Organic and Biological Chemistry: Functional Groups Drive Everything
Pattern recognition over memorisation
Organic and biological chemistry rewards pattern recognition. Learn the functional groups (alkanol, alkanal, alkanone, alkanoic acid, ester, amine, amide, alkene, haloalkane), IUPAC naming, structural formulae and the isomerism families: chain, position, functional-group, geometric (cis–trans) and optical (a chiral carbon bearing four different groups). Learn the reaction families too: addition across C=C (H₂/Ni, HX, Br₂ - the bromine decolourisation test), nucleophilic substitution of haloalkanes, elimination to alkenes, oxidation of alcohols (primary to aldehyde to carboxylic acid; secondary to ketone; tertiary resistant), esterification and hydrolysis, and addition versus condensation polymerisation. Biologically, link amino acids and peptide bonds to proteins and denaturation, mono- and disaccharides to reducing-sugar tests, and triglycerides to saponification.
Traps
- Naming esters backwards: the alcohol fragment comes first, as alkyl alkanoate.
- Forgetting that a primary alcohol must be distilled to stop at the aldehyde; reflux with excess oxidant gives the acid.
- Using bromine water to distinguish alkanes from alkenes without naming the correct addition product.
- Missing that cis–trans isomerism requires two different groups on each double-bond carbon.
- Confusing addition polymers (one monomer, no by-product) with condensation polymers (two monomers, small molecule lost).
Worked example
Predict the product when propan-1-ol is heated under reflux with acidified potassium dichromate(VI).
Propan-1-ol is primary, so oxidation gives propanal first. Under reflux with excess oxidant, the aldehyde is oxidised further to propanoic acid, and the dichromate colour changes from orange to green. If the aldehyde is wanted, distil it out as it forms before it can be oxidised again.