Chapter 13 Flashcards
What is meant by an alkene ?
Alkenes are unsaturated hydrocarbons containing a C=C bond comprising a π-bond (sideways overlap of adjacent p-orbitals above and below the bonding C atoms) and a σ-bond (overlap of orbitals directly between the bonding atoms); with restricted rotation of the π-bond
What is the double bond in alkenes made up of ?
The double bond in an alkene is made up two different types of bond: an ordinary single bond (a sigma bond) and a pi bond.
What is meant by a pi bond ( π-bond ) ?
In a double bond, each carbon atom uses 3 of its 4 outer shell
electrons to form 3 σ – bonds:
2 x C-H σ – bonds
1 x C-C σ – bond
The fourth bonding electron is in a p-orbital. These p-orbitals overlap sideways to form a π-bond.
The electron density in this bond is concentrated above and below the line of the σ – bond.
Pi bond = sideways overlap of adjacent p-orbitals above and below the bonding C atoms
What is meant by a sigma bond ( σ-bond ) ?
overlap of orbitals directly between the bonding atoms
A sigma bond is the overlap of atomic orbitals directly between bonding atoms.
Each sigma bond has two electrons shared between the bonding atoms.
A sigma bond is positioned on a line directly between the bonding atoms
Explain the trigonal planar shape and bond angle around each carbon in the C=C of alkenes in terms of electron pair repulsion
Electron density of pi bond is above and below sigma bond.
The shape around the double bond. Each carbon atom in a double bond has 3 areas of electron density (2 single bonds and 1 double bond). These repel each other and get as far apart as possible.
This means that the bond angles around each carbon are all 120° and the shape around each carbon is trigonal planar. All the atoms are in the same plane – the molecule is flat (planar).
Explain the term “ stereoisomers “
compounds with the same structural formula but with a different arrangement of atoms in space
What must a molecule contain to display stereoisomerism ?
To display stereoisomerism a molecule must contain a C=C double bond with two different groups attached to each carbon.
How does stereoisomerism refer to spacial difference ?
occurs due to the presence of C=C double bonds
restricted rotation around the double bond
groups attached are fixed relative to each other due to the π-bonds (electron density being
above and below the plane of the σ-bond.
Explain the term “ cis–trans isomerism “
a special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same
How does “ cis–trans isomerism “ work ?
“cis” means both of the same substituents are on the same side of the molecule, “trans” means they are on the opposite sides of the molecule.
Isomers where there are two different groups both sides of a double bond (rotation around the double bond is not possible).
Useful when each of the carbons involved in the C=C double bond has two things the same bonded to it.
Explain the term “E/Z isomerism”
an example of stereoisomerism, in terms of restricted
rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group
How does E/Z isomerism work ?
1) Each substituent is given a priority based on atomic
number.
2) Deciding between (E)- and (Z): treat the two sides of the double bond separately.
3) For each side decide which of the two groups takes priority based on atomic number ( highest atomic number takes priority )
4) If the two highest priority groups are on the same side it’s the (Z)-form ( both above or both below ) and if they are on opposite sides it’s the (E)-form. E/Z isomers have different physical and chemical properties.
What is the “extra rule” to E/Z isomerism ?
In cases where a carbon is attached to the same atom you need to continue until you find
the first point of difference.
What is meant by the Cahn–Ingold–Prelog (CIP) priority rules to identify the E and Z stereoisomers?
For each side decide which of the two groups takes priority based on atomic number -highest atomic number takes priority.
Properties of alkenes
Non-polar, immiscible with water, relatively low mp/bps increasing with M r /chain
length.
Describe the reactivity of alkenes in terms of the relatively low bond enthalpy of the π-bond
Electron density of pi bond above and below sigma bond.
The electrons in the π-bond are more exposed than those in the σ-bond. The π-bond breaks more readily. The bond enthalpy for the π-bond is lower than for a σ-bond and is broken more easily. ( less energy is required to break the bond )
What is meant by an addition reaction ?
two reactants join together to form one product.
What is meant by a substitution reaction ?
an atom or group of atoms is replaced by a different atom or
group of atoms.
What is meant by an elimination reaction ?
Elimination – these reactions involve the removal of a small molecule from a larger one. A single reaction forms two products.
Describe the addition reaction of alkenes with hydrogen
Alkenes react with hydrogen in the presence of a Nickel catalyst to produce an alkane. The double bond is broken.
Describe the addition reaction of alkenes with halogens
Form dihaloalkanes
What reaction is used as the standard test for unsaturation ?
when an alkene is shaken with a few drops of orange bromine water, an addition reaction takes place and the bromine water is
decolourised.
Describe the addition reaction of alkenes with hydrogen halides
Form haloalkanes
Describe the addition reaction of alkenes with steam ( water )
Reaction occurs in the presence of an acid catalyst ( eg H3PO4 ) to form alcohols.
Define the term “ electrophile “
An electron pair acceptor
What is meant by a carbocation ?
Any particle/species with a positively charged carbon atom is called a carbocation.
Explain the mechanism of electrophilic addition in alkenes by heterolytic fission? ( eg reaction between ethene and hydrogen bromide )
1) Bromine more electronegative than Hydrogen , so HBr is polar
2) The electron pair in the pi bond in ethene is attracted to the partially positive hydrogen atom, causing the double bond to break, HBr = electrophile
3) The H-Br bond breaks by heterolytic fission, with the elctron pair going to the bromine atom.
4) A negative bromide ion and carbocation form
5) The bromide ion reacts with the carbocation to form the addition product.
Curly arrows are used to show the movement of pairs of electrons
State Markownikoff’s rule
Markownikoff’s rule: When an unsymmetrical alkene reacts with hydrogen bromide, two products are possible. When an unsymmetrical alkene reacts with hydrogen bromide, two products are possible depending
on which of the two possible carbocations is the most stable.
When a hydrogen halide reacts with an assymetric alkene, the hydrogen atom of the hydrogen halide is more likely to bond to the carbon atom which is attached to a greater number of hydrogen atoms.
What is an alkyl group ?
A group containing carbon and hydrogen atoms.
What does the stability of a carbocation depend on ?
Depends on how many alkyl groups ( represented as “R” groups ) are bonded to the carbon atom with the positive charge.
Primary carbocation vs Secondary carbocation vs Tertiary carbocation
Primary : one positive carbon atom is bonded to only one alkyl group
Secondary : one positive carbon atom is bonded to two alkyl groups ( more stable than primary )
Tertiary : one positive carbon atom is bonded to three alkyl groups ( more stable than primary and secondary
What is a polymer ?
- long chain made up of many small monomer units bonded together
- single product molecule: addition polymer
- normally occurs when the monomer is unsaturated, ie with alkenes
What is a monomer ?
a small molecule that combines with many other monomers to form a polymer
Describe the addition polymerisation of alkenes and substituted alkenes,
Alkenes like ethene can undergo addition polymerisation. They join end to end to make eg poly(ethene). After polymerization all the double bonds have gone. The addition polymerisation is carried out at high temperature and pressure, and in the presence of
a catalyst. Addition polymers have high molecular masses.
How do you show a repeat unit of a polymer after addition polymerisation ?
The polymer must be written in square brackets. The letter ‘n’ indicates a large number of repeats. You must write the ns in the right place. The ‘end bonds’ extend outside the brackets.
How do you identify the monomer from the polymer chain ?
Identify the repeat unit by picking two carbons in the main carbon chain and from this the structure of the monomer can be deduced.
List and describe the benefits for sustainability of processing waste polymers by combustion
for energy production
When polymers are difficult to recycle, they may be incinerated as fuel. This is because they are made from crude oil or natural gas so have high stored energy.
The heat produced can be used to drive turbines and produce electricity.
List and describe the benefits for sustainability of processing waste polymers by use as an
organic feedstock for the production of plastics and other organic chemicals
Feedstock recycling - reclaiming monomers from polymers: the monomers are then reused.
Bioplastics - made from plant materials – starch, cellulose, plant oils or proteins.
Bioplastics are renewable – which conserves oil reserves.
Break down in the environment, meaning they protect the environment from the damage caused by
crude oil-based polymers.
They are biodegradable or “compostable”.
Biodegradable polymers are broken down by organisms in the environment. Water and carbon
dioxide are formed, as well as other biological compounds.
Mostly made from starch or cellulose. “Compostable” polymers leave no toxic residues.
List and describe the benefits for sustainability of processing waste polymers by removal of
toxic waste products formed during disposal by combustion of halogenated plastics (e.g. PVC)
Due to the chlorine and additives in PVC, it is hazardous to dispose of. When burnt it produces
hydrogen chloride gas, which is corrosive. Toxins, such as dioxins, are also produced.
Using waste polymers as fuel. When polymers are difficult to recycle, they may be incinerated as
fuel. This is because they are made from crude oil or natural gas so have high stored energy.
The heat produced can be used to drive turbines and produce electricity.
Care must be taken with chlorinated polymers such as PVC as they that can produce dioxins
(toxic) and hydrogen chloride (corrosive).
List and describe benefits to the environment of development of biodegradable and photodegradable polymers
Biodegradable polymers are broken down by organisms in the environment. Water and carbon dioxide are formed, as well as other biological compounds. Mostly made from starch or cellulose. “Compostable” polymers leave no toxic residues. Photodegradable plastics and many other plastics tend to break down into small particles rather than decomposing completely: they are often not biodegradable and so remain in the environment as microplastics. The long-term health effects of microplastics is unknown.
