Organic Chemistry
Dr Kennedy Ngwira
Dr Maya Makatini
Prof Amanda Rousseau
Self study: Background notes – green booklet
Chemistry the Central Science - Brown (Chapter 24)
Extra reading: Organic Chemistry – McMurry (any edition)
1
Revision of Semester 1
Functional groups
• Identify the functional groups in lopinavir
Revision of Semester 1
• Identify the functional groups in Candarel
Revision of Semester 1
Organic
compounds
C and H
Hydrocarbon
C and H plus
Heteroatoms
Saturated
Unsaturated
Saturated
Unsaturated
• C & H ONLY
• σ / single
bonds only
• C & H ONLY
• Multiple (π & σ)
bonds present
C=C, CΞC
• C, H & other
atoms such as
O, N, S, P etc
• σ / single
bonds only
• C, H & other atoms
such as O, N, S, P
• Multiple (π & σ)
bonds present –
C=C, CΞC, C=O, C=N,
CΞN etc
O14: Unique Properties of Carbon
Structural and bonding properties
1) BOND STRENGTHS
Strong bonds form between like elements
H-H
C-C
(436 kJ/mol)
(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 the unlike elements
C-H (413 kJ/mol)
This bond is very common in organic compounds
O14: Unique Properties of Carbon
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:
1s2
2s2 2px1 2py1 2pz0
Core valence electrons
Therefore there are 4 valence electrons and 4 valence
orbitals. This means carbon must always form 4 bonds.
No unused electrons
No unused orbitals
No “reactive spots”
O14: Unique Properties of Carbon
The carbon atom is small,
and it is screened by the
other atoms attached to it
and therefore it is difficult
to attack at the carbon
centre (in order to react
with the carbon).
O14: Unique Properties of Carbon
Reactivity
• For all these reasons
ALKANES ARE VERY UNREACTIVE
• At room temperature alkanes do not react with
acids, bases, or strong oxidizing agents
• Therefore alkanes make good non-polar solvents.
O14: Unique Properties of Carbon
Elemental Carbon
Elemental carbon has many allotropes (structural
forms), including diamond, graphite and fullerenes.
Buckyballs
[5]-fullerene-C20
[6,4]-fullerene-C24
[5,6]-fullerene-C60
O15: 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 cause the
reaction to go
O15: Organic Reactions
Reaction types: 3 basic types
1. Addition (increases saturation at C)
2. Elimination (increases unsaturation at C)
3. Substitution (degree of saturation at C is not changed)
O15: Organic Reactions
Example:
Identify the type of reaction in each of the following:
a)
b)
c)
O15: Organic Reactions
Link between structural features and reaction
type
only σ-bonds
(+H2O)
(-H2O)
π-bond
(-2H) Oxidation
only σ-bonds
plus heteroatom
(+2H) Reduction
π-bond
O15: Organic Reactions
Reaction Pathways
Substrates
[Intermediates]
Products
Reaction mechanism: a detailed description of the
reaction path
There can be several intermediates in a reaction
pathway and usually they are too unstable to isolate.
This year we will look at some basic mechanisms and
will also look at the substrate/product relationships.
O16: Types of reagents and substrates
Reagents and substrates can be classified according to
how their electrons are arranged:
1) Unpaired electrons -Radicals (homolytic reactions)
2) Paired electrons (electron poor or electron rich
species)
(heterolytic/polar reactions)
O16: Types of reagents and substrates
Electron poor
Electron rich
Character
+ or δ+
− or δ−
Examples
F
H2C
B
F
H
F
+
CH2
: NH3
Cl
-
Attracted to
Electron rich
sites of
molecules
Electron poor
sites of
molecules
Name
Electrophiles
Nucleophiles
Criterion
One or more
empty orbital
One or more
lone pair or π
bond
O16: Types of reagents and substrates
What are electrophiles and nucleophiles?
Electrophile: literally means “electron loving”
• it is a fully or partially positively charged site
• it is looking for electrons
• It is Electron-poor
Nucleophile: literally means “nucleus loving”
• it is a fully or partially negatively charged site
• it has got electrons to give
• It is Electron-rich
O17: Main principle underlying reactions
Predicting Organic Reactions
Reactions occur between sites of opposite polarity
(i.e.+ or δ+ will react with – or δ-)
Example:
O17: Main principle underlying reactions
Curly arrows are used to show movement of electrons
in the reaction
Examples:
Electrophilic carbon
Nucleophile
Substitution reaction: electrons
move from the nucleophile to
the electrophile
Addition reaction: electrons
move from the nucleophile to
the electrophile
O16 & O17: Nucleophiles and Electrophiles
Examples:
Identify electrophilic and nucleophilic sites in the
following molecules
O18 & O19: Typical Reactions of Alkanes
Two types of reaction:
Substitution
Elimination
O18: 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
CH3-Cl + Cl2
CH2-Cl2
CHCl3
CCl4
These reactions are difficult to control and therefore
there are mixtures of products.
O18: Halogenation of Methane
Radical Reaction
(Small %)
Initiation
Homolytic cleavage
Propagation
Propagation
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.
O20:Unsaturated hydrocarbons
Alkenes and Alkynes
σ and π bonds
H2C
CH2
π, π, and σ bonds
HC
CH
Alkenes and alkynes have bonds which:
1) Are non-polar 2) have no lone pairs
3) have no vacant orbitals, BUT
have π electrons that are exposed and therefore accessible.
The π component is weaker than the σ
C-C (348 kJ/mol) vs C=C (619 kJ/mol)
π bonds contribute 271 kJ/mol
This makes unsaturated hydrocarbons far more reactive
than saturated hydrocarbons.
O20:Unsaturated hydrocarbons
Alkenes and Alkynes
The typical reaction for unsaturated hydrocarbons is an
addition reaction, where the π electrons from the
double bond act as the nucleophile.
The obvious reagent to react with an alkene in an addition
reaction? An electrophile.
Substitution and elimination reactions are less important
for unsaturated systems.
O20:Unsaturated hydrocarbons
Examples of addition reactions to alkenes
1. Hydrogenation
H-H is very strong (436 kJ/mol), needs a catalyst, eg. 1% Pd/C
2. Halogenation
Cl-Cl is much weaker (254 kJ/mol) (also Br2 and I2).
O20:Unsaturated hydrocarbons
Examples of addition reactions to alkenes
3. Addition of hydrogen halides
Addition of HCl across the double bond
a. In the first slow step, a carbocation is formed
b. In the second fast step, Cl- reacts with the carbocation
O20:Unsaturated hydrocarbons
Examples of addition reactions to alkenes
4. Addition of water (hydration)
Water is a poor electrophile hence the need for trace
amounts of H+ (acid) catalyst and heat
O20:Unsaturated hydrocarbons
Addition reactions to unsymmetrical alkenes
MINOR PRODUCT
MAJOR PRODUCT
Alkyl substituents on the alkene speed up the rate of
reaction with electrophiles.
O20:Unsaturated hydrocarbons
Carbocation intermediates (relative stability)
CH3
H 3C
+
C
H
CH3
H3C
H
+
C
CH3
H 3C
H
+
C
H
H
+
C
H
Increasing Stability
In the addition of HX to an alkene, the H attaches to the
C where there are more H’s and the X attaches to the C
with less H’s (more alkyl substituents). The reason for
this is that the reaction proceeds via the more stable
carbocation intermediate.
O20:Unsaturated hydrocarbons
Alkynes
• Alkynes undergo many of the same reactions alkenes do.
• As with alkenes, the impetus for the reaction is the
replacement of -bonds with -bonds.
MINOR
MAJOR
When there is a halogen already attached to the alkene, the 2nd halide
ion will add to the same position (the side with fewer hydrogens).
O21:Chemistry of benzene
Benzene
A special type of unsaturated compound: C6H6
Molecule as a whole is PLANAR. Neither structure actually
exists, which is the real structure?
Auguste Kekule first realized that benzene has a ring
structure when he dreamed of snakes biting their own
tails.
O21:Chemistry of benzene
Benzene: the real structure
Represented as:
Resonance forms
Evidence: All the C-C bonds in benzene are the same
length and strength!
Normal C-C single bond: 1.54 Å, normal C=C double bond:
1.34 Å
Benzene bond lengths are all 1.40 Å: bond order between
single and double
O21:Chemistry of benzene
Let us learn to dream, gentlemen, then perhaps we shall
find the truth...”
O21:Chemistry of benzene
Benzene
• Unlike alkenes and alkynes, -electrons do not sit between
two atoms. Not alternating single and double bonds.
• Electrons are delocalised around the ring; this stabilizes
aromatic compounds.
O21:Chemistry of benzene
The Benzene Family
The benzene family includes all “aromatic compounds” or
“arenes” e.g. naphthalene, benzene etc. Many aromatic
hydrocarbons are known by their common names.
TO BE AROMATIC: THEY MUST HAVE RINGS – RINGS MUST BE PLANAR
O21:Chemistry of benzene
Aromatic compounds:
1) Cyclic structures (Rings)
2) The structure must be planar (only contains sp2
hybridized carbon atoms)
3) The double bonds must be able to delocalize around
the ring (e.g. Single double single etc.)
If the molecule meets these requirements then it is a very
stable molecule! And it will try to stay aromatic.
O21:Chemistry of benzene
Using an analogy to alkene chemistry you might expect
the following addition reaction to occur.
This never happens! Being aromatic offers such additional
stability that benzene will rather undergo a substitution
reaction
Nucleophile
X must be electron-poor (electrophile), eg. NO2+, Cl+, CH3+
O21:Chemistry of benzene
When there is more than one substituent:
ORTHO
META
PARA
O21:Chemistry of benzene
Electrophilic Substitution Reactions, eg.
1. Nitration
TNT
O21:Chemistry of benzene
Electrophilic Substitution Reactions, eg.
2. Halogenation
O21:Chemistry of benzene
Electrophilic Substitution Reactions, eg.
3. Alkylation (Friedel-Crafts)
4. Acylation (Friedel-Crafts)
O22:Comparison of reactions
Alkanes: Substitution reaction, radical mechanism
Alkenes: Addition reaction
Nucleophile
Also HCl, H2O
Arenes (special class of alkenes): Substitution reaction
Nucleophile
O22:Comparison of reactions
Example: Indicate the reagent and/or catalyst
required for the following transformations:
O23: Saturated Heteroatom Compounds
Overview of Reactivity
Types of saturated heteroatom compounds:
1) Haloalkanes (Substitution vs Elimination)
2) Alcohols (Substitution, Dehydration, Oxidation)
3) Ethers
4) Amines
5) Organometallics
• Ease of reactions (effect of nucleophile, leaving group,
electrophile, acid)
• Comparison with inorganics (water and ammonia)
• Acid-Base properties
O23: Saturated Heteroatom Compounds
Reactivity of C-X
1. The C-X bond is polar; δ+ and δ- sites
2. The heteroatom has one or more lone pairs (or
vacant orbitals) which are reactive centres (“Hot
Spots”).
3. C-X is weaker than C-H (413 kJ/mol)
e.g. C-Cl (326 kJ/mol)
C-O ( 335 kJ/mol)
C-N (293 kJ/mol)
Therefore it is easy to break a C-X bond
O23: Saturated Heteroatom Compounds
Can be Electrophilic or Nucleophilic!
Electrophilic C
Nucleophilic C
Haloalkanes
Organometallics
Alcohols
Ethers
Amines
O24: Comparison with inorganics
Water
Ammonia
Alcohol
Ether
Primary
Secondary
Tertiary
Amine
Amine
Amine
Alcohols, ethers and amines have chemical similarities
to water and ammonia, especially acid/base properties
O24: Comparison with inorganics
As Bases (H+ acceptors)
Nu
E
O24: Comparison with inorganics
As Bases (H+ acceptors)
Nu
E
NB Reaction for drug absorption
O24: Comparison with inorganics
Acid/Base properties
E.g. Codeine (R=CH3), or Morphine (R=H)
N.B. Basic properties are especially important for drugs as their
solubility in blood or their ability to cross cell membranes is
influenced by whether or not they are protonated.
O24: Comparison with inorganics
As Acids (H+ donors: give up H+)
O25 & O26: Substitution and elimination
HALOALKANES:
What type of reactions do we expect to see?!
Substitution or Elimination
C-Cl is the reactive bond
(Remember: Addition reactions require an unsaturated system)
O25: Substitution
HALOALKANES:
Br- is the leaving group
OH- is the nucleophile
The C of C-Br is the electrophile (C-Br is reactive bond)
O26: Elimination
HALOALKANES:
N.B. The nucleophile (NaOH) is acting as a base.
Substitution and Elimination reactions are always in
competition with each other, both can occur!!!
O27: What favours S vs E?
1)Temperature:
High temp. (> 200°C) favours Elimination, e.g. cracking
Hot, conc. NaOH favours Elimination
Low temp. favours Substitution
2) Size effects:
Substitution is favoured for small nucleophiles - attack
the carbon (inside the molecule).
Elimination is favoured by large nucleophiles – attack
hydrogen (on the surface of the molecule).
O27: What favours S vs E?
3) Steric Hindrance:
Absence of steric hindrance (small hydrogen atoms
around the carbon centre) favours Substitution
A sterically hindered carbon or a crowded carbon centre
(bigger groups around the carbon) favours Elimination.
Any hindrance favours elimination, eg. bulky nucleophile or crowded
carbon centre
O25 & O26: Substitution vs Elimination
ALCOHOLS:
Elimination? Or Substitution?
-OH is not as good a leaving group as the halogens
- C-OH is a strong bond 335 kJ/mol
C-Cl is a weaker bond 326 kJ/mol
Under the right conditions both substitution and
elimination can take place!
O25: Substitution
ALCOHOLS:
Effect of H+
CH3CH2OH + NaCl
CH3CH2OH + HCl
No reaction
CH3CH2Cl + H2O
Why?
We have modified a poor leaving group (OH-) into a
better leaving group (H2O).
O26: Elimination
ALCOHOLS:
Elimination (Loss of H2O is also called dehydration)
In the presence of an acid such as H2SO4 (and heat)
there isn’t a good nucleophile therefore dehydration is
favoured.
O26: Elimination
ALCOHOLS:
Where more than one elimination is possible, that
giving the most highly substituted alkene is favoured
MINOR
MAJOR
O26: Elimination
ALCOHOLS: Alternative eliminations
This is also an elimination reaction, with the loss of 2
hydrogens, therefore we call it a dehydrogenation or
oxidation reaction.
An Oxidant is required, for example:
Cr(VI) - K2Cr2O7
(Potassium dichromate)
Mn(VII) - KMnO4 (Potassium permanganate)
CrO3
(Chromium trioxide)
O25: Substitution
ORGANOMETALLIC COMPOUNDS: Reactivity of C-X
where C acts as a nucleophile
With organometallic reactions we always see Substitution
The carbon attached to the metal is always the
nucleophile, and it is such a good nucleophile that it will
attack even a poor electrophile.
O28: Unsaturated Heteroatom compounds
These include imines and nitriles
And more importantly the ketones and aldehydes
π-bond
R’=H, R=C (aldehydes) R’,R=C (ketones)
REACTIVITY: POLAR; LONE PAIRS ON HETEROATOMS;
EXPOSED π ELECTRONS – HOT SPOTS
O28: Unsaturated Heteroatom compounds
Reactivity
The bonds in these compounds are strong.
Compare
C-O 335 kJ/mol
C=O 704 kJ/mol
vs
C-C 348 kJ/mol
C=C 619 kJ/mol
The carbon-oxygen double bond is more than twice as
strong than the carbon-oxygen single bond, whereas the
carbon-carbon double bond is less than twice as strong
than carbon-carbon single bond.
O28: Unsaturated Heteroatom compounds
Classified as a nucleophilic addition reaction
What attacks first, nucleophile or electrophile?
A good nucleophile will attack first: it attacks the carbon
atom - the electrophilic centre.
A poor nucleophile needs catalysis with H+ (H+ is an
electrophile)
O28: Unsaturated Heteroatom compounds
Reactions with good nucleophiles, eg:
Overall addition of H2 : reduction
LiAlH4 and NaBH4 are both sources of “H-”
“work-up”
O28: Unsaturated Heteroatom compounds
Reactions with good nucleophiles, eg:
OR
O28: Unsaturated Heteroatom compounds
Acid-catalysed additions
O28: Unsaturated Heteroatom compounds
Nucleophilic addition
• Nucleophile can be neutral (H2O, R-OH, NH3), which
usually requires acid catalysis.
• Nucleophile can be negatively charged [OH-, NC-, H-,
R3C- (carbanion)] -doesn’t require acid catalysis.
Oxidation reactions
• Aldehydes are easily oxidized to carboxylic acids
• Ketones resist oxidation
Reduction reactions
• Both aldehydes and ketones are reduced to alcohols
when hydrogen adds to the carbonyl group
• Typical reducing agents include LiAlH4, NaBH4.
O29: Comparing addition reactions
Adding Hydrogen, H2
H-H is a very strong bond. We need to activate it with a
metallic catalyst: Pt, Pd, Rh (eg. 1% Pt/C)
H2
H H
metal surface
Forward reaction = reduction (addition)
Reverse reaction = oxidation (elimination)
O30: Composite functional groups
What are Composite Functional Groups?
A simple functional group has one “special feature”
(heteroatom or π bond) on a carbon atom
e.g. CH3-OH, H2C=CH2, H2C=O etc.
A composite functional group has more than one
“special feature” (heteroatom and a π bond) at a single
carbon atom
O30: Composite functional groups
Examples:
Carboxylic acid
Ester
Acid Chloride
Amide
O30: Composite functional groups
Reactions of Composite Functional Groups
The double bond leads us to predict an addition reaction.
The heteroatom leads us to predict a substitution or
elimination.
Remember addition + elimination = substitution, so
overall we see a substitution reaction.
X= halogen
X= OH
X= OR
X= NR2
Carboxylic halide
Acid
Ester
Amide
O30: Composite functional groups
Reactions of Composite Functional Groups, eg.
Substitution reaction
Substitution reaction
O30: Composite functional groups
Reactions of Composite Functional Groups
Mechanism:
1. First there is a nucleophilic addition
2. Then there is an elimination
Overall there is a substitution reaction
O30: Composite functional groups
Reactions of Composite Functional Groups
Mechanism:
1. First there is a nucleophilic addition
2. Then there is an elimination
Overall there is a substitution reaction
O30: Composite functional groups
Reactions of Composite Functional Groups
Ease of reaction:
substitution
Addition plus
elimination =
substitution
Second reaction proceeds much faster
O30: Composite functional groups
Don’t forget acid-base properties!
N.B. Esters don’t have acidic protons
O31: Modern Materials
POLYMERS
Polymers (greek for “many parts”) are giant molecules
that are made up of many, many identical, smaller
molecules.
They typically have molecular weights greater than 1000.
The building blocks for polymers are called monomers.
Synthetic Polymers: plastics, rubber, nylon etc.
Natural Polymers: wool, silk, natural rubber etc.
Biopolymers: proteins, polysaccharides, nucleic acids
We will examine 2 kinds of polymers:
1)Condensation polymers
2)Addition polymers
O32: Modern Materials
Condensation Polymers
Condensation Polymerisation: molecules are joined by the
elimination of a small molecule (e.g. water):
O
H O
N H + H O C
N C
H
+ H O H
Example of condensation polymerisation: formation of
nylon.
O32: Modern Materials
Condensation Polymers
In order to form a polymer, we need bifunctional
molecules, eg. amino acids, diamines, dicarboxylic acids
X
X
Y
Y
(-XY)
X
Y
O32: Modern Materials
Condensation Polymers
Eg. Nylon 6,6 is a polyamide
O32: Modern Materials
Condensation Polymers (eg. polyester)
repeat unit
O32: Modern Materials
Addition Polymers
Example: ethylene H2C=CH2, can polymerise by using
electrons from the C–C  bond to form C–C  bonds with
adjacent ethylene molecules (with the help of radicals).
See earlier!
The result: polyethylene.
This is called addition polymerisation because ethylene
molecules are added to each other.
O32: Modern Materials
Addition Polymers
Initiator (small %)
Monomer
Polymer
n > 106
O32: Modern Materials
Addition Polymers
Eg. Polystyrene
repeating unit
O32: Modern Materials
Addition Polymers
Eg. Polyvinyl chloride (PVC)
repeat unit
monomer
O32: Modern Materials
Addition Polymers
n CH2=CH2
−(CH2-CH2)n−
n can be greater than a million!
n H2C=CHX
−(CH2-CHX)n−
X=H
X=CH3
X=Cl
X=benzene
X= -OCOCH3
X= CN
F2C=CF2
CH2=C(CH3)CO2CH3
polyethylene
polypropylene
polyvinylchloride (PVC)
polystyrene
polyvinylacetate (PVA)
Orlon
Teflon
Perspex
O33: The Molecules of Life
We are going to look at five classes of biological
molecules:
Amino Acids and Proteins
Sugars and Carbohydrates
Nucleic Acids and DNA
Lipids
Steroids
O34 & O35: Amino Acids
• There are 20 naturally occurring amino acids that are
used by the body to make proteins
• Amino acids contain an amine, a carboxylic acid and
an R group side chain
• Have a stereogenic centre
There are four distinct classes of amino acids
1. Acidic
2. Basic
3. Hydrophobic (non-polar)
4. Hydrophilic (polar)
α-amino acid
O34 & O35: Amino Acids
1) Acidic amino acid (e.g. aspartic acid)
2) Basic amino acid (e.g. lysine)
O34 & O35: Amino Acids
3) Hydrophobic (non-polar) amino acid (e.g.
phenylalanine)
4) Hydrophilic (polar) amino acid (e.g. serine)
O34 & O35: Amino Acids
Zwitterions
Amino acids fall into a class of molecules known as
zwitterions. Zwitterions have both an acidic group
(e.g. CO2H) and a basic group (e.g. NH2) and can be
described as amphoteric (proton donor and proton
acceptor).
O34 & O35: Amino Acids
Zwitterions
In aqueous solvents at a specific pH (for each amino
acid) the amino acid exists mainly as a zwitterion. This
means they have one positive charge and one negative
charge and are neutral overall, but dipolar. This pH is
called the isolectronic point.
Low pH
Neutral pH
High pH
O34 & O35: Amino Acids and Proteins
• Amino acids link together in long chains to form
proteins.
• They link by means of amide bonds: the amine group on
one amino acid links to the carboxylic acid of another.
• Proteins are polymers made up of repeating amino acid
units.
• Short chains of amino acids are called peptides and
longer chains are called polypeptides.
O34 & O35: Amino Acids and Proteins
Formation of an amide bond
Enzyme catalysis
O34 & O35: Amino Acids and Proteins
Show the amino acids that would form on hydrolysis of the
following tripeptide:
O34 & O35: Amino Acids and Proteins
Protein Structure
The sequence of amino acids that make up the protein is
known as the primary structure of the protein.
Chains of amino acids making up the
protein coil or stretch in different ways.
How the segments of chain are arranged
in regular patterns (coiling, alpha helices,
beta sheets) is known as
secondary structure.
This is often determined by the Hydrogen Bonding
O34 & O35: Amino Acids and Proteins
Protein Structure
The overall shape of the protein is called the tertiary
structure and is determined by various intramolecular
bonding interactions, e.g. disulphide linkages: S-S
Cysteine (an amino acid), present in the polypeptide
facilitates cross linking by forming disulphide bridges.
O34 & O35: Amino Acids and Proteins
Tertiary Protein Structure
O33: Sugars and Carbohydrates
Sucrose (a dimer of
glucose and fructose)
amylopectin
amylose
STARCH (a glucose polymer - polysaccharide)
O33: Nucleic Acids and DNA
Heterocyclic base, sugar and phosphate: these building
blocks make up a nucleotide.
DNA and RNA are made up of long chains of repeating
nucleotide units.
O33: Nucleic Acids and DNA
Nucleic acids
O33: Fatty acids and lipids
O33: Fatty acids and lipids
O33: Steroids
Steroids all contain the characteristic four-ring system,
A-D
C
A
B
cholesterol
oestrone
D
testosterone
oestradiol