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?

[Maximum number: 2]

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

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

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

[Maximum number: 1]

Organic compounds have many industrial applications.

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

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

nucleophilic substitution

Marking guidance:

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

[Maximum number: 1]

1-bromopropane undergoes a substitution reaction with potassium cyanide.

Explain why the substitution occurs on the carbon atom that is marked as *C.

[Maximum number: 1]

When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.

\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92.6 \mathrm{~kJ}

Define the term base according to the Lewis theory.

[Maximum number: 2]

Urea, $\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}$ , is excreted by mammals and can be used as a fertilizer.

Describe the bond formation when urea acts as a ligand in a transition metal complex ion.

[Maximum number: 2]

Phosphoryl chloride, $\mathrm{POCl}_{3}$ , is a dehydrating agent.

$\quad \mathrm{PCl}_{3}$ and $\mathrm{Cl}^{-}$ can act as ligands in transition metal complexes such as $\mathrm{Ni}\left(\mathrm{PCl}_{3}\right)_{4}$ and $\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3} \mathrm{Cl}_{3}\right]$ .

Define the term ligand.

[Maximum number: 1]

Organic compounds have many industrial applications.

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

Draw the transition state produced in this mechanism.

Figure for Answer to Question 10 — IB Chemistry HL

[Maximum number: 1]

Organic compounds have many industrial applications.

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

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.

[Maximum number: 3]

Organic compounds have many industrial applications.

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

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

Figure for Answer to Question 12 — IB Chemistry HL

[Maximum number: 2]

Ethanol is obtained by the hydration of ethene, $\mathrm{C}_{2} \mathrm{H}_{4}$ .

Alternative synthetic routes exist to produce alcohols.

Explain why the major organic product is 2-bromopropane and not 1-bromopropane.

«2-bromopropane involves» formation of more stable «secondary» carbocation/carbonium ion/intermediate
OR
1-bromopropane involves formation of less stable «primary» carbocation/ carbonium ion/intermediate «increased» positive inductive/electron-releasing effect of extra -R group/- $\mathrm{CH}_{3}$ /methyl «increases stability of secondary carbocation»

[Maximum number: 5]

Benzene is an aromatic hydrocarbon.

(a)

State the typical reactions that benzene and cyclohexene undergo with bromine.
Benzene:
Cyclohexene:

[ 1 ]

benzene: electrophilic substitution/ $\mathrm{S}_{\mathrm{E}}$
AND
cyclohexene: electrophilic addition/ $\mathrm{A}_{\mathrm{E}}$

(b)

Explain the mechanism for the nitration of benzene, using curly arrows to show the movement of electron pairs.

[ 4 ]

curly arrow going from benzene ring to N of ${ }^{+} \mathrm{NO}_{2} / \mathrm{NO}_{2}{ }^{+}$ carbocation with correct formula and positive charge on ring curly arrow going from C-H bond to benzene ring of cation formation of organic product AND $\mathrm{H}^{+}$

Marking guidance:

Accept mechanism with corresponding Kekulé structures.

Do not accept a circle in M2 or M3.

Accept first arrow starting either inside the circle or on the circle.

M2 may be awarded from correct diagram for M3.

M4: Accept $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}+\mathrm{H}_{2} \mathrm{SO}_{4}$ if $\mathrm{HSO}_{4}{ }^{-}$ used in M3.

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Covered syllabus points

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