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Chemistry AS
Alkenes
| Question | Answer |
|---|---|
| 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
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