Chapter 3: Unsaturated Hydrocarbons

13-1 Unsaturated Hydrocarbons

  • Definition: a hydrocarbon that contains one or more carbon–carbon multiple bonds (double, triple, or both).
  • Physical properties: similar to saturated hydrocarbons; chemical properties are distinct and typically more reactive due to C=C or C≡C bonds.
  • Functional group: a part of a molecule responsible for most of its chemical reactions; in unsaturated hydrocarbons, the C=C and C≡C bonds are the key functional groups.
  • Examples of major subclasses discussed: alkenes, cycloalkenes, alkynes, and aromatic hydrocarbons.

13-2 Characteristics of Alkenes and Cycloalkenes

  • Alkenes: acyclic unsaturated hydrocarbons containing one or more C=C double bonds.
    • Functional group: C=C.
    • Naming: end with "-ene".
    • General formula: CnH{2n}
    • Simplest alkenes: ethene (C2H4), propene (C3H6).
    • Common names: ethylene (ethene), propylene (propene).
  • Carbon geometry in alkenes: about C=C, each carbon is trigonal planar (not tetrahedral as in alkanes).
  • Cycloalkenes: cyclic unsaturated hydrocarbons with at least one C=C within the ring.
    • General formula for cycloalkenes with one double bond: CnH{2n-2}
    • Simplest cycloalkene: cyclopropene (C3H4).
  • Cycloalkenes with more than one double bond exist but are not common.

13-3 Nomenclature for Alkenes and Cycloalkenes

  • IUPAC rules for alkenes and cycloalkenes:
    • Rule 1: Replace the -ane ending of the parent alkane with -ene.
    • Rule 2: Select as the parent chain the longest continuous chain containing both carbons of the double bond. If the double bond is equidistant from ends, number from the end nearer a substituent.
    • Rule 3: Number the chain so the double bond gets the lowest possible numbers; start from the end nearest to the double bond.
    • Rule 4: Give the position of the double bond as a single number (the lower-numbered carbon of the double bond).
    • Rule 5: Double bonds outrank alkyl groups and halogens in determining the main chain; exception for alcohol OH group.
    • Rule 6: Use suffixes -diene, -triene, -tetrene, etc., for multiple C=C bonds.
    • Rule 7: For unsubstituted cycloalkenes with one double bond, do not use a number to locate the double bond (assumed between C1 and C2).
    • Rule 8: For substituted cycloalkenes with one double bond, number so that substituent encountered first has the lower number.
    • Rule 9: For cycloalkenes with multiple double bonds, assign one as 1–2 and give other doubles the lowest possible numbers.
  • Example: 3-methyl-1,5-hexadiene; cyclohexene; 1,4-cyclohexadiene; 5-chloro-1,3-cyclohexadiene.
  • Optional: some slides show common names and line-angle representations to aid quick recognition.

13-4 Line-Angle Structural Formulas for Alkenes

  • Line-angle formulas represent carbon skeletons with lines for bonds and vertices for carbon atoms.
  • Examples given for 3–6 carbon alkenes (acyclic) such as propene, 1-butene, 1-pentene, 1-hexene.
  • Representative line-angle formulas for substituted alkenes: 3,5-dimethyl-1-hexene; 2-ethyl-3-methyl-1-pentene.
  • Dienes: line-angle representations for conjugated dienes like 1,4-pentadiene; 2-methyl-1,3-butadiene.

13-5 Constitutional Isomerism in Alkenes

  • Constitutional (structural) isomers: same molecular formula, different connectivity.
  • There are more alkene isomers than alkane isomers for a given carbon count.
  • Types:
    • Positional isomers: same carbon skeleton, different location of C=C.
    • Skeletal (chain) isomers: different carbon skeletons with possibly different hydrogen placements.
  • Visual comparison shows the number of possible isomers increases with carbon count (example: four- and five-carbon systems).

13-6 Cis–Trans Isomerism in Alkenes

  • Stereoisomers with a rigid C=C bond: cis (Z) and trans (E) configurations.
  • Conditions for cis/trans:
    • Each double-bonded carbon must have two different substituents.
  • Definitions:
    • Cis: identical or analogous substituents on the same side of the double bond.
    • Trans: identical or analogous substituents on opposite sides.
  • Examples: cis-2-butene vs. trans-2-butene; 1-butene and 2-methylpropene illustrate stereochemistry.
  • Important: some alkenes with multiple double bonds have more complex stereochemistry (E/Z notation used for multi-alkenes).
  • Notation: E (entgegen) and Z (zusammen) provide a general system based on CIP priority (see 13-25 below).
  • Oleic acid example: common cis configuration in natural products.
  • Ambiguity: when comparing analogous substituents, cis/trans can be ambiguous if substituents are not clearly distinct.

13-7 Naturally Occurring Alkenes

  • Terpenes: large class of natural products built from isoprene units (2-methyl-1,3-butadiene) as C5H8 building blocks.
  • Isoprene = 2-methyl-1,3-butadiene.
  • Terpenes are widely distributed in nature (>22,000 identified).
  • Examples shown include beta-carotene, zingiberene, racemic and enantiomeric limonene derivatives, alpha-farnesene, etc.

13-8 Physical Properties of Alkenes and Cycloalkenes

  • Solubility: not soluble in water; soluble in nonpolar solvents.
  • Density: less dense than water.
  • Phase behavior (based on carbon count):
    • 2–4 carbons: gases at room temp.
    • 5–17 carbons, one C=C: liquids.
    • >17 carbons: solids.
  • Dipole moments illustrate substituent effects:
    • Examples with Cl substituent show how dipole moments add up across C=C to give overall molecular dipole.
    • Methyl groups donate electron density to the double bond, affecting polarity.

13-9 Preparation of Alkenes

  • Not detailed in the transcript beyond general prep topics; typically includes dehydration of alcohols, elimination reactions, and dehydrohalogenation strategies.

13-10 Chemical Reactions of Alkenes

  • Addition reactions occur at the C=C; the π bond is broken and two σ bonds form.
  • Alkenes are electron-rich; they react with electrophiles rather than nucleophiles.
  • Types of additions:
    • Symmetrical addition: identical atoms/groups added to each carbon of the double bond (e.g., hydrogenation, halogenation).
    • Unsymmetrical addition: different atoms/groups added to each carbon (e.g., hydrohalogenation, hydration, sulfuric acid addition).
  • Hydrogenation (addition of H2):
    • Produces alkanes; uses metal catalysts (Pt, Pd, Rh, Ni).
    • Mechanistic steps (summary): surface H2 dissociation, alkene adsorption, transfer of H to each carbon.
    • Hydrogenation is a syn addition (both hydrogens add to the same face of the double bond).
  • Halogenation (X2, X = Cl, Br):
    • Rapid at room temperature; forms vicinal dihalides via anti addition.
    • Bromine in water decolorizes with C=C to indicate unsaturation.
  • Addition of hydrogen halides (HX):
    • Electrophilic H adds first (polar H–X bond), followed by X− addition.
    • Markovnikov’s rule: addition proceeds so that the hydrogen adds to the carbon with more hydrogens, and the halogen to the carbon with fewer hydrogens.
    • Examples:
    • Propene + HBr → 2-bromopropane (major).
    • 1-butene + HBr → 2-bromobutane (major).
  • Hydration (acid-catalyzed addition of water):
    • Markovnikov’s rule governs addition in aqueous acid (e.g., H2SO4/H2O).
    • Carbocation intermediates are formed and captured by water, yielding alcohols after workup.
    • Example: propene hydration gives 2-propanol; 2-methylpropene hydration gives tert-butanol.
  • Hydration of alkenes can also be described via formation of alkyl hydrogen sulfates in concentrated H2SO4, which hydrolyze to give alcohols.
  • Summary table (reactions):
    • Catalytic hydrogenation: alkene to alkane with H2 and metal catalyst.
    • Addition of HX: alkene to alkyl halide with Markovnikov regioselectivity.
    • Halogenation: X2 adds anti to yield vicinal dihalide.
    • Sulfuric acid addition and hydration: formation of alkyl hydrogen sulfates or alcohols, with Markovnikov control.

13-11 Polymerization of Alkenes: Addition Polymers

  • Definition: polymerization via addition of alkenes to form long chains, with no small-molecule byproducts.
  • Key terms:
    • Monomer: the starting alkene unit.
    • Polymer: high-molecular-weight material made from many monomer subunits.
    • -mer suffix denotes polymer units (dimer, trimer, tetramer, …).
  • Major polymer families from ethene and substituted ethenes:
    • Polyethylene (HDPE, LDPE, LLDPE) with varying degrees of branching; properties range from rigid to flexible.
    • Polypropylene, poly(vinyl chloride) (PVC), polystyrene are common addition polymers.
    • Substituted-ethene addition polymers: examples include vinyl chloride (PVC) and styrene.
  • Copolymers: polymers made from two different monomers (A and B) such as Saran Wrap (vinyl chloride and 1,1-dichloroethene) and styrene–butadiene rubber (SB rubber).
  • Structural representation: repeat unit notation and simplified line diagrams to illustrate polymer chains.

13-12 Alkynes

  • Alkynes: hydrocarbons with a C≡C triple bond.
    • General formula for noncyclic alkynes with one triple bond: CnH{2n-2}
    • Ethyne (acetylene) is the simplest alkyne (C2H2).
    • Terminal (monosubstituted) vs internal alkynes (disubstituted or more).
  • Structure and bonding:
    • Triple bond consists of two π bonds and one σ bond.
    • Both carbons are sp hybridized; linear geometry.
  • Cycloalkynes: small-ring cycloalkynes are unstable; cyclononyne is the smallest stable cycloalkyne.
  • Nomenclature (IUPAC): replace -ane with -yne; longest chain containing both triple-bond carbons; number to give the lowest locant for the triple bond; diynes/triynes for more than one triple bond.
  • Common names: acetylene (ethyne), methylacetylene (propyne), dimethylacetylene (increasing alkyl substitutions).
  • Isomerism: no cis/trans for most alkynes due to linearity; positional and skeletal isomerism exist.
  • Physical properties: similar to alkanes and alkenes; soluble in organic solvents, not in water; lower density than water; gas at room temperature for small alkynes.
  • Reactions: alkynes undergo addition reactions similar to alkenes, but can be halted at the alkene stage (via selective catalysts, e.g., Lindlar catalyst) to yield cis alkenes (syn hydrogenation).
  • Hydrogenation to alkanes (full hydrogenation) using Pt, Pd, Ni, Rh; hydrogenation to alkenes can be controlled with Lindlar catalyst to yield cis-alkenes.
  • Hydrogenation to trans alkenes via sodium in liquid ammonia (metal-ammonia reduction).
  • Electrophilic additions to alkynes with HX yield vinyl halides; with excess HX yield geminal dihalides.
  • Hydration of alkynes yields ketones or aldehydes via enol–keto tautomerization (acidic medium).
  • Halogenation of alkynes with Cl2 or Br2 yields dihaloalkenes and tetrahaloalkanes with sequential additions.
  • Tables summarize these reactions and typical products (e.g., 2,2-dibromopropane from propyne with HBr).

13-13 Aromatic Hydrocarbons

  • Aromatic hydrocarbons (arenes): unsaturated cyclic hydrocarbons based on a benzene ring; unusually stable due to conjugation.
  • Benzene (C6H6) as the prototypical aromatic; benzene does not undergo typical alkene-type additions; instead, substitution reactions predominate.
  • Kekulé structure vs. resonance:
    • Early models proposed alternating single/double bonds; benzene is better described as a resonance hybrid with delocalized π electrons over the ring.
    • Bond lengths in benzene are intermediate between typical single and double bonds, reflecting delocalization.
  • Substitution chemistry: substitution on the benzene ring rather than addition; examples include halogenation, Friedel–Crafts alkylation (or acylation).
  • Nomenclature and derivatives:
    • Monosubstituted benzenes named by prefix substituent + benzene (phenyl group when benzene is considered a substituent: phenyl group C6H5–).
    • Common substituent prefixes: o-, m-, p- for ortho-, meta-, para- disubstituted benzenes.
    • Xylenes: three isomers (o-, m-, p-) from 1,2-disubstitution with two methyl groups on benzene.
  • Special cases: when a substituent is a common name (e.g., toluene), the compound is named as a derivative of that parent; alphabetical ordering governs listing of substituents when neither substituent has a special name.
  • Common rearrangements and named derivatives: styrene, acetophenone, benzaldehyde, benzoic acid, anisole, phenol, aniline, etc.
  • Fused-ring aromatics: compounds containing two or more fused rings.

13-14 Nomenclature for Aromatic Hydrocarbons

  • Monosubstituted benzenes: substituent as prefix; some derivatives treat the ring as a parent with the substituent attached at position 1.
  • Disubstituted benzenes: prefixes o-, m-, p- to indicate positions; or use alphabetical priority to assign position 1 for substituents.
  • Trisubstituted or more: use numbers to indicate substituent positions; common examples include 1,2-dichlorobenzene, 1,4-dichlorobenzene, 1,3-dichlorobenzene, etc.
  • When the substituent group has a common name (e.g., methyl, ethyl, tert-butyl, -toluen- derivatives), the ring is named as a substituent of that group and vice versa; e.g., 4-bromotoluene instead of 1-bromo-4-methylbenzene.
  • Alphabetical order matters for listing substituents (ignoring prefixes like di-, tri-).

13-15 Properties of and Sources for Aromatic Hydrocarbons; Chemical Reactions

  • Physical properties: meling points and boiling points depend on symmetry and dipole; benzene derivatives are generally insoluble in water; dense than water; aromatic rings contribute to stability.
  • Chemical properties: tendency toward substitution rather than addition due to aromatic stabilization.
  • Common reactions: halogenation (requires Lewis acid catalysts like FeBr3/FeCl3; substitution only), Friedel–Crafts alkylation (requires Lewis acids to generate carbocations), and related electrophilic aromatic substitution reactions.

13-16 Fused-Ring Aromatic Hydrocarbons

  • Fused rings: compounds with two or more rings sharing carbon atoms.
  • Many fused aromatics are solids at room temperature.

25–26: Key Concepts and Notation (CIP, E/Z, Heats, etc.)

  • CIP priority rules (summary):
    1) Higher atomic number at first point of difference wins.
    2) If tied, compare next atoms outward from the point of attachment.
    3) Evaluate substituents atom-by-atom outward from the point of attachment.
    4) Treat multiple bonds as if duplicated substituents when ranking.
  • E/Z nomenclature (for alkenes with stereochemistry):
    • E (entgegen): higher-priority substituents on opposite sides.
    • Z (zusammen): higher-priority substituents on the same side.
  • Example: In RCH=CHR' systems, CIP priorities determine E or Z configuration; the CIP table (Rule 5–Rule 9) applies to labeling.
  • The table of typical reactions and products (summarized):
    • Hydrogenation of alkenes to alkanes (Pt, Pd, Rh, Ni catalysts).
    • Addition of HX to alkenes (Markovnikov regioselectivity).
    • Halogenation (anti addition) to give vicinal dihalides.
    • Hydration (acid-catalyzed) to form alcohols.
    • Sulfuric acid addition to form alkyl hydrogen sulfates and subsequent hydrolysis to alcohols.
    • Addition polymerization of alkenes (polymerization terminology: monomer → polymer; ethene as a key monomer).
  • Naturally occurring alkenes and their roles (Terpenes):
    • Terpenes are built from isoprene units; natural odorants and fragrances are often terpenoids.

29–32: Physical Properties, Stability, and Heats of Combustion

  • Stability trends in alkenes:
    • Degree of substitution: more highly substituted alkenes are generally more stable.
    • Van der Waals strain: cis alkenes experience steric hindrance, reducing stability relative to trans alkenes.
    • Substituent effects: alkyl groups stabilize C=C by donation of electron density.
  • Heats of combustion: used to compare stability; lower energy of combustion corresponds to greater stability for a given isomer.
  • General trend: trans-configured and more substituted alkenes tend to be more thermodynamically stable than their cis and less substituted counterparts.

37–41: Mechanistic Aspects of Alkene Reactions

  • Addition reactions involve the π bond; result in formation of two new σ bonds and loss of the C=C π bond.
  • Syn vs anti addition:
    • Hydrogenation on typical metals is syn-addition (both hydrogens added to same face).
    • Halogenation proceeds via anti-addition (anti stereochemistry).
  • Catalysis and stereochemistry:
    • Lindlar catalyst (Pd on CaCO3 poisoned with PbO2 and quinoline) enables syn hydrogenation of alkynes to cis-alkenes.
    • Metal-ammonia reduction can yield trans-alkenes from alkynes.

42–43: Special Addition Reactions and Problem-Solving

  • Hydrogenation of alkynes to alkanes or alkenes (via selective catalysts).
  • Hydration and hydrohalogenation provide practical routes to alcohols and haloalkanes, respectively.
  • Halogenation and Markovnikov additions offer predictable regiochemistry that can be exploited in synthesis.

48–55: Practice Problems and Applications

  • Examples include determining major products of Markovnikov additions, writing structures for alkynes and alkenes, and identifying stereochemical configurations (cis/trans).
  • Practice problems emphasize: identifying the major product by Markovnikov’s rule, applying CIP to assign E/Z, and drawing line-angle or condensed structures.

60–70: Recap of Hydrocarbons and Reactions (Aromatic Focus)

  • Alkanes, cycloalkanes, alkenes, alkynes, and aromatics form the major classes of hydrocarbons.
  • Aromatics emphasize substitution chemistry rather than additions; resonance stabilization and delocalization are key concepts for benzene and derivatives.
  • Common named derivatives: toluene, phenol, aniline, anisole, styrene, benzaldehyde, benzoic acid, acetophenone, etc.
  • Ortho-, meta-, para- nomenclature is used for disubstituted benzenes to denote relative positions of substituents.

71–76: Additional Alkyne Transformations (Selected Highlights)

  • Hydrogenation of alkynes to alkanes and to cis-alkenes via Lindlar catalyst; trans alkenes via metal-ammonia (Na/NH3) reduction.
  • Hydration of alkynes yields carbonyl compounds through enol–keto tautomerism; following Markovnikov additivity.
  • Halogenation of alkynes to dihalides and tetrahalides depending on equivalents of halogen.

83–89: Aromatic Nomenclature and Properties (Extended)

  • Substituted benzenes: naming rules based on substituent positions; use prefixes o-, m-, p- or apply numerals to locate substitutions.
  • The benzene ring can be treated as a substituent (phenyl group) or the parent (as in toluene or xylenes).
  • Physical properties, symmetry, and delocalization contribute to the distinctive behavior of arenes.

106–113: Practice and Problem Sets (Strategy)

  • Formulas and naming exercises for alkenes, alkynes, and arenes.
  • Distinguish between alkene, diene, and triene from molecular formulas such as C5H10, C6H12, etc.
  • Determine whether cis–trans isomerism exists for given structures and draw the isomers when applicable.
  • Identify whether given reactions are additions or substitutions and write balanced equations (including catalysts where indicated).

115–118: Advanced Nomenclature and Isomerism Problems

  • Assign IUPAC names for disubstituted benzenes including ortho-, meta-, para- prefixes.
  • Distinguish benzene as a substituent vs. the parent ring in complex molecules.
  • Draw skeletal or condensed structures for various unsaturated hydrocarbons and determine their common or IUPAC names.

119–123: Summary of Key Takeaways

  • Unsaturated hydrocarbons include alkenes, alkynes, and arenes, each with characteristic reactivity patterns centered on C=C or C≡C bonds and/or delocalized π systems.
  • IUPAC nomenclature for these compounds relies on longest chain containing the multiple bond, lowest locant for the multiple bond, and correct suffixes (-ene, -yne).
  • Stereochemistry (E/Z) is essential for multi-substituted alkenes; Markovnikov’s rule guides addition regioselectivity for HX and water additions.
  • Reactions of alkenes are dominated by additions to the double bond (hydrogenation, halogenation, hydrohalogenation, hydration); alkyne chemistry extends these patterns with selective stop points (cis alkenes via Lindlar catalyst, trans alkenes via Na/NH3).
  • Aromatic hydrocarbons resist addition and favor substitution; benzene’s resonance stabilizes the ring, influencing reactivity and directing electrophilic substitutions.
  • Polymers from alkenes (PE, PP, PVC, PS, etc.) illustrate the industrial importance of addition polymerization, with copolymers expanding material properties.
  • Nomenclature conventions for arenes (ortho/ meta/ para) and substituent naming cover a wide range of benzene derivatives encountered in organic synthesis.

Notes and examples referenced in the transcript include: isoprene as the building block of terpenes; terpenes like

beta-carotene, zingiberene, limonene, and alpha-farnesene; Markovnikov’s rule illustrated with 1-butene and propene reactions; hydration of 2-methylpropene to tert-butyl alcohol; and the various polymer examples (HDPE, LDPE, PVC, polystyrene).

Important formulas and rules (quick reference)

  • Alkenes: CnH{2n}
  • Cycloalkenes: CnH{2n-2}
  • IUPAC rules (highlights):
    • Suffix -ene for alkenes; -yne for alkynes.
    • Longest chain containing the multiple bond as parent.
    • Double bond locant is the lowest possible.
    • For cycloalkenes with one C=C, the bond is assumed between C1 and C2 unless specified.
  • Markovnikov’s rule: in HX additions to alkenes, H adds to the carbon with more hydrogens; X adds to the carbon with fewer hydrogens.
  • E/Z nomenclature: E (entgegen) = opposite; Z (zusammen) = together; assign by CIP priorities.
  • Reactions overview (additions):
    • Hydrogenation: ext{R}2C=CR2 + H2 ightarrow ext{R}2CH-CHR_2, with Pt, Pd, Rh, or Ni catalyst; syn addition.
    • Halogenation: ext{R}2C=CR2 + X_2
      ightarrow X-CH-CHX; anti addition; bromination/decolorization test with Br2.
    • HX addition: ext{RCH=CHR'} + HX
      ightarrow ext{RCH(X)–CH_2R'} with Markovnikov orientation.
    • Hydration: ext{RCH=CHR'} + H2O ightarrow ext{RCH2–CHOH–R'}
      ightarrow ext{RCH(OH)–CH_2R'} (tautomerization to alcohol).
  • Alkyne transformations: hydrogenation to alkanes or to cis-alkenes (Lindlar catalyst); sodium in liquid ammonia for trans-alkenes; hydration yields ketones/aldehydes; halogenation yields dihaloalkenes.
  • Aromatic chemistry: electrophilic substitution (halogenation with Lewis acids; Friedel–Crafts alkylation/acylation); resonance stabilizes benzene; substituent effects govern directing outcomes.