Organic Chemistry/Introduction to reactions/Alkenes

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Alkenes[edit]

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.

Pi-bonding.png

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.


Bond dissociation energy
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°.

Ethene-2D-flat.png

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.


Gallery of alkenes
IUPAC
common names
CH2=CH2 ethene ethylene
CH3CH=CH2 propene propylene
CH2=CH2CH2CH3 1-Butene 1-Butylene
CH3CH=CHCH3 2-Butene 2-Butylene
CH2=C(CH3)2 2-Methylpropene Isobutene
Cyclobutene
3-Methylcyclopropene

IUPAC alkene nomenclature[edit]

  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[edit]

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

(IUPAC name)

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

Gallery of alkenols[edit]

 
IUPAC
2-Cyclohexen-1,4-diol
4-Cyclohexen-1,3-diol


Naming alkane and alkene substituents

Alkyl means a saturated substituent (all single bonds).

Alkenyl means an unsaturated substituent (en indicates a double bond).


IUPAC name
Common name
(1-Methylcyclohexyl)ethene
1-Ethenylcyclohexanol 1-Vinylcyclohexanol
(a cyclopropane diol) 1-(1-Propenyl)-1,2-cyclopropanediol
1-(2-Propenyl)-1,2-cyclopropanediol
1-Allyl-1,2-cyclopropanediol
Naming alkenes with multiple double bonds
CH2=C=CH2 Propacanadienne Allene
CH2=C=CH-CH3 1,2-Butadiene
CH2=CH-CH=CH2 1,3-Butadiene
1,3-Cyclopentadiene
1,3,5-Hexatriene
3-Methylene-1,4-pendadiene

Preparation of alkenes, or how to make double bonds.[edit]

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 equation

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).

(pictures)

E1 reactions[edit]

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

Elimination vs Substitution ?[edit]

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


Elimination
Substitution
High temperature (the biggest factor) Low temperature
Strong base Good nucleophile (bulkiness interferes with nucleophilicity)

Tertiary halides favor E1, SN1.

Reactions of alkenes[edit]

Bromine[edit]

Electrophilic addition reaction between Bromine and Ethene gas:

CH2=CH2 + Br2 → CH2BrCH2Br

Complete Combustion[edit]

Like other hyrdrocarbons 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

Hydrogen Bromide[edit]

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

Sulphuric Acid[edit]

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


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