Alkenes Flashcards
General formula of alkenes
CnH2n
What are alkenes
Unsaturated hydrocarbons
What does bonding in alkenes involve
a double covalent bond and a centre of high electron density
What do alkenes consist of
They have a carbon-carbon double bond which consists of a sigma bond and a pi bond
Which is weaker the pi bond or the sigma bond
the pi bond
Why is a compound with the general formula CnH2n not necessarily an alkene
It could be a cycloalkane as they are isomeric with alkenes
How would you name the alkene if it has 2 double bonds
pent-2-ene becomes penta-1,2-diene
-DIENE
How would you name alkenes using IUPAC
select the longest carbon chain containing the double bond
Number the carbon atoms giving the lower number to the carbon atom containing the double bond
What is the arrangement of bond around the carbon-carbon double bond
Trigonal planar
What is the bond angles in an alkene
120 degrees
Why does ethene have a trigonal planar shape
Each carbon has 3 orbitals containing 1 unpaired electron. They repel each other as far apart as possible to a position of minimal repulsion and take up a trigonal planar shape
Why is no free rotation possible around the carbon-carbon double bond
The double bond restricts the rotation
How is a pi bond formed
By the sideways overlap of adjacent p orbitals above and below the bonding carbon atoms and there is high electron density above and below the line between the 2 nuclei in a pi bond
Explain why alkenes are more reactive than alkanes considering the strength of the bonds in these compounds
The C-C bonds in alkanes are sigma bonds.
The C-C double bond in alkenes has a pi bond.
A pi bond is weaker than a sigma bond so less energy is needed to break the pi bond and therefore electrophiles attack the pi bonds in alkenes
Why do electrophiles attack alkenes
The carbon-carbon double bond has a high electron density, so alkenes are attacked by electrophiles. The electrophile attacks the pi bond in carbon-carbon double bond.
What are the similarities between pi bonds and sigma bonds
- Both involve the overlap of 2 atomic orbitals each containing a single electron
- Both involve an attraction between a bonding pair of electrons and 2 nuclei
- In both bonds the 2 electrons are localised(fixed in position) between 2 atoms in the bond
What are the differences between pi and sigma bonds
- sigma bond is formed by the head on overlap of 2 atomic orbitals whereas a pi bond is formed by adjacent overlap of p orbitals
- There is high electron density along line between 2 nuclei in sigma bond but in a pi bond it is above and below the line between 2 nuclei
- sigma bond is formed by overlap of a single lobe, but pi bond is formed by overlap of 2 lobes from a single orbital
What is the shape of any alkane molecule
Tetrahedral
Why is the shape of all alkanes tetrahedral
There are 4 bonding pairs of electrons around the central C atom. These electron pairs repel each other as far apart as possible to a position of minimum repulsion
What is the shape of any alkene molecule
Trigonal planar
Why is the shape of any alkene molecule trigonal planar
There are 3 regions of electron density around the central C atom. These electron regions repel each other as far apart as possible to a position of minimum repulsion
Steroisomerism
Same structural formula but different spatial arrangement of atoms
Why does E-Z isomerism occur
Due to restricted rotation about the carbon-carbon double bond due to the pi bond preventing it
Why does geometric isomerism occur
Due to the non-rotation (restricted rotation) of the carbon-carbon double bond because the pi bond prevents it.
where are the H atoms in a cis isomer
Both the hydrogen atoms are on the same side of the carbon-carbon double bond
Conditions needed for geometric isomerism
There should be 2 different atoms/groups bonded to each unsaturated carbon atom
There should be carbon-carbon double bond which restricts rotation
How would you distinguish between 2 steroisomers
they have different BP/MP
There is a difference in their dipoles
They have different infrared spectra
Why does one isomer not convert to the other at room temperature
The carbon-carbon double bond restricts rotation
There isn’t enough energy to break the pi bond at room temperature
When can you not use the cis-trans system and have to use the E-Z nomenclature
When the 4 groups around the carbon-carbon double bond are different
Z-isomer
The highest priority groups on each carbon of the C=C double bond are on the same side of the carbon-carbon double bond. Z corresponds to cis
E-isomer
The highest priority groups on each carbon of the C=C double bond are on opposite sides of the C=C bond. E corresponds to trans
Types of structural isomerism
Chain, Position and Functional group isomerism
Types of steroisomerism
Geometric and optical isomerism
Electrophile
A species which accepts a pair of electrons
How do alkenes react
By electrophillic addition
What happens in electrophilic addition
A molecule adds across the C=C bond to give a single saturated product
Conditions needed for addition reaction of alkenes with hydrogen
Nickel catalyst and heat at 150 degrees
Explain why bromine molecules react with the double bonds in alkenes
C=C is an area of high electron density
Br-Br becomes polarised
The delta positive bromine molecule is attracted to the C=C double bond
How to test for unsaturation / double bond in an organic compound
Add bromine water to the compound and shake. It turns from orange to colourless.
What happens when a hydrogen halide is added to an unsymmetrical alkene
The more electropositive atom with be added to the unsaturated carbon atom with the greatest number of hydrogen atoms
Conditions needed for electrophilic reaction fo alkenes with hydrogen halides, forming haloalkanes
gas phase, room temperature
What does the reaction between alkenes and steam in the presence of a catalyst form
Alchohols
Conditions needed for reaction between alkenes and steam to form alcohols
High pressure and high temperature and H3PO4 catalyst
Explain why but-1-ene can form 2 products when it reacts with HBr
Hydrogen could add to either carbon in the double bond
How to determine which product is the major/minor product
teritary>secondary>primary
This means tertiary will always be major and primary will be minor product
Describe the steps of electrophilic addition when alkenes react with H, halogens and hydrogen halides
- Pair of electrons from pi bond is attracted the electropositive bromine atom and foems a dative covalent bond with bromine atom. Pi bond breaks
- Carbocation is attacked by bromide ion. Lone pair of electrons on Br- ion forms a dative covalent bond with the positively charged carbon atom
How does the stability of carbocations change with number of alkyl groups
Stability increases with number of alkyl groups because alkyl groups are electron-donating and therefore push the shared pair of electrons towards the carbon atom with the positive charge so positive charge on C atom reduces and carbocation is more stable.
Why is a tertiary carbocation the most stable
Due to the electron-donating character of three alkyl groups as opposed to 2 or 1
What is addition polymerisation
Many monomers react to form a large molecule (polymer) by an addition reaction.
The double bond breaks and only one product forms
What is the empirical formula of a poly(alkene)
The same as that of the monomer from which it is produced since no atoms are lost in the addition reaction
Uses of poly(chloroethene) / PVC
water pipes, electrical insulation and guttering
Uses of PTFE
non-stick coating for saucepans and burette taps
Are addition polymers reactive or unreactive
Unreactive
Problems with disposal of polymers
Addition polymers are mostly non-biodegradable since they are chemically inert and burning them produces toxic gases like hydrogen chloride. FIll up landfull sites
Conditions for reaction between alkenes and halogens
Room temperature
Why is the major product 2-chloropropane when propene reacts with hydrogen chloride?
The carbocation intermediate in the formation of 2-chloropropane is more stable than the carbocation in the formation of 1-chloropropane.
A secondary carbocation is more stable than a primary carbocation due to the electron donating character of 2 alkyl groups as opposed to one.
Suggest why it is not possible to quote an exact value for the molar mass of any addition polymer but only an average value
The polymer is made up of molecules of different chain lengths
Why does any polymer not have a sharp melting point
It is made up of molecules of different chain lengths and therefore melts over a range of temperatures
Why does poly(chloroethene) have a higher MP than poly(ethene)
There are permanent dipole-dipole attractions between the polymer chains in poly(chloroethene).
There are Van der Waals forces between the polymer chains in poly(ethene), which are weaker.
More energy is needed to break the stronger intermolecular forces between the polymer chains in poly(chloroethene).
