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Organic Chemistry
Table of Contents
Unit 7 Overview: Alkenes
7.1 Industrial Preparation and Use of Alkenes
7.2 Calculating the Degree of Unsaturation
7.3 Naming Alkenes
7.4 Cis–Trans Isomerism in Alkenes
7.5 Alkene Stereochemistry and the E,Z Designation
7.6 Stability of Alkenes
7.7 Electrophilic Addition Reactions of Alkenes
7.8 Orientation of Electrophilic Additions: Markovnikov’s Rule
7.9 Carbocation Structure and Stability
7.10 The Hammond Postulate
7.11 Evidence for the Mechanism of Electrophilic Additions: Carbocation Rearrangements

Alkene stability is crucial in organic chemistry. More substituted alkenes are generally more stable due to hyperconjugation and inductive effects. Trans alkenes are usually more stable than cis due to less steric strain.

Heats of hydrogenation help measure alkene stability, with more stable alkenes releasing less energy. Molecular orbital theory explains alkene bonding, while resonance structures contribute to overall stability. Understanding these factors is key to predicting alkene behavior.

Alkene Stability and Structure

Alkene stability and substitution

  • Alkene stability increases with the degree of substitution
    • Tetrasubstituted alkenes are most stable (2,2-dimethylpropene)
    • Trisubstituted alkenes are more stable than disubstituted alkenes (2-methylpropene vs 2-butene)
    • Disubstituted alkenes are more stable than monosubstituted alkenes (2-butene vs propene)
    • Monosubstituted alkenes are more stable than ethene (propene vs ethene)
  • Hyperconjugation contributes to alkene stability
    • Interaction between $\pi$ bond and adjacent $\sigma$ bonds allows electron density from $\sigma$ bonds donated to empty $\pi^*$ orbital
    • More substituted alkenes have more $\sigma$ bonds available for hyperconjugation (tetrasubstituted > trisubstituted > disubstituted > monosubstituted)
  • Alkyl groups stabilize alkenes through hyperconjugation and inductive effects
    • Alkyl groups are electron-donating and donate electron density to the electron-deficient $\pi$ bond (methyl, ethyl, propyl, etc.)
  • Conjugation can further stabilize alkenes through resonance effects

Cis vs trans alkene stability

  • Trans alkenes are generally more stable than cis alkenes
    • Trans alkenes have less steric strain or repulsion between substituents since bulky substituents are farther apart in trans configuration (trans-2-butene vs cis-2-butene)
  • Steric strain in cis alkenes increases with the size of substituents
    • Larger substituents cause greater steric strain than smaller substituents (tert-butyl vs methyl)
  • Energy difference between cis and trans isomers depends on substituent size
    • Larger substituents lead to a greater energy difference (tert-butyl)
    • Smaller substituents result in a smaller energy difference (methyl)
  • Cis-trans stability difference is more pronounced in cycloalkenes
    • Ring strain contributes to the instability of cis isomers in small and medium rings (cis-cyclooctene vs trans-cyclooctene)

Heats of hydrogenation for stability

  • Heat of hydrogenation is the energy released when an alkene is converted to an alkane
    1. Involves the addition of hydrogen ($\ce{H2}$) to the double bond
    2. Exothermic process where energy is released as heat
  • More stable alkenes have less negative heats of hydrogenation
    • Less energy is released when hydrogenating more stable alkenes (2-methylpropene vs propene)
    • More substituted alkenes have less negative heats of hydrogenation (2,3-dimethyl-2-butene vs 2-methyl-2-butene)
  • Comparing heats of hydrogenation allows for the determination of relative alkene stabilities
    • Alkene with the least negative heat of hydrogenation is the most stable (2,3-dimethyl-2-butene in a series)
  • Limitations of using heats of hydrogenation:
    • Differences in heats of hydrogenation can be small and experimental errors may affect the accuracy of measurements
    • Steric effects in heavily substituted alkenes can influence heats of hydrogenation (tetra-tert-butylethylene)
    • Conjugated and aromatic systems may not follow the same trends as isolated alkenes (1,3-butadiene, benzene)

Molecular Orbital Theory and Alkene Structure

  • Molecular orbital theory explains bonding in alkenes
    • Bond angles in alkenes are influenced by sp² hybridization
    • Bond lengths in alkenes are shorter than single bonds due to stronger π-bond overlap
  • Resonance structures contribute to overall alkene stability

Key Terms to Review (38)

Steric strain: Steric strain is the repulsion between adjacent atoms or groups in a molecule due to their physical size, causing a decrease in stability. This strain affects the molecule's spatial configuration and can influence its reactivity.
Bond Angles: Bond angles refer to the geometric arrangement of atoms around a central atom in a molecule, determined by the number and type of bonds formed. This concept is crucial in understanding the structures and properties of various organic compounds.
Resonance: Resonance is a fundamental concept in organic chemistry that describes the ability of certain molecules to exist in multiple equivalent structures or resonance forms. This phenomenon arises from the delocalization of electrons within the molecule, leading to the stabilization of the overall structure and the distribution of electron density across multiple atoms.
Molecular Orbital Theory: Molecular Orbital Theory is a model that describes the behavior of electrons in a molecule by considering the formation of molecular orbitals from the combination of atomic orbitals. This theory provides a more comprehensive understanding of chemical bonding compared to the earlier Valence Bond Theory.
σ Bonds: A σ bond is a type of covalent bond formed by the head-on overlap of atomic orbitals, resulting in a high electron density between the bonded atoms. These bonds are fundamental to the structure and stability of organic molecules.
Benzene: Benzene is a planar, aromatic hydrocarbon compound with the chemical formula C6H6. It is a key building block in organic chemistry and has a unique resonance structure that contributes to its stability and reactivity.
Conjugation: Conjugation refers to the overlap or sharing of atomic orbitals, resulting in the delocalization of electrons across a system of connected atoms. This concept is central to understanding resonance, the stability of certain molecules and ions, and the interpretation of various spectroscopic techniques in organic chemistry.
Hyperconjugation: Hyperconjugation is a type of conjugation in organic chemistry where the sigma bonds of alkyl groups (such as methyl or ethyl) interact with adjacent pi bonds, leading to increased stability of the molecule. This stabilizing effect is particularly important in understanding the stability of carbocations and the orientation of electrophilic additions.
Methyl: The methyl group is a simple alkyl group consisting of a single carbon atom bonded to three hydrogen atoms. It is denoted by the formula -CH3 and is the most basic and common alkyl group found in organic chemistry. The methyl group plays a crucial role in various organic reactions and structural features across several key topics in this course.
Alkyl Groups: Alkyl groups are hydrocarbon substituents derived from alkanes by the removal of one hydrogen atom. They are non-polar, saturated, and can be straight-chain or branched. Alkyl groups play a crucial role in understanding the properties and behavior of various organic compounds, including alkanes, alkenes, and benzene derivatives.
π Bond: A π bond is a type of covalent bond that forms between atoms when they share a pair of electrons in a side-to-side arrangement, rather than the head-to-head arrangement of a σ bond. π bonds are crucial in understanding the structure and reactivity of organic compounds.
Tert-Butyl: The tert-butyl group is a branched alkyl substituent with the chemical formula (CH3)3C-. It is a tertiary alkyl group, meaning the carbon atom to which the three methyl groups are attached is also bonded to another carbon atom. This structural feature gives the tert-butyl group unique properties that are relevant in the context of organic chemistry topics such as alkyl groups, the stability of alkenes, reactions of ethers, and conjugate carbonyl additions.
Steric Strain: Steric strain refers to the distortion or destabilization of a molecule caused by the repulsive interactions between bulky groups or atoms that are in close proximity within the molecule's structure. This concept is particularly relevant in the context of understanding the conformations and stability of cyclic compounds, such as cyclohexane, as well as the stability of alkenes.
2-methylpropene: 2-methylpropene, also known as isobutylene, is a branched-chain alkene with the molecular formula C₄H₈. It is an important organic compound that is relevant in the context of several topics in organic chemistry, including the addition of HBr to ethylene, the stability of alkenes, Markovnikov's rule, and the Hammond postulate.
Ethene: Ethene, also known as ethylene, is a simple unsaturated hydrocarbon with the chemical formula C₂H₄. It is the simplest alkene and plays a crucial role in various topics within organic chemistry, including calculating the degree of unsaturation, naming alkenes, understanding cis-trans isomerism, and evaluating the stability of alkenes.
Cis Alkenes: Cis alkenes are a type of alkene where the two largest substituents attached to the carbon-carbon double bond are on the same side of the molecule. This geometric arrangement contrasts with trans alkenes, where the largest substituents are on opposite sides of the double bond.
2-methyl-2-butene: 2-methyl-2-butene is an alkene with the molecular formula C₅H₁₀. It is a structural isomer of 2-butene, with a methyl group (CH₃) attached to the central carbon atom of the alkene. This term is important in the context of understanding the stability of alkenes and the stability of the allyl radical.
Tetrasubstituted Alkenes: Tetrasubstituted alkenes are organic compounds containing a carbon-carbon double bond where all four substituents attached to the double-bonded carbons are different. These unique alkenes are of particular interest in the context of alkene stereochemistry, the stability of alkenes, and the Wittig reaction.
Heats of Hydrogenation: Heats of hydrogenation refer to the amount of energy released or absorbed when a compound undergoes hydrogenation, the chemical reaction where hydrogen gas is added to a compound. This term is particularly relevant in the context of understanding the stability of alkenes and conjugated dienes.
2,3-dimethyl-2-butene: 2,3-dimethyl-2-butene is an alkene with two methyl groups (CH3) attached to the second and third carbon atoms, and a double bond between the second and third carbon atoms. It is a key term in the context of understanding the stability of alkenes.
2-butene: 2-butene is an unsaturated hydrocarbon with the molecular formula C4H8. It is an alkene with a carbon-carbon double bond located at the second carbon position of the four-carbon chain. This structural feature of 2-butene is central to understanding its behavior and properties in the context of various organic chemistry topics.
Inductive Effects: Inductive effects refer to the ability of substituents or functional groups to influence the distribution of electron density within a molecule through space. This phenomenon can have significant implications on the stability, reactivity, and orientation of various organic reactions.
Cis-2-butene: cis-2-butene is a geometric isomer of the alkene 2-butene, where the two largest substituents (in this case, methyl groups) are positioned on the same side of the carbon-carbon double bond. This structural arrangement contrasts with the trans isomer, where the largest substituents are on opposite sides of the double bond.
Propene: Propene, also known as propylene, is a simple unsaturated hydrocarbon with the molecular formula C3H6. It is the second member of the alkene family and is an important industrial chemical used in the production of various petrochemicals and plastics.
Monosubstituted Alkenes: Monosubstituted alkenes are organic compounds that contain a carbon-carbon double bond with a single substituent attached to one of the carbon atoms. These molecules are important in the context of understanding the stability of alkenes and the hydration of alkenes through the process of hydroboration.
π* Orbital: The π* orbital is an antibonding molecular orbital that arises from the constructive and destructive interference of atomic p-orbitals in a π-bond. It is higher in energy than the corresponding π-bonding orbital and is an important concept in understanding the stability and reactivity of alkenes.
Bond Lengths: Bond length refers to the distance between two bonded atoms in a molecule. It is a fundamental characteristic of chemical bonds and plays a crucial role in determining the stability and reactivity of organic compounds, particularly in the context of alkene stability.
Cis-cyclooctene: cis-Cyclooctene is a cyclic alkene with eight carbon atoms in the ring. The term 'cis' refers to the configuration of the double bond, where the two substituents are on the same side of the ring.
Trisubstituted Alkenes: Trisubstituted alkenes are organic compounds containing a carbon-carbon double bond with three substituents attached to one of the carbon atoms. These types of alkenes are important in the context of understanding alkene stereochemistry, the stability of alkenes, and the hydration of alkenes through hydroboration reactions.
Trans Alkenes: Trans alkenes, also known as trans-configured alkenes, are a type of alkene isomer where the two largest substituents are on opposite sides of the carbon-carbon double bond. This geometric arrangement contrasts with cis alkenes, where the two largest substituents are on the same side of the double bond.
Cycloalkenes: Cycloalkenes are a class of organic compounds that consist of a cyclic ring structure with at least one carbon-carbon double bond. These molecules are closely related to both cycloalkanes and alkenes, combining the cyclic nature of the former with the unsaturated character of the latter.
Disubstituted Alkenes: Disubstituted alkenes are organic compounds that contain a carbon-carbon double bond with two substituents attached to each carbon atom of the double bond. These types of alkenes are particularly relevant in the context of understanding the stability of alkenes and the hydration of alkenes through the process of hydroboration.
Alkene Stability: Alkene stability refers to the relative stability of different alkene structures, which is determined by the arrangement and substitution patterns of the carbon-carbon double bond. The stability of an alkene is an important concept in organic chemistry as it helps predict the reactivity and the preferred products of chemical reactions involving alkenes.
2,2-dimethylpropene: 2,2-dimethylpropene, also known as tert-butylethylene, is an alkene with a highly branched structure that exhibits unique stability and reactivity characteristics, particularly in the context of understanding the factors that influence the stability of alkenes.
1,3-Butadiene: 1,3-Butadiene is a simple conjugated diene, composed of four carbon atoms with two carbon-carbon double bonds separated by a single carbon-carbon bond. This structural feature gives 1,3-butadiene unique chemical properties and reactivity that are important in various organic chemistry topics.
Tetra-tert-butylethylene: Tetra-tert-butylethylene is an alkene compound with four tert-butyl groups attached to the carbon-carbon double bond. It is an important molecule in the context of understanding the stability of alkenes.
Trans-cyclooctene: trans-Cyclooctene is a cyclic alkene compound with eight carbon atoms in the ring and a trans configuration of the double bond. It is an important intermediate in organic chemistry, particularly in the context of studying the stability of alkenes.
Trans-2-butene: trans-2-butene is a geometric isomer of the alkene 2-butene, where the two largest substituents (in this case, methyl groups) are positioned on opposite sides of the carbon-carbon double bond. This structural arrangement has important implications for the stability and reactivity of the molecule.