Mock chem paper 2

IDK Junior College CANDIDATE NAME CHEMISTRY 9476/02 2 hours Additional Material(s): Data Booklet READ THE FOLLOWING FIRST Write in dark blue or black ink. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions in the spaces provided. You are reminded that there is extra working space on pages 15 & 16. Do write the question number and part clearly. The use of a scientific calculator is expected, where appropriate. You are reminded of the need for good English and legible handwriting. The number of marks is given in brackets [ ] at the end of each question or part question. For examiner’s use 1 /11 2 /15 3 /13 4 /12 5 /16 6 /8 Deductions sf: structures: units: Total /75 This document consists of 16 pages 1

Answer all questions 1 Squaric acid is a dibasic organic acid. Fig. 1.1. shows the skeletal formula of squaric acid. Fig. 1.1 (a) State the IUPAC name of this compound. You are told that priority goes to the carbonyl groups. [2] ……………………………………………………………………………………………………………… (b) Fig. 1.2 shows the delocalisation of the divalent squarate anion. Draw, on Fig 1.2, curly arrows and lone pairs to show how this resonance stabilisation is achieved, from left to right. [1] Fig. 1.2 (c) Hence, suggest why the divalent squarate anion is a perfect square. [1] .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… (d) Given that the 1st pKa of squaric acid is 1.5, draw the predominant species present at pH 3 [1] 2

(e) You are given squaric acid, ethanoic acid and ethanol at equal concentrations. Describe their relative acidities and explain. [3] ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. (f) A notable derivative of squaric acid is dibutyl squarate. (Fig. 1.3) In order to form C-O-C (ether) bonds, an S N 2 reaction takes place where the conjugate base of an alcohol acts as a nucleophile and reacts with a suitable halogenoalkane. Hence, state the reagents needed to form dibutyl squarate from squaric acid. [2] Fig. 1.3 ……………………………………………………………………………………………………………….. (g) The divalent squarate anion is an example of the oxocarbon anion, which is a negative ion consisting solely of carbon and oxygen atoms. Each oxocarbon anion can also form a corresponding hydrogenated anion, H k C x O y m⁻ . Find the average oxidation number of carbon in this ion, leaving your answer in terms of k, x, y, z and m. Let z be the average oxidation number. [1] 3

[Total: 11] 2 Alkynes are a class of organic compounds with the general formula C n H 2n⁻2 . Table 2.1 shows the carbon-hydrogen bond length in ethane, ethene and ethyne. Molecule Carbon⁻hydrogen bond length /Å Ethane 1.14 Ethene 1.09 Ethyne 1.06 Table 2.1 (1Å = 10 ⁻ 10 m) (a) Use the concept of hybridisation to explain the difference above. [2] ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… (b) Interestingly, terminal alkynes are more acidic than alkenes or alkanes, with a pKa of 25. By drawing the conjugate base of ethyne, suggest why this is so. [1] .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… (c) Using the information in Table 2.2, calculate the standard enthalpy change of formation of C 2 H 2 . [3] Table 2.2 4 Equation Δ H ⊖ / kJ mol ⁻1 3C(s) + H 2 O(l) → CO(g) + C 2 H 2 (g) +401 2C(s) + O 2 (g) → 2CO(g) ⁻221 2H 2 O(l) → 2H 2 (g) + O 2 (g) +572

(d) Suggest the type of reaction alkynes tend to undergo, briefly explain. [2] ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… (e) Alkynes are useful as due to their unusual acidity, it is possible to greatly increase the number of carbons on a molecule. A base known as NaNH 2 can be used to first deprotonate the terminal hydrogen, and the resultant species can act as a nucleophile to attack an alkyl halide in a S N 2, forming a new carbon-carbon bond. Using ethyne and 1⁻bromobutane, describe the mechanism of this reaction, showing all lone pairs, curly arrows and partial charges where necessary. [3] 5

(f) Alkynes can be subsequently reduced to cis-alkenes using a poisoned lead catalyst known as Lindlar’s catalyst and hydrogen. Hence, complete the synthesis route below, giving all reagents, conditions and intermediates clearly. [4] [Total: 15] 6

3 Huckel’s rule is such that a planar compound is aromatic if it contains (4n+2) pi electrons, where n is an integer, with a conjugated system. (a) Show that benzene is an aromatic compound. [1] Heterocyclic compounds are extremely important in the field of organic chemistry due to them acting as building blocks in many pharmaceutical compounds. Figure 3.1 shows the structures of pyridine and pyrrole. Fig. 3.1 (b) By drawing the atomic orbitals of the atoms in the rings of pyridine and pyrrole, predict whether both compounds are aromatic, given that both compounds are planar.[3] (c) Hence, deduce whether pyrrole is a strong or weak base. [2] ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… 7

(d) Given your answer in (c) , deduce whether the conjugate acid of pyridine is stronger or weaker than the conjugate acid of pyrrole. [2] ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. ..……………………………………………………………………………………………………………… (e) Pyridine does not undergo electrophilic aromatic substitution reactions as readily as benzene. Suggest a reason for this phenomenon. [1] ……………………………………………………………………………………………………………….. .………………………………………………………………………………………………………………. The Chichibabin reaction is an example of nucleophilic aromatic substitution, where a nucleophile attacks the aromatic ring. Figure