Name: alkene stability: heat and structure
Uploaded: 2023-04-30T18:28:36.000Z
Duration: 1 min 59 s
Channel: JoVE

The energy released when an alkene reacts with hydrogen is called the heat of hydrogenation.&#160;
The relative stability of alkenes is determined by comparing their heats of hydrogenation. The smaller the heat of hydrogenation, the higher the stability of the alkene.
Increasing the substitution across the double bond is found to decrease the heat of hydrogenation, revealing that the tetrasubstituted alkene is more stable than the tri-, di-, and monosubstituted alkenes.
The reason is that the more-substituted alkenes have a higher ratio of sp2&#8211;sp3 bonds than sp3&#8211;sp3 bonds. The sp2&#8211;sp3 bond is lower in energy and stronger than the sp3&#8211;sp3 bond since it has a higher contribution of s electrons, which are lower in energy.
Another factor responsible for the stability of substituted alkenes is the stabilizing interaction, called hyperconjugation, between the delocalized electron density of the pi bond and the carbon&#8211;hydrogen sigma bonds on the substituents.&#160;
Alkyl substituents provide additional hyperconjugation and lead to more stability. Therefore, 2,3-dimethyl-2-butene, with four substituents across the double bond, is more stable than propene, which has just one substituent.&#160;&#160;&#160;
Further,

relative stabilities of alkenes

Relative Stabilities of Alkenes

The relative stability of alkenes can be determined by comparing their heats of hydrogenation. The lower heat of hydrogenation indicates the more stable alkene.&#160; The three main factors determining the relative stability of alkenes are i) the number of substituents attached to the double-bond carbon atoms, ii) hyperconjugation, and iii) the stereochemistry of the double bond.
<ol>
	<li>Number of substituents across the double bond: An alkene with two smaller substituents is more stable than its isomer having one large substituent. For example, 2-butene is more stable compared to 1-butene. The highly substituted alkenes have a higher ratio of sp2&#8211;sp3 bonds, which are lower in energy and are stronger as compared to the sp3&#8211;sp3 bonds. Thus, a tetrasubstituted alkene is more stable than a tri-, di-, or monosubstituted alkene.</li>
	<li>Hyperconjugation: Hyperconjugation is a stabilizing interaction of the delocalized electron density between the carbon&#8211;carbon &#960; bond and the adjacent carbon&#8211;hydrogen &#963; bonds on the substituent. Thus, a higher number of alkyl substituents across the double bond suggests greater hyperconjugation, resulting in a more stable alkene.</li>
	<li>Stereochemistry: The spatial arrangement of the substituents also contributes to the stability of alkenes. The cis isomer exhibits steric strain because of the crowding of the substituents on the same side of the double bond and therefore is less stable compared to the trans isomer.</li>
</ol>

The relative stability of alkenes can be determined by comparing their heats of hydrogenation. The lower heat of hydrogenation indicates the more stable ...

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Organic Chemistry

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Alkene Structure and Reactivity

KEY TERMS AND CONCEPTS

alkene stability: heat and structure

Alkene stability depends on heats of hydrogenation and on the structure around the double bond. A lower heat of hydrogenation means the alkene is more stable. Three main factors help explain these differences: the number of substituents on the double-bond carbons, hyperconjugation, and double-bond stereochemistry.

Substitution across the double bond has a strong effect on stability. An alkene with two smaller substituents is more stable than its isomer with one large substituent. For example, 2-butene is more stable than 1-butene. In general, a tetrasubstituted alkene is more stable than a tri-, di-, or monosubstituted alkene because it has more sp2-sp3 bonds, and these bonds are lower in energy and stronger than sp3-sp3 bonds.

Hyperconjugation also adds stability. It is the stabilizing interaction between the carbon-carbon pi bond and nearby carbon-hydrogen sigma bonds on the substituents. When more alkyl substituents are attached across the double bond, hyperconjugation increases. That greater electron sharing makes the alkene more stable.

The stereochemistry of the double bond matters too. The cis isomer has steric strain because its substituents crowd the same side of the double bond. This crowding makes cis alkenes less stable than trans alkenes.

Alkene Stability: Heat and Structure

regioselectivity of electrophilic additions to alkenes: markovnikov's rule

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Markovnikov Addition Paths in Alkenes

Markovnikov addition in alkenes explains why one product often forms in greater amount during hydrohalogenation. When an unsymmetrical alkene reacts, two haloalkane constitutional isomers are possible. In most cases, hydrogen adds to the vinylic carbon with more hydrogens, and the halide part adds to the more substituted carbon. This preference is called regioselectivity. Regioselective reactions favor bond formation at one site over another, so one constitutional isomer becomes the major...

structure and bonding of alkenes

Alkene Bonding, Properties, and Uses

Alkenes are unsaturated hydrocarbons with one or more carbon-carbon double bonds. They are also called olefins, and they are broadly divided into alkenes and cycloalkenes. The general formula for an alkene is CnH2n. The carbons in a double bond are sp2 hybridized. This gives each carbon a trigonal planar shape. The double bond has two parts: a sigma bond from overlap of hybrid orbitals and a pi bond from side-by-side overlap of unhybridized 2p orbitals. Each carbon also bonds to two hydrogen...

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Degree of unsaturation helps count rings and double bonds in an organic molecule. It is also called the index of hydrogen deficiency, or IHD. The idea is based on comparing a compound with the acyclic alkane that has the same number of carbon atoms. Each double bond or ring removes two hydrogen atoms compared with a saturated analog. That loss equals one degree of unsaturation. For hydrocarbons, the formula is U = (2C + 2 - H) / 2, where C is the number of carbon atoms and H is the number of...

isomerism in alkenes

Cis, Trans, and E/Z Alkene Forms

Alkene isomers can differ in both bond position and spatial arrangement. 1-butene and 2-butene are constitutional isomers because the double bond is in a different place. 2-butene also shows stereoisomerism, which means its atoms are connected the same way but arranged differently in space. In 2-butene, the two methyl groups can sit on the same side of the double bond or on opposite sides. When the methyl groups are on the same side, the compound is called cis-2-butene. When they are on...

Alkene stability depends on heats of hydrogenation and on the structure around the double bond. A lower heat of hydrogenation means the alkene is more stable. Three main factors help explain these differences: the number of substituents on the double-bond carbons, hyperconjugation, and double-bond stereochemistry. Substitution across the double bond has a strong effect on stability. An alkene with two smaller substituents is more stable than its isomer with one large substituent. For example,...

introduction to electrophilic addition reactions of alkenes

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Alkene Addition Reactions and Redox Changes

Alkene addition reactions show how a carbon-carbon double bond reacts with electrophiles and changes during the process. A simple, unconjugated alkene has a double bond with high electron density, so it can act as a weak base or a nucleophile. Its filled π orbital, called the HOMO, can interact with the empty LUMO of an electrophile.

key terms and concepts

Compare alkene stability using heats of hydrogenation, substitution level, hyperconjugation, and cis versus trans stereochemistry in high school chemistry.

Alkene Stability: Heat and Structure | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Alkene Stability: Heat and Structure

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