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Chemistry AS

Alkenes

QuestionAnswer
what are alkenes? unsaturated hydrocarbons made of carbon and hydrogen only and have 1 or more c-c double bonds.
are alkenes more reactive than alkanes? yes the double bonds make alkenes more reactive because of the high concentration of electrons between 2 carbon atoms. - high electron density.
whats so special about ethene? it is the starting material for a large range of products including polymers such as: polyethene, PVC, polystyrene, anti freeze, paints.
How are alkenes produced? produced in large quantities when crude oil is thermally cracked.
What is the general formula of alkenes? CnH2n
What is the structure shape of alkenes? -triagonal planar -120 degrees
How does alkenes differ from alkane? no rotation in alkenes due to double bond
why can't double bonds rotate -2p orbitals overlap -this forms an orbital with a cloud of electron density above and below the single bond - this is called pi bond - restricted rotation
what is E-Z notation based on? atomic numbers
What are the physical properties of alkenes? very similar to alkanes. -double bond doesn't greatly affect properties of b.p and m.p. - VDW are they only IMF that act between alkene molecules.
What is the solubility of alkenes? not soluble in water.
How do alkenes react? -alkenes are more reactive than alkanes. - c=c forms an electron rich area in a molecule which can easily be attacked by positive charged reagents.
What are alkenes attacked by? Electrophiles. - electron pair acceptor e.g H+ ions.
What reaction do alkenes undergo? Electrophilic addition
combustion of alkenes: C2H4 + 3O2 -----> 2CO2 +2H20 -They burn in air - But not used as fuels because their reactivity makes them very useful for other purposes.
Describe the mechanism of electrophilic addition reaction: 1) electrophile is attracted to the double bond 2) electrophiles are positively charged and accept a pair of electrons from the double bond # 3) carbocation is formed 4) charged ions forms a bond with carbocation
Reaction between alkenes and hydrogen halides: -Br is more elcetrongative than H thus polar -electrophile is the H(delta +) of H-Br - H is attracted to carbon double bonds high electron density - carbocation formed - the bond in H-Br breaks hetrolytically
Explain the positve inductive effect: - arrows represent the direction of tendency to release e- - e- releasing effect tends to stabilise the + charge of the intermediate carbocation. - the more alkyl groups attached to the + charged cation the more stable the carbocation is.
which is the most stable carbocation? tertiary carbocation. most stable compounds produces majority.
Reaction between alkenes and halogens: -Alkenes react rapidly with chlorine gas to give dihalogenoalkanes: -halogen molecule act as an electrophile= likely to be instantaneous dipole (delta positive Br- delta negative Br) -e- in the c=c are attracted to delta +
How do you test for double bonds? Add few drops of bromine water - solution decolourises from orange
Reactions with concentrated sulfuric acid and alkene: -Room temperature -Exothermic reaction -H2SO4 is regenerated
Reaction of alkene with water: -Used industrially to make alcohols -uses steam - acidic catalyst : H3PO4 - Temp + pressure e.g CH2=CH2 ----> CH3CH2OH
What are polymers? large molecules built from smaller molecules called monomers
What is addition polymers? Made from monomers with c=c - when monomers are polymerised, the c=c opens and monomers bond togteher to from a back bone of C atoms.
Modifying plastics: Polymers can be modified using additives such as plasticisers, these are small molecules that get between the polymer chains forcing them apart and allowing them to slide across eachother.
Biodegradability: alkenes have a strong non polar bonds - unreactive molecules -thus they are not attacked by biological agents such as enzymes so they are not biodegradable. waste disposal is difficult.
Low density polyethene: made by polymerising ethene at high temp+pressure via free radical mechanism. produces polymers with certain amount of chain branching- branched chain do not pack together well thus they are more flexible and stretches well with low density.
High density polyethene: made at temp+pressure a little greater than room conditions. -uses ziegler-natta catalyst produces enzymes with less branches thus can pack together well making density + m.p . higher. Rigid. useful for buckets and milk craters.
What are the two ways to reduce pollutions by plastics? 1) Mechanical recycling 2) Feedstock recycling
Mechanical recycling: -First step is to separate different types of plastics. -Then wash them. -Then ground them up into small pellets. -Can be melted and remoulded.
Feedstock recycling: Plastics heated to a temp that will break the polymer bonds to produce monomers. These can then be used to make new plastics.
Created by: rulaalrikabi