Organic chemistry tricks that make nomenclature stick

Memory trick: My Excellent Professor Bought Purple High Heels On Nice Days — Meth, Eth, Prop, But, Pent, Hex, Hept, Oct, Non, Dec.

The suffix tells you the functional group: -ane=alkane, -ene=alkene, -yne=alkyne, -ol=alcohol, -al=aldehyde, -one=ketone, -oic acid=carboxylic acid, -amine=amine.

1) Find longest carbon chain. 2) Number from end closest to first substituent. 3) Name and number all substituents. 4) Name parent chain. 5) Alphabetize substituents. 6) Write full name.

When the same substituent appears more than once: 2=di, 3=tri, 4=tetra, 5=penta. Example: 2,2-dimethylpropane. These prefixes are NOT counted in alphabetical ordering.

Add 'cyclo' prefix before the parent chain name. Cyclohexane, cyclopentene, cyclobutanol. For substituents on ring, number to give lowest locants.

Two different stereodescriptors: R/S describes configuration at a chiral center (4 different groups on sp³ carbon). E/Z describes configuration of double bond (restricted rotation). A compound can have both: (E)-2-bromobutene has E geometry at double bond but no chiral center. (R)-2-bromobutane has R configuration at chiral center but no double bond. Some compounds have both R/S AND E/Z.

Cis/trans is older and simpler: cis = same side, trans = opposite side (works when substituents are obvious). E/Z is systematic and uses CIP priority rules — applies to all alkenes. They can DIFFER: cis-2-butene (same groups same side) = Z-2-butene (higher priority same side = Z). But for more complex alkenes, cis/trans is ambiguous — use E/Z only. On exams: always use E/Z for assignment unless specifically asked for cis/trans.

Find longest chain containing the OH group. Number from end nearest OH to give lowest locant. Replace -e ending of alkane with -ol. Examples: propan-1-ol (not 1-propanol in IUPAC 2013), butan-2-ol, 2-methylpropan-1-ol. Polyols: ethane-1,2-diol (ethylene glycol), propane-1,2,3-triol (glycerol). For phenols, parent is benzene: phenol (hydroxybenzene). Secondary/tertiary distinction: name and locant tell you which carbon.

Esters: alcohol part + acid part → [alkyl] [acyl-ate]. Example: CH₃COOC₂H₅ = ethyl acetate (ethyl ethanoate). Ethers: smaller alkyl + larger alkyl + ether. Simple: diethyl ether. IUPAC: alkoxy prefix (methoxy-, ethoxy-). Amides: replace -oic acid with -amide, or -COOH with -carboxamide. N-substituted amides: N-methyl name. Acid halides: replace -ic acid with -yl halide (e.g., acetyl chloride, benzoyl chloride).

Key groups as substituents: CH₃- = methyl. C₂H₅- = ethyl. n-C₃H₇- = propyl. (CH₃)₂CH- = isopropyl (1-methylethyl). n-C₄H₉- = n-butyl. (CH₃)₃C- = tert-butyl (1,1-dimethylethyl). C₆H₅- = phenyl. C₆H₅CH₂- = benzyl. CH₂=CH- = vinyl (ethenyl). HC≡C- = ethynyl. These names appear constantly — memorize them cold.

Many important compounds are known by common names: Formaldehyde (HCHO) = methanal. Acetaldehyde (CH₃CHO) = ethanal. Acetone (CH₃COCH₃) = propan-2-one. Acetic acid (CH₃COOH) = ethanoic acid. Formic acid (HCOOH) = methanoic acid. Ethylene = ethene. Acetylene = ethyne. Toluene = methylbenzene. Aniline = aminobenzene. Phenol = hydroxybenzene. These appear in lab, literature, and industry — know both names.

Bicyclic = two rings sharing two carbons (bridgehead carbons). Name: bicyclo[x.y.z]alkane where x, y, z = number of carbons in each bridge (largest to smallest). Total carbons = x + y + z + 2. Example: bicyclo[2.2.1]heptane = norbornane (7 carbons: bridges of 2, 2, 1 + 2 bridgeheads). Number: start at one bridgehead, go along longest bridge, through other bridgehead, along next longest, then shortest. Spiro compounds: two rings sharing ONE carbon — spirocyclic.

Disubstituted benzene names: ortho (o-) = 1,2-substitution (adjacent carbons). Meta (m-) = 1,3-substitution (one carbon apart). Para (p-) = 1,4-substitution (opposite). Examples: o-dichlorobenzene = 1,2-dichlorobenzene. p-nitrotoluene = 4-nitromethylbenzene. For three or more substituents, use numbers only. When one substituent is a principal group (COOH, OH), that carbon is C1. Alphabetize/number to give lowest locants.