Intuition The one core ideaEvery reaction in this topic is a flat carbon–carbon double bond (C=C) getting broken open so that two new groups clip onto the two carbons. The entire difficulty is answering only two questions: which carbon gets which group (regiochemistry) and from which side of the flat bond the groups arrive (stereochemistry).
Before you can read the parent note, you must own every mark on the page. We build each one from nothing — plain words first, then a picture, then why the topic even needs it . Nothing here is assumed; if the parent leaned on it, we construct it below.
Definition A single bond and a double bondA straight single line between two letters (like C–C one pair of electrons — this is one glue-joint." A double line (C=C two pairs — two glue-joints at once.
Definition σ (sigma) and π (pi) — the two kinds of glueσ sigma ) is the name chemists give the first, strong bond that lies directly on the line joining the two atoms — think of it as the wooden plank bolted flat between them. π pi ) is the name for the second bond of a double bond : its electrons do not sit on the line — they form a cloud floating above AND below the plank. So C=C σ π
Look at the figure. The single bond is a hinge: the two carbons can spin freely around it (only a σ locked plank — the second pair of electrons, the π jams the rotation . That is why a double bond has a definite "top face" and "bottom face."
Intuition Why the topic needs thisEvery reaction here consumes that red π π reactive part ; the black σ π π
Definition The atom letters
C carbon atom. Carbon always wants 4 bonds total.H hydrogen atom. It makes exactly 1 bond.R some carbon chain , don't worry which." It is a stand-in, like the x
When you see R–CH=CH 2 R H H H 2
Common mistake A subscript is a count, not a multiplier of the whole formulaCH 2 two hydrogens on it" — not "two CH units." The small number below only touches the atom directly to its left.
Definition More- vs less-substituted carbonTake one of the two carbons in the C=C. Count how many other carbons are attached to it. The one attached to more carbons is more substituted ; the one attached to more hydrogens (fewer carbons) is less substituted .
In the figure the red carbon has two carbon neighbours → more substituted (crowded). Its partner has two hydrogens → less substituted (open, roomy).
Intuition Why the topic needs thisHalf of the reactions say "the group goes to the more substituted carbon" and half say "the less substituted one." If you can instantly point at the crowded carbon, every regiochemistry rule becomes a one-second decision.
Definition The little deltaδ delta ) written as a superscript means "partial , a fraction of a full charge." δ + δ − full + −
Think of a full + δ + fuller glass of δ + there .
Definition Electron-poor vs electron-rich
Electrophile = "electron-loving." An atom that is short of electrons and therefore hunts for them. Written with δ + Nucleophile = "nucleus-loving." An atom rich in electrons that donates a pair. It goes toward the plus.
Nucleophile = the giver (has spare electrons). Electrophile = the taker (wants them). Arrows in mechanisms always fly from rich to poor — from the nucleophile's electron pair toward the electrophile.
Intuition Why the topic needs thisThe red π electron-rich — it is the built-in nucleophile of every alkene. Whatever attacks it (boron, ozone, osmium, bromine) is the electrophile . Once you label the two players, the first arrow of every mechanism draws itself.
Definition Two different arrows, never confuse them
A straight reaction arrow ⟶
A curved arrow (↷ where a pair of electrons moves . Its tail starts on the electrons that move (a lone pair or a bond like the red π head points to where those electrons end up (a new bond, or an atom).
Intuition Why the topic needs thisEvery mechanism in the parent note is a chain of curved arrows. The very first one always starts on the red π (electron-rich) and points at the incoming electrophile (electron-poor) — exactly the "rich → poor" rule from section 5. If you read a curved arrow as a reaction arrow, the whole mechanism becomes gibberish, so keep the two straight: straight = molecule becomes molecule; curved = electron pair moves.
Definition Planar double bondThe two carbons of a C=C and everything directly attached to them all lie in one flat plane — like four dots painted on a tabletop. That flatness gives the bond a top face and a bottom face .
syn addition = both new groups arrive on the same face (both from the top, say).anti addition = the two new groups arrive on opposite faces (one from top, one from bottom).
Intuition Why the topic needs thisThis single flat plane is the stage for the second big question of the whole topic. Hydroboration and cold-KMnO₄ are syn (same-face) because a ring-shaped intermediate grips one face and delivers both groups there. Epoxide-opening and halohydrin are anti because the second group is forced to attack from the back side . See Stereochemistry — syn vs anti addition, cis/trans, R/S for the full vocabulary of same-side/opposite-side.
Definition cis and trans (on a ring or across a bond)
cis = two groups on the same side of a ring or double bond.trans = two groups on opposite sides .
Definition E and Z — the upgrade for non-identical groups"cis/trans" only works cleanly when each double-bond carbon carries one obvious "same" group. When the four groups are all different, chemists rank the two groups on each carbon by priority (heavier atom = higher priority — the same idea used in R/S naming).
Z (from German zusammen = "together") = the two higher-priority groups are on the same side .E (from German entgegen = "opposite") = the two higher-priority groups are on opposite sides .
So Z is the careful version of "cis," and E is the careful version of "trans."
Vicinal = "on neighbouring carbons." A vicinal diol is two –OH groups on two carbons that sit right next to each other. ("Vicinal" comes from the same root as "vicinity.")
Common mistake syn/anti and cis/trans (or E/Z) are not the same wordsyn/anti describes how the groups arrived (the mechanism's faces). cis/trans and E/Z describe where they ended up on the final molecule. On a flat ring a syn addition often gives a cis product — but the words answer different questions, so keep them separate.
Definition Arrow with reagentsstart reagent product straight arrow means "turns into" (contrast the curved arrow of section 6, which moves electrons — never mix them up). Whatever is written above the arrow is what you add to make it happen. When steps are numbered ( 1 ) … ( 2 ) … one after another, in order — you finish step 1 completely, then add step 2's reagents.
Intuition Why the topic needs thisHydroboration is written as two separate numbered steps for a reason: first boron adds, then a totally different reagent (H 2 O 2 / OH −
Definition The cast of reagents
BH 3 borane , the electron-poor boron reagent (only 6 electrons around B → hungry electrophile). See Electrophilic Addition to Alkenes .H 2 O 2 / OH − hydrogen peroxide in base , the oxidiser that replaces a C–B bond with C–OH.O 3 ozone , three oxygens, the C=C-cleaver.Zn/H 2 O ( CH 3 ) 2 S gentle (reductive) work-up .KMnO 4 potassium permanganate , a purple oxidiser (cold-dilute = diol; hot-conc = cleaves). See Oxidation of Alkenes — KMnO4 and OsO4 .OsO 4 osmium tetroxide , forms a ring on one face → syn diol.mCPBA / peroxyacid — makes an epoxide (a 3-membered oxygen ring). See Epoxides — Formation and Ring Opening .X 2 halogen molecule (Br 2 Cl 2 X Halogenation of Alkenes (halonium ion) .
Definition The carbon–oxygen family
Carbonyl = a carbon double-bonded to oxygen: C=O Aldehyde = carbonyl carbon that still holds at least one H (–CHO Ketone = carbonyl carbon holding two carbons, no H (R–CO–R Carboxylic acid = carbonyl carbon carrying an –OH too (–COOH
Intuition Why the topic needs thisOzonolysis and hot KMnO₄ turn each half of the broken C=C into one of these. The single deciding factor is the number of H's on the original double-bond carbon — 0 H can only become a ketone (it has no H to lose, so it can't climb higher), while a carbon with an H can be pushed all the way up to an acid.
A carbocation is a carbon atom carrying a full positive charge (+ δ + one bond short — it has only 3 bonds and 6 electrons instead of the 4 bonds it wants. It is a hungry, unstable intermediate that a nucleophile rushes to fill.
Intuition Why its stability sets the regiochemistryA carbocation is more stable when it sits on a more-substituted carbon , because the surrounding carbon groups gently push electron density toward the positive centre and soothe it. So when a reaction goes through a carbocation, the positive charge prefers the more substituted carbon, and the nucleophile therefore ends up there . Full detail: Markovnikov's Rule and Carbocation Stability .
Definition The two regiochemistry labels
Markovnikov : the –OH (or the "rich" group) lands on the more substituted carbon.anti-Markovnikov : the –OH lands on the less substituted carbon.
Intuition The picture (and why)In acid-catalysed hydration a real carbocation forms; because it is more stable on the more-substituted carbon, that is where the positive charge (and then the –OH) lands → Markovnikov (see Acid-Catalyzed Hydration of Alkenes ). Hydroboration flips this because bulky boron , not H⁺, is the electrophile and there is no carbocation at all — boron simply dodges the crowded carbon → anti-Markovnikov.
Single vs double bond sigma and pi
The red pi cloud is electron-rich
Electrophile hunts, nucleophile gives
Partial charge delta plus and delta minus
Curved arrow moves electron pairs
More vs less substituted carbon
Regiochemistry Markovnikov
C=C is flat with two faces
Stereochemistry syn vs anti
cis trans E Z and vicinal diol
Carbonyl aldehyde ketone acid
Ozonolysis products by H count
The four reactions of 4.2.06
Every arrow that finally points at Z is a foundation the parent note silently assumes. Master the left-hand boxes and the parent topic reads like plain English.
Cover the right side and recite before moving on.
What are σ and π, and which one is the red reactive cloud? σ is the first bond lying on the line between the atoms; π is the second bond of a double bond, a cloud above and below — the red π is the reactive part.
Electrophile vs nucleophile — who has spare electrons? The nucleophile is electron-rich (the giver); the electrophile is electron-poor (the taker); arrows fly from nucleophile to electrophile.
What does δ + + δ + +
What does a curved arrow show, and how is it different from a reaction arrow? A curved arrow shows a pair of electrons moving (tail = source electrons, head = destination); a straight reaction arrow means one molecule turns into another.
How do you spot the more-substituted carbon of a C=C? Count carbon neighbours on each double-bond carbon; the one with more carbons (fewer H's) is more substituted.
What is a carbocation and why does it control Markovnikov regiochemistry? A carbon with a full positive charge and only 3 bonds; it is more stable on a more-substituted carbon, so the positive charge (and then the nucleophile) ends up there.
syn vs anti addition — in one line each? syn = both new groups arrive on the same face of the flat C=C; anti = they arrive on opposite faces.
Difference between syn/anti and cis/trans (or E/Z)? syn/anti describes how groups arrived (mechanism faces); cis/trans and E/Z describe where they ended up on the product.
When must you use E/Z instead of cis/trans? When the double-bond carbons carry non-identical groups; rank each carbon's groups by priority — Z = higher-priority groups same side, E = opposite sides.
What does "vicinal diol" mean? Two –OH groups on neighbouring (adjacent) carbons.
For ozonolysis, what single count decides aldehyde vs ketone vs acid? The number of H's on the original double-bond carbon (0 H → ketone always; 1+ H → aldehyde or acid depending on work-up).
In "( 1 ) … ( 2 ) … Do step 1 completely first, then add step 2's reagents — not all at once.
