Difficulty: Introductory | Prerequisites: Lewis structures, lone pairs, basic bonding concepts
Functional groups are the "vocabulary" of organic chemistry: once you can spot them in a molecule, you can predict how that molecule will react. Molecular geometry (from VSEPR theory) tells you the 3D shape around each atom, which matters for everything from boiling points to enzyme binding. This material is foundational and comes up in every unit of the course.
Functional groups are specific atom arrangements (like -OH, C=O, -NH2) that determine a molecule's chemical behaviour. VSEPR theory predicts the 3D geometry and bond angles around an atom based on how many bonding groups and lone pairs surround it.
Functional group
A specific grouping of atoms within a molecule that has its own characteristic chemical reactivity. The functional group is what makes an alcohol behave like an alcohol, regardless of the rest of the molecule.
Hydroxyl group (alcohol, -OH)
An oxygen atom bonded to a hydrogen and to a carbon. Written as -OH. The oxygen carries two lone pairs. Molecules containing a hydroxyl group are called alcohols.
Think of it as the group that makes ethanol (drinking alcohol) different from ethane (a gas).
Carbonyl group (C=O)
A carbon atom double-bonded to an oxygen atom. The oxygen is more electronegative, so the carbon is electrophilic (electron-poor) and the oxygen is nucleophilic (electron-rich). Found in aldehydes, ketones, carboxylic acids, esters, and amides.
Ketone
A carbonyl group bonded to two carbon groups (R-CO-R). Distinguished from an aldehyde, where the carbonyl is bonded to at least one hydrogen.
Amine (-NH2, -NHR, -NR2)
A nitrogen atom bonded to one, two, or three carbon groups (primary, secondary, or tertiary amine). The nitrogen carries a lone pair, making amines basic and nucleophilic.
Think of it as the nitrogen-based cousin of alcohols.
Amide (-CO-NH2, -CO-NHR, -CO-NR2)
A carbonyl group bonded directly to a nitrogen atom. The nitrogen's lone pair is partially delocalised into the carbonyl (resonance), making amides far less basic than amines. Found in proteins as the peptide bond.
VSEPR theory (Valence Shell Electron Pair Repulsion)
A model for predicting the 3D geometry around an atom by assuming that electron groups (bonding pairs and lone pairs) repel each other and arrange themselves as far apart as possible.
Bond angle
The angle between two bonds originating from the same atom. Determined by the number of electron groups (bonding + lone pairs) around that atom.
The worksheet presents a large organic molecule (resembling epinephrine/adrenaline) and asks students to label each functional group.
Amine: a nitrogen bonded to carbon groups and hydrogens, with the label pointing to the N(CH3) group. The nitrogen carries a lone pair.
Hydroxyl / alcohol: -OH groups on the aromatic ring and on the side chain. The oxygen in each is bonded to one hydrogen and one carbon, with two lone pairs.
Carbonyl / ketone: a C=O group where the carbon is bonded to two other carbons. The worksheet labels this as "carbonyl or ketone."
Amide: a C(=O)-NH2 group, where the carbonyl carbon is directly bonded to a nitrogen. Distinguished from a simple amine by the presence of the adjacent C=O.
The worksheet presents two molecules and asks students to predict bond angles at labelled atoms.
Molecule I (contains a double bond and an oxygen)
Atom a (carbon in a C=C double bond): 3 electron groups (the double bond counts as one group), trigonal planar geometry, bond angle approximately 120 degrees.
Atom b (carbon between the double bond and the oxygen): 3 electron groups, trigonal planar, bond angle approximately 117.5 degrees. The slight compression from 120 degrees is due to lone pairs on the adjacent oxygen.
Atom c (oxygen with two lone pairs and two bonds): 4 electron groups total (2 bonding + 2 lone pairs), bent geometry, bond angle approximately 104.5 degrees.
Molecule II (contains a triple bond and a carboxylic acid)
Atom d (carbon in a C triple bond, i.e. alkyne carbon): 2 electron groups, linear geometry, bond angle 180 degrees.
Atom e (carbonyl carbon in -COOH): 3 electron groups, trigonal planar geometry, bond angle approximately 120 degrees.
Electron groups | Lone pairs | Geometry | Approximate bond angle |
|---|---|---|---|
2 | 0 | Linear | 180 degrees |
3 | 0 | Trigonal planar | 120 degrees |
3 | 1 | Bent | Less than 120 degrees |
4 | 0 | Tetrahedral | 109.5 degrees |
4 | 1 | Trigonal pyramidal | About 107 degrees |
4 | 2 | Bent | About 104.5 degrees |
Functional groups determine a drug's behaviour in the body. Epinephrine's hydroxyl groups make it water-soluble enough to dissolve in blood, while its amine group lets it bind to adrenergic receptors. Bond angles and molecular shape determine whether a drug fits into an enzyme's active site, which is why 3D structure is central to medicinal chemistry and pharmacology.
Students often confuse amines and amides. The key difference: an amide has a carbonyl (C=O) directly bonded to the nitrogen. An amine does not.
A double bond counts as one electron group in VSEPR, not two. The same goes for a triple bond.
Lone pairs compress bond angles. An oxygen with two bonds and two lone pairs has a bond angle smaller than the tetrahedral 109.5 degrees (approximately 104.5 degrees), not larger.
Students sometimes think "trigonal planar" and "bent" are the same because both involve three things on an atom. Trigonal planar has three bonding groups and no lone pairs; bent has two bonding groups and one or two lone pairs.
Functional group identification is tested constantly. You will be given complex molecules and asked to circle and name each group.
VSEPR bond angle questions appear on nearly every exam. You must be able to look at any atom in a molecule, count its electron groups (remembering that double and triple bonds each count as one group), and state the geometry and approximate bond angle.
True or False: An amide is more basic than an amine.
Fill in the blank: A carbon with a triple bond has ___ electron groups and ___ geometry.
True or False: A double bond counts as two electron groups in VSEPR.
Fill in the blank: The bond angle around an sp3 oxygen with two lone pairs is approximately ___ degrees.
True or False: The carbonyl carbon in a carboxylic acid (-COOH) has trigonal planar geometry.
Q: Identify all functional groups in the following molecule: a benzene ring with an -OH group, a -C(=O)-CH3 side chain, and an -NH2 group.
A: Hydroxyl/alcohol (-OH), carbonyl/ketone (C=O bonded to two carbons), and primary amine (-NH2). If the -NH2 were bonded directly to a carbonyl carbon, it would instead be an amide.
Q: What is the bond angle around the nitrogen in a primary amine (e.g. CH3NH2)?
A: Nitrogen has 4 electron groups (3 bonding pairs + 1 lone pair), giving trigonal pyramidal geometry with a bond angle of approximately 107 degrees.
Q: Predict the geometry and bond angle at each carbon in HC triple bond C-COOH.
A: The alkyne carbon (C triple bond C) has 2 electron groups, linear geometry, 180 degrees. The carbonyl carbon in -COOH has 3 electron groups, trigonal planar geometry, approximately 120 degrees.
Q: Why is the bond angle around an oxygen in water (104.5 degrees) less than the tetrahedral angle (109.5 degrees)?
A: Oxygen has 4 electron groups (2 bonding + 2 lone pairs). Lone pairs occupy more space than bonding pairs and compress the bond angle below the ideal tetrahedral value.
Q: How do you distinguish a ketone from an aldehyde?
A: Both contain a carbonyl group (C=O). In a ketone, the carbonyl carbon is bonded to two other carbon groups. In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen.
Functional groups connect to every reaction type you will study: alcohols undergo substitution and elimination, carbonyls undergo nucleophilic addition and substitution, and amines act as nucleophiles and bases. Molecular geometry connects to hybridisation (sp, sp2, sp3), which you will cover next, and to stereochemistry, where 3D arrangement determines whether a molecule is chiral.
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