Difficulty: Intermediate | Prerequisites: Chapter 3 (conformational analysis, hybridisation basics)
Alkenes sit at the heart of organic chemistry reactivity. This chapter introduces how to identify degrees of unsaturation, name and classify alkene stereoisomers, and predict which alkenes are more thermodynamically stable. If you can read a Newman projection for an alkane, you already have the spatial reasoning this material builds on. If you skipped hybridisation, go back to that first.
Alkenes contain carbon-carbon double bonds, and every double bond, ring, or triple bond in a molecule raises its "index of hydrogen deficiency" (IHD) by one. Whether an alkene is labelled E or Z depends on which substituents sit on the same side of the double bond, using atomic-number priority rules. More substituted alkenes are more stable, trans (E) beats cis (Z) in most cases, and you can measure stability differences through heats of hydrogenation and formation.
Index of Hydrogen Deficiency (IHD) / Unsaturation Number / Degree of Unsaturation
The number of pairs of hydrogen atoms "missing" from a molecule compared with the fully saturated formula. Each ring or double bond adds 1; each triple bond adds 2. Formula: U = (2C + 2 + N - H - X) / 2. In simple terms, this number tells you how many rings and/or pi bonds are hiding in the molecular formula.
E (entgegen) Isomer
The stereoisomer of an alkene in which the two highest-priority substituents (one on each carbon of the double bond) sit on opposite sides. E is usually more stable than Z. Think of E as "trans-like": the bulky groups point away from each other.
Z (zusammen) Isomer
The stereoisomer in which the two highest-priority substituents sit on the same side of the double bond. Think of Z as "cis-like": the bulky groups are on the same side, which often causes steric strain.
Cahn-Ingold-Prelog (CIP) Priority Rules
The ranking system that assigns priorities to substituents on a stereocentre or double bond based on atomic number. Higher atomic number equals higher priority. In simple terms, heavier atoms win.
Newman Projection (for Alkenes)
A view looking down the carbon-carbon bond axis. For alkenes, this view shows why rotation around the double bond is blocked: the pi bond locks the geometry.
Heat of Hydrogenation
The enthalpy change when one mole of an unsaturated compound reacts with H2 to become fully saturated. Nearly all alkenes release roughly 30 kcal/mol per double bond. In simple terms, the less heat released, the more stable the starting alkene was.
Heat of Formation (deltaH_f)
The enthalpy change when one mole of a compound forms from its elements in their standard states. A more negative value means a more stable compound.
Vinyl Group
H2C=CH-, an ethenyl substituent. It is a primary unsaturated group.
Allyl Group
H2C=CH-CH2-, a propenyl substituent. It is a secondary unsaturated group.
Isopropenyl Group
H2C=C(CH3)-, a 2-methylethenyl substituent. It is a tertiary unsaturated group.
s-Character
The percentage of s orbital contributing to a hybrid orbital. More s-character means a shorter, stronger, and more electronegative bond. Think of it as: the more "s" in the mix, the tighter the electron cloud is held.
Tags: IHD, degree of unsaturation, hydrogen deficiency index, unsaturation formula
General formula (no nitrogen)
U = (2C + 2 - H - X) / 2
where C = number of carbons, H = number of hydrogens, X = number of halogens (F, Cl, Br, I). Halogens count the same as hydrogen in this formula.
When nitrogen is present
U = (2C + 2 + N - H - X) / 2
Nitrogen adds one to the numerator because each nitrogen can bond to three atoms rather than one.
What each unit of unsaturation means
1 degree = one ring or one double bond (pi bond)
2 degrees = one triple bond, or two double bonds, or two rings, or a ring plus a double bond
A benzene ring accounts for 4 degrees of unsaturation (3 double bonds + 1 ring)
Oxygen and sulfur
Neither oxygen nor sulfur changes the IHD formula. They are simply ignored in the count because each bonds to two atoms, which does not alter the hydrogen requirement.
Bond strengths to keep in mind
C=C double bond: roughly 150 kcal/mol. C-C single bond: roughly 90 kcal/mol. C triple bond C: roughly 200 kcal/mol. In terms of strength: sigma > pi, and triple bonds are the strongest overall. Sigma bonds are individually stronger than pi bonds because sigma overlap is head-on.
Tags: E/Z nomenclature, cis-trans, Cahn-Ingold-Prelog, stereoisomers, alkene stereochemistry
E is usually more stable than Z. The E isomer places the two highest-priority groups on opposite sides of the double bond, reducing steric strain.
How to assign E or Z
Look at each carbon of the double bond separately
On each carbon, rank the two substituents by CIP priority (highest atomic number = highest priority)
If the two high-priority substituents are on the same side: Z (zusammen, "together")
If they are on opposite sides: E (entgegen, "opposite")
There must be one substituent of each priority (high and low) on each side of the double bond for E/Z to apply
CIP priority rules in brief
Compare atoms directly attached to the double-bond carbon. Higher atomic number wins.
If there is a tie, move outward to the next set of atoms and compare.
Double and triple bonds are "expanded": a C=C is treated as if each carbon is bonded to a phantom second carbon. So C=C counts as C-C, C-C in the priority expansion; C triple bond C counts as C-C, C-C, C-C.
Relationship to cis/trans
Cis and trans are older labels that work only when each double-bond carbon has exactly one hydrogen. Cis means the two non-hydrogen groups are on the same side; trans means opposite sides. E/Z is the more general system and works for any substitution pattern.
No interconversion at ordinary temperatures. Cis-2-butene cannot spontaneously become trans-2-butene because rotation around the double bond requires breaking the pi bond, which costs roughly 60 to 65 kcal/mol of energy.
Tags: Newman projection, alkene rotation, restricted rotation, pi bond geometry
Newman projections for alkenes work the same way as for alkanes: you sight down a C-C bond and draw what you see. The key difference is that you cannot rotate around a double bond. The pi bond, formed by the side-on overlap of two p orbitals, locks the substituents in place.
Example: 3-methyl-3-hexene
Sighting down the C3-C4 bond, you draw the front carbon's three substituents and the back carbon's three substituents just as you would for a single bond. But unlike the single-bond case, there is only one conformation, not a set of rotamers. The groups are fixed.
Why this matters
Because rotation is blocked, the spatial arrangement of groups around a double bond is permanent (at room temperature). That is why E/Z isomers are distinct, isolable compounds rather than rapidly interconverting conformations.
Tags: delta H, enthalpy, thermodynamic stability, heat of hydrogenation, heat of formation, exothermic, endothermic
Heat of hydrogenation
When an alkene reacts with H2 (using a catalyst), the double bond becomes a single bond. The energy released is the heat of hydrogenation. Nearly all simple alkenes release about 30 kcal/mol per double bond. Alkynes release roughly 69 kcal/mol (for both pi bonds).
The key insight: a more stable alkene releases less energy on hydrogenation, because it started from a lower energy state. So a smaller (less negative) heat of hydrogenation means a more stable alkene.
Heat of formation (deltaH_f)
The enthalpy change when one mole of a compound forms from its elements in standard states. A more negative deltaH_f means a more thermodynamically stable compound.
Relationship: deltaH_reaction = deltaH_f(products) - deltaH_f(reactants).
deltaH < 0: exothermic (energy released)
deltaH > 0: endothermic (energy absorbed)
Worked example: cis-2-butene vs trans-2-butene
Using formation data:
4C + 4H2 forming trans-2-butene: deltaH_f = -11.6 kJ/mol (-2.72 kcal/mol)
4C + 4H2 forming cis-2-butene: deltaH_f = +7.4 kJ/mol (+1.77 kcal/mol)
Converting cis to trans: deltaH = -11.6 + 7.4 = -4.2 kJ/mol (or -2.72 + 1.77 = -0.95 kcal/mol). The negative value confirms trans-2-butene is more stable than cis-2-butene.
Energy diagram interpretation
Hydrogenation graph: more stable alkene sits lower on the energy axis, releases less energy, so deltaH_hydrogenation is smaller in magnitude
Formation graph: more stable product sits lower, deltaH_f is greater (more negative) for a more stable product
Tags: vinyl, allyl, isopropenyl, degree of substitution, alkene stability, hyperconjugation
Named substituent groups
Vinyl: H2C=CH- (primary)
Allyl: H2C=CH-CH2- (secondary)
Isopropenyl: H2C=C(CH3)- (tertiary)
Stability ranking by degree of substitution: tertiary > secondary > primary. That is, 3 degree > 2 degree > 1 degree.
Full stability ranking of alkenes (increasing stability)
R-CH=CH2 (monosubstituted) < R-CH=CH-R or R2C=CH2 (disubstituted) < R2C=CHR (trisubstituted) < R2C=CR2 (tetrasubstituted)
Within disubstituted alkenes, trans (E) is more stable than cis (Z).
Why more substitution means more stability
Alkyl groups donate electron density to the double bond through hyperconjugation (overlap of adjacent C-H sigma bonds with the pi system). More alkyl substituents provide more electron donation, which stabilises the electron-poor pi bond.
Real-world connection
This stability ordering directly predicts which product forms preferentially in elimination reactions (Chapter 5). When a reaction can form two different alkenes, the more substituted one is usually the major product (Zaitsev's rule).
Tags: s-character, p-character, sp2, sp3, hybridisation, electronegativity, bond polarity
Defining s-character
s-character is the percentage of s orbital in a hybrid orbital. For example, sp2 = 1/3 s, so 33% s-character and 67% p-character. sp3 = 1/4 s, so 25% s-character.
The rule: more s-character = stronger bond
s orbitals hold electrons closer to the nucleus than p orbitals do. A hybrid with more s-character pulls its bonding electrons in more tightly, making the bond shorter and stronger. Sigma bonds are stronger than pi bonds precisely because sigma bonding involves head-on overlap with significant s-character.
Electronegativity consequence
An sp2 carbon is somewhat more electronegative than an sp3 carbon, because the sp2 hybrid holds electrons closer to the carbon nucleus. This gives the C=C double bond a very slight polar effect.
Stability consequence (counterintuitive)
More s-character = less stable when comparing the alkene itself within a stability series. An alkene with more s-character in its C-H bonds holds those hydrogens more tightly, which means less hyperconjugative stabilisation of the pi system. This is one reason terminal (less substituted) alkenes are less stable: they have more C-H bonds with relatively high s-character and fewer stabilising alkyl groups.
Students often think cis and E/Z are interchangeable labels. They are not. Cis/trans only works when each carbon of the double bond carries exactly one hydrogen. E/Z uses CIP priority and works for all cases.
Students often assume a smaller heat of hydrogenation means a less stable alkene. The opposite is true: a smaller (less negative) value means the alkene was already more stable and had less energy to give up.
Students sometimes forget that oxygen and sulfur do not change the IHD formula. Only carbon, hydrogen, nitrogen, and halogens matter for the calculation.
Students often confuse "more s-character = stronger bond" with "more s-character = more stable compound." The bond itself is stronger, but the alkene as a whole may be less stable if it lacks the alkyl substituents that provide hyperconjugative stabilisation.
The IHD formula is a frequent first step on exam problems. You will be given a molecular formula and asked to determine how many degrees of unsaturation are present. Know the formula cold.
E/Z assignment is heavily tested. Expect problems where you must assign priorities using CIP rules and then determine E or Z. The expanded-bond treatment of double and triple bonds for priority assignment is a common source of exam mistakes.
Heat of hydrogenation comparisons are a classic exam question format: "rank these alkenes by stability" or "which alkene releases the most/least heat on hydrogenation?"
The stability ordering (tetra > tri > di > mono, and E > Z for disubstituted) appears in both multiple-choice and short-answer formats.
Know the three named substituent groups (vinyl, allyl, isopropenyl) and their degree of substitution.
True or false: A molecule with IHD = 3 must contain three double bonds. ______
Fill in the blank: The formula for IHD when nitrogen is present is U = (2C + 2 + ____ - H - X) / 2.
True or false: Trans-2-butene has a larger (more negative) heat of hydrogenation than cis-2-butene. ______
Fill in the blank: The E isomer places high-priority groups on ________ sides of the double bond.
True or false: An sp2 carbon is more electronegative than an sp3 carbon. ______
Answers: 1. False (could be rings, triple bonds, or combinations). 2. N. 3. False (trans is more stable, so it releases less energy). 4. Opposite. 5. True.
Q: Calculate the IHD for C8H14BrN.
A: U = (2(8) + 2 + 1 - 14 - 1) / 2 = (16 + 2 + 1 - 14 - 1) / 2 = 4/2 = 2. There are two degrees of unsaturation.
Q: Assign E or Z to the following: on C1 of a double bond, the substituents are -CH3 (lower priority) and -Cl (higher priority). On C2, the substituents are -H (lower priority) and -CH2CH3 (higher priority). The Cl and CH2CH3 are on the same side.
A: The two high-priority groups (Cl and CH2CH3) are on the same side, so the isomer is Z.
Q: Rank the following alkenes in order of increasing stability: 1-butene, trans-2-butene, cis-2-butene.
A: 1-butene (monosubstituted, least stable) < cis-2-butene (disubstituted, Z) < trans-2-butene (disubstituted, E, most stable).
Q: Alkene A has a heat of hydrogenation of -28 kcal/mol. Alkene B has a heat of hydrogenation of -30 kcal/mol. Which is more stable?
A: Alkene A is more stable. It releases less energy on hydrogenation, meaning it started from a lower energy (more stable) state.
Q: What is the s-character of an sp2 hybrid orbital, and how does it compare to sp3?
A: sp2 has 33% s-character (1/3). sp3 has 25% s-character (1/4). The sp2 hybrid has more s-character, making its bonds shorter, stronger, and the carbon slightly more electronegative.
This material connects directly to elimination reactions (E1 and E2 in Chapter 5 or equivalent), where predicting which alkene product is major requires understanding the stability ordering covered here. Zaitsev's rule favours the more substituted (more stable) alkene.
E/Z nomenclature and CIP priority rules reappear whenever stereochemistry is at play, including R/S assignment at sp3 stereocentres. If you are comfortable with priorities here, the chirality chapter will feel familiar.
Heat of hydrogenation data underpins the discussion of aromatic stability later in the course, where benzene's unusually low heat of hydrogenation is evidence for resonance stabilisation.
alkene, olefin, degree of unsaturation, index of hydrogen deficiency, IHD, unsaturation number, E/Z isomers, cis-trans isomers, geometric isomers, Cahn-Ingold-Prelog priority, CIP rules, Newman projection, heat of hydrogenation, heat of formation, enthalpy, thermodynamic stability, vinyl group, allyl group, isopropenyl group, degree of substitution, hyperconjugation, s-character, p-character, sp2 hybridisation, sp3 hybridisation, bond strength, sigma bond, pi bond, restricted rotation, double bond geometry, organic chemistry chapter 4, HEA 0004, University of Washington, Prof. Forrest Michael