7: Alkenes- Structure and Reactivity
After you have completed Chapter 7, you should be able to
- fulfill all of the detailed objectives listed under each individual section.
- describe the importance of alkenes to the chemical industry.
- use the concept of “degree of unsaturation” in determining chemical structures.
- describe the electronic structure and geometry of alkenes.
- describe the factors that influence alkene stability, and determine the relative stability of a number of given alkenes.
- write the IUPAC name of a given alkene, and draw the structure of any alkene, given its IUPAC name.
- determine whether a given alkene has an E configuration or a Z configuration.
- explain why alkenes are more reactive than alkanes.
- describe the reaction between an alkene and a hydrogen halide, and explain why one product is formed rather than another. Base your explanation on the concepts of carbocation stability and the Hammond postulate.
- define, and use in context, the key terms introduced in this chapter.
This, the first of two chapters devoted to the chemistry of alkenes, describes how certain alkenes occur naturally, then shows the industrial importance of ethylene and propylene (the simplest members of the alkene family). The electronic structure of alkenes is reviewed, and their nomenclature discussed in detail. After dealing with the question of cis-trans isomerism in alkenes, Chapter 7 introduces the reactivity of the carbon-carbon double bond. The chapter then focuses on one specific reaction—the addition of hydrogen halides to alkenes—to raise a number of important concepts, including carbocation stability and the Hammond postulate.
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- 7.5: Naming Alkenes
- Alkenes contain carbon-carbon double bonds and are unsaturated hydrocarbons with the molecular formula is CnH2n; this is also the same molecular formula as cycloalkanes. For straight chain alkenes, it is the same basic rules as nomenclature of alkanes except change the suffix to "-ene."
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- 7.6: Cis-Trans Isomerism in Alkenes
- Geometric isomerism (also known as cis-trans isomerism or E-Z isomerism) is a form of stereoisomerism. Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements (structural isomerism) which are simply due to the molecule rotating as a whole, or rotating about particular bonds. Structural isomerism is not a form of stereoisomerism, and is dealt with in a separate Module.
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- 7.7: Sequence Rules - The E,Z Designation
- The traditional system for naming the geometric isomers of an alkene, in which the same groups are arranged differently, is to name them as cis or trans. However, it is easy to find examples where the cis-trans system is not easily applied. IUPAC has a more complete system for naming alkene isomers. The R-S system is based on a set of "priority rules", which allow you to rank any groups. The IUPAC system for naming alkene isomers, called the E-Z system, is based on the same priority rules.
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- 7.8: Stability of Alkenes
- Alkene hydrogenation is the syn-addition of hydrogen to an alkene, saturating the bond. The alkene reacts with hydrogen gas in the presence of a metal catalyst which allows the reaction to occur quickly. The energy released in this process, called the heat of hydrogenation, indicates the relative stability of the double bond in the molecule.
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- 7.9: Electrophilic Addition Reactions of Alkenes
- This page looks at the reaction of the carbon-carbon double bond in alkenes such as ethene with hydrogen halides such as hydrogen chloride and hydrogen bromide. Symmetrical alkenes (like ethene or but-2-ene) are dealt with first. These are alkenes where identical groups are attached to each end of the carbon-carbon double bond.
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- 7.10: Orientation of Electrophilic Additions - Markovnikov's Rule
- This page looks at the reaction of the carbon-carbon double bond in alkenes such as ethene with hydrogen halides such as hydrogen chloride and hydrogen bromide. Symmetrical alkenes (like ethene or but-2-ene) are dealt with first. These are alkenes where identical groups are attached to each end of the carbon-carbon double bond.
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- 7.11: Carbocation Structure and Stability
- It is a general principle in chemistry that the more a charge is dispersed, the more stable is the species carrying the charge. Put simply, a species in which a positive charge is shared between two atoms would be more stable than a similar species in which the charge is borne wholly by a single atom.
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- 7.13: Evidence for the Mechanism of Electrophilic Additions - Carbocation Rearrangements
- Whenever possible, carbocations will rearrange from a less stable isomer to a more stable isomer. This rearrangement can be achieved by either a hydride shift, where a hydrogen atom migrates from one carbon atom to the next, taking a pair of electrons with it; or an alkyl shift, in which an alkyl group undergoes a similar migration, again taking a bonding pair of electrons with it. These migrations usually occur between neighboring carbon atoms.
Thumbnail: Ball-and-stick model of the ethylene (ethene) molecule, \(\ce{C2H4}\). (Public Domain; Ben Mills via Wikipedia )