Alkenes – Comprehensive Study Notes (AS 91165)

Alkenes – Classification + Name

• Alkenes are a homologous series of hydrocarbons that contain at least one C=C double bond.
• Naming rules:
  • Suffix = -ene.
  • The double bond must be on the lowest possible carbon number; the number is stated before "-ene" to indicate the location of the C=C bond.
  • Alkenes are unsaturated (contain fewer than the maximum number of hydrogens).
• Examples from the transcript:
  • Ethene (CH2=CH2)
  • Propene (CH3-CH=CH2)
  • But-1-ene (CH2=CH-CH2-CH3)
  • But-2-ene (CH3-CH=CH-CH3)
• Notes on structure:
  • A C=C bond prevents rotation, fixing the relative positions of groups around the double bond.
  • The location of the double bond is indicated in IUPAC naming by a number (e.g., but-2-ene).

Alkenes – Geometric Isomers

• Geometric isomers arise from the same order of bonding but different spatial arrangement around the C=C bond.
• Why they exist:
  • The double bond does not rotate; thus substituents on each carbon can be arranged on the same side (cis) or opposite sides (trans).
• Key features:
  • Each end of the C=C must have two different groups attached to be able to distinguish cis/trans forms.
  • The C=C angle around the double bond is effectively around 120° due to the sp2 geometry.
  • Geometric isomers may have different physical properties but often similar chemical properties.
• Nomenclature:
  • Use prefixes cis- and trans- before the alkene name, separated by a dash (e.g., cis-but-2-ene, trans-but-2-ene).
• Examples from the transcript:
  • cis-1,2-dichloroethene vs trans-1,2-dichloroethene.
  • But-2-ene examples showing different arrangements of CH3 groups around the double bond.

Alkenes – Physical Properties

• Similar to alkanes in many respects.
• As C-chain increases, melting point and boiling point increase.
• Insoluble in polar solvents like water; alkenes are nonpolar.
• Do not conduct electricity (no free electrons or ions).
• Density is lower than water; alkenes typically float on water.
• Physical state:
  • C2–C4: gases
  • C5+: liquids

Alkenes – Chemical Properties

Combustion

• Alkenes undergo complete and incomplete combustion depending on oxygen availability.
• Complete combustion products:
  • \text{CO}2 + \text{H}2 ext{O}
• Incomplete combustion products can include C, CO, H2O depending on oxygen.
• Alkenes tend to burn with a yellow, sooty flame (less cleanly than alkanes).

Preparation (from Alcohols)

• Alcohols can be dehydrated to form alkenes using concentrated sulfuric acid (H2SO4) with heat.
• Net reaction:
  • Alcohol → alkene + water
  • \text{Alcohol} \xrightarrow{H2SO4, heat} \text{alkene} + \text{H2O}

Zaitsev’s Rule (Say: “the poor get poorer”)

• In elimination reactions, especially with asymmetric substrates, the double bond forms preferentially at the carbon that has fewer attached hydrogens (the more substituted carbon).
• Major product: double bond between carbons with fewer hydrogens (more substituted).
• Minor product: double bond between carbons with more hydrogens (less substituted).
• This rule helps predict the major vs minor products in E2 eliminations from alkenes or haloalkanes.

Addition Reactions (to a C=C bond)

• Common reagents include H2, X2 (Br2, Cl2), H2O (acid-catalyzed hydration), ROH, and HX (HBr, HCl).
• General observation: the double bond is broken, and two atoms/groups are added across the two carbons.
• Markovnikov’s Rule (the “rich get richer”): in the addition of HX or H-OH to an asymmetric alkene, the hydrogen attaches to the carbon that already has more hydrogens, and the other group attaches to the other carbon.
  • E.g., addition of HBr to propene yields 2-bromopropane as the major product and 1-bromopropane as the minor product.
• Additional rules/examples mentioned in the transcript:
  • Hydration of alkenes (dilute acid) yields alcohols following Markovnikov's rule; major product often places the OH on the more substituted carbon.
  • Halogen addition (Br2, Cl2) across the double bond yields vicinal dihalides (e.g., ethene + Br2 → 1,2-dibromoethane).
  • Hydrogenation (H2 with Pt catalyst) across C=C yields alkanes (saturation).
  • Oxidation with oxidants like KMnO4 can convert alkenes to diols (e.g., ethene to ethane-1,2-diol) under certain conditions.
• Specific examples from the transcript:
  • Ethene + Br2 → 1,2-dibromoethane:
    $ext{CH}<em>2= ext{CH}</em>2 + ext{Br}<em>2 ightarrow ext{Br-CH}</em>2 ext{CH}_2 ext{Br}$
  • Ethene + H2O/H+ (hydration) → ethanol:
    $ext{CH}<em>2= ext{CH}</em>2 + ext{H}<em>2 ext{O} ightarrow ext{CH}</em>3 ext{CH}_2 ext{OH}$
  • Ethene + KMnO4 (oxidation) → ethene-1,2-diol (ethane-1,2-diol):
    $ext{CH}<em>2= ext{CH}</em>2 + ext{[O]} <br /> ightarrow ext{HO-CH-CH-OH} ext{ (ethylene glycol)}$
  • Propene + HCl (Markovnikov): major product is 2-chloropropane, minor product is 1-chloropropane.
  • Major: $ext{CH}<em>3- ext{CH(Cl)}- ext{CH}</em>3$
  • Minor: $ext{CH}<em>3- ext{CH}</em>2- ext{CH}_2 ext{Cl}$

Oxidation (of alkenes)

• Oxidation with oxidants like KMnO4 in dilute acid or basic conditions can add across the double bond to form diols (vicinal diols).
• Example: Ethene → ethane-1,2-diol (ethylene glycol) as noted above.

Hydrogen Halide Addition (Markovnikov and exceptions)

• Addition of HX (HCl, HBr) follows Markovnikov’s rule in most cases for asymmetric alkenes.
• Examples from the transcript include: propene + HCl → major product 2-chloropropane; minor product 1-chloropropane.
• This principle also applies to the addition of HBr and other hydrogen halides to alkenes.

Distinguishing Alkanes vs Alkenes (Bromine Water Test)

• Bromine water (Br2(aq)) test is used to distinguish alkanes from alkenes:
  • Alkenes: addition reaction occurs rapidly; orange/brown Br2 is decolorized quickly.
  • Alkanes: substitution with Br2 is slow; oxidation requires UV light to proceed noticeably.
• Observed color change:
  • Alkenes: orange to colorless quickly (rapid, adds across double bond).
  • Alkanes: slow decolorization under UV or light due to substitution mechanism.

Elimination Reactions: Haloalkanes → Alkenes

• Haloalkanes treated with KOH (alcohol) undergo elimination to form alkenes and water + potassium halide:
  • Example: haloalkane (e.g., 2-chloropropane) with KOH (alc) → propene (elimination) + KCl + H2O
• Zaitsev’s rule applies: in asymmetric haloalkanes, the major alkene product forms with the double bond between the carbon that has fewer hydrogens (more substituted alkene).
• Transcript examples:
  • Reaction of but-1-ene with HBr yields two products due to asymmetry of the alkene and reagent; major product corresponds to Markovnikov addition.
  • In elimination from haloalkanes, the more substituted alkene is the major product (Zaitsev).

Reactions (Examples from NCEA Assessments)

• NCEA 2018 Reactions (addition): But-1-ene + HBr (asymmetric) → two products; major: 2-bromobutane; minor: 1-bromobutane; base reasoning via Markovnikov’s rule and hydrogen distribution.
• NCEA 2018 Reactions (substitution vs elimination): 2-chloropropane with KOH under different conditions:
  • Dilute aqueous KOH: substitution to give propan-2-ol (primary or secondary alcohol depending on substrate).
  • Concentrated KOH (alc): elimination to give propene.
  • Reasoning: dilute KOH favors substitution; concentrated KOH favors elimination; symmetry of product affects whether a single product or mixture is formed.
• NCEA 2016 Reactions (addition): Ethene reacts with Br2(aq) and KMnO4(aq) and H2O/H+ and HBr under various conditions; observations include color changes (orange Br2 decolorization) and product types (diol, bromoalcohol, etc.).

Polymerisation (Alkenes to Polymers)

• Polymers are large molecules formed from many repeating units (monomers).
• Addition polymerisation: monomers add to form polymers, breaking C=C bonds in the process.
• Key idea: each monomer is derived from an alkene.
• General pattern:
  • Monomer: \ce{CH2=CH2} (ethylene) → Polymer: polyethene, with repeating unit \ce{(-CH2-CH2-)_n}.
• Example: Propene monomer yields polypropylene (polypropene).
• Naming of addition polymers is based on the monomer: ethene → polyethene; propene → polypropylene; etc.
• Polymerisation in practice builds long chains by repeating the unit from the monomer until n repeats occur.
• Example from transcript: Polystyrene involves the monomer styrene (vinyl benzene) and forms polystyrene via addition polymerisation; monomer is styrene, polymer is polystyrene.
• Monomer identification from polymers:
  • Example: Polystyrene monomer is styrene (C6H5-CH=CH2).
  • The polymer repeating unit is -CH2-CH(C6H5)-.

Test for Unsaturation and Real-World Relevance

• Unsaturation test cases (toluene vs cyclohexene vs cyclohexane) illustrate the presence of C=C bonds (unsaturation) in cycloalkenes vs saturated rings.
• Real-world relevance:
  • Alkenes are central to manufacturing polymers (polyethylene, polypropylene, polystyrene, etc.).
  • Geometric isomerism affects physical properties (melting point, boiling point, crystallinity) and can influence reactivity.
  • Markovnikov’s rule and Zaitsev’s rule guide product distributions in industrial alkene reactions, impacting yield and selectivity of chemicals like alcohols, halides, diols, and dihalides.

Key Formulas and Notations to Remember

• General alkene structure: $ext{R-CH=CH-R'}$
• Markovnikov addition (for HX or H2O across C=C): major product places H on the carbon with more hydrogens; example:
  • Propene + HBr → major: $ext{CH}<em>3- ext{CH(Br)}- ext{CH}</em>3$, minor: $ext{CH}<em>3- ext{CH}</em>2- ext{CH}_2 ext{Br}$
• Hydration (acid-catalyzed) on alkenes:
  • $ext{R-CH=CH-R'} + ext{H}<em>2 ext{O} ightarrow ext{R-CH(OH)-CH</em>2-R'}$ (major product follows Markovnikov’s rule)
• Bromination across C=C:
  • $ext{R-CH=CH-R'} + ext{Br}_2 <br /> ightarrow ext{R-CH(Br)-CH(R')Br}$
• Hydrogenation (addition of H2 to saturate):
  • $ext{R-CH=CH-R'} + ext{H}<em>2 ightarrow ext{R-CH}</em>2- ext{CH}_2 ext{-R'}$
• Oxidation of alkenes to diols (vicinal diols):
  • $ext{R-CH=CH-R'} + ext{[O]} <br /> ightarrow ext{R-CH(OH)-CH(R')OH}$
• Polymer repeating unit (polymer from an alkene):
  • For ethene: $ext{(-CH}<em>2- ext{CH}</em>2-)_n$
  • For styrene (polystyrene): monomer is $ext{CH}<em>2= ext{CH-Ph}$; polymer repeating unit is $ext{(-CH}</em>2- ext{CH(Ph)}-)_n$

Connections to Foundational Principles

• The double bond’s restricted rotation explains geometric isomerism and its impact on physical properties.
• The concept of unsaturation connects to NSA (n= number of repeating units) in polymers and to the degree of hydrogen deficiency in reactions.
• Catalysis and reaction conditions (temperature, solvent, acid concentration) govern substitution vs elimination and the selectivity of addition reactions per Markovnikov’s and Zaitsev’s rules.
• Redox chemistry (e.g., KMnO4 oxidation) demonstrates how functional groups are introduced or altered under oxidative conditions, essential for understanding transformations in organic synthesis.

Practical and Ethical Considerations

• Many of these reactions underpin industrial synthesis of plastics, solvents, and fine chemicals; understanding reaction conditions helps optimize safety, yield, and environmental impact.
• The use of strong acids (e.g., conc H2SO4) and oxidants (KMnO4) requires proper handling and waste management.
• Knowledge of reaction mechanisms and product distributions supports ethical decision-making in chemical manufacturing, including considerations of waste, byproducts, and energy use.