Question bank — Isomerism — structural (chain, position, functional, metamerism, tautomerism) and stereo (geometrical - cis-trans - E-Z,

False. Cis/trans compares groups "by eye" (are the similar ones together?), while E/Z uses CIP priority (atomic number). A high-priority group can sit on the "cis" side, so a cis alkene can be E. (See the figure below — same skeleton, two verdicts.)

False. It needs four different groups. $C{H}_{2}ClBr$ has two identical H's, so its mirror image superimposes — not chiral.

False. In resonance only electrons move and it is one molecule; in tautomerism a real H atom relocates giving two distinct molecules in equilibrium.

False. $2^n$ (here $n=2$, so $2^2=4$) is the maximum; internal symmetry (a meso form) collapses two of them into one achiral molecule, so tartaric acid has 3, not 4.

False. They interconvert by free rotation about a σ bond thousands of times a second at room temperature — you can never isolate a single one. (See the Newman sketch of staggered vs eclipsed below.)

False. Each enantiomer rotates strongly; a 50:50 mix is inactive because the equal and opposite rotations cancel (external compensation).

False. Restricted rotation is what matters; a ring (e.g. 1,2-dimethylcyclopropane) also locks groups into cis/trans without any C=C.

False. Each doubly-bonded carbon must carry two different groups. In $C{H}_2=CHCl$ one carbon has two H's, so there is no distinct "other side" — no geometrical isomers.

False. Position isomers move a group along the same skeleton; metamers redistribute the carbons on either side of a divalent atom (O, S, N), e.g. $C{H}_{3}O{C}_3{H}_7$ vs $C_2{H}_{5}O{C}_2{H}_5$.

False. Enantiomers share all scalar physical properties (b.p., m.p., density); they differ only in the sign of optical rotation and in reactions with other chiral things.

False. Epimers differ in configuration at only one of several stereocentres, so they are diastereomers (different physical properties); enantiomers differ at every stereocentre and are mirror images.

Acetone does have α-hydrogens on its $C{H}_3$ groups; the C=O next door makes them acidic (α-H acidity), so keto–enol tautomerism happens.

They have the same functional group (a C=C alkene); only its position differs, so they are position isomers, not functional.

One H is fine — chirality needs four different groups, and H, F, Cl, Br are all different, so it is a genuine chiral centre.

Formaldehyde has no α-carbon (nothing next to the C=O except two H's on the carbonyl carbon), so there is no α-H to shuttle — no tautomerism.

The coincidence only holds when the "similar" groups are also the higher-priority ones. Assign E/Z case by case with CIP, never by transferring a cis-2-butene result.

They have different functional groups (an –OH alcohol vs an –O– ether), so they are functional isomers; metamerism requires the same functional group.

A meso compound does contain chiral centres; it is achiral because an internal mirror plane makes one half the reflection of the other, cancelling the rotations inside one molecule (see figure below).

Differing at one of several stereocentres makes them epimers (a kind of diastereomer), not enantiomers; enantiomers must be opposite at every centre.

The π bond is a sideways overlap of p-orbitals (p-orbital overlap); rotating one carbon would break that overlap, costing a huge amount of energy, so the groups stay frozen.

Removing it leaves a carbanion stabilised by resonance onto the electronegative oxygen (an enolate); that stabilisation is exactly why the H can leave and return.

cis/trans becomes ambiguous once a carbon carries three or four different substituents ("which pair is cis?"); E/Z uses CIP priority to give one unique, non-arguable label.

R/S reads the 1→2→3 rotation as seen from the side opposite the smallest group; viewing it from the wrong side flips clockwise↔anticlockwise and inverts your answer.

For every molecule twisting light one way there is its mirror twin twisting it exactly the opposite way; the population-average rotation is zero.

Because atoms (an H) and a π bond actually change position — connectivity differs between keto and enol — and different connectivity is the definition of structural isomerism.

That formula counts configurational isomers fixed by bonds you'd have to break to interchange; conformers interconvert by mere σ-bond rotation (see Newman projections, illustrated above) and are not distinct configurations.

Because only one stereocentre is flipped, the two molecules are not complete mirror images, so they have genuinely different physical properties like a normal pair of diastereomers.

Yes — a ring restricts rotation just like a π bond, so 1,2-disubstituted rings give cis (same face) and trans (opposite faces) forms.

Yes — allenes and certain biphenyls are chiral through their overall shape (axial/atropisomerism); chirality is about non-superimposable mirror images, not about a labelled carbon.

There is nothing to compare on that carbon, so the alkene has no geometrical isomers and E/Z simply does not apply.

No — if the two centres are mirror images of each other within the molecule (meso), an internal mirror plane makes the whole molecule achiral despite the stereocentres.

They behave differently only toward other chiral things — polarised light, chiral catalysts, enzymes, taste/smell receptors — but are identical toward ordinary achiral reagents and instruments.

Yes — e.g. a β-keto system whose enol form has a C=C bearing two different groups on each carbon can display cis/trans enols while still equilibrating with its keto form.

Yes — if one is a meso form and the other a chiral diastereomer differing at a single centre; the "epimer" label is about single-centre difference, independent of whether each is optically active.