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Alcohols

Chapter 5: Alcohols

Updated Apr 17, 2026
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1. (5.1) A primary (1°) alcohol has:
A. Ethanol is 1°. Secondary = OH on a C with 2 carbons; tertiary = 3 carbons. Classification controls reactivity.
2. (5.1) The IUPAC name of CH₃CH₂CH₂OH is:
D. OH sits on C1. Isopropanol/2-propanol has the OH on C2 (CH₃CH(OH)CH₃).
3. (5.2) Alcohols have unusually high boiling points because of:
C. Compared to similarly sized alkanes, alcohols have much higher boiling points. Ethanol bp is 78 °C; ethane bp is -89 °C.
4. (5.2) Compared with an alkane of similar mass, an alcohol is:
B. Short-chain alcohols like methanol, ethanol, propanol are fully water-miscible. Higher alcohols become less soluble as the alkyl portion grows.
5. (5.3) The pKa of ethanol is approximately:
A. Alkoxides (RO⁻) are strong bases, capable of deprotonating most aldehydes and esters' α-H (pKa ~25).
6. (5.3) Phenol is more acidic than aliphatic alcohols because:
D. Phenol pKa ≈ 10 vs. ethanol pKa ≈ 16. Electron-withdrawing groups on the ring increase phenol acidity further.
7. (5.4) Converting R-OH to R-OMs or R-OTs with MsCl or TsCl:
C. Now the alcohol can undergo SN/E reactions that OH would not (unless protonated to OH₂⁺ by acid).
8. (5.4) Mesylates and tosylates are good leaving groups because:
B. The negative charge is delocalized over three oxygen atoms, making the anion very stable.
9. (5.5) An SN1 reaction proceeds via:
A. Rate = k[substrate]. The flat carbocation means the nucleophile can attack from either face, giving racemization.
10. (5.5) SN1 reactions are favored by:
D. The more substituted the carbon, the better the cation, and the more SN1 is preferred.
11. (5.6) An SN2 reaction:
C. Rate = k[Nu][substrate]. Walden inversion flips the stereocenter.
12. (5.6) SN2 reactions run fastest on:
B. DMSO, DMF, and acetone are common polar aprotic solvents that solvate cations but leave anions "naked" and nucleophilic.
13. (5.7) To distinguish SN1 from SN2:
A. A fast, strong nucleophile at a 1° carbon in DMSO screams SN2. A weak Nu in water on a 3° halide screams SN1.
14. (5.7) An SN2 reaction shows:
D. Backside attack passes the carbon through a trigonal-bipyramidal transition state, flipping like an umbrella in a gust of wind.
15. (5.8) E1 elimination:
C. E1 shares the cation intermediate with SN1; the two commonly compete.
16. (5.8) E1 products typically follow:
B. Zaitsev's rule: the most substituted alkene wins. Each additional alkyl group stabilizes the double bond via hyperconjugation.
17. (5.9) E2 elimination:
A. E2 has no intermediate. The antiperiplanar geometry is crucial; it controls product stereochemistry.
18. (5.9) E2 is favored by:
D. Bulk of base and substrate steric environment determine which β-H is accessible.
19. (5.10) The SN/E decision framework uses:
C. These four variables together essentially dictate which of the four mechanisms dominates.
20. (5.10) A bulky strong base at high temperature with a β-H tends to give:
B. Elimination beats substitution at high temperature because of the large positive ΔS of forming an alkene + small molecule.
21. (5.11) A primary alcohol treated with PCC (mild oxidant) gives:
A. PCC in dichloromethane is the classic reagent for this controlled oxidation. Jones reagent (CrO₃, H₂SO₄) goes all the way to the carboxylic acid.
22. (5.11) Secondary alcohols oxidized by PCC, Jones, or similar reagents give:
D. Secondary alcohols cannot be oxidized past a ketone without breaking a C-C bond. Tertiary alcohols do not oxidize at all.
23. (5.12) Grignard reagents (R-MgX) act as:
C. The C-Mg bond is very polarized, giving the carbon a high δ-. Grignards react violently with water, acids, and alcohols.
24. (5.13) Phenols:
B. Electron-withdrawing substituents on the ring (e.g., para-nitrophenol) lower the pKa further by stabilizing phenoxide.

Alcohols are the entry point to reactive organic chemistry. They show up in every biological molecule you will encounter - carbohydrates, amino acids with hydroxyl side chains, nucleic acids with ribose sugars, cholesterol, steroid hormones. They are everywhere. And for the MCAT, the reactions you learn with alcohols - substitution, elimination, oxidation, and Grignard addition - become templates for every reactive functional group in later chapters.

This chapter builds the single most-tested framework in MCAT organic chemistry: the SN1/SN2/E1/E2 decision. Given a substrate, a nucleophile or base, and a set of conditions, you should be able to predict which mechanism runs - and therefore which product forms. By the end of this chapter, you will have a flowchart in your head that solves 90% of the substitution and elimination questions the MCAT can throw at you.

We also cover how to convert alcohols into something useful (mesylates and tosylates), how to oxidize them selectively (PCC vs. Jones), and how Grignard reagents form new carbon-carbon bonds by attacking carbonyls. Finally, we look at phenols - alcohols attached to aromatic rings that behave surprisingly differently from regular alcohols.

The Central Analogy

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