Difficulty: Introductory | Prerequisites: General Chemistry (atomic bonding, electronegativity, Lewis structures). If you are shaky on how atoms share electrons, review covalent bonding first.
Functional groups are the single most important organising idea in organic chemistry. Every organic molecule is built on a carbon backbone, and the specific clusters of atoms bolted onto that backbone, the functional groups, determine how the molecule behaves: what it reacts with, how soluble it is, whether it is acidic or basic, and what role it plays in biology or industry. If general chemistry gave you the periodic table as an alphabet, functional groups are the vocabulary. Master them now and every reaction mechanism, synthesis problem, and spectroscopy question later in the course will make far more sense. If you skip this material, you will be guessing for the rest of the semester.
A functional group is a specific arrangement of atoms within a molecule that dictates its chemical behaviour. Organic Chemistry I covers roughly 15 core functional groups, from simple alkenes and alcohols to more complex amides and nitriles. Recognising each group on sight, knowing its formula, and understanding its typical reactivity is the foundation for every topic that follows in the course.
Functional group
A specific arrangement of atoms within a molecule that gives it characteristic chemical properties and reactivity. In simple terms, it is the "active site" of an organic molecule, the part that does the chemistry.
Alkene
A group of atoms containing a carbon-carbon double bond (C=C). Think of it as the reactive cousin of a plain carbon-carbon single bond: that extra bond makes alkenes far more chemically interesting.
Alcohol (hydroxyl group)
A hydroxyl group (-OH) attached to a carbon atom. In simple terms, this is the same -OH you see in water, just bolted onto a carbon skeleton. It makes the molecule more polar and water-soluble.
Amine
A nitrogen atom bonded to one, two, or three carbon atoms (-NH2, -NH-, or -N-). Think of it as ammonia (NH3) with one or more hydrogens swapped for carbon groups. Amines are often basic and have a characteristic fishy smell.
Carboxylic acid
A carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-COOH). This is the group that makes vinegar sour. The combination of C=O and -OH on the same carbon creates an acidic hydrogen.
Ester
A carbon atom double-bonded to an oxygen and single-bonded to another oxygen that bridges to a second carbon (-COO-). Think of it as a carboxylic acid that has swapped its acidic hydrogen for a carbon group. Esters often have fruity or floral smells.
Aldehyde
A carbon atom double-bonded to an oxygen and single-bonded to a hydrogen (-CHO). In simple terms, the carbonyl (C=O) sits at the end of a carbon chain, always with at least one hydrogen neighbour.
Ketone
A carbon atom double-bonded to an oxygen and flanked by two other carbon atoms (-CO-). Think of it as a carbonyl buried in the middle of a carbon chain rather than at the end. Acetone (nail polish remover) is the classic example.
Halogen (alkyl halide)
A halogen atom (F, Cl, Br, or I) bonded to a carbon atom (-X). In simple terms, one hydrogen on the carbon skeleton has been replaced by a halogen. The carbon-halogen bond is polar, which drives many substitution and elimination reactions.
Amide
A nitrogen atom bonded to a carbonyl carbon (-CONH2). This is the linkage that holds proteins together (the peptide bond is an amide). Unlike amines, amides are not basic.
Phosphoric acid (phosphate group)
A phosphorus atom bonded to four oxygen atoms (-PO4H2). Think of it as the connector in DNA and ATP. Phosphate esters are central to biochemistry.
Sulfonic acid
A sulfur atom bonded to a carbon and three oxygen atoms (-SO3H). In simple terms, a strong acid group used in detergents and dyes. Much stronger than a carboxylic acid.
Sulfide (thioether)
A sulfur atom bonded to two carbon atoms (-S-). Think of it as the sulfur analogue of an ether. Found in garlic and in the amino acid methionine.
Ether
An oxygen atom bonded to two carbon atoms (-O-). In simple terms, two carbon groups sharing one oxygen. Ethers are relatively unreactive, which makes them popular as solvents (diethyl ether, THF).
Nitrile (cyano group)
A carbon atom triple-bonded to a nitrogen atom (-C≡N). Think of it as the nitrogen analogue of an alkyne triple bond. Nitriles can be hydrolysed to carboxylic acids or reduced to amines, making them versatile intermediates.
Alkene (C=C)
Contains a carbon-carbon double bond: one sigma bond and one pi bond
Geometry: trigonal planar around each doubly bonded carbon (roughly 120° bond angles)
Key reactions: electrophilic addition (HBr, H2O, Br2), hydrogenation, oxidation
Example: ethylene (ethene, CH2=CH2), the simplest alkene and a major industrial feedstock
Alcohol (-OH)
Hydroxyl group bonded to an sp3-hybridised carbon
Classified as primary (1°), secondary (2°), or tertiary (3°) based on the carbon's substitution
Capable of hydrogen bonding, which raises boiling points relative to similar-mass hydrocarbons
Example: ethanol (CH3CH2OH)
Ether (-O-)
An oxygen atom bridging two carbon groups (R-O-R')
Relatively unreactive; commonly used as solvents
Cannot hydrogen-bond with itself (no O-H), so lower boiling point than comparable alcohols
Example: diethyl ether (CH3CH2OCH2CH3)
Aldehyde (-CHO)
Carbonyl group (C=O) at the terminal position, with one hydrogen
More easily oxidised than ketones (Tollens' test, Benedict's test)
Example: formaldehyde (HCHO), acetaldehyde (CH3CHO)
Ketone (-CO-)
Carbonyl group flanked by two carbon groups (R-CO-R')
Cannot be oxidised without breaking C-C bonds
Example: acetone (CH3COCH3)
Carboxylic acid (-COOH)
Carbonyl + hydroxyl on the same carbon
Acidic: can donate the O-H proton (pKa roughly 4 to 5 for simple carboxylic acids)
Example: acetic acid (CH3COOH), found in vinegar
Ester (-COO-)
Formed by condensation of a carboxylic acid and an alcohol (loss of water)
Often fragrant; common in fats, oils, and flavouring agents
Example: ethyl acetate (CH3COOCH2CH3)
Amine (-NH2, -NH-, -N-)
Classified as primary, secondary, or tertiary depending on how many carbons are bonded to nitrogen
Basic: the lone pair on nitrogen can accept a proton
Example: methylamine (CH3NH2)
Amide (-CONH2)
Nitrogen bonded to a carbonyl carbon
The lone pair on nitrogen is delocalised into the carbonyl, so amides are not basic
The peptide bond in proteins is an amide linkage
Example: acetamide (CH3CONH2)
Nitrile (-C≡N)
Carbon triple-bonded to nitrogen
Polar, with a linear geometry at the nitrile carbon
Versatile in synthesis: hydrolysis gives carboxylic acids, reduction gives amines
Example: acetonitrile (CH3CN)
Alkyl halide (-X, where X = F, Cl, Br, I)
Polar C-X bond, with carbon bearing a partial positive charge
Central to substitution (SN1, SN2) and elimination (E1, E2) reactions
Reactivity order: C-I > C-Br > C-Cl > C-F (bond strength increases, reactivity decreases)
Example: chloromethane (CH3Cl)
Sulfide / thioether (-S-)
Sulfur analogue of an ether
Sulfur is larger and more polarisable than oxygen, so sulfides are better nucleophiles
Example: dimethyl sulfide (CH3SCH3)
Sulfonic acid (-SO3H)
Very strong acid (much stronger than carboxylic acids)
Found in detergents, dyes, and pharmaceutical intermediates
Example: methanesulfonic acid (CH3SO3H)
Phosphoric acid / phosphate (-PO4H2)
Phosphorus bonded to four oxygens
Fundamental to biochemistry: DNA backbone, ATP energy transfer
Can form mono-, di-, and triesters
Example: the phosphodiester linkages in nucleic acids
Alcohols are the basis of antiseptics (isopropanol) and fuels (ethanol blends in petrol).
Carboxylic acids and esters underpin the food industry: vinegar is dilute acetic acid; artificial fruit flavours are esters.
Amines and amides are central to pharmaceuticals. Most drug molecules contain at least one nitrogen-based functional group, and the peptide bond that holds every protein together is an amide.
Alkyl halides are used as refrigerants, solvents, and starting materials in organic synthesis. CFCs (chlorofluorocarbons) famously damaged the ozone layer.
Phosphate groups drive cellular energy transfer (ATP) and form the structural backbone of DNA and RNA.
Sulfonic acid groups make surfactants work: sodium lauryl sulfate in shampoo is a sulfonate ester.
Confusing aldehydes and ketones. Both contain a carbonyl (C=O), but an aldehyde has at least one hydrogen on the carbonyl carbon (terminal position), while a ketone has two carbon groups flanking it (internal position). If you see -CHO, it is an aldehyde. If the C=O is between two carbons, it is a ketone.
Thinking amides are basic like amines. The nitrogen lone pair in an amide is delocalised into the adjacent carbonyl, making amides essentially non-basic. Amines are basic; amides are not.
Mixing up esters and ethers. Both contain C-O-C, but an ester also has a carbonyl (C=O) next to that oxygen. Ethers have no carbonyl.
Assuming all -OH groups are alcohols. A hydroxyl group on a carbonyl carbon is part of a carboxylic acid (-COOH), not an alcohol. The carbon's oxidation state and bonding environment matter.
Forgetting that halide reactivity depends on the halogen. C-I bonds are weakest and most reactive; C-F bonds are strongest and least reactive. Students often assume all alkyl halides behave the same.
⚠️ You will almost certainly be asked to identify functional groups in a drawn molecule. Practise spotting each group in complex structures, not just in isolation.
⚠️ Know the general formula for each group cold: -OH, -COOH, -CHO, -CO-, -COO-, -CONH2, -NH2, -C≡N, -X, -SO3H, -S-, -O-, -PO4H2, C=C.
⚠️ Expect questions that test whether you can distinguish similar groups: aldehyde vs. ketone, ester vs. ether, amine vs. amide, alcohol vs. carboxylic acid.
⚠️ Physical property questions (boiling point, solubility, acidity) tie directly to which functional group is present. Hydrogen bonding capability and polarity are the usual angles.
⚠️ Nomenclature: each functional group has a suffix in IUPAC naming (-ol for alcohol, -al for aldehyde, -one for ketone, -oic acid for carboxylic acid, -amine for amine, etc.). Exam questions frequently ask you to name a compound from its structure or draw a structure from its name.
True or false: An aldehyde and a ketone both contain a carbonyl group (C=O). True.
Fill in the blank: The general formula for a carboxylic acid functional group is ________. -COOH.
True or false: Amides are strongly basic because they contain a nitrogen atom. False. The nitrogen lone pair is delocalised into the carbonyl.
Fill in the blank: A sulfide functional group has the general formula ________. -S- (sulfur bonded to two carbon atoms).
True or false: An ether contains a C=O group. False. An ether is simply R-O-R' with no carbonyl.
Q: What distinguishes an aldehyde from a ketone?
A: Both contain a carbonyl group (C=O). In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen atom and sits at the end of the chain (-CHO). In a ketone, the carbonyl carbon is bonded to two other carbon atoms and sits within the chain (-CO-).
Q: A molecule contains a carbon double-bonded to oxygen and single-bonded to an -OH group. What functional group is this?
A: A carboxylic acid (-COOH).
Q: Why are amides not basic, even though they contain nitrogen?
A: The nitrogen lone pair in an amide is delocalised (resonance) into the adjacent carbonyl group, making it unavailable to accept a proton. In contrast, an amine's nitrogen lone pair is localised and freely available to act as a base.
Q: List the alkyl halides in order of decreasing reactivity by halogen type.
A: C-I > C-Br > C-Cl > C-F. Iodine forms the weakest C-X bond and is the best leaving group; fluorine forms the strongest and is the poorest leaving group.
Q: What is the structural difference between an ester and an ether?
A: An ester has a carbonyl group (C=O) adjacent to a C-O-C linkage, giving the general formula -COO-. An ether has only the C-O-C linkage with no carbonyl, giving the formula R-O-R'.
Q: Name the functional group with the formula -C≡N and give one reason it is synthetically useful.
A: A nitrile (cyano group). It is useful because it can be hydrolysed to a carboxylic acid or reduced to a primary amine, making it a versatile intermediate in synthesis.
Q: Which functional group is responsible for the backbone linkage in DNA?
A: The phosphodiester linkage, formed by the phosphoric acid / phosphate group (-PO4H2) connecting sugar units.
Q: A compound has a boiling point much higher than a hydrocarbon of similar molecular weight. It is water-soluble and contains only C, H, and O. Which functional group is most likely present?
A: An alcohol (-OH). The hydroxyl group enables hydrogen bonding, raising the boiling point and increasing water solubility.
This material connects directly to reaction mechanisms (SN1, SN2, E1, E2, electrophilic addition), because the functional group present determines which mechanism operates. You will revisit carbonyl chemistry in depth when studying nucleophilic addition (aldehydes, ketones) and nucleophilic acyl substitution (esters, amides, carboxylic acid derivatives).
Functional groups also link to spectroscopy: IR spectroscopy identifies groups by their characteristic absorption frequencies (broad O-H stretch for alcohols, sharp C=O stretch for carbonyls, C≡N stretch for nitriles). NMR and mass spectrometry build on the same vocabulary.
In biochemistry, every major biomolecule class maps to functional groups covered here: carbohydrates (alcohols, aldehydes, ketones), amino acids and proteins (amines, amides, carboxylic acids), lipids (esters, carboxylic acids), and nucleic acids (phosphate esters, amines, amides).
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