12 alkenes Flashcards
Physical properties of the alkenes
- Alkenes are unsaturated hydrocarbons containing at least 1 C=C.
- They have the general formula CnH2n.
- They typically take part in addition reactions (to the C=C).
- They exhibit E/Z isomers including the cis / trans isomerism.
- The C=C is the functional group in an alkene and is responsible for is reactions
- Ethene is one of the most important alkenes. It is obtained from the catalytic cracking of long chain alkanes. Ethene is used to make polymers for the
plastics industry.
Definition: A pi-bond
(π-bond) is the reactive part of the double bond formed above and below the plane of the bonded atoms by the sideways overlap of p-orbitals. It’s an area of high electron density
The C=C double bond
- The π-bond consists of 2 lobes one on each side of the sigma bond, the 2 lobes
sideways overlap to produce a π-bond. - To ensure maximum overlap, ‘intermittent ethane species’ must be a planar
molecule. - The asymmetric shape of the 2 bonds locks the molecule around the double bond.
This means that there is no free rotation about these bonds. - The π-bond changes the shape around the carbon atom to a trigonal planar with a
bond angle of 120o:
The shape of alkenes
Despite having a C=C - it is simply just one area of electron density.
So the explanation of the geometry is as follows:
Alkenes have three bonding ares in the same plane (3 areas of high electron density)
which repel equally to be as far apart as possible - TRIGONAL PLANAR (120o).
The π-bond changes the shape around the carbon atom to a trigonal planar with a
bond angle of 120o:
Cahn-Ingold-Prelog rules
To have E/Z isomerism, a molecule must have:
A carbon-carbon double bond as there is no free rotation around a C=C which
means the geometry is maintained. (rigidity occurs because of the π-bond’s
electron density).
Each carbon in the double bond must be attached to two different groups.
The groups are given priority with reference to their atomic number.
Definition: Stereoisomers
compounds with the
same structural formula but with a
different arrangement in space
E/Z stereosimerism aslo kown as
(geometric)
priority for stereoisomersim is based of
atomic number not mass number
Description of the nature of the double bond c=c
- The extra electrons in the C=C makes it more reactive as it is more electron rich (localised
electron density). - The C=C is it a lot stronger than just a C-C, the C=C is not however twice as strong as the C-C
(due to orbital overlap) - The sigma bond (σ) is a lot stronger than the pi bond (π) which means that the pi bond (π) breaks leaving the sigma bond (σ) intact when an alkene reacts. Pi bond has a low bond enthalpy
- The σ bond is stronger than the π bond the extent of overlap in sigma bonds is greaterthan that in pi bonds.
- Alkenes are more reactive than the alkanes due to the electron rich C=C.
- The C=C is the functional group made
from a sigma, σ bond and a pi, π-bond.
why do alkenes undergo
Addition reactions of alkenes:
Alkenes undergo addition reactions as they are unsaturated.
The reactant will add across the C=C forming a saturated product.
Two reactant molecules react to form one product.
x4 name all 4 and the reaction type
the addition reactions of alkenes
1 Addition of hydrogen- hydration/reduction
2 Addition of a halogen- halogenation
3 Addition of steam
4 Addition of hydrogen halides
alkene
Addition of hydrogen
Each hydrogen joins to each carbon either side of the C=C.
This reaction is also called hydrogenation
It is also an example of a reduction reaction as hydrogen is added.
conditions:
- H2
- Ni catalyst
- 150 .c
alkenes
Addition of a halogen
Each halogen joins to each carbon either side of the C=C.
This reaction is also called halogenation, it makes a dihalogeno alkane
It is also a test for unsaturation.
alkene -> alcohol
- The H joins to one of the carbons while the OH joins to the other carbon in the C=C
- This reaction is also called hydration as water is added to themolecule. This reaction makes an alcohol.
- conditons: steam , PHOSPHORIC
ACID CATALYST
alkenes
Addition of hydrogen halides
The hydrogen joins to one of the carbons while the halogen joins to the other
carbon in the C=C.
Any hydrogen halide will react, these are gases and are usually bubbled through
the alkene. This reaction makes a haloalkane.
Definition: A carbocation
Definition: A carbocation is
an organic ion in which a
carbon atom has a positive
charge.
alkenes
draw The electrophilic addition mechanism
cl-cl h-br
The C=C double bond is a region of high electron density due to
the π electrons. This means it will attract electrophiles to itself.
As all the reactions are addition reactions, the reactions are
described as electrophilic addition
Addition to unsymmetrical alkenes
- forms two products one major one minor
Markownikoff’s rule states…
‘A rule used to predict the major and minor product when adding a hydrogen halide to an unsymmetrical alkene’
Rule of thumb:
- Hydrogen adds to the C of the double bond with greatest number of hydrogenatoms already attached.
- Halide adds to other carbon.
draw the major product
Forming the major product:
Once the C=C attacks the H with a partial positive charge, causing the heterolytic fission of the
H-Br bond, a negative bromide ion and positive carbocation remain.
The carbocation formed is positive on the middle C as the hydrogen adds to the first Carbon (this
is called a secondary carbocation)
draw the minor product
Forming the minor product:
The carbocation formed is positive on the terminal C as the hydrogen adds to the second
carbon (this is called a primary carbocation)
Carbocation stability from most ro least stable
Most stable
– tertiary carbocation (3 alkyl groups attached)
– secondary carbocation (2 alkyl groups attached)
– primary carbocation (0, or 1 alkyl group attached)
Least stable
explain the trend in carbocation stability
WHY?
Carbocations are stabilised by alkyl groups.
Alkyl groups are considered to be electron donating – so they push electrons towards the
positive charge, thus spreading out the positive charge onto neighbouring carbons.
The more alkyl groups surrounding the positively charged carbon, the greater the spread of
charge – the more stable the carbocation.
Definition: A polymer
a
long molecular chain made
up from monomer units.
