Module 4: Chapter 13 - Alkenes Flashcards
What is hybridisation?
The mixing of atomic orbitals (of different energies) in order to form new hybrid orbitals (of equal energy) suitable for forming covalent bonds
Explain hybridisation in carbon atoms:
A carbon atom has the electronic arrangement 1s² 2s² 2p², however, when carbon bonds to other atoms the valence bonding electrons in shell 2 must exist at equivalent energies (2p is of higher energy than 2s). Therefore, the 2s and 2p shells “hybridise” to form an energy level which is between 2s and 2p (2sp)
What is 2sp³ hybridisation in carbon?
This is when carbon is surrounded by 4 electron regions. 1 2s orbital and 3 2p orbitals contribute to the hybridisation forming 4 2sp³ orbitals, these 4 orbitals directly overlap with another orbital when bonding to form a sigma bond
What is 2sp² hybridisation in carbon?
This is when carbon is surrounded by 3 electron regions. 1 2s orbital and 2 2p orbitals contribute to the hybridisation forming 3 2sp² orbitals, these 3 orbitals directly overlap with another orbital when bonding to form a sigma bond. The remaining unhybridised p orbital overlaps with another unhybridised p orbital to form a pi bond.
What is 2sp hybridisation in carbon?
This is when carbon is surrounded by 2 electron regions. 1 2s orbital and 1 2p orbital contribute to the hybridisation forming 2 2sp orbitals, these 2 orbitals directly overlap with another orbital when bonding to form a sigma bond. The remaining 2 unhybridised p orbitals each overlap with another unhybridised p orbital to form pi bonds.
When does 2sp³ hybridisation in carbon occur?
When a carbon has 4 σ bonds
When does 2sp² hybridisation in carbon occur?
When a carbon has 3 σ bonds (contains 1 double bond)
When does 2sp hybridisation occur in carbon?
When a carbon has 2 σ bonds (contains a triple bond)
Can hybridised orbitals form pi bonds?
No, hybridised orbitals can only form sigma bonds, pi bonds are always unhybridised
What are alkenes?
Unsaturated hydrocarbons containing at least 1 carbon-carbon double bond
How does the double bond in an alkene form?
For each carbon atom of the double bond, 3 of the 4 valence electrons are used to form sigma bonds (σ bonds). This leaves 1 electron on each carbon atom not involved in σ bonds. This electron is found in an unhybdridised p orbital. A pi bond (π bond) forms from the sideways overlap of 2 p-orbitals, one from each carbon of the double bond.
Where is the electron density located in a π bond?
Above and below the line joining the nuclei of the bonding atoms
Can a double bond rotate?
No, although a signma bond alone can rotate, the π bond locks the 2 carbon atoms in position and prevents them from rotating around the double bond
What is the shape around a double bond?
There are 3 regions of electron density around each of the carbon atoms, therefore they repel each other as far apart as possible, this causes a bond angle of 120°. This causes a trigonal planar shape
What is stereoisomerism?
Isomerism when molecules have both the same molecular formula and the same structural formula but the atoms are joined together in a different spacial arrangement - they occupy different positions in 3-dimensional space
What are the 2 types of stereoisomerism?
- Geometrical Isomerism
- Optical Isomerism
Why does geometrical isomerism occur in alkenes?
Due to the restricted rotation of the C=C double bond, groups on either side of the bond are fixed in position relative to one another. This produces 2 possibilities for the structure, either the groups are on the same side or opposite side
What is the cis isomer?
Non-hydrogen groups/atoms are on the same side of the double bond
What is the Z isomer?
Higher priority groups/atoms are on the same side of the double bond
What is the trans isomer?
Non-hydrogen groups/atoms are on opposite sides across the double bond
What is the E isomer?
Higher priority groups/atoms are on opposite sides across the double bond
What type of molecules can geometrical isomerism occur in?
Molecules with have limited rotation about bonds:
* Molecules containing a double bond
* Cyclic molecule
Why can geometrical isomerism not occur in alkanes?
Alkanes only contain single bonds, therefore there is free rotation about these bonds meanining that although it may look like a different strucutre, it is actually the same just rotated differently about the bond.
What does Z stand for in the z isomer?
Zusammen (german for “together”)
What does E stand for in the E isomer?
Entgegen (german for against)
How is the higher priority group determined in geometrical isomerism?
Using CIP rules
What does CIP rules stand for?
Cahn, Ingold, Prelog rules
When can E/Z nomenclature be used?
In any cases where there is geometrical isomerism
Use this in exams as it is harder to make mistakes using it
When can Cis/Trans nomenclature be used?
Cis/Trans isomerism should only be used when there are 2 H’s and 2 non-hydrogen groups attached to each carbon
What is cis isomerism equivalent (but not the same) as?
Cis ≈ Z
What is trans isomerism equivalent (but not the same) as?
Trans ≈ E
There are 3 structural isomers of C₄H₈, which one of them displays geometrical isomerism?
But-2-ene
Can geometrical isomerism occur if there are 2 of the same group attacted to one end of the C=C?
No, there must be 2 unique groups on each end of the C=C for geometrical isomerism to occur
What are optical isomers?
Isomers that occur when compounds have non-superimposable mirror images
What is the word equation for hydrogenation of alkenes?
Alkene + Hydrogen -> Alkane
What happens if a diene undergoes hydrogenation?
Every one of the double bonds will react with the hydrogen to form a saturated alkane
What is the word equation for the halogenation of alkenes?
Alkene + Halogen -> Dihaloalkane
What is the word equation for the reaction of an alkene with a hydrogen halide?
Alkene + Hydrogen Halide -> Haloalkane
What is the word equation for the hydration of alkenes?
Alkene + Water (g) -> Alcohol
What is the mechanism for the reaction of but-2-ene with hydrogen bromide?
(draw and explain each step)
IMPORTANT
- Bromine is more electronegative than hydrogen, so hydrogen bromide is polar and contains dipoles
- The electron pair in the π-bond is attracted to the partially positive hydrogen atom, causing the double bond to break
- A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was part of the double bond, causing the H-Br bond the break by heterolytic fission, with both the electrons going to the bromine atom
- This forms a Bromide ion and a carbocation
- The Bromide ion reacts with the carbocation to form the addition product
How can electrophilic addition occur with non-polar molecules such as Bromine?
When the non polar molecule (Bromine) approaches the alkene, the π-electrons interact with the electrons in the Br-Br molecule, inducing a dipole in the molecule (Induced dipole)
What is the mechanism for the reaction of propene with bromine?
(draw and explain each step)
IMPORTANT
- As the bromine molecule approaches the alkene, the π electrons interact with the electrons in the Br-Br molecule, inducing a dipole in the molecule
- The electron pair in the π-bond is attracted to the slightly positively charged bromine end of the molecule, causing the double bond to break
- A bond forms between one of the carbon atoms from the double bond and the slightly positively charged bromine atom, causing the Br-Br bond to break by heterolytic fission, with both electrons going to the slightly negatively charged bromine atom
- This forms a negative bromide ion and a carbocation
- The bromide ion reacts with the carbocation to form the addition product
What is a primary carbocation?
A carbocation where the positively charged carbon atom is at the end of a chain directly attached to only 1 other carbon atom
What is a secondary carbocation?
A carbocation where the positively charged carbon atom is on a carbon chain directly attached to 2 other carbon atoms
What is a tertiary carbocation?
A carbocation where the positively charged carbon atom is on a carbon chain directly attached to 3 other carbon atoms
What is the mechanism for the reaction between propene and hydrogen bromide?
(draw each step)
VERY IMPORTANT MECHANISM
Why are tertiary carbocations the most stable and primary carbocations the least stable?
Tertiary carbocations have 3 alkyl groups attatched to the carbocation, whereas primary carbocations only have 1. This makes tertiary carbocations more stable as each alkyl group donates and pushes electrons towards the positive charge of the carbocation, the positive charge being spread over all the alkyl groups. Therefore the more alkyl groups attached to the positively cahrged carbon-atom, the more the charge is spread out, and the more stable the carbocation. This is known as stabilisation by inductive effect.
What is the general equation for addition polymerisation?
