3.2.9 Alkenes - structure, bonding and reactivity
Students should:
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Alkenes
The structures are similar to those of the alkanes except two hydrogens on adjacent carbons are replaced by a double bond between those carbons. The number '1' in the names refers to the position of the carbon starting the double bond. No numbering is needed in the first two members as there can be no ambiguity.
The arrangement of atoms at the double bond is planar, wth bond angles of 120º. This is because the carbon atom has three regions of electron density around its nucleus that arrange themselves to minimise repulsion. This minimisation favoures a 120º angle.
The consequence of a double bond in a long hydrocarbon chain is to introduce a 'kink' which interrupts the zig-zag structure of the hydrocarbon. This hinders intermolecular attractive forces and decreases the boiling point with respect to the alkanes.
E-Z stereoisomerism
In this type of isomerism there is a lack of rotation between two asymmetric sides, which produces different chemical (and physical) environments. The simplest case is that of an alkene where there is no rotation possible about the double bond (because it would result in rupture of the pi orbital).
If there are two different attachments on each side of the double bond then geometrical isomerism results.
The simple naming system used to differentiate between isomers is the cis- trans- nomenclature. However, this is limited and a better system uses the Cahn, Ingold, Prelog system of priorities.
Cahn, Ingold, Prelog priority rules
In this system each bonded atom is prioritised according to its atomic mass. If this does not differentiate them, then the next atom in the chain is used etc..
If the two groups with the highest priority lie on the same side the molecule is designated the letter Z (German Zussamen = together). If the highest priority groups are on opposite sides of the double bond the isomer is labelled E (German Entgegen = opposite). Fopr example:
Z-dichloroethene and E-dichloroethene
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Compare Priorites : 1 -CH3 and 2 -CH2-CH3
Therefore, they are not differentiated. Go onto the second substituent:
Hence, the second chain has higher priority. |
Other situations where rotation is restricted may also produce geometric isomerism. This could be due to steric hindrance, or rings of carbon atoms.
Reactivity
The double bond comprises a region of high electron density that is attractive to particles with positive or partial positive charges (electrophiles), as well as electron deficient molecules.
This makes alkenes more reactive than alkanes.
The predominant reaction of the alkenes is addition, where the double bond electrons attach other species to the two carbon atoms which were involved in the double bond. This called electrophilic addition
Alkenes can add hydrogen halide molecules, halogens, sulfuric acid, water and other molecules that are, or can be polarised.
