How many asymmetric carbons are there in pentan-2-one?

Question

BlackTom AI · Accepted Answer

We need to determine how many stereocenters (asymmetric carbons) are present in pentan-2-one.
First, recall the structure of pentan-2-one: CH3–CO–CH2–CH2–CH3, where the carbonyl carbon is at position 2.
Option a: 2. To have two asymmetric carbons, two carbons would each need to be bonded to four different groups. In this molecule, none of the carbon atoms meet that criterion because most carbons are bonded to two hydrogens or have symmetry that prevents four distinct substituents. Therefore, this option is incorrect.
Option b: 3. Similar reasoning applies; there aren’t three distinct stereocenters in pentan-2-one since the carbonyl carbon isn’t chiral and the remaining carbons (CH3, CH2 next to carbonyl, CH2 in the middle, and the terminal CH3) do not possess four different substituents around them. This option is incorrect.
Option c: 0. Examining each carbon:
- The carbonyl carbon (C2) is attached to CH3, =O, and CH2; it has a double bond to oxygen, so it cannot be a stereocenter.
- C1 (terminal CH3) is connected to a single carbon and three hydrogens; not a stereocenter.
- C3 is a CH2 group adjacent to the carbonyl; it has two hydrogens, so not chiral.
- C4 is also CH2, flanked by C3 and C5; it has two hydrogens, not chiral.
- C5 is terminal CH3; not a stereocenter.
Thus, there are zero asymmetric carbons. This option is correct.
Option d: 1. This would claim there is exactly one stereocenter somewhere in the chain. Given the analysis above, no carbon meets the four-different-substituents condition, so this option is incorrect.
In summary, only pentan-2-one lacks any carbon with four different substituents, making the number of asymmetric carbons zero. The remaining options fail because they either assume the existence of stereocenters where there are none or miscount the possibilities based on the carbonyl-adjacent structure.

How many asymmetric carbons are there in pentan-2-one?

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