Intuition The one core ideaAn alkene is two carbon atoms glued together twice — a normal glue (σ bond) plus a springy extra glue (π bond) whose electrons stick out and are easy to reach. To make an alkene you strip two neighbouring atoms off so that second glue can snap into place; to react an alkene, something grabs those exposed springy electrons and the second glue pops open again.
Before you can follow one line of the parent note on Alkenes , you need every squiggle and word it uses to already feel obvious. This page builds them one at a time, each resting on the one before.
A bond is two electrons shared between two atoms . Picture a rope with a hand from each atom gripping it — the two hands are the two electrons, and the grip holds the atoms at a fixed distance.
We draw a single bond as a line : C − C is the shared pair.
Why the topic needs it: every reaction below is nothing but pairs of electrons moving from one place to another. If you don't see a bond as a movable pair , the arrows later will look like magic.
Carbon can grip a neighbour with two pairs at once. We draw it as two lines : C = C
σ π
The σ sigma ) is the first pair, sitting straight along the line joining the two carbons — a strong, head-on grip.
The π pie ) is the second pair, sitting above and below that line, like two clouds hovering over a see-saw.
The symbol π
π σ π outside , exposed and loosely held. That is why alkenes are reactive: the π
Why the topic needs it: "π density," "electron-rich," "the alkene attacks" — every one of these phrases points at this hovering pair.
When something is attached to a carbon and we want to talk about its neighbours, we label them with Greek letters.
α β
The α carrying the special group — the leaving group − X − OH
The β directly next-door to the α-carbon.
β C H − α C X
Read this as: the α-carbon holds X H
Why the topic needs it: "β-elimination" literally means "pull the group off α and pull an H off β." Without α/β the definition is unreadable.
Atoms and groups can carry electric charge, and we mark it with a small sign.
Definition Charge and radical symbols
H + lost its one electron . It is now positive and desperate for electrons. Picture an empty hand reaching out. Called a proton .OH − gained an extra electron . It is negative , electron-rich. Picture a full spare hand offering electrons.R + carbocation (say "carbo-cat-ion"). R Br ∙ ∙ one lonely unpaired electron : a radical , half a bond looking for its partner.
+ + missing an electron , not "has more." H + nothing left but its nucleus. Keep "positive = electron-hungry" in your head.
Why the topic needs it: the whole mechanism story is "H + R + OH − Br − are the plot.
Now that we have charges, we name the two personalities that drive every reaction.
Definition Electrophile and nucleophile
Electrophile = "electron-lover" (-phile = loving). It is electron-poor (often + seeks electrons. Example: H + Br + Nucleophile = "nucleus-lover." It is electron-rich (often − offers electrons. Example: OH − the alkene's π pair itself .
Intuition The picture: giver meets takerEvery step below is a rich hand (nucleophile) reaching toward a hungry empty hand (electrophile). "Electrophilic addition" just means: the electrophile is the one that makes first contact with our π electrons.
Why the topic needs it: the parent calls the alkene "a nucleophile / electron source" and the incoming H + / Br +
A curved arrow shows a pair of electrons moving , starting where the pair is now and pointing to where it goes . Tail = source of electrons, head = destination.
So when the parent says "the C–H electrons swing in to form the π bond," picture an arrow leaving the C–H line and landing between the two carbons.
Why the topic needs it: mechanisms are told entirely in curved arrows; they are the grammar of "what attacks what."
An alkyl group is a carbon-chain fragment like CH 3 − CH 3 CH 2 − R
Definition Degree of substitutionCount the alkyl groups (R to the carbon in question:
1° (primary): 1 alkyl group2° (secondary): 2 alkyl groups3° (tertiary): 3 alkyl groups
"More substituted" = more R
The symbols 3° > 2° > 1° two different things in the parent — both times meaning "more alkyl groups = better":
For a carbocation R + R more stable positive charge .
For an alkene : more R C = C more stable, lower-energy alkene (Zaitsev).
Why alkyl groups help (this is the deepest "why" in the topic): neighbouring C–H bonds can tilt their electron pair toward a nearby positive or π centre and share it — called hyperconjugation — plus alkyl groups gently push electron density along the bonds (+I / inductive effect ). More alkyl neighbours → more of this help → more stability. Both effects are unpacked in Hyperconjugation and Inductive effect and Carbocations — stability and rearrangement .
Why the topic needs it: both headline rules — Zaitsev and Markovnikov — are decided purely by counting alkyl groups. Master this count and you predict every major product.
Definition The reaction arrow and its labelsA above = reagent/catalyst below = conditions B → above the arrow is what you add (e.g. conc. H 2 SO 4 below is the condition (e.g. 443 K K 443 K ≈ 170° C
Why the topic needs it: every worked example is written this way. The reagent above the arrow (aqueous vs alcoholic KOH, peroxide or not) decides which product you get.
Bond = shared electron pair
Double bond sigma plus pi
pi electrons are exposed and reactive
Charges plus minus and radical dot
Electrophile taker vs Nucleophile giver
Curved arrow shows pair moving
Count alkyl groups 1 2 3 degree
Stability 3 beats 2 beats 1
Preparation = beta elimination
Addition = electrophilic addition
Markovnikov major product
Test yourself — cover the right side and answer out loud.
What does a single line between two atoms represent? One shared pair of electrons (a σ bond).
In C = C The π bond; it hovers above and below the line joining the carbons.
Why are π electrons the reactive part of an alkene? They stick out and are loosely held, so an electrophile can reach them first.
The α-carbon carries what? And the β-carbon? α carries the leaving group (− X − OH
What does the + H + It lost an electron — it is electron-hungry (a proton), not "extra of anything."
What does the dot in Br ∙ One unpaired electron — a radical.
Electrophile vs nucleophile in one word each? Electrophile = electron-taker; nucleophile = electron-giver.
Is an alkene's π pair an electrophile or nucleophile? A nucleophile (electron source).
What does a curved arrow show? A pair of electrons moving, from tail (source) to head (destination).
How do you tell 1°, 2°, 3°? Count alkyl (R) groups on the carbon: 1, 2, or 3.
Which is the more stable carbocation, 3° or 1°, and why? 3°, because more alkyl groups give more hyperconjugation and +I donation.
What do you read from above vs below a reaction arrow? Above = reagent/catalyst added; below = conditions like temperature.
