Alkenes

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Structure and Bonding

Alkenes are part of an homologous series. They have the functional group of a C=C (double bond between two carbons). and the general formula of CnH2n, where n is the number of carbon molecules.

There are two types of bonds in alkenes because of the different electron sub-shells. There is a σ bond (sigma) which is when two p suborbitals overlap, and the π bond (pi) is above and below the atoms, it tends to be weaker because of this and it also has no rotation which is why isomerism exists.

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diagram showing pi and sigma bonding

Because of this bonding, the double bond is a centre of high electron density, meaning it is a great place for electrophiles to attack.

Alkenes

Alkenes are unsaturated since they have a double covalent carbon bond. Alkenes have the general formula CnH2n. Below is a table of the first Alkenes. Note there is no 'methene' since, in order to have a carbon double bond, you need at least 2 carbon atoms. Once you get to butene, there are two locations the double bond could be placed (between the first and second, and third and fourth are considered the same since one could be flipped to make the other). This gives rise to isomerism.

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Name Formula Structure
Ethene C2H4
ethene.jpg
Propene C3H6
propene.png
Butene C4H8
butene.png

Cracking

This is the process of breaking down large fractions with high boiling points that are difficult to sell into smaller, more useful molecules.

An example of a cracking reaction:

Hexadecane ==>> Petrol + Butene + Ethene C16H34 ==>> C8H18 + C4H8 + 2C2H4

This reaction is done either by:

  • Passing the vapour over a catalyst at high pressure (catalytic cracking).
  • By the use of heat alone (thermal cracking) depending on the fraction used.

Cracking produces smaller alkanes, but also alkenes!

. Thermal Cracking Catalytic Cracking
Temperature 400-900oC 450oC
Pressure 7000 kPa Slight
How? Increased vibrations in atoms from heat splits the alkane. The higher the temperature, the further towards the end of the chain it splits at. Zeolite catalyst has pores in its crystal where the rection takes place. branched alkanes cannot fit into this.
Product Uses Smaller alkanes can be sold easily, and alkenes can be used to make plastics in a further process. This process produced branched alkanes, which burn more easily and therefore make good fuels.

Hydrogenation

This means adding hydrogen, and if you do this to alkenes then they will turn into alkanes. This is the process used to produce margarine. The conditions are: 150°C and a nickel catalyst. Here is the reaction.

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the hydrogenation of ethene

Electrophilic Addition

Because of this double bond, HBr for example, can attack the alkene and the mechanism for this is outlined below.

electrophillic addition onto alkenes

Not only HBr, but also other molecules will do this same process. For this reason, the two parts of the molecules have been color coded, and to get the mechanism for HSO4 and Br2 you merely substitute it.

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The test for alkenes is the bromine water test. The mechanism for this is the one with Br-Br. A solution of Br will be coloured orange, but when this reaction happens with an alkene, it will become colourless because it becomes a haloalkane. In the middle of this reaction, a carbocation (that is a cation of carbon) is formed; this is a C+. The position of this is important, depending on whether this is primary, secondary, or tertiary (see this page), determines the likelihood of different products being formed. A tertiary carbocation is the most stable, and therefore will form the major product, while secondary and primary carbocations will make the minor product.

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Categories : Organic

Recent edits by: Taylor (ScienceAid Editor), SpellBot, Jamie (ScienceAid Editor)

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