Alkenes Flashcards
Alkenes basic knowledge
unsaturated hydrocarbons- contain atleast 1 multiple bond between carbon atoms
In alkenes this is a c c double bond
general formular is CnH2n
Bonds in alkenes
The c=c double bond is made up of a sigma bond and a pi bond
pi bonds restrict rotation about the c=c bond
sigma-is a single covalent bond formed between 2 atoms by a direct head om overlap of a single orbital between bonded atoms
pi bond- a double bond, that consists of a pi and a sigma bond. it is formed by the double sideways overall of p orbitals above and below bonding C atoms
learn diagram
e.g. C2H4 has 1 pi and 5 sigma
shape around carbon atom (in double bond)
shape around each carbon is tig planar
120⁰ bond atom
3 bonding regions of electron density
3 bonding regions repel eachother equally to get as far apart as possible, so 120⁰
all atoms on same plane
C=C double bond locks atoms in place and restricts rotation around it
stereoisomerism definition
stereoisomers are compounds with the same structural formular but with a different arrangement of the atoms in space
E/Z stereoisomerism Definition
only occurs in molecules containing C=C, which can’t rotate
atoms attached to each C of C=C are fixed in position in space
Each Carbon in double bond must have 2 DIFF GROUPS attached to it
tip: show group in structural formular
Assigning priority in E/Z
CIP priority rules (cahn-ingold-prelog)
atoms attached to each C atom in double bond are given priority based on their atomic number
Z if high priority groups are on the same side of the double bond (up and down)
E if groups with higher priority are diagonally opposite sides of double bond (up and down)
1)decide which of the 2 groups attached to the carbon atom has high priority
high Atomic no = higher priority
2)if the 2 atoms attached are the same find the first point of diff and use whichever one had the high atomic no as higher priority
cis-trans stereoisomerism
special type of E/Z stereoisomerism
must have C=C cannot rotate
each C of the C=C must have 2 different groups attached
the two groups on the C=C must be identical e.g. a H and a Cl one and same on the other
trans- 2 same groups are opp the C=C bond
e.g Cl H
C=C
H Cl
cis- 2 groups are the same of the C=C
e.g. Cl Cl
C=C
H H
Chem reactions of Alkenes basic knowledge
Alkenes are more reactive then alkanes due to =
undergo addition type reactions- group added across the =
the pi bond breaks due to relatively low bond enthalpy
only 1 product is formed
100% atom economy as no waste product
unsaturated to saturated
reaction with H2
reagent- H2 and alkene
conditions- Nickel catalyst + 150⁰
produce alkane
Reaction with bromine
Test for double bond
orange to colourless if = present
no catalyst and room temp
reagents- bromine and alkene
H H H H
C=C + Br2 —> H C–C H
H H Shake Br Br
reaction with hydrogen Halide
reagent- hydrogen halide and alkene
no catalyst and room temp
produces haloalkane
e.g.
H H H H
c=c + Hbr —-> H C C H
H H H Br
reaction with steam
reagents- steam and alkene
condition- concentrated H3PO4 catalyst
High temp and high pressure
produces alchol
e.g.
H H H3PO4 H H
C=C + H20(g) ———-> H C C H
H H H OH
Electrophillic addition mechanism
Alkenes attract electrophiles due to C=C is region of high electron density
Alkenes react by electrophillic addition mech
uses curly arrow method
curly arrow shows movement of electron pair either make or break C bond
e.g. Carboncation intermediate
⏬️
H H SLOW H H
C=C ——> H C C H
H \ H H +
\ ⏫️- :Br-
⏬️
H🎶+
l —- \
Br🎶- < /
Fast H H
———-> H C C H
H Br
Addition reactions with unsymmetrical alkenes
forms 2 strutural isomers If unsym and 0 is sym
when hydrogen halide or steam is added to alkene unsym about = a mixture of 2 products is formed
e.g.
H H H H H H H H
C=C–C + HBr –> H C C C H + H C C C H
H H H H Br H Br H H
Markownikoff’s rule
when an asymet alkene reacts it forms 2 products 1 major and 1 minor
to determine which on is major or minor you look at the carboncation intermediates
this is classified by the no of alkyl groups (R) attached to the + C atom
the more groups the more spread the charge and the more stable
primary secondary tertiary
+ + +
R–C–H R–C–R R–C–R
l l l
H H R
stability increase –>
The major product is the one with the halogen or OH group on the more stable carboncation
Addition polymerisation of alkenes
alkene monomers can join to form long saturates molecules called addition polymers
in addition polymerisation
reagent: alkene monomer
Conditions: high temp and pressure + catalyst
A D ( A D )
general equation: n C=C —>( –C-C– )
B E ( B E ) n
Alkene monomer —> alkane polymer
Problems with disposal of addition polymers
NON biodegradable- non polar C chain doesnt breakdown by hydrolysis very stable dont breakdown naturally
Burning produces toxic gas e.g. Cl2 or HCl
processing waste addition polymers, benefits
benifitits for sustaiabiltiy by:
combustion for energy for production
removal of toxic waste products e.g. HCl
use as an organic feedstock for production of plastics and other organic chemicals
sorted and recycled
how to minimise environmental damage when managing polymers
develop biodegradable polymers and can be broken down by microorganisms
e.g. PLA made from lactic acid
develop photodegradable polymers- oil based polyemers which bonds weaken by absorbing light
use alkaline scrubber to neutralise toxic gases e,g, HCl produced when burning
