4.1.3 alkenes Flashcards
what is the reagent in a reaction between hydrogen and an alkene ?
hydrogen
what catalyst is needed in a reaction between hydrogen and an alkene?
nickel
give the basic product of a reaction between hydrogen and an alkene?
alkane
what is the reagent in a reaction between a halogen and an alkene?
halogen
give the basic product of a reaction between a halogen and an alkene
dihaloalkane
what is the test to test for alkenes and what is the outcome
reaction with bromine water where alkenes decolourise bromine water
what is the reagent in a reaction between a hydrogen halide and an alkene
hydrogen halide
what is hydration
addition of water to a molecule
give the three conditions needed for a reaction between an alkene and steam
- H3PO4 catalyst
- 300°C
- 60atm
what is the reagent in a reaction between an alkene and steam
steam
what type of reaction is the reaction between an alkene and steam
reversible
give the product of a reaction between an alkene and steam
alcohol
define alkene
unsaturated hydrocarbons contain at least one carbon to carbon double bond
give the shape and bond angle of a c=c bond
trigonal planar 120°
what two bonds are c=c bonds made up of
sigma and pi bonds
why are pi bonds vulnerable to attack from electrophiles
they are exposed and have high electron density
how is the pi bond formed
sideways overlap if two p orbitals on each carbon atom forming a pi bond above and below the plane of the molecule
which bond is weaker sigma or pi
pi
alkenes induce strong dipoles on which molecules
halogens
what is stereoisomerism
molecules that have the same structural formula but a different spacial arrangement of atoms
what kind of isomerism do alkenes exhibit
e-z
why do stereoisomers arise
- there is restricted rotation around the c=c double bond
* there are two different groups/atoms attached to both ends of the restricted double bond
give the steps of the cahn-ingold prelog rules
1) split in the middle
2) which atom or group of atoms has higher atomic number
why do cis-trans isomers arise
- if the alkene has two groups/atoms that are the same each side of the double bond
- if two groups are in the same plane the molecule is called cis and if not trans
define electrophile
electron pair acceptor
what is an addition reaction
where two molecules react to make one molecule
when the pi bond induces a dipole on a halogen what happens to the halogen
it becomes polar and electrophilic
what does an intermediate molecule have within it
a positively charged carbon atom called a carbocation
what type of fission do addition reactions of alkenes include
heterolytic
what are the mechanisms in the addition reactions of alkenes called
electrophilic addition mechanism
define addition
the adding of an atom or group of atoms to a molecule
describe the steps of the electrophilic addition mechanism
- c=c double bonds attract electrophiles because of the high electron density
- pi bond breaks and a pair of electrons form a bond to the electrophile from one carbon across the double bond
- the bond in the electrophile breaks and the electron pair forms a negative ion
- ion is attracted to the carbocation intermediate
what is the order of stability for carbocations
most stable
tertiary
secondary
primary
least stablewhy is the carbocation intermediate more stable
the methyl groups in either side of the positive carbon are electron releasing and reduce the charge on the ion that stabilises it
explain why a particular major product is formed
the tertiary/secondary carbonium ion intermediate is favoured because it is more stable
what is hydration
addition of water to a molecule
hydration is what type of reaction
reversible
why is hydration of alkenes done industrially
high pressure needed, no waste product- causes high atom economy, separation of products is easier
give the basic template of addition polymers
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n c=c ——>
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bracket -c-c- bracket n
| |why are polyalkenes unreactive
they have strong c-c and c-h bonds
how is margarine formed
catalytic hydrogenation of unsaturated vegetable oils using hydrogen and a nickel catalyst converts double bonds to single bonds therefore increasing melting point
what are the benefits or crude oil derived plastics
cheap -less energy needed
purer -less separation
what are drawbacks if crude oil derived plastics
non-biodegradable -landfill
crude oil is non-renewable -being used up hard to replace
how does recycling work
polymers are sorted into different types then each type can be recycled by melting and remoulding
how can waste polymers be used in cracking
as feedstock which allows for production of plastics and other chemicals
how can waste polymers be used for energy production
can be combusted and heat released can be used to generate electricity
what forms when waste polymers are combusted and how can this be overcome
toxic, acidic waste products
can remove the fumes formed by reacting with an alkaki eg CaO
give the names of four other types of polymer
biodegradable
compostable
soluble
photodegradable
what are the benefits of using renewable resources
easily replaced and won’t run out
derived from plants -carbon neutral
plant based polymers use less energy over their lifetime
what are the drawbacks of biodegradable polymers
must be composted in ideal conditions to degrade
must be separated from non-biodegradable polymers
more expensive
draw the mechanism for propene reacting with HBr and state the type of bond fission occurring
https://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwi2sMTwy4DaAhUEDcAKHfNwCOwQjRx6BAgAEAU&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3D-DdAxbfo0uM&psig=AOvVaw15uQEtNm81BsKLTP6SaXD0&ust=1521830749869917
why are alkenes highly reactive
- pi bonds are exposed
- have high electron density with 2 electron pairs in a double bond with a negative charge that electrophiles are attracted to
- can induce a strong dipole on halogen molecules such as bromine
what type of mechanism occurs when alkenes react with halides or hydrogen halides
electrophilic addition
which product is most likely to form in a reaction between hydrogen halides and alkenes
-the major product is the one where hydrogen adds to the carbon with the most hydrogens
give 2 uses of alkenes
- making margarine
- forming polymers
