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ACS: Organic Chemistry

ACS · Work through your exam, one topic at a time.

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Organic Chemistry

Organic Chemistry items on the ACS exam are mechanism- and structure-driven: assigning stereochemistry, predicting substitution and elimination outcomes, ranking acidity and carbonyl reactivity, and extracting structure from IR, NMR, and mass spectra.

Stereochemistry and conformation

  • CIP priorities follow atomic number; a doubly bonded atom is treated as bonded to two copies of the same atom, so a carbonyl carbon counts as two oxygens.
  • With the lowest-priority group pointing away, clockwise is R; if it points toward you, reverse the result.
  • Enantiomers differ at every stereocenter and rotate plane-polarized light equally and oppositely; diastereomers differ at some but not all; a meso compound has stereocenters plus an internal mirror plane.
  • In cyclohexane chairs the bulky substituent prefers equatorial; a tert-butyl group locks the ring.

Substitution and elimination

  • SN2 is concerted backside attack with inversion; rate = k[substrate][nucleophile]; methyl > primary > secondary; favored by strong nucleophiles in polar aprotic solvents.
  • SN1 proceeds through a carbocation, giving racemization and possible rearrangement; rate = k[substrate]; tertiary > secondary.
  • E2 requires an anti-periplanar β-hydrogen and a strong base; Zaitsev's rule gives the more substituted alkene, but bulky bases such as t-BuOK give the Hofmann product.
  • Heat plus strong base favors elimination; polar protic solvent and weak nucleophile favor SN1/E1.

Carbonyls and aromaticity

  • Nucleophilic acyl substitution reactivity: acid chloride > anhydride > ester > amide; aldehydes react faster than ketones.
  • Hückel aromaticity requires a cyclic, planar, fully conjugated ring with 4n + 2 π electrons; 4n electrons means antiaromatic.

Worked example

A compound analyzing as C8H8O2 has DBE = (2×8 + 2 − 8)/2 = 5. IR shows a strong 1720 cm^−1 band and no O–H stretch. The 1H NMR shows a 5H multiplet at 7.4 ppm and a 3H singlet at 3.9 ppm. Five DBE fit one benzene ring (4) plus one C=O (1). The 3.9 ppm singlet is an OCH3 deshielded by the adjacent ester oxygen, so the structure is methyl benzoate rather than a ketone, whose CH3 would appear near 2.1–2.5 ppm.

Traps

  • Reversing the CIP result when the lowest-priority group lies in the plane rather than pointing away.
  • Forgetting that SN2 inverts configuration.
  • Assuming a bulky base still delivers the Zaitsev alkene.
  • Counting symmetry-equivalent protons separately when integrating an NMR spectrum.
  • Calling a non-planar or partially conjugated ring aromatic.