Learning Arc 02 · Build electron, stereochemical and substitution grammar

Read electron movement in substitution

Board reaction equations become understandable only when every electron pair, changed bond and conserved quantity has a named place.

Foundation for this Concept9 symbols · 3 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

valence electron
An electron counted in an atom's outer bonding layer.
This Chapter uses the supplied school counting model
deeper atomic theory is not required.
lone pair
Two valence electrons placed on one atom and not shared in a bond
they are drawn as two dots.
A charge sign is not itself a lone pair.
formal charge
A whole-number electron-bookkeeping charge written as + or - on an atom or complete species.
A raised minus means the complete species has one more assigned electron than a neutral species.
partial charge
Delta plus or delta minus marks unequal sharing inside a polar covalent bond.
Delta does not mean a whole electron was transferred.
OH−
Read hydroxide ion: one O atom bonded to one H atom, with overall charge minus one.
The raised minus is a charge sign
it is not a lone pair.
Br−
Read bromide ion: one Br atom with overall charge minus one.
Br− is not neutral Br bonded to carbon.
Lewis drawing
A drawing that shows bond lines, lone-pair dots and whole-number charge signs.
The positions of the dots on the page do not show three-dimensional shape.
brackets around one ion
Square brackets can enclose one complete Lewis species.
A raised charge written outside applies to that whole bracketed species.
These are structure brackets.
In Concept 2.2, brackets such as [RX] instead mean concentration because they appear in a rate expression.
bounded model
A conclusion limited to the structures, reagent and conditions stated in the Question.
It is not a rule for every possible reaction.

Foundation 1

Read the electron and charge marks

  • A Lewis drawing shows bond lines, lone-pair dots and whole-number charge signs.
  • A bond line represents one shared electron pair.
  • Two dots on one atom represent one lone pair.
  • OH− means one O-H ion with overall charge minus one.
  • Br− means one bromine ion with overall charge minus one.
  • A raised minus is a charge sign, not a lone pair.

Do

  • Point to one bond line.
  • Point to one pair of dots.
  • Read each raised minus as overall charge minus one.
Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Hydroxide ion with one oxygen-hydrogen bond, three lone pairs on oxygen and formal charge minus one.
Bromide ion with formal charge minus one and four lone pairs.

Foundation 2

Read brackets around one ion

  • In [CH3-O(··)(··)(··)]−, the brackets enclose one complete methoxide ion.
  • The raised minus outside the bracket applies to the whole ion.
  • The three dot pairs belong to oxygen.

Do

  • Trace the opening and closing brackets.
  • Read the outside sign as overall charge minus one.
  • Point to one oxygen lone pair as the electron source.

Foundation 3

Keep two charge languages separate

  • Cδ+—Brδ− describes unequal sharing inside an unbroken bond.
  • OH− and Br− carry whole-number formal charges.
  • Only whole-number charges enter the reaction charge total.

Do

  • Place delta signs in a partial-charge row.
  • Place raised plus or minus signs in a formal-charge row.
Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Hydroxide ion with one oxygen-hydrogen bond, three lone pairs on oxygen and formal charge minus one.
Bromide ion with formal charge minus one and four lone pairs.

Entry check 1

In the bromoethane graph, identify the carbon directly bonded to Br and state what the C-Br line represents.

Show the recovery
  • The terminal carbon shares the C-Br bond
  • the line represents one shared electron pair.

Return to Concept 1.1 for one-bond tracing and the covalent-bond line.

Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.

Entry check 2

At the carbon bonded to Br, state the ordinary carbon bond-order limit and explain what Cδ+—Brδ− means.

Show the recovery
  • Ordinary neutral carbon has bond-order total four.
  • Delta marks unequal electron sharing, with the carbon end partially positive and Br partially negative
  • they are not free ions.

Return to Concepts 1.2 and 1.6 for bond-order completion, electronegativity, partial charge and C-X polarity.

Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.

Entry check 3

State what a molecular formula can prove and what a reaction comparison must preserve before any new reagent atom is included.

Show the recovery
  • A molecular formula proves atom inventory, not connectivity.
  • A lawful comparison tracks atom identities, direct-neighbour pairs and bond order.

Return to Concepts 1.1-1.2 for inventory versus connectivity and representation conservation.

1 · ObjectiveOpen

What you will understand

Use lone pairs, formal charges and curved electron-pair arrows to audit one complete substitution without assuming its timing.

Problems this Concept unlocks

  • read where a curved arrow starts and ends
  • separate partial charge from full formal charge
  • identify substrate, nucleophile, electrophilic atom and leaving group
  • write the bond-made and bond-broken ledger
  • audit atom, charge, carbon valence and carbon count
2 · MysteryOpen

The problem to explain

Read these words and symbols first

lone pair
Two valence electrons placed on one atom and not shared in a bond
they are drawn as two dots.
A charge sign is not itself a lone pair.
formal charge
A whole-number electron-bookkeeping charge written as + or - on an atom or complete species.
A raised minus means the complete species has one more assigned electron than a neutral species.
OH−
Read hydroxide ion: one O atom bonded to one H atom, with overall charge minus one.
The raised minus is a charge sign
it is not a lone pair.
Br−
Read bromide ion: one Br atom with overall charge minus one.
Br− is not neutral Br bonded to carbon.
  • The reactants are bromoethane and hydroxide, OH−.
  • The products are ethanol and bromide, Br−.
  • Two dots on oxygen show one lone pair.
  • The C-Br line shows one shared electron pair.
Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Hydroxide ion with one oxygen-hydrogen bond, three lone pairs on oxygen and formal charge minus one.
  • Where is each of these two electron pairs in the product state?
  • Does the total whole-number charge change?

Your decision

  • Circle one oxygen lone pair.
  • Circle the C-Br bond.
  • Point to the location of each pair in the products.
3 · InvestigationOpen

Investigation

Investigation 1

Name the electrons we track

Read these words and symbols first

valence electron
An electron counted in an atom's outer bonding layer.
This Chapter uses the supplied school counting model
deeper atomic theory is not required.

Do

Copy the table once, then retrieve O = 6 and Br = 7 without using atomic number or total electrons.

Check

The values are the outer-electron counts used by the formal-charge rule, not the total electrons in each atom.

Investigation 2

Read two dots as one pair

Read these words and symbols first

valence electron
An electron counted in an atom's outer bonding layer.
This Chapter uses the supplied school counting model
deeper atomic theory is not required.
lone pair
Two valence electrons placed on one atom and not shared in a bond
they are drawn as two dots.
A charge sign is not itself a lone pair.

Do

  • On hydroxide, circle one pair of dots on oxygen
  • do not circle the minus sign.
Hydroxide ion with one oxygen-hydrogen bond, three lone pairs on oxygen and formal charge minus one.

Check

  • One bond pair belongs between O and H
  • three lone pairs remain on O in hydroxide.

Investigation 3

Derive full charge; separate delta

Read these words and symbols first

partial charge
Delta plus or delta minus marks unequal sharing inside a polar covalent bond.
Delta does not mean a whole electron was transferred.
formal charge
A whole-number electron-bookkeeping charge written as + or - on an atom or complete species.
A raised minus means the complete species has one more assigned electron than a neutral species.
valence electron
An electron counted in an atom's outer bonding layer.
This Chapter uses the supplied school counting model
deeper atomic theory is not required.
lone pair
Two valence electrons placed on one atom and not shared in a bond
they are drawn as two dots.
A charge sign is not itself a lone pair.

Do

  • Calculate: OH− oxygen 6 - 6 - 1 = -1
  • Br− 7 - 8 - 0 = -1
  • cyanide carbon 4 - 2 - 3 = -1
  • cyanide nitrogen 5 - 2 - 3 = 0
  • a three-bond carbon with no lone pair 4 - 0 - 3 = +1.
  • Then keep Cδ+—Brδ− in a separate partial-charge row.
Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Hydroxide ion with one oxygen-hydrogen bond, three lone pairs on oxygen and formal charge minus one.
Bromide ion with formal charge minus one and four lone pairs.
Cyanide ion with a carbon-nitrogen triple bond and one lone pair on each endpoint. The Lewis ledger derives formal charge minus one on carbon and zero on nitrogen.

Check

  • Neutral checks: four-bond C gives 4 - 0 - 4 = 0
  • three-bond N with one lone pair gives 5 - 2 - 3 = 0
  • two-bond O with two lone pairs gives 6 - 4 - 2 = 0
  • bonded halogen gives 7 - 6 - 1 = 0.

Investigation 4

Identify the bond map that changes

Read these words and symbols first

substrate
The organic molecule whose bond map changes in the reaction.
The word names a reaction role, not a new type of atom.
substitution
One atom or group attached to the changing organic reactant is replaced by another.
This bond-change name does not decide the order or timing of the bond changes.
Later Concepts compare possible pathways.

Do

  • Compare bromoethane and ethanol
  • mark the one bond lost and one bond gained.
Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Two-carbon product in which oxygen is bonded to the carbon that was bonded to bromine.

Check

The C-C bond and both carbon identities are preserved.

Investigation 5

Find the electron-pair donor

Read these words and symbols first

nucleophile
An electron-rich atom or species that supplies a pair of electrons to form a bond.
Identify the exact donating atom, not only the name of the whole reagent.
lone pair
Two valence electrons placed on one atom and not shared in a bond
they are drawn as two dots.
A charge sign is not itself a lone pair.

Do

Name the species, then name its donating atom and point to the pair it supplies.

Hydroxide ion with one oxygen-hydrogen bond, three lone pairs on oxygen and formal charge minus one.

Check

The minus sign supports electron richness, but the arrow starts at the drawn lone pair.

Investigation 6

Find the electron-pair acceptor

Read these words and symbols first

electrophile
An electron-poor atom that accepts an electron pair to form a bond.
The role belongs to the accepting atom in the step under study.
partial charge
Delta plus or delta minus marks unequal sharing inside a polar covalent bond.
Delta does not mean a whole electron was transferred.

Do

Start at Br, cross the C-Br bond to carbon, then use the supplied delta signs to justify the target.

Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.

Check

The arrow targets carbon, not the Brδ− endpoint.

Investigation 7

Identify the departing endpoint

Read these words and symbols first

leaving group
An atom or group that departs with the electron pair from its broken bond.
Departure must be supported by a bond-to-atom electron arrow.
formal charge
A whole-number electron-bookkeeping charge written as + or - on an atom or complete species.
A raised minus means the complete species has one more assigned electron than a neutral species.

Do

Trace C-Br bond to Br and state the before/after formal charge on bromine.

Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Bromide ion with formal charge minus one and four lone pairs.

Check

Neutral bonded Br becomes bromide only after receiving the bonding pair.

Investigation 8

Read every arrow source first

Read these words and symbols first

curved electron-pair arrow
A full curved arrow traces one electron pair from its present location to its next location.
It does not show an atom travelling and does not by itself prove reaction timing.
lone pair
Two valence electrons placed on one atom and not shared in a bond
they are drawn as two dots.
A charge sign is not itself a lone pair.

Do

For each displayed move, say source pair, destination, immediate bond effect—in that order.

Electron-pair mechanism

Net electron and bond ledger for hydroxide substitution

Which electron pairs and bond changes must any complete pathway account for?

BeforeReactant inventory: hydroxide plus bromoethaneThis is a net electron ledger. It compares the complete reactant and product states. It does not decide whether the two bond changes occur together or in separate steps. Concept 2.2 tests the one-step pathway. Concept 2.3 tests the ionisation-first pathway.
Complete expanded bromoethane and hydroxide structures before substitution. Bromoethane has three hydrogens on C1 and two on C2. The carbon bonded to bromine is partially positive and bromine is partially negative. Hydroxide has O-H, three oxygen lone pairs and formal charge minus one.CCδ+Brδ−O−HHHHHH12
  • The carbon bonded to Br is electron-poor. OH− carries one whole negative charge.
  1. Arrow 1
    Pair starts atoxygen lone pairPair ends atcarbon bonded to bromine
    Why
    The oxygen lone pair is available and the carbon end of the polar C-Br bond is electron-poor.
    Immediate effect
    A new C-O bond is formed.
  2. Arrow 2
    Pair starts atC-Br bonding pairPair ends atbromine
    Why
    Carbon cannot keep both the new C-O bond and the old C-Br bond without exceeding ordinary valence.
    Immediate effect
    The C-Br bond breaks heterolytically and bromide is formed.
After both arrowsProduct inventory: ethanol plus bromide
Complete expanded ethanol and bromide structures after substitution. C-O has replaced C-Br, O-H and every C-H bond remain, oxygen has two lone pairs and bromide has four lone pairs and formal charge minus one.CCBr−OHHHHHH
Three compulsory checks
Atoms
The same C1, C2, O, Br and six H atom identities appear before and after.
Charge
Before: hydroxide is -1 and bromoethane is 0. After: ethanol is 0 and bromide is -1.
Carbon valence
C1 has four single bonds; C2 loses C-Br as it gains C-O and keeps four; O changes from one bond and three lone pairs at -1 to two bonds and two lone pairs at 0.

This is an equation-level ledger, not a time-ordered mechanism frame. It records an oxygen pair assigned to the new carbon-oxygen bond and the former carbon-bromine pair assigned to bromine; the carbon skeleton and total formal charge are conserved.

Check

An arrow from an atom label, partial-charge mark or empty space fails the source test.

Investigation 9

Break C-Br without losing electrons

Read these words and symbols first

heterolysis
A bond breaks and both bonding electrons go to the same endpoint.
This is different from splitting one electron to each endpoint.
formal charge
A whole-number electron-bookkeeping charge written as + or - on an atom or complete species.
A raised minus means the complete species has one more assigned electron than a neutral species.
curved electron-pair arrow
A full curved arrow traces one electron pair from its present location to its next location.
It does not show an atom travelling and does not by itself prove reaction timing.

Do

Follow the bond-to-Br arrow and compare formal charges before and after.

Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Bromide ion with formal charge minus one and four lone pairs.
Electron-pair mechanism

Net electron and bond ledger for hydroxide substitution

Which electron pairs and bond changes must any complete pathway account for?

BeforeReactant inventory: hydroxide plus bromoethaneThis is a net electron ledger. It compares the complete reactant and product states. It does not decide whether the two bond changes occur together or in separate steps. Concept 2.2 tests the one-step pathway. Concept 2.3 tests the ionisation-first pathway.
Complete expanded bromoethane and hydroxide structures before substitution. Bromoethane has three hydrogens on C1 and two on C2. The carbon bonded to bromine is partially positive and bromine is partially negative. Hydroxide has O-H, three oxygen lone pairs and formal charge minus one.CCδ+Brδ−O−HHHHHH12
  • The carbon bonded to Br is electron-poor. OH− carries one whole negative charge.
  1. Arrow 1
    Pair starts atoxygen lone pairPair ends atcarbon bonded to bromine
    Why
    The oxygen lone pair is available and the carbon end of the polar C-Br bond is electron-poor.
    Immediate effect
    A new C-O bond is formed.
  2. Arrow 2
    Pair starts atC-Br bonding pairPair ends atbromine
    Why
    Carbon cannot keep both the new C-O bond and the old C-Br bond without exceeding ordinary valence.
    Immediate effect
    The C-Br bond breaks heterolytically and bromide is formed.
After both arrowsProduct inventory: ethanol plus bromide
Complete expanded ethanol and bromide structures after substitution. C-O has replaced C-Br, O-H and every C-H bond remain, oxygen has two lone pairs and bromide has four lone pairs and formal charge minus one.CCBr−OHHHHHH
Three compulsory checks
Atoms
The same C1, C2, O, Br and six H atom identities appear before and after.
Charge
Before: hydroxide is -1 and bromoethane is 0. After: ethanol is 0 and bromide is -1.
Carbon valence
C1 has four single bonds; C2 loses C-Br as it gains C-O and keeps four; O changes from one bond and three lone pairs at -1 to two bonds and two lone pairs at 0.

This is an equation-level ledger, not a time-ordered mechanism frame. It records an oxygen pair assigned to the new carbon-oxygen bond and the former carbon-bromine pair assigned to bromine; the carbon skeleton and total formal charge are conserved.

Check

Total formal charge stays minus one across the complete reaction.

Investigation 10

Name only the structural change

Read these words and symbols first

substitution
One atom or group attached to the changing organic reactant is replaced by another.
This bond-change name does not decide the order or timing of the bond changes.
Later Concepts compare possible pathways.
curved electron-pair arrow
A full curved arrow traces one electron pair from its present location to its next location.
It does not show an atom travelling and does not by itself prove reaction timing.

Do

  • Write MADE and BROKEN on separate lines.
  • Leave timing blank until later Concepts supply pathway evidence.
Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Two-carbon product in which oxygen is bonded to the carbon that was bonded to bromine.
Electron-pair mechanism

Net electron and bond ledger for hydroxide substitution

Which electron pairs and bond changes must any complete pathway account for?

BeforeReactant inventory: hydroxide plus bromoethaneThis is a net electron ledger. It compares the complete reactant and product states. It does not decide whether the two bond changes occur together or in separate steps. Concept 2.2 tests the one-step pathway. Concept 2.3 tests the ionisation-first pathway.
Complete expanded bromoethane and hydroxide structures before substitution. Bromoethane has three hydrogens on C1 and two on C2. The carbon bonded to bromine is partially positive and bromine is partially negative. Hydroxide has O-H, three oxygen lone pairs and formal charge minus one.CCδ+Brδ−O−HHHHHH12
  • The carbon bonded to Br is electron-poor. OH− carries one whole negative charge.
  1. Arrow 1
    Pair starts atoxygen lone pairPair ends atcarbon bonded to bromine
    Why
    The oxygen lone pair is available and the carbon end of the polar C-Br bond is electron-poor.
    Immediate effect
    A new C-O bond is formed.
  2. Arrow 2
    Pair starts atC-Br bonding pairPair ends atbromine
    Why
    Carbon cannot keep both the new C-O bond and the old C-Br bond without exceeding ordinary valence.
    Immediate effect
    The C-Br bond breaks heterolytically and bromide is formed.
After both arrowsProduct inventory: ethanol plus bromide
Complete expanded ethanol and bromide structures after substitution. C-O has replaced C-Br, O-H and every C-H bond remain, oxygen has two lone pairs and bromide has four lone pairs and formal charge minus one.CCBr−OHHHHHH
Three compulsory checks
Atoms
The same C1, C2, O, Br and six H atom identities appear before and after.
Charge
Before: hydroxide is -1 and bromoethane is 0. After: ethanol is 0 and bromide is -1.
Carbon valence
C1 has four single bonds; C2 loses C-Br as it gains C-O and keeps four; O changes from one bond and three lone pairs at -1 to two bonds and two lone pairs at 0.

This is an equation-level ledger, not a time-ordered mechanism frame. It records an oxygen pair assigned to the new carbon-oxygen bond and the former carbon-bromine pair assigned to bromine; the carbon skeleton and total formal charge are conserved.

Check

  • The carbon skeleton C-C remains unchanged.
  • Later Concepts compare the timing models.

Investigation 11

Audit every proposed frame

Read these words and symbols first

formal charge
A whole-number electron-bookkeeping charge written as + or - on an atom or complete species.
A raised minus means the complete species has one more assigned electron than a neutral species.
substitution
One atom or group attached to the changing organic reactant is replaced by another.
This bond-change name does not decide the order or timing of the bond changes.
Later Concepts compare possible pathways.

Do

Build three rows—atoms, charge, carbon valence—and mark pass or repair.

Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Hydroxide ion with one oxygen-hydrogen bond, three lone pairs on oxygen and formal charge minus one.
Two-carbon product in which oxygen is bonded to the carbon that was bonded to bromine.
Bromide ion with formal charge minus one and four lone pairs.
Electron-pair mechanism

Net electron and bond ledger for hydroxide substitution

Which electron pairs and bond changes must any complete pathway account for?

BeforeReactant inventory: hydroxide plus bromoethaneThis is a net electron ledger. It compares the complete reactant and product states. It does not decide whether the two bond changes occur together or in separate steps. Concept 2.2 tests the one-step pathway. Concept 2.3 tests the ionisation-first pathway.
Complete expanded bromoethane and hydroxide structures before substitution. Bromoethane has three hydrogens on C1 and two on C2. The carbon bonded to bromine is partially positive and bromine is partially negative. Hydroxide has O-H, three oxygen lone pairs and formal charge minus one.CCδ+Brδ−O−HHHHHH12
  • The carbon bonded to Br is electron-poor. OH− carries one whole negative charge.
  1. Arrow 1
    Pair starts atoxygen lone pairPair ends atcarbon bonded to bromine
    Why
    The oxygen lone pair is available and the carbon end of the polar C-Br bond is electron-poor.
    Immediate effect
    A new C-O bond is formed.
  2. Arrow 2
    Pair starts atC-Br bonding pairPair ends atbromine
    Why
    Carbon cannot keep both the new C-O bond and the old C-Br bond without exceeding ordinary valence.
    Immediate effect
    The C-Br bond breaks heterolytically and bromide is formed.
After both arrowsProduct inventory: ethanol plus bromide
Complete expanded ethanol and bromide structures after substitution. C-O has replaced C-Br, O-H and every C-H bond remain, oxygen has two lone pairs and bromide has four lone pairs and formal charge minus one.CCBr−OHHHHHH
Three compulsory checks
Atoms
The same C1, C2, O, Br and six H atom identities appear before and after.
Charge
Before: hydroxide is -1 and bromoethane is 0. After: ethanol is 0 and bromide is -1.
Carbon valence
C1 has four single bonds; C2 loses C-Br as it gains C-O and keeps four; O changes from one bond and three lone pairs at -1 to two bonds and two lone pairs at 0.

This is an equation-level ledger, not a time-ordered mechanism frame. It records an oxygen pair assigned to the new carbon-oxygen bond and the former carbon-bromine pair assigned to bromine; the carbon skeleton and total formal charge are conserved.

Check

  • Before charge -1 equals after charge -1
  • C gains C-O as it loses C-Br.

Investigation 12

Name the attacking endpoint

Read these words and symbols first

ambident nucleophile
A nucleophile with two possible attacking atoms.
The reagent and stated syllabus condition decide which endpoint is used.
nucleophile
An electron-rich atom or species that supplies a pair of electrons to form a bond.
Identify the exact donating atom, not only the name of the whole reagent.
formal charge
A whole-number electron-bookkeeping charge written as + or - on an atom or complete species.
A raised minus means the complete species has one more assigned electron than a neutral species.
KCN
Read potassium cyanide: the reagent containing K+ and CN− ions.
In the supplied Board case, the carbon end of CN− forms the new bond.
AgCN
Read silver cyanide: the stated reagent containing silver and the C-N group.
In the supplied Board contrast, the nitrogen end forms the new bond.
nitrile
A product containing the bond path carbon-C triple-bond N, written carbon-C≡N.
The first C after the organic chain is the carbon supplied by CN−.
isocyanide
A product containing the bond path carbon-N triple-bond C, written carbon-N+≡C−.
Nitrile and isocyanide have different direct-neighbour maps.
terminal dash
The dash after an unfinished group shows the one bond that joins that group to the next atom.
The dash is not a minus sign and is not an extra atom.

Do

  • Write the unchanged chain first.
  • Use its terminal dash as the attachment bond.
  • For KCN attach cyanide C.
  • For AgCN attach N.
Cyanide ion with a carbon-nitrogen triple bond and one lone pair on each endpoint. The Lewis ledger derives formal charge minus one on carbon and zero on nitrogen.
Three-carbon nitrile product; the original two-carbon chain is bonded to the carbon endpoint of cyanide.
Neutral three-carbon isocyanide product. The original chain is bonded to N. Nitrogen has formal charge plus one; terminal carbon has one lone pair and formal charge minus one, so the whole molecule is neutral.

Check

  • The two products contain the same C and N inventory.
  • Their direct-neighbour paths differ: chain-C-N versus chain-N-C.
  • In chain-N+≡C−, +1 and -1 give zero overall charge.

Investigation 13

Count incoming carbon

Read these words and symbols first

carbon ledger
A before-and-after count that tracks every carbon source into the product.
It checks atom accounting
it does not replace a bond map.
ambident nucleophile
A nucleophile with two possible attacking atoms.
The reagent and stated syllabus condition decide which endpoint is used.

Do

  • Write the carbon count.
  • Then write the new direct-neighbour path on a separate line.
Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Cyanide ion with a carbon-nitrogen triple bond and one lone pair on each endpoint. The Lewis ledger derives formal charge minus one on carbon and zero on nitrogen.
Three-carbon nitrile product; the original two-carbon chain is bonded to the carbon endpoint of cyanide.
Neutral three-carbon isocyanide product. The original chain is bonded to N. Nitrogen has formal charge plus one; terminal carbon has one lone pair and formal charge minus one, so the whole molecule is neutral.

Check

  • For bromoethane, 2 + 1 = 3 in both products.
  • Atom count cannot distinguish their connectivity.
4 · MeaningOpen

Build the meaning

Meaning 1

Name the electrons we track

  • A covalent-bond line represents a shared pair of valence electrons.
  • For the school counting model in this Chapter, use this supplied outer-electron table: H 1
  • C 4
  • N 5
  • O 6
  • F, Cl, Br and I 7.

Check

The values are the outer-electron counts used by the formal-charge rule, not the total electrons in each atom.

Meaning 2

Read two dots as one pair

Two dots written on one atom represent one lone pair: two valence electrons not shared in a bond.

Check

  • One bond pair belongs between O and H
  • three lone pairs remain on O in hydroxide.

Meaning 3

Derive full charge; separate delta

  • For the displayed school structures: formal charge = outer-electron count - nonbonding electrons - bond-order share.
  • Each bond endpoint receives one share for a single bond, two for a double bond and three for a triple bond.
  • Delta charges describe unequal sharing and never enter this whole-number calculation.

Check

  • Neutral checks: four-bond C gives 4 - 0 - 4 = 0
  • three-bond N with one lone pair gives 5 - 2 - 3 = 0
  • two-bond O with two lone pairs gives 6 - 4 - 2 = 0
  • bonded halogen gives 7 - 6 - 1 = 0.

Meaning 4

Identify the bond map that changes

  • The substrate is the organic molecule whose connectivity changes.
  • Here bromoethane is the substrate because C-Br is replaced by C-O.

Check

The C-C bond and both carbon identities are preserved.

Meaning 5

Find the electron-pair donor

  • A nucleophile supplies an electron pair.
  • In hydroxide, oxygen is the donating atom because a lone pair on O begins the new C-O bond.

Check

The minus sign supports electron richness, but the arrow starts at the drawn lone pair.

Meaning 6

Find the electron-pair acceptor

  • An electrophile accepts an electron pair.
  • In bromoethane, the carbon endpoint of Cδ+—Brδ− is the accepting atom.

Check

The arrow targets carbon, not the Brδ− endpoint.

Meaning 7

Identify the departing endpoint

  • A leaving group departs with the electron pair from its broken bond.
  • Bromine receives the C-Br bonding pair and becomes Br−.

Check

Neutral bonded Br becomes bromide only after receiving the bonding pair.

Meaning 8

Read every arrow source first

  • A full curved arrow begins at an electron pair: either a lone pair or a bond.
  • Its head ends at the atom or bond that receives that pair.

Check

An arrow from an atom label, partial-charge mark or empty space fails the source test.

Meaning 9

Break C-Br without losing electrons

  • When the C-Br pair moves to Br, both bonding electrons reach one endpoint.
  • This heterolytic cleavage produces bromide.

Check

Total formal charge stays minus one across the complete reaction.

Meaning 10

Name only the structural change

  • For this displayed reaction, the product has C-O and the reactant has C-Br.
  • Therefore C-O is made and C-Br is broken.
  • This comparison does not say whether the changes occur together or separately.

Check

  • The carbon skeleton C-C remains unchanged.
  • Later Concepts compare the timing models.

Meaning 11

Audit every proposed frame

A lawful frame conserves atom identities and total formal charge, and no ordinary carbon exceeds bond-order total four.

Check

  • Before charge -1 equals after charge -1
  • C gains C-O as it loses C-Br.

Meaning 12

Name the attacking endpoint

  • Cyanide has a carbon endpoint and a nitrogen endpoint.
  • KCN means potassium cyanide.
  • In the supplied Board case, KCN connects the organic chain to cyanide C and gives a nitrile: chain-C≡N.
  • AgCN means silver cyanide.
  • In the supplied Board contrast, AgCN connects the chain to N and gives an isocyanide: chain-N+≡C−.

Check

  • The two products contain the same C and N inventory.
  • Their direct-neighbour paths differ: chain-C-N versus chain-N-C.
  • In chain-N+≡C−, +1 and -1 give zero overall charge.

Meaning 13

Count incoming carbon

  • Both cyanide products in the stated Board cases preserve the substrate C-C skeleton.
  • Both products contain the incoming cyanide carbon.
  • KCN and AgCN therefore give the same carbon total but different C-C-N and C-N-C paths.

Check

  • For bromoethane, 2 + 1 = 3 in both products.
  • Atom count cannot distinguish their connectivity.
5 · ExamplesOpen

Worked examples

Example 1 of 6

Read one arrow before following it

For the oxygen-to-carbon move, identify source, destination and immediate effect.

Two-carbon bromoethane heavy-atom graph; bromine is directly bonded to the terminal carbon.
Hydroxide ion with one oxygen-hydrogen bond, three lone pairs on oxygen and formal charge minus one.
Electron-pair mechanism

Net electron and bond ledger for hydroxide substitution

Which electron pairs and bond changes must any complete pathway account for?

BeforeReactant inventory: hydroxide plus bromoethaneThis is a net electron ledger. It compares the complete reactant and product states. It does not decide whether the two bond changes occur together or in separate steps. Concept 2.2 tests the one-step pathway. Concept 2.3 tests the ionisation-first pathway.
Complete expanded bromoethane and hydroxide structures before substitution. Bromoethane has three hydrogens on C1 and two on C2. The carbon bonded to bromine is partially positive and bromine is partially negative. Hydroxide has O-H, three oxygen lone pairs and formal charge minus one.CCδ+Brδ−O−HHHHHH12
  • The carbon bonded to Br is electron-poor. OH− carries one whole negative charge.
  1. Arrow 1
    Pair starts atoxygen lone pairPair ends atcarbon bonded to bromine
    Why
    The oxygen lone pair is available and the carbon end of the polar C-Br bond is electron-poor.
    Immediate effect
    A new C-O bond is formed.
  2. Arrow 2
    Pair starts atC-Br bonding pairPair ends atbromine
    Why
    Carbon cannot keep both the new C-O bond and the old C-Br bond without exceeding ordinary valence.
    Immediate effect
    The C-Br bond breaks heterolytically and bromide is formed.
After both arrowsProduct inventory: ethanol plus bromide
Complete expanded ethanol and bromide structures after substitution. C-O has replaced C-Br, O-H and every C-H bond remain, oxygen has two lone pairs and bromide has four lone pairs and formal charge minus one.CCBr−OHHHHHH
Three compulsory checks
Atoms
The same C1, C2, O, Br and six H atom identities appear before and after.
Charge
Before: hydroxide is -1 and bromoethane is 0. After: ethanol is 0 and bromide is -1.
Carbon valence
C1 has four single bonds; C2 loses C-Br as it gains C-O and keeps four; O changes from one bond and three lone pairs at -1 to two bonds and two lone pairs at 0.

This is an equation-level ledger, not a time-ordered mechanism frame. It records an oxygen pair assigned to the new carbon-oxygen bond and the former carbon-bromine pair assigned to bromine; the carbon skeleton and total formal charge are conserved.

Key evidence

  • drawn oxygen lone pair
  • electron-poor carbon endpoint

One lawful move at a time

One move at a time

Step 1 of 3
Action
Locate tail
Why
An arrow must begin at electrons
Result
one O lone pair is selected

Answer

A lone pair moves from O towards carbon, forming C-O.

Independent check

The after-scene contains C-O and no unexplained atom appears.

Counter-pattern

An arrow beginning at the O label or minus sign has no explicit electron-pair source.

6 · QuestionsOpen

Questions

Question 1 of 14

Start an electron-pair arrow at a drawn lone pair rather than at an atom label or charge sign.

On the displayed hydroxide ion, where must an electron-pair arrow begin?

Hydroxide ion with one oxygen-hydrogen bond, three lone pairs on oxygen and formal charge minus one.

Preview only.

Your selection is not checked or saved.

Select one answer.
7 · FeedbackOpen

Feedback is disabled in this preview

This preview does not register the Organic provider.

No attempt, working or comment is sent to the database.

Record review notes separately.

Persistent learner feedback becomes available only after the scientific and human-review gates close.