Difficulty: Introductory-Intermediate | Prerequisites: Chapters 1–4 (bonding, polarity, acid-base), Chapter 7 (substitution and elimination basics)
Big picture: Alcohols are among the most common functional groups in organic chemistry and biochemistry. This chapter introduces how to name them, explains why they behave so differently from alkanes (hint: hydrogen bonding), and covers their acid-base chemistry. Everything here feeds directly into the reactions covered in Part 2 of these notes, so nail the fundamentals before moving on.
Alcohols (R-OH) and thiols (R-SH) are named by finding the longest chain containing the functional group, numbering to give that group the lowest locant, and swapping the alkane ending for "-ol" or "-thiol." The -OH group makes alcohols polar and capable of hydrogen bonding, which is why they have much higher boiling points than comparably sized alkanes and are soluble in water. Alcohols are weakly acidic (pKa around 16), and treating them with metals or metal hydrides gives metal alkoxides, which are useful bases and nucleophiles in later reactions.
Alcohol
An organic compound containing a hydroxyl group (-OH) bonded to a saturated carbon atom. In simple terms, any molecule where an -OH sits on a carbon chain or ring.
Thiol
An organic compound containing a sulfhydryl group (-SH). Think of it as the sulfur analogue of an alcohol.
Primary (1°) alcohol
The carbon bearing the -OH is attached to one other carbon. Example: 1-propanol.
Secondary (2°) alcohol
The carbon bearing the -OH is attached to two other carbons. Example: 2-propanol (isopropanol).
Tertiary (3°) alcohol
The carbon bearing the -OH is attached to three other carbons. Example: 2-methyl-2-propanol.
Diol
A compound with two -OH groups. Named by adding "diol" to the parent chain and keeping the terminal "e" (e.g. propane-1,3-diol, also written 1,3-propanediol).
Alkoxide
The conjugate base of an alcohol (R-O⁻). Formed by deprotonation with a strong base, a metal, or a metal hydride. In simple terms, it is the anion you get when an alcohol loses its O-H proton.
Hydrogen bonding
An intermolecular force between a hydrogen bonded to an electronegative atom (O, N, F) and a lone pair on another electronegative atom. This is the main reason alcohols have elevated boiling points.
pKa
A measure of acid strength. Lower pKa means a stronger acid. Alcohols sit around pKa 16; thiols around pKa 10.5, making thiols more acidic.
Metal hydride
A compound such as NaH or LiH that acts as a strong base. Reacts with alcohols to produce alkoxides and H₂ gas.
Protic solvent
A solvent that can donate a hydrogen bond. Alcohols are protic because of their O-H group. This matters for understanding solubility and reaction mechanisms.
Find the longest carbon chain that includes the carbon bearing the -OH group. This is your parent chain.
Number the chain so that the carbon with the -OH gets the lowest possible number.
Replace the "-e" ending of the parent alkane with "-ol." For example, propane becomes propan-1-ol (or 1-propanol).
For cyclic compounds, numbering starts at the carbon bearing the -OH group.
List substituents alphabetically with their locants, as usual.
Carbons | Prefix |
|---|---|
2 | eth- |
3 | prop- |
4 | but- |
5 | pent- |
6 | hex- |
7 | hept- |
8 | oct- |
9 | non- |
10 | dec- |
Two -OH groups: add "diol" to the parent name. Keep the terminal "e" of the alkane name (e.g. propane-1,3-diol, cyclohexane-1,2-diol).
Three -OH groups: "triol," and so on.
CH₃CH₂CH₂OH: three-carbon chain, -OH on C1, so 1-propanol (propan-1-ol).
2-methyl-1-propanol: three-carbon parent chain, methyl branch at C2, -OH at C1.
2-bromo-5-methyl-1-cyclohexanol: cyclohexane ring, -OH at C1, bromine at C2, methyl at C5.
Same approach as alcohols but use the suffix "-thiol" instead of "-ol."
The parent chain must include the carbon bearing the -SH group.
Examples: 1-butanethiol, 1,2-ethanedithiol, 3-methyl-1-butanethiol.
Alcohols are polar, protic molecules. The O-H bond is polarised: oxygen carries a partial negative charge (δ-) and hydrogen carries a partial positive charge (δ+). This allows alcohols to form hydrogen bonds with each other and with water.
Hydrogen bonds are roughly 1.8 Å long and hold alcohol molecules together far more strongly than the London dispersion forces that hold alkanes together.
Hydrogen bonding has a dramatic effect on boiling points. Three molecules with nearly identical molecular weights illustrate this:
Compound | MW | BP |
|---|---|---|
Propane (CH₃CH₂CH₃) | 44 | -47 °C |
Dimethyl ether (CH₃OCH₃) | 46 | -24 °C |
Ethanol (CH₃CH₂OH) | 46 | +78 °C |
Ethanol's boiling point is over 100 °C higher than propane's, despite nearly the same molecular weight. The difference comes entirely from hydrogen bonding.
Alcohols dissolve in polar solvents such as water. The -OH group forms hydrogen bonds with water molecules, facilitating solubility. As the hydrocarbon portion of the molecule gets larger, water solubility decreases because the non-polar carbon chain increasingly dominates.
Hydrogen bonding from protic solvents (including alcohols and water) can stabilise anions by surrounding them with partial positive charges from O-H hydrogens. This is relevant when considering how leaving groups and conjugate bases behave in solution.
Alcohols are weak acids. Typical pKa for an alcohol is around 16, meaning they are far less acidic than carboxylic acids (pKa around 5) but more acidic than alkanes (pKa around 50).
Thiols are more acidic than alcohols, with a pKa around 10.5. Sulfur is larger and more polarisable than oxygen, so the thiolate anion (R-S⁻) is more stable than the alkoxide (R-O⁻), spreading the charge over a larger atom.
Alkali metals (Li, Na, K) react directly with alcohols. For example:
2 CH₃OH + 2 Na → 2 CH₃O⁻ Na⁺ + H₂
The metal donates an electron to deprotonate the alcohol. Hydrogen gas is evolved.
Metal hydrides such as NaH and LiH are strong bases. The hydride ion (H⁻) is the nucleophile/base:
CH₃OH + NaH → CH₃O⁻ Na⁺ + H₂
The alcohol acts as the acid (electrophilic O-H proton), and the hydride acts as the base (nucleophile). The reaction is irreversible because H₂ gas escapes.
Metal alkoxides are strong bases and good nucleophiles. They appear repeatedly in later chapters as reagents for elimination reactions (E2) and nucleophilic substitutions. If you can make an alkoxide, you can drive reactions that need a strong, non-bulky base.
Students often think that alcohols and ethers have similar boiling points because both contain oxygen. They do not. Alcohols can donate and accept hydrogen bonds; ethers can only accept them. This is why ethanol (BP +78 °C) boils far higher than dimethyl ether (BP -24 °C) despite nearly the same molecular weight.
Students sometimes forget to keep the terminal "e" when naming diols and triols. Propanediol, not propandiol. The "e" stays because the suffix begins with a consonant.
Confusing pKa values: a lower pKa means a stronger acid. Thiols (pKa ~10.5) are stronger acids than alcohols (pKa ~16), so thiols lose their proton more readily.
Students assume NaH is a nucleophile that attacks carbon. NaH is a base, not a nucleophile. It deprotonates the alcohol; it does not perform substitution at carbon.
⚠️ Naming alcohols and thiols is guaranteed to appear. Practice giving the lowest locant to the -OH or -SH group, including on cyclic structures.
⚠️ You will be asked to compare boiling points of molecules with similar molecular weights. Know the hydrogen bonding argument cold.
⚠️ Expect a question on which reagent (metal vs. metal hydride) you would use to convert an alcohol to an alkoxide, and why.
⚠️ pKa comparison between alcohols and thiols comes up frequently. Remember: thiols are more acidic because the thiolate anion is more stable.
True or false: Thiols are less acidic than alcohols. (False. Thiols have a lower pKa and are more acidic.)
Fill in the blank: To name an alcohol, replace the "-e" of the parent alkane with "___". (-ol)
True or false: NaH reacts with an alcohol to form an alkoxide and water. (False. The byproduct is H₂ gas, not water.)
Fill in the blank: Alcohols have higher boiling points than alkanes of similar MW because of ___. (hydrogen bonding)
True or false: When naming a diol, you drop the terminal "e" of the alkane name. (False. You keep the "e" before "diol.")
Q: Name the following compound: a cyclohexane ring with -OH at C1 and -Br at C4.
A: 4-bromocyclohexan-1-ol (or 4-bromo-1-cyclohexanol). Number the ring so that -OH gets position 1.
Q: Rank the following in order of increasing boiling point: pentane, 1-butanol, diethyl ether.
A: Pentane < diethyl ether < 1-butanol. Pentane has only London dispersion forces. Diethyl ether has dipole-dipole interactions but cannot donate hydrogen bonds. 1-butanol can both donate and accept hydrogen bonds.
Q: What products form when ethanol reacts with sodium metal?
A: Sodium ethoxide (CH₃CH₂O⁻ Na⁺) and hydrogen gas (H₂).
Q: Which is the stronger acid, ethanol or ethanethiol? Explain.
A: Ethanethiol (pKa ~10.5) is the stronger acid. Sulfur is larger and more polarisable than oxygen, so the resulting thiolate anion is more stable than the ethoxide anion, favouring deprotonation.
Q: A student wants to generate methoxide ion in solution. They add NaOH to methanol. Will this work?
A: Poorly. Methanol (pKa ~16) and water (pKa ~15.7) have very similar acidities, so the equilibrium lies only slightly to one side. To drive the reaction fully, use Na metal or NaH instead, both of which react irreversibly.
This connects to acid-base chemistry (Chapters 3-4) because understanding pKa values and conjugate bases is essential for predicting whether an alcohol will be deprotonated by a given base. It also sets the stage for substitution and elimination reactions (Chapter 7 concepts, extended here in Part 2) since the -OH group is a poor leaving group on its own, and much of Chapter 10's reaction chemistry is about converting it into something better.
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