Alkenes
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What is the functional group of alkenes?
C=C (carbon-carbon double bond)
Compared to alkanes, how reactive are alkenes?
Considerably more reactive
Why are C=C more reactive than C-C?
• The π bond formed is weaker • The π bond attracts electrophiles
How are σ (sigma) bonds formed?
From the direct overlap of two atomic orbitals
Where is the electron density in a σ (sigma) bond?
Mostly between the two nuclei
How are π bonds formed?
From the sideways overlap of two p atomic orbitals
Do π bonds have a higher electron density than σ (sigma) bonds?
Yes
Why do π bonds have a higher electron density than σ (sigma) bonds?
There are twice as many electrons in a smaller space
Are π bonds weaker than σ (sigma) bonds?
Yes
In a π bond, where are the electrons located?
Above and below the plane of the carbon atoms
What are bond angles measured between?
BONDS not atoms
Why are π bonds weaker than σ (sigma) bonds?
• the electron density of a π bond is above and below the plane of the molecule • whilst the electron density is directly between two carbon atoms in the σ (sigma) bond
What does the higher electron density of a π bond attract?
Electrophiles
Define electrophile
A species capable of accepting a pair of electrons
Define stereoisomer
Molecules that have the same structural formula but a different arrangement of atoms in space
What type of stereoisomerism do alkenes exhibit?
E/Z isomerism
Under what conditions does E/Z isomerism occur?
• no rotation about a bond (as in a C=C) • each of the carbon in the C=C is attached to two different groups
What is cis/trans isomerism?
A special case of E/Z isomerism in which one of the attached groups on each carbon atom of the double bond must be hydrogen
Are Cis isomers E or Z isomers?
Z
Are Trans isomers E or Z isomers?
E
Explain why cyclohexene is more reactive than cyclohexane
• cyclohexene has a double bond which is made up of a σ and a π bond • cyclohexane contains only σ bonds • the π bond has a higher electron density than the σ bond and so can attract electrophiles • π bonds are also weaker than σ bonds so π bonds are broken more easily
How can the presence of an alkene be tested? What is observed if an alkene is present?
• bromine water is added • the orange solution rapidly decolourises
Where must curly arrows originate from?
Either from a bond or a lone pair
What are the reaction conditions for an alkene and water?
Water must be steam Concentrated phosphoric acid catalyst