not from target size.

Meaning 4

Balance the equation

  • Wurtz: 2 R-X + 2 Na → R-R + 2 NaX.
  • Fittig: 2 Ar-X + 2 Na → Ar-Ar + 2 NaX.
  • Wurtz-Fittig: Ar-X + R-X + 2 Na → Ar-R + 2 NaX.

Check

Each equation contains two Na atoms and two X atoms on both sides.

Meaning 5

Test the mixed-Wurtz trap

R-X and R′-X can pair as R-R, R-R′ and R′-R′.

Check

Three distinct pairings are possible when R and R′ differ.

5 · ExamplesOpen

Worked examples

Example 1 of 4

Build butane

Which halide gives butane by a symmetric Wurtz coupling?

Every atom and bond in bromoethane is shown. The ethyl attachment carbon C2 is bonded to Br.
Every atom and bond in butane is shown. C2-C3 joins two ethyl fragments.

Key evidence

  • target bond
  • fragment types
  • carbon count

One lawful move at a time

One move at a time

Step 1 of 4
Action
Cut C2-C3
Why
The central cut divides the target into equal halves.
Result
Two two-carbon fragments

Answer

Bromoethane gives butane.

Independent check

Two plus two carbons equals four and the target central bond is rebuilt.

Counter-pattern

A four-carbon total alone is insufficient unless connectivity also matches.

6 · QuestionsOpen

Questions

Question 1 of 10

Mark the exact target bond before choosing a reaction name.

Which bond in butane is created when two ethyl fragments couple?

Every atom and bond in butane is shown. C2-C3 joins two ethyl fragments.

Preview only.

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Select one answer.
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