Structure and Synthesis of Alkenes: Elimination (Chapter 7 Study Notes)

Structure and Synthesis of Alkenes: Elimination

Bonding, Bond Strength, and Bond Length

Alkenes are hydrocarbons containing carbon-carbon double bonds. The double bond consists of one sigma (σ) bond and one pi (π) bond, resulting from the overlap of sp2 hybridized orbitals and unhybridized p orbitals, respectively.

• Bond Dissociation Energies:

  • C=C bond: ~146 kcal/mol (~611 kJ/mol)

  • C–C (σ bond): ~83 kcal/mol (~347 kJ/mol)

  • C=C (π bond): ~63 kcal/mol (~264 kJ/mol)

• Bond Length: The stronger the bond, the shorter the bond length.

  • C–C bond: 1.54 Å

  • C=C bond: 1.33 Å

  • $1~\text{Å} = 10^{-10}~\text{m}$

• Geometric Isomerism: The double bond restricts rotation, allowing for geometric (cis/trans) isomers.

Example: Cis and trans isomers of 2-butene.

Degree of Unsaturation

The degree of unsaturation indicates the total number of π bonds and rings in a molecule. It helps determine the structure from a molecular formula.

• Saturated Compound: Contains only single bonds (e.g., alkanes).

• Unsaturated Compound: Contains one or more double/triple bonds or rings (e.g., alkenes, alkynes).

• Formula for Degree of Unsaturation:

  • $\text{Degree of unsaturation} = (\text{number of C}) + 1 - \frac{1}{2}(\text{number of H and halogens}) + \frac{1}{2}(\text{number of N})$

  • Oxygen is ignored in the calculation.

• Interpretation:

  • One π bond = one degree of unsaturation

  • One ring = one degree of unsaturation

Example: C4H8Br has degree of unsaturation = 1.

E/Z Nomenclature of Geometric Isomers

Geometric isomers of alkenes are described using the E/Z system, which is based on the Cahn-Ingold-Prelog priority rules.

• Cis/Trans System: Used when each carbon of the double bond has a hydrogen.

• E/Z System: Used for more complex cases; unambiguous and current standard.

  • E (Entgegen): Higher priority groups on opposite sides of the C=C bond.

  • Z (Zusammen): Higher priority groups on the same side of the C=C bond.

Example: 2-bromo-2-butene: E and Z isomers based on group priorities.

Nomenclature of Alkenes

Alkene nomenclature follows IUPAC rules, with the longest chain containing the double bond as the parent. Numbering starts from the end nearest the double bond.

• Steps:

  1. Identify the longest carbon chain containing the double bond.

  2. Number the chain to give the double bond the lowest possible number.

  3. Name substituents and their positions.

  4. Indicate the position of the double bond.

  5. Use E/Z notation for geometric isomers when necessary.

Example: 2-methyl-2-butene, 3-hexene, cyclohexene.

Stability of Alkenes

Alkene stability is influenced by the number of alkyl groups attached to the double-bonded carbons and the isomeric form (cis/trans).

• General Trend: More alkyl substitution increases stability.

• Trans Isomer: More stable than cis due to less steric strain.

• Heat of Hydrogenation: Lower heat released indicates greater stability.

Alkene	ΔH° (kJ/mol)
Ethylene	-126.0
1-Butene	-117.7
cis-2-Butene	-113.8
trans-2-Butene	-111.6
2-Methyl-2-butene	-110.4

Example: trans-2-butene is more stable than cis-2-butene.

Elimination Reactions

Elimination reactions involve the loss of two atoms or groups from a substrate, forming a new π bond. They are key in alkene synthesis.

• Dehydrohalogenation: Elimination of a proton and a halide ion.

• Dehydration: Elimination of a water molecule.

• Mechanisms: E1 (unimolecular) and E2 (bimolecular).

The E1 Reaction (Elimination, Unimolecular)

The E1 reaction proceeds via a two-step mechanism involving carbocation formation. It is first-order and depends only on the substrate concentration.

• Rate Law: $\text{rate} = k[\text{substrate}]$

• Major Product: More substituted alkene (Zaitsev's Rule).

Example: Dehydrohalogenation of 2-bromobutane in methanol yields the more substituted alkene as the major product.

Positional Orientation of the E1 Reaction

• Zaitsev's Rule: In elimination reactions, the most substituted alkene usually predominates as product.

The E2 Reaction (Elimination, Bimolecular)

The E2 reaction is a single-step, concerted mechanism requiring a strong base. It is second-order, depending on both substrate and base concentrations.

• Rate Law: $\text{rate} = k[\text{substrate}][\text{base}]$

• Major Product: More substituted alkene (Zaitsev's Rule), unless a bulky base is used.

Example: Dehydrohalogenation of 2-bromopropane with sodium ethoxide yields the more substituted alkene.

Effect of Bulky Bases (Hofmann Product)

Bulky bases favor formation of the least substituted alkene (Hofmann product) due to steric hindrance.

• Common Bulky Bases:

  • Sodium t-butoxide

  • Diisopropylamine (LDA)

  • Triethylamine

Example: Elimination with sodium t-butoxide yields the Hofmann product.

Stereochemistry of the E2 Reaction

The E2 reaction requires an anti elimination—the hydrogen and leaving group must be anti (opposite sides) for elimination to occur.

• Newman Projection: Used to visualize anti-periplanar arrangement.

• Consequence: Only certain stereoisomers can undergo E2 elimination.

Dehydration of Alcohols

Alcohols can be dehydrated to form alkenes using strong acids as catalysts.

• Catalysts: Concentrated H2SO4 or H3PO4

• Reaction: $\text{Alcohol} \xrightarrow{\text{H}_2\text{SO}_4} \text{Alkene} + \text{H}_2\text{O}$

• Reversible: Equilibrium constant is not large.

• Product: Follows Zaitsev's rule (more substituted alkene favored).

Competition Between Nucleophilic Substitution and Elimination Reactions

SN1/SN2 and E1/E2 reactions can compete, depending on substrate, base/nucleophile strength, and reaction conditions.

• SN1 and E1: Both involve carbocation intermediates; compete when substrate is tertiary or stabilized.

• SN2 and E2: Compete when substrate is primary or secondary and base/nucleophile is strong.

• General Guidelines:

  • Good nucleophiles/strong bases favor SN2/E2.

  • Poor nucleophiles/weak bases favor SN1/E1.

  • Primary substrates favor SN2/E2; tertiary favor SN1/E1.

Substrate	Base/Nucleophile	Major Reaction
Primary	Strong	SN2/E2
Secondary	Strong	SN2/E2
Tertiary	Strong	E2
Primary	Weak	SN1/E1 (rare)
Secondary	Weak	SN1/E1
Tertiary	Weak	SN1/E1

Example: 2-bromopropane with sodium ethoxide yields E2 product; with ethanol yields SN1 product.

Additional info: These notes cover the essential concepts for Chapter 7: Structure and Synthesis of Alkenes, including bonding, nomenclature, stability, elimination mechanisms, and competition with substitution reactions. For detailed IUPAC nomenclature rules, refer to the "Nomenclature-Alkenes" document.