Organic Chemistry: Introduction
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Organic Chemistry: Introduction
Summary — Everything In This Chapter
- Organic reactions fall into 7 named categories: addition, elimination, substitution, oxidation, reduction, hydrolysis, and polymerisation.
- Addition = two things become one thing (no atoms lost). Elimination = one (or two) things become MORE things (a small molecule breaks off).
- Substitution = one atom/group is swapped out for another — nothing is gained or lost overall.
- [O] and [H] are shorthand for "an oxidising agent" and "a reducing agent" in equations — you don't need to know the exact reagent to write the equation.
- Hydrolysis is a specific case: it's literally a reaction *with water* that breaks a bond.
- Addition polymerisation: alkene monomers open up their C=C double bond and link into a long repeating chain.
- Reaction mechanisms use curly arrows to show electron movement — full-headed arrows for electron PAIRS, fish-hook (half) arrows for SINGLE electrons.
- Bond polarity predicts mechanism type: non-polar bonds → homolytic fission → free radicals. Polar bonds → heterolytic fission → electrophiles/nucleophiles.
Topic 1: Types of Reaction
Imagine every organic reaction as something happening to a LEGO structure. Addition = you snap two LEGO pieces together into one bigger structure. Elimination = you snap a small piece OFF a bigger structure (and it goes flying away). Substitution = you pop one LEGO piece out and click a different one in its place. That's genuinely most of what you need to tell these apart on sight.
1 Addition Reactions
An addition reaction is when two reactants combine to form ONE single product — nothing is left over, nothing is released as a by-product. Count the atoms on the left, count them on the right: they should all be accounted for in that one product.
This is the signature move of alkenes, because the C=C double bond is basically an "open door" — one of the two bonds in it can break and grab something new without disturbing the rest of the molecule.
2 Elimination Reactions
Elimination is basically the reverse idea: you usually start with one or two reactants and end up with more products than you started with. Typically, a larger organic molecule breaks apart, releasing a small molecule (often water, or a hydrogen halide) and leaving behind a new C=C double bond.
A really common trap: students see "OH⁻" as a reactant and instantly think "substitution!" But look closely at the conditions — here it's used with H⁺ as a catalyst, which is your clue that this is actually an elimination happening (the OH⁻ is acting as a base, removing an H, not swapping in).
3 Substitution Reactions
Substitution is the simplest to picture: one atom or group of atoms is replaced by a different atom or group. The "skeleton" of the molecule barely changes — you're just swapping out one attachment for another, like changing a phone case.
Here, the Br atom is swapped out and an OH group takes its place. Notice: exactly 2 reactants → exactly 2 products, and the "backbone" of the molecule is untouched.
4 Oxidation Reactions
In organic chemistry, oxidation usually means losing hydrogen or gaining oxygen. Rather than making you memorise exact reagents, exam equations often use [O] as shorthand for "an oxidising agent" — you just need to balance the equation using it like a normal atom of oxygen.
The [O] in that equation could be acidified potassium dichromate(VI) solution, Fehling's solution, or Tollens' reagent — all common oxidising agents you'll meet later when you study aldehydes and alcohols in detail.
5 Reduction Reactions
Reduction is the mirror image of oxidation: gaining hydrogen or losing oxygen. It's shown using [H] as shorthand for a reducing agent.
[H] here could represent lithium aluminium hydride (LiAlH₄) — a strong reducing agent used to turn carbonyl compounds (like aldehydes) into alcohols.
6 Hydrolysis Reactions
This one trips people up because of the name. "Hydro-lysis" sounds like it should mean "splitting with water" in a general sense — and actually, that's exactly right! Hydrolysis is simply a reaction with water that splits (lyses) a compound into two pieces.
Students often call ANY reaction that produces water "hydrolysis" — wrong direction! Hydrolysis means water is a reactant that breaks a bond, not a product being formed. If water appears on the right-hand side of the equation, that reaction is something else entirely (often condensation, which you'll meet later).
7 Polymerisation Reactions (Addition Polymerisation)
At this level, you need addition polymerisation: the C=C double bond in an alkene monomer opens up, and thousands of these monomers link together end-to-end into one giant repeating chain called a polymer.
The polymer is named by putting "poly" in front of the monomer's name, usually in brackets: 1,2-dichloroethene monomer → poly(1,2-dichloroethene) polymer.
Topic 2: Reaction Mechanisms — Introduction
Reaction equations tell you the "before and after" — what goes in, what comes out, and in what ratio (stoichiometry). But they don't tell you the story of HOW the reaction actually happens at the electron level. That's what a reaction mechanism shows you, using curly arrows to track exactly where electrons move.
1 Two Types of Curly Arrow
The unpaired electron on the chlorine radical and ONE electron from the C–H bond pair up to form the new Cl–H bond. The other electron from that original C–H bond is left behind as the new unpaired electron on the methyl radical (•CH₃). Notice how TWO separate fish-hook arrows are needed here — one for each single electron that moves.
Drawing single-headed arrows isn't actually required knowledge at this stage — free radical reactions are usually shown with written equations instead of full drawn mechanisms. But you DO need to understand what homolytic fission, radicals, and propagation mean conceptually.
The lone pair of electrons on the bromide ion swoops in and attacks the positively-charged carbon (the carbocation), forming a brand new C–Br covalent bond. One full-headed curly arrow, starting at the lone pair and ending at the positive centre, captures this whole step.
2 The Key Vocabulary
These words come up constantly in mechanism questions, so let's nail down exactly what each one means:
| Term | What it actually means |
|---|---|
| Homolytic fission | A covalent bond breaks EVENLY — each atom keeps one of the two shared electrons. This produces two radicals. Happens with non-polar bonds. |
| Heterolytic fission | A covalent bond breaks UNEVENLY — one atom takes BOTH electrons from the bond. This produces one positive ion and one negative ion. Happens with polar bonds. |
| Radical | A species with an unpaired (lone) electron, shown with a dot (•). Highly reactive because it "wants" to pair that electron up. |
| Electrophile | An electron-deficient species (positive or δ+) that is attracted to areas of high electron density. Literally means "electron-loving". |
| Nucleophile | An electron-rich species (negative, or has a lone pair) that is attracted to areas of positive charge. Literally means "nucleus-loving". |
3 Curly Arrows Belong to Three Reaction Types
Curly-arrow mechanisms are drawn for three main reaction categories (you met these back in Topic 1 — now you can see WHY each one happens at the electron level):
- Addition — e.g. ethene + bromine forming 1,2-dibromoethane. The π-bond electrons of the C=C attack the electrophilic bromine molecule.
- Substitution — e.g. bromoethane + hydroxide ion forming ethanol + bromide ion. A nucleophile's lone pair attacks the carbon bonded to the leaving group.
- Elimination — e.g. ethanol + acid catalyst forming ethene + water. A bond breaks and a small molecule leaves, with a new π-bond forming in its place.
4 Using Bond Polarity to PREDICT the Mechanism
Here's a genuinely powerful exam skill: you can look at an unfamiliar reaction, check whether the bonds involved are polar or non-polar, and predict what type of mechanism is most likely — without ever having seen that exact reaction before.
Worked example — Ethane + Chlorine:
A hydrogen is substituted by a chlorine atom. All the bonds involved (C–H, C–C, Cl–Cl) are non-polar or only very slightly polar. This points to homolytic fission, which points to free radicals, which means the mechanism is free radical substitution.
Worked example — Ethene + Hydrogen Bromide:
Hydrogen bromide is added across the double bond. The H–Br bond is strongly polar (Br is far more electronegative than H), pointing to heterolytic fission. The ethene's C=C double bond is electron-rich and will attract an electrophile. So the mechanism is electrophilic addition.
What to Memorise
| Term | Quick Definition |
|---|---|
| Addition | 2 reactants → 1 product. No atoms left over. |
| Elimination | 1–2 reactants → more products; small molecule breaks away, new C=C often forms. |
| Substitution | One atom/group replaced by another; skeleton stays the same. |
| Oxidation | Loss of hydrogen OR gain of oxygen. Shown using [O]. |
| Reduction | Gain of hydrogen OR loss of oxygen. Shown using [H]. |
| Hydrolysis | A reaction WITH water that splits a molecule apart. |
| Addition polymerisation | Alkene monomers' C=C bonds open up and link into a long repeating chain. |
| Homolytic fission | Bond breaks evenly → 2 radicals. Non-polar bonds. Fish-hook arrows. |
| Heterolytic fission | Bond breaks unevenly → + ion and − ion. Polar bonds. Full arrows. |
| Electrophile | Electron-deficient (+ or δ+); attacks electron-rich areas. |
| Nucleophile | Electron-rich (has a lone pair); attacks electron-poor/positive areas. |
| [O] / [H] | Shorthand for "an oxidising agent" / "a reducing agent" in balanced equations. |
Concepts Checklist
Exam Tips & Common Mistakes
Same reactants, different mechanism
Bromoethane + OH⁻ can be EITHER substitution OR elimination depending on conditions (solvent, temperature, whether H⁺ acts as catalyst). Always check the conditions given, not just the reactants!
Hydrolysis direction matters
Water must be a REACTANT for it to count as hydrolysis. If water appears as a product instead, you're looking at a different reaction type — don't automatically label it hydrolysis.
Arrow starting point errors
Curly arrows must start from a bond or a lone pair — NEVER from a single atom with no available electrons. A very common lost mark is starting the arrow from the atom itself rather than from its lone pair or bond.
Single vs. double-headed arrows
Mixing these up is a classic error. ONE electron moving = fish-hook (half arrowhead). A PAIR of electrons moving = full arrowhead. Radicals only ever use fish-hooks; ions/lone pairs use full arrows.
Bond polarity ≠ optional detail
When asked to "predict" or "suggest" a mechanism, examiners want you to explicitly reference bond polarity in your answer — not just name the mechanism. State the bond, say whether it's polar/non-polar, then link that to fission type.
Polymer repeating unit brackets
Don't forget the subscript "n" outside the square brackets when drawing a repeating unit — this shows the pattern repeats an indefinite number of times to form the full polymer chain.
If a question says "suggest" or "predict" a mechanism for an unfamiliar reaction, it is testing whether you can apply the bond-polarity logic chain — not whether you've memorised that specific reaction. Practise the reasoning, not just the answers.
- Exam Tips & Common Mistakes
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