I am glad they offer the chance for clarification. Below are the latest Q’s. Again, when I get the answers, I will post them here.

.

A few questions

Q1 a)

In ‘Edexcel A2 Chemistry’ By Fullick and McDuell, published by Pearson, on page 214, the reduction of 4-nitrophenol into 4-aminophenol, it is shown to occur with NaBH4 in alkaline medium, yet in the mark scheme for June 2010 Unit5 Q22 a iii) The answer sodium borohydride in alkaline solution does not appear yet “sodium borohydride” alone is rejected.

b) a further query on this point is: if alkaline solution was used, is it not the case that the deprotonated (phenolic) anion of 4-aminophenol would be the product, and in acidic medium from say, the traditional standard answer of using Sn(s) and conc HCl, that the protonated (amine) cation of 4-aminophenol would form and not the unionised molecule as shown.

c) A third point about this is even with post-reaction work-up (acidic or basic) an ionic product would be the likely produce and not the neutral molecule shown in the texts.

Could you please resolve this confusing state of affairs, and state what edexcel will credit and reject in future exams?

Response (Rod Beavon):

1a. Sodium borohydride does not reduce aromatic nitro compounds to amines on its own – it forms different products and this is well-documented (March, J., Advanced Organic Chemistry, 4th ed 1992, Wiley, pp1216 & 1233). That is why it is rejected – it does not work.

In the presence of a palladium catalyst in alkali it will work and that answer would have been credited. Other catalysts are possible – also referenced in March. The common method of reduction of aromatic nitro compounds, that is the use of iron or tin in hydrochloric acid, will also effect the same reduction.

Sodium borohydride with Pd/NaOH is referred to on p3 of the Context Study booklet, ‘The Pharmaceutical Industry for A2′ which is available to schools from the Edexcel website.

1b When synthetic pathways are shown they don’t necessarily show all the steps involved in the work-up of the materials concerned nor are they overly concerned whether the product is the anionic form of the phenol or the cationic form of the amine – it’s the overall idea of the functional group inter-conversions that is important.

1c Your comments are correct and if the structures were asked for and shown in their ionised form that would be credited.

Q2)

The pH of H2SO4 solution. Students are taught the second ionisation is suppressed due to the large [H+] from the first ionisation. How are students to calculate the pH of say a 0.01M solution of H2SO4? So they assume total (first and second) ionisation, or do they calculate pH based on a 1 H2SO4 : 1 H+ ratio, i.e. forst ionisation alone, or could they state the pH would be <2 as some secondary ionisation occurs?

Response (Rod Beavon):

2)  For a comprehensive answer to this see http://www.rod.beavon.clara.net/sulphuric_acid.htm . The question that you mention would not be asked, i.e. calculation of the pH of anything other than a monobasic acid.

Q3) Re Edexcel endorsed book by George Facer (page 259) gives:

“C2H5Cl + 2NH3 –> C2H5NH2 + NH4Cl

an excess of halogenoalkane will yield a secondary amine.”

The equation is accompanies with text saying: If the ammonia is not in excess, the amine salt is formed)

a) With excess R-X, would edexcel not expect the answer of a tertiary amine to be formed instead of a secondary amine, or would the students gain credit if the secondary amine answer was offered.

b) As the amine formed is a stronger base than ammonia, one might be forgiven for conceptualising any initially liberated HCl would in fact protonate the produced amine instead and not the ammonia, hence would edexcel give credit for a reaction equation as such:

“C2H5Cl + NH3 –> C2H5NH3Cl”. In fact, what would edexcel credit and what would it reject/consider non-meritable here?

Note The chemistry immediately above is reinforced by the likelihood if an ammonium ion was produced, the (more)basic amine would deprotonate the ammonium forming the amine salt and free ammonia.

Response (Rod Beavon):

3 The problem with halogenoalkanes and ammonia is complex – and not a useful way of making amines because of that. All of the outcomes you mention are possible. The answer accepted would depend on the question – so if a primary amine was asked about then the others would not be credited. Whether the product were to be the amine salt or the amine and ammonium chloride wouldn’t matter.

The answer that we would not like is CH3CH2Cl + NH3 –> CH3CH2HN2 + HCl since the acid would certainly not be produced.

I replied to his answers (in blue above) as follows:

Dear Sir.

Once more, thank you for your reply. Such clarifications are of great assistance. For the purposes of reciprocal assistance, I will give some explanation as to why these questions were asked. I’d like to say Even though we are looking at the same chemistry (albeit with different reference books perhaps) we are looking at it from a different vantage point so some of the fine points may appear slightly different.

1a) NaBH4 et al with aminophenol. It was peculiar to only refer to NaBH4 in the negative sense i.e. without an acceptable alternative. It is common for mark schemes to say wtte “reject if just … alone” but all there was here was the instruction to reject “sodium borohydride” it could be interpreted as a blanket-prohibition. One would at least expect the Edexcel/Pearson ‘book answer’ to be appear on the Edexcel mark scheme. And given the other questionable material in that book, doubts as to the exact situation will occur.

1b) We have expressed such thinking to the students, but I think I have occasionally seen a mark can be scored based on the ‘finishing touches’ of some reactions. {I can’t give specifics as currently I don’t have my reference materials to hand} and I presume the questions will be sufficiently worded for students to pick up on such cases.

2) Subtleties and the accuracy of models change with changing specifications. I wasn’t too sure on Edexcel’s stance on this in relation to this latest specification. Past years papers of other exam boards (AQA) imply students have been directed to make the assumption both H+ ions are liberated, and it is unknown as to whether the QCA require standardisation on various topics amongst exam boards.

3) I hope my students can accept this, and I do appreciate your referring to the specificity of the question – which can often quite focused and can cause these occasional points of confusion can be avoided.

Once more thank you for your attention in addressing these concerns. It is highly appreciated.