1
Previously...
• You have hopefully learnt about structure...
H
H
≡
C
H
H
H
H
C
H
H
• We have covered representation of organic compounds
•
and that bonds are 2 electrons (Lewis structure)
H
H N
H
≡
N
•
H
H
•
• We have started to look at reagents
H
δ+ δ–
C Y
δ– δ+
C
Z
•
electrophilic nucleophilic
site
•
site
• Lets put it all together and look at reactions...
2
The chemical reaction
• A huge topic - I'm not going to deal with the thermodynamics etc
• Look at reaction mechanism or the movement of electrons
• All about electron flow
• Electrons flow from high
electron density - the electron donor -
nucleophile
• To low electron density - the electron acceptor - electrophile
• Just like water flowing down a hill
3
The chemical reaction II
F B F
F
•
•
•
•
•
•
F
F B F
F
F
Electron flow is very important
Electrons flow from negative ( or δ-) to positive ( or δ+)
NEVER to
Like charges NEVER flow towards each other (Coulomb's law)
'Curly arrow' shows the movement of 2 electrons
Invariably shows the making and / or breaking of bonds
F
F
F
B
F
F
B
F
F
F
There is too much organic chemistry to learn it parrot fashion use the curly arrow to predict reactions instead
4
The 'curly arrow'
H
O
H
O
H δ– H
δ+
δ+
new σ
bond
• Curly arrow always starts with a covalent bond or unshared pair
•
•
•
of electrons
Curly arrow points towards a site of electron deficiency or
electronegativity
Charge is conserved - neither created or destroyed
You may not understand the reactions but by the end of this term
you should be have a good stab and complex mechanisms such
as...
O
Bn
R
O
O
N
R3
R2
R3
≡
Bn
R3
R
N
R2
new σ
bond
R
?
N
Bn
new π
bond
R2
broken π
bond
5
One arrow: anion + empty AO
H O
O
H
H
H
new
nucleophile
electrophile
3
empty
s orbital
sp lone pair
σ bond
• Reaction - overlap of a full orbital with an empty orbital
• So reaction is movement of 2 electrons
• Remember an orbital can contain a maximum of 2 electrons
H O
combined
charge = 0
• Just
H O H
H
charge = 0
moving 2 electrons so no change in overall charge
H O
H
H
O
new
H
σ bond
2 electrons
move
• Get
used to the last drawing - thats how chemists depict reactions!
6
One arrow: anion + empty AO
Reaction viewed
as orbital overlap
Electron
bookkeeping in a
reaction (and the
'curly' arrow)
'Curly' arrow
mechanism standard
representation
F
R
formal
charge on B
B is sp2
hybridised
F
B F
F
S
S
B
nucleophile
3(sp3) lone pair
F F
electrophile
empty p orbital
R
new
σ bond
F
B F
F
R S
B is sp3
hybridised
F
R S B F
F
nucleophile full octet
& negative charge
electrophile 6 valence
electrons needs 2
more for octet
combined
charge = -1
charge = -1
F
R
S
F
2 electrons
move
B
R
F
new
σ bond
S
B
F
F
F
charge
conserved
7
One arrow: lone pair + empty AO
O is sp3
orbital
representation
H O
Al is sp2
O is sp3
Al is sp3
Cl
H
Cl
Al Cl
Cl
O
Al
Cl Cl
H
nucleophile
2(sp3) lone pair
H
electrophile
empty 3p orbital
H
Lewis
structures
new
σ bond
Cl
Al Cl
H O
H O
Cl
H
vacancy for 2 electrons
electrophile
overall
charge = 0
2 electrons
moving
O
H
H
only drawing the
lone pair I need
Cl
Al
Cl
Cl
overall
charge = 0
H
Cl
Cl
Al Cl
all atoms have
full octet
full octet
lone pairs nucleophiles
standard
representation
molecule neutral
but formal charges
on atoms
Cl
O Al Cl
H
Cl
new
σ bond
new bond is
2 electrons
8
One arrow: σ bond & empty orbital
orbital
representation
Lewis
structures
H
B = sp2
& empty p
B = sp3
H B
H
H
nucleophile
sp3 σ bond
H
H
H H
new
electrophile
empty s orbital
H
H B H
H
full octet
nucleophile σ bond
σ bond
H
H B
H
H
overall
charge = 0
H
H B H
H
σ bond is
nucleophile
H H
empty s orbital
electrophile
overall
charge = 0
standard
representation
HH
B
H
H
H
B
H H
H
new
σ bond
9
Two arrows: lone pair + empty MO
σ* bond filled
so bond breaks
H
H N
H
nucleophile
sp3 lone pair
H
Br
Br
σ bond
electrons from
H–Br σ bond
H Br
H
H N H
H
Br
new
full octet
lone pair nucleophile
both full octets still
formal charge as electrons
shared differently
overall
charge = 0
full octet & duplet
Br electronegativity
makes σ* electrophile
overall
charge = 0
H
H Br
δ+ δ–
2 electrons move
to make new bond
H
H N
H
electrophile
empty σ*
bond polarised to Br
H
H N
H
H N
H
H
can't have 4
electrons on H so
break σ bond
H
H N H
H
new
σ bond
Br
Br
is a leaving group
(LG) - the more
electronegative or stable
an atom or molecule the
better a LG it is
10
2 arrows: lone pair and empty MO
H
H
H
H N
H
σ* bond filled
so bond breaks
O
σ bond
H
H
H N
H O
H
H
full octet
full shells
lone pair nucleophile do NOT attack O as we
can not have 10e so
attacks H (σ*)
overall
charge = +1
H N
H
2 electrons move
to make new bond
H
H O
H
2 electrons move
to break bond
H
new
electrophile
empty σ*
bond polarised to O+
H
H
H N
H
H
nucleophile
sp3 lone pair
H
H
lone pair
lectrons from O–
H bond
H
H
H N H
O
H
H
both full octets
formal charge changes as
electrons shared differently
overall
charge = +1
H
H N H
H
new
σ bond
O
H
O
H
nucleophile attacks H as
empty σ*
Can not attack O as full octet
H–O bond breaks and
electrons nolonger shared
but exclusively on O