Structure, Synthesis and Design
What you must understand
This domain integrates structure determination with purposeful synthesis. Know functional groups and IUPAC naming, isomerism (chain, position, functional group, and stereoisomerism including cis–trans and optical isomers with chirality), and how shape affects biological and material function. Learn spectroscopic and analytical techniques: infrared (identify bonds such as O–H, C=O, C–H), mass spectrometry (molecular ion and fragmentation), nuclear magnetic resonance (number of environments, chemical shift, integration, splitting), and chromatography (separation by polarity or size). For synthesis, plan multi-step routes using substitution, addition, oxidation, reduction, esterification and polymerisation, considering yield, atom economy, selectivity, safety and green chemistry principles.
Traps in questions
- IR absorption indicates a bond is present, not the full structure; combine with other evidence.
- In ¹H NMR, integration gives relative numbers of hydrogens per environment, and n+1 splitting arises from neighbouring non-equivalent hydrogens.
- A chiral centre needs four different groups; a molecule with a plane of symmetry is achiral.
- Cis–trans isomerism requires restricted rotation (C=C or a ring) plus two different groups on each carbon.
- Atom economy uses molar mass of desired product over total reactant molar mass; percentage yield is separate.
- Choosing a synthesis route must account for selectivity, not just whether a reaction works.
Rule of thumb and worked example
Rule: determine structure by combining evidence: molecular ion gives molar mass, IR confirms functional groups, NMR gives hydrogen environments and connectivity.
Example: A compound has molecular ion at m/z 60, an IR band near 1700 cm⁻¹, and ¹H NMR with a triplet at 1.2 ppm (3H), a quartet at 2.4 ppm (2H) and a singlet at 10.0 ppm (1H). The IR band suggests C=O; the singlet near 10 ppm suggests an aldehyde proton. The triplet–quartet pattern indicates an ethyl group adjacent to CH₂. Combining these gives propanal, CH₃CH₂CHO, with molar mass 58–60 consistent with the molecular ion.