Learning Arc 03 · Choose elimination and carbon-construction routes

Track carbon through magnesium and water

  • Board questions often show two arrows: haloalkane to R-MgX, then R-MgX to hydrocarbon.
  • The carbon fragment, proton source and condition must be tracked separately.
Foundation for this Concept11 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

R
Read R as the carbon fragment.
In CH3-CH2-Br, R is CH3-CH2-.
The terminal dash is its attachment bond.
R is not an element, a free radical or a complete isolated molecule.
X
Read X as one halogen atom.
X may be Cl, Br or I in the equations used here.
X is a placeholder.
Replace X by the halogen shown in each concrete equation.
Mg
The element symbol for magnesium.
Mg is one magnesium atom in the equation.
R-MgX
Read R-MgX as R bonded to Mg, with X attached to Mg in the Board model.
CH3-CH2-MgBr is one concrete example.
R-MgX is a connectivity and polarity model.
It is not a detailed metal-surface mechanism.
anhydrous
Containing no water.
It does not mean that every solvent is allowed.
dry ether
Ether solvent from which water has been excluded.
Dry ether is a reaction condition and is not a carbon source in these equations.
electron-rich carbon end
The C end of C-Mg has greater electron density in the accepted Board polarity model.
This phrase does not mean a free isolated carbon ion is present.
proton source
A substance with an H atom that can be transferred to the Grignard carbon end.
The complete proton source must be named, such as water, alcohol or ammonia.
D and D2O
D is deuterium, a heavier isotope of hydrogen.
D2O is water whose two H atoms are D.
D changes isotope identity, not the carbon skeleton.
→
The reaction arrow points from starting substances to products under the written condition.
It does not by itself reveal microscopic timing.
Mg(OH)Br
Read Mg(OH)Br as magnesium hydroxy bromide.
The parentheses keep O and H together as the OH group.
Mg(OH)Br is the final equation-level salt.
The two-arrow frame first shows OH− and a positive magnesium-bromide residue.

Entry check 1

Can you read the C-X polarity, electron-pair role and atom ledger used in this sequence?

Show the recovery
  • A more electronegative atom pulls shared density.
  • The electron-pair destination identifies its role.
  • Every reaction must conserve atoms and the carbon skeleton.

Reopen the Arc 1 C-X polarity card and the Arc 2 electron-pair and conservation cards.

Every atom and bond in bromoethane is shown. Br is bonded to C2.
1 · ObjectiveOpen

What you will understand

Track the carbon fragment while magnesium replaces the halogen and a later hydrogen source replaces the carbon-magnesium connection.

Problems this Concept unlocks

  • decode R-MgX
  • write Grignard formation equations
  • explain anhydrous conditions
  • predict hydrocarbons after water or ammonia
  • track D from D2O
  • write the complete Maharashtra HSC answer
2 · MysteryOpen

The problem to explain

Read these words and symbols first

R
Read R as the carbon fragment.
In CH3-CH2-Br, R is CH3-CH2-.
The terminal dash is its attachment bond.
R is not an element, a free radical or a complete isolated molecule.
X
Read X as one halogen atom.
X may be Cl, Br or I in the equations used here.
X is a placeholder.
Replace X by the halogen shown in each concrete equation.
Mg
The element symbol for magnesium.
Mg is one magnesium atom in the equation.
R-MgX
Read R-MgX as R bonded to Mg, with X attached to Mg in the Board model.
CH3-CH2-MgBr is one concrete example.
R-MgX is a connectivity and polarity model.
It is not a detailed metal-surface mechanism.
anhydrous
Containing no water.
It does not mean that every solvent is allowed.
dry ether
Ether solvent from which water has been excluded.
Dry ether is a reaction condition and is not a carbon source in these equations.
electron-rich carbon end
The C end of C-Mg has greater electron density in the accepted Board polarity model.
This phrase does not mean a free isolated carbon ion is present.
proton source
A substance with an H atom that can be transferred to the Grignard carbon end.
The complete proton source must be named, such as water, alcohol or ammonia.
D and D2O
D is deuterium, a heavier isotope of hydrogen.
D2O is water whose two H atoms are D.
D changes isotope identity, not the carbon skeleton.
→
The reaction arrow points from starting substances to products under the written condition.
It does not by itself reveal microscopic timing.
Mg(OH)Br
Read Mg(OH)Br as magnesium hydroxy bromide.
The parentheses keep O and H together as the OH group.
Mg(OH)Br is the final equation-level salt.
The two-arrow frame first shows OH− and a positive magnesium-bromide residue.
  • Bromoethane has an electron-poor carbon beside Br.
  • After Mg is inserted, the same carbon end behaves as electron-rich.
  • One drop of water then gives ethane.
Every atom and bond in bromoethane is shown. Br is bonded to C2.
Every carbon-bound H is shown. C2 is bonded to Mg, and Mg is bonded to Br in the bounded Board model.
Every atom and bond in ethane is shown. C2 now has H where the C-Mg bond occurred.
One oxygen atom is bonded to two hydrogen atoms.
  • Why does changing Br to Mg reverse the carbon-end polarity?
  • Where does ethane's new H come from?

Your decision

Track the bonded partner, the carbon fragment and the transferred H separately.

3 · InvestigationOpen

Investigation

Investigation 1

Decode R-MgX

Read these words and symbols first

R
Read R as the carbon fragment.
In CH3-CH2-Br, R is CH3-CH2-.
The terminal dash is its attachment bond.
R is not an element, a free radical or a complete isolated molecule.
X
Read X as one halogen atom.
X may be Cl, Br or I in the equations used here.
X is a placeholder.
Replace X by the halogen shown in each concrete equation.
Mg
The element symbol for magnesium.
Mg is one magnesium atom in the equation.
R-MgX
Read R-MgX as R bonded to Mg, with X attached to Mg in the Board model.
CH3-CH2-MgBr is one concrete example.
R-MgX is a connectivity and polarity model.
It is not a detailed metal-surface mechanism.
anhydrous
Containing no water.
It does not mean that every solvent is allowed.
dry ether
Ether solvent from which water has been excluded.
Dry ether is a reaction condition and is not a carbon source in these equations.
electron-rich carbon end
The C end of C-Mg has greater electron density in the accepted Board polarity model.
This phrase does not mean a free isolated carbon ion is present.
proton source
A substance with an H atom that can be transferred to the Grignard carbon end.
The complete proton source must be named, such as water, alcohol or ammonia.
D and D2O
D is deuterium, a heavier isotope of hydrogen.
D2O is water whose two H atoms are D.
D changes isotope identity, not the carbon skeleton.
→
The reaction arrow points from starting substances to products under the written condition.
It does not by itself reveal microscopic timing.
Mg(OH)Br
Read Mg(OH)Br as magnesium hydroxy bromide.
The parentheses keep O and H together as the OH group.
Mg(OH)Br is the final equation-level salt.
The two-arrow frame first shows OH− and a positive magnesium-bromide residue.

Do

Circle the repeated CH3-CH2 fragment in CH3-CH2-Br and CH3-CH2-Mg-Br.

Every atom and bond in bromoethane is shown. Br is bonded to C2.
Every carbon-bound H is shown. C2 is bonded to Mg, and Mg is bonded to Br in the bounded Board model.

Check

The two-carbon C1-C2 fragment is unchanged.

Investigation 2

Reverse the carbon-end polarity

Read these words and symbols first

R
Read R as the carbon fragment.
In CH3-CH2-Br, R is CH3-CH2-.
The terminal dash is its attachment bond.
R is not an element, a free radical or a complete isolated molecule.
X
Read X as one halogen atom.
X may be Cl, Br or I in the equations used here.
X is a placeholder.
Replace X by the halogen shown in each concrete equation.
Mg
The element symbol for magnesium.
Mg is one magnesium atom in the equation.
R-MgX
Read R-MgX as R bonded to Mg, with X attached to Mg in the Board model.
CH3-CH2-MgBr is one concrete example.
R-MgX is a connectivity and polarity model.
It is not a detailed metal-surface mechanism.
anhydrous
Containing no water.
It does not mean that every solvent is allowed.
dry ether
Ether solvent from which water has been excluded.
Dry ether is a reaction condition and is not a carbon source in these equations.
electron-rich carbon end
The C end of C-Mg has greater electron density in the accepted Board polarity model.
This phrase does not mean a free isolated carbon ion is present.
proton source
A substance with an H atom that can be transferred to the Grignard carbon end.
The complete proton source must be named, such as water, alcohol or ammonia.
D and D2O
D is deuterium, a heavier isotope of hydrogen.
D2O is water whose two H atoms are D.
D changes isotope identity, not the carbon skeleton.
→
The reaction arrow points from starting substances to products under the written condition.
It does not by itself reveal microscopic timing.
Mg(OH)Br
Read Mg(OH)Br as magnesium hydroxy bromide.
The parentheses keep O and H together as the OH group.
Mg(OH)Br is the final equation-level salt.
The two-arrow frame first shows OH− and a positive magnesium-bromide residue.

Do

Use the electronegativity card to draw the pull direction in C-Br and C-Mg.

Every atom and bond in bromoethane is shown. Br is bonded to C2.
Every carbon-bound H is shown. C2 is bonded to Mg, and Mg is bonded to Br in the bounded Board model.

Check

The bonded partner changed, so the polarity comparison changed.

Investigation 3

Form R-MgX

Read these words and symbols first

R
Read R as the carbon fragment.
In CH3-CH2-Br, R is CH3-CH2-.
The terminal dash is its attachment bond.
R is not an element, a free radical or a complete isolated molecule.
X
Read X as one halogen atom.
X may be Cl, Br or I in the equations used here.
X is a placeholder.
Replace X by the halogen shown in each concrete equation.
Mg
The element symbol for magnesium.
Mg is one magnesium atom in the equation.
R-MgX
Read R-MgX as R bonded to Mg, with X attached to Mg in the Board model.
CH3-CH2-MgBr is one concrete example.
R-MgX is a connectivity and polarity model.
It is not a detailed metal-surface mechanism.
anhydrous
Containing no water.
It does not mean that every solvent is allowed.
dry ether
Ether solvent from which water has been excluded.
Dry ether is a reaction condition and is not a carbon source in these equations.
electron-rich carbon end
The C end of C-Mg has greater electron density in the accepted Board polarity model.
This phrase does not mean a free isolated carbon ion is present.
proton source
A substance with an H atom that can be transferred to the Grignard carbon end.
The complete proton source must be named, such as water, alcohol or ammonia.
D and D2O
D is deuterium, a heavier isotope of hydrogen.
D2O is water whose two H atoms are D.
D changes isotope identity, not the carbon skeleton.
→
The reaction arrow points from starting substances to products under the written condition.
It does not by itself reveal microscopic timing.
Mg(OH)Br
Read Mg(OH)Br as magnesium hydroxy bromide.
The parentheses keep O and H together as the OH group.
Mg(OH)Br is the final equation-level salt.
The two-arrow frame first shows OH− and a positive magnesium-bromide residue.

Do

Compare the carbon atoms and C-C bonds in the two displayed structures.

Every atom and bond in bromoethane is shown. Br is bonded to C2.
Every carbon-bound H is shown. C2 is bonded to Mg, and Mg is bonded to Br in the bounded Board model.

Check

The same C1-C2 bond occurs before and after formation.

Investigation 4

Give dry ether a chemical job

Read these words and symbols first

R
Read R as the carbon fragment.
In CH3-CH2-Br, R is CH3-CH2-.
The terminal dash is its attachment bond.
R is not an element, a free radical or a complete isolated molecule.
X
Read X as one halogen atom.
X may be Cl, Br or I in the equations used here.
X is a placeholder.
Replace X by the halogen shown in each concrete equation.
Mg
The element symbol for magnesium.
Mg is one magnesium atom in the equation.
R-MgX
Read R-MgX as R bonded to Mg, with X attached to Mg in the Board model.
CH3-CH2-MgBr is one concrete example.
R-MgX is a connectivity and polarity model.
It is not a detailed metal-surface mechanism.
anhydrous
Containing no water.
It does not mean that every solvent is allowed.
dry ether
Ether solvent from which water has been excluded.
Dry ether is a reaction condition and is not a carbon source in these equations.
electron-rich carbon end
The C end of C-Mg has greater electron density in the accepted Board polarity model.
This phrase does not mean a free isolated carbon ion is present.
proton source
A substance with an H atom that can be transferred to the Grignard carbon end.
The complete proton source must be named, such as water, alcohol or ammonia.
D and D2O
D is deuterium, a heavier isotope of hydrogen.
D2O is water whose two H atoms are D.
D changes isotope identity, not the carbon skeleton.
→
The reaction arrow points from starting substances to products under the written condition.
It does not by itself reveal microscopic timing.
Mg(OH)Br
Read Mg(OH)Br as magnesium hydroxy bromide.
The parentheses keep O and H together as the OH group.
Mg(OH)Br is the final equation-level salt.
The two-arrow frame first shows OH− and a positive magnesium-bromide residue.

Do

Compare C2 in ethylmagnesium bromide with C2 in ethane and mark the changed bond.

Every carbon-bound H is shown. C2 is bonded to Mg, and Mg is bonded to Br in the bounded Board model.
Every atom and bond in ethane is shown. C2 now has H where the C-Mg bond occurred.
One oxygen atom is bonded to two hydrogen atoms.

Check

Water supplies H but supplies no carbon.

Investigation 5

Track the proton

Read these words and symbols first

R
Read R as the carbon fragment.
In CH3-CH2-Br, R is CH3-CH2-.
The terminal dash is its attachment bond.
R is not an element, a free radical or a complete isolated molecule.
X
Read X as one halogen atom.
X may be Cl, Br or I in the equations used here.
X is a placeholder.
Replace X by the halogen shown in each concrete equation.
Mg
The element symbol for magnesium.
Mg is one magnesium atom in the equation.
R-MgX
Read R-MgX as R bonded to Mg, with X attached to Mg in the Board model.
CH3-CH2-MgBr is one concrete example.
R-MgX is a connectivity and polarity model.
It is not a detailed metal-surface mechanism.
anhydrous
Containing no water.
It does not mean that every solvent is allowed.
dry ether
Ether solvent from which water has been excluded.
Dry ether is a reaction condition and is not a carbon source in these equations.
electron-rich carbon end
The C end of C-Mg has greater electron density in the accepted Board polarity model.
This phrase does not mean a free isolated carbon ion is present.
proton source
A substance with an H atom that can be transferred to the Grignard carbon end.
The complete proton source must be named, such as water, alcohol or ammonia.
D and D2O
D is deuterium, a heavier isotope of hydrogen.
D2O is water whose two H atoms are D.
D changes isotope identity, not the carbon skeleton.
→
The reaction arrow points from starting substances to products under the written condition.
It does not by itself reveal microscopic timing.
Mg(OH)Br
Read Mg(OH)Br as magnesium hydroxy bromide.
The parentheses keep O and H together as the OH group.
Mg(OH)Br is the final equation-level salt.
The two-arrow frame first shows OH− and a positive magnesium-bromide residue.

Do

For each arrow, point to its electron source and destination in the displayed frame.

Every carbon-bound H is shown. C2 is bonded to Mg, and Mg is bonded to Br in the bounded Board model.
Every atom and bond in ethane is shown. C2 now has H where the C-Mg bond occurred.
One oxygen atom is bonded to two hydrogen atoms.
Electron-pair mechanism

Grignard proton-transfer ledger

Which electron pair makes the new C-H bond?

BeforeEthylmagnesium bromide and waterThis is a bounded electron-pair accounting model for proton transfer. It is not a metal-surface formation mechanism.
Every bond in ethyl-Mg-Br and H-O-H is shown before proton transfer.CCMgBrOHHHHHHH12
  1. Arrow 1
    Pair starts atC2-Mg bonding pairPair ends atH1 of water
    Why
    The electron-rich carbon end forms the new C2-H1 bond.
    Immediate effect
    C2-H1 forms and C2-Mg breaks.
  2. Arrow 2
    Pair starts atO-H1 bonding pairPair ends atoxygen
    Why
    Hydrogen cannot retain both its O-H and new C-H bonds.
    Immediate effect
    O-H1 breaks and its pair returns to oxygen.
After both arrowsEthane, hydroxide and a magnesium bromide ion pair
The two shown electron-pair moves give ethane, hydroxide and a positive magnesium bromide residue. A later ion association is not drawn because no third electron move is shown.CCMg+BrO−HHHHHHH
Three compulsory checks
Atoms
The same C, Mg, Br, O and H atom identities remain.
Charge
O has formal charge minus one. The magnesium bromide residue has formal charge plus one. Total charge remains zero.
Carbon valence
C2 loses C-Mg as it gains C-H. H1 loses O-H as it gains C-H. No unshown O-Mg bond is added.

The C-Mg bonding pair forms C-H while the O-H bonding pair returns to oxygen. The carbon fragment remains unchanged.

Check

  • C-Mg and O-H are absent.
  • C-H is present.

Investigation 6

Run the carbon ledger

Read these words and symbols first

R
Read R as the carbon fragment.
In CH3-CH2-Br, R is CH3-CH2-.
The terminal dash is its attachment bond.
R is not an element, a free radical or a complete isolated molecule.
X
Read X as one halogen atom.
X may be Cl, Br or I in the equations used here.
X is a placeholder.
Replace X by the halogen shown in each concrete equation.
Mg
The element symbol for magnesium.
Mg is one magnesium atom in the equation.
R-MgX
Read R-MgX as R bonded to Mg, with X attached to Mg in the Board model.
CH3-CH2-MgBr is one concrete example.
R-MgX is a connectivity and polarity model.
It is not a detailed metal-surface mechanism.
anhydrous
Containing no water.
It does not mean that every solvent is allowed.
dry ether
Ether solvent from which water has been excluded.
Dry ether is a reaction condition and is not a carbon source in these equations.
electron-rich carbon end
The C end of C-Mg has greater electron density in the accepted Board polarity model.
This phrase does not mean a free isolated carbon ion is present.
proton source
A substance with an H atom that can be transferred to the Grignard carbon end.
The complete proton source must be named, such as water, alcohol or ammonia.
D and D2O
D is deuterium, a heavier isotope of hydrogen.
D2O is water whose two H atoms are D.
D changes isotope identity, not the carbon skeleton.
→
The reaction arrow points from starting substances to products under the written condition.
It does not by itself reveal microscopic timing.
Mg(OH)Br
Read Mg(OH)Br as magnesium hydroxy bromide.
The parentheses keep O and H together as the OH group.
Mg(OH)Br is the final equation-level salt.
The two-arrow frame first shows OH− and a positive magnesium-bromide residue.

Do

Count and trace every carbon through both arrows.

Every atom and bond in bromoethane is shown. Br is bonded to C2.
Every carbon-bound H is shown. C2 is bonded to Mg, and Mg is bonded to Br in the bounded Board model.
Every atom and bond in ethane is shown. C2 now has H where the C-Mg bond occurred.

Check

The carbon count and C-C bond map do not change.

4 · MeaningOpen

Build the meaning

Meaning 1

Decode R-MgX

  • R is the unchanged carbon fragment.
  • Mg is magnesium.
  • X is the original halogen.

Check

The two-carbon C1-C2 fragment is unchanged.

Meaning 2

Reverse the carbon-end polarity

  • Br pulls shared electron density more strongly than carbon.
  • Carbon pulls shared electron density more strongly than Mg.

Check

The bonded partner changed, so the polarity comparison changed.

Meaning 3

Form R-MgX

R-X reacts with Mg in dry ether to form R-MgX while the carbon skeleton stays unchanged.

Check

The same C1-C2 bond occurs before and after formation.

Meaning 4

Give dry ether a chemical job

Water transfers H to the electron-rich carbon end and converts R-MgX into R-H.

Check

Water supplies H but supplies no carbon.

Meaning 5

Track the proton

  • The C-Mg pair forms C-H.
  • The old O-H pair returns to oxygen.

Check

  • C-Mg and O-H are absent.
  • C-H is present.

Meaning 6

Run the carbon ledger

R-X, R-MgX and R-H contain the same connected carbon fragment R.

Check

The carbon count and C-C bond map do not change.

5 · ExamplesOpen

Worked examples

Example 1 of 3

Ethyl fragment

Convert bromoethane to ethane through its Grignard reagent.

Every atom and bond in bromoethane is shown. Br is bonded to C2.
Every carbon-bound H is shown. C2 is bonded to Mg, and Mg is bonded to Br in the bounded Board model.
Every atom and bond in ethane is shown. C2 now has H where the C-Mg bond occurred.

Key evidence

  • carbon fragment
  • proton source
  • dry condition

One lawful move at a time

One move at a time

Step 1 of 4
Action
Write the carbon fragment
Why
R is the part that stays connected.
Result
R is CH3-CH2

Answer

The hydrocarbon is ethane.

Independent check

C1-C2 remains present in all three structures.

Counter-pattern

Do not treat Mg or water as a carbon source.

6 · QuestionsOpen

Questions

Question 1 of 10

Explain R, Mg and X before using the general equation.

In R-MgX, what does R represent?

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