4 Alkenes Flashcards
Electrphilic Addition Intro- electrophile definition, why it happens, what happens, what is formed, main and minor product decided by, trick for major
Alkenes are susceptible to attack by
electrophiles (lone pair acceptors).
This is because the C=C double bond is very electron-rich due to the electron cloud of the Pi bond
In an addition reaction, the C=C double bond opens up and an atom or group of atoms joins onto each C of the C=C double bond.
During the mechanism, a carbocation is formed. Where more than one carbocation can be formed, the main product will be formed from the
more stable carbocation
Tertiary (3 alkyls)> Secondary (2 alkyls)> Primary Carbocation(1 alkyl)
For major, H goes to C with most Hs attached
Why are carbocations with the most alkyl groups (R) more stable?
Rs have +ve inductive effect (+I) (help stabilise) and push e- density towards C+ slightly reducing reactivity, conc builds up ∴ ends up reacting w/X- (second halogen) to form major product
Electrophilic Addition 1- reaction with Br2, Reagent, conditions, what happens, overall equation, mechanism, do Propene + Br2 and but-2-ene + Br2
Reagent: Br2
Conditions: aqueous (i.e. bromine water)
What happens: C=C opens up; and added onto the two C atoms of the C=C double bond are: Br & Br
Check rest in booklet
Electrophilic Addition 1- reaction with HBr, Reagent, conditions, what happens, overall equation, mechanism, do Propene + HBr and methylpropene + HBr check rest in booklet
Reagent: HBr
Conditions: Nothing
What happens: C=C opens up; and added onto the two C atoms of the C=C double bond are: H & Br
Electrophilic Addition 1- reaction with H2SO4, Reagent, conditions, what happens, overall equation, mechanism, do methylpropene + H2SO4 and but-1-ene + H2SO4 check rest in booklet. Extra: what its used for, industrial scale etc
Reagent H2SO4
Conditions Concentrated H2SO4, cold (typically at room temperature)
What happens C=C opens up; and added onto the two C atoms of the C=C double bond are:
H & O-SO2OH
Check rest in booklet
Polymers definition, what is the difference between addition and condensation polymerisation? Addition polymer properties
Polymers- large molceules made from lots of monomers joined together
Addition polymers are made from a monomer or monomers with a carbon-carbon double bond (alkenes and no other products are formed. In condensation, water is lost
Very unreactive
What is PVC’s polymer name? Draw structure, repeating unit and monomer unit. Describe structure, types of IMF and bonds. What are its properties and uses?
Poly(chloroethene)
Covalent bonds between C and Cl are polar
Permanent dipole-dipole forces between polymer chains
PVC hard but brittle – used to make drain pipes and window frames.
Addition Polymers Properties (saturation, reactivity,IMF, MP, biodegradability)
Polymers are saturated (single bonds between C atoms in the main chain)
The carbon chain back bone is usually non-polar
This makes them unreactive (chemically inert), so they are very hard to break down.
Strong covalent bonds within the polymer molecules, weak intermolecular forces (usually Van der Waals) between the polymer chains.
The stronger the intermolecular forces the higher the melting point.
Most polymers contain a mixture of chains of various lengths so melt over a range of temperatures.
Polyalkenes, in spite of their name, have a backbone which is a long chain saturated alkane molecule. Alkanes have strong non-polar C- C and C- H bonds. So, they are very unreactive molecules, which is a
useful property in many ways. However, this does mean that they are not attacked by biological agents - like enzymes - and so they are not biodegradable. This is an increasing problem in today’s world, where waste disposal is becoming more and more difficult.
Polyethene structure+IMF and description, two types, draw skeletal diagram
Has no branches (side chains)
Polymer chains pack closely together, attracted by Van der Waals forces.
Makes a strong (high density polythene) and rigid material.
Low density poly(ethene) LDPE 200°C, 2000 atm, O2 Molecules loosely packed due to branching Flexible & soft Bags, cling flim
High density poly(ethene) HDPE 0°C, 2 atm, Ziegla-Natta Molecules tightly packed Stiffer & harder Buckets, bottles
Two types of polypropene
isotactic poly(propene) - X group same side
properties: strong
uses: crates, ropes, carpet
atactic poly(propene)- X group alternating sides
properties: softer, lower melting point
uses: less uses (e.g. tape, sealants, waterproof coatings)
Polystyrene (+compare to polyethene)- draw structure
The benzene ring is a large branch
Makes it harder for the chains to pack together closely reducing the strength of the Van der Waals forces.
Polystyrene is more flexible than polyethene
What are alkenes? Describe structure, bonding (why can’t double bond rotate?),physical properties, reactivity (and why this is), how are they produced
What: Alkenes are unsaturated hydrocarbons. They are made of carbon and hydrogen only and have one or more carbon-carbon double bonds.
The double bond makes them more reactive than alkanes because of the high concentration of electrons (high electron density) between the two carbon atoms.
Ethene. the simplest alkene, is the starting material for a large range of products. including polymers such as polythene, PVC, polystyrene, and terylene fabric, as well as product.s like antifreeze and paints.
Structure: The double bond is planar (flat). This makes the angles between each bond roughly 120°.
Unlike the C- C bonds in alkanes, there is no rotation about the double bond. This is because of the make-up or a double bond. Any molecules in which a hydrogen atom in ethene is replaced by another atom or group wi ll have the same flat shape around the carbon- carbon double bond.
Why a double bond cannot rotate
As well as a normal C-C single bond there is a p-orbital (which contains a single electron) on each carbon. These two orbitals overlap to form an orbital with a cloud of electron density above and below the single bond. This is called a ‘Pi-Orbital (pronounced pi) and its presence means the bond cannot rotate. This is sometimes called restricted rotation.
Properties:
IMF: Van der Waals forces are the only intermolecular forces that act between the alkene molecules. This means that the physical properties of alkenes are very similar to those of the alkanes.
MP and BP: increase with the number of carbon atoms present.
Solubilty: NOT in water
Reactivity: More reactive than alkanes. The C=C forms an electron -rich area in the molecule, which can easily be attacked by positively charged reagents. These reagents are called electrophiles (electron liking). They are electron pair acceptors. An example of a good electrophile is the H+ ion. As alkenes are unsaturated they can undergo addition reactions. In conclusion, most of the reactions of alkenes are electrophilic additions
How produced: Alkenes are produced in large quantities when crude oil is thermally cracked.
How can plastics be modified? Using what? What do these do?
Using Plasticisers
- Added to make polymers more flexible
- molecules get in between the polymer chains pushing them apart
- Reduces the strength of the intermolecular forces making it easier for the chains to slide over each other.
- Plasticised PVC more flexible – used to make cable insulation, flooring tiles and clothing
