Which of the following best describes the difference between cis-trans isomerism and tautomerization?

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BlackTom AI · Accepted Answer

Let’s break down the question by restating what each term means and then evaluate each option against these definitions.

Question restatement: Which statement best distinguishes cis–trans isomerism from tautomerization?

Option 1: 'Cis-trans isomers always have different functional (chemical) groups, while tautomers always have the same functional groups.' This is incorrect. Cis–trans isomerism refers to the spatial arrangement around a restricted bond (usually a double bond), not to changes in the actual functional groups present. Two cis or trans isomers can indeed have the same set of functional groups; they differ in orientation about the bond. Tautomerization involves an intramolecular shift of atoms (often a proton and a double bond rearrangement) that can interconvert functional placements while often preserving the overall formula, but the key point here is that this option makes an overgeneralization about groups that does not capture the core distinction.

Option 2: 'Cis-trans isomerism involves restricted rotation around a double bond, while tautomerization is a rapid equilibrium between two structural forms.' This aligns with the standard definitions. Cis–trans (geometric) isomerism arises because rotation around a double bond is restricted, leading to distinct spatial arrangements (cis vs trans). Tautomerization is specifically an equilibrium process in which protons and pi bonds shift to yield constitutional isomers (tautomers) that rapidly interconvert under certain conditions. The emphasis on restricted rotation for cis–trans and rapid equilibrium for tautomerization accurately contrasts the two concepts.

Option 3: 'Tautomerization requires a chiral center, while cis-trans isomerism does not.' This is incorrect. Tautomerization does not require a chiral center; many tautomerizations occur without any stereocenters. Moreover, cis–trans isomerism can involve chiral molecules in some cases, but the presence or absence of chirality is not the defining difference between the two phenomena. The statement confuses stereochemistry with tautomerism.

Option 4: 'Cis-trans isomers can interconvert rapidly in solution, just like tautomers.' This is misleading. In many systems, cis–trans interconversion around a double bond is not rapid at room temperature and may require light (photoisomerization) or high energy to rotate around the double bond. In contrast, tautomerizations are often rapid equilibria under appropriate conditions. Stating that both interconvert rapidly in solution incorrectly generalizes the kinetics and mechanism of cis–trans isomerism relative to tautomerization.

In summary, option 2 captures the fundamental distinction: restricted rotation around a double bond underpins cis–trans isomerism, whereas tautomerization involves a rapid constitutional rearrangement between forms. The other options either misstate fundamental aspects of the two concepts or conflate related ideas without addressing the core difference.

2254 BIOSC 1544 SEC1000 Exam #2

Which of the following best describes the difference between cis-trans isomerism and tautomerization?

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