Alkenes Flashcards
Are alkenes saturated or unsaturated
Unsaturated because they contain at least one carbon - carbon double bond
What is the general formula for an alkene
CnH2n
Do alkenes form a homologous series
Yes
For a carbon of the double bond, how many sigma bonds are formed and between which atoms are they formed
3 sigma bonds, 1 between the carbon in which it’s double bonded too and 2 to the other atoms e.g. hydrogen or carbon
How is a Pi bond formed
Formed by the sideways overlap of 2 p-orbitals, one from each carbon atom of the double bond
Where is the Pi- electron density concentrated in a double bond
Above and below the line joining the nuclei of the bonding atoms
What bond (sigma or Pi) locks the carbon atoms in an alkene in place, preventing them from rotating around the double bond
Pi bond
How does the geometry differ in alkanes and alkenes
Rotation is possible in alkanes since there is no Pi bond whereas rotation cannot happen in alkenes due to the presence of a Pi bond.
What is the bond angle and shape around the carbon atoms in the double bond in an alkene
Trigonal planar (120)
- There are 3 regions of electron density around each carbon atom
What is a stereoisomer
A compound with the same structural formula but a different arrangement of atoms in space
What type of isomerism occurs in compounds with C=C double bond
E/Z isomers
Why does stereoisomers arise around a double bond
Because rotation around the double bond is restricted so the groups attached to each carbon is fixed
Why are the groups attached to each double bonded carbon restricted
Due to the position of the Pi bond above and below the C=C double bond
What 2 conditions are required for a molecule to have E/Z isomerism
- A C=C double bond
- Different groups attached to each carbon atom of the double bond
Why can But-1-ene not form E/Z isomers whereas But-2-ene can
Because one of the carbons in the double bond in But-1-ene is attached to 2 hydrogens meaning that there isn’t a different group attached to each carbon
What is cis-trans isomerism
The name commonly used to describe a special case of E/Z isomerism.
What extra rule differs E/Z isomerism and cis-trans isomerism
In cis-trans isomerism one of the attached groups on each carbon atom of the double bond must be hydrogen
Is But-2-ene an example of a molecule with E/Z isomers or cis-trans isomers
Both, because there is hydrogen attached to each carbon involved in the double bond
In a cis isomer, where are the hydrogens
On the same side of the molecule
In a trans isomer, where are the hydrogens
Diagonally opposite each other in the molecules
In an E isomer, are the same atoms/groups opposite each other in the molecule or on the same side of the molecule
Opposite
In a Z isomer, are the same atoms/ groups opposite each other in the molecule or on the same side of the molecule
The same side
In cis-trans isomerism, is the cis isomer the E or the Z
Z isomer
In cis-trans isomerism, is the trans isomer the E or the Z
E isomer
When naming isomers, how do you assigns priority
Upon their atomic number
If the groups of higher priority are on the same side of the double bond, what isomer is the compound
Z isomer
If groups of higher priority are diagonally placed across the double bond, what isomer is the compound
E isomer
If the 2 atoms attached to a carbon atom in the double bond are the same, what do you do when trying to identity the isomerism
Find the first point of difference
Why are alkenes much more reactive than alkanes
Because of the presence of the Pi bond
Why do Pi bonds readily break
Because they’re on the outside of the double bond, so they are more exposed than the electrons in the sigma bond
What are the 4 addition reactions of alkenes
- Hydrogen in the presence of a nickel catalyst
- Halogens
- Hydrogen halides
- Steam in the presence of an acid catalyst
What bond breaks when alkenes undergo addition reactions
The Pi bond
What are the 2 conditions required for the hydrogenation of alkenes
- Heat
- Nickel catalyst
What is the conditions required for the halogenation of alkenes
Room temperature
What element is used when testing for unsaturation
Bromine
What happens to the bromine water when it is added to an organic compound
The orange colour disappears and becomes colourless
If bromine is added to any C=C bond what will happen
It will be decolourised
What is the name of product when an alkene reacts with a hydrogen halide
Haloalkane
How many products are there when an unsymmetrical alkene reacts with an unsymmetrical molecule e.g. HCl
2 possible products
When alkenes react with steam what is the name of the group formed
Alcohols
What catalyst is required when alkenes react with steam
Phosphoric acid (H3PO4)
How many products can be produced when alkenes react with steam
2 products
What is an electrophile
An atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair
What is the name of the mechanism, where alkenes take part in addition reactions to form saturated compounds
Electrophilic addition
Does the double bond in alkenes, with the presence of a Pi bond, represent a region of high or low electron density
High electron density
In an electrophilic addition reaction, what does the high electron density of the Pi bond attract
It attracts electrophiles
Is an electrophile, usually an positive or negative ion or a molecule with a slightly positive or negative charge
Positive ion or a molecule with a slightly positive charge
What are the 6 steps in an electrophilic addition reaction of But-2-ene and HBr
- Br is more electronegative than hydrogen, so HBr is polar and contains dipoles
- The electron pair in the Pi bond is attracted to the slightly positive hydrogen atom, breaking the double bond
- A bond is now formed between the H and the C which had the initial double bond
- H-Br bond breaks by heterolytic fission, the electron pair goes to the bromine
- Leaving a Br- ion and a carbocation
- Br- ion reacts with the carbocation
What is a carbocation
A positively charged carbon atom
When a non-polar molecule wants to react in a elecrophilic addition reaction how are the dipoles decided
When one of the atoms in the non-polar molecule approaches the Pi bond, the electrons in the non-polar molecule bond are repelled, so polarity has been induced
What are the 3 types of carbocations
- Primary
- Secondary
- Tertiary
How many carbons is a primary carbocation bonded to
1 carbon
How many carbons is a secondary carbocation bonded to
2 carbons
How many carbons is a teritary carbocation bonded to
3 carbons
What is the order of stability of carbocations
Primary < Secondary < Tertiary
When drawing the electrophilic addition reaction mechanism what do the curly arrow represent
The bonds breaking
What is Markownikoff’s rule
The major product of the addition reaction will be the one formed via the more stable carbocation intermediate and the stability is in the order primary<secondary<tertiary
What is a polymer
Extremely large molecules formed from many thousands of repeat units of smaller molecules known as monomers
What type of polymerisation do unsaturated alkenes undergo to produce long, saturated chains containing no double bonds
Addition polymerisation
What 3 conditions are required in industrial polymerisation
- High pressure
- High temperature
- Using a catalyst
Do addition polymers have a high or low molecular mass
High molecular mass
When writing out an addition polymerisation equation, what types of brackets is the repeat unit always written in
Square brackets
What is the common prefix of addition polymers
Poly(alkene)
What is Poly(chloroethene) more commonly known as
PVC
Are the major of alkene-based products biodegradable
No, so they tend to grow in landfills
Why is recycling polymers good
It conserves finite fossil fuels and decreases the amount of waste going into landfill
What is the disposal of PVC so toxic
Due to the high chlorine content
What is released when PVC is burnt
HCl, which is a corrosive gas, and other pollutants like toxic dioxins
How do we use waste polymers as fuel
Waste polymers can be incinerated to produce heat, which generates steam to drive a turbine producing electricity
What is feedstock recycling
Describes the chemical and thermal processes that can reclaim monomers, gases, or oil from waste polymers
The products from feedstock recycling resemble products similar to what
Products produced from crude oil refineries
What is an advantage of feedstock recycling
It is able to handle unsorted and unwashed polymers
What are biodegradable polymers usually made from
- Starch
- Cellulose
- Additives that alter the structure
What does photodegradable mean
Can be broken down by light-driven chemical processes
