13 - Alkenes Flashcards
Structure of alkenes
Unsaturated hydrocarbons with at least 1 C=C bond - aliphatic alkenes have a formula of CnH2n
Do cyclic alkenes obey general formula?
No - but branched ones do ; draw out cyclic alkenes to see how many carbon and hydrogen atoms
Nature of double bond in alkenes
Alkenes have 4 electrons in outer shell - carbon in double bond has 3/4 forming sigma binds but 1 electron on each atom in double bond forms a pi bond is formed
How is a pi bond formed?
Through sideways overlap of 2 p orbitals ; pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms - sigma bond is the actual line between the two carbon atoms while pi bond above and below it
What does pi bond do?
Locks the two carbons in place and stops them from moving around the double bond making it different from ALKANES (where rotation is possible)
What is the angle around a carbon in C=C bond in alkenes?
There are 3 regions of electron density around each carbon atom which repel each other as far apart as possible so the bond angle around each carbon atom is 120 degrees - TRIGONAL PLANAR
Draw but-2-en-1-ol
Remember OH takes priority over double bond
What is stereoisomerism?
Same structural formula but a different arrangement of the atoms in space
E/Z isomerism?
Type of stereoisomerism that only occurs in compounds with a C=C bond
Why does stereoisomerism arise?
Because rotation about the double bond (pi + sigma) is restricted and the groups attached to each carbon are fixed relative to each other
Why is the double bond rigid?
Because of the pi bonds electron density above and below the sigma plane (p orbitals overlap above and below with more electrons than s or p in sigma)
What conditions must a molecule satisfy for E/Z isomerism?
1) A C=C BOND
2) DIFFERENT GROUPS ATTACHED TO EACH CARBON ATOM OF THE DOUBLE BOND
Why does but-1-ene not display stereoisomerism but but-2-ene does?
In but-1-ene the carbon atom is attached to two Hydrogen atoms which means that it does not satisfy the second condition of there having to be a different group on each carbon ; but-2-ene has a methylene group and a hydrogen atom on each of the carbon atoms so it has E/Z isomerism
What is cis trans isomerism?
A special case of E/Z isomerism where the molecules must have a C=C bond and each carbon must be attached to two different groups (like all E/Z isomer)s BUT in CIS TRANS ONE OF THE ATTACHED GROUP ON EACH CARBON ATOM OF THE DOUBLE BOND MUST BE THE SAME
Cis isomer?
Has the hydrogen atoms and methyl groups on each carbon on the same side of the molecule
Trans isomer
Has hydrogen atoms and methyl group diagonally opposite
Is cis Z or E?
Z
Is trans Z or E?
E
When can cis trans only be used?
When each carbon atom in double bond is attached to a single hydrogen atom
Cahn Ingold Prelog rules?
Given priority based upon atomic number
1) If groups of higher priority on same side of double bond then it is Z
2) If groups of higher priority are diagonally placed then it is E
How to compare higher priority?
Look at each carbon of the double bond carefully and compare BETWEEN THE ATOMS/GROUPS ATTACHED TO THE CARBON ATOM ITSELF
For single atoms bonded to each carbon atom how do we judge priority?
Higher priority = higher atomic number
What if the two atoms attached to the carbon are the same?
Then find the first point of difference WHICHEVER HAS HIGHER ATOMIC NUMBER THEN THIS IS GIVEN HIGHER PRIORITY
Why are alkenes more reactive than alkanes?
Due to the presence of the double bond which contains both the pi and sigma bond - pi bond has its electron density concentrated above and below sigma plane therefore they’re are more exposed and can thus break more readily (leaving sigma bond intact)
4 addition reactions of Alkenes
Hydrogen (nickel catalyst) Halogens Hydrogen halides Steam (acid catalyst) Each involves addition of small molecule across double bond causing pi bond to break and new ones to form
What is an addition reaction?
Two molecules react to form one saturated product
Hydrogenation of alkenes
Alkene + hydrogen + nickel catalyst + 423K
Addition reaction forming an alkane - double bond breaks and both H atoms are bonded on
NUMBER OF C=C BONDS = NUMBER OF H2 MOLECULES ; ALL C=C BONDS BREAK TO BECOME SATURATED
Halogenation of alkenes
Alkenes undergo a rapid addition reaction with the halogens Cl2 and Br2 at RTP ; similar to Hydrogenation resulting in haloalkane
Test for unsaturation
Alkene + bromine can be sued to test for presence of C=C bond - ORANGE BROMINE is added to alkene ; Orange colour becomes colourless if double bond is present. If test is carried out with a saturated compound then there is no addition reaction and no colour change
Alkenes + hydrogen halides
Alkenes + gaseous Hydrogen Halides at RTP to form halo alkanes ; hydrogen halide is always a gas therefore if alkene is liquid then hydrogen halide bubbled through it/with HCl.
UNSYMMETRICAL ALKENES WITH HYDROGEN HALIDE (ALSO UNSYMMETRICAL) PRODUCES 2 POSSIBLE PRODUCTS
Hydration reaction of alkenes
This forms alcohols - reaction takes place in presence of acid (Phosphoric/sulfuric) catalyst - addition of one OH and one H atom onto the 2 carbon in C=C bond ; 2 POSSIBLE PRODUCTS FORMED DUE TO UNSYMMETRICAL SHAPE
Mechanism for addition reactions of alkenes
Electrophilic addition - forms saturated compounds
What attracts electrophiles?
The high electron density of the pi bond - above and below the sigma plane ; this attracts “electron pair acceptors”
What is an electrophile?
Atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair - usually a posited ion or molecule containing a partial positive charge (dipole)
Draw Reaction mechanism between but-2-end and HBr
DRAWN (check pg 211)
Describe stages of Hydrogen Halide addition
Dipoles form due to difference in electronegativity on the Hydrogen Halide
Electron pair in pi bond is attracted to the partial positive end - causing double bond to break and both electrons to form a dative bond with Hydrogen
Heterocyclic fission of hydrogen halide means that the halogen is now negatively charged and attracted to positive carbon (one that is not bonded to H)
Halogen ion reacts with carbonation to form final plrduct
What is a carbocation?
Compound that contains a positively charged carbon atom
Can alkenes react with non-polar molecules too?
Yes - such as Br2, by the same method of electrophilic addition ; SINGLE PRODUCT CREATED BECAUSE BR2 IS SYMMETRICAL
Draw mechanism between propene and Bromine
DRAWN (check pg 212)
Describe mechanism of addition of halogen
Interaction between pi electrons and electrons in Halogen-Halogen diatomic bond causes polarisation of bond (induced dipoles)
Electron pair attracted to partially positive halogen atom ; form dative bond between carbon atom and halogen
Halogen bond geeks through heterocyclic fission with the other halogen atom becoming negatively charged - dative bond between that and carbonation
What is the term for the most common product created?
Major product
What do carbocations have?
A positive charge on a carbon atom
Markownikoff’s Rule
When a Hydrogen Halide reacts with an unsymmetrical alkene - HYDROGEN of hydrogen halide attaches itself to the carbon atom of the alkene with the greater numbers of hydrogen atoms/smaller number of carbon atoms
How does Markownikoff’s rule relate to electrophilic addition?
1) Carbocation formed - can be primary, secondary or tertiary ; in primary then positive carbon attached to single carbon, secondary then positive carbon attached to two carbons and tertiary = 3 carbons
2) THEREFORE SECONDARY PRODUCED IN A HIGHER YIELD COMPARED TO PRIMARY BECAUSE HYDROGEN ALREADY ATTACHED TO CARBON WITH THE GREATER NUMBER OF HYDROGEN ATOMS
What does the different yield of the two possible isomers produced depend on?
The stability of the intermediate carbocations formed
How are carbocations classified?
Number of alkyl groups attached to the positive carbon atom
Which carbocations are most/least stable?
Tertiary are most stable and primary are least stable
Reasoning behind carbocation stability?
Linked to electron-donating ability of alkyl groups - each group donates and pushes electrons towards positive charge of carbonation ; POSITIVE CHARGE SPREAD OVER ALKYL GROUPS THEREFORE MORE ALKYL GROUPS = MORE STABILITY
How does major and minor product link to carbocation stability?
Major product is always formed from the most stable carbocation available
Common addition polymers
Poly(vinylchloride) - PVC
What are polymers?
Large molecules formed from many thousands of repeat units of smaller molecules known as monomers
What is addition polymerisation?
Unsaturated alkene molecules undergo addition polymerisation to produce long saturated chains containing no double bonds
How does industrial polymerisation work?
High temperature and high pressure using catalysts - energy intensive
Properties of polymers?
Each polymer has its own specific properties depending on the monomer used - addition polymers have high molecular masses and synthetic polymers are just the monomers prefixed by “poly”
General equation for Addition Polymerisation
Monomer unit (alkene) Repeat unit of polymer - saturated with brackets on outside and “n” on outside
Repeat unit?
Specific arrangement of atoms in the polymer molecule that repeats over and over again (BRACKETS)
Poly(ethene)
Made by heating many ethene monomers at high pressure and temperature - ADDITION POLYMERISATION
Poly(chloroethene)
PVC can be prepared to make a polymer that is flexible or rigid
Addition polymerisation of chloroethene
C=C(H3Cl) ; brackets around them, now saturated and “n” on outside
Uses of PVC
Pipes, ducts, films and sheeting
Things to note about addition polymerisation
REPEAT UNITS ONLY CONTAIN 2 MAIN CARBON ATOMS (SATURATED)
Draw cyclic ring attached if needed
Brackets on outside with “n”
Identifying monomers from polymers
Identify repeating unit and monomer can be seen by changing single bond to double bond - LOOK AT WHERE IT REPEATS AND THEN IDENTIFY MONOMER
Positives of polymers?
Lack of reactivity that makes them suitable for storing food and chemicals safety BUT downside is that they are mainly non-biodegradable ; can cause environmental effects like killing marine life
Positive of polymers
Readily available, cheap to purchase and more convenient for our throwaway society than the alternatives
Recycling polymers?
Reduces environmental impact by conserving fossil fuels + decreasing amount of waste going to landfill - recycling process if undermined if polymers are mixed therefore they have to be sorted and once they are cut into pellets, they can be used by manufacturers to make new products
Downside of PVC recycling?
Hazardous due to high chlorine content and range of additives in polymer
Dumping PVC is not sustainable as when burnt it released HCl which is a corrosive gas
Old and modern PVC recycling?
Old - grinding into pellets and reusing it to manufacture new products
Modern - solvents dissolve polymer and then PVC recovered by precipitation from solvent (which can be used again)
Waste polymers?
Some are derived from petrol therefore high stored energy values - waste polymers can be incinerated to produce heat/generate steam to drive turbine for electricity
Feedstock recycling?
Reclaim monomers, gases or oil from waste polymers (same found in crude oil refineries) - these can then be used as raw materials for the production of new polymers
Advantage of feedstock recycling?
Can handle unsourced and unwashed polymers
What are bio plastics?
Produced from plant starch, cellulose and proteins to offer a renewable approach to oil based products - it helps conserve valuable oil reserves
Biodegradable polymers
Broken down by microorganisms into water, CO2 and biological compounds - polymers contain additives that alter the structure so that microorganisms can break them down
Compostable polymers?
They degrade and leave no visible residues
Future of polymers?
As technology advances, bioplastics are likely to be more extensively used in packaging
Photodegradable polymers
These polymers contain bond that are weakened by absorbing light to stop the degradation - light absorbing additives can too be used
