Answers to poll

Q1: Which reagent can be used to distinguish between methanol and ethanol.

Answer: (1) KMnO₄/H⁺ can be used. Both decolourises purple KMnO₄, but only methanol will produce a colourless and odourless gas which forms a white precipitate with Ca(OH)₂. This is not observed for ethanol.

(2) NaOH, I₂, warm. Ethanol produces a pale yellow crystal with antiseptic smell. While methanol doesn't. (Note: As I₂ is a mild oxidising agent, both primary alcohols will be oxidised, hence we will see the purple decolourising for both).

Q2: CH₃MgBr is a source of a the nucleophile CH₃^-. This nucleophile is similar to the cynaide ion. To which of the following will CH₃MgBr react and produce a chiral compound? (i) CH₃CHO (ii) CH₃CH₂CHO (iii) CH₃COCH₃ (iv) CH₃COCH₂CH₃

Answer: (1) Look for the asymmetrical carbonyl. Thus, (iii) is out. As the nucleophile is CH₃^-, when added to the carbonyl it is common to an R group found in (i) and (iv), hence despite both (i) and (iv) are asymmetrical, addition of the nucleophile does not produce a chiral compound. Hence answer is (ii).

Q3: Given that the numerical value of K_a of H₂CO₃ and phenol is 4.5 x 10^-7 and 1.3 x 10^-10. Which of the following reactions will take place? (i) phenol and carbonate (ii) carbon dioxide and phenoxide

Answer: (ii). This is because carbon dioxide is a stronger acid than phenol. Hence, if phenol and carbonate reacts, we will get phenoxide (anion of phenol) and CO₂ as the products. BUT, CO₂ is a stronger acid (than phenol) hence, it will react with the phenoxide ion (since phenol is a weaker acid than CO₂, this also implies that phenoxide is a stronger conjugate base than carbonate), therefore returning to the original reactants. Hence, this implies that phenol will not react with carbon dioxide. 

Q4. What would you expect when CH₃CH(Cl)CN is boiled with water in the presence of an acid?

Answer: The first reaction will be the acid hydrolysis of the -CN functional group to give -COOH. However, since water is a nucleophile and heat is added, there is nucleophilic substitution for the alkyl halide. This alkyl halide is a secondary alkyl halide, which has potential to undergo S_N1 mechanism.

This mechanism suggest that even if a weak nucleophile is used, the nucleophilic substitution can occur since the rate of reaction via S_N1 is independent on the nucleophile.

Hence, it is also possible for the Cl to be substituted by the OH. Hence, we would expect that CH₃CH(OH)COOH is produced.  This is because we assume that S_N1 has taken place.

Fortunately, for most of the time we expect student to consider about acid hydrolysis.  This is because water is a weak nucleophile and the reaction between a primary alkyl halide is not very favourable.

Q5. The more acidic the substituted benzoic acid, the less susceptible it is toward electrophilic attacks. True or False? Explain

Answer: True. This is because when a substituted benzoic acid is more acidic, it implies that its conjugate base is more stable. The conjugate base is made more stable because the substitutent must have been an electron withdrawing substituent that cause benzene ring to be even more electron withdrawing. This results in a greater extent of the delocalisation of the negative charge.

Since, the substituent is electron withdrawing, it pulls the pi electron cloud electron density making it less available for benzene to make use of it for electrophilic substitution.

Hope you got them right. :)

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^{Article written by Kwok YL 2010.}

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