Organic structure, bonding & nomenclature
The idea
Organic chemistry is built on carbon's reliable tetravalence: four bonds, arranged as chains, branches, and rings, with functional groups — alcohols, amines, carbonyls, halides, alkenes and the rest — serving as the reactive handles that determine behavior. Skeletons are mostly inert scenery; reactions happen at functional groups, so learning to spot and rank them is the first reading skill of the subject. Your covalent bonding and formula-writing background carries over directly; what is new is a systematic language for unambiguous structure.
IUPAC nomenclature runs on three moves: find the longest carbon chain that contains the principal characteristic group, number the chain so that group gets the lowest possible locant, then attach substituent names with locants in alphabetical order. The principal group claims the suffix (-ol, -al, -one, -oic acid) and outranks mere substituents in the numbering contest. A useful companion tool is the degree of unsaturation, (2C + 2 − H)/2 for a hydrocarbon-like formula, which counts rings plus π bonds before you draw anything.
The misconception to clear: two condensed formulas that look different on paper are not necessarily different compounds — the same molecule can be written from either end or with branches reordered. The systematic name is the unique identifier; if two structures generate the same name, they are the same substance.
Worked example
Give the IUPAC name of CH₃CH(OH)CH₂CH(CH₃)CH₃ and classify its functional group.
- Identify the functional group first: an OH bound to a carbon that carries two other carbons makes this a secondary alcohol, so the suffix is -ol and the OH carbon must get the lowest locant available.
- Find the longest chain through the OH-bearing carbon: tracing the backbone gives five carbons, so the parent is pentane, modified to pentanol.
- Number from the end nearer the OH: counting from that end places the OH on C2; numbering from the other end would put it on C4, so the first choice wins (the principal group outranks the substituent in this contest).
- Locate substituents on the chosen numbering: a methyl branch sits on C4.
- Assemble the name with locants: 4-methylpentan-2-ol. Double-check by re-deriving the structure from the name — a five-carbon chain, OH on C2, methyl on C4 — and confirming it reproduces the original formula.
Answer. The compound is 4-methylpentan-2-ol, a secondary alcohol.
Check your understanding
- Why does the principal characteristic group, rather than the substituents, control the numbering direction of the chain?
- How would the name change if the OH in this molecule were replaced by a C=O at the same position, and what suffix rules drive that?
- What does a degree of unsaturation of 1 allow structurally, and how would you decide between a ring and a double bond experimentally?
- How would you convince a friend that CH₃CH(CH₃)CH₂CH(OH)CH₃ is the very same compound as the one named above?
Build the foundations first
Organic structure, bonding & nomenclature builds on these concepts. If any feel shaky, start there.