How do chemists design efficient syntheses and use instrumental techniques to identify and measure substances?
Evaluate chemical synthesis using percentage yield and atom economy and green chemistry principles, and interpret instrumental analysis data to identify substances
A focused answer to the WACE Year 12 Chemistry dot point on percentage yield, atom economy, green chemistry and instrumental analysis, with a worked yield calculation and common mistakes.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
What this dot point is asking
Designing a chemical synthesis means choosing reactions that convert available starting materials into a target product efficiently, safely and with minimal waste. Two quantitative measures judge that efficiency, and modern practice is guided by green chemistry.
Percentage yield
The percentage yield compares the amount of product actually obtained with the maximum predicted by stoichiometry (the theoretical yield):
Real syntheses never reach 100 percent because of side reactions, incomplete reactions (equilibria), losses during purification, and impurities in the reactants.
Atom economy
Atom economy measures how much of the total mass of all reactants ends up in the desired product, rather than in by-products:
A reaction can have a high yield but poor atom economy if it produces large amounts of unwanted by-products. Addition reactions tend to have high atom economy (everything ends up in one product); substitution and elimination reactions have lower atom economy because they release small molecules.
Green chemistry
Green chemistry designs syntheses to reduce environmental impact: maximising atom economy and yield, using renewable feedstocks, avoiding toxic or hazardous reagents and solvents, using catalysts rather than stoichiometric reagents, minimising energy use, and designing products and by-products that are safe and degradable. The goal is to prevent waste rather than to clean it up afterwards.
Instrumental analysis
Chemists identify and measure substances using instrumental techniques. The WACE course expects you to interpret data from:
- Mass spectrometry (MS): measures the mass of the molecule (molecular ion peak gives the molar mass) and the masses of fragments, which help deduce structure.
- Infrared (IR) spectroscopy: absorption bands at characteristic wavenumbers reveal functional groups (for example a broad band near for O-H, a strong band near for C=O).
- Nuclear magnetic resonance (NMR) spectroscopy: the number of signals, their chemical shifts and the relative peak areas indicate the different proton (or carbon) environments and how many of each.
Combining these techniques lets a chemist confirm the identity and purity of a synthesised product.
When answering synthesis questions in the WACE examination, identify the limiting reagent before computing the theoretical yield, keep yield and atom economy separate, and quote IR and NMR evidence specifically (state the wavenumber or shift and the group it indicates).