Difficulty: Introductory to Intermediate | Prerequisites: Lewis structures, electron pairs, bond polarity (see Part 1 notes)
Resonance structures are different ways of drawing the same molecule's electron arrangement; the real molecule is a blend (hybrid) of all valid contributors. Functional groups are specific atom clusters (like alcohols, amines, ketones) that give a molecule its chemical behaviour, and recognising them on sight is a core organic chemistry skill. Hybridisation (sp³, sp², sp) tells you the geometry and bond angles around each atom, which in turn controls molecular shape and reactivity.
Resonance structure (resonance contributor)
One of two or more valid Lewis structures for the same molecule that differ only in the placement of electrons (not atoms). The real molecule does not flip between them; it is a weighted average of all contributors.
Think of it as: different sketches of the same building from different angles, none of which is the building itself.
Resonance hybrid
The actual electronic structure of a molecule, represented as a blend of all its resonance contributors. Bond lengths and charge distributions in the hybrid are intermediate between those shown in any single contributor.
Major contributor
The resonance structure that best represents the actual molecule. It is the one with the most atoms carrying complete octets, the fewest formal charges, and any unavoidable negative charge placed on the most electronegative atom.
Arrow-pushing (curved-arrow formalism)
A notation system where curved arrows show the movement of electron pairs. The tail of the arrow starts where the electrons are; the head points to where they move. A double-barbed arrow moves a pair; a single-barbed (fishhook) arrow moves one electron.
In simple terms: arrows trace where electrons go, not where atoms go.
Functional group
A specific grouping of atoms within a molecule that determines the molecule's chemical reactivity and properties. Organic chemistry is largely organised by functional group.
Alcohol (–OH)
A hydroxyl group bonded to a carbon. The oxygen carries two lone pairs. Alcohols can hydrogen-bond, making them more water-soluble than similar-sized hydrocarbons.
Amine (–NH₂, –NHR, –NR₂)
A nitrogen atom bonded to one, two, or three carbon/hydrogen atoms, carrying a lone pair. Amines are basic because that lone pair can accept a proton.
Amide (–CONR₂)
A carbonyl group (C=O) bonded directly to a nitrogen. The nitrogen's lone pair is delocalised into the carbonyl, making amides far less basic than amines. Found in proteins (the peptide bond is an amide).
Ketone (R–CO–R)
A carbonyl group flanked by two carbon atoms (not hydrogen, which would make it an aldehyde). The C=O bond is polar, with δ+ on carbon.
Hybridisation
The mixing of atomic orbitals (s, p, and sometimes d) to form new hybrid orbitals of equal energy. The type of hybridisation determines geometry and bond angles.
sp³ hybridisation
Four hybrid orbitals arranged in a tetrahedron. Bond angle approximately 109.5°. Found on any carbon with four single bonds (and no lone pairs involved in resonance).
sp² hybridisation
Three hybrid orbitals in a trigonal planar arrangement, plus one unhybridised p orbital. Bond angle approximately 120°. Found on carbons in double bonds and aromatic rings.
sp hybridisation
Two hybrid orbitals in a linear arrangement, plus two unhybridised p orbitals. Bond angle 180°. Found on carbons in triple bonds and allenes.
Resonance structures differ only in electron placement; the atoms stay in exactly the same positions. You generate a new resonance contributor by moving lone pairs into bonds or bond pairs into lone pairs, using curved arrows to show the electron flow.
Curved arrows always show electron-pair movement. The tail starts at the electron source (a lone pair or a bond). The head points to the destination (forming a new bond, or becoming a new lone pair).
Atoms do not move. If you have to move an atom to draw the structure, it is not a resonance structure.
All valid resonance structures must have the same number of electrons, the same net charge, and complete octets (where possible) on all second-row atoms.
When a molecule has two or more resonance structures, the major contributor is the one that best represents the real molecule. Use these rules in order:
More complete octets wins. A structure where every atom (especially C, N, O) has a full octet is preferred over one with an incomplete octet.
Fewer formal charges wins. A structure with no formal charges is preferred over one with separated charges.
Negative charge on the more electronegative atom wins. If formal charges are unavoidable, place the negative charge on the atom best able to stabilise it (higher electronegativity).
Charge separation costs stability. A contributor with a +1 and a –1 is less stable than one with all zeros, even if octets are satisfied in both.
Example (a): A molecule with two C=O groups separated by a CH₂ unit. One resonance contributor moves a lone pair from one oxygen into the adjacent C–C bond, pushing the other C=O’s bonding electrons onto that oxygen as a lone pair. The major contributor is the structure with the most atoms at full octet and the fewest formal charges.
Example (b): A molecule with an amine (NH₂) group adjacent to a carbonyl. The nitrogen’s lone pair can delocalise into the C=O system. The major contributor keeps the C=O double bond intact (oxygen is more electronegative, so it holds onto double-bond character more readily).
Example (c): Diazomethane-type structure (CH₂N=N). Curved arrows push electrons along the N=N system. The major contributor has the negative formal charge on the more electronegative terminal atom.
Tetracycline is a large molecule with multiple functional groups. The worksheet asks you to identify the circled ones:
Alcohol (–OH): the hydroxyl group bonded to the ring system. Oxygen with two lone pairs, single bond to carbon, single bond to hydrogen.
Amine (–N(CH₃)₂): a tertiary amine, nitrogen bonded to two methyl groups and one ring carbon, carrying a lone pair. This is the basic nitrogen in tetracycline.
Amide (–CONH₂): a carbonyl directly bonded to nitrogen. The nitrogen’s lone pair is partially delocalised into the C=O, reducing its basicity relative to the amine.
Ketone (C=O): a carbonyl flanked by two carbons within the ring system. No nitrogen or oxygen bonded to the carbonyl carbon on the other side.
The key skill here is pattern recognition: scan for the characteristic atom grouping, ignore the rest of the molecule, and name what you see.
Hybridisation is determined by counting the number of electron groups (bonds + lone pairs) around an atom. Each bond (single, double, or triple) counts as one electron group.
4 electron groups = sp³. Tetrahedral geometry. Bond angle ~109.5°.
Example from the worksheet: angle a = 109° (an sp³ carbon with four single bonds).
3 electron groups = sp². Trigonal planar geometry. Bond angle ~120°.
Example from the worksheet: angle b = 120° (a carbon in a double bond or aromatic ring). Angle e = 120° (the carboxylic acid carbon, which is sp²).
2 electron groups = sp. Linear geometry. Bond angle = 180°.
Example from the worksheet: angle d = 180° (a carbon in a triple bond, as in the alkyne portion of molecule 2).
To identify hybridised carbons in a structure:
Circle sp² carbons: any carbon involved in a C=C double bond, a C=O double bond, or an aromatic ring.
Box sp carbons: any carbon involved in a C≡C triple bond.
Electron Groups | Hybridisation | Geometry | Bond Angle | Example |
|---|---|---|---|---|
4 | sp³ | Tetrahedral | ~109.5° | CH₄, any saturated carbon |
3 | sp² | Trigonal planar | ~120° | C=C in alkenes, C=O in carbonyls, aromatic carbons |
2 | sp | Linear | 180° | C≡C in alkynes, C≡N in nitriles |
All atoms have complete octets (highest priority).
Fewest formal charges.
Negative formal charge on the most electronegative atom.
Least charge separation.
Functional groups are the reason organic chemistry can be systematic rather than molecule-by-molecule. Knowing that an amide behaves differently from an amine lets a medicinal chemist predict how a drug candidate will interact with enzymes. Tetracycline’s multiple functional groups each play a role: the amine provides water solubility at low pH, the amide and ketone groups coordinate metal ions, and the alcohol groups participate in hydrogen bonding with the ribosomal target.
Hybridisation controls molecular rigidity. An sp² carbon locks its three substituents into a flat plane (no rotation around a double bond), which is why cis/trans isomerism exists in alkenes. Drug designers exploit this rigidity to lock molecules into shapes that fit an enzyme’s active site.
Resonance arrows are not reaction arrows. Curved arrows in resonance show how you could redraw the electrons in a different valid Lewis structure. They do not mean electrons are physically moving or that a reaction is occurring. Reaction mechanism arrows look similar but describe an actual chemical transformation.
Resonance structures are not real, separate molecules. The molecule does not flicker between contributors. The actual molecule is the hybrid, a single structure with delocalised electrons. This is the single most common misconception in introductory organic chemistry.
Confusing sp² with sp³. Students often forget that a carbon in an aromatic ring is sp² (three electron groups, 120°), or that a carbonyl carbon is sp². If there is a double bond to the carbon, it is sp², not sp³.
Thinking amides are basic like amines. The nitrogen lone pair in an amide is delocalised into the carbonyl. Amides are not appreciably basic under normal conditions. Students often see N–H and assume basicity.
⚠️ Drawing resonance structures with correct curved arrows is heavily tested. You will almost certainly be asked to provide a resonance contributor and identify the major one.
⚠️ Functional group identification appears in multiple forms: circling groups on a complex molecule (as with tetracycline), naming them, or predicting reactivity based on which groups are present.
⚠️ Hybridisation and bond angles are often tested together. A common exam format: given a molecule, label each marked angle and identify which carbons are sp, sp², or sp³.
⚠️ Expect a question combining resonance with functional groups, such as explaining why the nitrogen in an amide is less basic than the nitrogen in an amine (the answer involves resonance delocalisation of the lone pair).
True or False: Resonance structures differ in the arrangement of atoms.
Answer: False. Only electrons move; atoms stay in the same positions.
Fill in the blank: An sp² carbon has a bond angle of approximately ______ degrees.
Answer: 120.
True or False: The nitrogen in an amide is a strong base because it has a lone pair.
Answer: False. The lone pair is delocalised into the adjacent carbonyl, making amides essentially non-basic.
Fill in the blank: A carbon with a triple bond is ______ hybridised with a bond angle of ______ degrees.
Answer: sp; 180.
True or False: The major resonance contributor is the one with the most formal charges.
Answer: False. The major contributor has the fewest formal charges.
Q: Given a molecule with a C=O adjacent to a nitrogen with a lone pair, draw one additional resonance structure using curved arrows. Which contributor is major?
A: The curved arrow starts at nitrogen’s lone pair and pushes into the C–N bond, forming a C=N double bond. Simultaneously, the C=O bonding electrons move onto oxygen as a lone pair, giving oxygen a formal charge of –1 and nitrogen a formal charge of +1. The major contributor is the original structure (the one with the C=O intact), because oxygen is more electronegative and better able to hold the double bond than nitrogen.
Q: Identify the hybridisation and approximate bond angle for a carbon in a benzene ring.
A: sp² hybridisation, approximately 120°. Each carbon in the aromatic ring has three electron groups (two C–C bonds and one C–H bond, with the delocalised pi system above and below the ring).
Q: Name the functional group present in –CONH₂ and explain why it is less basic than –NH₂.
A: It is an amide. The nitrogen lone pair is delocalised by resonance into the adjacent C=O, so it is far less available to accept a proton compared to a free amine, where the lone pair is fully localised on nitrogen.
Q: A molecule contains a carbon bonded to two other carbons by a triple bond on one side and a single bond on the other. What is the bond angle at that carbon, and what is its hybridisation?
A: 180°, sp hybridisation. The triple bond counts as one electron group and the single bond as another, giving two electron groups and a linear arrangement.
Q: In tetracycline, one circled group is a ketone. What distinguishes a ketone from an aldehyde?
A: Both have a C=O (carbonyl) group. In a ketone, the carbonyl carbon is bonded to two other carbon atoms. In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen.
Resonance connects directly to acidity and basicity: a conjugate base stabilised by resonance (charge delocalised over multiple atoms) makes the parent acid stronger. This is why carboxylic acids are more acidic than alcohols.
Functional groups are the organising principle for the rest of the course. Every reaction mechanism you learn from here on will be classified by functional group: alcohol reactions, carbonyl chemistry, amine chemistry, and so on.
Hybridisation connects to molecular orbital theory: the unhybridised p orbitals on sp² and sp carbons are the ones that form pi bonds and participate in conjugation, aromaticity, and the colour of organic dyes.
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