Difficulty: Intermediate | Prerequisites: Chapters 7 and 9 (elimination reactions, E1/E2 mechanisms, carbocation stability).
Alkenes are the first major unsaturated functional group you will study in detail. This chapter lays the groundwork for everything in Chapter 12, where alkenes become substrates for a dozen different addition reactions. If you are coming into this cold, you need a solid grasp of hybridisation (sp2 vs sp3), how sigma and pi bonds differ, and the basics of E1/E2 elimination. If those terms do not ring a bell, go back to Chapters 7 and 9 first. The payoff here is learning how carbon-carbon double bonds form and why they behave as nucleophiles, which drives the entire next chapter.
Alkenes are hydrocarbons with at least one carbon-carbon double bond. The double bond consists of one strong sigma bond and one weaker pi bond; the pi bond is what makes alkenes reactive and nucleophilic. Alkenes are prepared mainly through E1 and E2 elimination reactions, where Zaitsev's rule predicts the more substituted (more stable) alkene as the major product, and Hofmann's rule applies in specific base-controlled cases.
Alkene
An unsaturated hydrocarbon containing at least one carbon-carbon double bond (C=C). In simple terms, it is like an alkane that has "lost" two hydrogen atoms to form a double bond.
Degree of unsaturation (DoU)
A formula-based count of how many rings or pi bonds a molecule contains. Calculated as DoU = (2C + 2 + N - H - X) / 2. Think of it as a quick way to figure out whether a molecule has double bonds, triple bonds, or rings without drawing it.
Sigma (σ) bond
A bond formed by direct, head-on overlap of atomic orbitals along the internuclear axis. Bond energy approximately 108 kcal/mol for a C-C sigma bond. This is the strong, stable backbone of the double bond.
Pi (π) bond
A bond formed by side-to-side overlap of p orbitals above and below the plane of the sigma bond. Bond energy approximately 65 kcal/mol. This is the weaker, more reactive part of the double bond, and it is why alkenes behave as nucleophiles.
sp2 hybridisation
The hybridisation state of each carbon in a C=C double bond. The carbon uses three sp2 hybrid orbitals (arranged in a trigonal planar geometry at roughly 120°) for sigma bonds, and retains one unhybridised p orbital for the pi bond.
Zaitsev's rule
In elimination reactions, the more substituted alkene (more alkyl groups on the double-bond carbons) is the major product, because it is more thermodynamically stable.
Hofmann's rule
The less substituted alkene forms as the major product, typically when a bulky, non-nucleophilic base (such as tert-butoxide or LDA) is used, or in E1cb-type pathways. Think of it as the steric exception to Zaitsev.
Cis/trans isomerism
Stereoisomerism arising from restricted rotation about a double bond. Cis means same-side substituents; trans means opposite-side. Alkenes in small rings (C3 to C7) can only be cis because the ring strain makes the trans geometry impossible.
Carbocation rearrangement
A shift of a hydride or alkyl group to an adjacent carbocation centre, producing a more stable carbocation. Relevant in E1 elimination pathways where alkene preparation may give unexpected regiochemistry.
Alkenes are unsaturated hydrocarbons; general formula CₙH₂ₙ (for acyclic, mono-ene)
Compared to alkanes (all sp3, CₙH₂ₙ₊₂), alkenes are "missing" 2H per degree of unsaturation
Each double-bond carbon is sp2 hybridised
Trigonal planar geometry, bond angles near 120°
One sigma bond (head-on overlap, ~108 kcal/mol) and one pi bond (side-to-side p orbital overlap, ~65 kcal/mol)
The pi bond is weaker than the sigma bond and is therefore the reactive site
Pi electrons sit above and below the molecular plane
The alkene behaves as a nucleophile (electron-rich pi cloud attacks electrophiles)
Restricted rotation about the C=C: breaking the pi bond costs ~65 kcal/mol, so cis/trans isomers do not interconvert at room temperature
More substituted alkenes are more stable (hyperconjugation and inductive effects stabilise the double bond)
Tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted
Trans alkenes are generally more stable than cis alkenes (less steric strain between substituents on opposite sides)
Exception: alkenes in small rings (C3 to C7) can only be cis. Trans geometry would introduce too much ring strain.
Classification by substitution count: mono-, di-, tri-, and tetrasubstituted refers to the number of non-hydrogen groups attached to the C=C carbons
Alkenes are prepared primarily through elimination reactions (review Ch. 7 and 9)
Two main pathways:
E2 (bimolecular elimination): one-step, concerted. Requires an antiperiplanar arrangement of the leaving group and the beta-hydrogen. Zaitsev's rule gives the more substituted alkene as the major product with standard bases. Hofmann's rule applies with bulky bases (tert-butoxide, LDA), favouring the less substituted alkene.
E1 (unimolecular elimination): two-step. Substrate ionises to form the most stable carbocation first, then a base removes a proton. Zaitsev product dominates. Watch for carbocation rearrangements (hydride and methyl shifts) that can change the carbon skeleton before elimination.
Dehydration of alcohols (Ch. 9): acid-catalysed (H₂SO₄) removal of water from an alcohol to form an alkene. E1 mechanism for secondary and tertiary alcohols. Carbocation rearrangements are common.
Selective alkene formation:
Choose E2 with a strong, bulky base for Hofmann (less substituted) product
Choose E2 with a standard base or E1 conditions for Zaitsev (more substituted) product
Always check for possible carbocation rearrangement in E1 pathways
Degree of unsaturation: DoU = (2C + 2 + N - H - X) / 2
Each DoU corresponds to one ring or one pi bond; a triple bond counts as 2
C-C sigma bond energy: ~108 kcal/mol
C-C pi bond energy: ~65 kcal/mol
Total C=C double bond energy: ~173 kcal/mol (sigma + pi, but the pi bond alone is what breaks in addition reactions)
Alkane general formula: CₙH₂ₙ₊₂
Alkene general formula (acyclic, one double bond): CₙH₂ₙ
Alkenes are the backbone of the petrochemical industry. Ethylene (the simplest alkene) is the most-produced organic compound in the world; it is polymerised to make polyethylene (plastic bags, bottles, packaging). Propylene becomes polypropylene. The reactivity of the pi bond is also why vegetable oils (which contain alkene-rich fatty acid chains) can be hydrogenated to produce margarine and solid fats.
Students often think the double bond is simply "twice as strong" as a single bond. It is not. The sigma component is about 108 kcal/mol and the pi component only about 65 kcal/mol, so the total (~173) is well short of double.
Students confuse degree of unsaturation with number of double bonds. A ring also counts as one DoU, so a molecule with DoU = 2 could have two double bonds, one double bond and one ring, a triple bond, or two rings.
Students sometimes forget that E1 reactions can produce rearranged products. If the carbocation intermediate can rearrange to a more stable one, it will, and the resulting alkene may have a different carbon skeleton from what you expected.
Thinking that cis is always less stable than trans. This is the general trend, but in small rings (C3 to C7), the trans isomer cannot exist at all because of ring strain.
⚠️ You will almost certainly be asked to predict the major alkene product of an elimination reaction. Know when Zaitsev applies and when Hofmann applies.
⚠️ Calculating degree of unsaturation from a molecular formula is a standard exam question. Practise until it is automatic.
⚠️ Carbocation rearrangements in E1 are a favourite trick question. If the initial carbocation is secondary, check whether a hydride or methyl shift would produce a more stable tertiary carbocation.
⚠️ Know the structural requirements for E2: antiperiplanar geometry of the H and the leaving group. If there is only one antiperiplanar H available, that determines which alkene forms, regardless of Zaitsev.
True or false: A pi bond is stronger than a sigma bond. (False. Pi bonds are weaker, ~65 vs ~108 kcal/mol.)
Fill in the blank: The general formula for an acyclic alkene with one double bond is ______. (CₙH₂ₙ)
True or false: Zaitsev's rule always predicts the major product of an E2 reaction. (False. Bulky bases such as tert-butoxide favour the Hofmann product.)
Fill in the blank: Alkenes in small rings (C3 to C7) can only adopt ______ geometry. (Cis)
True or false: In E1 elimination, the carbocation intermediate can rearrange before the alkene forms. (True.)
Q: A secondary alkyl bromide is treated with sodium ethoxide (NaOEt) in ethanol. What is the expected major product, and by what mechanism?
A: The major product is the more substituted (Zaitsev) alkene, formed via an E2 mechanism. Sodium ethoxide is a strong, moderately sized base that promotes bimolecular elimination with antiperiplanar geometry.
Q: The same substrate is treated with potassium tert-butoxide (KOtBu). How does the product change?
A: The major product shifts to the less substituted (Hofmann) alkene. The bulky tert-butoxide base has difficulty abstracting the more sterically hindered proton, so it preferentially removes the less hindered one.
Q: A tertiary alcohol is heated with H₂SO₄. What mechanism governs the elimination, and what should you watch for?
A: E1. The alcohol is protonated and water leaves to form a tertiary carbocation. Watch for possible carbocation rearrangements (hydride or methyl shifts) before the proton is lost to form the alkene. The Zaitsev product is favoured.
Q: Calculate the degree of unsaturation for C₅H₁₀. What structural features might the molecule have?
A: DoU = (2(5) + 2 - 10) / 2 = 1. The molecule has one degree of unsaturation, which could be one double bond (e.g. a pentene) or one ring (e.g. cyclopentane).
Q: Why can cyclohexene exist as a cis alkene but not a trans alkene?
A: The six-carbon ring is too small to accommodate the geometric constraints of a trans double bond without extreme ring strain. Trans-cyclohexene would require the ring carbons to bridge across opposite sides of the double bond, which is geometrically impossible for a six-membered ring.
This connects directly to Chapter 12 (reactions of alkenes), where the nucleophilic pi bond you learned about here becomes the substrate for hydrogenation, halogenation, hydration, and many other addition reactions. Understanding alkene stability also feeds back into Chapter 7 and 9 elimination reactions, because the thermodynamic product of elimination is the more stable alkene. Later in the course, the same sp2 hybridisation and pi-bond concepts reappear in conjugated dienes, aromatic systems, and carbonyl chemistry.
Alkene, olefin, unsaturated hydrocarbon, C=C double bond, pi bond, sigma bond, sp2 hybridisation, sp2 carbon, degree of unsaturation, index of hydrogen deficiency, DoU, Zaitsev rule, Saytzeff rule, Hofmann rule, E1 elimination, E2 elimination, dehydration, dehydrohalogenation, carbocation rearrangement, hydride shift, methyl shift, cis-trans isomerism, geometric isomers, restricted rotation, alkene stability, substituted alkene, antiperiplanar, bulky base, LDA, tert-butoxide