Transcript Organic Reactions

Organic Reactions
1
Point #1
3 basic kinds of reactions
A) Addition Reactions (like synthesis reactions)
• Hydrogenation
– saturating an unsaturated carbon chain
– alkene/yne to alkane
• Hydration
– alkene to alcohol
• Halogenation/Hydrohalogenation
– alkane to haloalkane
– alkene to haloalkane
2
B) Elimination Reactions (like synthesis reactions
because they result in larger compounds)
• Condensation
– Esterification
– Formation of alkene
– Formation of amide (peptide bond)
3
C) Substitution Reactions (like single or double
replacement reactions where one
atom/ion/functional group is replaced by another)
• S N1
• S N2
4
Point #2 Basic process of organic
reactions is through attraction of
positively (electrophile) and negatively
(nucleophile) charged parts of
molecules
5
Electrophiles
 “loves electrons” =
attracted to negative
charge
 maybe positively charged
or have deficit of electrons
b/c atom is attached to
very electronegative atom
Nucleophiles
 “loves nuclei” = attracted
to positive charge
 carbon of carbonyl group
 acids




 often negatively charged or
 lone pairs
 high electronegativity
alkenes
Hydroxide –OH
Chloride –Cl
Ammonia – NH3
6
– many organic reactions happen through the
attraction of electrophiles for nucleophiles
– in reaction mechanisms, curly arrows show how
electrons move – generally electrons from
nucleophile move to electrophile
7
Point #3 Alkanes/Alkenes are relatively
inert compared to other functional
groups
A) Alkenes have pi bonds in which electrons are
easily accessible b/c they aren’t trapped between
two nuclei as sigma bonding electrons are.
8
B) Other functional groups have highly
electronegative atoms like O, N or halogens
9
The table below gives the characteristic
reactions for several functional groups
Functional
Group
Addition
Elimination
Alkane
Alkene





Halogenation
(haloalkanes)
Hydrohalogenation
(mono-haloalkanes,)
Hydration (alcohols)
Halogenation (dihaloalkanes)
Hydrogenation
(alkanes)
Oxidation (-OH,
C=O, COOH)
Alcohol
Condensation
 w/ COOH to (ester)
 w/ conc. acid or
catalyst (alkene)
Carboxylic Acid
Condensation with –OH
(ester)
Condensation w/ COOH
(amide)
Amine
Substitution
Oxidation (aldehyde,
ketone, COOH)
10
Reactions
Example #1-Halogenation of alkane
– Alkane + halogen gas  haloalkane
– Need ultraviolet light for rxn to occur
– Depending on time and amount of reactants,
more than one halogen can added to the alkane
H
H
H
+
Cl Cl
H
h
Cl
H
H
H
methane
chloromethane
h
H
Cl
Cl
H
h
H
dichloromethane
Cl
Cl
h
Cl
Cl
chloroform
also: trichloromethane
Cl
Cl
Cl
tetrachloromethane
also: carbontetrachloride
11
-Reaction occurs through homolytic fission to form a
free radical
• Free radical is a
element or molecule
with an unpaired
electron
• Homolytic fission vs
Heterolytic fission:
Formation of free radicals often
results in chain reactions –
reaction keeps occurring until all
reactant is used up. See
polymerization notes form
mechanism.
– Fission means
splitting apart
– Heterolytic means
that one atom takes
both electrons in the
bond and two ions
are formed.
– Homolytic means
the bond is split in
half – each side takes
1 electron and 2 free
radicals are formed
12
Example #2-Hydrohalogenation
Hydrohalogenation of
ethene
– Alkene + acid
halide 
monohaloalkane
– Halide ion adds
to larger side
(more
substituted side
of alkene) if
there is one
H
H
H
H
+
H Cl
H Cl
H
H
H H
Hydrohalogenation of 1-propene
H
H
H
CH3
+
H Cl
H Cl
H
H
H CH3
13
• Reaction occurs through heterolytic fission to
form an ion
• The first step is the attraction of the electrophile
(Hydrogen ion) to the electrons in the pi bond. This
forms a carbocation.
• The carbocation that is more substituted
(has more carbons attached to it) is the
most stable.
• The negatively charged halogen
(nucleophile) adds to the carbocation to
form the halogenated alkene.
H
H
CH 3
+
H
H
H
Cl
H
C
H
H
CH 3
+
+
H
Cl-
CH 3
H
H
H
Cl
14
Example #3-Hydration
– Alkene + water in acidic
solution  alcohol
– Acid acts as catalyst in rxn
– –OH group adds to larger
side (more substituted
side) of alkene
– Uses: hydration is used
for commercial
manufacture of ethanol
Hydration of ethene
H
H
H
H H
acid
+
H
H
O
H
H
O
H
H H
Hydration of 1-propene
H
H
H
CH3
acid
+
H
O
H H
H
H
O H
H CH3
15
Example #4 -Halogenation
– Alkene + halogen gas  1,2-dihaloalkane
– Diatomic gas has two atoms – both add to opposite sides
of the double bond (and opposite sides of the molecule)
– Uses: Chlorine + ethane  1,2-dichloroethane: used as
starting material for PVC
– Uses: Br2 dissolved in dichloromethane is used to
distinguish between alkenes and alkanes. If reddish-brown
color of Br2 disappears when added to unknown, the
unknown has alkenes in it.
H
H
H
H
+
Cl Cl
Cl Cl
H
H
H H
16
Example #5 Hydrogenation
–
–
–
–
Alkene + Hydrogen gas (with catalyst)  alkane
Hydrogenation is saturating an unsaturated hydrocarbon
Addition Reaction
Heterogeneous Catalyst: Pd or PtO2 (rxn occurs on a metal
surface)
– Uses: unsaturated vegetable oils are saturated to produce
saturated fats (more solid at room temp than unsaturated) for
margarines
H
H
H
H
PtO2/Pd
+
H H
H H
H
H
H H
17
Example #6 Esterification
– Carboxylic acid + alcohol  ester + water
– Reaction conditions: acidic solution
– The OH group on the carboxylic acid is replaced by the alcohols
O-R group
– Condensation reaction: produces water
– Uses: flavouring agents, plasticizers, as solvents in perfume,
polyesters
O
OH
+
O
propionic acid
OH
O
+
O
propan-1-ol
acetic acid
OH
acid
+
OH
H2O
ethyl acetate
acid
O
+ H2O
O
propan-1-ol
propyl propionate
18
Examples #7 Amide formation
–
–
–
–
–
Carboxylic acid + amine  amide + water
Reaction condition: difficult to conduct in simple steps
The OH group on the carboxylic acid is replaced by the amine (NH-R)
Condensation rxn: produces water
Uses: peptide bond formation, polymerization reactions to make
nylons
O
O
+
H2N CH3
acetic acid
also: ethanoic acid
methanamine
H3C
OH
H3C
NH
CH3
N-methylacetamide
O
+
OH
butyric acid
also: butanoic acid
CH3
HN
CH3
N-methylmethanamine
O
CH3
N
CH3
N,N-dimethylbutanamide
19
Example #8 Oxidation of alcohol
– Alcohol + oxidizing agent  COOH (1, complete)
/Aldehyde (1, partial)/Ketone (2)
– Obviously an Oxidation reaction
– Reaction condition: aqueous, acidic solution. The
carboxylic acid and the aldehyde can be obtained
through different experimental set-ups
• Distill to get aldehyde
• Heat under reflux to get COOH
20
Oxidation of alcohol (continued)
– Complete Oxidation: primary alcohol + oxidizing
agent  carboxylic acid
OH
propan-1-ol
+
K2Cr2O7
acid
O
OH
propanoic acid
21
Oxidation of alcohol (continued)
• Partial Oxidation: primary alcohol + oxidizing agent
 aldehyde
OH
propan-1-ol
+
K2Cr2O7
acid
O
H
1-propanal
22
Oxidation of alcohol (continued)
• Secondary alcohol + oxidizing agent  ketone
OH
butan-2-ol
+
K2Cr2O7
acid
O
butan-2-one
23
Example #9 Condensation of alcohol
– Condensation of alcohol  alkene
– Reaction conditions:
• 170 and concentrated sulfuric acid or
• H3PO4 and a catalyst or Al2O3 and a catalyst
H
H3C
CH OH
H3C
H
H
H
H
H
C
C
H3C
O
CH2
OH
H3C
CH
24