Summary
Summary Notes 3.3.4 - Alkenes
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- Unit 3.3.4 - Alkenes
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- AQA
Summary Notes 3.3.4 - Alkenes
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Taylor’s Notes AlkenesBonding in Alkenes
Bonding in alkenes does not consist of two identical bonds
One of the bonds is a normal bond with the electron cloud lying
between the two atoms This is called a sigma/σ bond
The other bond has an electron cloud which is placed above and
below the two atoms This is called a pi/π bond
Geometric Isomerism
Alkenes exhibit a type of stereoisomerism called geometric isomeric
Stereoisomerism are compounds with the same structural formula but a different arrangement of atoms in
space
The geometry of the C=C bond is determined by the π bond Ethene is a flat molecule
The bond angles are 120°
The rotation of the C=C axis is restricted as the carbon to carbon double bond is rigid
The restriction of rotation produces geometric isomers called E-Z isomers (cis-trans isomers)
For E-Z isomerism to occur there must be a double bond and two different groups on each of the carbon
atoms that make up the double bond
E-Z Nomenclature
The E-Z system breaks down with examples like:
The naming of an isomer is determined by priorities (Cahn-Ingold-Prelog priority rules)
Each atom connected to a carbon on the double bond is given a priority determined by its atomic
number The smaller the atomic number the lower the priority
If atoms with the lowest priority on zee zame zide on each end of the C=C the isomer is called the Z-
isomer
If atoms with the lowest priority are on the opposite side on each end of the C=C the isomer is
called the E-isomer
Unsaturated Isomers of Pentene
, Taylor’s Notes Alkenes
IUPAC Name Structure Exhibits Geometric E Isomer Z Isomer
Isomerism
Skeletal Formula E or Z Isomer? IUPAC Name
Reactions of Alkenes
Alkenes are more reactive than alkanes because of the C=C bond
It is possible for the double bond to break, allowing each carbon to form a new bond, which is often
energetically favourable Halogens, hydrogen halides, hydrogen and conc sulphuric acid will react with
alkenes in addition reactions
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