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 geometrical isomerism?
A type of stereoisomerism in which different groups attatched to each carbon of a C=C double bond may be arranged differently in space due to the restricted rotation of the C=C bond
How would you name geometrical isomers?
Put which isomer it is before the name, for example:
* cis-but-2-ene / trans-but-2-ene
* (Z)-but-2-ene / (E)-but-2-ene
Does 2-methylpent-2-ene show E/Z isomerism?
No
Does hex-3-ene show E/Z isomerism?
Yes
Does pent-1-ene show E/Z isomerism?
No
What 2 conditions are required for E/Z isomerism?
- A C=C double bond
- Different groups attached to each carbon on the double bond
Explain the 3 step process of CIP nomenclature:
- Examine the atoms attached directly to the carbon atoms of the double bond, the atom with the higher atomic number has the higher priority
- If the 2 atoms attached to a carbon atom in the double bond are the same, then you must find the first point of difference. The group which has the higher atomic number at the first point of difference is given the higher priority (at this first point of a difference, a double/triple bond to an atom of the same type is given higher piority but when the atoms are the same ONLY)
- Repeat for the other carbon atom of the double bond
Why are alkenes much more reactive than alkanes?
Alkenes are much more reactive due to the presence of the π-bond. Being on the outside of the double bond, the π-bond electrons are more exposed than electrons in the σ-bond, allowing it to readily break and allowing alkenes to undergo addition reactions relatively easily
the π bond is weaker than the σ and is therefore broken more readily
What do alkenes undergo addition reactions with?
- Hydrogen in the presence of a nickel catalyst
- Halogens
- Hydrogen halides
- Steam in the presence of an acid catalyst
What are the conditions for hydrogenation of alkenes?
- Nickel catalyst
- 423K
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 test for an unsaturated compound?
Add bromine water dropwise to a sample of the solution being tested:
* If the bromine water remains orange, then the solution is saturated as no addition reaction has taken place and there is still aqueous bromine.
* If the mixture turns colourless, then the solution is unsaturated as an addition reaction has taken place and there is no longer aqueous bromine.
What is the word equation for the reaction of an alkene with a hydrogen halide?
Alkene + Hydrogen Halide -> Haloalkane
When will the reaction of an alkene with a hydrogen halide form 2 products?
When the alkene is unsymmetrical
What are the conditions for the hydration of alkenes?
- Steam
- In the presence of a phosphoric acid catalyst
What is the word equation for the hydration of alkenes?
Alkene + Water (g) -> Alcohol
What are examples of electrophilic addition reactions?
- Halogenation of alkenes
- The reaction of alkenes with hydrogen halides
- Hydration of alkenes
NOT HYDROGENATION
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 carbocation?
An ion that contains a positively charged carbon atom (is formed as an intermediate during electrophilic addition)
What is the intermediate in electrophilic addition?
A carbocation
What is an intermediate?
A molecule that is formed from two or more reactants and then quickly reacts further to give the products of the reaction
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
From which carbocation will the major product form?
The most stable carbocation
From which carbocation will the minor product form?
From the least stable carbocation
What is the mechanism for the reaction between propene and hydrogen bromide?
(draw each step)
VERY IMPORTANT MECHANISM
What is markownikoff’s rule?
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide is more likely to bond to the carbon atom which is attached to the greater number of hydrogen atoms
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 yield of the major product compared to the minor product and why?
The yield of the major product is much higher as it is formed from the more stable carbocation. Therefore as there will be more of the more stable carbocation, more of the major product will form increasing the yield compared to the minor product
What is a polymer?
A large moleculwe formed from many thousands of repeating units of smaller molecules known as monomers
What is addition polymerisation?
Formation of a very long molecular chain, by repeated addition reactions of many unsaturated alkene molecules (monomers)
How do you name and addition polymer?
poly-(monomer name)
What is the general equation for addition polymerisation?
What is a repeat unit?
The specific arrangement of atoms in the polymer molecule that repeat over and over again
What is feedstock?
A raw, unprocessed material to supply or fuel a machine or industrial process
What are bioplastics?
Polymers produced from plant starch, cellulose, plant oils, and proteins that offer a renewable and sustainable alternative to oil-based products
What are the benefits of bioplastics?
- Protects the environment from long term polymer waste
- Conserves valuable oil reserves
What are photodegradeable polymers?
Photodegradeable polymers are oil-based polymers that contain bonds that are weakened byu absorbing light in order to stard the degredation process. Alternatively, light-absorbing additives are used
What are biodegradeable polymers?
Polymers that can be broken down by microorganisms into water, carbon dioxide, and other biological compounds. These polymers are usually made of starch or cellulose, or may contain additives that alter the structure of traditional polymers so that microorganisms can break them down
What is feedstock recycling?
Feedstock recycling describes the chemical and thermal processes that can reclaim monomers, gases, or oil form waste polymers. The products from feedstock recycling resemble those produced from crude oil in refineries. These materials can then be used as raw materials for the production of new polymers
What is the advantage of feedstock recycling over traditional recycling?
Feedstock recycling is able to handle unsorted or unwashed polymers, unlike traditional recycling which requires the polymers to be sorted and separated
How can waste polymers be used as fuel?
Some polymers are difficult to recycle, however due to the fact that they are derived from petroleum or natural gas they have high stored energy value. Therefore waste polymers may be incinerated in order to produce heat, generating steam to drive a turbine and generate electricity
Why is the disposal and recycling or PVC hazardous?
PVC (polychloroethene) is hazardous due to the high chlorine content and the range of additives present in the polymer. Dumping PVC into landfill is not sustainable and, when burnt, PVC releases hydrogen chloride gas, a corrosive gas, and other toxic pollutants
How is PVC recycled?
- PVC may be ground up into small pieces and then reused to manufacture new products
- Solvents can eb used to dissolve the polymer, the PVC can the be recovered by precipitation from the solvent
Why is recycling beneficial?
Recycling polymers reduces their environmental impact by conserving finite fossil fuels as well as decreasing the amount of waste going to landfill
How are polymers recycled?
DIscarded polymers must be sorted by type, once sorted, the polymers are chopped into flakes, washed, dried, and melted. The recycled polymer is cur into pellets and used by manufacturers to make new products
