Difficulty: Intermediate | Prerequisites: IUPAC naming conventions, functional groups, acid-base equilibria (pKa), hydrogen bonding basics
Alcohols (-OH), diols (two -OH groups), and thiols (-SH) are named using the IUPAC suffix system. Their physical properties, particularly boiling points and solubility, are dominated by hydrogen bonding. Acidity depends on how well the conjugate base (alkoxide) is stabilised, with electron-withdrawing groups increasing acidity and bulky alkyl groups decreasing it.
Alcohol (R-OH)
An organic compound containing a hydroxyl group (-OH) bonded to a saturated carbon atom. In simple terms, it is a carbon skeleton with an -OH stuck on.
Diol
A compound bearing two hydroxyl groups. Think of it as a molecule with two alcohol functional groups.
Thiol (R-SH)
The sulphur analogue of an alcohol, where -SH replaces -OH. Sometimes called a mercaptan. In simple terms, swap the oxygen in an alcohol for sulphur and you have a thiol.
Alkoxide (RO-)
The conjugate base formed when an alcohol loses a proton. Think of it as the deprotonated form of an alcohol, e.g. ethoxide (CH3CH2O-) from ethanol.
Hydroxy
The prefix used when -OH is named as a substituent rather than the principal functional group.
Mercapto
The prefix used when -SH is named as a substituent rather than the principal functional group.
pKa
A measure of acid strength. A more positive pKa means a weaker acid (less willing to donate H+). In simple terms, higher pKa = harder to pull off the proton.
Inductive effect
The electron-withdrawing or electron-donating influence of nearby atoms transmitted through sigma bonds. In simple terms, electronegative atoms like fluorine tug electron density away through the bonds, stabilising a nearby negative charge.
The IUPAC suffix for alcohols is -ol. For diols, use -diol.
Identify the longest carbon chain containing the -OH group as the parent chain.
Number from the end that gives the -OH the lowest possible locant.
List substituents alphabetically in the prefix.
Example: CH3CH2CH2-OH = propan-1-ol (propanol). A six-carbon chain with -OH on carbons 2 and 4 plus a methyl at carbon 5 = 5-methyl-2,4-hexanediol.
When -OH or -SH is a substituent (not the highest-priority group):
-OH is named hydroxy
-SH is named mercapto
-OH outranks -SH in naming priority, so if both are present, the compound is named as an alcohol with mercapto as a prefix.
Common examples from the chapter:
CH3OH = methanol (toxic, "wood alcohol")
CH3CH2OH = ethanol (non-toxic, the drinkable one)
2-butanol, 1-methyl-1,2-cyclohexanediol, 4-phenyl-2-butanol, 2-mercapto-4,4-dimethylcyclohexanol
Alcohols, like water, have very polar O-H bonds and are capable of hydrogen bonding.
This gives low-molecular-weight alcohols higher boiling points than you would predict from molecular weight alone. Boiling requires pulling molecules apart; hydrogen bonds hold them together more tightly.
Alcohols can act as both proton donors (H-bond donors via O-H) and proton acceptors (H-bond acceptors via oxygen lone pairs).
Solubility in water:
Small alcohols (methanol, ethanol, isopropanol, tert-butanol) are fully water-soluble because the -OH dominates the molecular character.
As the hydrocarbon chain grows, solubility drops because the non-polar portion increasingly outweighs the polar -OH.
Real-world connection: This is why ethanol mixes with water in any proportion (your drinks), but long-chain alcohols like 1-decanol barely dissolve. It is also why low-molecular-weight alcohols feel "watery" while higher alcohols feel "oily".
Alcohols are amphoteric: they can donate a proton (act as acids) or accept one via their lone pairs (act as bases).
As acids:
Deprotonation by a base B- gives an alkoxide: RO-H + B- ⇌ RO- + BH
Common alkoxides: methoxide (CH3O-), ethoxide (CH3CH2O-), propoxide, tert-butoxide ((CH3)3CO-)
The key to acidity is conjugate-base stability. The more stabilised the alkoxide RO-, the stronger the acid RO-H.
Trends in alcohol pKa:
tert-Butanol has a higher pKa (less acidic) than ethanol, which has a higher pKa than water. Fluorinated alcohols like CF3CH2OH and (CF3)3COH are more acidic (lower pKa) because of the electron-withdrawing inductive effect of fluorine.
Electron-withdrawing groups (e.g. fluorine) stabilise the alkoxide by pulling electron density away from the negative oxygen.
Bulky alkyl groups destabilise the alkoxide by making it less well solvated ("greasier") in water, so branched alcohols are weaker acids.
Leaving-group activation:
HO- is a poor leaving group in substitution reactions. Protonating the -OH with an acid (H-X) converts it to H2O, which is a much better leaving group. This is a key principle from the SN2 chapter applied here.
Reactions with strong bases:
Strong bases such as NaH, NaNH2, R-Li, and R-MgBr are basic enough to fully deprotonate alcohols (and thiols), generating the corresponding alkoxide (or thiolate) salts.
Students often think all alcohols are water-soluble. They are not. Only those with short carbon chains (up to about four carbons) dissolve readily; longer-chain alcohols are effectively insoluble.
Students sometimes confuse pKa direction: a higher pKa means a weaker acid, not a stronger one. tert-Butanol is a weaker acid than water despite both having -OH groups.
Many students forget that -OH must be protonated before it can leave in a substitution reaction. Free hydroxide (HO-) is a terrible leaving group; water (H2O) is a good one.
The inductive effect of electronegative substituents increases acidity. Students sometimes think fluorine makes an alcohol less acidic because fluorine is "electron-rich." The opposite is true: fluorine withdraws electron density and stabilises the alkoxide.
⚠️ Naming is heavily tested. Be able to assign IUPAC names to alcohols, diols, and thiols with multiple substituents, and know when to use -ol vs. hydroxy vs. mercapto.
⚠️ pKa ordering questions are common. You should be able to rank a set of alcohols by acidity and explain why (inductive effects, solvation, alkyl branching).
⚠️ Leaving-group activation (protonating -OH to make H2O leave) appears in mechanism questions throughout the course. This concept recurs in every substitution and elimination chapter.
⚠️ Hydrogen-bonding explanations are a staple of "explain the trend" questions on boiling point, solubility, and physical property comparisons.
True or False: The IUPAC suffix for thiols is -ol.
Fill in the blank: When -OH appears as a substituent, it is called ______.
True or False: tert-Butanol is a stronger acid than water.
Fill in the blank: Protonating the -OH group of an alcohol converts it into ______, which is a good leaving group.
True or False: Methanol, ethanol, isopropanol, and tert-butanol are all water-soluble.
Answers: 1. False (thiols use -thiol). 2. Hydroxy. 3. False (tert-butanol has a higher pKa, making it weaker). 4. H2O (water). 5. True.
Q: Give the IUPAC name for a six-carbon chain with -OH groups on carbons 2 and 4, and a methyl group on carbon 5.
A: 5-Methyl-2,4-hexanediol.
Q: Rank the following in order of increasing acidity: (CH3)3COH, CH3CH2OH, H2O, CF3CH2OH.
A: (CH3)3COH < CH3CH2OH < H2O < CF3CH2OH. tert-Butanol is the weakest acid (bulky alkoxide, poorly solvated). The trifluoroethanol is the strongest because fluorine stabilises the conjugate base by inductive withdrawal.
Q: Why does ethanol have a higher boiling point than fluoroethane (CH3CH2F), even though fluoroethane has a larger dipole moment?
A: Ethanol can form hydrogen bonds (O-H to O lone pair) while fluoroethane cannot. Hydrogen bonding is a much stronger intermolecular force than dipole-dipole alone.
Q: Why is HO- a poor leaving group, and how do you convert it into a good one?
A: HO- is a strong base and a poor leaving group. Protonating the alcohol with an acid (H-X) converts -OH to -OH2+, and water (H2O) is a good leaving group.
Q: What product forms when cyclohexanol reacts with NaH?
A: Sodium cyclohexanolate (the sodium alkoxide) plus H2 gas. NaH is a strong enough base to deprotonate the alcohol.
Nomenclature rules here build directly on IUPAC naming from the alkane and alkene chapters. The acidity and leaving-group concepts tie back to SN1/SN2 and E1/E2 mechanisms, which you will use again in the reactions-of-alcohols material (Part 3 of these notes). Hydrogen bonding is the same principle covered in intermolecular forces and will recur in the biochemistry of proteins and DNA.
Related Terms / Search Tags
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