Foundations — α,β-Unsaturated carbonyls — Michael addition, 1,2 vs 1,4 addition

Before you can read the parent note, you need to see every symbol it throws at you. Below, each item is built from nothing: plain words → a picture → why the topic needs it. Read top to bottom; each rung uses only the rungs above it.

1. A bond, a line, a shared pair of electrons

Picture two people holding one rope between them — the rope is the shared pair, and holding it keeps them together. A double bond $C=C$ is two ropes (two shared pairs); it is stiffer and holds the atoms closer.

Why the topic needs it: the entire story is about where electron pairs move. If a line = a shared pair, then drawing a new line means "a new bond just formed" and erasing a line means "a bond just broke." Everything else is bookkeeping on these ropes.

2. δ⁺ and δ⁻ — the "slightly plus / slightly minus" symbols

Why does an atom lean? Because in a bond, one atom can hog the shared electron pair more than the other. Oxygen is a famous hog. In $C=O$, oxygen pulls the rope toward itself, so:

• oxygen ends up electron-rich → ${\delta }^{-}$
• carbon ends up electron-poor → ${\delta }^{+}$

Why the topic needs it: a ${\delta }^{+}$ carbon is a target. Anything electron-rich will want to attack there. The parent note's whole claim — "there are two ${\delta }^{+}$ carbons" — is a claim about two targets. You cannot understand that sentence without $\delta$.

3. Nucleophile and Electrophile — attacker and target

Why the topic needs it: the carbonyl carbon and the β-carbon are both electrophiles (${\delta }^{+}$). The nucleophile is whatever we throw at the molecule — a Grignard, a cuprate, an enolate. "1,2 vs 1,4" is literally the question: which electrophilic carbon does the nucleophile pick?

4. The curved arrow — how we draw electrons moving

Picture a rope being thrown from where it starts to where it lands. Tail = old home, head = new home.

Why the topic needs it: every mechanism in the parent — every "Nu attacks," every "electrons go onto O," every "tautomerise" — is a sequence of curved arrows. If you can't read the arrow, the resonance and the mechanism are just cryptic squiggles.

5. Conjugation — why the "hunger" travels down the chain

This is the keystone. Everything above was to build this.

Why the topic needs it: conjugation is the entire reason the β-carbon is electrophilic. Without conjugation you'd have one target; with it you have two. This is the machinery behind "resonance pushes the positive charge out to the β-carbon."

6. Resonance — snapshots of the same sliding electrons

Why the topic needs it: the parent's key line $\stackrel{\beta }{C}=\stackrel{\alpha }{C}-C=O⟷{}^{+}\stackrel{\beta }{C}-\stackrel{\alpha }{C}=C-O^{-}$ is exactly two snapshots. The second snapshot shows the β-carbon wearing a full $+$ and oxygen wearing a full $-$. Blend them and you get the real molecule: ${\delta }^{+}$ leaking onto β. Resonance is the proof that the β-carbon is a target.

7. Naming the carbons: C1, α, β

Why the topic needs it: the whole topic is a fight over addresses. "1,2-addition = attack C1." "1,4-addition = attack β." "α gets the H." If you don't have a fixed naming scheme, you can't say where anything landed.

8. The enolate and tautomerisation — the "after" of a 1,4 hit

Picture a seesaw: on one side sits "$C=C$ plus $O-H$" (enol); tip it and you get "$C-C$ plus $C=O$" (keto). Same atoms, rearranged.

Why the topic needs it: when the nucleophile bites the β-carbon (1,4), the first thing formed is an enolate, not the final product. It then tautomerises to the stable carbonyl. This is exactly why the parent says "1,4 keeps $C=O$, loses $C=C$" — the seesaw tips to the keto side at the end. See Enols and enolates for the full machinery.

9. Kinetic vs thermodynamic — "fastest" vs "most stable"

Why the topic needs it: the parent's whole decision table runs on this. 1,2 = kinetic = fast. 1,4 = thermodynamic = stable. Temperature and reversibility just decide which valley the reaction settles into. Full detail in Kinetic vs thermodynamic control.

10. Hard vs soft — the personality of a nucleophile

Why the topic needs it: the carbonyl carbon has a tight, localised ${\delta }^{+}$ → it's a hard target. The β-carbon's ${\delta }^{+}$ is spread out → a softer target. So hard nucleophiles bite C1 (1,2); soft nucleophiles bite β (1,4). This single rule predicts most of the selectivity. Details: HSAB principle — hard and soft acids/bases.

Prerequisite map

Equipment checklist

Cover the right side; can you answer each before revealing?

Return to the parent: main topic note.