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Alkenes

Alkenes

Background:


Ethylene, the simplest alkene
Ethylene, the simplest alkene
Alkenes like alkanes are pure hydrocarbons made up exclusively of carbon and hydrogen molecules. The single distinguishing feature is that alkenes contain at least one double carbon-carbon bond, whereas alkanes, by definition have only single carbon-carbon bonds.

Alkenes are a natural biproduct of hydrocracking since the breakup a long chain alkane results in the production of two shorter chain products, one an alkane and the other an alkene.

Other common names alkenes include olefin, or olefine.

The general formula for an alkene is CnH2n.

Physical Properties:


The physical properties of alkenes are comparable with alkanes. The physical state depends on molecular mass. The simplest alkenes, ethylene, propylene and butylene are gases. Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids.

Chemical Properties:


Alkenes are relatively stable compounds, but are more reactive than alkanes due to their double carbon-carbon bond. Although stronger than the single carbon-carbon bond in alkanes, the majority of the reactions of alkenes involve the rupture of this double bond, forming two new single bonds.

Uses and Applications:


Alkenes are particularly useful in the production of "designer compounds" since the double bonds are prone to opening up to allow in new attachments to the carbon chain.

By choosing and alkene with the appropriate double carbon bond location we can control were reagent atoms are to be added..

Important Compounds and Derivatives:


Alkenes can readily be converted into polymers by opening up the double bond to form chains of repeating molecules of indefinite length. An important example it the polymerization of ethene to form polyethelyne.

Alkenes are also useful in controlled reactions designed to place functional groups at specific locations on an alkane chain. These reactions include:

Halogonation

In the following reaction the double bond in the ethene molecule is opened up to allow absorption of an bromine atom (Br2), by placing one Bromine ion on each side of the previous double carbon bond.

CH2=CH2 + Br2 → CH2Br-CH2Br

Hydrogenation

Ethene reacts with hydrogen in the presence of a finely divided nickel catalyst at a temperature of about 150°C. Ethane is produced. This is typical of alkene reactions with hydrogen gas.

CH2=CH2 + H2 → CH3-CH3

This addition of a H2 molecule to an alkene chain is generally referred to as "hydrogenation."

Hydrohalogenation

This process describes the absorption of a hydrogen halide molecule, such as HBr or HCl by an alkene, again by the opening up of the double carbon carbon bond to allow in the hydrogen and halide ions.

CH3-CH=CH2 + HBr → CH3-CHBr-CH3

The placement of the halide ion is governed by Markovnikov's Rule which states that the hydrogen ion will be placed on the carbon which had the most hydrogens prior to the reaction.

Hydration

This process describes the absorption of a water molecule by an alkene, again by the opening up of the double carbon carbon bond to allow in the hydrogen and OH ions.

CH3-CH=CH2 + H2O → CH3-CHOH-CH3

The placement of the hydroxyl ion is governed by Markovnikov's Rule which states that the hydrogen ion will be placed on the carbon which had the most hydrogens prior to the reaction.

Laboratory Preparation:


Alkenes can be synthesized from alcohols via an elimination reaction that removes one water molecule:

H3C-CH2-OH + H2SO4 → H3C-CH2-O-SO3H + H2O → H2C=CH2 + H2SO4

Industrial Preparation:


The most common industrial synthesis path for alkenes is cracking of petroleum.

Naming Conventions:


  1. The ene suffix (ending) indicates an alkene or cycloalkene.
  2. The longest chain chosen for the root name must include both carbon atoms of the double bond.
  3. The root chain must be numbered from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
  4. The smaller of the two numbers designating the carbon atoms of the double bond is used as the double bond locator. If more than one double bond is present the compound is named as a diene, triene or equivalent prefix indicating the number of double bonds, and each double bond is assigned a locator number.

Bibliography:


Article on alkene reactions PART I
Article on alkene reactions PART II
Wikipedia article on alkenes
Wikipedia article on polyethylene

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