Chapter 13 Flashcards
What is the general formula of alkenes?
CnH2n
What is the main structural characteristic of alkenes?
They contain one or more C=C double bond hence their unsaturated structure
What is the suffix for alkenes?
-ene
What is the functional group of an Alkene?
C=C or a triple C-C bond
Are alkenes immiscible in water? why?
No because they are non-polar
What is the state of C1-C4 alkenes?
Gases
What is the state of C5-C15 alkenes?
Liquids
What is the state of C16+ alkenes?
Solids
What are alkenes useful for?
They are good solvents for other organic compounds
What is the melting point of alkenes and how does it change as chain length increases?
Relatively low melting point, increasing with Mr and chain length
What is a polyunsaturated alkene?
Alkenes that contain two or more C=C double bonds
When are structural isomers possible in alkenes?
After propene
When is a sigma bond formed?
A sigma bond if formed by head on overlap of the orbitals on the carbon atoms.
When is a pi-bond formed?
A pi-bond is formed by sideways overlap of the p-orbitals on the carbon atoms, above and below the plane of the carbon atoms.
Can C=C bonds rotate?
No, there is no free rotation about these bonds
What can alkenes show if there is no rotation around the C=C bond?
Stereoisomerism
Define Stereoisomers
Molecules with the same molecular formula and structural formula but with a different arrangement of the atoms in 3D-shape.
Define E/Z isomerism
Molecules have the same molecular formula but different special arrangement due to the restricted rotation around the C=C double bond
When does E/Z isomerism occur?
It occurs when:
- There is a C=C bond, which prevents free rotation
- AND there are two different groups on each C-atom of the C=C bond.
What are the first 2 Cahn-Ingold-Prelog (CIP) rules?
.These assign a priority (based on atomic number) for each atom attached to each carbon of the C=C bond.
.Higher atomic number results in high priority.
What is meant by the ‘first point of difference’?
If the two atoms attached to the carbon at in the double bond are the same, then you need to find the first point of difference. The atom which has the higher atomic number at the first point of difference is given the higher priority.
What is Cis-trans isomerism?
A special case of E/Z isomerism in which two of the substituent groups are the same
Are E/Z and Cis/trans always consistent?
No
Why are alkenes more reactive than alkanes?
Due to the presence of the Pi-bond
What is stronger, a pi-bond or an sigma bond?
The sigma bond is stronger than a pi-bond
What is the definition of a Addition reaction?
Alkenes undergo addition reactions which is when two reactants join together to form one product.
What is the atom economy of an addition reaction?
100%
What is the conditions and catalyst for the hydrogenation of alkenes?
Nickel catalyst, 150° and 5 atm
What is the conditions for the halogenation of alkenes reaction?
RTP
What is the test for alkenes?
Orange bromine solution is decolourised
What is the conditions for the addition of hydrogen halides reaction?
RTP
What is the conditions and catalyst for the hydration of alkenes reaction?
Catalyst is H3PO4, 300° and 60 atm
What is the definition for an addition to unsymmetrical alkenes reaction?
When an unsymmetrical alkene reacts with an unsymmetrical compound two products/ isomers are possible
What is Markovnikov’s Rule?
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen adds to the carbon that already has the most hydrogen.
What is a cycloalkene ?
Ring structured alkenes that have the prefix ‘cyclo-‘ at the beginning of their names.
What is the nature of the C=C bond?
It consists of a sigma bond and a pi bond
Why can alkenes exhibit stereoisomerism?
There is no free rotation around the C=C bonds.
What is the shape around the C=C bond in ethene?
Each C-atom of the C=C bond has 3 regions of electron density, all bonded pairs which repel each other as far apart as possible to minimise repulsion.
Define an electrophilic addition reaction in alkenes
The C=C bond is a region of high electron density due to the presence of the pi bond. The high electron density attracts electrophiles.
Define an Electrophile?
An electrophile is an atom or group of atoms that is attracted to an electron rich centre and accepts a pair of electrons to form a new covalent bond. Usually a positive ion or molecule containing an atom with a partial positive charge.
Define a carbocation?
A positively charged carbon atom. It is very reactive intermediate, reacting rapidly with any species with a lone pair of electrons.
What are the 5 steps to the mechanism of electrophilic addition of hydrogen bromide to ethene?
- The HBr molecule is polar
- The partial positive charge on the H atom acts as an electrophile
- The HBr bond breaks heterolytically
- A carbocation intermediate and bromide ion form
- The Bromide halogen bonds with the alkene
Why does the Br-Br bond break heterolytically during the mechanism of electrophilic addition of bromine to an alkene?
Because it becomes polarised on its approach to the alkene
What is the pattern in the stability of carbonations?
The stability of carbonations increases as the number of alkyl groups on the positively charged carbon atom increases.
Why does the stability of carbonations increase as the number of alkyl groups on the positively charged carbon atom increases?
This is because the alkyl group contains a greater electron density than hydrogen atoms. This electron density is attracted towards, and reduces, the positive charge on the carbon atom, which has a stabilising effect.
What effect does electrophilic addition to unsymmetrical alkenes have?
Adding a halogen to an unsymmetrical alkene means the halogen can add in two ways forming two different products/ isomers (a major and minor product)
How is it decided which is the major produce and which is the minor product after the electrophilic addition to unsymmetrical isomers?
The major product is formed via the more stable carbocation.
Define a Polymer?
Very long chain molecules made from many small molecules called monomers
Define polymerisation ?
The joining of many monomers to make a polymer
What are addition polymers and when do they form?
Monomers that join together to make one product, the polymer with a higher Mr. They form when the Pi- bond in alkenes break homolytically and new bonds are made with neighbouring Caron atoms.
What are the general conditions for addition polymerisation?
High temperature and pressure. Presence of a catalyst.
Define a repeat unit?
Repeat unit is the specific arrangement of atoms in the polymer molecule that repeats over and over again
What process occurs during separation and recycling of polymers?
The waste is sorted into each different type of polymer. Some are washed, chopped and melted and remoulded into new products.
What process occurs during incineration of polymers?
Some waste polymers can be incinerated and the heat released can be used to generate electricity.
What process occurs during PVC recycling of polymers?
Disposable is hazardous as can lead to the formation of toxic HCl gases. Now dissolved in solvents and recovered by precipitation.
What process occurs during feedstock recycling of polymers?
Waste polymers can be used as a feedstock for the cracking process, which produces new plastics and other chemicals.
What is the purpose of biodegradable and photodegradable polymers?
A range of biodegradable polymers, compostable polymers, soluble polymers and photodegradable polymers are being developed. Usually plant based eg corn starch, sugar cane etc.
