alkenes Flashcards
describe the bonding in an alkene
-C=C double bond made up of a sigma and pie bond
-restricted rotation of pie bond
-sigma bond formed directly between 2 carbon atoms by direct single overlap of orbitals directy between bonding atoms
-pie bond is fomed by the double sideways overlap of adjecent p-orbitals above and below the bonding c atom
why is the shape around each C atoms in each C=C double bond is trigonal planar?
-3 bonding regions of electron density around each carbon atom
-3 bonding regions repel each other equally to get as far apart as possible - bond angle 120
-all of the atoms are in the same plane
define structural isomer
compounds with the same molecular formula but different structural formulae
what 3 ways can structural isomers occur?
chain length
functional group position
different functional group
define stereoisomers
compounds with the same structural formula but different arrangements of atoms in 3D space
whats the criteria for a compound to show E/Z?
-mus have a C=C double bond because cant rotate
-each carbon of the C=C must have 2 different groups attached
whats the criteria for a compound to show cis-trans ?
-must have C=C double bond because cant rotate
-each carbon of C=C must have 2 diff groups
-2 groups must be identical
why are alkenes more reactive than alkanes?
due to the alkene functional group, the C=C double bond
describe the chemical test for an alkene functional group
-add bromine and shake
-if bromine is decolourised from orange to colourless alkene present
what type of reaction does alkenes undergo?
electrophillic addition
describe electrophilic additon with hydrogen
hydrogen gas + alkene
nickel catalyst
high temp 150degrees
forms alkane
describe the electrophilic addition with steam H2O(g)
steam+alkene
conc H3PO4 catalyst
high temp, high pressure
makes alcohol
define electrophile
an electron pair acceptor
why do alkenes attract electrophiles?
because the C=C double bond is a region of high electron density
what does a curly arrow show?
the movement of an electron pair to either break or make a new covalent bond
how is a covalent bond broken?
by heterolytic fission
how is the minor product formed?
from the less stable primary carbocation intermediate
how is the major product formed?
from the more stable secondary or tertiaty carbocation intermediate
what is an addition polymerisation?
when many alkene monomers join together to form one long saturated molecule
what are the conditions needed for addition polymerisation?
high temp
high pressure
catalyst
what are 2 problems with disposal of addition polymers?
-non-biodigradable:
carbon chain is non-polar so cant be broken down by hydrolysis
-burning (incineration) produces toxic gases
poly(chloroethene) (PVC) contains chlorine which produces toxic chloro-gases like HCl which dissolves in water and produces acid rain
how can waste addition polymers be processed?
-combustion for energy
needs removal of toxic waste product such as an alkaline scrubber (NaOH) to neutralise HCl
-use as an organic feedstock for production of plastics and other organic chemicals by cracking
how can chemists limit environmental damage?
-develop biodegradable polymers = broken down by microorganisms
-develop photodegradable polymers = oil-based, bonds weakened by absorbing light
-alkaline scrubbers to neutralise toxic HCl gas when chloropolymers are burned
