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.