- Define and explain Organic Synthesis in your own words
- Use key terms such as functional group accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Organic Synthesis: plan reaction routes and identify functional groups.
Plan reaction routes and identify functional groups.
Key ideas
Synthesis is planned backwards
Planning a synthesis means working backwards from the target molecule, choosing reactions that build each functional group in turn. Each step must use conditions that do not destroy groups already present — for instance, oxidise an alcohol before introducing an alkene that the oxidiser would attack. Good routes are short, high-yielding and use cheap starting materials.
The double bond dominates alkene chemistry
Alkenes contain a C=C double bond: one sigma bond plus one pi bond, and the pi bond's exposed electron density attracts electrophiles. In electrophilic addition, the pi bond breaks and both carbons gain new atoms — for example, ethene + HBr → bromoethane. With unsymmetrical alkenes, Markownikoff's rule predicts the major product: the hydrogen adds to the carbon already carrying more hydrogens, giving the more stable carbocation.
Key term — functional group: The atom or group of atoms responsible for a compound's characteristic reactions, e.g. –OH in alcohols.
Plan a two-step synthesis of ethanoic acid from ethene.
Step 1: ethene + steam (phosphoric acid catalyst, heat, pressure) → ethanol. Step 2: reflux ethanol with acidified potassium dichromate → ethanoic acid.
Answer: Step 1: ethene + steam (phosphoric acid catalyst, heat, pressure) → ethanol. Step 2: reflux ethanol with acidified potassium dichromate → ethanoic acid.
- Oxidising a tertiary alcohol Tertiary alcohols have no hydrogen on the carbon bearing the –OH group, so they cannot be oxidised without breaking carbon–carbon bonds. Acidified dichromate stays orange with a tertiary alcohol — a classic test result.
- Drawing curly arrows starting at atoms Curly arrows show electron pairs moving: they start at the electron source (a bond or lone pair) and point to where the electrons go. An arrow starting at an atom rather than the bond is meaningless.
Practice
Ethanoic acid; the orange dichromate turns green as it is reduced.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
The orange bromine water decolourises; the product is 1,2-dibromoethane.
H⁺ adds to C1, giving a secondary carbocation on C2, which is more stable than the primary alternative; Br⁻ then attacks C2 (Markownikoff's rule).
Quick check
Which of these best defines "functional group"?
Name the structural isomers of C₄H₁₀.
- Organic Synthesis: plan reaction routes and identify functional groups.
- Synthesis is planned backwards: Planning a synthesis means working backwards from the target molecule, choosing reactions that build each functional group in turn.
- homologous series: A family of compounds with the same functional group and a general formula, e.g.
- Watch out for: oxidising a tertiary alcohol