Download File - Mr Weng`s IB Chemistry

IB CHEMISTRY
Topic 10 Organic chemistry
Higher level
10.1 Fundamentals of organic chemistry
OBJECTIVES
• A homologous series is a series of compounds of the same family, with the same general
formula, which differ from each other by a common structural unit.
• Structural formulas can be represented in full and condensed format.
• Structural isomers are compounds with the same molecular formula but different
arrangements of atoms.
• Functional groups are the reactive parts of molecules.
• Saturated compounds contain single bonds only and unsaturated compounds contain double
or triple bonds.
• Benzene is an aromatic, unsaturated hydrocarbon.
• Explanation of the trends in boiling points of members of a homologous
• Identification of different classes: alkanes, alkenes, alkynes, halogenoalkanes, alcohols, ethers,
aldehydes, ketones, esters, carboxylic acids, amines, amides, nitriles and arenes.
• Identification of typical functional groups in molecules eg phenyl, hydroxyl, carbonyl, carboxyl,
carboxamide, aldehyde, ester, ether, amine, nitrile, alkyl, alkenyl and alkynyl.
• Construction of 3-D models (real or virtual) of organic molecules.
• Application of IUPAC rules in the nomenclature of straight-chain and branched-chain isomers.
• Identification of primary, secondary and tertiary carbon atoms in halogenoalkanes and
alcohols and primary, secondary and tertiary nitrogen atoms in amines.
• Discussion of the structure of benzene using physical and chemical evidence.
Father of Organic Chemistry
Friedrich Wöhler
Terminology
Hydrocarbon
compounds that contain mostly hydrogen and carbon
Homologous series
compounds with the same general formula
Molecular formula
shows the number of atoms only
Structural formula
shows how the atoms are arranged
Empirical formula
shows lowest whole number ratio of atoms
Examples
Homologous series = CnH(2n+2)
Molecular formula = C4H10
H H
Structural formula =
H
H
H
C
C
C
C
H
H
H
H
(condensed structural CH3CH2CH2CH3)
Empirical formula = C2H5
H
Homologous series
The general trend for all molecules in a
homologous series is an increase in the boiling
point. This is because of the effect of London
Dispersion Forces and the molecular weights
of the lengthening carbon chains.
Structural Isomers
Structural isomer
same molecular formula but different structure
H
H
H
H
C
C
C
H
H
H
H
C
H
H
BRANCHED CHAIN
CH3CH2(CH2)CH3
H
H
H
H
H
C
C
C
C
H
H
H
H
STRAIGHT CHAIN
CH3CH2CH2CH3
H
Functional groups
Atoms or groups of atoms attached to a
hydrocarbon
They are usually the reactive groups on a stable
carbon chain so form the important part of the
molecule
Examples:
• alcohols
• aldehydes
• ketones
• carboxylic acids
• halides (halogenoalkanes)
Class name
Formula
Functional group
name
Functional group
alkane
CnH2n+2
alkyl
none
Alkanes
Suffix
-ane
Alkanes
Name
Number of
carbons
Condensed
Structural Formula
Methane
1
CH4
Ethane
2
CH3CH3
Propane
3
CH3CH2CH3
Butane
4
CH3CH2CH2CH3
Pentane
Hexane
5
6
CH3CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH3
Alkyl Groups
Branches on carbon chains
H
C
H
H
H
H
C
C
H
H
H
CH3
methyl
CH2CH3
ethyl
IUPAC Naming Summary
1. Count the C’s in the longest chain.
2. Name each attached group.
3 Count the longest carbon chain to give the
first attached group the smallest number.
4. Name and locate each group.
Naming Branched Alkanes
CH3 methyl branch
CH3CH2CH2CHCH2CH3
6
5
4 3 2 1
Count
3-methylhexane
on third C
CH3
group
six carbon chain
Class name
Formula
Functional group
name
alkene
CnH2n
alkenyl
Functional group
Alkenes
Suffix
-ene
Alkenes
Name
Number of
carbons
Condensed
Structural Formula
Ethene
2
CH2CH2
Propene
3
CH2CHCH3
Butene
4
CH2CHCH2CH3
Pentene
Hexene
5
6
CH2CHCH2CH2CH3
CH2CHCH2CH2CH2CH3
Naming Branched Alkanes
• select the longest chain of C atoms containing the double
bond and number the chain from this end
• place the ending ENE on the basic name
• use a number to indicate the lower number carbon of the
C=C
• as in alkanes, prefix with substituents
• side chain positions are based on the number allocated to
the first C of the C=C
e.g. CH3 - CH = CH - CH2 - CH(CH3) - CH3
5-methylhex-2-ene
Structural isomerism in alkenes
Different structures are possible due to...
Different positions for the double bond
pent-1-ene
pent-2-ene
Branching
3-methybut-1-ene
Class name
Formula
Functional group
name
alkyne
CnH2n-2
alkynyl group
Functional group
Alkynes
Suffix
-yne
Alkynes
Name
Number of
carbons
Condensed
Structural Formula
Ethyne
2
CHCH
Propyne
3
CHCCH3
Butyne
4
CHCCH2CH3
Pentyne
Hexyne
5
6
CHCCH2CH2CH3
CHCCH2CH2CH2CH3
Saturated vs unsaturated
Saturated hydrocarbons are hydrocarbons
that contain no double or triple bonds
(alkanes).
Unsaturated hydrocarbons are alkenes and
alkynes.
Class name
Formula
Functional group
name
halogenoalkane
CnH2n+1X
X = F, Cl, Br, I
Functional group
Halogenoalkanes
Suffix
Prefix
none
flouro-, chloro-,
bromo-, iodo-
Halides and naming
Different positions for the halogen and branching of the carbon chain
1-chlorobutane
2-chloro-2-methylpropane
2-chlorobutane
1-chloro-2-methylpropane
Halides
The number of carbons that are joined to the carbon
with the halogen group determine if it is a 1⁰, 2⁰,
or 3⁰ halide.
PRIMARY 1°
SECONDARY 2°
If you remove this carbon it is still primary
TERTIARY 3°
Class name
Formula
Functional group
name
alcohol
ROH
hydroxyl
Functional group
Alcohols
Suffix
-ol
Naming Alcohols
Full structure
formula
Skeletal
Name
Formula
Ethanol
Propan-1-ol
Propan-2-ol
Full structure
formula
Skeletal
Name
Formula
Butan-1-ol
Pentan-1-ol
Butan-2-ol
Pentan-3-ol
Alcohols
• The number of carbons that are joined to the
carbon with the alcohol group determine if it
is a 1⁰, 2⁰, or 3⁰ alcohol.
Class name
Formula
Functional group
name
ether
ROR’
ether
Functional group
Ethers
Suffix
Midfix
none
-oxy-
Naming Ethers
methoxy methane
2-ethoxy-2-methylpropane
Class name
Formula
Functional group
name
aldehyde
RCHO
aldehyde
Functional group
Aldehydes
Suffix
-al
Naming Aldehydes
CARBONYL COMPOUNDS - NOMENCLATURE
CH3CHO
ethanal
CH3CH2CHO
propanal
CH3CH2CH2CHO
butanal
CH3CH2CH2CH2CHO
pentanal
Class name
Formula
Functional group
name
ketone
RC(O)R’
carbonyl
Functional group
Ketones
Suffix
-one
Naming Ketones
CH3COCH3
propanone
CH3CH2COCH3
butanone
CH3COCH2CH2CH3 pentan-2-one
CH3CH2COCH2CH3 pentan-3-one
Class name
Formula
Functional group
name
ester
RCOOR’
ester
Functional group
Esters
Suffix
-oate
Naming Esters
•The first part is from the alcohol group (of a
carboxylic acid), the second part is from the
acid (of a carboxylic acid).
•Add oate
e.g. methyl ethanoate CH3COOCH3
METHYL ETHANOATE
ETHYL METHANOATE
Class name
Formula
Functional group
name
carboxylic acid
RCOOH
carboxyl
Functional group
Carboxylic acids
Suffix
-oic acid
Carboxylic acids
Carboxylic acids form a homologous series
HCOOH
CH3COOH
C2H5COOH
With more carbon atoms, there can be structural isomers
C3H7COOH
(CH3)2CHCOOH
NAMING CARBOXYLIC ACIDS
Naming Carboxylic acids
• select the longest chain of C atoms containing the COOH
group;
• remove the e and add oic acid after the basic name
• number the chain starting from the end nearer the COOH
group
• as in alkanes, prefix with alkyl substituents
• side chain positions are based on the C in COOH being 1
e.g.
CH3 - CH(CH3) - CH2 - CH2 - COOH
4-methylpentanoic acid
Class name
Formula
Functional group
name
amine
RNH2 (H=R)
amino
Functional group
Amines
Suffix
-amine
Naming Amines
Nomenclature Named after the groups surrounding the nitrogen +
amine
C2H5NH2
ethylamine
(CH3)2NH
dimethylamine
(CH3)3N
trimethylamine
C6H5NH2
phenylamine (aniline)
Class name
Formula
Functional group
name
amide
RCONH2
carboxamide
Functional group
Amides
Suffix
-amide
Naming Amides
• Solids named from the corresponding acid
• Remove oic acid, add amide
CH3CONH2
ethanamide (acetamide)
C2H5CONHC6H5
N - phenyl propanamide
- the N tells you the substituent is on the nitrogen
Class name
Formula
Functional group
name
nitrile
RCN
cyano
Functional group
Nitriles
Suffix
-nitrile
Naming Nitriles
CH3CH2CH2C≡N butanenitrile
ethanenitrile
Class name
Formula
Functional group
name
arene
CnH2n-6
phenyl
Functional group
Arenes
Suffix
none
Benzene
or
or
Due to the resonance energy or
stabilization energy of benzene,
it is reluctant to undergo
addition reactions, but will
undergo substitution reactions.
Delocalization minimizes the
repulsion between electrons.
Physical evidence for benzene stability
Naming Arenes
Arenes are compounds with a benzene ring.
Aliphatics on the other hand are compounds
without a benzene ring such as alkanes and
alkenes.
Bromobenzene
1,2-diphenylethyne
Physical Characteristics of the
functional groups
• Explain volatility between functional groups (Hbonding and Van der Waals)
• Explain solubility
Effects of lengthening chain and branching
10.2 Functional group chemistry
OBJECTIVES
Alkanes:
• Alkanes have low reactivity and undergo free-radical
substitution reactions.
• Writing equations for the complete and incomplete
combustion of hydrocarbons.
• Explanation of the reaction of methane and ethane with
halogens in terms of a free-radical substitution mechanism
involving photochemical homolytic fission.
Alkenes:
• Alkenes are more reactive than alkanes and undergo
addition reactions. Bromine water can be used to
distinguish between alkenes and alkanes.
• Writing equations for the reactions of alkenes with
hydrogen and halogens and of symmetrical alkenes with
hydrogen halides and water.
• Outline of the addition polymerization of alkenes.
• Relationship between the structure of the monomer to
the polymer and repeating unit.
Benzene:
• Benzene does not readily undergo addition reactions but
does undergo electrophilic substitution reactions
Alcohols:
• Alcohols undergo nucleophilic substitution reactions with
acids (also called esterification or condensation) and some
undergo oxidation reactions.
• Writing equations for the complete combustion of
alcohols.
• Writing equations for the oxidation reactions of primary
and secondary alcohols (using acidified potassium
dichromate(VI) or potassium manganate(VII) as oxidizing
agents). Explanation of distillation and reflux in the
isolation of the aldehyde and carboxylic acid products.
• Writing the equation for the condensation reaction of an
alcohol with a carboxylic acid, in the presence of a catalyst
(eg concentrated sulfuric acid) to form an ester.
Halogenoalkanes:
• Halogenoalkanes are more reactive than alkanes. They
can undergo (nucleophilic) substitution reactions. A
nucleophile is an electron-rich species containing a lone
pair that it donates to an electron-deficient carbon.
• Writing the equation for the substitution reactions of
halogenoalkanes with aqueous sodium hydroxide.
Polymers:
• Addition polymers consist of a wide range of monomers
and form the basis of the plastics industry.
.
Writing conventions
Double barbed arrow to indicate
movement of an electron pair.
Single barbed arrow to indicate the
movement of a single electron.
Single large dot means a radical. Not
a single electron but an incomplete
valence shell.
Square brackets means the charge is
distributed over the complex.
Dashed lines to indicated electrons
shared over more than one bond.
[Co(NO2)6]3-
Writing conventions
Line is on the page
Dash is going
behind the page
Wedge is coming
out of the page
Alkane reactions
Combustion
Photochemical substitution
(halogenation)
Alkanes – chemical properties
Carbon in general:
CATENATION is the ability to form bonds between atoms of the same
element.
Carbon forms chains and rings, with single, double and triple covalent
bonds, because it is able to FORM STRONG COVALENT BONDS WITH
OTHER CARBON ATOMS
In particular alkanes:
- fairly unreactive; (old family name, paraffin, meant little reactivity)
- have relatively strong, almost NON-POLAR, SINGLE covalent bonds
-they have no real sites that will encourage substances to attack them
Alkanes – chemical properties
Carbon forms a vast number of carbon compounds because of the
strength of the C-C covalent bond. Other Group IV elements can do
it but their chemistry is limited due to the weaker bond strength.
BOND
ATOMIC RADIUS
BOND ENTHALPY
C-C
0.077 nm
+348 kJmol-1
Si-Si
0.117 nm
+176 kJmol-1
The larger the atoms, the weaker the bond. Shielding due to filled inner
orbitals and greater distance from the nucleus means that the shared
electron pair is held less strongly.
Alkanes – combustion reactions
2O2(g)  CO2(g) +
complete
combustion
CH4(g)
+
incomplete
combustion
CH4(g)
+ 1½O2(g)  CO(g)
2H2O(l)
+
2H2O(l)
Homolytic and heterolytic fission
There are 3 ways to split the shared electron pair in an unsymmetrical covalent bond.
UNEQUAL SPLITTING
produces IONS
known as HETEROLYSIS or
HETEROLYTIC FISSION
EQUAL SPLITTING
produces RADICALS
known as HOMOLYSIS or
HOMOLYTIC FISSION
•
•
•
•
If several bonds are present the weakest bond is usually broken first
Energy to break bonds can come from a variety of energy sources - heat / light
In the reaction between methane and chlorine either can be used, however...
In the laboratory a source of UV light (or sunlight) is favoured.
Alkanes – free radical mechanism
(substitution reactions)
Homolytic fission
substitution reaction where electrons are shared when a
bond is broken
Free radical
atom or molecule with one free unpaired electron
Mechanism to be known
Methane + Cl2
Cl2 ——> 2Cl•
Initiation
RADICALS CREATED
Fish hook arrows
represent single
electrons
During initiation, the WEAKEST BOND IS BROKEN as it requires less energy.
There are three possible bonds in a mixture of alkanes and chlorine.
412
348
242
Average bond enthalpy kJ mol-1
The Cl-Cl bond is broken in preference to the others as it is the weakest and requires requires
less energy to separate the atoms.
Mechanism to be known
Methane + Cl2
1. Initiation
2. Propagation
Must start
and end with
a radical.
3. Termination
Mechanism to be known
Methane + Br2
Homolytic fission
making 2 free radicals
Bromomethane
CH4 + Br2  CH3Br + HBr
UV
(Hydrogen bromide is a colourless gas) ie. Brown to clear
Alkene reactions
Addition
Polymerization
Properties of alkenes
Carbon atoms are sp2 hybridized at 120⁰ with an outer π
bond that is the site of reactivity allowing range of
addition reactions.
As the bonds in double carbon bonds are not as stable as
single bonds (C-C 348 kJ mol-1, C=C 612 kJ mol-1 ie. not
2x348) they are energetically favourable to be converted
to single bonds. Also they have a high electron density
making their activation energy low.
Addition of hydrogen to alkene
Addition of HX to alkene
Bromine water
• Alkenes react with Bromine water.
• The bromine water changes from brown to clear.
• Tube A must contain an alkene
Alkene
Alkane
Bromine
Water
Addition of halogen to alkene
Addition of water to alkene
Polymerization
Alkenes can be added together to make long polymer
molecules. Here ethene makes the plastic polyethene.
Alkene summary
Alcohol reactions
Combustion
Oxidation
Esterification
Complete combustion of alcohols
• This is an extreme form of oxidation
• Like all organic compounds they give CO2 & H2O – in excess
O2(g)
• They burn more cleanly than their equivalent alkanes – O in
the compound is available for combustion products so less CO
made in limited O2 conditions (incomplete combustion).
Alcohols with longer chains have greater molar enthalpies,
but may not be able to burn cleanly.
C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(l)
NB: Watch to count the O in the alcohol when balancing!
Oxidation of alcohols
Oxidation of primary alcohols
Primary alcohols are easily oxidised to aldehydes
e.g.
CH3CH2OH(l) + [O]
ethanol
——>
KMnO4 or K2Cr2O7
CH3CHO(l) + H2O(l)
ethanal
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) + [O] ——> CH3COOH(l)
ethanal KMnO4 or K2Cr2O7
ethanoic acid
Practical details
•
•
•
•
•
the alcohol is dripped into a warm solution of acidified KMnO4 or K2Cr2O7
K2Cr2O7 is reduced from orange Cr(VI) to green Cr(III)
aldehydes have low boiling points - no hydrogen bonding - they distil off immediately
if it didn’t distil off it would be oxidised to the equivalent carboxylic acid
to oxidise an alcohol straight to the acid, reflux the mixture
compound
formula
intermolecular bonding
boiling point
ETHANOL
C2H5OH
HYDROGEN BONDING
78°C
ETHANAL
CH3CHO
DIPOLE-DIPOLE
23°C
ETHANOIC ACID
CH3COOH
HYDROGEN BONDING
118°C
Oxidation of primary alcohols
Controlling the products
e.g.
CH3CH2OH(l) + [O]
——>
CH3CHO(l) + H2O(l)
then
CH3CHO(l) + [O]
——>
CH3COOH(l)
KMnO4 or K2Cr2O7
OXIDATION TO ALDEHYDES
DISTILLATION
OXIDATION TO CARBOXYLIC ACIDS
REFLUX
Aldehyde has a lower boiling point so
distils off before being oxidised further
Aldehyde condenses back into the
mixture and gets oxidised to the acid
Oxidation of secondary alcohols
Secondary alcohols are easily oxidised to ketones
e.g.
CH3CHOHCH3(l) + [O] ——>
propan-2-ol KMnO4 or K2Cr2O7
CH3COCH3(l) + H2O(l)
propanone
The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment
with a powerful oxidising agent they can be further oxidised to a mixture of acids
with fewer carbon atoms than the original alcohol.
Oxidation of tertiary alcohols
Tertiary alcohols are resistant to normal oxidation
Condensation - Esterification
Halogenoalkene reactions
Nucleophic substitution
Terminology
Nucleophile
Reactants with a non-bonding electron pair that are attracted to a
positive carbon (a form of electrophile – loves negative)
Heterolytic fission
Formation of a carbocation and a negative ion, due to carbon losing
it’s shared electron
As the carbon-halogen bond means carbon is slightly
positive, halogenoalkanes are reactive and undergo
nucleophilic substitution reactions from nucleophile
attack.
Halogenoalkane substitution reactions
General formula:
Eg. with NaOH
CH3CH(Cl)CH2CH3 + NaOH  CH3CH=CHCH3 + NaCl + H2O
Conditions: Heat (boil)
Benzene reactions
Electrophilic substitution
Electrophilic substitutions
Just know
electrophiles
will substitute.
Specific
reactions do
not need to be
memorized.
OBJECTIVES
Nucleophilic Substitution Reactions:
• SN1 represents a nucleophilic unimolecular substitution reaction and SN2 represents a nucleophilic bimolecular substitution reaction. SN1involves a
carbocation intermediate. SN2 involves a concerted reaction with a transition state.
• For tertiary halogenoalkanes the predominant mechanism is SN1and for primary halogenoalkanes it is SN2. Both mechanisms occur for secondary
halogenoalkanes.
• The rate determining step (slow step) in an SN1reaction depends only on the concentration of the halogenoalkane, rate = k[halogenoalkane]. For
SN2, rate = k[halogenoalkane][nucleophile]. SN2 is stereospecific with an inversion of configuration at the carbon.
•SN2 reactions are best conducted using aprotic, non-polar solvents and SN1reactions are best conducted using protic, polar solvents.
• Explanation of why hydroxide is a better nucleophile than water.
• Deduction of the mechanism of the nucleophilic substitution reactions of halogenoalkanes with aqueous sodium hydroxide in terms of SN1and SN2
mechanisms. Explanation of how the rate depends on the identity of the halogen (ie the leaving group), whether the halogenoalkane is primary,
secondary or tertiary and the choice of solvent.
• Outline of the difference between protic and aprotic solvents
Electrophilic Addition Reactions:
• An electrophile is an electron-deficient species that can accept electron pairs from a nucleophile. Electrophiles are Lewis acids.
• Markovnikov’s rule can be applied to predict the major product in electrophilic addition reactions of unsymmetrical alkenes with hydrogen halides
and interhalogens. The formation of the major product can be explained in terms of the relative stability of possible carbocations in the reaction
mechanism.
•Deduction of the mechanism of the electrophilic addition reactions of alkenes with halogens/interhalogens and hydrogen halides.
Electrophilic Substitution Reactions:
• Benzene is the simplest aromatic hydrocarbon compound (or arene) and has a delocalized structure of π bonds around its ring. Each carbon to
carbon bond has a bond order of 1.5. Benzene is susceptible to attack by electrophiles.
• Deduction of the mechanism of the nitration (electrophilic substitution) reaction of benzene (using a mixture of concentrated nitric acid and
sulfuric acid).
Reduction Reactions:
• Carboxylic acids can be reduced to primary alcohols (via the aldehyde). Ketones can be reduced to secondary alcohols. Typical reducing agents are
lithium aluminium hydride (used to reduce carboxylic acids) and sodium borohydride.
• Writing reduction reactions of carbonyl containing compounds: aldehydes and ketones to primary and secondary alcohols and carboxylic acids to
aldehydes, using suitable reducing agents.
• Conversion of nitrobenzene to phenylamine via a two-stage reaction.
Higher level
20.1 Types of organic reactions
Higher level
Nucleophilic Substitution
Reactions
SN1 and SN2
Polar – have dipole moments due to different
electronegativities
Non-polar – similar electronegativities
Protic – polar solvents with O-H or N-H bonds allowing
hydrogen bonding and a source of protons eg. water,
ethanol
Aprotic – a polar or non-polar solvent that do not have
O-H or N-H bonds nor provide a source of protons eg.
acetone (propanone) (polar), ethanenitrile (polar),
benzene (non-polar), hexane (non-polar)
SUMMARY: 3 main types of solvents, non-polar, polar
protic, polar aprotic, no such thing as a non-polar protic
Higher level
Solvents
SN2 reactions involve heterolytic fission and
nucleophilic substitution with mainly primary
halogenoalkanes.
An unstable transition state is created meaning the
reaction is bimolecular requiring two molecules:
rate = k[halogenoalkane][nucleophile]
Polar aprotic solvents are preferred which are
those unable to form hydrogen bonds, eg.
propanone, ethyl ethanoate. Otherwise the solvent
would bind to the nucleophile inhibiting its action.
Higher level
SN2 reaction mechanism
SN2 reaction mechanism
Inversion at 180⁰ must be shown.
Higher level
Mechanism to be known
SN1 reactions involve heterolytic fission and nucleophilic substitution with
mainly tertiary halogenoalkanes.
Due to steric hindrance the halogen must be heterolytically removed to
create a carbocation before the nucleophile can attached.
This creates a more stable carbocation intermediate so the reaction is
unimolecular:
rate = k[halogenoalkane]
Polar protic solvents are preferred
which are able to form hydrogen
bonds which stabilize the
carbocation by ion-dipole interactions
eg. water, alcohol
Higher level
SN1 reaction mechanism
The positive induction is the stabilization of a
carbocation because the other alkyl groups can
unevenly share their electrons with the positive
centered carbon:
Higher level
Positive induction
SN1 reaction mechanism
Higher level
Mechanism to be known
As halogen-carbon bonds become less polar you
would expect nucleophilic attack to be less and
so rate to decrease down the group:
However we must also consider how much
energy it takes to break bonds:
And this is the deciding factor meaning rate
increases down the group:
Higher level
Effect of leaving group
In general SN1 tertiary reaction rates are faster
than SN2 primary due to the stability of the
formation of carbocations:
Higher level
Effect of mechanism
Higher level
Electrophilic Addition Reactions
Alkenes
Ethene and bromine
Higher level
Mechanism to be known
Ethene and HBr
Higher level
Mechanism to be known
Markovnikov’s rule: the hydrogen will attach to
the carbon that is already bonded to the greater
number of hydrogens.
In the following situation:
b. Will be more stable due to positive induction:
Hence reaction b. prevails.
Higher level
Asymmetric alkenes
Higher level
Propene and HBr
Higher level
Electrophilic Substitution
Reactions
Benzene
Nitration of benzene
Firstly sulfuric acid is stronger so protonates the nitric acid:
With heat, the electrophilic substitution by the nitronium ion
causes a momentary loss of symmetry of the electron structure of
benzene.
Higher level
Mechanism to be known
Higher level
Reduction Reactions
Alcohols
Nitrobenzene
The oxidation of alcohols can be reversed by reduction as follows:
[+H-]
[+H-]
Needs lithium aluminium hydride (LiAlH4) in dry ether.
[+H-]
Needs heat with sodium borohydride (NaBH4).
Higher level
Reduction of carbonyl compounds
Reduction of nitrobenzene
Higher level
Mechanism to be known?
OBJECTIVES
• The synthesis of an organic compound stems from a readily available starting material
via a series of discrete steps. Functional group interconversions are the basis of such
synthetic routes.
• Retro-synthesis of organic compounds.
• Deduction of multi-step synthetic routes given starting reagents and the product(s).
Higher level
20.2 Synthetic routes
A synthetic route is a series of discrete chemical steps to change a
reactant to a desired product. Efficient design has been the result
of retrosynthetic analysis where you think of the target molecule
and work backwards through precursors to the starting material.
Higher level
Synthetic routes
aldehyde
phenylamine (aniline)
Reflux Oxidation
M
Reduction
LiAlH4 dry ether
Cr2O72- H+
Cr2O72- H+
Reduction
1⁰ Oxidation
LiAlH4 dry ether
carboxylic acid
NaOH
2⁰ Oxidation
alcohol
ketone
Reduction
Condensation
phenylammonium ion
M
NaBH4
halogenoalkane
M
1⁰ SN2, 3⁰ SN1
H2O/H2SO4
Substitution
Addition
M
Sn conc. HCl
dihalogenoalkane
Cl2 UV
HCl
Free radical
Addition
nitrobenzene
M
Cl2
M
Addition
Homolytic fission
alkane
M
ester
NaOH
Conc. HNO3O/H2SO4
Ni H2
Addition
Addition
alkene
polymerization
polymer
benzene
Higher level
Cr2O72- H+
Higher level
Problem 1: Produce ethanol from ethane.
OBJECTIVES
• Stereoisomers are subdivided into two classes—conformational isomers,
which interconvert by rotation about a σ bond and configurational isomers
that interconvert only by breaking and reforming a bond.
Configurational isomers are further subdivided into cis-trans and E/Z isomers
and optical isomers.
• Cis-trans isomers can occur in alkenes or cycloalkanes (or heteroanalogues)
and differ in the positions of atoms (or groups) relative to a reference plane.
According to IUPAC, E/Z isomers refer to alkenes of the form R1R2C=CR3R4
(R1 ≠ R2, R3 ≠ R4) where neither R1 nor R2 need be different from R3 or R4.
• A chiral carbon is a carbon joined to four different atoms or groups.
• An optically active compound can rotate the plane of polarized light as it
passes through a solution of the compound. Optical isomers are enantiomers.
Enantiomers are non-superimposeable mirror images of each other.
Diastereomers are not mirror images of each other.
• A racemic mixture (or racemate) is a mixture of two enantiomers in equal
amounts and is optically inactive.
• Construction of 3-D models (real or virtual) of a wide range of
stereoisomers.
• Explanation of stereoisomerism in non-cyclic alkenes and C3 and C4
cycloalkanes.
• Comparison between the physical and chemical properties of enantiomers.
• Description and explanation of optical isomers in simple organic molecules.
• Distinction between optical isomers using a polarimeter.
Higher level
20.3 Stereoisomerism
S
C
Higher level
Isomers
Cis isomers are those
with the same groups on
the same side of the
double bond or cyclical
compounds.
Trans isomers have the
groups on the opposite
sides of the double bond.
Higher level
Cis-trans isomers
Higher level
Example:
Higher level
E/Z isomers
For isomers with more than 2 different groups, the group with the
highest priority on the left hand side is determined and then the group
with the highest priority on the right hand side is determined.
E isomers have these two groups opposite. (entgegen in German)
Z isomers have these two groups on the same side. (above) (zusammen
in German)
Z is S in a mirror
Priority rules:
1. The atom with the highest atomic number has the highest priority.
2. If the atom is the same, apply this rule to the next bonded atom in
the chain.
Higher level
Example:
Higher level
Different compounds - different properties.
Optical isomers are those which
contain a chiral carbon, that is, it
contains four different groups.
These compounds are nonsuperimposable on each other and
exist in pairs called enantiomers eg.
amino acids. Diastereomers
contain two or more chiral carbons
and are not mirror images.
Higher level
Optical isomers
• isomers differ in their reaction to plane-polarised light
• plane polarised light vibrates in one direction only
• one isomer rotates light to the right, the other to the left (degree of
angle equal)
• rotation of light is measured using a polarimeter
• rotation is measured by observing the polarised light coming out
towards the observer
• If the light appears to have
turned to the right
DEXTROROTATORY
d or + form
turned to the left
LAEVOROTATORY
l or - form
A racemate or racemic mixture is a 50-50 mixture of the two
enantiomers (dl) or (±). The opposite optical effects of each isomer
cancel each other out.
Higher level
Optical activity
Higher level
Polarimeter
A
B
C
D
E
F
A
B
C
D
E
F
Light source produces light vibrating in all directions
Polarising filter only allows through light vibrating in one direction
Plane polarised light passes through sample
If substance is optically active it rotates the plane polarised light
Analysing filter is turned so that light reaches a maximum
Direction of rotation is measured coming towards the observer
If the light appears to have:
turned to the right
DEXTROROTATORY
turned to the left
LAEVOROTATORY
• The formation of racemic mixtures is more likely in a
laboratory reaction than in a chemical process occurring
naturally in the body.
• If a compound can exist in more than one form, only one
of the optical isomers is usually effective.
• The separation of isomers will make manufacture more
expensive.
• A drug made up of both isomers will require a larger
dose and may cause problems if the other isomer is
‘poisonous’ like thalidomide, which is a teratogen –
causes birth defects.
Higher level
Optical isomers – Other points
(which is different to natural selection!)
Only L-forms of amino acids are used in cells.
Aren’t proteins a violation of entropy and enthalpy anyway?
If there are no proteins, how did the DNA reproduce?
Which came first the chicken or the egg?
Higher level
TOK - problems for evolution?