Views: 22

ALKENES AND ALKYNES

When hydrogen is removed from an alkane multiple carbon-carbon bonds result. Hydrocarbons that contain a carbon-carbon double bond are called alkenes, and have the general formula

                    CnH2n

The simplest alkene is ethene (also known as ethylene) (C2H4). Ethylene is emitted by green plants in substantial quantities. It has hormonal activity and has been implicated in the control of many physiological processes in plants. The natural atmospheric concentration of ethylene is low due to its high reactivity with ozone and other atmospheric chemicals, but in polluted environments concentrations can be much higher. It is a product of the combustion of wood, coal, oil, natural gas and petroleum.

Hydrocarbons that contain a carbon-carbon triple bond are called alkynes. These compounds have the general formula

CnH2n-2

The simplest alkyne is ethyne (which is also known as acetylene) which has the formula C₂H ₂. Ethyne is a flammable and explosive gas, when burnt in the presence of oxygen enough heat is produced to cut and weld metals (the basis of the oxyacetylene welding torch). Alkynes are not normally found in the environment because they are highly reactive.

Nomenclature of Alkenes and Alkynes

The systematic nomenclature for alkenes is quite similar to that for alkanes. Important differences are listed below.

• The root hydrocarbon name ends in –enerather than –ane.
• In alkenes with more than three carbon atoms, the lowest numbered carbon atom involved in the double bond indicates the location of the double bond.

Thus CH₂=CHCH₂CH₃ is called 1- butene, and

CH₃CH=CHCH₃ is called 2 – butene.

The restricted rotation around a double bond means that alkenes exhibit cis-trans isomerism (they have the same molecular formula but different structural formula). For example in the case of 2 – butene we can draw

structures (a) and (b) below. Identical substituents in the same side of the double bond are referred to as cis- (or Z), whilst identical substituents on the opposite side of the double bond are designated trans- (or E). Hence structure (a) is called cis-2-butene, whilst structure (b) is called trans-2-butene. Cis-trans isomers have different physical properties.

The nomenclature for alkynes involves the use of –yne as a suffix to replace the -ane of the parent alkane. Thus the molecule CH₃CH₂C=CCH ₃ has the name 2-pentyne.

Physical and Chemical Properties of Alkenes and Alkynes

Physical Properties

The physical properties of alkenes and alkynes are very similar to alkanes with the same number of carbon atoms and branching pattern.

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.

1. Addition Reactions

Unsaturated hydrocarbons can participate in a number of different addition reactions across their double or triple bonds.

2. Addition reactions: Alkenes participate in a variety of addition reactions.

These addition reactions include catalytic hydrogenation (addition of H₂), halogenation (reaction with X₂, where X is a halogen ), and hydrohalogenation (reaction with H-X, where X is a halogen), among others.

3. Cycloaddition

Alkenes undergo diverse cycloaddition reactions. Most notable is the Diels–Alder reaction with 1,3-dienes to give cyclohexenes.

This general reaction has been extensively developed, and electrophilic alkenes and alkynes are especially effective dienophiles. Cycloaddition processes involving alkynes are often catalyzed by metals.

4. Oxidation

Oxidation of alkynes by strong oxidizing agents such as potassium permanganate or ozone will yield a pair of carboxylic acids. The general reaction can be pictured as:

RC≡CR′−−−−→RCO₂H+R′CO₂H

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:

(H₃C)2C=CHCH₃−−−−→H₃CCOCH₃+H₃CCO₂H

Here, we have 3-methyl-2-butene oxidizing to form acetone and acetic acid.

5. Hydrogenation

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.

6. Halogenation

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.

7. Hydrohalogenation

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. If the alkene is asymmetric, the reaction will follow Markovnikov’s rule—the halide will be added to the carbon with more alkyl substituents.

8. 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 the catalyst.