tetrahedral, no unused p orbital
🪐ACS ORGANIC CHEMISTRY STUDY GUIDE
Structure, Orbitals, and Aromaticity for the ACS Exam
Review orbital mixing, formal charge, electron delocalization, conjugation, and aromaticity with ACS-style recall checks.
8 key ideas8 flashcards5 practice questions
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Geometry tells you where the p orbitals can meet.
planar with one p orbital
linear with two p orbitals
Key ideas to know
Start with the relationships between ideas. Then close the notes and explain each one from memory.
- 01
Carbon commonly forms four bonds by using sp3, sp2, or sp orbital mixing.
- 02
An sp3 center usually has tetrahedral electron geometry, an sp2 center is trigonal planar, and an sp center is linear.
- 03
Formal charge compares assigned valence electrons with the neutral atom's valence count.
- 04
Equivalent electron-delocalized forms describe one species rather than rapidly switching molecules.
- 05
Conjugation requires adjacent p orbitals that can overlap across three or more atoms.
- 06
Aromatic rings are cyclic, planar, fully conjugated, and contain 4n + 2 pi electrons.
- 07
Antiaromatic systems are cyclic, planar, fully conjugated, and contain 4n pi electrons.
- 08
Bond length, acidity, and stability can reflect electron delocalization that is not obvious from one Lewis form.
PART OF THE ACS ORGANIC CHEMISTRY GUIDE
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Open the full ACS Organic Chemistry guide →Two ideas worth correcting now
NOT QUITE
→Aromatic means a molecule has an odor
USE THIS INSTEAD
In organic chemistry, aromaticity describes a cyclic electron arrangement, not smell.
NOT QUITE
→More electron-delocalized forms always mean greater stability
USE THIS INSTEAD
The forms must be valid, similar in energy, and represent actual orbital overlap.
Flashcards
Answer before opening each card. The effort to retrieve is part of the learning.
1What geometry is usual at an sp2 carbon?Show answer ＋
Trigonal planar, with bond angles near 120 degrees.
2What does a curved arrow track?Show answer ＋
Movement of an electron pair from its starting location to a new bond or atom.
3What is conjugation?Show answer ＋
Continuous overlap of adjacent p orbitals.
4How many pi electrons satisfy the aromatic rule?Show answer ＋
4n + 2 pi electrons, where n is a nonnegative integer.
5Does an electron-delocalized form represent a separate compound?Show answer ＋
No. The forms are bookkeeping views of one electron distribution.
6Why is an allylic carbocation often more stable than a simple primary carbocation?Show answer ＋
Its empty p orbital joins an adjacent pi system, spreading electron deficiency across more than one atom.
7What can interrupt conjugation?Show answer ＋
An sp3 atom with no suitable p orbital between two pi systems.
8Why is cyclooctatetraene not antiaromatic in its usual form?Show answer ＋
It adopts a nonplanar conformation, so continuous planar p-orbital overlap is absent.
Explain it in your own words
Use the answer as a check after you have written or spoken your response.
01How do you test a ring for aromaticity?
Check that it is cyclic, planar, fully conjugated, then count pi electrons and apply the 4n + 2 rule.
02Why are all carbon-carbon bonds in benzene the same length?
The pi electrons are distributed around the ring, giving each bond the same intermediate bond order.
03How can orbital mixing predict acidity?
More s character holds electrons closer to the nucleus, which can stabilize a conjugate base at carbon.
04Why must curved arrows begin at electrons?
The arrow records electron movement, so its tail begins at a bond or lone pair rather than at a positive charge.
05How does a formal charge check catch a bad Lewis form?
It exposes atoms with implausible electron assignments and confirms that all charges add to the species' total charge.
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