Alkene

Alkenes ( formerly olefins ) are chemical compounds selected from the group of aliphatic hydrocarbons having at least one carbon -carbon double bond in the molecule at any position. An example with more C = C double bonds of butadiene, which has two double bonds in the molecule. Alkenes are unsaturated compounds, in contrast to the alkanes, which covered all valencies of the carbon atom ( saturated) is. Alkenes are found in small scale in the petroleum, natural they are used as pheromones and plant hormones. They are the most important basic products of industrial organic chemistry.

Alkenes with one double bond form a homologous series with the general formula CnH2n sum starting with the ethene. The deprecated name olefins results from the old name of ethylene olefin, since it forms with halogens oily, water-insoluble liquids, which consist of haloalkanes. There are also cyclic alkenes, cycloalkenes, whose main representative is the cyclohexene.

  • 4.1 Manufacturing process generally
  • 4.2 β - eliminations 4.2.1 dehydration
  • 4.2.2 dehydrohalogenation
  • 4.2.3 Saytzeff rule
  • 4.2.4 dehalogenation
  • 4.2.5 dehydration
  • 4.2.6 pyrolysis of quaternary ammonium hydroxides
  • 5.1 Addition of hydrogen halides
  • 5.2 Reaction with concentrated sulfuric acid
  • 5.3 reaction with hypochlorous acid
  • 5.4 Reaction with an oxidizing agent
  • 5.5 Ozonation of C = C double bonds
  • 5.6 Catalytic Hydrogenation
  • 5.7 Conversion of cis- trans -alkenes and vice versa

Properties

The alkenes are ethene (C2H4 ) to butene ( C4H8 ) gas and thus highly volatile. Pentene with 5 to 15 carbon atoms with pentadecene the alkenes are liquid. Alkenes with more than 15 carbon atoms are fixed (under standard conditions). In water alkenes are sparingly soluble, they burn with a sooty flame. The alkenes are reactive. The weak double bond provides a point of attack for reagents, more precisely, it is the π bond which is electrophilic attack. Alkenes react with halogens to form haloalkanes. This is done by an electrophilic addition. The most important alkenes with ethene ( C2H4 ) to decene ( C10H20 ) with names and molecular formulas:

  • Ethene: C2H4
  • Propene C3H6
  • Butene: C4H8
  • Pentene: C5H10
  • Witches: C6H12
  • 1-heptene, 2 -heptene, 3 -heptene: C7H14
  • 1-octene, 2 -octene, 3 -octene, 4-octene: C8H16
  • 1 -nonene: C9H18
  • Decene-1: C10H20

The empirical formula of the unsubstituted alkenes is: CnH2n

Nomenclature and isomers

Nomenclature generally

General alkenes are named according to IUPAC analogous to alkanes, with the suffix -an is replaced by - s.

The position of the double bond in the carbon chain is indicated in the name by a number denoting the carbon atom to which the double bond starts. It is seen as a functional group and must be taken into account for the order of the numbering, so get the smallest possible number. In molecules with multiple functional groups, the number is usually placed directly in front of the -en, even before the name. Multiple double bonds obtained provided the corresponding Greek word number before the suffix.

In addition to the structural isomerism in which the carbon atoms are arranged differently, may also be cis -trans isomerism occur in alkenes in the C = C double bond.

Since the double bond in contrast to the single bond is not freely rotatably, it may occur at the double bond to two possible arrangements in appended atoms or atom groups. Cis -trans isomers differ in their physical and chemical properties. They can be distinguished on the dipole moment and IR spectroscopy. While the cis is mentioned in connection name, you can also omit the trans.

The example of the isomeric but-2- ene, the cis -trans isomerism can understand. When cis- but-2 -en both methyl groups are present as chain residues on this side (Latin cis ), that is on the same page. When trans-but -2-ene, the methyl groups lie on top of each other (from the Latin trans) side of the double bond.

From the IUPAC, the cis / trans designation was replaced (since it can easily lead to more than two substituents astray;, consider only (E )-2 -bromo-1 -chloro -1 -fluoro -ethene! ) By E / Z, where (E) (opposite ) usually - but not always - is trans and (Z ) (together ) for cis. The mutual position of the substituent highest CIP priority is specified. More precise details see (E, Z)- isomerism.

Traditionally, some simple substances with (E, Z)- isomerism different names: fumaric acid [( E)- butenedioic ] and maleic acid [( Z)- butenedioic ] and their derivatives are examples.

Polyenes

Alkenes having two double bonds is called dienes, trienes, with three double bonds. In general, these polyenes are called. The naming of the molecule follows the same rules here as in the mono-unsaturated alkenes, see 1,3-butadiene and isoprene. The number of possible cis -trans isomers rises drastically, since there are for each of the double bonds of the cis- trans isomers.

Use

Due to the very reactive double bond alkenes are important starting materials for many other raw materials in the chemical industry.

Alkenes are used as a fuel and for the preparation of halogenated hydrocarbons, alcohols, ketones, glycols, olefin oxides, plastics and detergent components. Propene is used for the synthesis of, for example, glycerol, phenol, isopropyl alcohol, epoxy resins, and required for the polymerization of polypropylene.

Manufacturing process

Manufacturing process generally

Alkenes can be prepared by various methods. One possibility is the pyrolytic dehydrogenation and cracking of alkanes ( cracking). The short-chain alkanes are cleaved in the presence of mixed oxide catalysts in alkenes and hydrogen at 450-500 ° C. At higher alkanes, this method is, however, little sense, as well many different isomers may arise that separation very costly, if not impossible.

Another way to produce a alkenes, the partial hydrogenation of alkynes. In this case, acetylenes are hydrogenated in the presence of Lindlar catalyst. This easily poisoned catalyst, the further hydrogenation of alkenes to alkanes is prevented. The fact that the hydrogen molecule from one side approaches to the triple bond, arising exclusively (Z)- alkenes.

β - eliminations

General compounds containing the structural fragment CH -CX can be converted by elimination of HX to an alkene with the structural fragment C = C. In the starting material with the structural fragment CH -CX, the H and the X group are linked to directly adjacent C atoms.

Dehydration

The dehydration of alcohols, takes place in an acid medium. For tertiary alcohols is much lighter than water can be eliminated from secondary or even primary alcohols.

Alcohols can be dehydrated to alkenes at elevated temperatures (eg, about 200 .. 250 ° C) over porous catalysts with high surface area ( such as aluminum oxide Al2O3). Secondary alcohols of the type CH3 -CHOH -CH 2- R ( R = alkyl group) give it a mixture of 1 -olefins, cis -2 and trans-2- olefins:

Dehydrohalogenation

Similarly, you can transform haloalkanes to alkenes. Known as the dehydrohalogenation reaction is carried out, in contrast to the dehydration of alcohols under basic conditions. Here too, tertiary hydrogen halides easier dehydrohalogenate as secondary, and these in turn more easily than primary hydrogen halides.

Saytzeff rule

These β -elimination regioselectivity is observed. Under certain circumstances, several products may be produced. The rule according to Saytzeff:

In addition to the OH group, respectively, the halogen atom of the hydrogen is eliminated from that of the neighboring carbon atom, which has at least hydrogen atoms. In other words, it creates the highest substituted alkene.

Dehalogenation

As a third possible β -elimination, there is the dehalogenation of 1,2- dihalo alkanes. In this case, during the alcohols in the presence of zinc, two identical halogen atoms are eliminated from adjacent carbon atoms. In this case, the alkene and the halogen in molecular form is created:

Dehydration

Alkenes can be obtained by partial dehydrogenation of alkanes. It eliminates this hydrogen from the corresponding alkane. The number of carbon atoms remains the same.

Pyrolysis of quaternary ammonium hydroxides

Alkenes formed during the pyrolysis of the quaternary ammonium, where at least one alkane group on the nitrogen atom is 2 or more carbon atoms has. The implementation is as follows:

Typical reactions

The typical reaction of alkenes mechanism is the Electrophilic Addition. This reaction is also based on the typical detection method for alkenes Bromwasserprobe: In this test, alkenes and other unsaturated hydrocarbons with brown colored bromine together, wherein bromine is added to the alkene and enters a rapid discoloration of the mixture.

Addition of hydrogen halides

The addition of hydrogen halides is similar, as the addition of halogen - such as bromine. Although in the addition to unsymmetrical alkenes theoretically different reaction products are possible, depending on to which carbon atom involved in the double bond, the halogen atom is attached, the addition reaction preferably proceeds regioselectively from according to the Markovnikov rule.

With a regioselective reaction, refers to a chemical change, which occurs preferably at one of several locations. It is formed by reaction with the proton H hydrogen halide acid as an intermediate always the most stable carbenium ion. Mesomerieeffekte are for this consideration is always more important than inductive effects. The halide then adds to the C of the carbenium ion. If no resonance stabilization of the carbenium ion is possible, the Markovnikov rule may be applied: For unsymmetrical alkenes, the electrophilic addition of hydrogen halides is done so that the halogen preferentially binds to the carbon atom having the fewest hydrogen atoms; however, the hydrogen atom on the carbon atom most hydrogen. Reason for this is that alkyl groups act as electron donors ( I effect and hyperconjugation ) and thus encourage the distribution ( delocalization ) of the positive charge. The stability of the positive carbenium ions is greater, the more alkyl groups are bound to the charged carbon atom. Therefore, a tertiary carbenium ion is more stable than a secondary and primary. The same also applies to radicals ( see NC # 2 ), as they also suffer from lack of electrons.

Deviations from the Markovnikov rule occur among others in radical addition and hydroboration. In these reactions, the anti -Markovnikov product is formed:

  • In the hydroboration the boron atom ( electrophilic) is ( substituted lower) to the negative part of charged carbon atom of the double bond is added, whereas the hydrogen atom ( nucleophile ) to the positive part of the charged carbon atom ( substituted above) is added. Since the hydrogen atom is usually the electrophile, this role reversal causes a break with the rule.
  • In free-radical addition of hydrogen bromide, for example, does not attack the double bond of the electrophile (hydrogen ) to, but a bromine radical, the double bond. In this case the bromine atom binds to the less-substituted carbon atom, so that the radical is formed at the more substituted. By hyperconjugation with the substituents and by I effect can be stabilized so the electron deficiency at the radical. Only then uses this radical to another H -Br molecule and thus gains its hydrogen atom which is then located at the more substituted carbon atom and thus violates the rule.

Reaction with concentrated sulfuric acid

The addition takes place after the Markovnikov rule, unless Mesomerieeffekte dominate. Technically used this method for the preparation of alcohols from olefins. Alkyl sulfuric acid is easily converted by hydrolysis into alcohols.

Reaction with hypochlorous acid

This reaction is regioselective. The chlorine atom binds to the carbon atom which carries the majority of hydrogen atoms, if no mesomeric effects. Similarly, the reaction with nitrosyl chloride and nitrosyl bromide to alkenes under addition.

Reaction with an oxidizing agent

The oxidation is carried out either with osmium tetroxide or with alkaline potassium permanganate solution. First formed from a cis-addition, a cyclic ester, the hydrolysis results in a cis -1 ,2- diol.

Ozonation of C = C double bonds

When ozone is introduced into anhydrous alkenes to ozonides, which are explosive when dry form. Ozonolysis cleaves the double bond completely and is thus for the determination of the structure of a carbon chain is an important reaction since the fission products give information about the position of the double bond. The ozone can be generated by discharge of air oxygen in an ozone generator.

Catalytic hydrogenation

The hydrogenation is the addition of hydrogen to the double bond which is characterized to a single bond, with the aid of a catalyst. The reaction occurs at room temperature in the presence of platinum or palladium, which is saturated with hydrogen. example:

Conversion of cis to trans -alkenes and vice versa

(E)- alkenes [Example: (E) -stilbene, mp 124 ° C] can be photochemically in the (Z)- alkenes [ example: ( Z) -stilbene, mp 1 ° C] convert. The reaction is reversible. - In some rare cases (for example, ranitidine ), the energy barrier for the conversion of specifically substituted alkenes at room temperature is so low that the ( E) form spontaneously converts into the (Z)- form and vice versa. In other words, (E )-isomer and (Z)- isomer can in rare cases are in equilibrium with each other.

Proof

The non-specific detection of the double bond, especially for the differentiation of the alkanes, an alkene is introduced into brown bromine (e.g., ethene ). The reaction can also be without the addition of energy, such as light, to proceed. Alkene molecule is added to each C atom of the C = C double bond in accordance with the reaction mechanism of the electrophilic addition of a bromine atom; as a reaction product, the appropriate haloalkane forms. For example:

The bromine water is decolorized due to this reaction, closed an alkane would not discolor the brown bromine water. Phenols and many reducing compounds but also decolorize a solution of bromine.

The so-called Baeyer- sample used for general detection of C = C double bonds and alkenes. The alkene is introduced into a solution of potassium permanganate in a weakly alkaline or acidic environment, whereupon the solution turns brown or colorless. It created an alcohol and Braunstein and manganese (II ) ions.

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