Organic Chemistry, Alkanes and Alkenes (Chapter 11, 12 and 13) Flashcards
What happens during the hydrogenation of alkenes if two double bonds are present?
Both double bonds are hydrogenated, so 2H2 is needed
Cis =
Z
How do photodegradable polymers decompose?
Carbonyl (C=O) bonds in the backbone of a polymer absorb light energy and break, fracturing the polymer chain
They are initially converted to waxy compounds when exposed to light, and then to carbon dioxide and water in the presence of bacteria
What is the equation for initiation?
Br — Br =(UV)= Br• + Br•
What is an alkene?
An unsaturated hydrocarbon containing a carbon-carbon double bond comprising a pi bond and a sigma bond
What happens during the first propagation step in radical substitution?
A bromine radical reacts with a C-H bond in the methane, forming a methyl radical (CH3•) and HBr
What is an elimination reaction?
The removal of a small molecule from a saturated compound to make an unsaturated compound
How does bond enthalpy affect the reactivity of the pi bond?
- A pi bond has a lower bond enthalpy than a sigma bond
- Therefore, the pi bond is weaker and is broken more easily
Why are radicals very reactive intermediates and what does this result in?
Because they have a single unpaired electron
This results in reactions occurring via a radical mechanism which is very difficult to control
What is an electrophile?
1) An electron pair acceptor
2) Atoms or groups of atoms that are attracted to an electron-rich centre and can accept an electron pair
3) They are always positively charged or partially positively charged
Via what does addition of a hydrogen halide to an unsymmetrical alkene form the major product?
The most stable carbocation
What is the molecular formula?
The number and type of each element present in a molecule e.g. C3H8O
What is the hydration of alkenes mainly used for?
To produce ethanol from ethene
How reactive are alkanes?
Not very - alkanes do not react with most common reagents
Explain carbocation stability
1) Each alkyl group donates and pushes electrons towards the positive charge of the carbocation (which is spread over the alkyl groups)
2) The more alkyl groups attached to the positively charged carbon atom, the more the charge is spread out, making the ion more stable
3) Therefore, the tertiary carbocation is more stable than the secondary, which is more stable than the primary
What do alkenes undergo addition reactions with?
Hydrogen ( + nickel catalyst)
Halogens
Hydrogen halides
Steam ( + acid catalyst)
What is the major product?
The more common product
The product formed from the secondary (or largest e.g tertiary etc) carbocation e.g. 2-bromopropane
What is the equation for the hydration of propene?
Propene + H2O (g) = propan-1-ol or propan-2-ol
Why is UV light needed for the reaction of alkanes and halogens?
The high-energy UV radiation present in sunlight provides the initial energy for a reaction to take place
Define aromatic
A compound containing a benzene ring
What are bioplastics?
They are made from plant starch, cellulose, plant oils and proteins
They offer a renewable and sustainable alternative to oil-based products
The use of them protects our environment and conserves valuable oil reserves
When the n is written in front of the monomer, what does it signify?
Many units of the monomer
What are compostable polymers?
Biodegradable polymers that degrade and leave no visible or toxic residues
Define aliphatic
A compound containing carbon and hydrogen joined together in straight chains, branched chains or non-aromatic rings
What is the ‘more highly substituted carbon atom’?
The carbon atom of the alkene with the greater number of hydrogen atoms and smaller number of carbon atoms
When do alkenes not obey their general formula?
When they are cyclic or polyunsaturated
How are photodegradable polymers made?
By blending the polymer with light-sensitive additives that catalyse the breakdown of the polymer
What is carbocation stability linked to?
The electron donating ability of alkyl groups
What is an alkane?
A saturated hydrocarbon containing single C–C and C–H bonds as σ-bonds
What is the minor product?
The less common product
The product formed from the primary carbocation e.g. 1-bromopropane
What is a sigma bond?
- A single covalent bond
- The overlap of orbitals directly between the bonding atoms
- It has free rotation
What happens during hydration reactions of alkenes?
Alkenes react with steam ( H2O (g) ) to form alcohols, in the presence of a phosphoric acid catalyst (H3PO4)
What is an example of a compostable polymer?
Poly(lactic acid) - it is being used in packaging and is made from corn starch (a renewable resource)
What are addition reactions of alkenes?
They involved the addition of a small molecule across the double bond, causing the pi bond to break and new bonds to form
Why is substitution at different positions in the carbon chain not a limitation for methane or ethane?
For methane, all four hydrogen atoms are bonded to the same carbon atom so only one monobromo compound, CH3Br is possible
Bromoethane could not have any structural isomers
When does heterolytic fission happen?
In a polar bond with an electronegativity difference
What is the skeletal formula?
The simplified organic formula, shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional groups
- angle in C bonds = 109.5
- — = C1 - C2
What makes alkenes much more reactive than alkanes?
Because of the presence of the pi bond
What is electrophilic addition?
The mechanism for addition reactions to form saturated compounds
What is a monomer?
A small molecule that combines with many other monomers to form a polymer
What is cis isomerism?
When the hydrogen atoms on each carbon atom are on the same side of the double bond
What happens when polymers are recycled?
1) Discarded polymers have to be sorted by type
2) Once sorted, the polymers are chopped into flakes, washed, dried and melted
3) The recycled polymer is cut into pellets and used by manufacturers to make new products
What is an example of a biodegradable polymer?
Poly(glycolic acid) - it is being used in surgical stitches and is made from sugar cane and unripe grapes (renewable resources)
What is the displayed formula?
It shows the relative positioning (in 2D) of all the atoms and the bonds between them
What is a C=C double bond?
A sigma bond and a pi bond
What is trans isomerism?
When the hydrogen atoms on each carbon atom are diagonally opposite
What happens during the second propagation step in radical substitution?
Each methyl radical reacts with another bromine molecule, forming CH3Br and a new bromine radical
What are carbocations classified by?
The number of alkyl groups (represented by ‘-R’) attached to the positively charged carbon atom
What is Markownikoff’s rule?
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the more highly substituted carbon atom
Define hydrocarbon
A compound containing hydrogen and carbon only
Where is the positive charge on the primary carbocation?
On a carbon atom at the end of a chain
How does a pi bond form?
A pi bond is formed by the sideways overlap of 2p orbitals on adjacent carbons, with electron density concentrated above and below the plane of the carbon sigma bond
Why are alkanes used as fuels?
Because they are readily available, easy to transport and don’t release toxic products during complete combustion
What is a reaction mechanism?
It tells us what is happening to the bonding electrons during a chemical reaction
What is the problem of disposing PVC?
Dumping PVC in a landfill is not sustainable
PVC releases HCl(g) (a corrosive gas) and other pollutants such as toxic dioxins when burnt
Why is CO harmful?
CO is a colourless and odourless toxic gas which combines irreversibly with haemoglobin, preventing it from transporting O2 around the body
Explain why subsitution at diferent positions in the carbon chain is a limitation of radical substitution
If the carbon chain is long enough (e.g. pentane), there will be a mixture of monosubstituted isomers
What kind of reaction is the halogenation of alkanes?
A substitution reaction - a H atom in the alkane is substituted by a halogen atom
What reaction happens in the propagation stage of radical substitution?
The reaction propagates through 2 steps in a chain reaction
What happens in practice during propagation?
Propagation is terminated whenever 2 radicals collide
What are the two carbocations possible in the reaction between propene and a hydrogen halide?
Primary and secondary carbocation
What is the bond enthalpy of a sigma bond?
347 KJ/mol
When to alkanes react with the halogens and which halogens?
In the presence of sunlight (UV light), alkanes react with halogens (only Cl2 and Br2)
What is poly(propene) used in?
Children’s toys, packing crates and guttering
What is the equation for the first propagation step in radical substitution?
CH4 + Br• = •CH3 + HBr
What is a radical?
A species with an unpaired electron
What does the type of polymer and its specific properties depend on?
The monomer used in addition polymerisation
Why are alkanes tetrahedral with a bond angle of 109.5?
1) Electron pairs repel each other
2) Each carbon atom in an alkane is surrounded by 4 bonding pairs of electrons in four sigma bonds
3) They repel each other to give the 3D tetrahedral shape around each carbon atom with each bonding angle being 109.5
When the n is written after the repeat unit, what does it signify?
A large number of repeats
Where is the positive charge on the secondary carbocation?
On a carbon atom with two carbon chains attached
Explain what happens during the electrophilic addition reaction ethene and HBr
1) Bromine tis more electronegative than hydrogen, so HBr is a polar molecule and contains the dipole Hdelta+ - Brdelta-
2) The electron pair in the pi bond is attracted to the partially positive hydrogen atom, causing the double bond to break
3) A bond forms between the hydrogen atom of H-Br molecule and a carbon atom that was part of the double bond
4) So the H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom
5) This results in two charged intermediates - a carbocation and a bromide ion
6) The bromide ion reacts with the carbocation to form the addition product (bromoethane)
Describe the addition reaction of alkenes with hydrogen halides?
1) Alkenes react with gaseous hydrogen halides at room temp to form haloalkanes
2) If the alkene is a gas, the two gases are mixed
3) If the alkene is a liquid, the gas is bubbled through the liquid
4) Alkenes also react with concentrated HCl or HBr acid
5) There are sometimes more than one product
What happens during the hydrogenation of alkenes?
When an alkene is mixed with hydrogen and passed over a nickel catalyst at 423K (150 C), an addition reaction takes place, to form an alkene
What is the structural formula?
It shows the arrangement of atoms in a molecule with minimal detail
- functional groups are clear and any branches are shown in brackets e.g. CH3CH(OH)CH3
Define saturated
Single carbon-carbon bonds only
Why does the double bond in an alkene represent a region of high electron density?
Because of the pi electrons in the pi bond which attract electrophiles
What are photodegradable polymers?
Polymers that contain bonds that are weakened by absorbing light to start the degradation
They are designed to become weak and brittle when exposed to sunlight for prolonged periods
What are biodegradable polymers?
Polymers that are broken down by microorganisms into water, carbon dioxide and other biological compounds
Why does E/Z isomerism only occur in compounds with a C=C double bond?
Because rotation about the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other
This rigidity is due to the position of the pi-bond’s electron density above and below the plane of the sigma bond
What is 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
What is poly(ethene) used in?
Supermarket bags, children’s toys and shampoo bottles
How are waste polymers used as fuel?
- Waste polymers can be incinerated to produce steam to drive a turbine, producing electricity
- Waste gases from the combustion (e.g. HCl) are passed through scrubbers which can neutralise gases such as HCl by allowing them to react with a base
What are stereoisomers?
Compounds with the same structural formula but a different arrangement of the atoms in space
What is the bromination of alkanes an example of?
Radical substitution
What is the empirical formula?
The simplest whole number ratio of the atoms of each element present in a molecule
What is an addition reaction?
When two reactants join together to form one product
Adding a small molecule across an unsaturated compound to make a saturated compound
What are the two limitations of radical substitution in organic synthesis?
1) Further substitution
2) Substitutions at different positions in the carbon chain
What is a substitution reaction?
When an atom or group of atoms is replaced by another atom or group of atoms
How will bioplastics be used in the future?
In packaging, electronics and more fuel-efficient and recyclable vehicles
Why are polymers so common?
They are readily available, cheap to purchase and more convenient than alternatives e.g. glass bottles and paper bags
Their lack of reactivity makes polymers suitable for storing food and chemicals safely
How does recycling polymers reduce their environmental impact?
It conserves finite fossil fuels
It decreases the amount of landfill waste
What are biodegradable polymers made from?
Starch or cellulose or they contain additives that alter the structure of traditional polymers so that microorganisms can break them down
What is the algebraic equation for complete combustion?
CxHy + (x+y/4)O2 = xCO2 + y/2H2O
Define homologous series
A family of compounds with the same functional group and each successive member differs by a CH2 unit (and similar chemical properties)
Explain what happens during further substitution as a limitation of radical substitution
1) Another halogen radical (Br’) can collide with the haloalkane (CH3Br), substituting a further hydrogen atom to form CH2Br2
2) Further substitution can continue until all hydrogen atoms have been substituted
3) The result is a mixture of CH3Br, CH2Br2, CHBr3 and CBr4
Describe the the electrons in an alkene before a pi-bond is formed
1) Carbon has 4 electrons in its outer shell
2) For each carbon atom in the double bond, 3 of the 4 electrons are used in 3 sigma bonds with other atoms
3) The fourth electrons remains in a 2p orbital, perpendicular to the plane of the C-C sigma bond
Describe sigma bonding in alkanes
1) A sigma bond is the result of the overlap of two atomic orbitals, one from each bonding atom
2) Each overlapping orbital contains one electron, so the sigma bond has two electrons that are shared between the bonding atoms
3) A sigma bond is positioned on a line directly between bonding atoms
4) As the bonds are non-polar, the electron density is located equidistant between the two atoms
What happens during the halogenation of alkenes?
Alkenes undergo a rapid addition reaction with chlorine or bromine at room temperature with no catalyst
What happens when PVC is recycled?
Solvents are used to dissolve the PVC
High-grade PVC is then recovered by precipitation from the solvent, and the solvent is used again
What is a polymer?
A long-chain molecule made up of many repeating units of monomers
Explain what happens during the electrophilic addition between ethene and bromine
1) Bromine is a non-polar molecule. When Br approaches an alkene, the pi electrons interact with the electrons in the Br-Br bond
2) The interaction causes the polarisation of the Br-Br bond with one end becoming Brdelta+ and the other Brdelta- (this is an induced dipole)
3) The electron pair in the pi bond is attracted to the Brdelta+ end of the molecule, causing the double bond to break
4) A single bond forms between a carbon atom from the double bond and the Brdelta+ in Br-Br
5) The Br-Br bond breaks by heterolytic fission, with the electron pair going to the Brdelta- end of the molecule
6) This results in two charged intermediates - a carbocation and a bromide ion
7) The bromine ion reacts with the carbocation forming a new single bond (C-Br) and the addition product (1,2-dibromoethane)
What is the problem with polymers?
Their lack of reactivity makes them hard to dispose of
Many alkene-based polymers are non-biodegradable which has serious environmental effects e.g. killing marine life
What happens during the termination stage in radical substitution?
1) Two radicals collide, forming a molecule with all electrons paired
2) When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction
What happens during the initiation stage of radical substitution?
1) The reaction is started when the covalent bond is broken by homolytic fission (in the presence of UV)
2) Each Br atom takes one electron from the pair, forming two highly reactive bromine radicals
Define alicyclic
An aliphatic compound arranged in non-aromatic rings with or without side-chains
When are there two possible products from the addition reaction of alkenes and hydrogen halides?
Unsymmetrical alkene + unsymmetrical compound
What is heterolytic fission?
When a bond is broken and one of the bonded atoms takes both of the electrons from the bonding pair, resulting in charged ions
What does the incomplete combustion of alkanes produce?
With limited supplies of oxygen CO or C is formed instead of CO2
How is industrial polymerisation carried out?
At high temperatures and high pressure using a catalyst
Why is the disposal and recycling of PVC hazardous?
Because of the high chlorine content and the range of additives in the polymer
What is a key point to remember about electrophiles?
An electrophile wants electrons, therefore it is not a random collision
What type of bond joins the atoms in an alkane?
A sigma bond
How do you identify E/Z isomers using the Cahn-Ingold-Prelog rules?
1) The atom attached to a carbon with the highest atomic number = the highest priority group
2) E = 2 highest priority groups are diagonal on a double bond
Z = 2 highest priority groups are on the same side of a double bond
3) If the atoms are the same, find the first point of difference in the group
What are the three stages of radical substitution?
Initiation, propagation and termination
What is the equation for the bromination of propene?
Propene + Br2 = 1,2-dibromopropane
When does homolytic fission happen?
In non-polar bonds
Why are some polymers used as fuel?
Some polymers are difficult to recycle but as they are derived from petrol or natural gas, they have a high stored energy value
Why is the shape around each carbon atom in the double bond trigonal planar?
1) Electron pairs repel each other
2) There are three regions of electron density around each of the carbon atoms
3) The three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120
4) All of the atoms are in the same plane so the shape is trigonal planar
What is the bond enthalpy of a C=C double bond?
612 KJ/mol
What does the pi bond mean for the alkene?
1) The pi bond locks the two carbon atoms in position and prevents them from rotating around the double bond
2) This makes the geometry of the alkenes different from that of the alkanes, where rotation is possible around every atom
3) The 2p orbitals must be parallel to overlap sideways so any rotation means that the pi bond can’t form
What is the bond enthalpy of a pi bond?
612-347 = 265 KJ/mol
How are synthetic polymers named?
After the monomer that reacts to form their giant molecules, prefixed by poly with the monomer in brackets
What is the general formula for aliphatic alkenes with one double bond (can be branched)?
CnH2n
What effect does chain length have on the boiling point of alkanes?
Increased chain length = greater SA of contact = stronger and more London forces
1) London forces act between molecules that are in close surface contact
2) As the chain length increases, the molecules have a larger surface area, so more surface area contact is possible between molecules
3) The London forces between molecules will be greater and so more energy is required to overcome the forces
How do you test for unsaturation (the presence of a double bond)?
1) If bromine water is aded to an unknown organic sample and the bromine water is decolourised from orange to colourless then a C=C double bond is present
2) If there is no colour change, there is no C=C double bond
What can poly(chloroethene) (PVC) used in?
Pipes, bottles, films and sheeting, insulation and flooring
Why does boiling point increase?
The greater the London forces, the higher the boiling point
What is homolytic fission?
When a bond is broken and each of the bonded atoms takes one electron from the (shared) bonding pair, resulting in two radicals
When does a molecule have cis-trans isomerism?
1) When it has a C=C double bond
2) When it has different groups attached to each carbon atom of the double bond
3) When one of the attached groups on each carbon atom of the double bond is hydrogen
What does a curly arrow show?
The movement of an electron pair
- shows heterolytic fission/the formation of a covalent bond
Why are are alkenes more reactive than alkanes?
1) A C=C double bond = a sigma bond and a pi bond
2) Pi electron density is concentrated above and below the plane of the sigma bond
3) Because they are on the outside of the double bond, pi electrons are more exposed than the sigma electrons
4) A pi bond readily breaks and so alkenes undergo addition reactions relatively easily
When does a molecule have E/Z isomerism?
1) When it has a C=C double bond (i.e. restricted rotation)
2) When it has different groups attached to each carbon atom of the double bond (one carbon has two different groups)
Why are alkanes not very reactive?
1) The C-C and C-H sigma bonds are strong and have a high bond enthalpy
2) C-C bonds are non-polar
3) The electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered as non-polar
What is effect does branching have on the boiling point of alkanes?
1) Branched isomers of an alkane have lower boiling points
2) There are fewer surface points of contact between molecules of the branched alkanes, so the London forces will be weaker
3) The branches also get in the way and prevent the branched molecules getting as close together as straight-chain molecules, decreasing the London forces further
What could happen in theory during propagation?
The two steps can continue to cycle through a chain reaction
What are the three possible termination steps with different radicals in the reaction mixture (equations) ?
1) Br• + Br• = Br2
2) •CH3 + •CH3 = C2H6
3) •CH3 + •Br = CH3Br
Addition polymers have?
High molecular masses
What is feedstock recycling?
The chemical and thermal processes that can reclaim monomers, gases or oil from waste polymers
Waste polymers are not sorted by type but are recycled to produce feedstock chemicals
What is a carbocation?
A carbon atom that has lost it’s electrons that it had in the double bond, so it is positively charged
What is the equation for the second propagation step in radical substitution?
•CH3 + Br2 = CH3Br + Br•
What is a repeating unit?
The specific arrangement of atoms in the polymer that repeats
What is poly(tetrefluoroethene) (Teflon) used in?
Permeable membrane for clothing and shoes, coating for non-stick pans and cable insulation
What is the equation for the reaction of propene with HCl?
Propene + HCl = 1-chloropropane OR 2-chloropropane
What are the advantages of feedstock recycling?
These materials can be used as raw materials for the production of new polymers
It is able to handle unsorted and unwashed polymers
How did PVC used to be recycled?
It was ground down and reused to manufacture new products
What is the general formula?
The simplest algebraic formula for a member of a homologous series e.g. CnH2n+1OH
When unsaturated alkene molecules undergo addition polymerisation, what do they produce?
Long, saturated chains containing no double bonds
What does the complete combustion of alkanes produce?
Carbon dioxide and water (+heat)
What does Markownikoff’s rule allow you to do?
Predict which of the two possible isomers produced by an unsymmetrical alkene and a hydrogen halide is the major product
Learn diagram for electrophilic addition
kk
Define unsaturated
The presence of multiple carbon–carbon bonds, including C=C, C≡C, and aromatic rings
Give two examples of how biodegradable polymers are being used
1) Bags made from plant starch can be used as bin liners for food waste so that when they are disposed of, they can all be composted together
2) Compostable plates, cups and food trays made from sugar cane fibre are replacing expanded polystyrene
Define functional group
A group of atoms responsible for the characteristic reactions of a compound
In the reaction of bromine and ethene, how does bromine have a dipole and therefore undergo electrophilic addition?
As bromine approaches the pi-bond, electrons in the bromine bond are repelled so polarity has been induced
Otherwise there is no dipole in bromine and the electrons are equally distributed
Trans =
E
What is the equation for the hydrogenation of propene?
Propene + H2 = propane
What is the general formula of an alkyl group?
CnH2n+1
What are structural isomers?
Compounds with the same molecular formula but different structural formulae
What is a pi bond?
- The sideways overlap of adjacent p-orbitals above and below the bonding C atoms
- It has restricted rotation
Why do you add an acid catalyst slowly?
To avoid a huge rise in temperature which can result in side products
What is the trend in physical properties as the chain length of alkanes increases?
- Higher viscosity
- Darker colour
- Higher volatility
At what alkane do they start becoming liquids at room temp?
Pentane (so hexane = liquid, methane-butane = gas)
Why would you use a water bath to heat rather than a direct flame?
Because the reactants are flammable
What would the role of an anhydrous substance in an experiment be?
As a drying agent
Why might you need to redistill an organic product?
To remove any impurities e.g. an organic side-product or unreacted alcohol
How is margarine made?
By the hydrogenation of vegetable oil - turning many long unsaturated carbon chains into saturated carbon chains with a lower melting point in the presence of a nickel catalyst
