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Analyzing Organic Reactions

Chapter 4: Analyzing Organic Reactions

Updated Apr 17, 2026
Read the entire chapter on one page Every section in order, with the sidebar tracking where you are as you scroll.
🎯 Diagnostic: Test Your Starting Level 24 questions (2 per section). No prior reading required, see what you already know.

Aim to answer every question before checking. Missed questions point you to the sections you need most.

1. (4.1) In organic chemistry, a nucleophile is:
D. Nucleophiles carry lone pairs or π electrons that attack electron-poor sites. "Nucleo" = loves positive charge.
2. (4.1) A Lewis acid is:
C. BF₃, AlCl₃, and H⁺ are Lewis acids. Brønsted acids (proton donors) are a subset of Lewis acids.
3. (4.2) A Brønsted acid-base reaction:
B. Stronger acids go to weaker conjugate acids; stronger bases go to weaker conjugate bases. Compare pKas to predict direction.
4. (4.2) Acetic acid has pKa ~4.76, meaning it is:
A. Lower pKa means stronger acid. HCl has pKa ≈ -7; water has pKa ≈ 15.7.
5. (4.3) Acidity is increased (pKa lowered) by:
D. ARIO mnemonic: Atom (electronegativity/size), Resonance, Induction, Orbital (more s-character). EWGs stabilize negative charge inductively.
6. (4.3) A stable conjugate base (and thus a more acidic parent) is favored by:
C. Acetate (from acetic acid) is resonance-stabilized across two O atoms, which is why acetic acid is much more acidic than an alcohol.
7. (4.4) Nucleophilicity is:
B. Basicity is thermodynamic (equilibrium); nucleophilicity is kinetic (rate). Good nucleophiles often are strong bases, but not always.
8. (4.4) Within a period, nucleophilicity:
A. NH₂⁻ is a better nucleophile than OH⁻ in a given period. Down a group in protic solvents, nucleophilicity increases because solvation is weaker for bigger anions.
9. (4.5) An electrophile is:
D. Typical electrophiles: H⁺, carbonyl carbon, alkyl halide carbon with a leaving group, BF₃.
10. (4.5) A carbonyl carbon is:
C. The nucleophile typically attacks the carbon face of C=O; electrons flow onto oxygen to form an alkoxide intermediate.
11. (4.6) A good leaving group is one whose conjugate form is:
B. "Happy leaving group" = "Stable on its own." OH⁻ is a poor LG; OTs⁻ is excellent.
12. (4.6) Among F⁻, Cl⁻, Br⁻, I⁻, the best leaving group is:
A. Larger halides hold negative charge better (diffuse orbitals), making them more stable and better leaving groups.
13. (4.7) The oxidation state of carbon in CH₄ is:
D. Each C-H bond donates a formal electron to C (since C is more electronegative than H). Methane is the most reduced form of carbon.
14. (4.7) The oxidation state of carbon rises when:
C. Adding an electronegative bond takes electrons from C, which raises its oxidation state.
15. (4.8) Curved arrows in mechanisms show:
B. The tail is where electrons come from (lone pair, σ bond, π bond); the head is where they end up (forming a new bond or lone pair).
16. (4.8) A double-headed curved arrow (two-barbed) represents:
A. Single-barbed "fishhook" arrows are used for radical mechanisms.
17. (4.9) Carbocation stability generally follows:
D. Allylic and benzylic cations are even more stable thanks to resonance.
18. (4.9) Adjacent lone pairs or π systems stabilize a carbocation via:
C. Allyl and benzyl cations are resonance-stabilized and much more stable than their saturated counterparts.
19. (4.10) Carbanion stability:
B. Carbanions are opposite to carbocations in every way. Alkyl groups destabilize carbanions. sp hybridized > sp² > sp³ in stability.
20. (4.10) The most stable carbanion among the following is:
A. Enolates are the workhorse nucleophile in C-C bond formation (aldol, Claisen, alkylation).
21. (4.11) A carbon radical is stabilized by:
D. Radicals mirror carbocations in the sense that alkyl donation stabilizes both. Allyl and benzyl radicals are especially stable.
22. (4.11) Free-radical halogenation of an alkane proceeds by:
C. Light or heat triggers homolytic cleavage. Bromination is more selective (slower, more product-determining) than chlorination.
23. (4.12) Kinetic control in a reaction:
B. Classic example: 1,2- vs. 1,4-addition to butadiene at low T gives the kinetic (1,2) product.
24. (4.12) Thermodynamic control favors:
A. Warm the butadiene system up and the 1,4-product (thermodynamic) dominates.

Every organic reaction in this book - and every one the MCAT will ask you about - is a variation on the same four-step play: an electron-rich species finds an electron-poor species, they share electrons, something leaves, and a new bond forms. That is it. If you can spot the nucleophile, the electrophile, the leaving group, and the factors that stabilize the intermediates, you can predict the product of almost any reaction without memorizing a single mechanism.

This chapter is the reason later chapters make sense. Alcohols, aldehydes, ketones, carboxylic acids, amines - every reaction chapter in this book is just an application of the toolkit built right here. Students who try to memorize reactions without first mastering acids, bases, arrow pushing, and intermediate stability get buried in flashcards. Students who understand this chapter can reason their way to the answer even when they have never seen the specific reaction before.

The test-writers know this too. That is why MCAT passages routinely drop a reaction you have never seen, give you reagent and conditions, and expect you to predict the product. They are testing whether you have the tools in this chapter, not whether you memorized a lookup table.

The Central Analogy

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