What a Double Bond Really Is
Up to now your hydrocarbons have been alkanes — saturated chains where every carbon wears four single bonds and the whole molecule is stuffed full of hydrogen. An alkene breaks that pattern: somewhere along the chain two carbons share a carbon-carbon double bond, drawn C=C. Reach back to the degree-of-unsaturation idea from an earlier rung — each double bond is one degree of unsaturation, two fewer hydrogens than the matching alkane. But the two lines of C=C are not two identical bonds. They are two very different bonds living in the same place, and telling them apart is the key to everything that follows.
Bring back hybridization from the Foundations rung. In an alkane each carbon is sp3 — four equal hybrid orbitals pointing to the corners of a tetrahedron. At a double bond, each of the two carbons instead mixes only one s and two p orbitals to make three sp2 hybrids, leaving one whole unmixed p orbital standing untouched. The three sp2 hybrids spread out flat in a plane, 120 degrees apart like the spokes of a peace sign, and they form the ordinary sigma framework: one sigma straight down the C-C axis plus the bonds to the attached hydrogens or carbons. That leftover p orbital is what makes the second bond — and it makes it in a completely different way.
Stand the two carbons side by side and let each one's leftover p orbital point straight up and down, perpendicular to the flat sp2 plane. These two parallel p orbitals overlap not end-to-end but side-on, and that sideways overlap is the pi bond. So a double bond is one sigma (strong, along the axis, the same kind of bond holding an alkane together) plus one pi (a weaker, looser embrace formed sideways above and below the plane). Picture it as a hot-dog bun: the sigma is the sausage running down the middle, and the pi is the two halves of the bun — a cloud of electron density sitting above the line of atoms and a matching cloud below.
Why the Bond Cannot Turn
Here is the consequence that makes alkenes geometrically interesting. In an alkane, the two carbons of a C-C single bond can spin freely around their shared axis — that is the whole story of conformations you met earlier, where groups twirl past one another with only a small energy cost. A double bond cannot do that. To rotate one carbon relative to the other, you would have to twist the two parallel p orbitals out of alignment, and a sideways pi bond only exists when its p orbitals are parallel. Twisting them apart breaks the pi bond outright. So the double bond is rigid: the groups on each end are locked into one flat arrangement, frozen in place.
That rigidity is exactly why cis-trans (E/Z) isomers exist — a topic from the Stereochemistry rung that suddenly clicks here. Because the ends cannot rotate, putting two methyl groups on the same side of the C=C gives a genuinely different molecule from putting them on opposite sides, and the two cannot interconvert without snapping the pi bond. A single bond would just spin between the two arrangements and make them the same compound. Restricted rotation is not a quirk to memorize; it falls straight out of the picture of two p orbitals that must stay parallel to hold hands.
The Triple Bond: Three Bonds, One Line
Push the same idea one notch further and you get an alkyne, a hydrocarbon with a carbon-carbon triple bond, C#C (we write the triple bond as # in plain text). Now each of the two carbons hybridizes even less: it mixes one s with just one p to make two sp hybrid orbitals, which point in exactly opposite directions, 180 degrees apart — a straight line. That leaves two whole unmixed p orbitals on each carbon, perpendicular to the axis and perpendicular to each other. So the triple bond is one sigma (along the axis) plus two pi bonds (one in the up-down plane, one in the front-back plane), wrapping the axis in a cylinder of pi electron density rather than two separate clouds.
- sp3 carbon (alkane): four sp3 hybrids, tetrahedral, no leftover p orbital — only sigma bonds, so the C-C is the longest (~154 pm) and the most sluggish.
- sp2 carbon (alkene C=C): three flat sp2 hybrids at 120 degrees plus one leftover p orbital — that p makes one pi bond, so C=C is shorter (~134 pm) and bears an exposed pi cloud.
- sp carbon (alkyne C#C): two sp hybrids in a straight 180-degree line plus two leftover p orbitals — those two p orbitals make two pi bonds, giving the shortest, strongest C-C of all (~120 pm).
- The thread tying them together: the more s character in the hybrid (sp3 to sp2 to sp), the shorter and stronger the bond and the more tightly the electrons are held — and the leftover p orbitals are exactly what become the pi bonds.
The geometry is the giveaway. An sp carbon and its two neighbors are perfectly linear, so a triple bond and everything one atom out from it sit on a straight rod — which is why an alkyne like 2-butyne is a stiff little stick. Two extra facts fall out of the high s character of an sp orbital, where the bonding electrons sit closer to the nucleus: the C#C bond is the shortest and strongest carbon-carbon bond of the three (around 120 pm), and a hydrogen attached directly to an sp carbon is unusually acidic for a hydrocarbon — pKa around 25, versus roughly 44 for an alkene C-H and about 50 for an alkane. That acidity is small by acid standards but enormous for a hydrocarbon, and a later guide will use it to rip off that proton and turn a terminal alkyne into a nucleophile.
Naming the Unsaturation
Naming alkenes and alkynes reuses every IUPAC habit from the alkanes rung, with three small twists. First, the ending changes to flag what kind of bond you have: an -ane becomes -ene for a double bond and -yne for a triple bond. So ethane (CH3CH3) becomes ethene (CH2=CH2), and the two-carbon alkyne is ethyne (HC#CH, the gas you may know as acetylene). Second, the multiple bond rules the chain: you must choose the longest chain that contains the C=C or C#C, even if a longer carbon chain exists that skips it, and the bond gets the lowest possible locant.
Third, you locate the bond by the lower-numbered carbon it starts on, written just before the suffix in modern style: CH3-CH=CH-CH3 is but-2-ene (the double bond starts at C2), while CH2=CH-CH2-CH3 is but-1-ene. For alkenes that can be cis or trans, prepend the E/Z (or cis/trans) label you learned in stereochemistry. None of this is new machinery — it is the alkane naming game with the multiple bond as the new top priority for both the chain choice and the numbering. The one honest caution is that older names linger everywhere (ethylene, propylene, acetylene), so you will meet both systems in the real world.
Which Alkene Is More Stable?
Not all double bonds are equally content. Chemists rank alkene stability by how many carbon groups (rather than hydrogens) hang on the two sp2 carbons — the more substituted the C=C, the more stable it is. The trend runs from a tetrasubstituted alkene (four carbons on the double-bond carbons) at the top, down through trisubstituted, disubstituted, and monosubstituted, to a terminal CH2=CH- group at the bottom. Among disubstituted alkenes there is a finer split: the trans (E) isomer is usually a touch more stable than the cis (Z), because cis crowds two bulky groups onto the same side and they bump into each other.
Why should adding alkyl groups steady a double bond? Two reasons, both familiar from earlier rungs. The first is hyperconjugation: a neighboring C-H sigma bond can lean its electron pair into the empty side of the pi system, spreading the electrons over more atoms — and electrons spread out are electrons at lower energy. The more alkyl groups around the C=C, the more of these stabilizing C-H bonds donate in. The second is a subtler electronic effect: an sp2 carbon is a little electron-hungry, and alkyl groups are mildly electron-releasing, so they comfort the double bond the way a good sigma donor always does. You can measure all this directly: burning a more-substituted alkene releases less heat, proving it started from a lower, more stable energy.
Why These Are the Hungriest Hydrocarbons
Now the payoff — why this whole rung exists. An alkane is a closed, contented molecule: its electrons are buried inside strong sigma bonds, tucked between the nuclei where nothing can easily reach them, which is why alkanes are so sluggish and need harsh radical conditions to react at all. The pi bond is the opposite. Its electrons do not sit between the nuclei; they bulge out above and below the molecular plane (or in a cylinder around an alkyne), loosely held and fully exposed. That soft, accessible cloud of electron density is a beacon to anything electron-poor.
In the language you built two rungs back, the pi bond makes an alkene a nucleophile — an electron-rich region looking to share. Anything electron-poor, an electrophile like H+ of a strong acid or the positive end of a polarized bond, is drawn to that exposed cloud. The pi electrons reach out, grab the electrophile, and in doing so sacrifice the weaker pi bond to forge a new, stronger sigma bond. This is the master reaction of the rung: electrophilic addition, the exact reverse of the elimination you just finished. The double bond opens up and a reagent adds across it, H going to one carbon and the rest to the other.
the master move of this rung (electrophilic addition):
CH2=CH2 + H-Br --> CH3-CH2-Br
(pi bond, (an (one weak pi bond traded
nucleophile) electrophile) for two strong sigma bonds)
alkene reactivity: exposed pi electrons = easy to attack
alkane reactivity: buried sigma electrons = hard to attackHold onto that frame as you climb the rest of this rung. The exposed pi electrons make the carbon-carbon multiple bond the most reactive functional group among the hydrocarbons — not because the double bond is fragile, but because its loose outer electrons are an open invitation. The next guides cash this in: electrophilic addition step by step, Markovnikov's rule (which is really just the carbocation-stability idea from the substitution rung in a new costume), and a whole toolkit of reagents you can add across a C=C to build bigger, more useful molecules. The structure you just learned is the lock; the reactions ahead are the keys.