What do alkenes and alkynes have in common

The world would be a much less colorful place without alkenes. Alkynes The simplest alkyne—a hydrocarbon with carbon-to-carbon triple bond—has the molecular formula C2H2 and is known by its common name—acetylene Fig 8.

Acetylene is used in oxyacetylene torches for cutting and welding metals. The flame from such a torch can be very hot. Most acetylene, however, is converted to chemical intermediates that are used to make vinyl and acrylic plastics, fibers, resins, and a variety of other products.

Alkynes are similar to alkenes in both physical and chemical properties. For example, alkynes undergo many of the typical addition reactions of alkenes. Benzene Next we consider a class of hydrocarbons with molecular formulas like those of unsaturated hydrocarbons, but which, unlike the alkenes, do not readily undergo addition reactions.

These compounds comprise a distinct class, called aromatic hydrocarbons. Aromatic hydrocarbons are compounds that contain a benzene ring structure.

The simplest aromatic compound is benzene C6H6 and it is of great commercial importance, but it also has noteworthy deleterious health effects. The formula C6H6 seems to indicate that benzene has a high degree of unsaturation. Hexane, the saturated hydrocarbon with six carbon atoms has the formula C6H14—eight more hydrogen atoms than benzene. However, despite the seeming low level of saturation, benzene is rather unreactive.

This is due to the resonance structure formed from the alternating double bond structure of the aromatic ring. Most of the benzene used commercially comes from petroleum. It is employed as a starting material for the production of detergents, drugs, dyes, insecticides, and plastics. Once widely used as an organic solvent, benzene is now known to have both short- and long-term toxic effects.

The inhalation of large concentrations can cause nausea and even death due to respiratory or heart failure, while repeated exposure leads to a progressive disease in which the ability of the bone marrow to make new blood cells is eventually destroyed.

This results in a condition called aplastic anemia, in which there is a decrease in the numbers of both the red and white blood cells. Substances containing the benzene ring are common in both animals and plants, although they are more abundant in the latter. The sigma bond is formed by end-to-end overlap of sp 2 hybrid orbitals, and the pi bond by side-to-side overlap of the p orbitals. A pi bond has two lobes of electron density above and below the plane of the molecule.

There are a number of consequences to this arrangement: 1 the resulting region of the molecule is planar the molecule is said to have trigonal planar geometry , 2 the electron density between the two carbons is high because there are four electrons in this region instead of two, and 3 rotation around a double bond is constrained in contrast to rotation around a single bond.

Rotation around a double bond requires breaking the overlap of the pi bond and its subsequent reformation. As with all bond-breaking phenomena, the bond-breaking step requires energy; in fact, significantly more energy than is required to bring about rotation around a single bond where no bond-breaking occurs.

As we will see, these three factors have a marked effect on the behavior of alkenes. Alkynes are compounds that contain triple bonds. The triple bond consists of one sigma bond formed from end-to-end overlap of sp-hybrid orbitals and two pi bonds formed from side to side overlap. The carbons are sp-hybridized and the molecule is linear in the region of the triple bond; again rotation around a triple bond is constrained—two pi bonds must be broken for it to occur which requires an input of energy.

This bonding arrangement results in a very electron rich C-C region with the sigma bond inside what looks like a cylinder of pi electron density. For example, in 2-butene there is a methyl and an H bonded to each of the double-bonded carbons carbons 2 and 3 of the molecule. As the groups attached to each carbon get more complex, such nomenclature quickly becomes confusing. To cope, we turn to another established naming scheme; in this case, the Cahn-Ingold-Prelog convention we previously used with chiral centers.

This involves ranking the groups linked to each double-bond carbon. If the high groups are together same side , the name is prefixed by Z from the German word for together: zusammen. If they are on opposite sides, they are labeled E entgegen; away.

E and Z isomers are diastereoisomers: they have the same connectivity but neither can be superimposed on its mirror image. In Ebromopentene, the CH 3 and CH 2 CH 3 groups are closer to one another than they are in Zbromopentene; the result is that they have different physical and chemical properties. Stability of alkenes: Elimination reactions that produce alkenes tend to favor the most substituted alkene as the major product. The more alkyl groups attached to the double bond, the more stable less reactive the alkene is, and therefore a lower amount of energy is released.

Molecular stability in alkenes is attributed to the same causes as the relative stabilities of carbocations; alkyl groups stabilize the pi bond by hyperconjugation and induction.

The double-bonded carbons of an alkene are electron-rich, that is, the electron density is high in the region of the double bond. Instead of a substitution, alkenes undergo electrophilic addition , a reaction in which a two-component reactant adds across the double bond. The reaction begins with an electrophilic attack by the double bond onto the reactant which produces a carbocation that then undergoes nucleophilic attack.

In the case of unsymmetrical alkenes where the groups attached to the double-bonded carbons are not exactly the same , the most stable carbocation is produced. This reaction is regioselective , that is, we can predict the orientation of reactant addition across the double bond. If we designate the reagent as E for electrophile or N for nucleophile , the reaction would proceed as outlined below. This pattern of reaction is referred to as Markovinkov addition, after the person [1] who first discovered that HBr adds in this way to a double bond.

We can classify many reagents as combinations of electrophile and nucleophile and, in this way, predict how they will add across the double bond.

Rather than memorizing the product of every type of addition across a double bond, it is much more productive to write a mechanism by determining which part is the electrophile, adding it to give the most stable carbocation, followed by the nucleophile. Additions to alkenes are reversible: Let us now take a closer look at the addition of water across a double bond.

Such a reaction can be accomplished by reacting the alkene with dilute sulfuric acid at low temperatures. The first step is addition of a proton to produce the most stable carbocation—which is then attacked by water the nucleophile. The final product is the alcohol that forms after a proton is transferred to water. By contrast, alkenes can be oxidized at low temperatures to form glycols. At higher temperatures, the glycol will further oxidize to yield a ketone and a carboxylic acid:.

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. In practice, it is difficult to isolate the alkene product of this reaction, though a poisoned catalyst—a catalyst with fewer available reactive sites—can be used to do so. As the hydrogen is immobilized on the surface of the catalyst, the triple or double bonds are hydrogenated in a syn fashion; that is to say, the hydrogen atoms add to the same side of the molecule.

Alkenes and alkynes can also be halogenated with the halogen adding across the double or triple bond, in a similar fashion to hydrogenation. The halogenation of an alkene results in a dihalogenated alkane product, while the halogenation of an alkyne can produce a tetrahalogenated alkane. Alkenes and alkynes can react with hydrogen halides like HCl and HBr. Hydrohalogenation gives the corresponding vinyl halides or alkyl dihalides, depending on the number of HX equivalents added.

The addition of water to alkynes is a related reaction, except the initial enol intermediate converts to the ketone or aldehyde. Water can be added across triple bonds in alkynes to yield aldehydes and ketones for terminal and internal alkynes, respectively. Hydration of alkenes via oxymercuration produces alcohols.

This reaction takes place during the treatment of alkenes with a strong acid as the catalyst. Privacy Policy. Skip to main content. Organic Chemistry. Search for:. Alkenes and Alkynes Naming Alkenes and Alkynes Alkenes and alkynes are named similarly to alkanes, based on the longest chain that contains the double or triple bond.

Learning Objectives Translate between the structure and the name of an alkene or alkyne compound. Key Takeaways Key Points Alkenes and alkynes are named by identifying the longest chain that contains the double or triple bond. Collectively, they are called unsaturated hydrocarbons because they have fewer hydrogen atoms than does an alkane with the same number of carbon atoms, as is indicated in the following general formulas:. The double bond is shared by the two carbons and does not involve the hydrogen atoms, although the condensed formula does not make this point obvious.

Note that the molecular formula for ethene is C 2 H 4 , whereas that for ethane is C 2 H 6. Ethylene is a major commercial chemical. The US chemical industry produces about 25 billion kilograms of ethylene annually, more than any other synthetic organic chemical. More than half of this ethylene goes into the manufacture of polyethylene, one of the most familiar plastics.

Propylene is also an important industrial chemical. It is converted to plastics, isopropyl alcohol, and a variety of other products.