Edexcel IAL Chemistry · Topic 5.6
Organic Synthesis
Big idea: Every organic molecule is a puzzle piece — once you know how functional groups
convert into each other, you can work out an unknown structure from data, or design a step-by-step route
from any starting material to any target molecule.
1. Summary — What This Chapter Covers
- Deducing structures: using combustion analysis, % composition, functional group tests and spectra (IR, MS, NMR) to work backwards to a structure.
- Planning reaction schemes: a map of how every functional group converts into every other one — this is the toolkit for synthesis questions.
- Increasing carbon chain length: using Grignard reagents (or nitriles) when the target molecule has more carbons than the starting one.
- Preparation techniques: simple distillation, steam distillation, and heating under reflux — knowing when to use which.
- Purification techniques: separating funnels, drying agents, and recrystallisation.
- Purity techniques: melting point and boiling point determination as evidence of how pure (and what) your product is.
2. Deducing Organic StructuresWorking from raw data to a full structure
The four-step method
Whenever you're given a mystery compound and a pile of data, always work through the same sequence — don't
jump straight to guessing a structure. Examiners reward this order because each step narrows down the possibilities:
- Find the empirical formula — the simplest whole-number ratio of atoms.
- Determine the molecular formula — usually using the given relative molecular mass (Mr) to scale up the empirical formula.
- Identify the functional groups present — from test-tube reactions and/or spectra.
- Deduce the overall structure — put the pieces together into one consistent skeleton.
Why this order matters
You can't sensibly guess functional groups until you know the molecular formula, and you can't know the
molecular formula until you have the empirical formula. Skipping ahead is the #1 reason students get the
wrong final structure — even when all their individual working is technically correct.
Combustion analysis
This is how you get from "burn a known mass of the compound" to "percentage composition of C, H and O."
A known mass of the compound is burned completely in excess dry oxygen. All the carbon ends up as
CO₂ and all the hydrogen ends up as H₂O — you then work backwards from the masses of those two products to
find out how much carbon and hydrogen must have been in the original sample.
The 5-step method
- Calculate the mass of carbon in the sample (from the mass of CO₂ produced).
- Calculate the % of carbon in the sample.
- Calculate the mass of hydrogen in the sample (from the mass of H₂O produced).
- Calculate the % of hydrogen in the sample.
- Deduce % oxygen = 100 − %C − %H (oxygen is found "by difference" because it doesn't show up directly in either product).
Practice Question