Alkynes and alkenes sit at the heart of Exam 4: you need to name them (IUPAC), draw them from names, rank their stability, and predict their acidity. The key ideas are that more substituted alkenes are more stable (lower heat of hydrogenation), sp-hybridised C-H bonds are the strongest and most acidic, and carbocation stability follows the usual order of 3° > 2° > 1°.
Difficulty: Intermediate | Prerequisites: CHEM 2301 material through Chapter 8 (functional groups, hybridisation, Lewis structures, electronegativity trends). Familiarity with IUPAC naming conventions for alkanes and alkenes is assumed.
This material sits in the second half of Organic Chemistry I, where the course shifts from structure and bonding into reactivity. You have already covered functional groups, hybridisation, and acid-base chemistry. Now the focus is on unsaturated hydrocarbons (alkenes and alkynes), their naming, their relative stability, and the physical properties that follow from differences in hybridisation. If you are comfortable drawing Lewis structures, assigning hybridisation, and using electronegativity to predict bond polarity, you have the foundation you need. Everything here feeds directly into the reaction mechanisms and product-prediction questions that make up the bulk of Exam 4.
Alkyne
A hydrocarbon containing at least one carbon-carbon triple bond (C≡C). Think of it as the "extra-unsaturated" cousin of an alkene, with two pi bonds instead of one.
Terminal alkyne
An alkyne where the triple bond is at the end of the carbon chain, giving a C≡C-H unit. In simple terms, this means there is an exposed, acidic hydrogen on the sp carbon.
Internal alkyne
An alkyne where the triple bond sits between two carbon substituents (R-C≡C-R). No terminal hydrogen is present.
Enyne
A molecule containing both a double bond (ene) and a triple bond (yne). IUPAC rules require giving the lowest set of locants to both unsaturations, with the double bond receiving priority for the lower number when there is a tie.
IUPAC nomenclature (alkynes)
The systematic naming system: find the longest chain that includes the triple bond, number from the end nearest the first point of unsaturation, and use the suffix "-yne." For diynes, use "-diyne."
Carbocation
A carbon atom bearing a positive formal charge and only three bonds (six electrons). Stability increases with substitution: 3° > 2° > 1° > methyl, because alkyl groups donate electron density through hyperconjugation and induction.
Hyperconjugation
The stabilising interaction between a filled C-H (or C-C) sigma bonding orbital on an adjacent carbon and the empty p orbital of a carbocation (or radical). In simple terms, neighbouring bonds "share" their electrons to help stabilise the electron-poor centre.
Heat of hydrogenation (ΔH_hydrog)
The enthalpy change when an alkene (or alkyne) is fully hydrogenated to an alkane. A lower absolute value of ΔH means the starting alkene was more stable (it had less energy to release).
Degree of substitution (alkenes)
The number of carbon substituents attached to the C=C. More substituted alkenes are more stable, so they have a lower heat of hydrogenation.
sp, sp², sp³ hybridisation (and C-H bond strength)
The hybridisation of a carbon determines the s-character in its bonds: sp has 50% s-character, sp² has 33%, sp³ has 25%. Higher s-character means electrons are held closer to the nucleus, making the bond shorter, stronger, and more acidic.
Acidity of terminal alkynes
Terminal alkynes (pKa ≈ 25) are far more acidic than alkenes (pKa ≈ 44) or alkanes (pKa ≈ 50). The conjugate base of a terminal alkyne is an sp-hybridised carbanion, which is stabilised by the high s-character of the orbital holding the lone pair.
Anti addition
An addition reaction in which two groups add to opposite faces of the double bond. The classic example is bromination of an alkene via a bromonium ion intermediate.
Find the longest continuous carbon chain that contains the triple bond. This is the parent chain.
Number from the end that gives the triple bond the lowest possible locant.
Replace the "-ane" ending with "-yne" (or "-diyne" for two triple bonds, "-en-yne" for a compound with both a double and a triple bond).
When both a double bond and a triple bond are present (an enyne), give the lowest set of locants overall. If there is a tie, the double bond gets the lower number.
Stereochemistry (Z/E) applies only to double bonds, not triple bonds (which are linear). For enynes, assign Z/E at the alkene portion based on priority rules (Cahn-Ingold-Prelog).
Worked examples from the exam:
Propyne: three-carbon chain with a terminal triple bond. HC≡C-CH₃.
3,3-Dimethylpent-1,4-diyne: five-carbon chain, triple bonds at C1 and C4, two methyl groups on C3.
(Z)-3-Methylpent-2-en-4-yne: five carbons, a double bond at C2 (Z-configuration) and a triple bond at C4, with a methyl branch at C3.
4-Chloro-2-butyn-1-ol: four carbons, a triple bond at C2, a chlorine on C4, and a hydroxyl on C1.
Carbocations are stabilised by electron donation from surrounding groups.
Order of stability: 3° > 2° > 1° > methyl.
Allylic and benzylic carbocations gain extra stability from resonance delocalisation of the positive charge.
On the exam, you were shown two carbocations and asked which is more stable. The one with greater substitution or with allylic/benzylic resonance wins.
The strength of a C-H bond depends on the hybridisation of the carbon:
sp³ C-H: weakest (~105 kcal/mol). 25% s-character.
sp² C-H: intermediate (~111 kcal/mol). 33% s-character.
sp C-H: strongest (~131 kcal/mol). 50% s-character.
Higher s-character pulls the bonding electrons closer to the carbon nucleus, shortening and strengthening the bond.
The exam showed a molecule with three labelled C-H bonds (one on an sp³ carbon, one on an sp² carbon, one on an sp carbon) and asked you to identify the weakest (sp³) and strongest (sp).
Heat of hydrogenation is the energy released when H₂ adds across a double bond (or triple bond) with a metal catalyst.
A more stable alkene releases less energy upon hydrogenation, so it has a lower |ΔH|.
Stability ranking for alkenes: tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted.
Trans disubstituted alkenes are more stable than cis (less steric strain).
On the exam, three alkene structures were shown. The least substituted has the highest heat of hydrogenation (least stable), and the most substituted has the lowest (most stable).
Terminal alkynes (sp C-H, pKa ≈ 25) are the most acidic hydrocarbons. Their conjugate base is stabilised because the lone pair sits in an sp orbital with high s-character.
Alkenes (sp² C-H, pKa ≈ 44) and alkanes (sp³ C-H, pKa ≈ 50) are far less acidic.
When comparing compounds for acidity, always consider: hybridisation of the atom bearing the proton, electronegativity effects, resonance stabilisation of the conjugate base, and inductive effects.
The exam asked which of several compounds (including a terminal alkyne, an alkene, and a cycloalkane) is most acidic. The terminal alkyne wins.
Anti addition means the two new groups end up on opposite faces of what was the double bond.
Reagents that give anti addition include:
Br₂ (or Cl₂) alone, or Br₂/H₂O (halohydrin formation), both via a cyclic halonium ion intermediate that blocks one face.
Reagents that do NOT give exclusively anti addition:
H₂O/H₂SO₄ (acid-catalysed hydration, goes via carbocation, not stereospecific).
HBr with peroxides (radical addition, not stereospecific).
H₂/Pd/C (syn addition).
The exam listed four reagent sets and asked which give only anti addition. The answer is Br₂/H₂O.
Students often think that a triple bond is always numbered C1. It is not. You number from the end that gives the lowest locant to the first point of unsaturation, considering all double and triple bonds together.
Students confuse bond strength with bond dissociation energy of the overall molecule. A stronger C-H bond (sp) does not mean the molecule itself is harder to break apart; it means that specific C-H bond requires more energy to cleave homolytically.
Many students assume "most acidic" means "most reactive." Acidity here is strictly about how easily the proton is lost and how stable the conjugate base is. A terminal alkyne is the most acidic hydrocarbon, but it is not the most reactive compound in every context.
Students sometimes think anti addition means "the reaction does not happen." Anti addition is a stereochemical descriptor: both groups do add, they simply land on opposite faces of the original double bond.
⚠️ IUPAC naming of alkynes, diynes, and enynes appears as a drawing question (8 points on this exam). You must be able to go from name to structure and back, including Z/E stereochemistry on the alkene portion.
⚠️ Carbocation stability ranking is tested directly and also underlies the regiochemistry of every electrophilic addition reaction (Markovnikov's rule).
⚠️ C-H bond strength by hybridisation is a recurring short-answer topic. Know the order sp > sp² > sp³ and the reason (s-character).
⚠️ Heat of hydrogenation is the standard exam tool for ranking alkene stability. Remember: more stable = lower |ΔH|.
⚠️ Acidity of terminal alkynes comes up in synthesis problems too, because deprotonation with NaNH₂ generates an acetylide nucleophile for C-C bond formation.
⚠️ Identifying which reagents give anti addition (vs syn or non-stereospecific) is tested both as a standalone question and within mechanism/product problems.
True or false: A terminal alkyne is more acidic than an alkene because the sp orbital has more s-character, stabilising the conjugate base. (True)
Fill in the blank: The order of C-H bond strength by hybridisation is sp ___ > sp² ___ > sp³ ___. (strongest, intermediate, weakest)
True or false: A tetrasubstituted alkene has a higher heat of hydrogenation than a monosubstituted alkene. (False, it has a lower |ΔH| because it is more stable.)
Fill in the blank: In IUPAC naming, when a molecule has both a double bond and a triple bond, the ___ bond receives the lower locant when there is a tie. (double)
True or false: Br₂/H₂O addition to an alkene proceeds through a bromonium ion and gives anti addition. (True)
Q: Draw the structure of (Z)-3-methylpent-2-en-4-yne. What functional groups are present?
A: A five-carbon chain with a Z-configured double bond between C2 and C3, a methyl group on C3, and a triple bond between C4 and C5 (terminal alkyne). Functional groups: alkene and terminal alkyne.
Q: Two carbocations are shown: one is a secondary allylic carbocation, the other is a secondary non-allylic carbocation. Which is more stable and why?
A: The secondary allylic carbocation is more stable because the positive charge is delocalised by resonance into the adjacent pi system, spreading the charge over two carbons.
Q: In a molecule with sp, sp², and sp³ C-H bonds, rank the C-H bonds from weakest to strongest.
A: sp³ C-H (weakest) < sp² C-H < sp C-H (strongest). Higher s-character means the bonding electrons are held more tightly, making the bond harder to break.
Q: Three alkenes are shown: one monosubstituted, one disubstituted (trans), and one trisubstituted. Which has the highest heat of hydrogenation?
A: The monosubstituted alkene. It is the least stable, so it releases the most energy upon hydrogenation.
Q: Which of the following reagents gives exclusively anti addition to an alkene: H₂O/H₂SO₄, HBr/peroxides, Br₂/H₂O, H₂/Pd/C?
A: Br₂/H₂O. The reaction proceeds through a bromonium ion, which forces the nucleophile (water) to attack from the opposite face.
Q: Why is a terminal alkyne more acidic than an alkene or an alkane?
A: The C-H in a terminal alkyne is on an sp carbon (50% s-character). When the proton is removed, the resulting carbanion holds its lone pair in an sp orbital close to the nucleus, stabilising the negative charge. Alkenes (33% s, sp²) and alkanes (25% s, sp³) produce less stable conjugate bases.
This material connects directly to electrophilic addition reactions (the next study notes doc), because carbocation stability determines Markovnikov regiochemistry, and the concept of anti vs syn addition determines the stereochemistry of products. Acidity of terminal alkynes feeds into synthesis, where NaNH₂ deprotonation creates acetylide anions for SN2 alkylation, a key strategy for building carbon-carbon bonds.
alkyne naming, IUPAC nomenclature alkynes, diyne, enyne, terminal alkyne acidity, sp hybridisation bond strength, C-H bond dissociation energy, heat of hydrogenation, alkene stability ranking, carbocation stability order, hyperconjugation, anti addition, bromonium ion, syn addition, Markovnikov, pKa terminal alkyne, CHEM 2301, organic chemistry I, Novikov exam 4, University of Minnesota