Organic Chemistry/Introduction to reactions/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.
|C-C → C° + C°||≈ 85 kcal/mole|
|C=C → C° + C°||≈ 145 kcal/mole|
Ethene is the smallest, simplest alkene. It is planar and the angle between its bonds is approximately 120°.
The C-H bonds are stronger in ethene than in ethane because the π 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
- find the longest chain of carbons containing both carbons of the double bond
- number this chain left to right and right to left and choose the method that gives the double bond the lowest numbers
- 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 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.
|CH2=CH2CH-OH||2-Propen-1-ol or Prop-2-en-1-ol|
The "2" refers to the double bond and can go at the beginning of the name or right before the "en".
Gallery of alkenols
Naming alkane and alkene substituents
Alkyl means a saturated substituent (all single bonds).
Alkenyl means an unsaturated substituent (en indicates a double bond).
|(a cyclopropane diol)||1-(1-Propenyl)-1,2-cyclopropanediol|
Preparation of alkenes, or how to make double bonds.
R-H + X2 + (heat or light) → R-X
R-X + H2O (by SN1 or SN2) → R-OH
CH3CH2Br → H2C=CH2 + HBr
CH3CH2-OH → CH2=CH2 + H2O
CH3CH2Br → CH2=CH2
H2C-CH2Br + (strong base) + Δ → CH2=CH2 (one step) (Δ ≡ heat)
What is "Δ"?
Δ is the capital Greek letter "Delta" and is used in science to indicate a change in something. It is used as a shorthand in organic chemistry and in this book to indicate a change in heating conditions, that is, the addition of heat.
Rate = K2[CH3CH2Br][base]
1°, 2°, 3° all work for E2 reactions (remember that the methyl group doesnt have a second carbon for an elimination to function).
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.
|High temperature (the biggest factor)||Low temperature|
|Strong base||Good nucleophile (bulkiness interferes with nucleophilicity)|
Tertiary halides favor E1, SN1.
Reactions of alkenes
Electrophilic addition reaction between Bromine and Ethene gas:
CH2=CH2 + Br2 → CH2BrCH2Br
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:
CH2=CH2 + 3O2 → 2CO2 + 2H2O
Electrophilic addition reaction between concentrated Hydrogen Bromide and Ethene gas:
CH2=CH2 + HBr → CH3CH2Br
The general formula for the reaction of any alkene with HBr is:
RCH=CHR' + HBr → RCH2-CHR'Br
For asymmetrical alkenes such as But-1-ene the following reaction is also feasible:
RCH=CHR' + HBr → RCHBr-CH2R'
Where R and R' are alkyl groups e.g. CH3 or CH3CH2
Electrophilic addition reaction between cold concentrated Sulphuric Acid and Ethene gas:
CH2=CH2 + H2SO4 → CH3CH3SO3H
Ethyl Hydrogensulphate is hydrolysed if warmed with water:
CH3CH3SO3H + H2O → CH3CH2OH + H2SO4
Overall reaction for the two steps:
CH2=CH2 + H2O → CH3CH2OH