IB Chemistry HL 3.4 Electron Pair Sharing Reactions Questions

Use IB Chemistry HL questions to identify electron-pair donors and acceptors, draw curly-arrow mechanisms and predict organic products.

Exam points

Identify electron-rich nucleophiles and electron-deficient electrophiles, and classify Lewis bases and Lewis acids by the electron pair they donate or accept.
Show heterolytic fission with a full double-barbed curly arrow, assigning the correct ionic fragments and distinguishing it from radical-forming homolysis.
Choose and represent SN1 or SN2 nucleophilic substitution from substrate and solvent evidence, including curly arrows, stereochemical inversion, carbocations and leaving-group effects.
Analyse electrophilic addition to alkenes by drawing the first attack, charged intermediate and nucleophilic completion, then use carbocation stability to justify the major product.
Represent electrophilic substitution in benzene with E+, the sigma-complex, curly-arrow electron movement, restored aromaticity and the substituted product.
Represent coordination-bond formation and complex ions by identifying ligand lone-pair donors, the central Lewis-acid ion, oxidation state, coordination number and supported geometry.

Covered syllabus points

3.4.1—Nucleophiles 3.4.2—Nucleophilic substitution 3.4.3—Heterolytic fission 3.4.4—Electrophiles 3.4.5—Electrophilic addition to alkenes 3.4.6—Lewis acid-base theory 3.4.7—Coordination bond formation 3.4.8—Complex ions 3.4.9 (HL)—Nucleophilic substitution mechanisms 3.4.10 (HL)—Leaving group effects 3.4.11 (HL)—Electrophilic addition mechanisms 3.4.12 (HL)—Carbocation stability 3.4.13 (HL)—Electrophilic substitution in benzene

[Maximum number: 8]

Some reactions of but-2-ene are given below.

(a)

Deduce the full structural formula of compound A.

[ 1 ]

(b)

Describe the colour change observed when excess but-2-ene reacts with bromine to form compound A.

[ 1 ]

(c)

Compound $\mathbf{C}, \mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}$ , can also be formed by reacting compound B, $\mathrm{CH}_{3} \mathrm{CHBrCH}_{2} \mathrm{CH}_{3}$ , with aqueous potassium hydroxide. This reaction proceeds by both $\mathrm{S}_{\mathrm{N}} 1$ and $\mathrm{S}_{\mathrm{N}} 2$ mechanisms. Explain the $\mathrm{S}_{\mathrm{N}} 2$ mechanism, using curly arrows to represent the movement of electron pairs.

[ 4 ]

(d)

Explain why the hydroxide ion is a better nucleophile than water.

[ 2 ]

$\mathrm{OH}^{-}$ has a negative charge/higher electron density;
stronger attraction to the carbon atom with the partial positive charge / OWTTE;

[Maximum number: 8]

Organic compounds have many industrial applications.

(a)

An alkene such as ethene can be used as starting material for a range of compounds.

[ 5 ]

(i)

Predict the product of the reaction between ethene and bromine.

[ 1 ]

(ii)

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

[ 3 ]

Figure for Answer to Question (ii) — IB Chemistry HL

curly arrow from C=C bond to Br AND curly arrow showing Br-Br bond breaking

carbocation with charge on correct atom

curly arrow from lone pair/negative charge on $\mathrm{Br}^{-}$ to $\mathrm{C}^{+}$

(iii)

Outline why unsaturated molecules, such as ethene, readily undergo this type of reaction.

[ 1 ]

(b)

1-Bromobutane reacts with aqueous sodium hydroxide, NaOH(aq), to form butan-1-ol.

[ 3 ]

(i)

State the name of the mechanism by which this reaction occurs.

[ 1 ]

nucleophilic substitution

Marking guidance:

Allow $S_{N} 2$ but not $S_{N} 1$ .

(ii)

Draw the transition state produced in this mechanism.

[ 1 ]

Charge and square bracket must be included.

Accept two partial negative charges on Br AND O in HO, shown either as $\delta-$ or $\frac{1}{2-}$ .

Partial bonds from HO to C AND from C to Br must be shown.

Accept any angle for the partial bonds.

Accept carbocation, or a valid transition state, if $S_{N} 1$ named in (d)(l).

Accept any correct representation including skeletal structures.

(iii)

Outline how the rate of reaction of 1-bromobutane with sodium hydroxide compares with the rate of reaction of 1-chlorobutane with sodium hydroxide under the same conditions.

[ 1 ]

[Maximum number: 1]

Which correctly shows heterolytic bond fission?

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Covered syllabus points

Question 1

Question (a)

Question (b)

Question (c)

Question (d)

Question 2

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (b)

Question (i)

Question (ii)

Question (iii)

Question 3