alkenes

Question 1

describe the bonding in an alkene

Answer 1

-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

Question 2

why is the shape around each C atoms in each C=C double bond is trigonal planar?

Answer 2

-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

Question 3

define structural isomer

Answer 3

compounds with the same molecular formula but different structural formulae

Question 4

what 3 ways can structural isomers occur?

Answer 4

chain length
functional group position
different functional group

Question 5

define stereoisomers

Answer 5

compounds with the same structural formula but different arrangements of atoms in 3D space

Question 6

whats the criteria for a compound to show E/Z?

Answer 6

-mus have a C=C double bond because cant rotate
-each carbon of the C=C must have 2 different groups attached

Question 7

whats the criteria for a compound to show cis-trans ?

Answer 7

-must have C=C double bond because cant rotate
-each carbon of C=C must have 2 diff groups
-2 groups must be identical

Question 8

why are alkenes more reactive than alkanes?

Answer 8

due to the alkene functional group, the C=C double bond

Question 9

describe the chemical test for an alkene functional group

Answer 9

-add bromine and shake
-if bromine is decolourised from orange to colourless alkene present

Question 10

what type of reaction does alkenes undergo?

Answer 10

electrophillic addition

Question 11

describe electrophilic additon with hydrogen

Answer 11

hydrogen gas + alkene
nickel catalyst
high temp 150degrees
forms alkane

Question 12

describe the electrophilic addition with steam H2O(g)

Answer 12

steam+alkene
conc H3PO4 catalyst
high temp, high pressure
makes alcohol

Question 13

define electrophile

Answer 13

an electron pair acceptor

Question 14

why do alkenes attract electrophiles?

Answer 14

because the C=C double bond is a region of high electron density

Question 15

what does a curly arrow show?

Answer 15

the movement of an electron pair to either break or make a new covalent bond

Question 16

how is a covalent bond broken?

Answer 16

by heterolytic fission

Question 17

how is the minor product formed?

Answer 17

from the less stable primary carbocation intermediate

Question 18

how is the major product formed?

Answer 18

from the more stable secondary or tertiaty carbocation intermediate

Question 19

what is an addition polymerisation?

Answer 19

when many alkene monomers join together to form one long saturated molecule

Question 20

what are the conditions needed for addition polymerisation?

Answer 20

high temp
high pressure
catalyst

Question 21

what are 2 problems with disposal of addition polymers?

Answer 21

-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

Question 22

how can waste addition polymers be processed?

Answer 22

-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

Question 23

how can chemists limit environmental damage?

Answer 23

-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