Structural Isomers and Stereochemistry

These differ in which atoms are bonded to which. Chain isomers differ in branching (butane vs 2-methylpropane). Position isomers move a group along the chain (1-propanol vs 2-propanol). Functional-group isomers belong to different families altogether (ethanol vs dimethyl ether, both C₂H₆O).

To enumerate an acyclic alkane's constitutional isomers, first draw the longest unbranched skeleton, then shorten the longest chain and place branches at inequivalent positions. For C₅H₁₂: a five-carbon chain gives pentane; a four-carbon chain plus one methyl gives 2-methylbutane; a three-carbon chain with two methyls at the center gives 2,2-dimethylpropane. A branch at an end just lengthens the parent, and flipping a drawing can repeat a previously counted skeleton. Confirm every candidate has five carbons and twelve hydrogens and compare connectivity, not page orientation.

For position isomers, keep the skeleton and family but move the group: propan-1-ol and propan-2-ol both have C₃H₈O. Functional-group isomers instead change the local connections: ethanol and methoxymethane share C₂H₆O but are alcohol and ether. Matching atom counts is required before calling any pair isomers.

Stereoisomers have identical bonds but differ in how those bonds point in space. The two kinds you need are cis-trans (geometric) isomers around a rigid double bond or ring, and enantiomers, which are non-superimposable mirror images.

Rotation about an ordinary C–C single bond changes conformation without changing connectivity. Conformers interconvert by such rotations and are not new constitutional isomers. Restricted rotation around C=C can give separately distinguishable geometric isomers because the π overlap must be disrupted to interconvert them. Ring constraints can also produce cis/trans arrangements, but use the ring's geometry rather than the alkene's two-substituents-per-carbon test.

A C=C cannot rotate, because the pi bond would have to break. Two different groups on each alkene carbon allow geometric isomerism. When a suitable matching pair of groups is present, cis and trans compare those groups on the same and opposite sides; use E/Z for the general case. If either carbon carries two identical groups, no cis-trans pair exists.

For but-2-ene, each alkene carbon carries H and CH₃, so the two methyl groups can be on the same or opposite sides. But-1-ene has two H on one alkene carbon and cannot give that pair. With four different substituents, the simple cis/trans names may be ambiguous; E/Z uses priority rules separately at each carbon. At first comparison, higher atomic number has higher priority; if tied, compare outward along the groups. Higher-priority groups together give Z and opposite give E. Geometric isomerism requires distinct substituents at both ends, regardless of which notation names it.

A tetrahedral carbon bonded to four different groups is a stereogenic center. A molecule with one such center and no compensating symmetry has nonsuperimposable mirror images, giving a pair of enantiomers. With multiple centers, inspect the entire molecule for internal symmetry. Enantiomers share almost every physical property but rotate plane-polarised light in opposite directions and can behave very differently in biology, which is why drug chirality matters.

At C₂ in butan-2-ol, list H, OH, CH₃ and CH₂CH₃: all four are different. Wedge and dashed bonds show bonds projecting toward and behind the page; exchanging two groups changes the configuration at that center. Enantiomers are nonsuperimposable mirror images, with equal ordinary properties in an achiral environment but opposite optical rotations under matching conditions. A racemic equal mixture has no net optical rotation. The sign of rotation is not determined just by an R or S label.

One isolated tetrahedral center with four different groups commonly gives an enantiomeric pair. Multiple stereocenters require checking the whole molecule: internal symmetry can produce a meso compound, as the existing 2,3-dibromobutane example shows. Conversely some molecules are chiral without a tetrahedral carbon center; the center-count test is an introductory tool, not the definition of all chirality.