Comprehensive Study Guide on Alkenes, Dienes, and Alkynes

Functional Groups and the Nature of Alkenes

• Definition of Functional Group: The functional group is the reactive center in a molecule. It determines the general properties of a class of compounds to a large extent. For alkenes, the functional group is the carbon-carbon double bond ($C=C$).

• Unsaturated Hydrocarbons: Alkenes are characterized as unsaturated hydrocarbons because they contain less than the maximum quantity of hydrogen compared to alkanes ($C_nH_{2n+2}$). They can be converted into alkanes by the addition of hydrogen. This unsaturation gives rise to the characteristic chemical properties of the alkene family.

Structure of Ethylene and the Carbon-Carbon Double Bond

• Ethylene ($C_2H_4$): The simplest member of the alkene family. To satisfy the octet rule for both carbon atoms while maintaining neutrality, the carbon atoms share two pairs of electrons, creating a double bond.

• $sp^2$ Hybridization: Carbon makes use of three equivalent hybrid orbitals formed by mixing one $s$ and two $p$ orbitals.

  • Geometry: These $sp^2$ orbitals lie in one plane and are directed toward the corners of an equilateral triangle.

  • Angle: The angle between any pair of orbitals is exactly $120^\circ$. This trigonal arrangement minimizes mutual repulsion between orbitals.

• The Sigma ($\sigma$) Bond: Cylindrically symmetrical bonds formed by the overlap of $sp^2$ orbitals between the two carbons and between carbon and hydrogen atoms.

• The Pi ($\pi$) Bond: Formed by the overlap of the remaining $p$ orbital on each carbon atom. These $p$ orbitals consist of two lobes lying above and below the plane of the $sp^2$ orbitals.

  • Structure: The $\pi$ bond consists of two electron clouds: one above and one below the plane of the atoms.

  • Properties: The $\pi$ bond is weaker than the carbon-carbon $\sigma$ bond due to lesser overlap. Overlap can only occur when all six atoms (two carbons and four hydrogens) lie in the same plane, making ethylene a flat molecule.

• Bond Energy and Length:

  • Total Energy: The total bond energy of the ethylene double bond is $146\text{ kcal/mol}$, which is greater than the carbon-carbon single bond in ethane ($88\text{ kcal/mol}$).

  • Individual strengths: The $\sigma$ bond in ethylene is estimated at $95\text{ kcal/mol}$, and the $\pi$ bond is estimated at $51\text{ kcal/mol}$.

  • Distance: The $C=C$ distance is $1.34\text{ …}$, compared to $1.53\text{ …}$ in ethane. The carbon-hydrogen distance in ethylene is $1.103\text{ …}$.

Hybridization, Orbital Size, and Bond Strengths

• $s$ Character and Bond Length: As the $s$ character of a hybrid orbital increases ($sp^3$ to $sp^2$ to $sp$), the effective size of the orbital decreases, and the resulting bond becomes shorter.

  • Ethane ($sp^3$): $C-H$ bond is $1.112\text{ …}$.

  • Ethylene ($sp^2$): $C-H$ bond is $1.103\text{ …}$.

  • Benzene ($sp^2$): $C-H$ bond is $1.10\text{ …}$.

  • Acetylene ($sp$): $C-H$ bond is $1.079\text{ …}$.

• Bond Dissociation Energies: Shorter bonds are generally stronger.

  • Ethylene $C-H$: $108\text{ kcal/mol}$.

  • Ethane $C-H$: $98\text{ kcal/mol}$.

  • Propylene $C-C$ (single bond): $92\text{ kcal/mol}$, which is greater than ethane's $88\text{ kcal/mol}$.

The Butylenes and Geometric Isomerism

• Isomeric Structures of $C_4H_8$:

  1. 1-Butene: $CH_3CH_2CH=CH_2$ (b.p. $-6^\circ\text{C}$, m.p. $-195^\circ\text{C}$).

  2. cis-2-Butene: Methyl groups on the same side (b.p. $+4^\circ\text{C}$, m.p. $-139^\circ\text{C}$).

  3. trans-2-Butene: Methyl groups on opposite sides (b.p. $+1^\circ\text{C}$, m.p. $-106^\circ\text{C}$).

  4. Isobutylene: $(CH_3)_2C=CH_2$ (b.p. $-7^\circ\text{C}$, m.p. $-141^\circ\text{C}$).

• Hindered Rotation: Conversion between cis and trans isomers requires rotating the carbon-carbon double bond, which necessitates breaking the $\pi$ bond. Breaking this bond requires approximately $70\text{ kcal/mol}$, creating a high energy barrier that prevents interconversion at room temperature.

Nomenclature (IUPAC Rules)

1. Parent Chain: Select the longest continuous chain containing the double bond. Change the suffix from -ane to -ene.

2. Numbering: Number from the end nearest the double bond. Indicate the double bond position by the number of the first doubly bonded carbon.

3. Substituents: Indicate the positions of alkyl groups by numbers.

4. Special Groups:

   • Vinyl Group: $CH_2=CH-$

   • Allyl Group: $CH_2=CH-CH_2-$

5. Priority: In alcohols containing a double bond (alkenols), the -ol suffix takes priority over -ene and is given the lower number.

Physical Properties of Alkenes

• Solubility and Density: Insoluble in water; soluble in non-polar solvents (benzene, ether, chloroform). Less dense than water.

• Boiling Points: Rise by $20\text{-}30^\circ\text{C}$ for each added carbon. Branching lowers the boiling point. The boiling point of an alkene is close to that of the corresponding alkane.

• Polarity: Alkenes are weakly polar. The $\pi$ electrons are easily displaced, giving larger dipole moments than alkanes. Alcohols/alkyl groups are electron-releasing toward the double bond.

  • cis-2-Butene: Small dipole moment due to methyl groups on one side ($b.p. +4^\circ\text{C}$).

  • trans-2-Butene: Zero dipole moment due to cancellation ($b.p. +1^\circ\text{C}$).

The Organic Chemistry of Vision

• Rhodopsin: A conjugated protein in mammalian rod cells containing 11-cis-retinal, an unsaturated aldehyde derived from vitamin A.

• Visual Process: When light strikes the eye, it brings energy to rhodopsin, effectively opening the $C=C$ bond to allow rotation. This transforms 11-cis-retinal into 11-trans-retinal.

• Biological Trigger: The change in shape (the molecule straightens out) alters the conformation of the protein, affecting membrane permeability and allowing $Ca^{2+}$ ions to pass. This triggers the nerve impulse to the brain. The eye can detect as few as five photons.

Preparation of Alkenes (Elimination Reactions)

1. Dehydrohalogenation of Alkyl Halides

Reaction with hot concentrated alcoholic solution of a strong base (like $KOH$).

• Equation: $R_2CH-CXR_2 + KOH \xrightarrow{\text{alcohol}} R_2C=CR_2 + KX + H_2O$

• Reactivity Order: $3^\circ > 2^\circ > 1^\circ$

2. Dehydration of Alcohols

Catalyzed by acid (e.g., $H_2SO_4$ or $H_3PO_4$) and heat.

• Equation: $R_2C(OH)-CHR_2 \xrightarrow{\text{acid, heat}} R_2C=CR_2 + H_2O$

• Reactivity Order: $3^\circ > 2^\circ > 1^\circ$

3. Dehalogenation of Vicinal Dihalides

Treatment with zinc.

• Equation: $-CX-CX- + Zn \rightarrow -C=C- + ZnX_2$

4. Reduction of Alkynes

• Syn-addition: $(BH_3)_2$ or Lindlar catalyst ($H_2$) yields cis-alkenes.

• Anti-addition: $Na$ or $Li$ in liquid $NH_3$ yields trans-alkenes.

5. Carbonyl Conversions

• Wittig Reaction: Uses a phosphorus ylide (phosphorane) to replace $C=O$ with $C=C$. Driven by the formation of $Ph_3PO$.

• Peterson Reaction: Uses $\alpha$-silyl carbanions to form $\beta$-hydroxy silanes, which eliminate to form alkenes.

• Shapiro Reaction: Converts ketones (via tosylhydrazones) to alkenes using strong base via a vinyl anion intermediate.

Mechanisms of Elimination: E2 and E1

The E2 Mechanism (Elimination, Bimolecular)

• Kinetics: Second-order (Rate = $k[RX][:B]$).

• Process: A single-step reaction where the base pulls a proton from the $\beta$-carbon while the halide ion departs and the double bond forms simultaneously.

• Evidence: No rearrangements; large hydrogen isotope effect ($k_H/k_D \approx 7$); element effect (reactivity $I > Br > Cl > F$).

• Orientation (Saytzeff's Rule): The preferred product is the more stable alkene (the one with the greater number of alkyl groups attached to the doubly bonded carbons).

The E1 Mechanism (Elimination, Unimolecular)

• Kinetics: First-order (Rate = $k[RX]$).

• Process: Two steps. Step 1: Slow heterolysis to form a carbocation. Step 2: Fast loss of a proton to a base.

• Evidence: No primary hydrogen isotope effect; rearrangements are common (1,2-shifts of methyl or hydrogen); reactivity order follows carbocation stability ($3^\circ > 2^\circ > 1^\circ$).

Addition Reactions of Alkenes

1. Catalytic Hydrogenation

Addition of $H_2$ in the presence of $Pt, Pd,\text{ or }Ni$. It is exothermic ($\Delta H \approx -30\text{ kcal/mol}$ per double bond).

2. Electrophilic Addition of Hydrogen Halides ($HX$)

• Mechanism: Step 1: Transfer of $H^+$ to the alkene to form a carbocation (slow). Step 2: Combination of the carbocation with a halide ion (fast).

• Markovnikov's Rule: In the addition of an acid to an alkene, the hydrogen attaches to the carbon holding the greater number of hydrogens (forming the more stable carbocation intermediate).

• Reactivity: $3^\circ > 2^\circ > 1^\circ$ carbocation formation.

3. Peroxide Effect (Anti-Markovnikov Addition)

Applies only to $HBr$. In the presence of organic peroxides, a free-radical chain mechanism occurs where the bromine atom adds first to the terminal carbon to form the more stable secondary or tertiary radical.

4. Halogen Addition ($X_2$)

Addition of $Cl_2$ or $Br_2$ involves the formation of a cyclic halonium ion intermediate, leading to anti-addition.

5. Solvomercuration-Demercuration

• Oxymercuration: Reaction with mercuric acetate and water to form a hydroxymercurial compound.

• Demercuration: Reduction with $NaBH_4$ replacement of $-HgOAc$ with $-H$.

• Result: Markovnikov addition of water without rearrangement.

6. Hydroboration-Oxidation

Reaction with diborane $(BH_3)_2$ followed by alkaline $H_2O_2$.

• Result: Anti-Markovnikov, syn-addition of water.

7. Ozonolysis

Cleavage of the double bond by ozone followed by hydrolysis in the presence of zinc to yield aldehydes and ketones. Used for structure determination.

Dienes: Conjugation and Resonance

• Classes of Dienes:

  1. Conjugated: Alternating double and single bonds ($-C=C-C=C-$).

  2. Isolated: Double bonds separated by more than one single bond.

  3. Cumulated (Allenes): Double bonds sharing a single carbon ($C=C=C$).

• Stability: Conjugated dienes are more stable than isolated dienes. The heat of hydrogenation for 1,3-butadiene is $57\text{ kcal/mol}$ (expected $60\text{ kcal/mol}$), indicating a resonance energy of about $3\text{ kcal/mol}$.

• The Allyl Radical and Cation: Resonance hybrids where electrons are delocalized over three carbon atoms.

  • Allyl Radical Resonance Energy: Estimated at $10\text{ kcal/mol}$.

  • Bonding: p orbitals of all three carbons overlap to form a continuous p cloud.

• 1,4-Addition: Conjugated dienes undergo both 1,2-addition and 1,4-addition.

  • Kinetic Control: At low temperatures (e.g., $-80^\circ\text{C}$), the 1,2-adduct predominates because it forms faster.

  • Thermodynamic Control: At higher temperatures (e.g., $40^\circ\text{C}$), equilibrium is reached and the 1,4-adduct predominates because it is the more stable isomer.

Alkynes ($C_nH_{2n-2}$)

• Structure of Acetylene ($C_2H_2$):

  • Hybridization: $sp$ hybridized carbon.

  • Geometry: Linear molecule ($180^\circ$ angles).

  • Bonds: One strong $\sigma$ bond and two $\pi$ bonds forming a cylindrical sheath.

  • Length: $C\equiv C$ distance is $1.21\text{ …}$.

• Acidity: Hydrogens attached to triply bonded carbons (terminal alkynes) are significantly more acidic than those on alkenes or alkanes.

  • Acidity Order: $H_2O > ROH > HC\equiv CH > NH_3 > RH$.

  • Formation of Acetylides: Acetylene reacts with $NaNH_2$ or Grignard reagents to form metal acetylides ($RC\equiv C^-M^+$).

• Preparation:

  • Dehydrohalogenation of vicinal dihalides.

  • Reaction of metal acetylides with primary alkyl halides ($RC\equiv C^-Li^+ + R'X \rightarrow RC\equiv CR' + LiX$).

• Reactions: Electrophilic addition occurs in two stages. Hydration of alkynes ($H_2O, H_2SO_4, HgSO_4$) yields ketones (except for acetylene, which yields acetaldehyde).