ORGANIC CHEMISTRY
Classification of Reactions and Reagents
H
N
O
O
N
HO
O
N3
Winston Nxumalo (lab 317)
www.gh.wits.ac.za/chemnotes
1
Unique Properties of Carbon
Elemental Carbon
Carbon atoms are unique in their ability to form strong
covalent bonds to each other
Elemental carbon has many allotropes (structural forms),
including diamond, graphite, polyynes and fullerenes.
Buckyballs
[5]-fullerene-C20
[6,4]-fullerene-C24
[5,6]-fullerene-C602
Unique Properties of Carbon
Elemental Carbon
Carbon is also able to form strong covalent bonds with nonmetals e.g. Hydrogen (H) and Oxygen (O)
Carbon can form long chains of more than 1000 carbon
units, with each carbon holding hydrogen atoms e.g.
plolymers
Atoms in the same group e.g. Si can only have only have a
chain of 8 atoms while holding hydrogen atoms
3
Unique Properties of Carbon
Structural and bonding properties
1) Bond strengths
• Strong bonds form between like elements e.g. H-H (436 kJ/mol)
and C-C (348 kJ/mol). Therefore it is favourable for C atoms to link
up into chains (catenation). Note: Homonuclear bonds are rare
amongst other elements.
• Carbon has the unique ability to form long chains and rings.
• Another strong bond is between unlike elements C-H (413 kJ/mol)
and is very common in organic compounds
2) Nature of bonds
• There are a variety of possible bonds involving carbon e.g. C-C,
C=C, C≡C, C-X, C=O, C≡N (X= halogen or OH)
• The electron configuration of C: 1s22s22p2 therefore there are 4
valence electrons and 4 valence orbitals. This means carbon must
always form 4 bonds.
4
Unique Properties of Carbon
Reactivity
Let’s look at the simplest organic molecule, methane:
H
H
C
H
H
•
There are no unused electrons and no unused orbitals
therefore there are no “reactive spots”.
•
The carbon atom has 4 strong covalent bonds to
hydrogen which are not easily broken
5
Unique Properties of Carbon
Reactivity
•
Extend this analogy to all other saturated hydrocarbons,
and remembering that C-C and C-H bonds are very
strong, we arrive at the conclusion that ALKANES ARE
VERY UNREACTIVE.
•
At room temperature alkanes do not react with acids,
bases, or strong oxidizing agents.
•
Therefore alkanes make great non-polar solvents.
6
Organic Reactions
Introduction to Organic Reactions
The key idea is to look for “patterns” of reactivity. In
order to do this you must recognize 3 key things:
1) Identify the structural features in the molecules
2) Identify the types of reactions they might
undergo
3) Identify the types of reagents that will
help/cause the reaction to go
7
Organic Reactions
Reaction types: 4 basic types
1. Addition (increases saturation at C)
H
H
C
C
AB
H
H
C
Y
C
C
A
B
XY
C
A
X
H
B
H
2. Elimination (increases unsaturation at C)
X
H
Y
H
C
H
C
A
C
H
A
C
+
H
B
C
H
+
AB
XY
B
C
3. Substitution (degree of saturation at C is not changed)
H
H
C
R
H
X
Y
H
C
Y
+ X
R
4. Oxidation/Reduction (addition of oxygen/removal of hydrogen)
8
Organic Reactions
Example:
Identify the type of reaction in each of the following:
H
H
a)
H
C
Cl
+ H 2O
H
C
H
c)
H
H
C
C
H
OH
H
H
H
H
H
C
C
+
H
C + H2O
H
H
H
C
+ H Cl
R
R
b)
OH
HCl
H
H
H
C
C
H
Cl
H
9
Organic Reactions
Link between structural features and reaction type
H
E
H
H
C
C
H
H
A
H
H
C
H
H
S
S
A
H
C
H
H
H
C
C
H
H
E
A
OH
E
H
H
C
H
O
C
H
10
Organic Reactions
Reaction Pathways
Substrates
[Intermediates]
Products
A detailed description of the reaction path is known as a
reaction mechanism and usual goes from a substrate
(starting material) via an intermediate to a product.
There can be several intermediates in a reaction pathway
and usually they are too unstable to isolate.
This year we will seldom go into detailed mechanisms and
will mainly look at the substrate/product relationships.
11
Nucleophiles, electrophiles and curly
arrows
What are electrophiles and nucleophiles?
Electrophile: literally means “electron loving”,
therefore it is a fully or partially positively charged site
and attracts towards negative electrons. Electronpoor
Nucleophile: literally means “nucleus loving”,
therefore it is a fully or partially negatively charged
site and attracts towards positive nuclei. Electronrich
12
Nucleophiles, electrophiles and curly
arrows
What are electrophiles and nucleophiles?
Character
Examples
Electron poor
Electron rich
+ or δ+
− or δ−
F
H3C
B
X
CH2
: NH3
F
+
F
H2C
H
Cl
-
H3C
M
Attracted to
Electron rich sites
of molecules
Electron poor sites
of molecules
Name
Electrophiles
Nucleophiles
Criterion
One or more empty One or more lone
orbital
pair or π bond
13
Nucleophiles, electrophiles and curly
arrows
Predicting Organic Reactions
Reactions occur between sites of opposite polarity
(i.e.+ or δ+ will react with – or δ-)
Example:
CH3
H3C
Br
+ HC
3
N
H
CH3
H3C
+
N
HBr
CH3
14
Nucleophiles, electrophiles and curly
arrows
Predicting Organic Reactions
Examples
Electrophile
Nucleophile
F
B
: NH3
F
F
F
O
H
-B
N
F
H
O
C
H3C
F
+
CH3
Nu -
H3C
+
H
-
C
Nu
CH3
15
Nucleophiles, electrophiles and curly
arrows
Examples:
Identify electrophilic and nucleophilic sites in the following
molecules
O
H
H
C
C
C
Cl
H3C
CH3
H
CH3
R
H3C
CH2
C
HO
H
CH3
H3C
C
O
CH2
Li
H3C
CH3
CH3
NH2
CH2
H3C
CH2
C
CH2
H
16
Nucleophiles, electrophiles and curly
arrows
Examples
Identify the structural type and characteristic reactions of
the following:
O
H
H
C
C
C
Cl
H3C
CH3
H
CH3
R
H3C
CH2
C
HO
H
CH3
H3C
C
O
CH2
Li
H3C
CH3
CH3
NH2
CH2
H3C
CH2
C
CH2
H
17
Nucleophiles, electrophiles and curly
arrows
Types of Reagents and Substrates
Reagents and substrates can be classified according to
how their electrons are arranged;
1) Unpaired electrons (H, Cl) radicals (these
undergo homolytic reaction pathways)
2) Paired electrons (electron rich or electron poor)
(these undergo heterolytic/polar reaction pathways)
18
Halogenation of Methane
Radical Reaction
These reactions don’t use nucleophiles or electrophiles,
but instead they use “free radicals” which have unpaired
electrons. Both reactants donate one electron to form
the new bond. E.g. H Cl
Substitution
e.g. CH4 +Cl2
CH3-Cl + Cl2
CH3-Cl
CH2-Cl2
CHCl3
CCl4
These reactions are difficult to control and therefore
there are mixtures of products.
19
Halogenation of Methane
Radical Reaction
Pathway (homolytic) to form a small percentage of free
radicals.
heat
Cl
Cl
or light
H
H
Cl
+ H
C
H
HCl + C
H
Cl
C
H
H
Propagation
H
H
Cl
Initiation
Cl + Cl
H
H
Cl
C
H +
Cl
Propagation
H
Termination (two radicals collide and combine)
Both propagation steps alternate until the reactants are completely
consumed (termination). This is a chain reaction and the chlorine
radical can undergo further reactions.
20
Halogenation of Methane
Example:
Write the equation for when 1mol CH4 (methane) reacts
with 1 mol Cl2
CH4 + Cl2
CH3Cl + HCl
Write the equation for when I mol CH4 (methane) reacts
with 2 mol Cl2
CH4 + 2Cl2
CH2Cl2 + 2HCl
21
Halogenation of Methane
Example:
Write the equation for when 1mol CH3CH3 (ethane) reacts
with 1 mol Cl2
CH3CH3 + Cl2
CH3CH2Cl + HCl
Write the equation for when I mol CH3CH3 (ethane) reacts
with 2 mol Cl2
CH3CH3 + 2Cl2
CH3CHCl2 + CH2ClCH2Cl + 2HCl
22
Cracking of alkanes
Eliminations from Alkanes
H
H
H
C
H
H
H
C
CH3
H
C
800 `C
H
C
+
H
H
CH3
This process is known as “cracking”.
thermal cracking: Alkanes are very unreactive, but at
extremely high temperatures they can undergo
elimination reactions.
catalytic cracking: Alkanes will also crack at 400 -600°C
when a catalyst such as aluminosilicate is added.
This is of industrial importance as long-chain alkanes
are broken into more useful shorter chains that can be
used for fuels and to make alkenes.
23
Cracking of alkanes
Eliminations from Alkanes
Where will hydrocarbons crack?
H2C
CH
CH2 CH3
H
H3C
H2C
+
CH
H3C
H
H
H
H
C
C
C
C
H
H
H
H
CH
CH
CH3
H
H
H2C
CH3
H3C
+
CH2
CH3
C-C (348 kJ/mol) bonds are weaker than C-H (413
kJ/mol) bonds so the major products are the bottom ones
24
Cracking of alkanes
Eliminations from Alkanes
The cracking mechanism is a radical mechanism
(disproportionation).
H
H
H
C
C
H
CH3
H
H
H
H
C
C
H
CH3
H
H
H
C
H
C
+
H
H
CH3
NB industrial importance is to make unreactive alkanes
into reactive alkenes –synthesize more complex
molecules.
25
Cracking of alkanes
EXAMPLE
Using structural formulae, show all possible compounds
formed when propane is cracked
H
H
H
H
H
C
C
C
H
H
H
H
H
C
C
C
H
H
H
H
+
C
H
+
H2
H
H
C
H
H
H
C
H
H
26
Unsaturated hydrocarbons
Alkenes and Alkynes
σ and π bonds
H2C
CH2
π, π, and σ bonds
HC
CH
Alkenes and alkynes have bonds which are;
1) Non-polar
2) have no lone pairs
3) have no vacant orbitals
but have π electrons which are exposed and therefore
accessible.
The π component is weaker than the σ C-C (348 kJ/mol)
vs C=C (619 kJ/mol) therefore π bonds contribute 271
kJ/mol.
This makes unsaturated hydrocarbons far more reactive
than saturated hydrocarbons.
27
Unsaturated hydrocarbons
Alkenes and Alkynes
The typical reaction for unsaturated hydrocarbons is an
addition reaction, where the π electrons from the double
bond are the nucleophile.
H
H
C
H
C +
H
X
Y
Addn
H
H
H
C
C
X
Y
H
The obvious reagent for an addition reaction? An
electrophile.
Substitution and elimination reactions are less
important for unsaturated systems.
28
Unsaturated hydrocarbons
Alkenes and Alkynes
Halogenation
In this addition reaction:
™Two atoms (e.g., bromine) add across the double
bond.
™One π-bond and one σ-bond are replaced by two σbonds; therefore, ΔH is negative.
Unsaturated hydrocarbons
Alkenes and Alkynes
Addition of hydrogen halides
The mechanism for the addition reaction is two-step:
™First step is slow, rate-determining step.
™Second step is fast.
Unsaturated hydrocarbons
Alkenes and Alkynes
Hydrogenation
H
H
C
H
C + H
H
H
H-H is very strong (436 kJ/mol), needs a catalyst
Halogenation
H
C
H
C + Cl
H
H
C
C
H
H
Pt or Pd (catalyst)
H
H
H
Cl
H
H
H
C
C
Cl
Cl
H
H
Cl-Cl is much weaker (254 kJ/mol) (also Br2 and I2).
31
Unsaturated hydrocarbons
Alkenes and Alkynes
Examples
H
H
C
C + H
H
+
Cl
H
C
C
H
Cl
easy
Cl
H
H
H
H
H
-
H
H
H
C
C
+
H
Cl
H
-
This reaction goes easily due to the weak H-Cl bond
32
Unsaturated hydrocarbons
Alkenes and Alkynes
Hydration of alkenes
Water is a poor electrophile hence the need for trace
amounts of H+ catalyst and heat.
H
H
C
+
C + H
H
+
H
C
C
H
OH
H (catalyst)
OH
H
H
H
H
H
-
HO
H
H
H
C
C
H
+
H
-
HO
33
Unsaturated hydrocarbons
Alkenes and Alkynes
Hydration of alkynes
Water is a poor electrophile hence the need for trace
amounts of H+ catalyst and heat.
H
H
C
C
H
+
-
H
+ H
H2SO4/HgSO4
OH
H
C
H
HO
O
C
H
-
HO
H
C
H
H
OH
+
C
C
H
H
C
H
34
Unsaturated hydrocarbons
Examples
Write equations using structural formulae to represent
the reaction of i) propene and water ii) 1-butene and H2
iii) ethene and HCl
i)
CH
CH2
H3C
+
H2O
H3C
CH2
+
OH
CH2
OH
H3C HC
CH3
ii)
iii)
CH
H2C
H2C
CH3
CH2
CH2
+
+
H2
HCl
H3C
CH2
CH3
CH2
CH2
H3C
Cl
35
Unsaturated hydrocarbons
Alkenes and Alkynes
What happens with unsymmetrical alkenes?
CH3 H
H3C
H
C
H
+ H
C
H
Cl
C
H
H
+
H
Minor
H
Major
Cl
CH3 H
H
C
C
Cl
H
CH3 H
H
C
+
C
C
CH3 H
H
H
More stabilized
H
C
+
C
H
H
Less stabilized
36
Unsaturated hydrocarbons
Alkenes and Alkynes
Carbocation intermediates (relative stability)
CH3
H3C
+
C
H
CH3
H3C
H
+
C
CH3
H 3C
H
+
C
H
H
+
C
H
Increasing Stability
Markovnikov’s rule: In the addition of HX to an
alkene, the H attaches to the C with fewer alkyl
substituents and the X attaches to the C with more
alkyl substituents.
I.e. via the more stable carbocation intermediate
37
Unsaturated hydrocarbons
Alkenes and Alkynes
When there is a halogen already attached to the alkene,
the second halide ion will add to the same position (the
side with fewer hydrogens).
H
HC
CH
H
Cl
H
C
H
H3C
CHCl2
Major
C
Cl
ClH2C
CH2Cl
Minor
Alkyl substituents on alkenes speed up the rate of
reaction with an electrophile.
38
Reactions of Alkynes
Reactivity of Alkynes
• Alkynes undergo many of the same reactions alkenes do.
• As with alkenes, the impetus for the reaction is the
replacement of π-bonds with σ-bonds.
Reactions of Alkynes
Example
Write equations using structural formulae to represent
the following reactions i) 1 mol ethyne with 1 mol Br2 ii)
1 mole ethyne with 1 mol H2O
Br
HC
CH
+
Br Br
CH
CH
Br
OH
HC
CH
+
HO
CH
H
H
CH
Reactions of Alkynes
Example
Write equations using structural formulae to represent
the following reactions iii) I mol propyne with 2 mol HCl
(NB. 3 products possible)
Cl
H3C
C
CH
+ HCl
H3C
+
C
H3C
C
H
HCl
H2
C
Cl
CH
H3C
CH2
CH
+
H3C
Cl
CH
Cl
H
C
H3C
CH2
Cl
Cl
Cl
H3C
H
C
+
C
Cl
Cl
C
H
Cl
H3C
CH3