Intuition The ONE core ideaAn alkyne is two carbons clamped together by a triple bond — a bundle of extra electrons that behaves in two ways: it is a magnet for anything positive (so it adds reagents across itself), and, at the very end of a chain, it lets go of its hydrogen more easily than any other C–H (so it acts as a weak acid). Everything in this chapter is just these two behaviours, dressed up in different reagents.
Before you can read a single reaction arrow in the parent note, you need to own the symbols. Below, each symbol is defined in plain words, drawn as a picture, and justified — why does the topic even need it? They are ordered so each one leans on the one before.
Definition How to read the::: reveal lines
Later on, the Equipment checklist uses lines of the form Question ::: Answer. In your vault this renders as a flashcard: the part left of the ::: is the prompt shown to you, and the part right of it is the hidden answer. Cover the right side, say your answer aloud, then reveal. The ::: is just the divider — it is notation for this study system , not chemistry.
Definition A chemical bond (the dash)Two atoms sharing a pair of electrons are "bonded". We draw one shared pair as a single line: C − C one pair. The dash is not decoration — each dash is literally one shared pair of electrons .
Intuition Why we care about counting bondsEverything an alkyne does comes from having more than the usual number of shared pairs between two carbons. So step one is learning to read how many pairs a line-drawing shows.
C − C single bond, one shared pair.C = C double bond, two shared pairs.C ≡ C triple bond, three shared pairs. This is the alkyne.
Figure 1. Three side-by-side C–C bonds drawn with one, two, and three lines. As the number of shared-pair lines grows, the two carbons are pulled closer together, so the bond gets shorter: C ≡ C ( 120 pm ) < C = C ( 134 pm ) < C − C ( 154 pm )
Look at the figure: as we stack more lines, the two carbons are pulled closer (the bond gets shorter) and there is more electron "cloud" packed between them. That extra cloud is the treasure the whole chapter chases.
Not all shared pairs are the same shape. The parent note says a triple bond is "one strong σ two weaker π
σ directly on the line joining the two atoms, like a rod end-to-end. It is strong and holds the atoms together. Every bond (single, double, triple) has exactly one σ
π above and below the line joining the atoms, like two loaves of bread sandwiching the axis. It is weaker and more exposed — the electrons stick out into space.
Figure 2. Left: a σ on the axis between the two carbons (a rod). Right: a π above and below the axis — exposed clouds that electrophiles can grab. A triple bond is one σ π
Intuition Why split them?Because the exposed π σ two π twice . The whole "addition reactions" section is just π
Single bond: 1 σ
Double bond: 1 σ + 1 π
Triple bond: 1 σ + 2 π this is why alkynes add two equivalents of reagent.
Before the acidity story, we must name where the triple bond sits, because only one kind is acidic.
Definition Terminal and internal alkyneA terminal alkyne has its triple bond at the end of the chain, so one of the triple-bond carbons still carries a hydrogen: the ≡ C − H C H 3 C ≡ C H internal alkyne has the triple bond inside the chain, with a carbon group on both sides and no ≡ C − H C H 3 C ≡ C C H 3
Intuition Why this distinction mattersOnly the terminal alkyne owns an ≡ C − H that hydrogen is the acidic one the whole acidity section (and the silver/copper lab tests) is about. An internal alkyne has no such H, so it cannot be deprotonated and gives no precipitate. Keep this label in hand for Section 4.
This is the deepest prerequisite, and the parent note's acidity argument collapses without it. See Hybridisation and s-character for the full build; here is the essential picture.
A region of space where an electron is likely to be found. Two shapes matter: the s orbital is a small round ball hugging the nucleus; the p orbital is a long dumbbell reaching far out.
When a carbon bonds, it blends its one round s orbital with some of its dumbbell p orbitals to make identical "hybrid" orbitals. The label counts how many p's joined the mix:
s p 50% s-character (used by triple-bond carbon).s p 2 33% s-character (double bond).s p 3 25% s-character (single bond).
Intuition Why s-character controls acidityMore s-character means more of that hug-the-nucleus round ball in the mix, so the hybrid orbital sits closer to the nucleus . Electrons parked there are held tightly and comfortably. When a terminal alkyne loses its H⁺, the leftover electron pair lands in the s p stable , and a stable conjugate base means a stronger acid. That is the entire "s p > s p 2 > s p 3
Figure 3. Three carbon hybrid orbitals drawn as clouds around a nucleus, shrinking left to right: s p 3 s p 2 s p
Definition Bond angle and linearityThe bond angle is the corner-angle between two bonds meeting at an atom. An s p σ 18 0 ∘ linear .
Definition pm (picometre)A ruler unit for bond length: 1 pm = 1 0 − 12 m 120 pm C ≡ C ( 120 ) < C = C ( 134 ) < C − C ( 154 )
Definition Ions and formal chargeA superscript + − C 2 2 − [ A g ( N H 3 ) 2 ] + carbanion is a carbon carrying a lone pair and a −
Definition The heat symbolΔ Δ apply heat " — the reaction needs warming to go. It is not a quantity here, just an instruction: heat this mixture . You will see it in the parent note's elimination steps, e.g. KOH (alc.) Δ
Definition Reaction arrows
⟶ ⇌ equilibrium — both directions happen; which side "wins" depends on stability (this is exactly the enol ⇌ keto balance).above below above , conditions (Δ below .↑ ↓
Definition The "approximately" symbol≈ ≈ approximately equal to " — the value that follows is a rounded, close-enough figure, not an exact number. So p K a ≈ 25 p K a p K a ballpark matters for comparisons.
Definition Nucleophile vs electrophileA nucleophile ("nucleus-lover") is electron-rich and seeks a positive partner — the alkyne's π electrophile ("electron-lover") is electron-poor and seeks electrons — like H + Alkenes — electrophilic addition .)
p K a p K a H + Lower p K a It is a logarithmic scale, so each step of 1 is a factor of 10.
The picture below shows how each foundation feeds the next, ending at the alkyne's reactions. Read arrows as "is needed for".
Figure 4. Prerequisite flow. The dash (shared pair) splits into bond-counting and orbital shapes; orbital shapes give hybridisation, which gives both s-character (→ acidic C–H via a stable carbanion) and linear geometry. The two exposed π p K a alkyne reactions .
count bonds single double triple
two pi clouds react twice
orbitals s ball and p dumbbell
s-character closeness to nucleus
stable carbanion means acidic C-H
180 degree linear geometry
nucleophile meets electrophile
pKa rule pick the right base
Each line below is a Question ::: Answer reveal card (see the note near the top for how to read :::). Cover the right side; you are ready for the parent note only when each is instant.
How many electron pairs does one dash represent exactly one shared pair
How many σ π one σ π
Which bond type has electron clouds above and below the axis the π
Why can an alkyne add two reagent equivalents it has two π
Terminal vs internal alkyne terminal has ≡ C − H
s-character of s p s p 2 s p 3 50%, 33%, 25%
More s-character means the orbital is… closer to the nucleus, holds electrons tighter
Why is the s p its lone pair sits in a high-s, close-to-nucleus orbital
Lower p K a stronger acid (gives up H +
What does ≈ approximately equal to (a rounded value)
What does Δ apply heat
Rule for whether a base deprotonates an acid base's conjugate acid must have higher p K a
Bond angle and shape at an s p 18 0 ∘
What ⇌ an equilibrium, both directions, stability decides the winner
What ↓ it precipitates out as a solid
Parent topic Hybridisation and s-character Alkenes — electrophilic addition Markovnikov rule and carbocation stability Keto–enol tautomerism 
