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
