A-level CHEMISTRY 7405/1 Paper 1 Inorganic and Physical Chemistry

2 Organic structures Where students are asked to draw organic structures, unless a specific type is required in the question and stated in the mark scheme, these may be given as displayed, structural or skeletal formulas or a combination of all three as long as the result is unambiguous. In general • Displayed formulae must show all of the bonds and all of the atoms in the molecule, but need not show correct bond angles. • Skeletal formulae must show carbon atoms by an angle or suitable intersection in the skeleton chain. Functional groups must be shown and it is essential that all atoms other than C atoms are shown in these (except H atoms in the functional groups of aldehydes, secondary amines and N-substituted amides which do not need to be shown). • Structures must not be ambiguous, e.g. 1-bromopropane should be shown as CH3CH2CH2Br and not as the molecular formula C3H7Br which could also represent the isomeric 2-bromopropane. • Bonds should be drawn correctly between the relevant atoms. This principle applies in all cases where the attached functional group contains a carbon atom, eg nitrile, carboxylic acid, aldehyde and acid chloride. The carbon-carbon bond should be clearly shown. Wrongly bonded atoms will be penalised on every occasion. (see the examples below) • The same principle should also be applied to the structure of alcohols. For example, if students show the alcohol functional group as C ─ HO, they should be penalised on every occasion. • Latitude should be given to the representation of C ─ C bonds in alkyl groups, given that CH3─ is considered to be interchangeable with H3C─ even though the latter would be preferred. • Similar latitude should be given to the representation of amines where NH2─ C will be allowed, although H2N─ C would be preferred. • Poor presentation of vertical C ─ CH3 bonds or vertical C ─ NH2 bonds should not be penalised. For other functional groups, such as ─ OH and ─ CN, the limit of tolerance is the half-way position between the vertical bond and the relevant atoms in the attached group. MARK SCHEME – A-LEVEL CHEMISTRY – 7405/1 – JUNE 2021 7 By way of illustration, the following would apply. CH3 C C CH3 C CH3CH2 OH C C OH allowed allowed not allowed not allowed not allowed NH2 C C NH2 NH2 NH2 NO2 allowed allowed allowed allowed not allowed CN C C CN COOH C C COOH C COOH not allowed not allowed not allowed not allowed not allowed CHO C C CHO C CHO COCl C C COCl not allowed not allowed not allowed not allowed not allowed • Representation of CH2 by C−H2 will be penalised • Some examples are given here of structures for specific compounds that should not gain credit (but, exceptions may be made in the context of balancing equations) CH3COH for ethanal CH3CH2HO for ethanol OHCH2CH3 for ethanol C2H6O for ethanol CH2CH2 for ethene CH2.CH2 for ethene CH2:CH2 for ethene • Each of the following should gain credit as alternatives to correct representations of the structures. CH2 = CH2 for ethene, H2C=CH2 CH3CHOHCH3 for propan-2-ol, CH3CH(OH)CH3 MARK SCHEME – A-LEVEL CHEMISTRY – 7405/1 – JUNE 2021 8 • In most cases, the use of ‘sticks’ to represent C ─ H bonds in a structure should not be penalised. The exceptions to this when “sticks” will be penalised include • structures in mechanisms where the C ─ H bond is essential (eg elimination reactions in halogenoalkanes and alcohols) • when a displayed formula is required • when a skeletal structure is required or has been drawn by the candidate. 3.13 Organic names As a general principle, non-IUPAC names or incorrect spelling or incomplete names should not gain credit. Some illustrations are given here. Unnecessary but not wrong numbers will not be penalised such as the number ‘2’ in 2methylpropane or the number ‘1’ in 2-chlorobutan-1-oic acid. but-2-ol should be butan-2-ol 2-hydroxybutane should be butan-2-ol butane-2-ol should be butan-2-ol 2-butanol should be butan-2-ol ethan-1,2-diol should be ethane-1,2-diol 2-methpropan-2-ol should be 2-methylpropan-2-ol 2-methylbutan-3-ol should be 3-methylbutan-2-ol 3-methylpentan should be 3-methylpentane 3-mythylpentane should be 3-methylpentane 3-methypentane should be 3-methylpentane propanitrile should be propanenitrile aminethane should be ethylamine (although aminoethane can gain credit) 2-methyl-3-bromobutane should be 2-bromo-3-methylbutane 3-bromo-2-methylbutane should be 2-bromo-3-methylbutane 3-methyl-2-bromobutane should be 2-bromo-3-methylbutane 2-methylbut-3-ene should be 3-methylbut-1-ene difluorodichloromethane should be dichlorodifluoromethane