The Cahn-Ingold-Prelog priority designation for the following groups, CH3 (i), OH (ii), CHO (iii), and COOH (iv), is

Question

BlackTom AI · Accepted Answer

Cahn–Ingold–Prelog priority is assigned by comparing the atomic numbers of the atoms directly bonded to the stereocentre; if those are the same, compare the sets of atoms bonded to those atoms (treat multiple bonds as if the atom were bonded to duplicate “ghost” atoms).

1) Relevant knowledge points
- Higher atomic number = higher priority.
- For tie at the first atom, list the atoms bonded to that atom in descending atomic number order; for multiple bonds, count the bonded atom twice (or as duplicates) and compare lists lexicographically.

2) Step-by-step reasoning and intermediate lists
- Group (ii) OH: first atom = O (Z = 8). This immediately outranks any group whose first atom has lower Z.
- Groups (i), (iii), (iv) start with C (Z = 6), so compare their substituent-lists (including duplicates for multiple bonds).

Construct the lists of atoms attached to the first carbon (ordered by decreasing Z), including duplicated atoms for multiple bonds:
- (iv) COOH (carboxyl carbon): bonded to a double-bond O (counted twice), a single-bond O, and the stereocentre C. List: O, O, O, C (i.e. 8, 8, 8, 6).
- (iii) CHO (formyl carbon): bonded to a double-bond O (counted twice), an H, and the stereocentre C. List: O, O, C, H (8, 8, 6, 1).
- (i) CH3 (methyl carbon): bonded to three H and the stereocentre C. List: C, H, H, H (6, 1, 1, 1).

3) Lexicographic comparison
- Compare (iv) vs (iii): at the third entry O (8) vs C (6) → COOH (iv) wins over CHO (iii).
- Compare (iii) vs (i): at the first entries O (8) vs C (6) → CHO (iii) wins over CH3 (i).
- OH (ii) had O at the first position, so OH > any C-starting group.

4) Why the correct answer is correct
Combining the results gives OH (ii) highest, then COOH (iv), then CHO (iii), and last CH3 (i): ii > iv > iii > i.

5) Why other options are incorrect
- i > ii > iii > iv: places CH3 above OH (wrong because O (Z=8) > C (Z=6)).
- iv > iii > ii > i and iv > iii > i > ii: place COOH above OH (wrong for the same reason: OH has O directly bonded so it must outrank any group whose attached atom is C).

6) Conclusion
The CIP priority order is ii > iv > iii > i, which matches the provided answer.

The Cahn-Ingold-Prelog priority designation for the following groups, CH3 (i), OH (ii), CHO (iii), and COOH (iv), is单项选择题

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