alkene

Examples of alkene in the following topics:

• Organic Reactions Overview

  • What starts as an ether with two alkene units is converted to a ketone with just one alkene functionality.
  • However, the arrows show that the alkene bonds all shift circularly: electrons from one bond in the diene are used to form a bond to the alkene, breaking the alkene's bond.
  • Simultaneously, the alkene attaches to the opposite side of the diene, forcing an alkene bond to rotate:
  • There are three alkene bonds in the reactants, that are broken.
  • In their place come two new single C-C bonds and a new alkene bond.

• Addition Reactions

  • For instance, an alkene's hydration reaction adds water to an alkene, and an alcohol's dehydration removes water from the alkene; these two reactions are opposites and are considered addition-elimination pairs.
  • Most addition reactions to alkenes follow the mechanism of electrophilic addition.
  • In halogenation, adding elementary bromine or chlorine to alkenes yields dibromo- and dichloro-alkanes, respectively.
  • Addition of hydrohalic acids like HCl or HBr to alkenes yields the corresponding haloalkanes.
  • Top to bottom: electrophilic addition to alkene, nucleophilic addition of nucleophile to carbonyl, and free radical addition of halide to alkene.

• Catalytic Hydrogenation

  • From the heats of hydrogenation, shown in blue in units of kcal/mole, it would appear that alkynes are thermodynamically less stable than alkenes to a greater degree than alkenes are less stable than alkanes.
  • Independent studies of hydrogenation rates for each class indicate that alkenes react more rapidly than alkynes.
  • However, careful hydrogenation of an alkyne proceeds exclusively to the alkene until the former is consumed, at which point the product alkene is very rapidly hydrogenated to an alkane.
  • Before hydrogen can add to a multiple bond the alkene or alkyne must be adsorbed on the catalyst surface.
  • In this respect, the formation of stable platinum (and palladium) complexes with alkenes has been described earlier.

• Hydrogenation

  • The simplest source of two hydrogen atoms is molecular hydrogen (H2), but mixing alkenes with hydrogen does not result in any discernible reaction.
  • First, the alkene must be absorbed on the surface of the catalyst along with some of the hydrogen.
  • For example, the following table lists the heats of hydrogenation for three C5H10 alkenes which give the same alkane product (2-methylbutane).
  • Similar complexes have been reported for nickel and palladium, metals which also function as catalysts for alkene hydrogenation.
  • Examples of alkene reductions by both procedures are shown below.

• Properties of Alkenes

  • Due to the presence of a double bond in their carbon skeletons, alkenes are more reactive than their related alkanes.
  • Rotation around the double bond is disfavored, so alkenes form fairly stable isomers depending on the positioning of substituents on the same (cis) or opposite (trans) sides of the double bond.
  • The melting and boiling points of alkenes are determined by the regularity of the packing, or the closeness, of these molecules.
  • A space-filling model of ethylene, the simplest alkene, showing its planar structure.

• Hydroboration Reactions and Oxidations

  • Diborane reacts readily with alkynes, but the formation of substituted alkene products leaves open the possibility of a second addition reaction.
  • Consequently, large or bulky electrophilic reagents add easily to the triple-bond, but the resulting alkene is necessarily more crowded or sterically hindered and resists further additions.
  • The bulky hydroboration reagent needed for this strategy is prepared by reaction of diborane with 2-methyl-2-butene, a highly branched alkene.
  • Because of the alkyl branching, only two alkenes add to a BH3 moiety (steric hindrance again), leaving one B-H covalent bond available for reaction with an alkyne, as shown below.
  • As with alkenes, the B-H reagent group adds in an apparently anti-Markovnikov manner, due to the fact that the boron is the electrophile, not the hydrogen.

• Properties of Aldehydes and Ketones

  • A comparison of the properties and reactivity of aldehydes and ketones with those of the alkenes is warranted, since both have a double bond functional group.
  • Because of the greater electronegativity of oxygen, the carbonyl group is polar, and aldehydes and ketones have larger molecular dipole moments (D) than do alkenes.
  • We expect, therefore, that aldehydes and ketones will have higher boiling points than similar sized alkenes.
  • The polarity of the carbonyl group also has a profound effect on its chemical reactivity, compared with the non-polar double bonds of alkenes.
  • The C=C of alkenes has an average bond energy of 146 kcal/mole.

• Synthesis of Addition Polymers

  • All the monomers from which addition polymers are made are alkenes or functionally substituted alkenes.
  • The most common and thermodynamically favored chemical transformations of alkenes are addition reactions.

• Nucleophilic Addition Reactions & Reduction

  • The sp-hybrid carbon atoms of the triple-bond render alkynes more electrophilic than similarly substituted alkenes.
  • This mode of reaction, illustrated below, is generally not displayed by alkenes, unless the double-bond is activated by electronegative substituents, e.g.
  • Electron addition to a functional group is by definition a reduction, and we noted earlier that alkynes are reduced by solutions of sodium in liquid ammonia to trans-alkenes.
  • Returning to the reducing capability of the blue electron solutions, we can write a plausible mechanism for the reduction of alkynes to trans-alkenes, as shown below.

• Reactions of Alkenes and Alkynes

  • By contrast, alkenes can be oxidized at low temperatures to form glycols.
  • In the presence of a catalyst—typically platinum, palladium, nickel, or rhodium—hydrogen can be added across a triple or a double bond to take an alkyne to an alkene or an alkene to an alkane.
  • Hydration of alkenes via oxymercuration produces alcohols.
  • This reaction takes place during the treatment of alkenes with a strong acid as the catalyst.
  • Give examples of the various reactions that alkenes and alkynes undergo