2.7 Acids and Bases: The BrØnsted-Lowry Definition
Acidity and bacidity are related to a molecules electronegativity and polarity.
Acids donate protons, and bases accept protons
Water can act either as an acid or a base
2.8 Acid and Base Strength
Acids differ in their ability to donate protons, some reacting almost completely and others
only slightly. The strength is indicated by the acidity constant, which resembles the degree
to which an acid is ionised in solution.
Stronger acids have their equilibria more towards the right and have larger acidity constants.
A stronger acid has a smaller pKA and a weaker acid has a larger pKa.
There is an inverse relationship between the acid strength of an acid and base strength of its
2.9 Predicting Acid-Base Reactions from pK Values
An acid will donate a proton to the conjugate base of a weaker acid, and the conjugate base
of a weaker acid will remove the proton from a stronger acid.
The product conjugate acid in an acid-base reaction must be weaker and less reactive than
the starting base.
2.10 Organic Acids and Organic Bases
Organic acids are characterized by the presence of a positively polarized hydrogen atom, and
are of two main kinds:
(1) Contain a hydrogen tom bonded to an electronegative oxygen atom (i.e.
methanol and acetic acid)
(2) Contain a hydrogen atom bonded to a carbon atom next to a C=O bond (i.e.
In both cases acidity is due to the fact that the conjugate base resulting from loss of proton
is stabilized by having its negative charge on a strongly electronegative oxygen atom. It is
additionally stabilized by resonance.
The acidity of ketones is due to the conjugate base resulting from loss of a proton is
stabilised by resonance. Additionally one of the resonance forms stabilises the negative
charge by placing it on an electronegative oxygen atom.
Organic bases are characterized by the presence of an atom with a lone pair of electrons
that can bond to protons. Nitrogen containing compounds are most common, followed by
oxygen containing compounds.
5.2 How Organic Reaction Occur: Mechanisms
A reaction mechanism is an overall description of how a reaction occurs- i.e. exactly what
takes place at each stage of a chemical transformation.
Considers which bonds are broken an in what order.
A bond can break in an electronically symmetrical way so that one electron remains with
each product fragment. This is called homolytic. A bond can also break in an electronically unsymmetrical way so that both bonding electrons
remain with one product fragment, leaving the other with a vacant orbital. This is called
A bond can form in an electronically symmetrical way if one electron is donated to the new
bond by each reactant or in an unsymmetrical way if both bonding electrons are donated by
Radical (neutral chemical species that contains an odd number of electrons and has single
unpaired electron) reactions involve symmetrical bond-breaking and bond-making.
Polar reactions involve unsymmetrical bond-breaking and bond-making and are the most
5.4 Polar Reactions
Polar reactions occur because of the electrical attraction between positive and negative
centres on functional groups in molecules.
Most organic compounds are electrically neutral, they have no net charge.
Bond polarity is a consequence of an unsymmetrical electron distribution in a bond and is
due to the difference in electronegativity of the bonded atoms.
Metals are less electronegative than carbons.
As the electric field around a given atom changes because of changing interactions with
solvent or other polar molecules nearby, the electron distribution around that atom also
changes. The measure of this response to an external electrical influence is called the
Polarizability of the atom.
Large atoms with more, loosely held electrons are more polarisable, and smaller atoms less
The fundamental characteristic of all polar organic reactions is that electron rich sites react
with electron poor sites.
A nucleophile is a substance that is “nucleus-loving”, has a negatively polarized, electron-rich
atom and can form a bond by donating a pair of electrons to a positively charged; ammonia,
water, hydroxide ion.
An electrophile has a positively polarized, electron poor atom and can form a bond by
accepting a pair of electrons from a nucleophile.
5.6 Using Curved Arrows in Polar Reaction Mechanisms
An electron pair moves from the atom at the tail of the arrow to the atom at the head of the
Rules of arrows:
(1) Electrons move from a nucleophilic (Nu: or Nu: ) source to an electrophilic sink
(E or E )
(2) The nucleophile can be either negatively charged or neutral
(3) The electrophile can be either positively charged or neutral
(4) The octet rule must be followed. 6.8 Orientation of Electrophilic Additions: Markovnikov’s Rule
Reactions in which an unsymmetrical substituted alkene has given a single addition product
are regiospecific, when only one of the two possible orientations occur.
In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents
and the X attaches to the carbon with more alkyl substituents.
6.9 Carbocation Structure and Stability
Carbocations are planar, with the pi orbital being unoccupied.
The stability of carbocations increases with increasing substitution so that the stability order
is tertiary > secondary > primary > methyl.
To determine carbocation stabilities one may measure the energy required to form the
carbocation by dissociated of the corresponding alkyl halide.
Inductive effects result from the shifting of electrons in a sigma bond in response to the
electronegativity of nearby atoms, thus the more alkyl groups there are attached to the
positively charged carbon, the more electron density shifts toward the charge and the more
inductive stabilisation of the cation occurs.
Hyperconjugation is the stabilising interaction between a vacant p orbital and properly
oriented C-H bonds on neighbouring carbons.
6.10 The Hammond Postulate
Electrophilic addition to an unsymmetrically substituted alkene gives the more highly
substituted carbocation intermediate.
A more highly substituted carbocation is more stable than a less highly substituted one.
The more stable intermediate is forms faster than the less stable one.
Transition states are high-energy activated complexes that occur transiently during the
course of a reaction and represent an energy maximum.
Hammond Postulate: The structure of a transition state resembles the structure of the
nearest stable species. Transition states for endergonic steps structurally resemble products,
and transition states for exergonic steps structurally resemble reactants.
The transition state for alkene protonation structurally resembles the carbocation
The transition state is stabilised by hyperconjugation and inductive effects in the same way
as the product carbocation.
7.4 Addition of Water to Alkenes: Oxymercuration
Water adds to alkenes to yield alcohols, in the presence of a strong acid catalyst (HA).
Results in a carbon intermediate which reacts with water to yield a protonated alcohol
Biological hydration requires that the double bond be adjacent to a carbonyl group for
reaction to proceed.
Alkenes are often hydrated by Oxymercuration
Nucleophilic addition of water as in halohydrin formation, followed by loss of a proton, then
yields a stable organo-mercury product. 7.7 Reduction of Alkenes: Hydrogenation
In the presence of a metal catalyst alkenes react with H - th2 hydrogen bond has been
hydrogenated or reduced.
In organic chemistry a reduction is a reaction that results in a gain of electron density by
carbon, caused either by bond formation between carbon and a less electronegative atom or
by bond-breaking between carbon and a more electronegative atom.
Catalytic hydrogenation is a heterogenic process rather than a homogenous one- takes place
on the surface of insoluble catalyst particles.
Molecular hydrogen and the alkene adsorb to catalyst surface and dissociate, a hydrogen ion
is transferred between them forming an artificially reduced intermediate with a C-H bond. A
second hydrogen is transferred from the metal to the second carbon, giving the alkane
product and regenerating the catalyst.
Aldehydes, ketones, esters and nitriles survive normal alkene hydrogenation conditions
unchanged, although reaction with these groups does occur under more vigorous
11.1 The Discovery of Nucleophilic Substitution Reactions
Nucleophilic substitution reactions involve the substitution of one nucleophile or hydroxide
ion with another. R-X + Nu:- R-Nu + X:-
The inversion of stereo chemical configuration must therefore take place in the second step,
the nucleophilic substitution of tosylate ion by acetate ion.
The nucleophilic substitution reaction of a primary or secondary alkyl halide or tosylate
always proceeds with inversion of configuration.
11.2 The SN2 Reaction
There is a direct relationship between the rate at which the reaction occurs and the
concentrations of the reactants. Measuring this is an indication of the kinetics of the
Substitutions occurs at different rates depending upon temperature, concentrations and pH.
Second order reaction: reaction rate is linearly dependent on the concentrations of two
Reaction rate = rate of disappearance of reactant
= k X [RX] X [OH-]
SN2 short for, substitution, nucleophilic and bimolecular.
Takes place in a single step without intermediates when the incoming nucleophile reacts
with the alkyl halide or tosylate (the substrate) from a direction opposite the group that is
displaced (the leaving group)
Nucleophile must approach from the opposite end of the molecule to the leaving group.
Stereo chemical configuration is reversed compared the original molecule.
11.3 Characteristics of the N 2 Reaction
The rate of a chemical reaction is determined by the energy difference between reactant
ground state and transition state.
The transition state for reaction of a sterically hindered (i.e. 3 methyl groups) alkyl halide,
whose carbon atom is shielded from approach of the incoming nucleophile is higher in energy and forms more slowly than the corresponding transition state for a less hindered
Nucleophile attacks carbon from the back
SN2 reactions occur only at relatively unhindered sites, usually only primary and a few
The exact nucleophilicity of a species in a given reaction depends on the substrate, the
solvent, and even the reaction conditions.
Nucleophilicity roughly parallels basicity: strong bases often make strong nucleophiles.
Nucleophilicity usually increases going down a column of the periodic table as larger atoms
hold their valence electrons less tightly are and consequentially more likely to react.
Negatively charged nucleophiles are usually more reactive than neutral ones (reactions
generally carried out under basic conditions)
As the leaving group is expelled with a negative charge the best leaving groups are those
that best stabilize the negative charge in the transition state; i.e. Cl .
An alcohol can be treated with para-toluenesulfonyl chloride to form a tosylate to improve
its reactivity towards nucleophilic substitution.
Protic solvents (containing an –OH or –NH group) are generally the worst for SN2 as they
undergo salvation of the reactant nucleophile and polar aprotic solvents (polar but neither
of above groups) are the best as they raise the ground state energy of the nucleophile.
Substrate characteristics: methyl and primary substrates.
Nucleophile: basic, negatively charged with a higher ground state energy.
Leaving group: more stable anions.
Polar aprotic solvents which surround the accompanying cation.
11.4 The S 1 Reaction
Tertiary halide substrates are most effective in SN1 reactions, and is the only factor upon
which the reaction rate is dependent.
Unimolecular rate limiting step. Reaction takes place by loss of the leaving group before the
Reaction occurs through a carbocation intermediate, with products showing a 1:1 ratio of
stereo chemical isomers of original asymmetric molecule.
11.5 Characteristics of the SN1 Reaction
Reaction is favoured whenever a stable carbocation is formed. As a result of resonance
stabilization, tertiary carbocations are the most stable and thereby the most likely to react.
Most stable leaving groups are more reactive, i.e. tertiary alcohols.
Nucleophiles do not affect the rate of SN1 reactions.
Solvent effects in SN1 reaction are due largely to stabilization or destabilization of th