Constitutional (Structural) Isomers

Constitutional isomers share a molecular formula but differ in their actual bonding SEQUENCE, and this section works through five named varieties (a sixth, tautomerism, is developed on its own in the next section given how much depth it needs).

1. Chain (also called nuclear or skeletal) isomerism: the isomers differ purely in how their carbon atoms are strung together -- a straight chain versus a branched one -- while the molecular formula stays fixed. The classic C5H12 trio illustrates it fully: n-pentane (a straight five-carbon chain), isopentane / 2-methylbutane (one branch point), and neopentane / 2,2-dimethylpropane (a fully-branched, symmetric structure) are three genuinely different compounds sharing one formula.
2. Position isomerism: here the isomers belong to the same homologous series, share the same molecular formula AND the same carbon skeleton, but differ in exactly WHERE a substituent, functional group or unsaturated (double/triple) bond sits along that skeleton. Pent-1-ene and pent-2-ene (both C5H10) differ only in where the double bond falls; 1-chlorobutane and 2-chlorobutane (both C4H9Cl) differ only in where the chlorine sits; pentan-2-one and pentan-3-one (both C5H10O) differ only in where the carbonyl sits.
3. Functional isomerism: the isomers share a molecular formula but carry entirely DIFFERENT functional groups. Propanal (an aldehyde) and propanone (a ketone) are both C3H6O; propanoic acid (a carboxylic acid) and methyl acetate (an ester) are both C3H6O2 -- in each pair, the two compounds belong to completely different chemical families despite the identical formula.
4. Metamerism: a more specialised kind of structural isomerism, arising only in compounds whose functional group sits BETWEEN two alkyl groups -- ethers, ketones, esters and secondary amines are the families that show it. Metamers share a molecular formula and the same functional group, but differ in how the carbon atoms are distributed on the two sides of that functional group (unequal alkyl groups on either side). Methyl propyl ether and diethyl ether (both C4H10O) are metamers, as are diethyl ketone, methyl propyl ketone and methyl isopropyl ketone (all three C5H10O) -- each pair/set has the identical functional group (ether-oxygen, or ketone-carbonyl) but a different split of carbons on its two sides. …