Electron Pair Sharing Reactions

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Nucleophiles

A nucleophile is an electron-rich species that can donate a pair of electrons.
'Nucleophile' means 'nucleus/positive charge loving' — nucleophiles are attracted to positively charged species.
Neutral nucleophiles include H₂O, NH₃, ROH (alcohols) and RNH₂ (amines).
Charged nucleophiles include OH⁻, Cl⁻, CN⁻ and carbanions (R⁻).
OH⁻ is a stronger nucleophile than H₂O because it carries a full negative charge, while the oxygen in H₂O has only a partial negative charge.
This makes OH⁻ more reactive and more likely to donate its lone pair quickly.

Nucleophilic Substitution

Nucleophilic substitution is a reaction in which a nucleophile attacks a carbon atom carrying a partial positive charge.
An atom with a partial negative charge is replaced by the nucleophile.
Halogenoalkanes undergo nucleophilic substitution because the C–X bond is polar — the carbon is δ⁺ and the halogen is δ⁻.
The bond that forms and the bond that breaks must both involve the carbon atom bonded to the leaving group.
In the hydrolysis of a halogenoalkane, the nucleophile is OH⁻ and an aqueous solution of NaOH or KOH with ethanol is used.
The reaction is very slow at room temperature, so the mixture is warmed.
Example: CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻ (bromoethane → ethanol), with Br⁻ as the leaving group.
Halogens make good leaving groups because they form relatively weak bonds with carbon and their high electronegativity pulls electron density away from carbon.

Rate of Nucleophilic Substitution

The rate depends on the halogen: the stronger the C–X bond, the slower the reaction.
Bond enthalpy trend: C–F > C–Cl > C–Br > C–I.
Fluoroalkanes are unreactive, while iodoalkanes react very quickly.
When the nucleophile is neutral (e.g. H₂O), the initial product is positive and then deprotonates, losing H⁺ to form a neutral product.
Example: CH₃CH₂Cl + H₂O → CH₃CH₂OH₂⁺ → CH₃CH₂OH + H⁺.

Heterolytic Fission

Heterolytic fission is the breaking of a covalent bond so that both bonding electrons are taken by the more electronegative atom.
This forms a negative ion (which gains both electrons) and a positive ion (left with none).
In mechanism diagrams, a curly double-headed arrow shows the movement of a pair of electrons from the bond to one atom.
For example, when an H–Cl bond breaks heterolytically, the electrons move to chlorine forming Cl⁻, leaving H⁺.
The negative ion is electron-rich, can donate a pair of electrons and is a nucleophile.
The positive ion is electron-deficient, can accept a pair of electrons and is an electrophile.

Nucleophile–Electrophile Interaction

A nucleophile can donate a pair of electrons to an electrophile, forming a coordinate covalent bond.
This is the opposite direction to heterolytic bond breaking and is key to many organic mechanisms, such as nucleophilic substitution.
A nucleophile 'loves' a positive charge and an electrophile 'loves' a negative charge.

Electrophiles

An electrophile is a species that forms a covalent bond when it reacts with a nucleophile by accepting electrons.
Electrophiles are electron-deficient and typically have a full or partial positive charge.
Neutral electrophiles include HX (hydrogen halides), X₂ (halogens), H₂O and RX (halogenoalkanes).
Positively charged electrophiles include H⁺, NO₂⁺ (nitronium), NO⁺ (nitrosonium) and carbocations (R⁺).

Electrophilic Addition Reactions

Electrophilic addition is the reaction of an electrophile with a carbon–carbon double bond, C=C.
The C=C double bond is an area of high electron density, making it attractive to electrophiles.
The C=C bond breaks, forming a single C–C bond and a new bond from each carbon to the electrophile.
Electrophilic addition includes addition of steam (H₂O) to form alcohols, hydrogen halides (HX) to form halogenoalkanes, and halogens (X₂) to form dihalogenoalkanes.
Alkenes are unsaturated and the atoms around the C=C bond are planar with a bond angle of 120°.
The C=C bond consists of one σ-bond and a weaker π-bond; the π-bond can be broken and replaced by stronger σ-bonds.
This allows alkenes to undergo addition reactions, which are not possible for saturated alkanes, making alkenes more reactive than alkanes.

Addition across a carbon–carbon double bond

Addition of Water (Hydration)

Hydration occurs when alkenes are treated with steam at 300 °C and 60 atmospheres pressure.
An acid catalyst such as sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄) is used.
Water is added across the double bond, converting the alkene into an alcohol.
Example: CH₂=CH₂ + H₂O → CH₃CH₂OH (ethene → ethanol).
The reaction proceeds via an intermediate in which H⁺ and HSO₄⁻ ions are added across the double bond.
The intermediate is quickly hydrolysed by water, regenerating the sulfuric acid catalyst.
This industrial route to ethanol is faster and more efficient than fermentation.

Addition of Halogens (Halogenation)

Alkenes react with halogens in an electrophilic addition called halogenation.
The π-bond breaks and is replaced by two new C–X bonds, forming a dihalogenoalkane.
This reaction occurs readily at room temperature.
Bromine water is an orange/yellow solution of Br₂(aq) and is used to test for unsaturation.
If a C=C bond is present, an addition reaction occurs and the coloured solution becomes decolourised.
If no C=C bond is present, there is no reaction and the solution remains orange/yellow.
The bromine water test is the standard test for unsaturation in alkenes.

Addition of Hydrogen Halides (Hydrohalogenation)

Alkenes react with hydrogen halides (e.g. HCl, HBr) to form halogenoalkanes.
This is an electrophilic addition reaction that occurs rapidly at room temperature.
All hydrogen halides react with alkenes in this way.
The reaction is fastest in the order HI > HBr > HCl because weaker H–X bonds break more easily.

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1.Which of these compounds would act as a nucleophile?
Easy
- AC6H12
- BH+
- COH-
- DAl
2.Ethene reacts with steam in the presence of sulfuric acid: C2H4 + H2O → CH3CH2OH. What type of reaction is this?
Easy
- Aacid / base
- Baddition
- Chydrolysis
- Dsubstitution
3.Which reaction is an example of nucleophilic substitution?
Easy
- AC6H6 + Br2 → C6H5Br + HBr
- BCH2=CH2 + HBr → CH3CH2Br
- CC3H7Br + H2O → C3H7OH + HBr
- DC2H6 + Br2 → C2H5Br + HBr
4.Bromomethane, CH3Br, can be made by reacting methanol with hydrogen bromide: CH3OH + HBr → CH3Br + H2O. What type of reaction is this?
Easy
- Acondensation
- Belectrophilic substitution
- Cfree radical substitution
- Dnucleophilic substitution
5.What reagents and conditions are needed to convert but-2-ene into butane?
Easy
- AConcentrated sulfuric acid, steam, 300 °C
- BHydrogen gas, Ni catalyst, 150 °C
- CAcidified potassium dichromate, heating under reflux
- DHydrogen bromide, room temperature
6.Which reaction(s) can alkenes undergo? I. Addition II. Hydration III. Reduction
Medium
- AI and II only
- BI and III only
- CII and III only
- DI, II and III
7.In the hydrolysis of bromoethane by aqueous sodium hydroxide, what is the nature of the attacking group and of the leaving group?
Medium
- Aattacking group: electrophile; leaving group: electrophile
- Battacking group: electrophile; leaving group: nucleophile
- Cattacking group: nucleophile; leaving group: electrophile
- Dattacking group: nucleophile; leaving group: nucleophile
8.The synthesis of ethyl butanoate can be carried out in three steps: C4H9Br → C4H9OH → C3H7COOH → C3H7COOC2H5. What is the correct classification of the steps I, II and III?
Medium
- AI substitution; II oxidation; III condensation
- BI addition; II substitution; III condensation
- CI oxidation; II substitution; III condensation
- DI substitution; II oxidation; III substitution

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9.Which compound could be X in this two-step reaction pathway? C3H8 → X → C3H7OH
Medium
- AC2H5CN
- BC3H7Cl
- CC3H6Cl2
- DC2H5COOH
10.Which molecule can react with both sodium borohydride, NaBH4, and warm acidified potassium dichromate(VI)?
Hard
- ACH3CHOHCH3
- BCH3C(CH3)2CHO
- C(CH3)3COH
- DCH3C(CH3)2CH2OH
11.When an excess of bromine, Br2(l), reacts with limonene at room temperature in the dark, which product is formed?
Medium
- AA product in which one C=C bond has been brominated
- BA product in which both C=C bonds have been brominated
- CA product in which the C=C bonds are unchanged
- DA product in which bromine substitutes a hydrogen atom
12.Which compound has an Mr of 84.18 and will react with HBr to give a product with an Mr of 165.09?
Hard
- AC6H12 (Mr = 84.18)
- BC6H14 (Mr = 86.20)
- CC5H10 (Mr = 70.15)
- DC6H6 (Mr = 78.12)
13.Which of the following species are nucleophiles? (select all that apply)
Medium
- AH2O
- BNH3
- COH-
- DCN-
- EH+
14.Which statements about the C=C double bond in alkenes are correct? (select all that apply)
Medium
- AIt consists of one sigma bond and one π bond
- BThe π bond is weaker than the sigma bond
- CThe atoms around the C=C bond are planar with a bond angle of 120°
- DThe π bond is stronger than the sigma bond
- EIt is an area of low electron density
15.Heterolytic fission of a covalent bond produces one positive ion and one negative ion.
Easy
True or false?
16.A nucleophile is an electron-deficient species that accepts a pair of electrons.
Easy
True or false?
17.Bromine water is decolourised when shaken with an alkene but remains orange/yellow with an alkane.
Easy
True or false?
18.Match each term with its correct definition.
Medium
- Nucleophile
- Electrophile
- Heterolytic fission
- Leaving group
- Electron-rich species that donates a pair of electrons
- Electron-deficient species that accepts a pair of electrons
- Bond breaking in which both electrons go to one atom
- Atom or group that is replaced and departs with a pair of electrons
19.Match each addition reaction of an alkene with the product it forms.
Medium
- Addition of steam, H2O(g)
- Addition of a halogen, X2
- Addition of a hydrogen halide, HX
- Addition of hydrogen, H2
- Alcohol
- Dihalogenoalkane
- Halogenoalkane
- Alkane
20.Place the steps of the mechanism for the nucleophilic substitution of bromoethane by hydroxide ions in the correct order.
Medium
- The C-Br bond breaks heterolytically, with Br taking both electrons
- The hydroxide ion donates a pair of electrons to the δ+ carbon
- A bromide ion is released as the leaving group
- A C-O bond forms and ethanol is produced
21.Arrange these carbon-halogen bonds from strongest to weakest.
Medium
- C-F
- C-Cl
- C-Br
- C-I
22.Which pair of products is formed when propene reacts with HCl?
Medium
- A1-chloropropane and 2-chloropropane
- B1-chloropropane and 1,2-dichloropropane
- C2-chloropropane and propan-2-ol
- Dpropane and chloropropane
23.Which of the following species are classified as neutral electrophiles? (select all that apply)
Medium
- AHBr
- BBr2
- COH-
- DH3O+
- ENH3
24.Match each species with its correct role in organic reactions.
Medium
- Nucleophile
- Electrophile
- Heterolytic fission
- Leaving group
- Electron-rich species that donates a pair of electrons
- Electron-deficient species that accepts a pair of electrons
- Bond breaking where both electrons go to one atom
- Atom or group that is replaced, taking a pair of electrons
25.Place the steps of the electrophilic addition mechanism of HBr to propene in the correct order.
Medium
- The π-bond breaks and H+ adds to a carbon, forming a carbocation
- The C=C double bond induces a dipole in HBr, making H δ+
- Br- donates a pair of electrons to the carbocation, forming a C–Br bond
26.Which statements about heterolytic fission are correct? (select all that apply)
Medium
- AIt produces a positive ion and a negative ion.
- BBoth bonding electrons are taken by the more electronegative atom.
- CIt is shown in mechanisms using a curly double-headed arrow.
- DIt produces two radicals.
- EThe positive ion formed acts as a nucleophile.
27.Which of the following is the best nucleophile?
Medium
- AH2O
- BOH-
- CNH3
- DCH3OH
28.What is the bond angle around each carbon atom in the C=C double bond of an alkene?
Easy
- A90°
- B109.5°
- C120°
- D180°
29.Which of the following statements about the hydrolysis of bromoethane with aqueous sodium hydroxide is correct?
Medium
- AThe hydroxide ion acts as an electrophile and bromide acts as a nucleophile.
- BThe hydroxide ion acts as a nucleophile and bromide acts as a leaving group.
- CThe hydroxide ion acts as a leaving group and bromide acts as a nucleophile.
- DBoth hydroxide and bromide act as nucleophiles.
30.In the reaction CH3CH2Br + OH- → CH3CH2OH + Br-, the hydroxide ion is the nucleophile.
Easy
True or false?
31.Which of the following statements about the reaction of alkenes with hydrogen halides are correct? (select all that apply)
Medium
- AThe reaction is an electrophilic addition.
- BThe reaction occurs rapidly at room temperature.
- CThe reaction is fastest with HI and slowest with HCl.
- DThe reaction is a nucleophilic substitution.
- EThe product is a dihalogenoalkane.
32.Match each addition reaction of an alkene with the reagent used and the product formed.
Medium
- Hydration
- Halogenation
- Hydrohalogenation
- Steam, H2O(g), with acid catalyst → alcohol
- Halogen, X2 → dihalogenoalkane
- Hydrogen halide, HX → halogenoalkane
33.Arrange the following carbon–halogen bonds from strongest to weakest.
Medium
- C–I
- C–F
- C–Br
- C–Cl
34.A student shakes an unknown liquid with bromine water and the orange colour disappears. Which conclusion is best supported by this observation?
Medium
- AThe liquid contains a C=C double bond and has undergone electrophilic addition
- BThe liquid is an alkane that has undergone free radical substitution
- CThe liquid contains a C–X bond that has undergone nucleophilic substitution
- DThe liquid is an alcohol that has been oxidised by the bromine
35.Which statement explains why the reaction of an alkene with HI is faster than with HCl?
Medium
- AThe H–I bond is weaker than the H–Cl bond, so it breaks more easily
- BThe H–I bond is stronger than the H–Cl bond, so it breaks more easily
- CIodine is a better leaving group than chlorine because it is more electronegative
- DHI is a stronger acid than HCl so it protonates the alkene more readily
36.A reaction pathway is shown: but-1-ene → X → butan-1-ol. Which statements about this pathway are correct? (select all that apply)
Medium
- AThe first step could be addition of HBr to form 1-bromobutane
- BThe second step could be hydrolysis with aqueous NaOH, which is nucleophilic substitution
- CThe second step could be addition of steam with an acid catalyst
- DThe first step must be a reduction with hydrogen and a nickel catalyst
- EThe second step involves an electrophile attacking the halogenoalkane
37.A halogenoalkane R–Br (Mr = 137.0) reacts with aqueous NaOH to form an alcohol R–OH (Mr = 74.0). What is the relative molecular mass of R? Use H = 1.01, O = 16.00, Br = 79.90.
Hard
- A57.1
- B41.1
- C73.1
- D137.0
38.In the reaction CH3CH2Cl + H2O → CH3CH2OH + HCl, the water molecule acts as a nucleophile and the initial organic product is positively charged before deprotonation.
Easy
True or false?
39.Match each reaction condition with the type of organic reaction it produces.
Medium
- Alkene + H2O(g), H2SO4 catalyst, 300 °C
- Halogenoalkane + aqueous NaOH, warm
- Alkene + Br2, room temperature
- Alkene + HBr, room temperature
- Electrophilic addition forming an alcohol
- Nucleophilic substitution forming an alcohol
- Electrophilic addition forming a dihalogenoalkane
- Electrophilic addition forming a halogenoalkane
40.A student investigates the rate of hydrolysis of three halogenoalkanes with aqueous NaOH. Arrange the halogenoalkanes from fastest to slowest reaction.
Medium
- iodoethane
- bromoethane
- chloroethane
- fluoroethane
41.The relative atomic masses are H = 1.01, Br = 79.90. Calculate the mass of one HBr unit added to an alkene during electrophilic addition. Give your answer to 2 decimal places.
Medium
42.Calculate the relative molecular mass, Mr, of C6H12. Use C = 12.01 and H = 1.01.
Medium
43.Complete the definition of a nucleophile.
Easy
A nucleophile is an electron-____ species that can ____ a pair of electrons.
44.Complete the sentence about heterolytic fission.
Easy
In heterolytic fission, both bonding electrons are taken by the more ____ atom, forming a ____ ion and a positive ion.
45.Name the three types of addition reaction that alkenes can undergo with the reagents listed.
Easy
Alkenes undergo ____, ____, and ____ reactions.
46.State the type of reaction that occurs when propene is converted into chloropropane.
Medium
47.State the type of isomerism exhibited by these chloropropane products.
Medium
48.Calculate the relative molecular mass, Mr, of propene, C3H6. Use C = 12.01 and H = 1.01.
Medium
49.Name the type of reaction that occurs when propene reacts with bromine water.
Medium
50.Using H = 1.01 and Br = 79.90, calculate the relative molecular mass, Mr, of hydrogen bromide, HBr. Give your answer to 2 decimal places.
Medium
51.Complete the table by naming the reagent added across the C=C bond to form each product.
Medium
To form an alcohol, the reagent is ____. To form a dihalogenoalkane, the reagent is ____. To form a halogenoalkane, the reagent is ____.
52.Name the industrial process in which ethene is converted into ethanol using steam at 300 °C and 60 atm with an acid catalyst.
Medium
53.State the meaning of the term nucleophile.
Easy
54.Explain why the hydroxide ion, OH-, is a stronger nucleophile than water.
Medium
55.Explain why halogenoalkanes are more reactive than alkanes.
Hard
56.An aromatic organic compound with molecular formula C7H8 reacts with bromine in the presence of UV light to produce a compound with molecular formula C7H7Br. Name the type of reaction taking place and give the formula of an additional organic product that could be obtained.
Hard
57.State the reagents and conditions required for the formation of propan-1-ol from 1-bromopropane.
Medium
58.The relative atomic masses are H = 1.01, Br = 79.90. Calculate the mass of one HBr unit added to an alkene during electrophilic addition. Give your answer to 2 decimal places.
Medium
- A72.82 g/mol
- B89 g/mol
- C80.91 g/mol
- D97.09 g/mol
59.Calculate the relative molecular mass, Mr, of C6H12. Use C = 12.01 and H = 1.01.
Medium
- A75.76
- B92.6
- C84.18
- D101.02
60.Calculate the relative molecular mass, Mr, of propene, C3H6. Use C = 12.01 and H = 1.01.
Medium
- A42.09
- B50.51
- C37.88
- D46.3
61.Using H = 1.01 and Br = 79.90, calculate the relative molecular mass, Mr, of hydrogen bromide, HBr. Give your answer to 2 decimal places.
Medium
- A97.09
- B89
- C72.82
- D80.91

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