Alkenes Flashcards
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
What are the reaction conditions for an alkene and hydrogen?
150°C
Nickel catalyst
What are the reaction conditions for an alkene and a halogen?
Room temperature and pressure
What are the reaction conditions for an alkene and a hydrogen halide?
Room temperature and pressure
Define addition reaction
A reaction in which a reactant is added to an unstaurated molecule to make one saturated molecule
Define heterolytic fission
The breaking of a covalent bond forming a cation and an anion
Why does the halogen/water/hydrogen/hydrogen halide become polarised during the reaction with an alkene?
The high electron density of the C=C polarises the molecule
Why can the H-Br molecules be considered an electrophile in the 1st step of electrophilic addition?
The H-Br molecule accepts a pair of electrons from the π bond to form a new C-H bond
Why does one of the carbon atoms have a positive charge during the intermediate of electrophilic addition?
One of the electrons in the π bond orginally belonged to this Carbon atom and this electron has now been lost as it is part of the new C-H bond
