Unit One Part 6: analysing chemical reactionsgjr-–-
• Write & balance chemical equations for simple organic reactions• Identify addition, substitution & elimination reactions• Recognise electrophilic & nucleophilic centres in molecules• Interpret reaction profiles
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dr gareth rowlands; [email protected]; science tower a4.12http://www.massey.ac.nz/~gjrowlan
gjr-–-
ClH3C
H3CH3C + H2OOH
H3C
H3CH3C + HCl
ZnCl2 (aq)
Introduction
• Reactions are all about the movement of electrons• It is a vast (& in places, complicated) topic - we will just look at the basics
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Lucas test
water solubleH-bond
water solubleion-dipole
water insolubletwo layers
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Simple reaction: definitions3
ZnCl2 (aq)OH
H3C
H3CH3C + HCl Cl
H3C
H3CH3C + H2O
reactantscompound put into reaction
productscompound formed in reaction
stoichiometryratio of compounds involved in reaction
(1 : 1 : 1 : 1)
ArrowsYes, chemists are out to confuse you!
reactionindicates reaction will go to
completion
retrosynthesisthinking backwards
(advanced!)
equilibriumreversible reaction; both reactants &
products present
'curly arrow'movement of two electrons
(your friend)
resonancenot a reaction! indicates different
resonance forms
'curly fish hook'movement of one electron
(not your friend)
gjr-–-
+ H Br
O OMe
MeO
HO
HO+ Br
O OMe
MeO
O
O
H
O OMe
MeOH
HO
HO+ Br
O OMe
MeO
O
O
OMe
HO3S
OMe
HO3SHO SO3H
OMe
H
+ HO
HH2SO4+OMe
Simple reactions: examples
• Must start to keep track of atoms• In a reaction they are neither created nor destroyed - just moved• We commonly draw reactions as below...but a product (unwanted) is missing
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H3CCH3
OH+
ZnCl2 (aq)
H3CCH3
Cl+H Cl H
OH
• Stoichiometry is very important - you need to able to balance equations• There should be the same number of atoms on both sides of the arrow
1 13 3
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Bond formation and bond cleavage
• One bond is broken and two bonds are formed• As each bond is two electrons this is just the flow / movement of electrons
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Cl– H+
bond broken
bonds formed
• Or in our normal simplified fashion...Please note: charges on both side of the reaction balance (0 both sides)
OHH3C
H3CH3C + Cl + H Cl
H3C
H3CH3C + H
OH
C
C
C
CH
HH
HHH
HH
HO H Cl H C
C
C
CH
HH
HHH
HH
HO HH
Cl
gjr-–-
fluoxetineProzac®
F3C
Cl+ NHCH3
ONa
+ NaCl
O
F3CNHCH3
fluoxetineProzac®
F3C
Cl+ NHCH3
ONa
+ NaCl
O
F3CNHCH3
CH3NH2Br
OH
+ NHCH3
OH
+ H BrCH3NH2Br
OH
+ NHCH3
OH
+ H Br
Types of reactions: substitution reactions
• Substitution reactions - involve 1 atom or group being replaced by another• The Lucas reaction was a substitution reaction• As before, identify the bonds broken & formed to observe substitution
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A B + C A C + B
gjr-–-
Ph
O+ H3C MgBrCH3 Ph
CH3H3C O MgBr
Ph
O+ H3C MgBrCH3 Ph
CH3H3C O MgBr
H3C CH3
CH3+ H BH2 H3C CH3
H3C H
BH2H3C CH3
CH3+ H BH2 H3C CH3
H3C H
BH2
CH3 + H Br BrH
CH3CH3 + H Br BrH
CH3
Types of reactions: addition reactions
• Addition reactions - adding a new atom or group (with no loss of atoms)• Most commonly occur to double bonds...
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A + Y A Y
gjr-–-
NCH3
SeO
Ph
NCH3
+ PhSeO
HH
NCH3
SeO
Ph
NCH3
+ PhSeO
HH
H3C
CH3H3C
H
Br
H3C
CH3H3C + H BrH3C
CH3H3C
H
Br
H3C
CH3H3C + H Br
CH3OH
H
CH3+ H
OH
CH3OH
H
CH3+ H
OH
Types of reactions: elimination reactions
• Elimination reactions - involve the loss of atoms or groups from a molecule• Often the reverse of an addition reaction (but not always)
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A X A + X
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Reactants: nucleophiles
• Nucleophiles - electron-rich atoms or groups that seek +ve charge• Form covalent bonds by donating 2 electrons
• Anions and lone pairs can be good nucleophiles
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H O ≡ OH
3 lone pairs
Br Br≡4 lone pairs
H3CPr C
H
H1 lone pair
≡
Anions
Nucleophilic lone pairs
HO
Hwater
HN
HH
ammoniaH3C
SCH3
dimethyl sulfide (DMS)
lone pairs as nucleophile
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Reactants: nucleophiles II (it gets harder...)10
Nucleophilic single (σ) bonds
Nucleophilic double (π) bonds
C Yδ+δ–
nucleophilic site
Y=Li, MgR CH3
O+ Et MgBr
R CH3
HO Et
HB
HHH R CH3
O+
HB
HHH R CH3
HO H
HC
HC
H
Hhigh electron density
nucleophile
HC
HCH
H+ Br Br Br
Br
RR = electron
donating groupR
+ Br BrR
Br
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Reactants: electrophiles• Electrophiles - electron-deficient atoms or groups that seek –ve charge• Form covalent bonds by accepting 2 electrons
• Cations (positively charged) & polar groups can be good electrophiles
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H ≡ H
empty 1s orbital (no e–)
HO
HH
HO
H
H
Cation
Neutral molecules
Cl AlClClF B
FF
empty 2p orbital
empty 3p orbital
F BF
F
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Reactants: electrophiles II
• Neutral molecules - with highly polarised bonds• Positive (δ+) end - readily accepts electrons
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R CH3
Br+ C N
R CH3
CNC Yδ+ δ–
electrophilic site
Y = Cl, Br, N, O
R CH3
O+ Et MgBr
R CH3
HO EtC Y Y = O, NR
δ+ δ–
electrophilic site
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Is it that easy??• Yes...and no• The vast majority of reactions you will meet are simply:
nucleophile + electrophile ➔ product
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• The polar bond in t-BuOH suggests it is either a nucleophile or a electrophile• In the Lucas reaction it is a nucelophile as it attacks a proton (H+)• The product of this reaction is an electrophile
δ+ δ–
gjr-–-
OH
H3C
CH3H3C
H
Reaction mechanism• Reaction mechanism - the order bonds are broken & formed• Not all reactions occur in a single step...• Here is the Lucas test (again...)
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Step 1
CH3CCH3
CH3O H H CH3C
CH3
CH3O HH
H3C O
CH3H3C
H + H ClH3C Cl
CH3H3C +
HO
H
OH
H3C
CH3H3C
H
O lone pairnucleophile
H+ no electronselectrophile
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ClH3C
CH3H3C
CH3
CH3H3CCl
C
C
C
CH
HH
HHH
HH
H
O HH
C
C
C
CH
HH
HHH
HH
HO HH
CH3
CH3H3C
HOH
Reaction mechanism II15
Step 2
Step 3
C
C
C
CH
HH
HHH
HH
HCl C
C
C
CH
HH
HHH
HH
HCl
OH
H3C
CH3H3C
H
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Reaction profile
• Reaction profile - shows the energy of the system vs. the progress of the reaction
• Useful when thinking about what factors effect the rate / ease of a reaction
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ener
gy
reaction progressOH + HCl
O + ClH
H
+ ClH2O
Cl + H2O
gjr-–-
HO + CH3Br CH3OH + Br
Reaction profile II17
ener
gy
reaction progress
HOCH3Br
CH3OHBr
HBr
HH
H
HHHO Brδ– δ–
+ + BrH
NuH
H
transition state
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12SLOW
RDSHO
Overviewgjr-–-
What have we learnt?• To write & balance reactions• Learnt to analyse simple reactions• Learnt to classify reactants as either electrophiles or nuclephiles• Started to look at the murky world of reaction mechanisms
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What's next?• Looking at the shape of molecules• Noting how important 3D structure is for the course of a reaction• ...stereochemistry (& its evil minions)
dr gareth rowlands; [email protected]; science tower a4.12http://www.massey.ac.nz/~gjrowlan
