3.4 Electron-pair sharing reactions

A nucleophile is an electron-rich species that donates an electron pair to an electron-deficient centre.

Look for an available electron pair: OH⁻ and CN⁻ use a negative charge and lone pair, while NH₃ uses a lone pair without being an anion. A curly arrow must start at that pair and point toward the atom where the new bond forms.

Representative question

[Maximum number: 1]

Identify a nucleophile which could be used for this reaction.

$\mathrm{OH}^{-}$

Marking guidance:

Accept water / $\mathrm{H}_{2} \mathrm{O}$

Accept "hydroxide"/ "sodium hydroxide / NaOH"

The nucleophile donates a pair to carbon while the leaving group departs with its bonding pair. Deduce the product by replacing the leaving group with the nucleophile.

For CH₃CH₂Br + OH⁻, the C–O bond forms as the C–Br bond breaks, giving CH₃CH₂OH + Br⁻. Account for charge and every atom in the product; the leaving group takes the bonding pair rather than departing as a neutral bromine atom.

Representative question

[Maximum number: 4]

Explain the mechanism of the reaction, using curly arrows to represent the movement of electron pairs.

curly arrow from lone pair/negative charge on O in OH to C attached to Br curly arrow from C-Br bond to Br

transition state showing negative charge AND partial bonds products ( $\mathrm{Br}^{-}$ AND $\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}$ )

Award [3 max] if $S_{N} 1$ mechanism is given.

Accept curly arrows in the transition state.

Do not penalize if HO and Br are not at $180^{\circ}$ .

Accept NaBr as part of the products only if $\mathrm{Na}^{+}$ is shown at the start.

In heterolytic fission both bonding electrons remain with one fragment, producing ions. Curly arrows show movement of an electron pair.

Place the curly-arrow tail on the bond being broken and its head on the fragment receiving both electrons. Then assign charges from electron ownership: heterolysis creates ions, unlike homolysis, which gives one electron to each radical.

Representative question

[Maximum number: 2]

Contrast homolytic and heterolytic fission.

Homolytic fission:

Heterolytic fission:

An electrophile is an electron-deficient species that accepts an electron pair from a nucleophile.

Identify the electron-poor atom, not merely a positive-looking formula. H⁺ and carbocations are electrophiles, and the δ⁺ carbon in a polar C–X bond can also accept a pair. The incoming curly arrow ends at this acceptor.

Representative question

[Maximum number: 1]

Which species is the electrophile?

\mathrm{CH}_{3} \mathrm{Br}+\mathrm{OH}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{OH}+\mathrm{Br}^{-}

$\mathrm{OH}^{-}$

$\mathrm{Br}^{-}$ c. $\mathrm{CH}_{3} \mathrm{OH}$ D. $\mathrm{CH}_{3} \mathrm{Br}$

The electron-rich C=C attacks an electrophile. Deduce addition products with water, halogens or hydrogen halides within the SL mechanism boundary.

Treat the C=C as the reactive site and place the two added groups on its two carbon atoms. Bromine addition removes the double bond and forms a dibromoalkane; hydration forms an alcohol. At SL, deducing these products does not require a mechanism.

Reagent	Groups added across C=C	Product check
X₂ (for example Br₂)	X and X	vicinal dihalogenoalkane; C=C becomes C–C
HX	H and X	halogenoalkane; conserve the H and halogen from HX
H₂O/steam under acid-catalysed hydration conditions	H and OH	alcohol; conserve the carbon skeleton

At SL, use reagent and atom conservation to deduce products; curly-arrow mechanisms are not assessed in this card.

Representative question

[Maximum number: 1]

Predict the product of the reaction between ethene and bromine.

A Lewis acid accepts an electron pair; a Lewis base donates an electron pair. Nucleophiles correspond to Lewis bases and electrophiles to Lewis acids.

In BF₃ + NH₃ → F₃B←NH₃, NH₃ donates the pair and is the Lewis base; BF₃ accepts it and is the Lewis acid. Classify the roles from electron-pair movement rather than from whether H⁺ appears.

Representative question

[Maximum number: 1]

What is the role of the $\mathrm{CN}^{-}$ ion in the reaction of 1-chloropropane with excess KCN in ethanol?

\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{Cl}+\mathrm{KCN} \rightarrow \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{CN}+\mathrm{KCl}

Electrophile and Lewis base

Nucleophile and Lewis acid

Electrophile and Lewis acid

Nucleophile and Lewis base

A ligand acts as a Lewis base and donates an electron pair to a Lewis-acid transition-metal cation, forming a coordinate bond.

Show a coordination bond with an arrow from a ligand lone pair to the metal ion. The arrow records the origin of the shared pair; after formation the bond is not a different electrostatic species from other covalent bonds.

Representative question

[Maximum number: 1]

Outline how ammonia acts as a Lewis base when it forms the complex ion

\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right]^{2+}(\mathrm{aq}) .

it donates an electron/lone pair «to $\mathrm{Cu}^{2+}$ »

Marking guidance:

Accept diagram showing coordination
bond from lone pair on N to $\mathrm{Cu}^{2+}$ .

Identify the central transition-metal cation and the surrounding ligands. Each ligand donates an electron pair to the metal centre.

Read [Cu(NH₃)₄]²⁺ as one Cu centre with four NH₃ ligands and coordination number 4. Use ligand charges and the overall bracket charge to deduce the metal oxidation state; do not confuse coordination number with oxidation state.

Representative question

[Maximum number: 1]

Which statements are correct for the complex ion $\left[\mathrm{FeCl}_{4}\right]^{2-}$ ?

I. Chloride ions are behaving as ligands.
II. The oxidation state of iron is +3 .
III. Iron ion forms coordination bonds with chloride ions.

I and II only

I and III only

II and III only

I, II and III

Retrieve the route: classify nucleophiles and electrophiles, show heterolysis, write substitution and addition mechanisms, map Lewis coordination, compare SN1/SN2, and restore aromaticity in benzene substitution.

Check electron-pair arrow origin and destination, leaving-group departure, intermediate identity, carbocation stability and the assessed mechanism boundary.

Objective notes

8 learning objectives

3.4.1Nucleophiles• Donate electron pair to electrophile• Recognize nucleophiles as electron-rich speciesView 3.4.2Nucleophilic substitution• Nucleophile donates pair, forms new bond• Another bond breaks, leaving group departs• Deduce substitution products from nucleophile and leaving groupView 3.4.3Heterolytic fission• Both electrons stay with one fragment• Forms ions• Use curly arrows to show electron-pair movementView 3.4.4Electrophiles• Accept electron pair from nucleophile• Recognize electrophiles as electron-deficient speciesView 3.4.5Electrophilic addition to alkenes• High electron density at C=C bond• Reactions: + H₂O, + halogens, + hydrogen halides• Deduce alkene addition equations; SL mechanisms are not assessedView 3.4.6Lewis acid-base theory• Lewis acid = electron-pair acceptor• Lewis base = electron-pair donor• Classify nucleophiles/electrophiles using Lewis acid-base theoryView 3.4.7Coordination bond formation• Lewis base + Lewis acid• Nucleophiles = Lewis bases• Electrophiles = Lewis acids• Explain coordination bond formation with electron-pair donationView 3.4.8Complex ions• Ligands donate electron pairs to transition metal cations• Identify ligands and central transition metal cations in complex ionsView

3.4 Electron-pair sharing reactions

Recognizing Nucleophiles

Identifying Nucleophiles

Assessment in practice

Nucleophilic Substitution

Deducing Substitution Products

Assessment in practice

Heterolytic Fission

Showing Electron-Pair Movement

Assessment in practice

Recognizing Electrophiles

Recognizing Electrophiles

Assessment in practice

Electrophilic Addition to Alkenes

Deducing Alkene-Addition Products

Assessment in practice

Lewis Acids and Bases

Classifying Lewis Acids and Bases

Assessment in practice

Coordination Bonds

Explaining Coordinate-Bond Formation

Assessment in practice

Ligands and Complex Ions

Identifying Complex-Ion Components

Assessment in practice

Electron-Pair Sharing Summary

Objective notes

Question 1

Question 1

Question 1

Question 1

Question 1

Question 1

Question 1

Question 1