Alkenes, Alkynes, Cis–Trans Isomerism & Addition Reactions – Comprehensive Notes

Alkenes & Alkynes – General Characteristics

• Families within the hydrocarbon series that are unsaturated
  • Alkenes: contain at least one carbon–carbon double bond ($C=C$)
  • Alkynes: contain at least one carbon–carbon triple bond ($C\equiv C$)
• Unsaturation means fewer than the maximum number of H atoms per C; can undergo addition reactions to become alkanes
• React readily with $H_2$, halogens, water, etc., because the $\pi$ bonds are electron-rich and easily broken
• Industrial relevance:
  • Ethyne (acetylene) $C<em>2H</em>2$ burns in O$_2$ with flame temperatures >3300\,^{\circ}\text{C} (welding)
  • Ethene (ethylene) $C<em>2H</em>4$ is a plant hormone that ripens fruit, accelerates cellulose breakdown → wilting & leaf fall

Molecular Geometry & Bond Angles

• Alkene C atoms (each sp$^2$-hybridized)
  • Trigonal planar; bond angle $\approx 120^{\circ}$
• Alkyne C atoms (each sp-hybridized)
  • Linear; bond angle $180^{\circ}$

Identifying Structures

• Double bond present → classify as alkene
• Triple bond present → classify as alkyne

IUPAC Nomenclature – Linear Chains

• Base name comes from corresponding alkane length; replace ending:
  • Alkanes → -ane
  • Alkenes → -ene
  • Alkynes → -yne
• Three-step naming procedure (illustrated with solutions 2-A & 2-B):
  1. Longest chain containing the multiple bond = parent
  • 5 C with double bond ⇒ pentene
  • 6 C with triple bond ⇒ hexyne
  1. Number chain from end closest to the multiple bond
  • Gives 2-pentene, 2-hexyne
  1. Identify & alphabetize substituents + locate them
  • Example: 4-methyl-2-pentene
  • 2-hexyne has no substituents

Naming – Cyclic Alkenes (Cycloalkenes)

• Ring is numbered so the double bond is automatically between C-1 & C-2
• Only locate substituents; omit 1,2 for the double bond position
  • Ex: 3-methylcyclopentene (ring double bond assumed at 1-2)
• Double bond gets priority for lowest numbering before substituents

Practice Examples (Learning Checks & Solutions)

• Recognize structures as cyclohexene, 3,3-dimethylcyclopentene, etc.
• Determine cis/trans when appropriate (details below)

Cis–Trans (Geometric) Isomerism in Alkenes

• Double bond is rigid (no free rotation), locking substituents on fixed sides
• Requires each double-bond C to carry two different groups
  • cis: identical (or similar priority) groups on same side of $C=C$
  • trans: on opposite sides
• Different physical/chemical properties (bp, mp, reactivity)
• Demo: make a double bond with fingers—thumbs on same vs. opposite sides

Example – Butene

• cis-2-butene: CH$_3$ groups on same side
• trans-2-butene: CH$_3$ groups opposite

Biological Significance: Pheromones

• Activity can depend on precise cis/trans arrangement
• Bombykol (silkworm moth sex pheromone) has one cis and one trans C=C; stereochemistry essential for signaling

Naming Conventions

• Prefix cis- or trans- precedes full alkene name with locants
  • cis-1,2-dibromoethene vs. trans-1,2-dibromoethene
  • cis-2,3-dichlorobutene; trans-2-butene, etc.

Addition Reactions of Alkenes & Alkynes

• $\pi$ bonds provide electrons; reagents add across the double or triple bond
• Key commercial process: hydrogenation of veg. oils → saturated fats (margarine, shortening)

Summary Table of Common Additions

• Hydrogenation: $\text{Alkene} + H_2 \xrightarrow[\text{Pt/Ni/Pd}]{\text{catalyst}} \text{Alkane}$
• Hydration: \text{Alkene} + H_2O \xrightarrow[\text{H^+}]{\text{acid}} \text{Alcohol}
  • Follows Markovnikov’s rule: H from H$_2$O adds to C with more H’s; OH to C with fewer H’s

Hydrogenation Details

• Requires finely divided Pt, Pd, or Ni; syn addition of H atoms
• Example exercise: 1-butene + H$_2$ (Pt) → butane
• Oils → higher m.p. solids; partial hydrogenation may produce trans fats (health relevance, though not deeply discussed in slide)

Hydration Details

• Acid-catalyzed (H$2$SO$4$ commonly)
• Example problem (Learning Check 3) expects prediction of the corresponding alcohol following Markovnikov orientation

Worked Problems (Summaries)

1. Identify compounds as alkene/alkyne
   • Provided line structures → answer: A = alkene, B = alkyne
2. Name structures → 4-methyl-2-pentene; 2-hexyne, etc.
3. Cis/Trans naming → cis-2,3-dichlorobutene; trans-2-butene
4. Hydrogenation/ Hydration products → convert unsaturated reagent to saturated/alcohol per rules above

Numerical & Miscellaneous Data

• Welding flame temperature for acetylene-oxygen: >3300\,^{\circ}\text{C}
• Geometry:
  • Alkenes $120^{\circ}$ bond angles (sp$^2$)
  • Alkynes $180^{\circ}$ (sp)

Concept Connections & Implications

• Relation to previous alkane nomenclature: same root + changed suffix; substituent & locant rules identical but multiple bonds get priority for numbering.
• Environmental/Biological: small structural differences (cis vs. trans) create huge functional changes (pheromones, flavor compounds, nutrition—trans fats).
• Industrial: hydrogenation critical in food science; acetylene’s high flame temperature useful in metal fabrication.

Key Equations & Formulas (LaTeX)

• Ethene: $C<em>2H</em>4$
• Ethyne: $C<em>2H</em>2$
• Hydrogenation general: $RCH=CHR' + H<em>2 \xrightarrow{Pt} RCH</em>2CH_2R'$
• Hydration general: $RCH=CH<em>2 + H</em>2O \xrightarrow{H^+} RCH(OH)CH_3$