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H2 CHEMISTRY
Foundational & Organic chemistry
Topics covered:
1
...
Arenes
2
...
Halogen derivatives
3
...
Alcohols & phenols
4
...
Carboxylic acids
5
...
Carbonyl Compounds
6
...
Nitrogen compounds
7
...
8
a = 0
...
2
...
4
...
02 x 1023
Percentage composition by mass:
Percentage mass
= Mass of element (compound) / Mass of total compound
= η of element (compound) / η of total compound
Determining empirical and molecular formula:
C
H
Mass in 100g /g
No of mol /mol
Mole ratio
Hence empirical formula is CxHyOz
Let molecular formula of compound be (CxHyOz)n
Mr = n[ 12(n) + 6(y) + 16(z) ]
∴n=…
!
O
Calculations using combustion data:
Using formula:
CxHy + (x + y/4)O2 ! xCO2 + (y/2)H2O
Percentage yield:
Percentage yield = (Actual mass of product formed / Theoretical mass) x 100%
Calculations using volume of gas:
Condition
s
...
p
...
t
...
(room temperature and pressure)
Note: For a gas, volume ration = mole ratio
Temperature
273K
298K
Pressure
1 atm
1 atm
Molar volume
22
...
Determine amount of C required in titration
2
...
Hence, amount of A that reacted with B = total amount of A (in excess) – amount that reacted
with C
4
...
g
...
g
...
g
...
g
...
g
...
g
...
g
...
2
...
4
...
6
...
Construct half equation for A
2
...
Calculate mole ration of B and electron lost or gained, and thus number of moles of electron
transferred per mole of B
4
...
Proportional to charge on the particle
2
...
Sigma bonds: Formed through the “head-on” overlap of two atomic orbitals
2
...
Dative covalent bonds
• Involves one atom to contribute both electrons for sharing
• Requires donor to have one lone pair of electrons and acceptor having vacant orbial
Bond polarity: If difference in electronegativity, bond is considered to be polar
Drawing dot-and-cross diagram
General rules:
• Positively-charged ions, assign positive charge to less electronegative
• Negative-charged ions, assign negative charge to more electronegative
Normal diagram
Electron deficient molecule
Expansion of octet
Odd electron species
Conjugated system: Happens when p-orbitals of 2 or more adjacent atoms are in alignment,
electrons are then free to move about the entire conjugated system
Whenever a double bond occurs, or presence of elements with p-orbitals (N or C), there is a
possibility of conjugation occurring
Importance is on the p-orbital being in the same geometry and alignment
!
VSEPR Theory:
Works on the theory of
[Lone pair-lone pair repulsion > lone pair-bond pair repulsion > bond pair-bond pair repulsion]
• This is to adopt a geometry to minimize the repulsion
Hybridisation:
Shape
Tetrahedral
Type of hybridisation
sp3 (shape: bi-lobed)
Trigonal planar
sp2
Linear
sp
!
Drawing
Molecule polarity:
Non-polar molecule is one that has no overall net dipole after resolving all dipole moments
• Molecule contains no polar bonds
• Molecule has polar bonds but dipole moments cancel out due to geometry of molecule
o Linear, Trigonal planar, tetrahedral, square planar, Trigonal bipyramidal and octahedral
Intermolecular forces:
3 main types
1
...
Permanent dipole-permanent dipole
3
...
01 x 105 Pa
OR 101 kPa
1cm3 = 1 x 10-6 m3
1dm3 = 1 x 10-3 m3
0K = -273oC
-
Manipulation of gas equation:
Parameter
Mass
Concentration in moles
Molar mass
Density
Equation
m = PVMr / RT
c = P / RT
Mr = mRT / PV
ρ = PMr / RT
Gas mixtures and partial pressure:
Ptotal = Pa + Pb + Pc + …
In order to derive partial pressure of a specific gas in a mixture, we can use:
P1V1 = P2V2
• P1 – Pressure of gas A
• V1 – Volume of gas A
• P2 – Partial pressure of gas A in gas mixture
• V2 – Total volume of gas mixture
Also, taking into account mole fraction
Pa = (ηa / ηtotal) x Ptotal
!
Assumptions of ideal gas:
1
...
Gas particles have negligible volumes compared to volume of the container
3
...
Collision between gas particles and walls of container are perfectly elastic
5
...
High temperatures
2
...
Displayed formula / full structural formula
O
C
O
2
...
Skeletal formula
4
...
Aliphatic compounds – compounds with open chains of carbon (ends are not joined)
2
...
Aromatic compounds – compounds with one aromatic system (benzene)
Types of organic reactions:
Addition
Two substances react together to form a single
product
Elimination
Small molecule such as water is removed from a
larger one
Substitution
One atom or group of atoms is substituted by
another (always at least 2 products)
Oxidation
Oxygen is added or hydrogen is removed
Reduction
Oxygen is removed or hydrogen is added
Condensation
Two molecules come together to form bigger
molecule, elimination of water
Hydrolysis
Molecule is split into two by the action of water,
catalyzed by dilute acid or alkali
Mechanisms of organic reactions:
Homolytic fission
Involves the splitting of a single bond to give an equal share of bonding electrons, resulting in the
formation of radicals
!
Heterolytic fission
Involves the splitting of a bond resulting in unequal sharing of bonding electrons, resulting in the
formation of ions
Further use of the curly arrows:
Electrophiles and Nucleophiles:
Electrophiles – Electron deficient species that accepts an electron pair from electron rich species to
form a new covalent bond (usually has positive charge / partial positive charge)
Nucleophiles – Species that donates an electron pair to an electron-deficient species in a reaction to
form a new covalent bond (all possess lone pair of electrons, can be neutral or anions)
Reaction intermediates:
Formed in one of the steps of the mechanism reaction and consumed in later steps, it can be stable
and isolated (contrast to transition state)
Types:
1
...
Carbocation – contains carbon atom bearing a positive charge, bonded to three other atoms
3
...
Structural isomerism – atoms are linked in different ways
2
...
branched
Different carbon skeleton
Positional isomerism – pentan-1-ol vs
...
ethyl propanoate
Differ in functional group ! CH3CH2CH2CH2CO2H vs
...
Contains a plane of symmetry within the molecule itself – known as achiral (meso compound)
2
...
Locate longest continuous chain of carbon atoms
2
...
Use the number allocated to determine the position of the substituent
4
...
When 2 substituents are present on the same carbon, use that number twice
6
...
Probability: Ratio of primary (1o) hydrogen : secondary (2o) hydrogen
2
...
Hence, bromination is slower than
chlorination
!
Combustion of alkanes:
Alkanes react with excess oxygen (complete combustion) to give carbon dioxide and water
!! !! + ! + !
!
!
!! ! → !"!! + ! !! !
4
2
Alkanes only burn in the gaseous state
...
Alkanes are mainly separated through
fractional distillation, carried out at atmospheric temperature
Cracking
Alkanes can be converted to smaller alkanes through the process of cracking, in which the C–C
bonds in long-chain alkane molecules are broken to form smaller alkanes and alkenes
Thermal cracking – Heating alkane mixture at 800oC and produces alkane and alkenes
• Preceded by free radical substitution
• Involves very little rearrangement
Catalytic cracking – Involves heating mixture at 500oC and passing over a catalyst called zeolites
(aluminum and silicon oxides) and produces alkanes, alkenes and H2
• Involves formation of carbocation and undergoes rearrangement before forming final product
• Branched-chain alkanes produced are useful components of high-octane petrol
!
Catalytic Reforming
Reforming straight-chain alkanes is done by changing straight-chain alkanes into branched-chain
alkanes and cyclic hydrocarbons without loss of any carbon atoms
Done by passing vaporized alkane mixture over platinum-coated aluminum oxide catalyst at 500oC
and moderately high pressure
Environmental consequences of burning fuel:
Carbon monoxides and unburnt hydrocarbons
They arise primarily due to incomplete combustion of fuel
Carbon monoxide: binds irreversibly to haemoglobin in blood and forms a stable complex, thus
reducing capacity of haemoglobin to transport oxygen
Unburnt hydrocarbons: in the presence of sunlight, they react with oxygen, ozone and oxides of
nitrogen to form photochemical smog
Oxides of nitrogen
NOx are formed inside the combustion chambers of motor vehicles
...
They also result
in the formation of photochemical smog which has a harmful effect on plants
Formation of nitrous and nitric acid: 2NO2 + H2O ! HNO2 + HNO3
Formation of sulfuric acid: 2SO2 + NO2 ! 2SO3 + NO; NO + ½O2 ! NO2
HNO3 and H2SO4 are strongly acidic and form acid rain which causes corrosion of buildings and
man made structures, and also harms marine life and agriculture
!
Greenhouse effect
It is the heating of the earth due to the presence of greenhouse gases, which causes radiation to be
trapped in the atmosphere and increase in global temperature
Greenhouse gases:
• Natural: Carbon dioxide, Methane, Water vapor and Nitrous oxides
• Manmade: Chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs)
Catalytic converters
It removes pollutant gases from the exhaust by oxidizing or reducing them, where the gases passes
through a converter containing an alloy of platinum or rhodium (heterogeneous catalyst)
2NO + 2CO ! N2 + 2CO2
2CO + O2 ! 2CO2
However, catalytic converters only work with unleaded petrol as lead poisons the catalyst by
binding permanently to it
~ END ~
!
Chapter 7: Alkenes
Introduction:
Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond, CnH2n
Alkenes are named similarly to alkanes –
1
...
Number the double bond
3
...
If more than 1 double bond is present, indicate the position using –diene, -triene,
5
...
Due to the positive charge, it strongly
attracts any nucleophile that may attack from either side of the plane
Alkyl groups are electron donating, thus the more alkyl group the carbocation has, the more stable it
is as electron donating groups disperses the positive charge vs
...
This reaction is known as electrophilic addition
Electrophilic addition of hydrogen halides (H–X):
Step 1: Rate determining step
Polar HBr approaches the π electron cloud, the high electron density of π electron cloud “attacks”
electron deficient H atom on HBr
1
...
of hydrogen atoms on carbon Structure
Products after oxidation
2
1
0
Usefulness of oxidative cleavage:
• Establish the location of the alkene double bond
• Helps distinguish between alkenes with terminal double bonds and those without as those with
terminal ends would evolve CO2 gas
~ END ~
!
Chapter 8: Arenes
Introduction:
Arenes are aromatic hydrocarbons possessing the ring structure of benzene or its analogues
Benzene is a planar aromatic molecule –
1
...
Three π bonds alternate in the ring to allow each of the six unhybridized electrons to move
through the entire π system, so the electrons are said to be delocalized ! giving benzene extra
stability
Effects of π electron delocalization on the ring structure:
• All C–C bond lengths are equal
• Benzene undergoes substitution rather than addition reactions
o Addition destroys the delocalized π electron system and requires a lot of energy
Physical properties of benzene:
Benzene is a colourless liquid with a characteristic ‘aromatic’ odor
Melting and boiling point – being a non-polar molecule, only a small amount of energy is required
to break the weak dispersion forces between molecules, thus it has low mp/bp
Solubility – benzene is insoluble in polar solvents but soluble in non-polar solvents
Uses – benzene was frequently used as an industrial solvent, but now it is an intermediate to making
other chemicals
Heath effects – the major effect is to the blood: it causes harmful effects to the bone marrow and
result in a decrease in red blood cells; it could also lead to leukemia
Nomenclature:
Mono-substituted
Parent
When the benzene is attached
to an alkyl group <6 carbon
atoms
Usual naming method -benzene
Multi-substituted
Substituent
When the benzene is attached
to an alkyl group >6 carbon
atoms
Named as prefix of phenyl
More than one functional group
attached to it
Order according to –CO2H >
-OH > -CH3 > -halogen > -NO2
Reactions of benzene:
The main reaction undergone by benzene is electrophilic substitution
There is a rich source of π electrons above and below the benzene ring, which is highly reactive
towards electrophiles
Addition reactions would permanently destroy the aromaticity and extra stability associated with the
delocalized π electrons, whereas substitution would retain the delocalized system
!
Nitration:
Step 1 – formation of electrophile NO2+
2H2SO4 + HNO3 ! NO2+ + H3O+ + 2HSO4H2SO4 is a stronger acid than and protonates HNO3 to give the electrophile NO2+ and H2O; the
water molecule is then protonated by another molecule of H2SO4
Step 2 - Delocalized π electron cloud attacks the electrophile
Step 3 – Intermediate loses a proton and aromaticity is restored
Note: under more rigorous conditions, nitrobenzene can be further nitrated to give 1,3dinitrobenzene (95oC) or 1,3,5-trinitrobenzene (100oC)
!
Halogenation (chlorination or bromination):
Things to note –
1
...
They are used to generate a strong electrophile
3
...
This can be explained through the resonance and inductive effect
Effects on reactivity of benzene ring:
Activating groups: Electron-donating substituents increase electron density of benzene ring and thus
increase reactivity
Deactivating groups: Electron-withdrawing substituents decrease electron density
There are 2 ways which electrons can be donated or withdrawn from the benzene ring –
1
...
Through the pi bond ! resonance effect
Substituent
Inductive effect
Resonance effect
Overall effect on
reactivity
Alkyl groups
-OH, -NH2, OCH3
-F, -Cl, -Br, -I
Electron donating
Electron
withdrawing
Electron
withdrawing
Electron
withdrawing
Electron donating
Activating
Activating
Electron donating
Weakly
Deactivating
Deactivating
-CHO, -NO2,
-CO2H, -CN
!
Electron
withdrawing
Overall effect on
position of
incoming group
2,4-directing
2,4-directing
2,4-directing
3-directing
*For halogens, as the 3p orbital overlaps less effectively with the 2p orbital of the carbon atom,
compared to the more effective 2p-2p overlap between orbitals on O or N atoms; thus electron
withdrawing inductive effect outweighs the electron donating resonance effect
Side-chain free radical substitution of methylbenzene:
More than one chlorine or bromine atom can be added if excess halogen is used
Side chain oxidation:
Benzoic acid is always formed regardless of the length of the alkyl side chain
!
If a basic medium was used instead, sodium benzoate is formed and purple KMnO4 decolorizes and
brown precipitate of MnO2 is formed
For ethylbenzene, the second carbon in the side chain will be oxidized to CO2
If the side chain contains of a propyl group or longer, the products will consist of benzoic acid and a
carboxylic acid
Summary of reactions of benzene:
!
Summary of reactions of methylbenzene:
~ END ~
!
Chapter 9: Halogen Derivatives
Halogenoalkanes:
They are alkanes with one of more H atoms replaced by the halogen atom Cl, Br or I
They are classified as primary, secondary or tertiary depending on the number of alkyl groups:
They have higher boiling points compared to the alkanes as the main intermolecular forces of
attraction is dispersion forces with a small amount of pd-pd
Halogenoarenes:
Compounds that have a halogen atom attached directly to an aromatic carbon ring
!
Preparation of halogenoalkanes and halogenoarenes:
Halogenoalkanes
Free Radical Substitution
Reagents and conditions: Cl2(g) or Br2(l); UV light
Observations: Greenish-yellow Cl2 or reddish-brown Br2 decolorizes
Electrophilic addition of hydrogen halide
Reagents and conditions: gaseous HCl, HBr or HI; room temperature
Electrophilic addition of halogen
Reagents and conditions: Cl2(g) or Br2(l) or dissolved in CCl4; Room temperature without light
Observations: Greenish-yellow Cl2 or reddish-brown Br2 decolorizes
Preparation of chloroalkanes
Phosphorus pentachloride PCl5: ROH + PCl5 ! RCl + POCl3(l) + HCl
Reagents and conditions: PCl5(s); room temperature
Observations: White fumes of HCl(g)
Sulfur dichloride oxide SOCl2: ROH + SOCl2 ! RCl + SO2 + HCl
Reagents and conditions: SOCl2(l)
Observations: SO2(g) and white fumes of HCl(g)
Hydrogen chloride HCl: ROH + HCl ! RCl + H2O
Reagents and conditions: HCl(g) or concentrated HCl [tertiary alcohols]; with anhydrous ZnCl2
Preparation of bromoalkanes
Phosphorus tribromide PBr3: 3ROH + PBr3 ! 3RBr + H3PO3
Reagents and conditions: PBr3
Preparation of iodoalkanes
Phosphorus tri-iodide PBr3: 3ROH + PI3 ! 3RI + H3PO3
Reagents and conditions: PI3
Halogenoarenes
Electrophilic substitution
Reagents and conditions: X2, Fe or FeX3 catalyst
Reactions of halogenoalkanes:
They undergo primarily two types of reactions: nucleophilic substitution and elimination
Halogens are more electronegative than carbon, so the C-halogen bond is polar and the carbon
acquires a partial positive charge, causing it to be attractive to nucleophiles
!
Formation of alcohols (alkaline hydrolysis):
Formation of amines:
A primary amine is formed in the above reaction
...
The electrondeficient carbon atom is susceptible to attack by an electron-rich species
Primary halogenoalkanes undergo nucleophilic substitution by a one step mechanism (SN2)
Tertiary halogenoalkanes undergo nucleophilic substitution by a two step mechanism (SN1)
Secondary halogenoalkanes can undergo either
SN2 mechanism (bimolecular nucleophilic substitution)
•
•
•
•
•
C–Br bond is polar and thus allowing nucleophile (OH-) to attack the partially positive carbon
from the side opposite the bromine anion
The OH- donates one of its lone pair od electrons to the carbon so that a bond begins to form
between the oxygen on the OH- and the carbon
The C–Br is then weakened and begins to break
The transition state is formed where the entering nucleophile and leaving Br- ion are partially
bonded to the same carbon atom
Rate = k[RBr][OH-]
Note: If the original RX molecule is chiral, the stereochemistry will be inverted after substitution
SN1 mechanism (unimolecular nucleophilic substitution)
!
•
•
•
•
C–Br bond is polar and heterolytic fission of C–Br bond occurs, giving a carbocation
intermediate and BrIn the second step, the highly reactive carbocation is readily attacked by the nucleophile (OH-)
This is a two step mechanism
Rate = k[RBr]
Note: Since the carbocation is trigonal planar, the nucleophile is able to attack the top and bottom
face of the carbocation with equal probability, hence forming a racemic mixture
SN2 mechanism
Attacking nucleophile approaches carbon from
the side opposite the leaving halide
...
Hence,
tertiary halogenoalkanes undergo the SN1
mechanism
Rates of nucleophilic substitution:
It depends on the relative ease of breaking the C–X bond
...
The C–X bond breaks at the same time
...
The p-orbital of the halogen atom overlaps with the π electron cloud, allowing double bond
character in the C–X bond, hence its bond strength is much stronger and harder to break
2
...
They are inert and non-flammable
2
...
They are compounds that liquefy under pressure and thus vaporize readily when pressure is
released
4
...
Reduce the usage of CFCs
2
...
Phenols are compounds in which the OH
group is directly attached to an aromatic ring
Alcohols are named the following manner –
1
...
Number the hydroxyl group
3
...
If more than 1 hydroxyl group is present, indicate the position using –diol, -triol
5
...
Using alkenes through hydration
2
...
Using aldehydes and ketones from reduction
4
...
The acidity of the compound is
dependent on the stability of the conjugate base
• The more stable the conjugate base, the greater the extent of dissociation
• Thus a more acidic compound
Alcohols
In alcohols, the O–H bond breaks to give H+ and ROEffect of substituents
• Presence of electron withdrawing groups [–NO2, –F, –Cl, –CO2H, –CN, –OH] helps to
stabilize the alkoxide ion by dispersing the negative charge on the alkoxide ion, promoting
ionisation process
• Presence of electron donating groups [–CH3] intensifies the negative charge on the alkoxide
ion, thus destabilizing it and deprotonation does not take place as easily
Phenols
Phenols are more acidic than alcohols as the negative charge on the phenoxide ion can be
delocalized over the benzene ring, stabilizing the phenoxide ion
The p-orbital of the oxygen is able to overlap with π-orbital of the carbon atom, allowing the
negative charge to spread across the phenoxide ion, thus it is stabilized by resonance
Effect of substituents
• Similar to that of alcohols, except that OH group is now electron donating as resonance effect is
now more predominant
!
Halogenation:
Alcohols can be converted to halogenoalkanes through nucleophilic substitution of the OH group
Using hydrogen halides
!
Using phosphorus halides
!
REACTIONS OF ALCOHOLS
Dehydration:
Dehydrating agents and conditions
• Excess concentrated H2SO4, heated to 170oC
• Aluminum oxide, Al2O3, heat
Reaction with sodium metal:
Note: Alcohols are not reactive enough to react with NaOH or carbonates
Esterification:
With carboxylic acids
The reaction is slow and reversible
...
Methyl group (CH3)
2
...
Hydroxyl group
Note: It is useful as a step-down reaction as it removes one CH3 group
...
Find the longest parent chain containing the –CO2H group and name with the suffix –oic acid
2
...
Write the full name, numbering the substituents according to their position
4
...
H-bonding is stronger due to presence of C=O group (electron withdrawing), causing the O–H
bond to be more polarized
2
...
Probability of carboxylic acid molecules forming more extensive H-bonds with neighboring
molecules is higher
Solubility – carboxylic acids are soluble due to the ability to form H-bonds with water
• As the length of the hydrocarbon tail increases, solubility decreases
• The longer hydrocarbon chain disrupts formation of H-bonds, thus only weak dispersion forces
predominate between water and acid molecule
!
Comparing acid strengths:
Compounds
Aliphatic alcohols
(ROH)
Phenols
Carboxylic acids
(RCO2H)
Comparison of acid strength
Aliphatic alcohol (ROH) dissociates to form alkoxide ion
• Electron donating alkyl group (R) intensifies negative charge on
alkoxide ion, thus destabilizing it
Phenol dissociates to form phenoxide ion
• Negative charge on the phenoxide ion can be delocalized into the
benzene ring
• Negative charge is dispersed and phenoxide anion is stabilized
Carboxylic acids dissociate in water to form carboxylate ion
• Negative charge on carboxylate ion is delocalized over 2 highly
electronegative O atoms
• Negative charge is dispersed and carboxylate anion is greatly stabilized
Charge dispersal is greater than in phenoxide as the negative charge is
delocalized and equally distributed over carbon and 2 O atoms, but for
phenol it is only slightly delocalized into the ring
Aliphatic alcohols (ROH)
Phenols
Carboxylic acids (RCO2H)
Na
✔
✔
✔
NaOH
✖
✔
✔
Na2CO3 / NaHCO3
✖
✖
✔
Relative acidity of carboxylic acids:
Effect of substituents
• Presence of electron withdrawing groups [–NO2, –F, –Cl, –CO2H, –CN, –OH] helps to
stabilize the carboxylate anion by dispersing the negative charge on the carboxylate anion,
promoting ionisation process
• Presence of electron donating groups [–CH3] intensifies the negative charge on the
carboxylate anion, thus destabilizing it and deprotonation does not take place as easily
Effect of proximity of electron withdrawing groups
• The nearer the electron withdrawing groups to the carboxyl group, the greater the effect and the
stronger the acid
!
Preparation of carboxylic acids:
There are 5 main ways to prepare a carboxylic acid
1
...
Side chain oxidation of alkyl benzenes
3
...
Oxidative cleavage of alkenes
5
...
Acid-metal/Acid-base reaction
2
...
Reduction of the CO2H group
!
Acid-metal/Acid-base reaction:
Nucleophilic acyl substitution:
Carboxylic acids and their derivatives can undergo nucleophilic acyl substitutions as
• Electronegative O results in partial positive charge on the C=O carbon
• The C=O carbon has a planar geometry and is relatively unhindered
• It makes it susceptible towards nucleophilic attack, and the presence of a good leaving group
Formation of ester:
Note that phenols cannot work in this reaction
!
Formation of acyl chlorides:
Acyl chlorides are highly reactive and could in turn be easily converted into other compounds
Note: HCl cannot react with carboxylic acids as HCl is a stronger acid and will react like in an acidbase reaction
Reduction to primary alcohols:
!
Special case – oxidation of certain carboxylic acids:
Only TWO carboxylic acids undergo further oxidation
Distinguishing test for carboxylic acids:
Add aqueous Na2CO3 or NaHCO3 to the compound
...
Some might be observed
after addition of NaOH (aq)
+ heat
Least electron deficient
carbon on C–Cl due to
delocalization of lone
pair into benzene ring
Bulky aromatic ring
makes it very hindered
No ppt observed
Example
Reasons
Hydrolysis
H2O, room temperature
condition
Electronic C-Cl cleaves easily
factor
without heating
Steric
factor
Reaction with
aq AgNO3
!
Highly electron
deficient carbon on
C–Cl due to
electronegative Cl & O
Planar geometry makes
it less hindered
White ppt of AgCl
observed immediately
Acid hydrolysis of esters:
Base hydrolysis of esters:
Commercial uses of esters:
Esters are prepared industrially mainly for use as essence, perfumes or artificial flavoring
...
For aldehydes, find the longest parent chain containing the –CHO group and number the
carbonyl carbon
2
...
For ketones, parent chain is the longest one containing the ketone group and numbering begins
at the end nearer the carbonyl carbon
4
...
Oxidation of alcohols
Note: Only K2Cr2O7 is selective enough to oxidize primary alcohols to aldehydes
2
...
Add trace amount of KCN as catalyst
• HCN is a weak acid that only partially ionizes in water to give CN–
• Reaction takes place very slowly if only HCN is used thus KCN is added since it is able to
dissociate completely to provide free CN– ions
2
...
Thus the carbonyl carbon
has a higher partial +ve charge in aldehydes, making it more susceptible towards nucleophilic attack
Importance of cyanohydrins
• Nucleophilic addition with CN– ion is an important method of lengthening the carbon chain by 1
carbon (step-up reaction)
• Cyanohydrin group can be easily converted to other usable forms since the nitrile group can be
converted to other functional groups
!
Condensation reactions:
These reactions involve a group of nucleophiles with 2 hydrogen atoms on a nucleophilic nitrogen
atom (–NH2) and thus eliminate water to give an unsaturated product
Reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH)
This is a characteristic test for the presence of the carbonyl functional group in aldehydes & ketones
Reductions:
They undergo reduction to form primary and secondary alcohols
!
Functional groups
Alkenes
Carboxylic Acids
Esters
Nitriles
Aldehydes
Ketones
H2 / Ni
+
+
+
+
Ability to be reduced by
LiAlH4 in dry ether
+
+
+
+
+
NaBH4 in methanol
+
+
Oxidation:
With acidified potassium dichromate (VI)
Only for aldehydes as they contain a hydrogen attached directly to the carbonyl carbon
With Tollens’ Reagent (Silver Mirror Test)
All aldehydes reduce Ag+ in Tollens’ reagent to silver metal and deposit on the walls
Tollens’ reagent contains [Ag(NH3)2]+ ions and is highly unstable, thus it should be freshly
prepared using the following method
• 1 drop of dilute NaOH to about 3cm3 of AgNO3 to produce brown ppt of Ag2O
• Dilute NH3 then added drop wise until brown ppt first formed dissolves – contains [Ag(NH3)2]+
!
With Fehling’s solution
Aldehydes (except benzaldehyde and its derivatives) reduce the copper(II) in Fehling’s solution to
reddish-brown copper(I) oxide, which is precipitated
Fehling’s solution is an alkaline solution of copper(II) tartrate and the reagent deteriorates on
keeping, thus it is prepared in two parts
• Fehling’s solution A (CuSO4 solution)
• Fehling’s solution B (sodium potassium tartrate + excess NaOH) where both are mixed to
produce a deep blue solution due to formation of copper(II) tartrate
Tri-iodomethane (Iodoform) Test:
This test specifically identifies aldehydes and ketones with the following structure
Note: The tri-iodomethane reaction is a method of breaking the C-C bond and removes a methyl
group (step-down reaction)
!
Summary of distinguishing tests:
Reagents & conditions
2,4-DNPH
K2Cr2O7 /
Dilute H2SO4
Heat
Tollens’ Reagent
Heat
Fehling’s solution
Heat
Aldehyde
+
Benzaldehyde
+
Ketone
+
+
+
-
+
+
-
+
-
-
~ END ~
!
Chapter 13: Nitrogen Compounds
AMINES AND AMIDES
Introduction:
Amines
They are nitrogen-containing compounds that can be derived from ammonia, where organic groups
replace one or more hydrogen atoms
Amides
They are carboxylic acid derivatives where the –OH group is replaced by a –NH2 group
Carbonyl compounds are named the following manner –
1
...
Number it beginning at the end nearer the amine group
3
...
For amides, they are named by replacing the –oic acid of parent carboxylic acid with –amide
5
...
However, quaternary ammonium salts have the
highest boiling point
• But they have lower boiling points than alcohols since the N–H bond is less polar than the O–H
bond
Compound
Primary Amine
Secondary Amine
Tertiary Amine
Quaternary Ammonium Salt
Relative Boiling Points
High
Medium
Low
Highest
Type of bonding
H-bonding
H-bonding
Pd-Pd
Ionic bond
Solubility – Lower aliphatic amines are readily soluble in water due to their ability to form strong
H-bonds with water
!
Basicity:
Amines are weak bases and when they dissolve in water, and equilibrium is established where water
donates a hydrogen ion to the amine
...
It also convers considerable double bond
character to the C–N bond
In summary, in terms of basicity
Secondary amine > Primary amine > Tertiary amine > Ammonia > Phenylamine > Amide
!
Synthesis of amines:
From Nucleophilic substitution of halogenoalkanes
Under these conditions, ammonia acts as a nucleophile and displaces the halide to produce primary
amine
Note: If halogenoalkane is in excess, further substitution can occur
!
From Reduction of Nitriles
In the proces the triple bond between carbon and nitrogen is broken
From Reduction of Nitrobenzene
Preparation of Phenylamine
Synthesis of amides:
From Acyl chlorides
N-substituted amides can be made by reaction of acyl chloride with a primary or secondary amine
!
Reactions of amines:
The lone pair of electrons on N atom dominates the chemistry of amines as they behave as
• Lewis bases (donating lone pair to H+ atom)
• Good nucleophiles
• Excellent ligands (transition metals)
Reaction with acids (as a base)
Amines (both aliphatic and phenylamines) react with acids to form salts
Because the salts are ionic, they are soluble in water but insoluble in organic solvents and can thus
be separated from other organic compounds by converting it to water-soluble ammonium salt
!
Reaction with halogenoalkanes (as a nucleophile)
Primary, secondary and tertiary amines can react with halogenoalkanes to form secondary amines,
tertiary amines and quaternary ammonium salts
Reaction with acyl chlorides (as a nucleophile)
They can react with acid chlorides to form amides
Reaction with bromine
Only for phenylamines due via an electrophilic substitution mechanism to form a white ppt
!
Reactions of amides:
Unlike amines, lone pair on the nitrogen atom in amides is in a p orbital that can overlap with the π
orbital of the adjacent carbonyl group
• Hence electron density is reduced and it is unable to act as a nucleophile
• However the delocalization gives it good stability & least reactive of carboxylic acid derivative
Reaction with acids or bases
AMINO ACIDS
Amino acids contain both an acidic carboxyl group –CO2H and a basic amino group –NH2 group
General features of amino acids:
They are building blocks for all proteins in the human body and the 20 amino acids have similar
structure, differing only in their R group
Acid-base behavior:
Amino acids can undergo and intramolecular acid-base reaction, and exist as dipolar ions called
zwitterions
!
Electrophoresis:
As amino acids are charged at certain pH values, we can use an electric field to separate them
through a process known as electrophoresis
A streak of amino acid mixture is placed in the center of a piece of filter paper wet with a buffer
solution and two electrodes are placed in contact with the edges of the paper
!"#$%&'(!!"#$%&!"! ∝ !
Titration of amino acids:
!
!ℎ!"#$
!"##
Physical properties of amino acids:
Formation of zwitterions gives amino acids some unusual properties
• Amino acids are crystalline solids with high melting points due to strong electrostatic forces of
attraction between dipolar zwitterions in solid lattice structure
• They are more soluble in water than organic solvents due to strong ion-dipole interaction
Peptide bond formation:
Amide linkage between amino acids is called a peptide bond and is formed from the condensation
reaction between –CO2H and –NH2
Features of polypeptide chain:
• Backbone is the main chain of polypeptide which includes everything but the side chains that
are the R-groups of each amino acid residue
• Amino acid residues are amino acids condensed together in a polypeptide chain
• Peptide bonds which arise from the condensation reaction between adjacent amino acids
• N- and C- termini where there is an uncondensed amino group and one uncondensed carboxylic
aid group
• Polypeptide chains are always written from the N-terminus to the C-terminus
PROTEINS
They are formed when a large number of amino acids are condensed together to form a long
polypeptide chain
Hydrolysis of proteins:
Proteins can be hydrolyzed into their constituent amino acids by an enzyme or heating in the
presence of dilute acid or alkali for several hours
!
Hierarchy of protein structure:
Proteins have three or four levels of structural organization and complexity
Structure
Primary
Secondary
Tertiary
Quaternary
Description
Sequence of amino acid residues in polypeptide chain
Regular arrangement of sections of polypeptide chain where common repeating
structural patterns in proteins are α-helices and β-pleated sheet stabilized by H-bonds
Overall 3D shape of a protein formed by folding of polypeptide chain
Characteristic manner in which individually folded polypeptide subunits are grouped
Primary structure
It is the sequence of amino acid residues in the polypeptide chain(s) of the protein
Secondary structure
It is the regular arrangement of sections of the polypeptide chains where the most common
structures are the α-helix and β-pleated sheet
...
6 amino acid residues
• N–H and C=O bonds point along the axis where each point in one direction opposite to the other
• C=O group of one amino acid is hydrogen bonded to an N–H group 4 amino acid residues
further along the chain (H-bonds are parallel to the axis)
• R groups extend outwards from the core
Note when drawing α-helix
1
...
Make sure C=O and N–H bonds
are properly aligned
3
...
Add in R-groups pointing
outwards from the helix if
required
!
β-pleated sheet: Forms when two or more sections of the peptide chain line up side by side
• The C=O and N–H bonds lie in the plane of the sheet
• Hydrogen bonding often occurs between the C=O and N–H groups of nearby amino acid
residues
• R groups are oriented above and below the plane of the sheet, alternating along a strand
• Can be either anti-parallel or parallel
Note: Anti-parallel is more common
Tertiary structure
It is the overall three-dimensional shape of a protein formed by the folding of the polypeptide
chains
There are 4 ways in which side chains can interact to form the tertiary structure of the protein
1
...
Ionic attractions: Stabilized by ionic interactions between side chains of opposite charges such
as the –NH3+ and the –CO2– groups
3
...
Disulfide (S–S) bridges: If folding of protein brings two cysteine residues together, the –SH
side chains can be oxidized to form a covalent S–S bond
!
Illustration of bonding in a tertiary protein:
Quaternary structure
Applies to proteins that consist of more than one polypeptide chain
Haemoglobin: It is a protein found in red blood cells and transports O2 in blood, taking it from the
lungs and delivering it to tissues around the body
It has a quaternary structure
• Made up of four polypeptide subunits; two α-chains and two β-chains, each with an associated
haem group
• They are held together by the same type of interactions that stabilize the tertiary structure
• Each haem group has a central Fe2+ ion which binds reversibly to one O2 molecule via a dative
bond, and thus haemoglobin can carry up to four oxygen molecules
• Fe2+ is held in place by 4 N atoms in the center of the haem ring and is firmly bonded to one of
the amino acid residues in the protein below the ring
• The 6th position is either a H2O molecule (deoxyhaemoglobin) or O2 molecule, giving rise to an
octahedral structure around the central Fe2+ ion
!
Functions of proteins:
Proteins are essential for life in all organisms and have many biological functions
1
...
Antibodies are specific protein molecules produced by specialized cells of the immune system
in response to foreign antigens
3
...
Regulatory proteins control many aspects of cell function – metabolism and reproduction
5
...
Movement proteins necessary for all forms of movement
7
...
Specific interactions between the side chain R-groups of the active site of the
enzyme and substrate results in binding
During enzymatic hydrolysis of proteins
• Enzyme provides active site for protein substrate to bind to, and hence helps water molecule to
bond to the polar carbonyl group of the peptide bond
• When an enzyme is denatured, conformation of the active site is altered, and thus catalytic
activity of the enzyme is lost
Denaturation:
It is a process that alters the conformation (3D shape) of the protein that results from possible
disruption of interactions that maintain the 3D-shape of the protein – thus losing its ability to
perform specific functions
Possible disruptions are
1
...
Side chain R-group interactions – destroying the tertiary and quaternary structures
Note: primary structure remains intact
!
Summary of types of denaturation:
Disruption of side chain R-group interactions by external stress such as
Heat increases kinetic energy of atoms in the protein, giving them
sufficient energy to disrupt the VDW forces in the tertiary and
quaternary structure
Heat
It also disrupts the H-bonds in the secondary, tertiary and quaternary
structures of the protein
–COO– becomes –COOH
–NH2 becomes –NH3+
Low pH
Disrupts original ionic interactions
in the tertiary and quaternary
structure
–NH3+ becomes –NH2
Disrupts original hydrogen bonds
in the tertiary and quaternary
structure
–COOH becomes –COO–
High pH
Disrupts original ionic interactions
in the tertiary and quaternary
structure
Metal ions form ionic interactions
with the –COO– and cause
precipitation of insoluble protein
Disrupts original hydrogen bonds
in the tertiary and quaternary
structure
Ag+ ions have high affinity for
sulfur and bind tightly to the –SH
group of cysteine residues
Extreme pH
Metal ions and heavy
metal ions
(Transition Metals)
Alcohol
Disrupts original ionic interactions Disrupts original disulfide bridges
in the tertiary and quaternary in the tertiary and quaternary
structure
structure
–OH group of the alcohol can form hydrogen bonds which disrupts
original hydrogen bonds in the secondary, tertiary and quaternary
structure
~ END ~
!