13 - Alkenes Flashcards
Define alkenes - simple way
• Unsaturated hydrocarbons
Define alkenes - complicated
• Aliphatic alkanes that have 1 double carbon bond = CnH2n
Bonding in alkenes
• Each C has 4 electrons in its outer shell which can be used for bonding.
• 3 of the electrons are used in sigma bonds
o One to the other C atom
o Two to other atoms = alkene so (C or H)
• 1 electron remaining = forms a π bond between 2 C atoms
What are pi bonds
• Sideways overlap of two P orbitals – one from each bonding atom
• Each orbital contributes one electron to the electron pair
• The pi bond density is concentrated above and below the line joining the nuclei of the bonding atom
Where is pi bond density concentrated
above and below the line joining the nuclei of the bonding atom
Which one is weaker - pi or sigma
Pi
• Sigma bonds results from head on overlapping whereas pi bonds can only partially overlap
• In sigma bonds there is more overlapping between orbitals compared to pi bonds
Does the pi bond lock the C atoms in place
YES - preventing rotation around the double bond. = therefore diff geometry than alkanes
What is stereoisomerism
• have the same structural formula but a different arrangement of atoms in space.
Types of stereoisomerism
o E/Z isomerism
o Optical isomerism – A2
Where does stereoisomerism occur
• occurs around a double bond
Why does stereoisomerism round a double bond
because rotation about the double bond is restricted
o Therefore the groups attached to each carbon atom are fixed relative to each other.
• The reason for this rigidity is the position of the π bonds electron density above and below the plane of the σ bond.
• For a molecule to show e/z isomerism it must have…
o A c=c double bond
o Different groups attached to each carbon atom of the double bond
What is cis - trans isomerism
• A special type of E/Z isomerism
• Occurs when one of the attached groups of each carbon atom of the double bonds is a hydrogen
Cis
• H atoms on the same side of C=C
• Z isomer
Trans
• H atoms on opposite sides of C=C
• E isomer
Example of cis / trans
How are atoms prioritised for E/Z isomerism
• Atoms on each carbon are prioritized based upon their atomic number
How does this prioritisation effect stereoisomerism
• The higher the atomic number…
The higher priority
Why are alkenes more reactive than alkanes
The pi bond
How does the pi bond make alkenes more reactive
• As the electron density is above and below the bond, the π-electrons are more exposed than the electrons in the σ-bond.
• A π-bond readily breaks and alkenes undergo addition reactions relatively easily.
What does hydrogenation form
(alkene –> alkane)
How does hydrogenation work
What type of reaction is hydrogenation
Addition
What does halogenation form
alkene -> haloalkane
How does halogenation work
How to test for unsaturation
• Use bromine water (an orange solution)
• When added to alkene, bromine adds across C=C, goes colourless
• When added to alkane, no addition reaction, so no change = stays orange
• Any compound with a C=C will decolourise bromine water
What does Halogenation with hydrogen halides form
alkene -> haloalkane
How does Halogenation with hydrogen halides work
• Alkenes react with hydrogen halides (g) at room temperature.
If the alkene is a gas… (halogenation with hydrogen halides)
the two gases are mixed
If the alkene is a liquid - (halogenation with hydrogen halides)
the HX is bubbled through it
When is a mixture possible when reacting alkenes with hydrogen halides
in UNSYMMETRICAL alkenes
What is Markownikoff’s Rule
• When a HX reacts with an unsymmetrical alkene, the hydrogen of the halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms, and the smaller number of carbon atoms.
The major product is…
The product where the H reacts with the C with the most number of Hs
What does Hydration form
alkene -> alcohol
How does hydration work
How can C=C attract electrophiles
Because of • The high electron density of the π-electrons
Define electrophiles
• An electrophile is an atom/group of atoms that is attracted to an electron-rich centre and accepts an electron pair = an electron pair acceptor
All of these reactions are - alkenes
Electrophiles addition
How to draw electrophiles addition mechanism
Where MUST the curly arrow go
• Must go from an area of high electron density to an area of low electron density
Draw the reaction mechanism (electrophilic mechanism) between propene and bromine
How is carbocation stability classified by
the number of alkyl groups attached to the positively charged carbon atom.
Tertiary carbocation
3 alkyl groups
Secondary carbocation
2 alkyl grouls
Most stable to least stable carbocation
Why is tertiary carbocation most stable
Each alkyl group can donate electrons to the positive charge, stabilising the carbocation.
How are hydrogen halides added to an alkene
• Addition of a hydrogen halide to an unsymmetrical alkene forms via the most stable carbocation.
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