Organic Chemistry/Introduction to reactions/Alkenes

Alkenes
Alkenes are organic molecules that contain at least one carbon-carbon double bond, which is referred to as unsaturation. Saturation is when all of the carbons have all single bonds. If the double bonds are just next to each other, they are called cummulated dienes. If there is only one C-C bond between two double bonds, it is called a conjugated diene. If more than one single bond intervenes it is an isolated diene.



The double bonds in alkenes are higher in energy than single bonds and are more reactive. Molecules will favor single bonds over double or triple ones when given a chance.

Bond dissociation energy is the energy needed to break a bond.

Ethene is the smallest, simplest alkene. It is planar and the angle between its bonds is approximately 120&deg;.



The C-H bonds are stronger in ethene than in ethane because the &pi; bond draws electron density away from from the carbon, which draws electron density away from the hydrogens. This makes ethene unreactive and gives the carbons sp2 character, which is more s-like and lower in energy.

IUPAC alkene nomenclature

 * 1) find the longest chain of carbons containing both carbons of the double bond
 * 2) number this chain left to right and right to left and choose the method that gives the double bond the lowest numbers
 * 3) using the smallest of the two numbers identifying the double bond as prefix, followed by a hyphen without spaces, name the chain as for an alkane changing the -ane to -ene

Then follow the rest of the rules for naming halogenoalkanes

Alkenols
Alkenols are compounds containing both a double bond and an OH group.

To name, find the longest carbon chain that contains both the C=C and the OH-. The OH group gets the higher priority (lower number). Oxygen takes precedence over carbon.

The "2" refers to the double bond and can go at the beginning of the name or right before the "en".

Preparation of alkenes, or how to make double bonds.
1&deg;, 2&deg;, 3&deg; all work for E2 reactions (remember that the methyl group doesnt have a second carbon for an elimination to function).

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E1 reactions
Elimination reactions are one way to produce alkenes. Learn more about them here.

Elimination vs Substitution ?
There are ways to predict if a reaction will follow an elimination or a substitution mechanism or pathway.

Tertiary halides favor E1, SN1.

Bromine
Electrophilic addition reaction between Bromine and Ethene gas:

Complete Combustion
Like other hydrocarbons alkenes will combust in excess air or oxygen provided that there is sufficient activation energy for the combustion reaction. The following reaction is the complete combustion of ethene with oxygen:

Hydrogen Bromide
Electrophilic addition reaction between concentrated Hydrogen Bromide and Ethene gas:

The general formula for the reaction of any alkene with HBr is:

For asymmetrical alkenes such as But-1-ene the following reaction is also feasible:

Where R and R' are alkyl groups e.g. CH3 or CH3CH2

Sulphuric Acid
Electrophilic addition reaction between cold concentrated Sulphuric Acid and Ethene gas:

Ethyl Hydrogensulphate is hydrolysed if warmed with water:

Overall reaction for the two steps:

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