IB DP Chemistry Structure 2.4 From models to materials HL Paper 2- Exam Style Questions - New Syllabus
Question
Alkanes form a homologous series.
(a)
(i) Outline the meaning of homologous series. [\(\,1\,\)]
(ii) State the preferred IUPAC name for the following compounds. [\(\,2\,\)]
(iii) Identify one chiral carbon atom present in one of the following structures with an asterisk \((*)\). [\(\,1\,\)]
(iv) But-2-ene can be polymerized. Draw a section of the resulting polymer showing two repeating units. [\(\,1\,\)]
(b) Chloroethane can be converted into ethanoic acid in a two-step process.
\(\text{Chloroethane}\ \xrightarrow[\text{Step 1}]{}\ \text{Ethanol}\ \xrightarrow[\text{Step 2}]{}\ \text{Ethanoic acid}\)
Identify reagents for each step. [\(\,2\,\)]
Step \(1\): \(\quad\) ………………………………………………………………………………………………….
Step \(2\): \(\quad\) ………………………………………………………………………………………………….
(c)
(i) Identify the type of reaction that takes place in step \(1\) of part \((b)\). [\(\,1\,\)]
(ii) Sketch the mechanism of the reaction for step \(1\) in part \((b)\), using curly arrows to show the movement of electron pairs. [\(\,4\,\)]
(iii) Identify the products formed from the reaction of ethanol and ethanoic acid in the presence of an acid catalyst. [\(\,1\,\)]
▶️ Answer/Explanation
(a)(i)
A homologous series is a family of compounds with the same functional group and general formula in which successive members differ by \(\mathrm{CH_2}\).
A homologous series is a family of compounds with the same functional group and general formula in which successive members differ by \(\mathrm{CH_2}\).
(a)(ii)
First structure: \(\ \mathbf{2\text{-}chlorobutane}\).
Second structure: \(\ \mathbf{1\text{-}chloro\text{-}2\text{-}methylpropane}\).
First structure: \(\ \mathbf{2\text{-}chlorobutane}\).
Second structure: \(\ \mathbf{1\text{-}chloro\text{-}2\text{-}methylpropane}\).
(a)(iii)
Example: in \(\mathrm{2\text{-}chlorobutane}\), the \(\mathrm{C_2}\) atom is chiral because it is attached to four different groups: \(\mathrm{CH_3}\), \(\mathrm{CH_2CH_3}\), \(\mathrm{H}\), and \(\mathrm{Cl}\). Mark \(\mathrm{C_2}\) with an asterisk \((*)\).
\(\mathrm{CH_3{-}^*\!CH(Cl){-}CH_2CH_3}\)
Example: in \(\mathrm{2\text{-}chlorobutane}\), the \(\mathrm{C_2}\) atom is chiral because it is attached to four different groups: \(\mathrm{CH_3}\), \(\mathrm{CH_2CH_3}\), \(\mathrm{H}\), and \(\mathrm{Cl}\). Mark \(\mathrm{C_2}\) with an asterisk \((*)\).
\(\mathrm{CH_3{-}^*\!CH(Cl){-}CH_2CH_3}\)
(a)(iv)
Polymer of but-2-ene (two repeating units shown):
\(\displaystyle \mathrm{-[-CH(CH_3)-CH(CH_3)-]_{2}-}\)
Polymer of but-2-ene (two repeating units shown):
\(\displaystyle \mathrm{-[-CH(CH_3)-CH(CH_3)-]_{2}-}\)
(b)
Step \(1\): \(\ \mathrm{KOH_{(aq)}}\) or \(\mathrm{NaOH_{(aq)}}\) (i.e., \(\mathrm{OH^- (aq)}\)) to substitute \(\mathrm{Cl}\) and form ethanol.
Step \(2\): Oxidation to ethanoic acid using \(\mathrm{acidified\ K_2Cr_2O_7}\) (with \(\mathrm{H^+}\), e.g. \(\mathrm{H_2SO_4}\)) or \(\mathrm{KMnO_4}\).
Step \(1\): \(\ \mathrm{KOH_{(aq)}}\) or \(\mathrm{NaOH_{(aq)}}\) (i.e., \(\mathrm{OH^- (aq)}\)) to substitute \(\mathrm{Cl}\) and form ethanol.
Step \(2\): Oxidation to ethanoic acid using \(\mathrm{acidified\ K_2Cr_2O_7}\) (with \(\mathrm{H^+}\), e.g. \(\mathrm{H_2SO_4}\)) or \(\mathrm{KMnO_4}\).
(c)(i)
\(\mathbf{\text{Nucleophilic substitution}}\) \((\mathrm{S_N2})\).
(Primary halogenoalkane \(\Rightarrow\) concerted backside attack.)
\(\mathbf{\text{Nucleophilic substitution}}\) \((\mathrm{S_N2})\).
(Primary halogenoalkane \(\Rightarrow\) concerted backside attack.)
(c)(ii)
Mechanistic outline for step \(1\) (from \(\mathrm{CH_3CH_2Cl}\) to \(\mathrm{CH_3CH_2OH}\)):
\(\mathrm{HO^-}\) attacks the electrophilic carbon bearing \(\mathrm{Cl}\) from the backside while \(\mathrm{C{-}Cl}\) bond breaks:
\(\displaystyle \mathrm{HO^- \;+\; CH_3CH_2Cl \;\xrightarrow[\text{S\!N2}]{}\ [CH_3\!\cdot\!\overset{\ddagger}{\!C\!}\!\cdots\!OH]^{-}\!\cdots\!Cl \;\longrightarrow\; CH_3CH_2OH \;+\; Cl^-}\)
Features to include in a sketch:
— Curly arrow from the O lone pair on \(\mathrm{HO^-}\) to the electrophilic carbon.
— Curly arrow from the \(\mathrm{C{-}Cl}\) bond to \(\mathrm{Cl}\) (leaving group).
— Transition state with partial bonds to \(\mathrm{O}\) and \(\mathrm{Cl}\), overall negative charge in square brackets, then products \(\mathrm{CH_3CH_2OH}\) and \(\mathrm{Cl^-}\).
Mechanistic outline for step \(1\) (from \(\mathrm{CH_3CH_2Cl}\) to \(\mathrm{CH_3CH_2OH}\)):
\(\mathrm{HO^-}\) attacks the electrophilic carbon bearing \(\mathrm{Cl}\) from the backside while \(\mathrm{C{-}Cl}\) bond breaks:
\(\displaystyle \mathrm{HO^- \;+\; CH_3CH_2Cl \;\xrightarrow[\text{S\!N2}]{}\ [CH_3\!\cdot\!\overset{\ddagger}{\!C\!}\!\cdots\!OH]^{-}\!\cdots\!Cl \;\longrightarrow\; CH_3CH_2OH \;+\; Cl^-}\)
Features to include in a sketch:
— Curly arrow from the O lone pair on \(\mathrm{HO^-}\) to the electrophilic carbon.
— Curly arrow from the \(\mathrm{C{-}Cl}\) bond to \(\mathrm{Cl}\) (leaving group).
— Transition state with partial bonds to \(\mathrm{O}\) and \(\mathrm{Cl}\), overall negative charge in square brackets, then products \(\mathrm{CH_3CH_2OH}\) and \(\mathrm{Cl^-}\).
(c)(iii)
\(\mathrm{Ethanol +\ ethanoic\ acid \ \xrightarrow[\text{acid cat.}]{}\ ethyl\ ethanoate\ +\ water}\)
Products: \(\ \mathbf{ethyl\ ethanoate\ (CH_3CH_2OOCCH_3)}\) and \(\mathbf{H_2O}\).
\(\mathrm{Ethanol +\ ethanoic\ acid \ \xrightarrow[\text{acid cat.}]{}\ ethyl\ ethanoate\ +\ water}\)
Products: \(\ \mathbf{ethyl\ ethanoate\ (CH_3CH_2OOCCH_3)}\) and \(\mathbf{H_2O}\).