Download Organic Reactions

Organic Reactions
Organic Reactions
• Terms:
– electrophile “love electrons” and
– nucleophiles “love nuclei”
• Reactions occur between electrophiles and
nucleophiles.
• Just about any part of a molecule can be either
electrophilic or nucleophilic depending one what
you are comparing it to
Electrophiles
 “loves electrons” = attracted to
negative charge
Nucleophiles
 “loves nuclei” = attracted to
positive charge
 maybe positively charged or
 often negatively charged or
have deficit of electrons b/c atom
 lone pairs
is attached to very electronegative
 high electronegativity
atom
 carbon of carbonyl group
 acids




alkenes
Hydroxide –OH
Chloride –Cl
Ammonia – NH3
Kinds of Reactions
• Just like in Chem 1 – there are classifications of
organic reactions
– Addition
• like synthesis
• Example: Electrophilic Addition
– Elimination
• kind of a mix between synthesis and decomposition
– Substitution
• like replacement reactions
• Example: Nucleophilic Substitution
Underlying principles of Organic
• In order to predict reactions
– and organic is all about predicting reactions
so that you can control the product that’s
formed
• Identify the nucleophiles and electrophiles
• Determine how electrons will move
• Determine stability of reactants, products,
and intermediates
Reactions of Alkanes
• Alkanes are relatively stable compared to
other functional groups
• Because there is no polarity in the
molecule, no area of high electron density,
the molecule doesn’t act as a nucleophile
or an electrophile
• Because of their stability, alkanes undergo
a limited number of reactions:
Reactions of Alkanes: Combustion
•
Complete:
–
•
Reaction with enough oxygen to form CO2 and
water
Incomplete:
– Reaction with not enough oxygen to make CO, C,
and water
•
incomplete combustion = smoky, yellow flame
–
•
CO is toxic for humans b/c it
–
•
•
The black soot is carbon and the yellow flame comes from glowing
carbon atoms
binds to hemoglobin in blood like oxygen and causes suffocation
Unburned hydrocarbons react with other molecules in the air (like
NO2) and make ozone which is also toxic for humans to breath.
Particulates in the air from unburned hydrocarbons can also
impair breathing ability.
Reactions of Alkanes: Halogenation
• Addition of Halogen
– homolytic fission: molecule breaks evenly
• Free Radical formed
– Free radical is a molecule with an unpaired electron
– Chlorination of methane
H
h

H H+C
lC
l
H
m
e
th
a
n
e
H
H
h

l
H C
C
l
C
l
H
H
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lo
ro
m
e
th
a
n
e
h

d
ich
lo
ro
m
e
th
a
n
e
H
C
l
C
l
C
l
ch
lo
ro
fo
rm
C
l
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
C
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C
l
C
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te
tra
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a
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o
:ca
rb
o
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te
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Steps in Free Radical Mechanism
• Initiation: UV light
separates Cl2 into free
radicals
• Propagation: more free
radicals formed (2 steps)
• Termination: two free
radicals come together.
UV
Cl
Cl
Cl
+
CH4
CH3
+
Cl
Cl
+
(many ways for this to happen) Cl +
Cl
Cl
Cl
CH3
+
CH3
H3C
Cl
H3C
Cl
+
HCl
Cl
+
Cl
Cl
Cl
Reactions of Haloalkanes:
Substitution
• Similar to single replacement reactions, the
halogen on an organic molecule is replaced
with another halogen or an ion:
H3C
H3C
Br
Br
-
+
Cl
+
-
HO
H3C
H3C
Cl
OH
+
+
-
Br
-
Br
Reactions of Alkenes
• Alkenes are a lot
more reactive
because of the double
bond.
• That area of high
electron density acts
as a nucleophile.
Rxns of Alkenes: Addition
•
Mechanism
–
Occurs through heterolytic fission
•
•
formation of 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 H
to form the halogenated alkene.
•
•
H
+H
H
H
H
CH
3
CH
3
C
l
+
H
+
C
H
H
Cl-
CH
3
H
H
H
C
l
Rxns of Alkenes: Addition
•
Hydrohalogenation
–
–
–
Alkene + acid halide  monohaloalkane
Halide ion adds to larger side (more substituted
side of alkene)
Secondary and tertiary carbocations are always
more stable than a primary
H
H
H
CH3
+
H Cl
H Cl
H
H
H CH3
Rxns of Alkenes: Addition
•
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
H
H
H
H
H H
acid
+
H
O
H
H
O
H H
H
Rxns of Alkenes: Addition
•
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. MOVIE
H
H
H
H
+
Cl Cl
Cl Cl
H
H
H H
Rxns of Alkenes: Addition
•
Hydrogenation
–
–
–
–
–
Alkene + Hydrogen gas (with catalyst)  alkane
Hydrogenation is saturating an unsaturated hydrocarbon
Also called reduction (carbon is reduced in this reaction but
is also reduced in many of the reactions above)
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
PtO2/Pd
H H
+ H H
H
H
H
H
H H
Addition Polymerization
• Animation (Bag It)
• Same Steps
(Trash vortex)
– Initiation (Free radical formed from UV)
– Propagation
• Initiator reacts with alkene to form a larger free
radical
• Propagation continues until it becomes more likely
for free radical to find another free radical to react
w/ rather than another alkene to react w/.
– Termination
• Two free radicals combine
Trans isomer
Cis isomer