Alkenes

Intermediate Level Alkenes

Introduction

Alkenes are organic compounds which possess a double bond between two adjacent Carbons, which would be sp² hybridised. Due to this fact, an alkene, such as ethene would be drawn as follows:

This would have two different types of bonds, a sigma bond which is a bond between two adjacent nuclei and a π bond, which is a bond that would be above the nuclei.

Due to the fact that two pairs of electrons are used the bond length between the two Carbon atoms would be smaller than that of alkenes, although it must be noted that this length is not half that of alkenes. This is due to the fact of the new π bonds, which are electron cloud over and under the nucleus. This electron cloud produces a totally new set of reactions that can be performed by alkenes and not by alkanes.

Properties

The boiling points and solubilities are similar to those of alkanes.

The boiling point of alkenes is slightly lower than that of alkanes. This is due to the fact that in alkenes there are 2 fewer C-H bonds that can take part in Var der Waals interactions.

Geometrical Isomerism

In alkenes a new type of isomerism is encountered, this is cis-trans isomerism, and arises due to the fact that a double bond is rigid, and cannot rotate. this would thus create different possibilities for same structural compounds. An example would be 1,2-dibromoethene, where the two possibilities are the following:

Due to the fact that the double bond cannot rotate these two compounds are not equivalent, and thus a different type of naming would have to be introduced in alkenes to make up for this geometrical isomerism.

This naming including the prefixes cis and trans, where cis is used for compounds which would have similar groups on the same side of the Carbon atoms, while trans is used to indicating two similar groups which are on opposite sides of the Carbon atoms.

The example used would thus give:

Addition reactions

In addition reaction through a double bond, it is of utter importance to have an electrophile present. An electrophile is any compound, element or ion which is electron loving, and this would not normally either possess a positive charge or/and not possess a freely available lone pair of electrons.

The mechanism of addition is as follows:

where the first step is the use of the electrophile which would open up the double bond. Once the double bond is open a cation would be produced (a cation is a positive charge Carbon atom) and this would be open for a nucleophilic attack. A nucleophile is any compound or ion which has got a freely available lone pair of electrons, which can normally be seen as a negative sign on the ion. This would be attracted to a positive charge.

Reactivity of alkenes

As seen in the mechanism of addition reaction a carbocation is produced and the stability of the carbocation is very important in the rate of reaction. In chemistry, there are groups which stabilise a negative charge and other which stabilise the positive charge, and a hydrocarbon is a positive charge stabiliser, due to the fact that it can donate electrons which would produce a stable intermediate. The reactivity increases as follows:

Markovnikov’s rule

In an addition reaction, the additive molecule RH adds as H and R, with the R going to the carbon atom with the lesser number of hydrogen atoms bonded to it.

For more information about Markovnikov’s Rule click here.

Production of alkenes

Alkenes can be produced by elimination reactions which will be discussed in halogenoalkanes and alcohols, although these will be briefly discussed in this chapter.

Alkenes can be produced from alcohols by dehydrating the alcohols producing a double bond. A dehydrating reaction is any reaction which loses water as a by-product. The dehydrating agents used are normally acids, such as sulfuric acid and phosphoric acid.

CH₃CH₂OH  CH₂=CH₂

where the ethane would then be collected.

Simple alkenes can also be produced by passing them overheated Aluminium oxide. The process is as follows:

 CH₃CH₂OH  CH₂=CH₂

Hydrogenation

There are two types of hydrocarbons, those that are saturated and those that are unsaturated. Saturated hydrocarbons are those hydrocarbons that have got no C to C double bonds, while unsaturated hydrocarbons would be those were C to C double bonds are present. Hydrogenation would thus be a process of transforming an unsaturated hydrocarbon to a saturated hydrocarbon.

The reaction would need a catalyst and normally a moderate temperature, where the catalyst can be Ni or Pt while the temperature would be around 140^oC

CH₃CH=CH₂  CH₃CH₂CH3

This reaction can be used to find the number of double bonds in a compound by reacting a known volume of the gas (most alkenes are gases) with an excess of Hydrogen. The change in volume would be equivalent to the Hydrogen used. The number of double bonds can then be worked out.

Fats and oils are two similar compounds, where fats are saturated and oils are unsaturated, and knowing that oils are much healthier then fats most oils were reacted with Hydrogen to produce an unsaturated oil, such as margarine.  This process was found to have some health hazards due to the fact that it was found that cis and trans unsaturated oil have different effects on human health and although in nature only the cis- form is found it was being noticed that trans addition was being performed due to a flip of the double bond due to the temperatures used in the addition.

Halogenation

As seen in the addition mechanism an electrophile is necessary to open up the double bond. A halogen is an electrophile, although it must be noted that the halides are nucleophiles. A halide is a halogen ion.

CH₂=CH₂ + Br_2(l) → CH₂BrCH₂Br

It must be noted that for a halogenation reaction to occur as seen in the above reaction it must be noted that the halogen should not be dissolved in water, since once the halogen is dissolved in water another nucleophile would be present, which would be OH^– which would be in excess and therefore the reaction  produced would be the following:

CH₂=CH₂ + Br_2(aq) → CH₂BrCH₂OH

This reaction can be used to distinguish an alkene from an alkane since alkenes decolourise bromine water while alkanes do not.

Apart from halogen halogenation can occur using a hydrogen halide, such as Hl,  HBr and HCl, with the HCl reaction being the fastest reaction to the highest charge separation between the H and Cl, and thus it would be attracted more by the electron cloud on the alkene. The reaction would be as follows:

CH₂=CH₂ + HX_(aq) → CH₂HCH₂X

Hydration

Putting a hydroxyl group on a double bond is not an easy process, and this normally would have an equilibrium between the reactants. The reactant would be the alkene and water, which can be seen as an H-OH molecule. The reaction is as follows:

CH₂=CH₂ + H₂O  CH₃CH₂OH

and due to the fact that an equilibrium is produced the reaction is not commonly used.

The reaction would proceed as follows:

which can then be reacted with water to form an alcohol.

Reaction with Potassium Permanganate

This reaction is an oxidation reaction which can produce a diol.

CH₂=CH₂  CH₂OHCH₂OH

Polymerisation

Unsaturation gives the possibility of polymerisation. Although this topic will be tackled in another chapter some ideas will be shown here.

In polymerisation the double bond will be opened using an initiator which will produce a radical. This radical would combine with other alkenes, opening the double bonds and polymerising.

Formation of epoxyethane.

Epoxyethane is a cyclic ether composed of an Oxygen and two Carbons. It is used as an antifreeze in cars and as a precursor in the formation of esters.

Preparation is as follows:

where the temperature would be between 200^oC and 300^oC with pressures ranging from 10 – 20 atmospheres using silver as a catalyst.

Ozonolysis

Alkenes can be oxidised by ozone to form aldehydes, or if the reactions favour complete oxidation carboxylic acids.

The conditions are as follows:

• dilute HCl + heat for complete oxidation
• Zn and CH₃COOH + heat to stop the reaction at the formation of aldehydes

where the marker would depend on the conditions used. This would either be an aldehyde or a carboxylic acid.