Chemistry · Lesson

Electron-pushing notation, nucleophiles and electrophiles

Use curved arrows to record electron redistribution, then check the resulting bonds, charges and atom identities. Treat a consistent electron ledger as a proposed step rather than proof of a real pathway.

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By the end, you can

Distinguish a two-electron curved arrow from a one-electron fishhook.
Name the electron source and destination in a proposed polar step.
Check formal charge, electron count and ordinary valence together.
Keep equilibrium basicity separate from relative nucleophilic reaction rate.

A mechanism proposes how the net change occurs

A net equation compares starting and ending species. A mechanism proposes a sequence of elementary events or a more detailed pathway consistent with evidence. The same balanced net can be compatible with several mechanisms. A plausible arrow drawing is a bookkeeping hypothesis; observed rates, stereochemistry, isotope effects and other evidence help test it.

A straight reaction arrow separates the species of a step. A curved electron-flow arrow within a mechanistic drawing has a different job: it starts at an electron source and points toward the location where those electrons contribute after the proposed redistribution. It is not a measured trajectory of an electron through space or time.

The arrowhead states how many electrons are being followed

A full-headed curved arrow represents a pair of electrons in familiar polar-reaction notation. Its source can be a lone pair or a bond; its destination can be an atom or the place where a new bond is formed. A single-barbed curved fishhook represents one electron in a radical ledger. State which atom or bond each tail belongs to rather than starting an arrow at a positive charge with no electron source.

For heterolytic bond cleavage, one full-headed arrow sends the original bonding pair to one partner. For homolytic cleavage, two fishhooks allocate one original bond electron to each partner. Charge and unpaired-electron counts need independent checks. A curved arrow and a resonance arrow also have different meanings: resonance contributors redraw one entity rather than form successive intermediates.

A nitrogen pair can become the new N–H bond

In the original symbolic step CH₃NH₂+H⁺→CH₃NH₃⁺, neutral amine N has three ordinary bonds and one lone pair. The pair contributes to a new bond with the incoming proton. Product N has four ordinary bonds, no lone pair and formal charge +1. The pair is neither destroyed nor donated as two free electrons outside the product.

The combined C₁H₆N₁ nuclear count, total charge +1 and total valence-electron count 14 are retained. H⁺ supplies a nucleus but no electron; carbon’s group is unchanged. The figure separates the green curved pair arrow from the straight net arrow. Positions are original schematic choices, not molecular coordinates or an actual solution protonation protocol.

An electron pair becomes the new N–H bond

Original symbolic CH₃NH₂ + H⁺ → CH₃NH₃⁺ electron ledger. Two green dots represent the N lone pair; the full-headed green curved arrow follows that pair toward the partner proton. The white straight arrow summarizes the species change. Total C₁H₆N₁ nuclei, charge +1 and 14 valence electrons are retained. No measured electron trajectory, molecular geometry or operating conditions are represented.

Text equivalent: a nitrogen lone pair forms a bond to the incoming H⁺. N goes from three bonds/one pair/formal charge 0 to four bonds/no pair/formal charge +1. The net step preserves every nucleus and electron.

Underlying electron and charge ledger

Bookkeeping before and after the selected step
Quantity	Combined reactants	Product
Ordinary bonds at N	3	4
Nonbonding electrons at N	2	0
Formal charge at N	0	1
Total species charge	1	1
Total valence electrons	14	14

Account for every simultaneous bond-order change

For a conceptual hydroxide addition at an ordinary carbonyl carbon, one pair forms the new C–O bond and the original C=O π pair moves toward its oxygen. If only the incoming bond were added while the original double bond and other attachments stayed unchanged, that carbon would have ten electrons in its ordinary Lewis shell. The accompanying π redistribution retains an eight-electron carbon octet.

After that selected addition ledger, the original carbonyl oxygen has alkoxide formal charge −1 and the attacking hydroxide oxygen has its stated OH attachment. Subsequent proton transfer requires its own partner and arrow bookkeeping. Do not silently fill a missing water, proton, leaving-group pair or formal charge merely to make a drawing look familiar.

Basicity and nucleophilicity ask different comparisons

Brønsted basicity refers to a proton-transfer equilibrium in a stated medium; the corresponding conjugate-acid pK_a is one way to describe it. Relative nucleophilicity normally compares reaction rate constants toward a common substrate under matched conditions. Charge alone and a universal pK_a list do not determine those rates.

An invented counterexample has conjugate-acid pK_a values 9.00 for X and 7.00 for Y in one matched convention, but second-order rate coefficients 1.00 and 2.00 L mol⁻¹ s⁻¹ toward the same substrate. X has greater proton-accepting equilibrium strength in that comparison, while Y has twice the specified nucleophilic rate coefficient. These are stipulated abstract inputs, not a real compound ranking or a general solvent-independent rule.

Worked examples

Check the charge and electrons of the protonation step

For the stated CH₃NH₂+H⁺→CH₃NH₃⁺ ledger, check N’s formal charge and the total valence-electron count.

Before, N charge is 5−2−3=0: five valence electrons, two nonbonding, one assigned per ordinary bond.
After, N charge is 5−0−4=+1.
For either combined side, total electrons are C 4 + N 5 + H 6 − positive-charge 1 = 14.
The new bond uses the original N pair.

N changes formal charge 0→+1 while the total electron count remains 14.

Return a bond pair to the atom losing a proton

For the abstract alcohol dissociation CH₃CH₂OH→CH₃CH₂O⁻+H⁺, where does the O–H pair go?

Use a full-headed curved arrow from the O–H bond to oxygen.
O changes from two ordinary bonds/four nonbonding electrons to one ordinary bond/six nonbonding electrons.
Its formal charge changes 0→−1 while H⁺ carries +1.

The pair remains on oxygen and total charge stayszero. A complete medium-specific proton-transfer pathway requires a real accepting partner.

Avoid replacing a rate comparison by pKₐ

Use the stipulated X/Y acid and rate inputs under their matched conventions.

The two-unit conjugate-acid pK_a difference corresponds to a factor 10²=100 in the compatible proton-accepting equilibrium comparison.
The nucleophilic rate-coefficient ratio is k_Y/k_X=2.00/1.00=2.00.
The rankings therefore differ in this invented case.

X is the stronger stipulated Brønsted base, while Y has twice the stated nucleophilic coefficient toward the same substrate.

Try it yourself

How many electrons does a full-headed curved arrow represent in the stated polar notation?
Show the answer and reasoning
Two.
It follows an electron pair from a specified source toward its destination.
How many fishhook arrows allocate both electrons of one homolytically cleaved bond to different partners?
Show the answer and reasoning
Two.
Each original bond electron is assigned to one partner.
In the stated carbonyl addition, may the new bond be added while retaining the original C=O π pair at carbon unchanged?
Show the answer and reasoning
No, within the ordinary second-row Lewis model.
That would give ten electrons around carbon; the coupled π-pair redistribution must also be accounted for.
Does a charge-balanced curved-arrow proposal by itself prove the actual mechanism?
Show the answer and reasoning
No.
It must also be compatible with kinetic, stereochemical and other relevant evidence.

What to carry forward

Begin an electron-flow arrow at a real pair or single-electron source.
Retain nuclei, electrons, charge and ordinary valence together.
Keep net, electron-flow and resonance arrows distinct.
Use matched rate constants for nucleophilicity comparisons.
A plausible ledger still needs mechanistic evidence.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.