IB Chemistry HL 3.4 Electron Pair Sharing Reactions Questions

Practise IB Chemistry HL 3.4 with HL questions on organic mechanisms, SN1/SN2 substitution, electrophilic addition and benzene.

Exam points

Deduce electrophilic addition products and mechanisms, including carbocation stability and major-product prediction.
Distinguish S_N1 and S_N2 substitution using stereochemical and leaving-group rate evidence.
Describe electrophilic substitution in benzene with curly-arrow movement and a charged electrophile.

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