IB DP Chemistry Reactivity 3.4 Electron-pair sharing reactions HL Paper 2- Exam Style Questions - New Syllabus

(a)(i)
The monomer is propene: \( \mathrm{CH_2{=}CH{-}CH_3} \)

\(\boxed{\mathrm{CH_2{=}CH{-}CH_3}}\)

(a)(ii)
The polymer is chemically inert/unreactive, making it useful for containers and packaging.
\(\boxed{\text{Chemically inert/unreactive}}\)

(b)(i)
The functional group is an aldehyde.
\(\boxed{\text{Aldehyde}}\)

(b)(ii)
The systematic name is \( \mathrm{3,5,5\text{-}trimethylhexanal} \).
\(\boxed{\text{3,5,5-trimethylhexanal}}\)

(b)(iii)
Number of signals: \( \mathrm{6} \)
Relative areas: \( \mathrm{9:2:1:3:2:1} \)
\(\boxed{\mathrm{6\ signals\ with\ areas\ 9:2:1:3:2:1}}\)

(b)(iv)

A ketone isomer: \( \mathrm{CH_3COC_8H_{17}} \) ( \( \mathrm{3,5,5\text{-}trimethylhexan\text{-}2\text{-}one} \) )
\(\boxed{\text{Any ketone with formula } \mathrm{C_9H_{18}O}}\)

(b)(v)
Oxidation product: \( \mathrm{RCOOH} \) (carboxylic acid)
Reduction product: \( \mathrm{RCH_2OH} \) (primary alcohol)
\(\boxed{\text{Oxidation: } \mathrm{RCOOH;\ \ Reduction:}\ \mathrm{RCH_2OH}}\)

(c)(i)
Product: \( \mathrm{1,2\text{-}dibromoethane} \)
\(\boxed{\mathrm{CH_2Br{-}CH_2Br}}\)

(c)(ii)

Electrophilic addition mechanism with:
• Curly arrow from \( \mathrm{C{=}C} \) to Br in \( \mathrm{Br_2} \)
• Curly arrow showing \( \mathrm{Br{-}Br} \) bond breaking
• Formation of bromonium ion intermediate
• Nucleophilic attack by \( \mathrm{Br^-} \)
\(\boxed{\text{Standard electrophilic addition mechanism}}\)

(c)(iii)
High electron density of the \( \mathrm{C{=}C} \) double bond makes it susceptible to electrophilic attack.
\(\boxed{\text{High electron density in } \mathrm{C{=}C}\text{ bond}}\)

(c)(iv)

\(C_nH_{2n}\)

(c)(v)
• Only London/dispersion forces present
• Strength increases with increasing chain length/molecular size/surface area
\(\boxed{\text{Increasing London forces with molecular size}}\)

(d)(i)
Mechanism: nucleophilic substitution (\( \mathrm{S_N2} \))
\(\boxed{\text{Nucleophilic substitution }(\mathrm{S_N2})}\)

(d)(ii)

Transition state with partial bonds to both \( \mathrm{OH} \) and \( \mathrm{Br} \), and partial negative charges.
\(\boxed{\text{[HO}\dots\mathrm{CH_2}\dots\mathrm{Br]^- \ transition\ state}}\)

(d)(iii)
\( \text{Rate} = k[\mathrm{CH_3(CH_2)_3Br}][\mathrm{OH^-}] \)
\(\boxed{\text{Rate }=k[\text{1-bromobutane}][\mathrm{OH^-}]}\)

(d)(iv)
Rate doubles because the reaction is first order with respect to \( [\mathrm{OH^-}] \).
\(\boxed{\text{Rate doubles; first order in }[\mathrm{OH^-}]}\)

(d)(v)
1-bromobutane reacts faster because the \( \mathrm{C{-}Br} \) bond is weaker/longer than the \( \mathrm{C{-}Cl} \) bond.
\(\boxed{\text{1-bromobutane faster; weaker } \mathrm{C{-}Br}\text{ bond}}\)

(e)(i)

cis-1-chlorobut-2-ene: \( \mathrm{Cl} \) and \( \mathrm{H} \) on the same side of the \( \mathrm{C{=}C} \) bond
\(\boxed{\text{Cl and H cis to each other}}\)

(e)(ii)
The cis-isomer is polar because the bond dipoles do not cancel due to the asymmetric arrangement.
\(\boxed{\text{Dipoles don’t cancel in cis-isomer}}\)

(e)(iii)

Structure with chiral centre: any \( \mathrm{C_4H_7Cl} \) chloroalkene containing a chiral carbon marked \( \mathrm{*} \).
\(\boxed{\text{Any } \mathrm{C_4H_7Cl} \text{ with chiral carbon}}\)

(f)(i)
Using bond enthalpies (keep reference: section 12 of the data booklet):
Bond enthalpies used (from section 12): \( \mathrm{C{=}C}=614 \), \( \mathrm{C{-}H}=414 \), \( \mathrm{O{-}H}=463 \), \( \mathrm{C{-}C}=346 \), \( \mathrm{C{-}O}=358 \) (all in \( \mathrm{kJ\ mol^{-1}} \)).
Bonds broken: \( (1\times 614) + (4\times 414) + (2\times 463) = 3196 \ \mathrm{kJ\ mol^{-1}} \)
Bonds formed: \( (1\times 346) + (1\times 358) + (5\times 414) + (1\times 463) = 3237 \ \mathrm{kJ\ mol^{-1}} \)
\( \Delta H = 3196 – 3237 = -41 \ \mathrm{kJ\ mol^{-1}} \)
\(\boxed{-41 \ \mathrm{kJ\ mol^{-1}}}\)

(f)(ii)
Using enthalpies of formation (keep reference: section 13 of the data booklet) and given \( \Delta H_f^{\circ} \) of \( \mathrm{CH_3CH_2OH(g)} = -235 \ \mathrm{kJ\ mol^{-1}} \):
From section 13: \( \Delta H_f^{\circ}(\mathrm{C_2H_4(g)}) = +52 \ \mathrm{kJ\ mol^{-1}} \), \( \Delta H_f^{\circ}(\mathrm{H_2O(g)}) = -242 \ \mathrm{kJ\ mol^{-1}} \).
\( \Delta H^{\circ} = \sum \Delta H_f^{\circ}(\text{products}) – \sum \Delta H_f^{\circ}(\text{reactants}) \)
\( \Delta H^{\circ} = (-235) – \bigl[(+52) + (-242)\bigr] = -45 \ \mathrm{kJ\ mol^{-1}} \)
\(\boxed{-45 \ \mathrm{kJ\ mol^{-1}}}\)

(f)(iii)
Bond enthalpies are average values (so the result is approximate), whereas formation enthalpies are specific to the substances in their stated standard states.
\(\boxed{\text{Bond enthalpies are average values; } \Delta H_f^{\circ} \text{ values are substance-specific.}}\)