AQA A-Level Chemistry - Alkenes

describe the structure of a C=C double bond

• made up of a single sigma bond (planar)

• and a Pi bond (above and below plane)

• Pi bond weaker than sigma bond so bond enthalpy of double bond is not double of single bond

• bond is an area of high electron density do is attractive to electrophiles

describe the test for alkenes

• add to bromine water

• goes from orange-brown to colourless

what are the properties of alkenes?

• low mp/bp - similar to matching alkane (only VDW)

• insoluble in water

• soluble in organic solvents

what form of polymerisation involves only alkenes?

addition

how do you name an addition polymer?

poly (alkene monomer)

draw the formation of poly (ethene) from ethene

what are the properties of addition polymers?

• unreactive - saturated, long alkane molecules

  • good for packaging and electrical insulation

• non biodegradable - end up in landfill, litter, harm small animals

  • have to be recycled - if burnt they release toxic gases - HCl

what are the two types of poly (ethene)?

• LDPE

• HDPE

describe LDPE

• properties : more flexible, used for plastic bags and bottles

• structure : molecules are loosely packed due to being highly branched - less VDW, meaning low BP and more flexible

• polymerisation : 200C, 2000 atm, O2 - radical polymerisation

describe HDPE

• properties: harder, used for kitchenware, buckets

• structure: very little branching so molecules are tightly packed

• polymerisation: 60C, 2 atm, Ziegler-Natta process (uses TiCl3 catalyst)

what is the IUPAC name of PVC (poly vinyl chloride)?

poly (chloroethene)

describe PVC

• without plasticiser → rigid plastic, used for doors+window frames (uPVC)

• with plasticiser → more flexible, used for clothing or electrical insulation

describe the boiling points of addition polymers

• in general, high mp/bp → long molecules → high Mr → lots of VDW forces

• more branching → lower BP as molecules can’t pack together as well so VDW forces are weaker

• less branching → higher BP as molecules can pack very close together so lots of VDW

what is an electrophile?

electron pair acceptor

give the general equation for reaction of an alkene and a halogen

alkene + halogen molecule → dihaloalkane

give the mechanism for the reaction to form a dihaloalkane

give the general equation for the reaction of an alkene and a hydrogen halide

alkene + hydrogen halide → haloalkane

give the mechanism for the formation of a haloalkane by electrophilic addition

what can occur if the alkene reacting in electrophilic addition is unsymmetrical?

• a major and minor product can be formed

• due to markovnikov rule

what is markovnikov rule?

• if an unsymmetrical alkene reacts in electrophilic addition, a major and minor product will be formed

• the major product will be the one with the more stable carbocation intermediate (tertiary > secondary > primary)

why are tertiary carbocations more stable and why do they form the major product?

• they are more stable because of the inductive effect of 3 alkyl groups (better than 2)

• this means they remain in the reaction mixture for longer so are more likely to form a product

what is the general equation for the electrophilic addition to an alkene of concentrated sulfuric acid at RTP?

alkene + conc H2SO4 → alkyl hydrogen sulfate

give the mechanism for the electrophilic addition of sulfuric acid to an alkene?

give the equation for hydration of ethene?

what are the conditions for industrial manufacture of ethanol (hydration of ethene)?

• concentrated phosphoric acid

• catalyst

• 60 atm

• 600 K

what are the advantages of industrial manufacture of ethanol?

• 100% atom economy

• continuous process

• fast

• high yield

what are the disadvantages of industrial manufacture of ethanol?

• uses fossil fuels

• uses medium amount of energy

what is the mechanism for hydration of ethene?

what are the equations for combustion of an alkene?

• alkene + O2 → CO2 + H2O

• alkene + O2 → C + H2O

• alkene + O2 → CO + H2O