Examiner's Report - Organic Paper 2

• • Although most students identified compound J as propan-1-ol, a surprising number failed to show the O−H bond as is necessary in a displayed formula
• Describe how you would distinguish between separate samples of the two stereoisomers of CH3CH2CH2CH2CH(OH)CN [2 marks]

  • Although this question was answered well by most, an appreciable number of students were careless with language and appeared to say that the enantiomers themselves rotate, or that the plane polarised light is ‘refracted’ or ‘bent’
  • rather than ‘rotated in opposite directions’.
• State in general terms what determines the distance travelled by a spot in TLC. [1 mark]

  • This question tested understanding of the balance between solubility in the moving phase and retention by the stationary phase.
  • Both parts were needed in the answer.
• • The full IUPAC name of isomer P includes the letter Z to indicate its stereochemistry. Only 10% of students answered this difficult question correctly.

  
• • Amine F = benzene+CH2CH2NH2
  • The use of chlorine or bromine in UV in the first step was usually well known
  • but use of the reagent KCN in a mixed aqueous/alcoholic solvent was less often correct in the next step.
  • The expected reducing agent/condition for the final step was hydrogen with a nickel or platinum catalyst.
  • LiAlH4 in dry ether is not required knowledge for this specification, but was an acceptable answer.

  
• • This part was challenging and a surprising proportion of students (8%) made no attempt.
  • Sadly, some of those who did gain the first mark failed to gain the second as this required two statements, not only that the C=C bond cannot rotate
  • but also that each carbon in the double bond has two different groups attached to it.

  
• Justify the statement that adding more water to the equilibrium mixture in Question 04.3 will lower the amount of A in the mixture. [3 marks]

  • This question was poorly answered and showed that students were unfamiliar with the idea of
  • the equilibrium moving to the side with a larger amount, in moles
  • to oppose the fall in concentrations of all reagents when more water was added to the equilibrium mixture.
  • Credit was given to those few students who adopted a mathematical approach.

  
• • Most students knew that carbon formed three bonds but only a few were able to discuss that the extra outer shell p electrons overlap to form the pi cloud.
  • Shape was less commonly discussed, although many noted that the molecule was planar.
  • All but the weakest students were able to calculate the expected enthalpy of hydrogenation of –360 kJ mol–1 and the consequent 152 kJ mol–1 extra stability of benzene and explain this in terms of electron delocalisation.

  
• • This question proved challenging.
  • Some students gave a wrong value that was more exothermic than –240 kJ mol−1 .
  • Of those who gave a suitable value, many were unable to explain it in terms of some extra stability due to delocalisation.
  • Only 8.7% of students were able to justify this in terms of alternate single and double bonds forming a conjugated (sharing of electrons) system and so gain full marks.

  
• • a number failed to gain the mark due to a failure to label 1-phenyl in an otherwise correct name.
  • Name of mechanism is nucleophilic addition with NaBH4
  • NOT reduction (type of reaction)

  
• Explaining how hydrogen bonds form in a beta-pleated sheet (secondary structure) of a protein. [2 marks]

  • Most students scored the first mark for correctly identifying that the hydrogen on the nitrogen is electron deficient.
  • The second mark proved to be more difficult and many failed to identify that the attraction was between the lone pair on the oxygen and the electron deficient hydrogen

  
• This question was not well answered and 12.6% of the students made no attempt at all.

  • Of those who did, very few seemed to be aware of disulphide bridges.
  • Others did not show the bridge as a covalent bond,
  • omitted the two CH2 groups
  • or had an ADDITIONAL HYDROGEN attached to each sulfur. (Must remove the hydrogens!)

  
• • TRAILING BONDS were rarely seen.
  • It was common to see structures that had no phosphate groups attached at all, with students simply copying the sugar straight from the data sheet and adding the base A.
  • A few scripts gave a structure with two phosphate groups rather than just one. Chemical Error = 0 (because question states the nucleotide)

  
• • Both marks were gained by the 28.9% of the students who stated that further substitution reactions would occur following the formation of a primary amine from a halogenoalkane
  • and also that no further reaction would occur when primary amines were formed from nitriles.
  • The second mark was less often gained, sometimes because students made no comment about nitriles at all.
  • MUST mention BOTH sides

  
• • Only 7.7% could describe the formation of the phenylammonium ion and so
  • explain the solubility of phenylamine in the aqueous acidic solution
  • C6H5NH2 present as an ionic salt