Part B: Systematic Aliphatic
Chemistry
Simbarashe Sithole
mandlasithole@gmail.com
Hybridization of Atomic Orbitals
• Formation of four equivalent pairs of electrons
• Combination of carbon 2s and 2p orbitals to make four
new orbitals
• sp3 hybrid orbitals to show the proportions of the AOs
in each
• In ethane, the carbons use three sp3 AOs to bond with
3 H atoms then one sp3 orbital for the C–C bond
Hybridization of Atomic Orbitals
• In ethene (ethylene), combine the 2s orbital on each
carbon atom with two p orbitals to make the three
bonds
• This forms three degenerate sp2 orbitals, leaving an 2p
orbital unchanged
• The unhybridized p orbital give a π orbital
Hybridization of Atomic Orbitals
• Ethyne (acetylene) has a C≡C triple bond
• The carbon 2s and 2px hybridize to form sp hybrid
• The remaining 2py and 2pz to form two mutually
perpendicular π
Organic reactions and mechanisms
• The curly arrow to represent the detailed movement of
electrons—the mechanism of the reaction
• A minimum energy is required for reaction to occur—
known as the activation energy
• Reactions happen when electrons flow between molecules
• the molecule that accepts the electrons is termed an
electrophile
• the molecule that donates the electrons is called the
nucleophile
Organic reactions and mechanisms
• the molecule that accepts the electrons is termed an
electrophile
• the molecule that donates the electrons is called the
nucleophile
Nucleophiles
• A nucleophile is an electron-rich atom that is capable
of donating a pair of electrons
• negatively charged or neutral species with a pair of
electrons in the high-energy orbital e.g. OH-, OR, SR,
CN-, Hal-, NH3, H2O, ROH, RLi, LiAlH4,RMgBr etc.
Electrophiles
• An electrophilic centre is an electron-deficient atom that is
capable of accepting a pair of electrons
• are neutral or positively charged species with a low-energy
antibonding orbital that can easily accept electrons e.g. H3O+,
H+, NO2+, BF3, AlCl3, I2, ZnCl2, Br2, RCOCl etc
Free Radicals
• Homolytic bond cleavage generates two uncharged
species, called radicals, each of which bears an unpaired
electron
For the following mechanism, identify the sequences of arrow
pushing patterns:
For the following mechanism, identify the nucleophile or
electrophile:
Organic reactions
• There are 4 fundamental organic reactions:
i.
Substitution [displacement] reactions
ii.
Addition reactions
iii.
Elimination reactions
iv.
Rearrangements
v.
Pericyclic reactions
vi. Polymerization reactions
Substitution [displacement] reactions
• one group is exchanged for another
i. Electrophilic substitution: attack by electrophile E+
ii. Nucleophilic substitution: attack by nucleophile Nu-
a. In a concerted process, nucleophilic attack and loss of the leaving
group occur simultaneously (SN2).
b. In a stepwise process, loss of the leaving group occurs first
followed by nucleophilic attack (SN1).
Substitution [displacement] reactions
The SN2 Mechanism
• has a single step in which the substrate and the nucleophile
collide with each other (two chemical entities i.e.
bimolecular)
• rate is dependent on the:
• the nucleophile (HO– > RCO2– > H2O > RSO2O–)
• the carbon skeleton
• the leaving group
The SN2 Mechanism
• SN2 reactions are sensitive to the nature of the starting alkyl halide
The SN2 Mechanism
• the nucleophile is expected to encounter
steric hindrance as it approaches the
substrate
• in the transition state the nucleophile is
in the process of forming a bond with
the substrate, and the leaving group is in
the process of breaking its bond with
the substrate
• By replacing H atoms with alkyl groups,
steric interactions cause the transition
state to be higher in energy, raising Ea for
the reaction
The SN1 Mechanism
a stepwise process in which there is:
i. loss of the leaving group to form a carbocation intermediate
followed by
ii. nucleophilic attack on the carbocation intermediate
The SN1 Mechanism
• The rate of an SN1 reaction
depends on:
• the carbon skeleton
• the leaving group
The SN1 Mechanism
• The rate of an SN1 reaction
depends on:
• the carbon skeleton
• the leaving group
• the step with the highest
energy transition state
determines the rate of the
overall process i.e. ratedetermining step
The SN1 Mechanism
• the efficiency of an SN1
reaction is determined by the
stability of any carbocation
that may have been formed
• an alkyl substituent stabilise
carbocation
SN2 / SN1 Stereochemistry Consequences
• SN1 reactions involve formation of an intermediate
carbocation, which can then be attacked from either side,
leading to both inversion of configuration and retention of
configuration
• In SN2 the nucleophile approaches the carbon bearing the
leaving group from the backside
• the reaction proceeds with inversion of configuration
(Walden inversions) e. g.
Factors Affecting the Rates of SN1 and SN2
Reactions
• A number of factors affect the relative rates of SN1 and
SN2 reactions. The most important factors are:
i. the structure of the substrate,
ii. the concentration and reactivity of the nucleophile (for
SN2 reactions only),
iii. the effect of the solvent, and
iv. the nature of the leaving group.
The relative strengths of nucleophiles can be correlated with
three structural features:
• A negatively charged nucleophile is always a more reactive
nucleophile than its conjugate acid.
• In a group of nucleophiles in which the nucleophilic atom is
the same, nucleophilicities parallel basicities.
• RO- > HO- >> RCO2- > ROH > H2O
• When the nucleophilic atoms are different, nucleophilicities
may not parallel basicities.
• HS- > NC- > I- > HO-
Solvent Effects in SN2 and SN1 Reactions
• SN2 reactions are favored by polar aprotic solvents (e.g.,
acetone, DMF, DMSO).
• SN1 reactions are favored by polar protic solvents (e.g.,
EtOH, MeOH, H2O).
• minimizing the solvent’s interaction with the nucleophile in SN2
reactions
• facilitating ionization of the leaving group and stabilizing ionic
intermediates by solvents in SN1 reactions
Factors Favouring SN1 versus SN2 Reactions
Factor
Substrate
SN1
3° (requires formation of a relatively
stable carbocation)
Weak Lewis base, neutral molecule,
Nucleophile may be the solvent
(solvolysis)
SN2
Methyl > 1° > 2° (requires
Unhindered substrate)
Nucleophile
Strong Lewis base, rate
increased by high
concentration of
nucleophile
Solvent
Polar protic (e.g., alcohols, water)
Polar aprotic (e.g., DMF,
DMSO)
Leaving group
I > Br > Cl > F for both SN1 and SN2
(the weaker the base after the group departs, the better the leaving
group)
Part B: Systematic Aliphatic
Chemistry
Simbarashe Sithole
mandlasithole@gmail.com
Heat of Reaction and Stabilities of
Alkenes
• On conversion to butane, 1-butene liberates the most heat (127
kJmol-1), followed by cis-2-butene (120 kJmol-1), with trans-2-butene
producing the least heat (115 kJmol-1)
• trans isomer is more stable than the cis isomer
• the terminal alkene, 1-butene, is less stable than either of the
disubstituted alkenes
Heat of Reaction and Stabilities of
Alkenes
• The greater the number of attached alkyl groups (i.e., the more highly
substituted the carbon atoms of the double bond), the greater is the
alkene’s stability
(E)–(Z) System
• If an alkene is trisubstituted or tetrasubstituted, the terms cis and
trans are either ambiguous or do not apply at all
• the (E )–(Z ) system, that applies priorities of groups in the Cahn–
Ingold–Prelog convention to alkene diastereomers is used
1. Examine the two groups attached to one carbon atom of the
double bond and decide which has higher priority.
2. Repeat that operation at the other carbon atom
3. Compare the group of higher priority on one carbon atom with
the group of higher priority on the other carbon atom.
(E)–(Z) System
(E)–(Z) System
Elimination Reactions
• a proton from the beta (β) position is removed together
with the leaving group, forming a double bond (π bond)
• a beta elimination, or 1,2-elimination
• E2 reaction is bimolecular in the rate-determining step
• stepwise process, first the leaving group leaves, and then
the base abstracts a proton
• E1 reaction is unimolecular in the rate-determining step
• concerted process, a base abstracts a proton and the
leaving group leaves simultaneously;
Elimination Reactions
• A substitution reaction occurs when the reagent
functions as a nucleophile and attacks an electrophilic position,
while an elimination reaction occurs when the reagent
functions as a base and abstracts a proton.
• With a tertiary substrate, steric hindrance prevents the
reagent from functioning as a nucleophile at an appreciable
rate, but the reagent can still function as a base without
encountering much steric hindrance
• Tertiary substrates react readily in E2 reactions
• These react even more rapidly than primary substrates
E2 mechanism
• a base removes a β hydrogen from the β carbon, as the
double bond forms and a leaving group departs from the α
carbon
NB: Reaction conditions that favour elimination by an E1
mechanism should be avoided because the results can be too
variable
How To Favour an Elimination Reaction
Use a secondary or tertiary alkyl halide if possible.
• steric hindrance in the substrate will inhibit substitution.
When a synthesis must begin with a primary alkyl halide,
use a bulky base.
• the steric bulk of the base will inhibit substitution.
Use a high concentration of a strong and nonpolarizable
base such as an alkoxide.
• a weak and polarizable base would not drive the reaction
toward a bimolecular reaction, thereby allowing unimolecular
processes (such as SN1 or E1 reactions) to compete.
How To Favour an Elimination Reaction
Sodium ethoxide in ethanol (EtONa/EtOH) and potassium
tert-butoxide intert-butyl alcohol (t-BuOK/t-BuOH)
• they meet criterion 3 above. NB: alkoxide base is dissolved in its
corresponding alcohol. (Potassium hydroxide dissolved in ethanol or
tertbutyl alcohol is also used, the active base includes both the
alkoxide and hydroxide species at equilibrium.)
Use elevated temperature because heat generally favors
elimination over substitution.
• elimination reactions are entropically favored over substitution
reactions (because the products are greater in number than the
reactants). Hence ΔS° in the Gibbs free-energy equation, ΔG° =
ΔH° - TΔS° is significant, and ΔS° will be increased by higher
temperature since T is a coefficient, leading to a more negative
(favourable) ΔG°.
Zaitsev’s Rule
• Dehydrohalogenation of 2-bromo-2-methylbutane can yield
two products: 2-methyl-2-butene and 2-methyl-1-butene
• The elimination occurs to give the more stable, more
highly substituted alkene
Zaitsev’s Rule
• Both products are formed, but the more substituted alkene
is generally observed to be the major product
• This is an example of regiochemistry (the double bond
can form in two different regions of the molecule)
Zaitsev’s Rule
Zaitsev’s Rule
allylbenzene
but-1-ene-2,3-diyldibenzene
(Z)-prop-1-en-1-ylbenzene
(E)-but-2-ene-2,3-diyldibenzene
E1 mechanism
• a base removes a β hydrogen from the β carbon, as the
double bond forms and a leaving group departs from the α
carbon
NB: Reaction conditions that favour elimination by an E1
mechanism should be avoided because the results can be too
variable
Electrophilic Addition Reactions
• Simple, unconjugated alkenes are nucleophilic and react with
electrophiles
• addition of two groups across a double bond
• Electrophiles are molecules or ions that can accept an electron
pair
• Electrophiles include proton donors such as Brønsted–Lowry acids,
neutral reagents such as bromine (because it can be polarized so that
one end is positive), and Lewis acids such as BH3, BF3, and AlCl3.
• Metal ions that contain vacant orbitals—the silver ion (Ag+), the
mercuric ion (Hg2+), and the platinum ion (Pt2+)—also act as
electrophiles
Electrophilic Addition Reactions
Regioselectivity of Hydrohalogenation
• Markovnikov’s rule says that in the addition of HX to an
alkene, the hydrogen atom adds to the carbon atom of
the double bond that already has the greater number of
hydrogen atoms
• order of reactivity of the hydrogen halides in alkene addition
is
HI > HBr > HCl > HF
Regioselectivity of Hydrohalogenation
• π electrons of the alkene form a bond with a proton from HX to
form a carbocation and a halide ion
• halide ion reacts with the carbocation by donating an electron pair;
the result is an alkyl halide
Regioselectivity of Hydrohalogenation
• π electrons of the alkene form a bond with a proton from HX to
form a carbocation and a halide ion
• halide ion reacts with the carbocation by donating an electron pair;
the result is an alkyl halide
Regioselectivity of Hydrohalogenation
• Addition of the electrophile determines the overall orientation of the
addition, because it occurs first (before the addition of the
nucleophilic portion of the adding reagent).
• Because of the greater electronegativity of chlorine, the positive
portion of this molecule is iodine
Regioselectivity of Hydrohalogenation
• protonation can occur with either of two regiochemical possibilities
• It can either occur to form the less substituted, secondary
carbocation, or occur to form the more substituted, tertiary
carbocation
or
Hammond–Leffler postulate
• The transition state that leads to the tertiary carbocation is lowest in
free energy because it resembles the carbocation that is lowest in
energy.
• the transition state for formation
of the tertiary carbocation will
be significantly lower in energy
than the transition state for
formation of the secondary
carbocation
• energy barrier for formation of the
tertiary carbocation will be smaller
than the energy barrier for
formation of the secondary
carbocation, and as a result, the
reaction will proceed more rapidly
via the more stable carbocation
intermediate
An Exception to Markovnikov’s Rule
• addition of HBr to alkenes when the addition is carried
out in the presence of peroxides (i.e., compounds with
the general formula ROOR)
• an anti-Markovnikov addition occurs in the sense that the
hydrogen atom becomes attached to the carbon atom with the
fewer hydrogen atoms
NB: This anti-Markovnikov addition occurs only when HBr is
used in the presence of peroxides
Electrophilic Addition Examples
Give the product(s) for the 3 addition reactions below:
• pent-2-ene + HBr in ether;
• 1-methylcyclohex-1-ene + HBr;
• 1-ethylcyclopent-1-ene + HCl;
• 2-methylbut-2-ene + ICl.
Part B: Systematic Aliphatic
Chemistry
Simbarashe Sithole
mandlasithole@gmail.com
Polymerisation
• Synthetic organic polymers are commonly formed via one of
three possible mechanistic pathways: radical
polymerisation, anionic polymerisation, or cationic
polymerisation
• Polymers are substances that consist of very large
molecules called macromolecules that are made up of
many repeating subunits
• The molecular subunits that are used to synthesize polymers
are called monomers
Polymerisation
Monomer
Polymer
Names
Polyethylene
Polypropylene
Poly(vinyl chloride), PVC
Polyacrylonitrile, Orlon
Poly(tetrafluoroethene), Teflon
Poly(methyl methacrylate), Lucite,
Plexiglas, Perspex
Polystyrene
Radical Polymerization of Alkenes: Chain
Growth Polymers
• ethylene polymerizes by a radical mechanism when heated at
a pressure of 1000 atm with a small amount of an organic
peroxide (called a duacyl peroxide)
Radical Polymerization of Alkenes: Chain
Growth Polymers
Chain Initiation
Step 1: diacyl peroxide dissociates and releases carbon dioxide gas
Step 2: alkyl radicals are produced, which in turn initiate chains
Radical Polymerization of Alkenes: Chain
Growth Polymers
Chain Propagation
Step 3: propagation of chains by adding successive ethylene units, until
their growth is stopped by combination or disproportionation
Radical Polymerization of Alkenes: Chain
Growth Polymers
Chain Termination
Step 4: radical at the end of the growing polymer chain can also
abstract a hydrogen atom from itself by what is called “black biting.”
This leads to chain branching
Radical Polymerization of Alkenes: Chain
Growth Polymers
Chain Branching
Radical Polymerization of Alkenes: Chain
Growth Polymers
• Polyethylene can be produced in a different way using
catalysts called Ziegler–Natta catalysts
• no radicals are produced, no back biting occurs, and,
consequently, there is no chain branching
• the polyethylene produced is of higher density, has a higher
melting point, and has greater strength
•
Polymers: How Useful
• Polyethylene can be produced in a different way using
catalysts called Ziegler–Natta catalysts
• no radicals are produced, no back biting occurs, and,
consequently, there is no chain branching
• the polyethylene produced is of higher density, has a higher
melting point, and has greater strength
•
Polymers: How Useful
• Primary packaging: food and beverages, pharmaceuticals,
chemicals replacing GLASS
• Secondary packaging: competing/ replacing wood, metal.
• Fabric: competing with cotton, silk etc
• Structural material: competing with steel, wood and metals
• Electrical/ Thermal insulation: competing with natural
rubber, wood, cotton etc.
• Pipes: competing/ replacing metal, clay/ cement
Domestic Products: MEGA PAK
International Products
FOOD & BEVERAGE
TRANSPORTATION
CONSTRUCTION
WEARABLES
MEDICAL
ELECTRONICS
Plastics have built the modern world - but
do they belong to our future?
• Since around 1950, the mass production of plastics has revolutionized
the food industry
• Plastic packaging keeps food hygienic, fresh and safe, and offers a
convenience that distribution networks and supermarkets rely on
• plastic has housed us, clothed us and enabled medical innovation
• How we dispose of plastics is a problem
• single-use depletes natural resources, adds greenhouse gases to the
atmosphere, harms wildlife and litters our landscapes and oceans
• REDUCE, RECYCLE, REUSE
Polymer degradation
• a change in the properties tensile strength, colour, shape, etc
• degradation under the influence of one or more environmental
factors: heat, light or chemicals (acids, alkalis and some salts)
• changes that occur are usually undesirable, such as cracking and
chemical disintegration of products
• Loss in physical properties
• Discolouration
Types of Degradation
• Chemical Degradation
• Thermal Degradation
• Biodegradation
• Radiolytic Degradation
• Mechanical Degradation
• Photodegradation
Photodegradation
• degradation of a photodegradable molecule caused by
absorption of photons,
• particularly those wavelengths found in sunlight, such as
infrared radiation, visible light, and ultraviolet light
• polymers can be degraded by photolysis to give lower
molecular weight molecules
• e.g. photo degradation of poly alpha –methyl styrene
Factors Causing Photodegradation
Internal Impurities
External Impurities
Hydroperoxide
Traces of solvent, catalyst,etc.
Compounds from a polluted
urban atmosphere and smog
Additives
Carbonyl
Unsaturated bonds
Catalyst residue
Traces of metal & metal oxides
Change transfer (CT) complexes from processing equipments &
containers, such as Fe, Ni or Cr
with oxygen
Photodegradation
• degradation of a photodegradable molecule caused by
absorption of photons,
• particularly those wavelengths found in sunlight, such as
infrared radiation, visible light, and ultraviolet light
• polymers can be degraded by photolysis to give lower
molecular weight molecules
• e.g. photo degradation of poly alpha –methyl styrene
Photostabilisation
• Photostabilisation is the retardation or elimination of the
Photodegradation of any polymer
• Stablisers are classified as:
• Light screeners
• UV absorbers
• Excited state quenchers
• Peroxide decomposers
• Radical scavengers
Photostabilisation
• Adding hindered amine light stabilisers (HALS)
• scavenge free radicals formed by photo-oxidation, preventing them
from chemical reaction
• Adding UV absorbers
• Convert UV into heat (benzophenones and benzotriazoles)
• Adding antioxidants which terminate chain reactions
• Adding blockers e.g. carbon black, titanium dioxide
Oxidative degradation
• This degradation usually leads hardening, discoloration as well as
surface change.
• degree of oxidative degradation of the polymer depends on its
structure
• Thus unstructured polymer such as polyisoprene or polybutadiene
containing double bonds are easily attacked by oxygen
Mechanism of Oxidative degradation
Part B: Systematic Aliphatic
Chemistry
Simbarashe Sithole
mandlasithole@gmail.com
Alcohols and Ethers
Acid-Catalyzed Hydration of Alkenes
• alkenes add water in the presence of an acid catalyst to yield alcohols
• addition takes place with Markovnikov regioselectivity
Hydroboration–Oxidation
• alkene reacts with BH3:THF or diborane to produce an alkylborane
• oxidation and hydrolysis of the alkylborane with hydrogen peroxide
and base yield an alcohol
Reactions of alcohols
• The reactions of alcohols have mainly to do with the following:
• The oxygen atom of the hydroxyl group is nucleophilic and weakly
basic.
• The hydrogen atom of the hydroxyl group is weakly acidic.
• Alcohols have acidities similar to that of water
• The hydroxyl group can be converted to a leaving group so as to
allow substitution or elimination reactions.
• alkyl halides are formed from the reaction of alcohols with hydrogen
halides
• The order of reactivity of alcohols is 3° > 2° > 1°
• The order of reactivity of the hydrogen halides is HI > HBr > HCl (HF is
generally unreactive).
Synthesis of ethers
• The alkoxide ion reacts with the substrate in an SN2 reaction, with the
resulting formation of an ether.
• The substrate must be unhindered and bear a good leaving group.
• Typical substrates are 1° or 2° alkyl halides, alkyl sulfonates, and dialkyl
sulfates:
• —LG = —Br , —I , —OSO2R’, or —OSO2OR”
The Williamson Ether Synthesis
Synthesis of ethers
1. Outline two methods for preparing isopropyl methyl ether by a
Williamson synthesis.
2. One method gives a much better yield of the ether than the other.
Explain which is the better method and why.
Reactions of ethers
• are generally unreactive hence their use as solvents
• resist attack by nucleophiles and by bases
• able to solvate cations
• E.g. Dimethyl ether, Ethyl methyl ether, Diethyl ether, 1,2Dimethoxyethane (DME), Oxirane, Tetrahydrofuran (THF), 1,4Dioxane
Aldehydes and Ketones
• Aldehydes have a carbonyl group bonded to a carbon atom on one
side and a hydrogen atom on the other side
• Ketones have a carbonyl group bonded to carbon atoms on both
sides
• The most characteristic reaction of aldehydes and ketones is
nucleophilic addition to the carbon–oxygen double bond
Aldehydes and Ketones
• The R group has an effect on proneness to attack of the C
atom
• R group with negative inductive effect increases reactivity
• R group with positive inductive effect reduces reactivity
• electron-withdrawing substituents cause the carbonyl carbon
to be more positive causing the addition reaction to be
more favourable
Synthesis of Aldehydes and Ketones
• Swern Oxidation used for the synthesis of aldehydes and
ketones from primary and secondary alcohols, respectively
• ozonolysis of alkene double bond also produces are
aldehydes and ketones
• Ketones can be produced from arenes by Friedel–Crafts
acylations
Nucleophilic Addition to the Carbon–Oxygen
Double Bond
• Addition of a strong nucleophile to an Aldehyde or Ketone
Trigonal planar
In the first step, the nucleophile forms
a bond to the carbon by donating an
electron pair to the top or bottom
face of the carbonyl group. An electron
pair shifts out to the oxygen.
Tetrahedral
intermediate
Tetrahedral
product
In the second step, the alkoxide oxygen,
because it is strongly basic, removes a
proton from H-Nu or some other acid.
Acid-Catalyzed Nucleophilic Addition to an
Aldehyde or Ketone
• In this step an electron pair of the carbonyl oxygen accepts a proton
from the acid (or associates with a Lewis acid), producing an oxonium
cation.
• The carbon of the oxonium cation is more susceptible to nucleophilic
attack than the carbonyl of the starting ketone.
Acid-Catalyzed Nucleophilic Addition to an
Aldehyde or Ketone
• In the first of these two steps, the oxonium cation accepts the electron
pair of the nucleophile.
• In the second step, a base removes a proton from the positively charged
atom, regenerating the acid.
Relative Reactivity: Aldehydes vs Ketones
• aldehydes are more reactive in nucleophilic additions than are ketones
Steric factors
• In aldehydes, the central carbon of the tetrahedral product formed from the
aldehyde is less crowded and the product is more stable.
Electronic Factors
• Because alkyl groups are electron releasing, aldehydes are more reactive
• Aldehydes have only one electron-releasing group to partially neutralize, and thereby
stabilize, the positive charge at their carbonyl carbon atom.
• Ketones have two electron-releasing groups and are stabilized more.
• Greater stabilization of the ketone (the reactant) relative to its product means that
the equilibrium constant for the formation of the tetrahedral product from a ketone
is smaller and the reaction is less favourable
Addition of Hydrogen Cyanide: Cyanohydrins
• Hydrogen cyanide adds to the carbonyl groups of aldehydes and most
ketones to form compounds called cyanohydrins
• A mechanism for the reaction cyanohydrin formation????
• Acidic hydrolysis converts cyanohydrins to a-hydroxy acids or to a, bunsaturated acids
ALDOL Condensation
• Condensations are reactions where two molecules combine with the
loss of another small molecule—usually water.
• In the aldol reaction, two carbonyl compounds combine with the loss
of water
• the enolate or enol of one carbonyl compound reacts with the
carbonyl group of another to join the two reactants
• The product is an aldehyde with a hydroxy (ol) group and it
has the trivial name aldol
• The mechanism for an aldol addition has three steps:
ALDOL Condensation
• Deprotonation of carbonyl to form an enolate
• the enolate attacks the carbonyl group to form an alkoxide
• alkoxide ion is then protonated to yield the product
In this step the base (a hydroxide
ion) removes a proton from the a
carbon of one molecule of
acetaldehyde to give a resonancestabilized enolate
The enolate then acts as a
nucleophile and attacks the
carbonyl carbon of a second
molecule of acetaldehyde, producing
an alkoxide anion.
The alkoxide anion now
removes a proton from a
molecule of water to form
the aldol product
The Acid-Catalysed Aldol Reaction
The mechanism begins with the acid-catalyzed formation of the enol
Then the enol adds to the protonated carbonyl group of another molecule of
acetone.
The Acid-Catalysed Aldol Reaction
Finally, proton transfers and dehydration lead to the product.
Carboxylic Acids and their Derivatives
• -CO2H or –COOH is one of the most widely occurring functional
groups in chemistry and biochemistry
• Most unsubstituted carboxylic acids have Ka values in the range of 104–10-5 (pKa = 4–5)
• Carboxylic acids having electron-withdrawing groups are stronger
than unsubstituted acids
• Delocalization of the negative charge in trichloroacetate by the
electron-withdrawing effect of its three chlorine atoms contributes to
its being a stronger acid than acetic acid
• the more delocalization of charge in the conjugate base, the more
stable is the anion, and the stronger the acid
Acidity of Carboxyllic Acids
• Stabilisation (by resonance or induction) of the R-CO2- increases the
acidity
• Electronegative atoms, Cl, F, Stabilise by withdrawing electrons =
Negative Induction.
• Benzene ring tends to have positive induction.
• An aliphatic chain has a positive inductive effect.
Amines
• Amines are derivatives of ammonia in which one or more of the
protons have been replaced with alkyl or aryl groups
• classified as 1°, 2°, or 3°, depending on the number of groups attached
to the nitrogen atom
• molecules of 1° and 2° amines can form strong hydrogen bonds to
each other and to water
• 3° amines cannot form hydrogen bonds to each other, but they can
form hydrogen bonds to molecules of water or other hydroxylic
solvents
• 3° amines generally boil at lower temperatures than 1° and 2° amines
of comparable molecular weight, but all low-molecular weight amines
are very water soluble
Basicity of Amines
• A high pKa indicates that the amine is strongly basic, while a low
pKa indicates that the amine is only weakly basic
• an electron-releasing ability such as an alkyl group stabilizes
the alkylaminium ion
• Aromatic amines are much weaker bases than alkylamines
• delocalisation of the unshared electron pair of the nitrogen
over the ring, makes it less available to a proton
• electron-withdrawing effect of a phenyl group also leads to
weak basic nature of aromatic amines
FINIS