3.3.4 Alkenes Flashcards
what are alkenes
unsaturated hydrocarbons
what type of bonding do alkenes have
a double covalent bond
what is the area of the double covalent bond refered to
a centre of high electron density
what is the general formula for alkenes
CnH2n
is there rotation in the carbon double bond if alkenes
no
what is used for the test of alkenes
bromine water
what is the positive result for the presence of an alkene
orange to colourless
what is a molecule with a positive charge called
cation
explain why there is an attraction between a C=C double bond and Br2.
(3 marks)
- the sigma bond between the C=C bonds is an area of high electron density.
- the Br2 becomes polariseed
- the delta positive Br iis attracted to the C=C bond
why is there a colour change between thee bromine water and an alkene
cause the Br2 is an electrophilie which is attracted to the areas of high electron density.
what reactions do alkenes undergo
electrophilic addition
what is the name of the mechanism where the double bond is eliminated
electrophilic addition
what are the elements that undergo electrophilic addition with alkenes
HBr
H2SO4
Br2
what is the first stage of electrophilic addition
- the double bond attacks the delta positive element
what is the 2nd stage of electrophilic addition
the bond breaks between the 2 elements.
[so the arrow is from the bond to the element]
what happens after the 2nd stage of electrophilic addition
the delta positive joins the alkene
where the pi bond is broken already
and theres aleady sigma bonds
what happens in the 3rd stage of electrophilic addition
the delta negative element with lone pairs and a negative charge creates a bond with the carbocation
what forms after the delta positive binds to the new molecule
a carbocation
what shouldnt you forget for the carbo cation
the plus
by the lonley carbon
what type of forces do alkenes have
van Deer Waal forrces
what is attracted to the high electron density region of the double bond
an electrophile
what is an electrophile (1 mark)
electron pair acceptor
what happens to the molecules when they come in contact with the alkene
an induced dipole
kinda describe the structure of sulphuric acid
s in the middle
with two double bonds on either side
each double bond is attached to an oxygen
up and below the s is OH binded.
the O binds with the S
what forms from unsymmetrical alkenes
a major and minor product
what are the major products
carbocations that are bonded to more alkyl groups
what are primary carbocations
bondedd to 1 alkyl group
what are secondary carbocations
bonded to 2 alkyl groups
what are tertiary carbocations
bonded to 3 alkyl group
what is the role of thee alkyl groups
the donate electron density
which stabilises the carbocation
they have a positive inductive effect
what is the most stable carbocation? And why is this
a tertiary carbocation
it has more alkyl groups which have more positive inductive effects
(b) State what you would observe if bromine water was added to
poly(chloroethene). Explain this observation.
no colour change
the polymer is saturated and does not contain any double bonds
explain why more of isomer E than isomer F is formed in this reaction. (2 marks)
isomer E is the major product as its formedd from a more stable secondary carbon cation.
secondary carbocations are more stable than primary carbocations
how are addition poylmers formed
from alkenes and subsitiuted alkenes
what is PVC known as?
poly(chloeothene)
why are addition polymers unreactive
the carbon-carbon single bonds aree strong and polar
what are properties of addition polymers
they are rigid and strong
why do addition polymers have high melting and boiling points
due to an increase in van-Deer Waal forces
cause there are more electrons in the long chain
what hydrocarbon is more stable
a saturated hydrocarbon
what are the forces present in poly(chloroethene)
permanenet dipole dipole forces
van-Deer waal forces
how can we modify PVC
using plasticiers
what do plasticisers do to PVC
it makes it flexible
how do plasticsiers work
they reduce the intermolecular forces between C and Cl
