Difficulty: Introductory | Prerequisites: General chemistry (Lewis structures, electronegativity, bonding basics)
This chapter is the foundation for the entire organic chemistry course. Before you can study reactions, you need to recognise the cast of characters: functional groups are how organic chemists classify molecules by reactivity. Nomenclature gives you the shared language for naming them. Conformational analysis and physical properties (boiling points, melting points, intermolecular forces) explain why molecules behave the way they do in real, physical terms. If you are coming in cold, make sure you are comfortable with Lewis structures and electronegativity from general chemistry.
Organic molecules are sorted by their functional groups, which determine reactivity. Naming follows IUPAC rules layered on a base of common names you simply have to memorise. Physical properties like boiling point and melting point come down to molecular shape, size, and the strength of intermolecular forces.
Functional group
A specific arrangement of atoms within a molecule that determines its chemical reactivity. Think of it as the "business end" of the molecule: the part that actually does something in a reaction.
Alkane
A hydrocarbon containing only single C–C bonds. Mostly inert (unreactive). Covered in Chapter 2.
Alkene
A hydrocarbon with at least one C=C double bond. Undergoes nucleophilic additions. Covered in Chapters 4–5.
Alkyne
A hydrocarbon with at least one C≡C triple bond. Also undergoes nucleophilic additions. Covered in Chapter 14.
Arene (benzene ring)
A ring structure with alternating double bonds (aromatic). Undergoes nucleophilic substitutions. Covered in Chapters 15–16.
Alcohol (R–OH)
An –OH group bonded to a carbon. Versatile in reactions. One C–O bond. Covered in Chapter 10. In simple terms, this is the hydroxyl group, not to be confused with carboxylic acids, which also contain –OH.
Aldehyde
A carbonyl group (C=O) at the end of a carbon chain, with one hydrogen attached. An electrophile. Covered in Chapter 19.
Ketone
A carbonyl group (C=O) flanked by two carbon groups (no hydrogen on the carbonyl carbon). Also an electrophile. Covered in Chapter 19.
Ether
Two carbon groups joined through an oxygen (R–O–R). Two C–O bonds. Mostly inert. Covered in Chapter 11.
Amine (R–NH₂)
A nitrogen bonded to carbon(s) and hydrogen(s). Acts as a nucleophile and a base, replacing oxygen groups with nitrogen. Covered in Chapter 23.
Carboxylic acid (R–COOH)
A carbon bearing both a C=O and an –OH. Acid and electrophile. Covered in Chapter 20.
Ester
A carboxylic acid derivative where the –OH is replaced by –OR. Electrophile, undergoes substitutions. Covered in Chapter 21.
Amide
A carboxylic acid derivative where the –OH is replaced by –NR₂. Electrophile, undergoes substitutions. Covered in Chapter 21.
Acyl halide (acyl chloride)
A carboxylic acid derivative where the –OH is replaced by a halogen (e.g. –Cl). Electrophile, undergoes substitutions. Covered in Chapter 21.
Alkyl halide (haloalkane, R–X)
A carbon bonded to a halogen (F, Cl, Br, I). Electrophile; undergoes substitutions and eliminations. Covered in Chapter 9.
Thiol (R–SH)
Sulphur analogue of an alcohol. Nucleophile, undergoes substitutions. Covered in Chapter 10.
Sulfide (R–S–R)
Sulphur analogue of an ether. Mostly inert. Covered in Chapter 11.
Functional groups are organised into families based on the type of bonding and the heteroatoms present.
Hydrocarbons (C and H only)
Alkane: all single bonds, mostly inert
Alkene: at least one C=C double bond
Alkyne: at least one C≡C triple bond
Arene: aromatic ring with delocalised electrons
Simple oxygen heteroatomics
Alcohol (R–OH): one C–O bond, versatile reactivity
Ether (R–O–R): two C–O bonds, mostly inert
Carbonyl compounds (C=O)
Aldehyde: carbonyl at the end of the chain (H on the carbonyl carbon)
Ketone: carbonyl flanked by two carbon groups
Carboxylic acids and derivatives
Carboxylic acid (R–COOH)
Ester (R–COOR')
Amide (R–CONR'₂)
Acyl halide (R–COX)
Other heteroatomics
Haloalkane / alkyl halide (R–X)
Amine (R–NH₂, R₂NH, or R₃N)
Thiol (R–SH), the sulphur analogue of an alcohol
Sulfide (R–S–R), the sulphur analogue of an ether
Identifying alcohols: a common exam trap
A molecule must have an –OH bonded directly to a saturated carbon to count as an alcohol. A carboxylic acid contains –OH, but it is not an alcohol because the –OH is bonded to a carbonyl carbon. When a molecule has both an –OH on a saturated carbon and a ketone elsewhere, identify both functional groups separately.
Common alkyl group names you must know
These come up constantly. The naming pattern is based on the carbon count and the point of attachment.
n-butyl: a four-carbon chain attached at the terminal carbon
sec-butyl: a four-carbon group attached at the second carbon in the chain
isobutyl: a three-carbon chain with a methyl branch, attached at the terminal carbon
tert-butyl: a central carbon bonded to three methyl groups
isopropyl: a three-carbon group attached at the central carbon
IUPAC vs. common naming
IUPAC names are systematic: find the longest chain, number it so substituents get the lowest locants, then list substituents alphabetically.
Common names use the prefixes above. For example, sec-butylcyclohexane can also be called (1-methylpropyl)cyclohexane under IUPAC rules. "sec-butyl" tells you the attachment point is the second carbon of the butyl chain.
Naming cycloalkanes
The prefix "cyclo-" indicates a ring. When a ring is the larger structural unit, substituents are named relative to it: butylcyclohexane means a butyl group attached to a cyclohexane ring.
Key pattern: combining base names with substituents
The base name reflects the longest continuous chain or the ring. Substituents are named as branches (methyl, ethyl, propyl, etc.) with locant numbers. When there are multiple identical substituents, use di-, tri-, tetra- prefixes.
Students often assume any molecule with an –OH is an alcohol. A carboxylic acid has an –OH, but it is bonded to a carbonyl carbon, which changes its chemistry entirely. Only an –OH on a saturated carbon counts as an alcohol.
Confusing sec-butyl with isobutyl. "sec" refers to attachment at the secondary carbon. "iso" refers to the branching pattern of the chain itself. Draw them out side by side until the distinction is automatic.
Thinking that aldehydes and ketones are the same functional group. Both contain C=O, but the position matters: aldehydes have at least one hydrogen on the carbonyl carbon, ketones do not. This difference changes their reactivity.
Treating amines and amides as interchangeable. An amine is nitrogen bonded to carbons and hydrogens. An amide is nitrogen bonded to a carbonyl (C=O). Amides are far less basic than amines.
⚠️ "Which of these molecules is an alcohol?" is a classic exam question. You will be given several structures and expected to pick only the ones with –OH on a saturated carbon.
⚠️ You will need to convert between common names (sec-butyl, tert-butyl) and IUPAC names fluently. Expect both directions on the exam.
⚠️ Functional group identification is tested repeatedly throughout the course, not just in Chapter 2. Every reaction chapter assumes you can spot the functional groups at a glance.
⚠️ Know which functional groups are electrophilic vs. nucleophilic. This distinction drives the logic of every reaction mechanism you will learn.
True or false: A carboxylic acid is a type of alcohol because it contains –OH.
Fill in the blank: An aldehyde differs from a ketone in that the carbonyl carbon of an aldehyde is bonded to at least one ______.
True or false: sec-butyl means the butyl group is attached through a tertiary carbon.
Fill in the blank: The sulphur analogue of an alcohol is called a ______.
True or false: Ethers have two C–O bonds and are generally unreactive.
Answers: 1. False (the –OH is on a carbonyl carbon, making it a carboxylic acid). 2. Hydrogen. 3. False (it is attached through a secondary carbon). 4. Thiol. 5. True.
Q: A molecule has both a ketone and an –OH group on a saturated carbon. What functional groups are present?
A: Both a ketone and an alcohol. Each is identified separately.
Q: Give the IUPAC name for sec-butylcyclohexane.
A: (1-Methylpropyl)cyclohexane. The sec-butyl group is a four-carbon substituent attached at its second carbon.
Q: What is the key structural difference between an ester and a carboxylic acid?
A: In a carboxylic acid, the carbonyl carbon is bonded to –OH. In an ester, the –OH is replaced by –OR (an alkoxy group).
Q: Classify the following as electrophile or nucleophile: alkyl halide, amine, acyl chloride.
A: Alkyl halide: electrophile. Amine: nucleophile (and base). Acyl chloride: electrophile.
Q: Why is an amide much less basic than an amine?
A: In an amide, the nitrogen's lone pair is delocalised into the adjacent C=O, reducing its availability to accept a proton. An amine's lone pair is fully available.
Functional group recognition feeds directly into reaction mechanisms. Every reaction chapter (SN1, SN2, E1, E2, additions, substitutions) begins by identifying the functional group present, then predicts behaviour from there. Nomenclature is the shared language for all of it.
This connects to Chapter 9 (substitutions and eliminations of alkyl halides), Chapters 4–5 (alkene additions), Chapters 10–11 (alcohols, ethers, thiols, sulfides), and Chapters 19–21 (carbonyl chemistry). Getting functional groups right here saves enormous time later.
Functional groups, organic nomenclature, IUPAC naming, common names, alkane, alkene, alkyne, arene, aromatic, alcohol, hydroxyl, aldehyde, ketone, carbonyl, ether, amine, carboxylic acid, ester, amide, acyl halide, acyl chloride, haloalkane, alkyl halide, thiol, sulfide, n-butyl, sec-butyl, isobutyl, tert-butyl, isopropyl, cycloalkane, electrophile, nucleophile, organic chemistry chapter 2, UW organic chemistry, Prof. Forrest Michael