alkenes Flashcards
general formula of an alkene
CnH2n
functional group alkene
c=c
what is the double covalent bond a centre of
high electron density
unsaturated
contain one or more carbon-carbon double bonds
what is the shape around each carbon in the c=c double bond
triganol planar
what is the bond angle around each carbon in the c=c double bond
120
what forms when the p orbitals overlap in the c=c group
a pi bond
what does the presence of a pi bond create
restricted rotation about the planar c=c bond
how are alkenes named
like alkanes based on longest unbranched carbon chain with suffix ‘ene’
what must be indicated in alkenes with 4 or more carbon atoms in the longest chain
the position of the c=c bond
diene
2 c=c bonds
triene
three c=c bonds
what is the difference between open chain alkenes and cycloalkenes
cycloalkenes have 2 less hydrogen atoms than open chain alkenes
how are cycloalkenes named
according to the largest ring containing the c=c bond
structural isomers
compound with same molecular formula but different structural formula
stereoisomers
compounds with the same structural formula but different arrangement of atoms in space
what type of stereoisomerism is exhibited by alkenes
E/Z isomerism
z
together
e
opposite
why does E/Z isomerism occur
as a result of restricted rotation about the c=c double bond
what must both carbon atoms of the c=c double bond be for an alkene to exhibit E/Z isomerism
both carbon atoms must be bonded to different groups
what are cycloalkanes
functional group isomers of alkenes
how to determine whether an alkene is the E or Z isomer
identify the highest priority group on each C atom of the c=c double bond
if both highest priority groups are on same side- z isomer
if both highest priority groups are on opposite sides of C=c bond- E isomer
which atom has the highest priority
the atom with the highest atomic NUMBER
what happens if two atoms have the same atomic number
next atom n chain should be compared (first point of difference used to assign priority)
why are alkenes reactive
high electron density of c=c double bond
what type of reactions do alkenes undergo
addition reactions in which a small molecule is added across C=C group
what does the addition reaction of an alkene form
a saturated molecule (pi bond breaks)
electrophile
electron pair acceptor
why do alkenes react with electrophiles
electrophile are attracted to high electron density of C=C bond
what occurs during the reaction between alkenes and electrophiiles
covalent bonds broken and formed
what is produced as a result of the reaction between an electrophile and an alkene
a reactive positively charged intermediate called a carbocation
what does a mechanism show
the breaking and forming of covalent bonds using curly arrows
curly arrow
shows movement of pair of electrons
where must curly arrows start from
a lone pair or a covalent bond
what is the reagent in the reaction of alkanes with halogens
the halogen eg bromine
what does the reaction between ethane and bromine produce
1,2- dibromo ethane
what observations could be made during the reaction between ethene and bromine
bromine decolourises
what is the name of the mechanism in the reacton between ethene and bromine
electrophilic addition
what acts as the electrophile in the reaction between bromine and ethene
bromine
what does the high electron density in the c=c group do
polarises the bond in the bromine molecule
why does the br-br bond break
a pair of electrons is accepted by the delta plus br atom
how does the br-br bond break
heterolytic fission
what does the lone pair of electrons on the bromide ion form
a new c-br bond with the carbocation
what is bromine used for
to test for the presence of a C=C group
what is the reagent in the reaction of alkenes with hydrogen halides (same applies as halogen)
the hydrogen halide
What do hydrogen halides contain
A permanent dipole, h atom always delta plus
What is the reagent in the electrophilic addition reaction between sulphuric acid and lakebed
Concentrated sulphuric acid
Organic hydrogen sulfates are readily hydrolysed with warm water to form…
An alcohol
Hydrolysis
Breaking a bond using water
What are carbocations classified as
Primary secondary or tertiary
What is the classification of carbocations based on
The number of carbon atoms directly attached to the carbon with the positive charge
What does a primary carbocation have
1 variable group
What does a secondary carbocation have
2 variable groups
What does a tertiary carbocation have
3 variable groups
What do alkyl groups have a tendency to do
Release electrons
What does the tendency of alkyl groups to release electrons help with
Stabilising the positive charge of carbocations- called the positive inductive effect
What carbocations are the most stable and why
Tertiary as they have the most alkyl groups attached
Order of stability of carbocations
Tertiary>secondary>primary
Most Least
stable Stable
What does addition to an unsymmetrical Alkene produce
Two different organic products
Major product
Produced in the largest amount
Minor product
Produced in the smallest amount
What is the yield of a particular product related to
The stability of the carbocation it is formed from
What is produced when a hydrogen is added across a c=c bond
An alkane is produced (hydrogenation)
What is hydrogenation used for
Important industrial process e.g. Production of margarine
What catalyst is used in hydrogenation of alkenes
Nickel
When are polymers formed
When monomers join together to form a very large molecule
What do alkenesact as in addition reactions
Monomers, the c=c bond breaks allowing the monomers to add together
Repeating unit
A specific arrangement of atoms that occurs in the structure over and over again
What are simple polyalkenes
Unreactive
What does the backbone of single c-c bonds make polyalkenes
Saturated
What do the properties of polyalkenes depend on
Their structure and the intermolecular forces between neighbouring polymer chains
What does polyethene have between neighbouring chains
VDW forces
How can the properties of polymers like polyethene be modified
By increasing branching which decreases points of contact so the VDW forces are weakened- increased flexibility and lowers boiling point
What is polychloroethene
Rigid and brittle
What does polychloroethene contain
Polar c-Cl bonds and therefore has permanent dipole-dipole forces between polymer chains
Addition of what molecules make polymers like polychloroethene more flexible
Plasticisers
How do plasticisers make polychloroethene more flexible
They force neighbouring chains apart- allowing them to slide over each other
Where do plasticisers go when added to polyalkenes like polychloroethene
Get in between neighbouring chains and reduce intermolecular forces
