lecture6: 123.101
TRANSCRIPT
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Unit One Part 6:analysing chemical reactions
at last...chemical reactions!
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Unit OnePart6Balanced reactions (pg69-70)Reaction types (pg71-72)Reagents (pg72-77)
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NUCLEOPHILEelectron richhigh electron
density
ELECTROPHILEelectron poorlow electron
density
Chemistry...
...it’s that simple
this slide sums up the majority of
chemical reactions (even the funkier ones
follow these principles)
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NUCLEOPHILEelectron richhigh electron
density
ELECTROPHILEelectron poorlow electron
density
Chemistry...
...it’s that simple
...so that was a quick lecture...
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lets look at a simple reaction...
lets just show its that simple (we do have 50 minutes to
kill after all)
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OHH3C
H3CH3C + HCl
ZnCl2 (aq)
the Lucas test
water solubleH-bond
water solubleion-dipole
water insolubletwo layers
ClH3C
H3CH3C + H2O
the Lucas test is simple...you’ll do it lab this week...two water soluble compounds
react...
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OHH3C
H3CH3C + HCl
ZnCl2 (aq)
the Lucas test
water solubleH-bond
water solubleion-dipole
water insolubletwo layers
ClH3C
H3CH3C + H2O
to give an insoluble compound. Why
insoluble? (no longer able to H-bond)
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ZnCl2 (aq)Cl
H3C
H3CH3C + H2OOH
H3C
H3CH3C + HCl
the Lucas test} }reactants products
stoichiometry1 11 1 ::: simple
definitions
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ZnCl2 (aq)Cl
H3C
H3CH3C + H2OOH
H3C
H3CH3C + HCl
the Lucas test} }reactants products
stoichiometry1 11 1 :::
ratio of compounds in
reaction
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...simply count atoms (or electrons)...
sorting out the stoichiometry is
easy...just make sure you have the same number of atoms on both sides of
the equation and remember...
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NOatoms (or electrons)
created or destroyed
we only move them (or change
the bonds)
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unfortunately, chemists are lazy...
a quick word of warning...organic chemists are a little lazy and often
miss side products off their reaction schemes!
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OMeH2SO4
OMe
HO3S
depiction of a standard reaction
...so, here is a standard reaction as shown in many text
books...
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OMeH2SO4
OMe
HO3S
depiction of a standard reaction
...problem is, it doesn’t show
everything...so lets have a closer look...
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so what bonds are broken?
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OMe
depiction of a standard reaction
obviously lost OH
OMe
HO3SHO SO3H
redraw sulfuric acid and it becomes
clear that we will loose an OH...
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OMe
depiction of a standard reaction
obviously lost OH
OMe
HO3SHO SO3H
...we can’t just add it to the aryl ring or one C would have too many
electrons...
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OMe OMe
HO3SH
HO SO3H
depiction of a standard reaction
remember we don’t always draw H
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depiction of a standard reaction
which means we ‘forgot’ to add HOH
OMe OMe
HO3SHO
HH
HO SO3H
...overall we have lost H from
aromatic and OH from acid...or water
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depiction of a standard reaction
which means we ‘forgot’ to add HOH
OMe OMe
HO3SHO
HH
HO SO3H
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depiction of a standard reaction
O OMe
MeOH
HO
HO
Br
O OMe
MeO
O
O
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depiction of a standard reaction
all the bonds broken & formed
O OMe
MeO
HO
HO
Br
O OMe
MeO
O
O
H
...break O–H bonds...
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depiction of a standard reaction
all the bonds broken & formed
O OMe
MeO
HO
HO
Br
O OMe
MeO
O
O
H
...break C–Br bond...
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depiction of a standard reaction
...and the ‘mystery’ product
O OMe
MeO
HO
HO
Br
O OMe
MeO
O
O
H Br
H
...form C–O bond AND H–Br
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depiction of a standard reaction
don’t forget the stoichiometry
1 13 3
O OMe
MeO
HO
HO
Br
O OMe
MeO
O
O
H Br
H
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what is happening in a reaction?
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it is the movement of electrons
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Lewis structures
OHH3C
H3CH3C + Cl + H Cl
H3C
H3CH3C + H
OH
as each bond is two electrons we have just moved electrons...
here is the Lucas reaction again...
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C
C
C
CH
HH
HHH
HH
HO H
Cl
HC
C
C
CH
HH
HHH
HH
H
O HH
Cl
Lewis structures
OHH3C
H3CH3C + Cl + H Cl
H3C
H3CH3C + H
OH
as each bond is two electrons we have just moved electrons...
...and here is the Lewis structures
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C
C
C
CH
HH
HHH
HH
HO H
Cl
HC
C
C
CH
HH
HHH
HH
H
O HH
Cl
Lewis structures
OHH3C
H3CH3C + Cl + H Cl
H3C
H3CH3C + H
OH
as each bond is two electrons we have just moved electrons...
...we take O with its complete octet of
electrons and share two of them with H
to...
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C
C
C
CH
HH
HHH
HH
HO H
Cl
HC
C
C
CH
HH
HHH
HH
H
O HH
Cl
Lewis structures
OHH3C
H3CH3C + Cl + H Cl
H3C
H3CH3C + H
OH
as each bond is two electrons we have just moved electrons...
...form water...
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C
C
C
CH
HH
HHH
HH
HO H
Cl
HC
C
C
CH
HH
HHH
HH
H
O HH
Cl
Lewis structures
OHH3C
H3CH3C + Cl + H Cl
H3C
H3CH3C + H
OH
as each bond is two electrons we have just moved electrons...
...then we share the electrons of the chloride octet to make sure the original C is still an a
happy octet...
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C
C
C
CH
HH
HHH
HH
HO H
Cl
HC
C
C
CH
HH
HHH
HH
H
O HH
Cl
Lewis structures
OHH3C
H3CH3C + Cl + H Cl
H3C
H3CH3C + H
OH
as each bond is two electrons we have just moved electrons...
so just swapped electrons around...
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what types of reaction are there?
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3basic reactions
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substitution reactions
A B + C A C + Bdoes what it says...the reagent swaps with a
functional group in our molecule...
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CH3NH2Br
OH
NHCH3
OH
H Br
substitution reactions
here’s an example from the synthesis of prozac® where an amine substitutes / displaces /
exchanges with a bromide...
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fluoxetineProzac®
F3C
Cl
NHCH3
ONa
NaCl
O
F3CNHCH3
substitution reactions
here is another example finishing the synthesis of prozac®; an alkoxide (alcohol derivative) substitutes a
chloride...
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addition reactions
A + Y A Ytwo molecules add
together or combine...all atoms in both starting
materials are found in the product
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addition reactions
here hydrogen bromide adds across an alkene to give us a
new bromide
CH3 + H Br BrH
CH3
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CH3 + H Br BrH
CH3
addition reactions
at some point you’ll have to learn why it adds the bromide
to the more hindered end of the alkene...but that’s someone
else’s job!
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Ph
OH3C MgBrCH3 Ph
CH3H3C O MgBr
addition reactions
here a Grignard reagent is adding to a ketone to
give (eventually) an alcohol
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elimination reactions
A X A + X
elimination is the opposite of addition...we rip a bit of
the molecule off.
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CH3OH
H
CH3HO
H
elimination reactions
this example shows the elimination of
water (dehydration) to form an alkene
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H3C
CH3H3C
H
Br
H3C
CH3H3C H Br
elimination reactions
or the elimination of hydrogen bromide
(hydrobromic acid) to form an alkene
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what reagents are involved in these
reactions?
now we know what reactions we can perform...what reagents can we use?
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nucleophileselectron rich molecules
donate2electrons
nucleophiles are electron rich compounds that
donate two electrons to form a new bond...examples include...
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anionsH O ≡ OH
3 lone pairs
Br Br≡4 lone pairs
H3CPr C
H
H1 lone pair
≡
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anionsH O ≡ OH
3 lone pairs
Br Br≡4 lone pairs
H3CPr C
H
H1 lone pair
≡
negatively charged compounds that possess a lone pair of electrons that
can form a new bond
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HO H H
OH
anionsnucleophilic reaction
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HO H H
OH
anionsnucleophilic reaction
hydroxide donates a pair of electrons to form a new bond to
proton
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HO H H
OH
anionsnucleophilic reaction
note: both sides of equation have same
charge (both are neutral overall)
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HO H H
OH
anionsnucleophilic reaction
this is a good sign that you’ve got the reaction right!
note: both sides of equation have same
charge (both are neutral overall)
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lone pairs
HO
Hwater
HN
HH
ammoniaH3C
SCH3
dimethyl sulfide (DMS)
lone pairs as nucleophile
lone pairs on neutral molecules are also good
nucleophiles...
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HN
HH
H
Cl
lone pairsnucleophilic reaction
HN
HH
H Cl
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HN
HH
H
Cl
lone pairsnucleophilic reaction
HN
HH
H Cl
ammonia donates a pair of electrons to form a new bond to hydrogen chloride
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HN
HH
H
Cl
lone pairsnucleophilic reaction
HN
HH
H Cl
can’t have two bonds to H (4 electrons) so break H–
Cl bond with electrons flowing towards most
electronegative element
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HN
HH
H
Cl
lone pairsnucleophilic reaction
HN
HH
H Cloverall its been a substitution
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HN
HH
H
Cl
lone pairsnucleophilic reaction
HN
HH
H Cl
note: overall both sides are neutral (have same
charge)
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bondsC Y
δ+δ–
nucleophilic site
Y=Li, MgHB
HHH
HC
HC
H
Hhigh electron density
nucleophile
RR = electron
donating group
reactive σ bond
bonds can also be a source of electrons...and these cause confussion!
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electrophileselectron poor molecules
accept2electrons
the other kind of reagents are...electrophiles are electron
poor compounds that accept two electrons to form a new bond...
examples include...
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H ≡ H
proton
empty 1s orbitalno electrons!
simplest electrophile...it has no electrons and
desperately wants some!
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HO
H
HHO
HH
protonelectrophilic reagent
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HO
H
HHO
HH
protonelectrophilic reagent
nucleophilic water donates two electrons to
proton, which accepts them to form new bond
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Group 13F B
FF
empty 2p orbital
Cl AlClCl
empty 3p orbital
F BF
F
only have 6 valence electrons...need 8 to obey
octet rule so happily accept a pair of electrons
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C Yδ+ δ–
electrophilic site
Y = Cl, Br, N, O
bondsC Y Y = O,
NRδ+ δ–
electrophilic site
bonds can be electrophiles if they are polarised and have a δ+
centre
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H3C CH3
O OH
HHO
H H3C CH3
O
bondselectrophilic reagent
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is it that easy?
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nucleophile + electrophile product
yup, this equation sums up the majority of reactions...
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yes!
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...and no...
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OH3C
H3CH3C
H
δ+ δ–Cl
H3C
H3CH3C
δ+ δ–
polarity in these two compounds looks the
same...so do they behave in the same way??
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OH3C
H3CH3C
H
δ+ δ–Cl
H3C
H3CH3C
δ+ δ–
of course not!this is chemistry after all...
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OH3C
H3CH3C
H
δ+ δ–
nucleophile
C ClH3C
H3CH3C
δ+ δ–
electrophile
alcohol uses lone pair to act as a
nucleophile
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OH3C
H3CH3C
H
δ+ δ–
nucleophile
C ClH3C
H3CH3C
δ+ δ–
electrophilechloride is an electrophile
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...you will learn to to identify which is which...
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so what actually happens in a reaction?
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H3C O
CH3H3C
H + H ClH3C Cl
CH3H3C +
HO
H
what order are the bonds made and broken?
so here’s the Lucas test again...overall
it’s a substitution of hydroxyl for chlorine
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H3C O
CH3H3C
H + H ClH3C Cl
CH3H3C +
HO
H
what order are the bonds made and broken?
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OH
H3C
CH3H3C
HO
HH3C
CH3H3C
H
additionstepstep
one
CH3CCH3
CH3
O H H CH3CCH3
CH3
O HH
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OH
H3C
CH3H3C
HO
HH3C
CH3H3C
H
additionstepstep
one
CH3CCH3
CH3
O H H CH3CCH3
CH3
O HH
nucleophilic alcohol donates 2 electrons to a proton from H–Cl in an addition reaction
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OH
H3C
CH3H3C
HO
HH3C
CH3H3C
H
additionstepstep
one
CH3CCH3
CH3
O H H CH3CCH3
CH3
O HH
note: octet rule is obeyed and the
charge is the same on both sides
(positive)
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C
C
C
CH
HH
HHH
HH
HO HH
C
C
C
CH
HH
HHH
HH
H
O HH
CH3
CH3H3C
HOH
OH
H3C
CH3H3C
H
eliminationsteptwo
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C
C
C
CH
HH
HHH
HH
HO HH
C
C
C
CH
HH
HHH
HH
H
O HH
CH3
CH3H3C
HOH
OH
H3C
CH3H3C
H
eliminationsteptwo
elimination of water...molecule splits in two...electrons go with most electronegative
atom
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C
C
C
CH
HH
HHH
HH
HO HH
C
C
C
CH
HH
HHH
HH
H
O HH
CH3
CH3H3C
HOH
OH
H3C
CH3H3C
H
eliminationsteptwo
charge is the sameon both sides (positive)...we now have a reactive
intermediate(charged species with 6
electrons)
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C
C
C
CH
HH
HHH
HH
HCl C
C
C
CH
HH
HHH
HH
HCl
ClH3C
CH3H3C
CH3
CH3H3CCl
additionstepstep
three
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C
C
C
CH
HH
HHH
HH
HCl C
C
C
CH
HH
HHH
HH
HCl
ClH3C
CH3H3C
CH3
CH3H3CCl
additionstepstep
three
finally, second addition step to complete octet
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C
C
C
CH
HH
HHH
HH
HCl C
C
C
CH
HH
HHH
HH
HCl
ClH3C
CH3H3C
CH3
CH3H3CCl
additionstepstep
three
nucleophile donates two electrons to electrophile
that accepts them to form new bond
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C
C
C
CH
HH
HHH
HH
HCl C
C
C
CH
HH
HHH
HH
HCl
ClH3C
CH3H3C
CH3
CH3H3CCl
additionstepstep
three
note: the charge is the same on both
sides (neutral)
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3steps
1substitutionreaction
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another representation...
we can follow the change of energy during a reaction...in a
reaction profile
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ener
gy
reaction progress
OH + HCl
O + ClH
H
+ ClH2O
Cl + H2O
reaction profile
![Page 93: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/93.jpg)
ener
gy
reaction progress
OH + HCl
O + ClH
H
+ ClH2O
Cl + H2O
reaction profileeach hill represents
one step in our reaction
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ener
gy
reaction progress
OH + HCl
O + ClH
H
+ ClH2O
Cl + H2O
reaction profileeach dip
represents an intermediate we
can see
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ener
gy
reaction progress
OH + HCl
O + ClH
H
+ ClH2O
Cl + H2O
reaction profilehigher the hill, the
harder the step
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ener
gy
reaction progress
OH + HCl
O + ClH
H
+ ClH2O
Cl + H2O
reaction profile
the lower in energy any stage the more
stable it is
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ener
gy
reaction progress
HOCH3Br
CH3OHBr
just one hill, so just one step
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ener
gy
reaction progress
HOCH3Br
CH3OHBr
no dip so no intermediates
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ener
gy
reaction progress
HOCH3Br
CH3OHBr
HO + CH3Br CH3OH + Br
direct substitution
would look like this
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HBr
HH
H
HHHO Brδ– δ–
+ + BrH
NuH
H
transition state
12
12SLOW
RDSHO
ener
gy
reaction progress
HOCH3Br
CH3OHBr
once again, you will learn more about this but not
from me! Why don’t I get to teach the cool stuff?
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what have....we learnt?
Picture: © Pittsburgh Supercomputing Center
•analyse reactions•classify reagents•react ion t ypes
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readpart7
![Page 103: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/103.jpg)
moreexamples
![Page 104: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/104.jpg)
what order are the bonds made and broken?
H BrHBr
![Page 105: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/105.jpg)
H BrHBrH
H HH
what order are the bonds made and broken?First, remember that our
skeletal figures are simplifications...these are all the atoms around the site of the
reaction
![Page 106: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/106.jpg)
substitutionstepstep
one
H BrH
Br
CH3C
CH
HH Br C
HH
HBr
H3CC
H3C
H3Cthe alkene is the nucleophile. Lots of
electrons between two carbon atoms
![Page 107: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/107.jpg)
substitutionstepstep
one
H BrH
Br
CH3C
CH
HH Br C
HH
HBr
H3CC
H3C
H3C
it participates in a substitution reaction,
replacing the bromide...
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substitutionstepstep
one
H BrH
Br
CH3C
CH
HH Br C
HH
HBr
H3CC
H3C
H3C
the alkene attacks the proton and not the
bromide. Why do the electrons go for the less electronegative atom?
![Page 109: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/109.jpg)
substitutionstepstep
one
H BrH
Br
CH3C
CH
HH Br C
HH
HBr
H3CC
H3C
H3CIf we attacked the bromide first then, to obey the octet rule, we
would have to give two electrons to H and it would leave as H–. The
hydride (H–) is not stable as it is not electronegative
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substitutionstepstep
one
H BrH
Br
CH3C
CH
HH Br C
HH
HBr
H3CC
H3C
H3C
electrons flow towards the most electronegative
element
![Page 111: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/111.jpg)
substitutionstepstep
one
H BrH
Br
CH3C
CH
HH Br C
HH
HBr
H3CC
H3C
H3C
reaction is not over as we have a C with only 6 electrons. It is not obeying the octet rule so is
very reactive
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additionsteptwo
HBr
HBr
CH
HH
BrCH3C
H3CCH
HH
CH3CH3C
Br
![Page 113: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/113.jpg)
additionsteptwo
HBr
HBr
CH
HH
BrCH3C
H3CCH
HH
CH3CH3C
Br
Second step is addition. The bromide shares 2 electrons with the carbon so that they
both obey the octet rule
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2steps
1additionreaction
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ener
gy
reaction progress
reaction profile
H Br
Br
Br
![Page 116: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/116.jpg)
ener
gy
reaction progress
reaction profile
H Br
Br
Br
the cation (positive charge) is the intermediate
![Page 117: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/117.jpg)
what order are the bonds made and broken?
Br
HHOHOH Br
![Page 118: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/118.jpg)
eliminationstepone
Br
HHOHOH Br
C HH
CH3CH3C
HOH
Br
OHH
CH
HC
H3C
H3CBr
![Page 119: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/119.jpg)
eliminationstepone
Br
HHOHOH Br
C HH
CH3CH3C
HOH
Br
OHH
CH
HC
H3C
H3CBr
once again the electrons are flowing towards the most electronegative
element
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eliminationstepone
Br
HHOHOH Br
C HH
CH3CH3C
HOH
Br
OHH
CH
HC
H3C
H3CBr
all atoms maintain the octet rule
![Page 121: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/121.jpg)
1step
1eliminationreaction
![Page 122: Lecture6: 123.101](https://reader037.vdocuments.us/reader037/viewer/2022102517/555d08a1d8b42a08668b5a5e/html5/thumbnails/122.jpg)
ener
gy
reaction progress
reaction profile
Br
HHO
HOH
Br
Br
HHOδ–
δ–
transition state
no intermediate as the reaction occurs by a
single step