I85PREP
⌘ K
GTLE Subject Specialism Chemistry study materials
GTLE Subject Specialism Chemistry study resources

GTLE Subject Specialism Chemistry: Organic Chemistry: Structure, Bonding, and Functional Groups

GTLE Subject Specialism Chemistry · Work through your exam, one topic at a time.

All guide sections

Organic Chemistry: Structure, Bonding, and Functional Groups

Organic structure questions test whether you can map a formula onto a bonding model, name the compound, and predict its physical and acid-base behaviour from functional-group effects. Expect hybridisation, resonance, aromaticity, isomer counting and systematic naming.

Bonding and geometry

  • sp3: four domains, 109.5°, tetrahedral. sp2: three domains, 120°, planar, the basis of alkenes, carbonyls and aromatic rings. sp: two domains, 180°, linear alkynes and nitriles.
  • More s character means shorter, stronger bonds and more acidic C-H: ethyne (pK_a about 25) > ethene (about 44) > ethane (about 50).
  • Resonance delocalises charge, so both carboxylate C-O bonds are equal rather than one double and one single.
  • Aromaticity requires a cyclic, planar, fully conjugated ring with 4n + 2 pi electrons. Benzene (6), pyridine (6) and the cyclopentadienyl anion (6) qualify; cyclooctatetraene (8 pi, tub-shaped) does not.

Naming priorities

Suffix priority: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine, with alkene, alkyne, halide and ether as prefixes. Isomer families include chain, position, functional group, cis-trans (E/Z when all four groups differ) and optical, where a stereocentre carries four different groups.

Worked example

Rank ethanoic acid, 4-nitrophenol, phenol and ethanol by acidity. Ethanol: the alkoxide has no resonance stabilisation, pK_a about 16, least acidic. Phenol: the phenoxide is delocalised into the ring, pK_a about 10. 4-Nitrophenol: the nitro group withdraws electron density by induction and resonance, stabilising the anion, pK_a about 7.2. Ethanoic acid: the carboxylate is shared over two equivalent oxygens, pK_a 4.76, most acidic. Order: ethanoic acid > 4-nitrophenol > phenol > ethanol.

Traps

  • Calling a ring aromatic without checking planarity and conjugation.
  • Confusing the stability of the conjugate base with the reactivity of the acid.
  • Missing a stereocentre because two substituents look similar at a glance.
  • Using the wrong suffix priority or omitting locants for the principal group.
  • Forgetting that electron-withdrawing groups raise acidity while electron-donating groups lower it.