# Reactions of Alkenes and Alkynes

## Alkenes and alkynes are more reactive than alkanes due to their pi bonds.

#### Terms

• A long or larger molecule consisting of a chain or network of many repeating units, formed by chemically bonding together many identical or similar small molecules called monomers. A polymer is formed by polymerization, the joining of many monomer molecules.

• The rule states that, with the addition of a protic acid HX to an alkene, the acid hydrogen (H) becomes attached to the carbon with fewer alkyl substituents, and the halide (X) group becomes attached to the carbon with more alkyl substituents.

#### Figures

Alkenes participate in a variety of addition reactions.

2. ##### Hydrogenation

Hydrogen can added across an unsaturated system—such as the olefin in maleic acid shown—by utilizing a catalyst, such as palladium.

3. ##### Markovnikov's Rule

This rule dictates that the addition of a protic acid (HX, in the case of HBr) to an alkene will lead to a product where the hydrogen is attached to the carbon with fewer alkyl substituents, whereas the halide group is attached to the carbon with more alkyl substituents.

## Reactions of Alkenes and Alkynes

Alkenes and alkynes are generally more reactive than alkanes due to the electron density available in their pi bonds. In particular, these molecules can participate in a variety of addition reactions and can be used in polymer formation.

Unsaturated hydrocarbons can participate in a number of different addition reactions across their double or triple bonds. A generic reaction scheme is shown in Figure 1.

These addition reactions include catalytic hydrogenation, halogenation, hydrohalogenation, and others.

Alkynes undergo diverse cycloaddition reactions. Most notable is the Diels–Alder reaction with 1,3-dienes to give 1,4-cyclohexadienes. This general reaction has been extensively developed, and electrophilic alkynes are especially effective dienophiles. The 'cycloadduct,' derived from the addition of alkynes to 2-pyrone, eliminates carbon dioxide to give the aromatic compound. Other specialized cycloadditions include multicomponent reactions such as alkyne trimerisation, which gives aromatic compounds and the [2+2+1] cycloaddition of an alkyne, alkene, and carbon monoxide in the Pauson–Khand reaction. Non-carbon reagents also undergo cyclization (Azide alkyne Huisgen cycloaddition) to give triazoles. Cycloaddition processes involving alkynes are often catalyzed by metals (enyne metathesis and alkyne metathesis), which allows the scrambling of carbyne (RC) centers.

$RC≡CR + R'C≡CR' 2 \rightleftharpoons RC≡CR'$

Oxidative cleavage of alkynes proceeds via cycloaddition to metal oxides. Most famously, potassium permanganate converts alkynes to a pair of carboxylic acids. Alkenes and alkynes can be oxidized to form epoxides, vicinal diketones, and carboxylic acids.

## Hydrogenation

In the presence of a catalyst—typically platinum, palladium, nickel, or rhodium—hydrogen can be added across a triple bond to take an alkyne to an alkene and 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. 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 come from the same side of the molecule. For an example, see Figure 2.

## Halogenation

Alkenes and alkynes can also be halogenated with the halogen adding across the double or triple bond. The halogenation of an alkene results in a dihalogenated alkane product, while the halogenation of an alkyne can produce a tetrahalogenated alkane.

## Hydrohalogenation

Alkenes and alkynes can react with hydrogen halides like HCl or 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. If the alkene is asymmetric, the reaction will follow Markovnikov's rule—the halide will be added to the carbon with more alkyl substituents (Figure 3).

## Hydration

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 catalyst.

#### Key Term Glossary

acid
an electron pair acceptor; generally capable of donating hydrogen ions
##### Appears in these related concepts:
an organic reaction where two or more molecules combine to form a larger one
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alcohol
class of organic compounds containing a hydroxyl functional group
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aldehyde
Any of a large class of reactive organic compounds (R·CHO) having a carbonyl functional group attached to one hydrocarbon radical and a hydrogen atom.
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alkane
Any of the saturated hydrocarbons—including methane, ethane, and compounds with long carbon chain known as paraffins, etc.— that have a chemical formula of the form CnH2n+2.
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alkene
An unsaturated, aliphatic hydrocarbon with one or more carbon–carbon double bonds.
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Alkenes
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond.
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alkyl
Any of a series of univalent radicals of the general formula CnH2n+1 derived from aliphatic hydrocarbons.
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alkyne
a hydrocarbon containing at least one carbon–carbon triple bond
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aromatic
Having a closed ring of alternate single and double bonds with delocalized electrons.
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atom
the smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons
##### Appears in these related concepts:
bond
a link or force between neighboring atoms in a molecule
##### Appears in these related concepts:
carboxylic acid
Any of a class of organic compounds containing a carboxyl functional group (or a carbon with one double bond to an oxygen and another single bond to another oxygen, which is in turn bonded to a hydrogen).
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catalyst
a substance that increases the rate of a chemical reaction without being consumed in the process
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compound
A substance made from any combination elements.
##### Appears in these related concepts:
density
a measure of the amount of matter contained by a given volume
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double bond
A covalent bond in which two electron pairs (instead of the usual one) are shared between two atoms; most common between carbon atoms and carbon, oxygen, or nitrogen atoms, but several other forms are known.
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electron
The subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor constitutes electricity.
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halide
a salt of any halogen acid
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halogen
Any element of group 7, i.e. fluorine, chlorine, bromine, iodine and astatine, which form a salt by direct union with a metal.
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halogenation
The reaction of a halogen with something, especially the replacement of a hydrogen atom of an organic compound with a halogen one.
##### Appears in these related concepts:
halogens
a group in the periodic table consisting of five chemically related elements, fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At)
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hydration
the incorporation of water molecules into a complex with those of another compound
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hydrocarbon
A compound consisting only of carbon and hydrogen atoms.
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hydrogenation
The chemical reaction of hydrogen with another substance, especially with an unsaturated organic compound, and usually under the influence of temperature, pressure, and catalysts.
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ketone
a compound containing an oxygen atom joined to a carbon atom by a double bond
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metal
Any of a number of chemical elements in the periodic table that form a metallic bond with other metal atoms; generally shiny, somewhat malleable and hard, often a conductor of heat and electricity
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molecule
the smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds
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oxide
a binary chemical compound of oxygen with another chemical element
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pi bond
covalent chemical interactions where two lobes of one involved atomic (p) orbital overlap two lobes of the other involved atomic (p) orbital
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pi bonds
In chemistry, pi bonds (π bonds) are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital. These orbitals share a nodal plane which passes through both of the involved nuclei.
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polymer
A long or larger molecule consisting of a chain or network of many repeating units, formed by chemically bonding together many identical or similar small molecules called monomers. A polymer is formed by polymerization, the joining of many monomer molecules.
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product
a chemical substance formed as a result of a chemical reaction
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strong acid
A strong acid is one that completely ionizes (dissociates) in water; in other words, one mole of a strong acid HA dissolves in water yielding one mole of H+ and one mole of the conjugate base, A−.
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substituents
Any atom, group, or radical substituted for another, or entering a molecule in place of some other part which is removed.
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triple bond
A covalent bond in which three electron pairs (instead of the usual one) are shared between two atoms; most common between carbon atoms and carbon or nitrogen atoms, but a few other forms are known. Symbolized in formulae as ≡.