Difficulty: Introductory-Intermediate | Prerequisites: Chapters 1-7 (bonding, structure, alkanes, stereochemistry)
This material is the gateway to organic reactivity. Chapter 8 introduces the major functional groups you will see for the rest of the course, from alcohols and halides to ethers and thiols. If you can identify and name these groups confidently, everything that follows (substitution, elimination, addition) will make far more sense. You should already be comfortable with Lewis structures, hybridisation and basic IUPAC naming of alkanes.
Organic molecules are sorted by the functional group attached to their carbon skeleton: alcohols have -OH on an sp3 carbon, alkyl halides have a halogen, thiols have -SH, ethers have an oxygen bridging two carbon groups, and so on. The carbon bearing the functional group is called the alpha-carbon, and counting how many other carbons are attached to it tells you whether the compound is primary, secondary or tertiary. Three naming systems exist (substitutive, radicofunctional, common), but substitutive (IUPAC) nomenclature is the one you will use most.
Alkyl group
The fragment that remains when one hydrogen is removed from an alkane chain. In simple terms, think of it as the hydrocarbon "stub" that a functional group hangs off.
Alkyl halide
A compound in which a halogen (F, Cl, Br or I) is bonded to the carbon of an alkyl group. Think of it as an alkane with one hydrogen swapped for a halogen.
Alcohol
A compound containing a hydroxy group (-OH) bonded to an sp3-hybridised carbon of an alkyl group. In simple terms, it is a carbon skeleton with an -OH attached.
Thiol (mercaptan)
A compound in which a sulfhydryl group (-SH), also called a mercapto group, is bonded to the carbon of an alkyl group. Think of it as the sulfur cousin of an alcohol.
Phenol
A compound in which an -OH group is bonded to a carbon of an aromatic (aryl) ring, rather than to an sp3 carbon. This distinction matters because phenols are more acidic than ordinary alcohols.
Enol
A compound in which an -OH group is bonded to an sp2-hybridised carbon that is part of a C=C double bond. In simple terms, the -OH sits on a carbon involved in a double bond, not a single-bonded carbon.
Ether
A compound in which an oxygen atom is bonded to two carbon groups (alkyl or aryl). Think of it as two hydrocarbon pieces joined through an oxygen bridge.
Sulfide (thioether)
The sulfur analogue of an ether: a sulfur atom bonded to two carbon groups.
Alpha-carbon (a-carbon)
The carbon directly bonded to the functional group (the halogen in a halide, the oxygen in an alcohol or ether). It is the reference point for classifying a compound as primary, secondary or tertiary.
Primary (1 degree)
The alpha-carbon is bonded to one other alkyl group.
Secondary (2 degree)
The alpha-carbon is bonded to two other alkyl groups.
Tertiary (3 degree)
The alpha-carbon is bonded to three other alkyl groups.
Substitutive nomenclature (IUPAC)
The most broadly applicable naming system. You name the parent chain, number it to give the lowest locants to substituents and functional groups, then list substituents alphabetically.
Radicofunctional nomenclature
Names the alkyl group(s) as a separate word followed by the functional-class name (e.g. "ethyl chloride", "diethyl ether").
Common nomenclature
Informal names used only for the simplest, most widely known compounds (e.g. "methanol" or "ethanol").
Alcohols: -OH bonded to an sp3 carbon of an alkyl group. Alkanes with four or fewer carbons are gases at room temperature; once a hydroxyl group is added, boiling points rise sharply because of hydrogen bonding.
Thiols (mercaptans): -SH bonded to an alkyl carbon. Sulfur is larger and less electronegative than oxygen, so thiols are weaker hydrogen-bond donors than alcohols.
Phenols vs. alcohols: In a phenol, the -OH is on an aryl (benzene-ring) carbon. In an alcohol, the -OH is on an sp3 carbon. This matters for acidity and reactivity.
Enols vs. alcohols: In an enol, the -OH sits on an sp2 carbon that is part of a C=C double bond. In an alcohol, the -OH is on an sp3 carbon.
Ethers: An oxygen bridging two carbon groups (alkyl or aryl). No O-H bond, so ethers cannot donate hydrogen bonds, though the oxygen lone pairs can accept them.
Sulfides (thioethers): The sulfur analogues of ethers, with sulfur in place of oxygen.
The alpha-carbon is the carbon directly attached to the functional group (the halogen in a halide, the oxygen in an alcohol or ether). You classify halides and alcohols by counting how many other carbon groups are bonded to that alpha-carbon:
Primary (1 degree): one alkyl group on the alpha-carbon (e.g. ethanol, CH3CH2OH)
Secondary (2 degree): two alkyl groups on the alpha-carbon (e.g. isopropanol)
Tertiary (3 degree): three alkyl groups on the alpha-carbon (e.g. tert-butyl alcohol)
Methyl compounds (e.g. CH3Br, methyl bromide) sit outside this scale because the alpha-carbon carries no alkyl groups at all.
Naming alkyl halides (substitutive/IUPAC):
Identify the longest chain containing the halogen-bearing carbon.
Number the chain to give the halogen the lowest possible locant.
Name the halogen as a prefix: F = fluoro, Cl = chloro, Br = bromo, I = iodo.
Example: 1-chlorobutane, 2-fluoropentane, 2-iodo-2-methylbutane.
Naming alcohols (substitutive/IUPAC):
The -OH group is the principal group, so the parent chain must include the carbon bearing it.
Replace the final "-e" of the alkane name with "-ol" and number to give -OH the lowest locant.
Example: 4-bromo-2-methylpentan-1-ol.
Naming ethers and sulfides (substitutive/IUPAC):
The ether oxygen or sulfide sulfur is treated as a substituent on the longer parent chain.
Oxygen substituent = alkoxy prefix (methoxy, ethoxy, isopropoxy, etc.).
Sulfur substituent = alkylthio prefix (methylthio, etc.).
Example: 2-ethoxy-3-methylpentane, 4-isopropxypentan-2-ol, 1-(methylthio)butane.
Radicofunctional names are still widely used for simple ethers and sulfides: name both alkyl groups alphabetically and add "ether" or "sulfide" (e.g. diethyl ether, methyl propyl sulfide).
Cyclic compounds: Use the ring as the parent when naming cyclic halides (e.g. fluorocyclohexane) or cyclic alcohols. For complex ring systems with multiple substituents, list substituents alphabetically: e.g. 2-ethyl-3-methoxy-2-methylcyclopentan-1-ol.
Students often confuse phenols with alcohols. A phenol has -OH on an aromatic ring carbon (sp2); an alcohol has -OH on an sp3 carbon. They behave differently in acid-base chemistry and in reactions.
Students sometimes classify a compound as "secondary" just because the -OH is on the second carbon of the chain. Classification depends on the number of alkyl groups attached to the alpha-carbon, not the locant number.
Thiols and alcohols look similar on paper, but sulfur is larger, less electronegative and far more polarisable than oxygen. This changes hydrogen-bonding ability, acidity and reactivity.
When naming ethers, students sometimes try to use an "-ol" suffix. Ethers have no O-H bond; the oxygen is a substituent (alkoxy prefix), not the principal group.
Expect to be asked to classify a given structure as primary, secondary or tertiary. Draw the alpha-carbon and count the attached alkyl groups.
Naming questions frequently combine multiple substituents (halogen + alkyl + ether) on a ring or chain. Practise alphabetical ordering and lowest-locant rules.
Know the difference between substitutive names (IUPAC) and radicofunctional names. Exam questions may give one and ask for the other.
The phenol vs. alcohol distinction appears in acid-base and spectroscopy questions later in the course. Nail it now.
True or false: A tertiary alcohol has three -OH groups. (False. It has one -OH, but the alpha-carbon is bonded to three alkyl groups.)
Fill in the blank: The sulfur analogue of an ether is called a ______ (also known as a ______). (Sulfide; thioether.)
True or false: In an enol, the -OH group is bonded to an sp3-hybridised carbon. (False. It is bonded to an sp2 carbon that is part of a C=C double bond.)
Fill in the blank: The prefix used for a bromine substituent in IUPAC nomenclature is ______. (Bromo.)
True or false: Methyl bromide (CH3Br) is classified as a primary alkyl halide. (False. It is a methyl halide: the alpha-carbon has zero alkyl groups attached.)
Q: Classify 2-bromo-2-methylpropane as methyl, primary, secondary or tertiary. Explain your reasoning.
A: Tertiary. The alpha-carbon (C-2, the one bearing bromine) is bonded to three methyl groups.
Q: Give the IUPAC (substitutive) name for a six-membered ring with a fluorine at position 1.
A: Fluorocyclohexane. The "1-" locant is implied and often omitted when there is only one substituent on the ring.
Q: What is the structural difference between a phenol and a benzyl alcohol?
A: In a phenol, the -OH is bonded directly to a carbon of the aromatic ring. In benzyl alcohol (C6H5CH2OH), the -OH is bonded to a -CH2- group that is attached to the ring, so the alpha-carbon is sp3. Benzyl alcohol is an alcohol; phenol is not classified as a simple alcohol.
Q: Name the compound CH3CH2-O-CH2CH3 using both radicofunctional and substitutive nomenclature.
A: Radicofunctional: diethyl ether. Substitutive: ethoxyethane.
Q: A compound has an -SH group bonded to a two-carbon chain. Name it using substitutive nomenclature and give the older common class name.
A: Ethanethiol. The older class name for thiols is mercaptans.
The functional-group vocabulary introduced here is the foundation for Chapters 8.4-8.6, where you will see how these groups determine intermolecular forces (van der Waals, dipole-dipole, hydrogen bonding) and, in turn, physical properties such as boiling point and solubility.
Primary, secondary and tertiary classification becomes critical in substitution and elimination reactions (SN1, SN2, E1, E2) covered later in the course. Whether a halide or alcohol is 1 degree, 2 degree or 3 degree often dictates which mechanism dominates.
Ethers and crown ethers reappear in the solubility and solvation section (Ch. 8.6), where their ability to coordinate metal cations has practical and biological significance.
alkyl group, alkyl halide, haloalkane, alcohol, hydroxyl, -OH group, thiol, mercaptan, sulfhydryl, -SH group, phenol, aryl alcohol, enol, vinyl alcohol, ether, thioether, sulfide, alpha carbon, a-carbon, primary secondary tertiary classification, IUPAC nomenclature, substitutive nomenclature, radicofunctional nomenclature, common names, fluoro chloro bromo iodo, alkoxy, methoxy, ethoxy, isopropoxy, methylthio, crown ether, organic chemistry chapter 8, CHEM 301, functional groups