Download Unit - III - E

ALLIED CHEMISTRY PAPER – I
UNIT - III
1. Explain Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of
pairs of electrons between atoms, or between atoms and other covalent bonds. In short,
attraction-to-repulsion stability that forms between atoms when they share electrons is
known as covalent bonding.
Covalent bonding includes many kinds of interaction, including σ-bonding, π-bonding,
metal to non-metal bonding, agostic interactions, and three-center two-electron bonds.
The term covalent bond dates from 1939. The prefix co- means jointly, associated in
action, partnered to a lesser degree, etc.; thus a "co-valent bond", essentially, means that
the atoms share "valence", such as is discussed in valence bond theory. In the molecule
H2, the hydrogen atoms share the two electrons via covalent bonding. Covalency is
greatest between atoms of similar electronegativities. Thus, covalent bonding does not
necessarily require the two atoms be of the same elements, only that they be of
comparable electronegativity. Although covalent bonding entails sharing of electrons, it
is not necessarily delocalized. Furthermore, in contrast to electrostatic interactions ("ionic
bonds") the strength of covalent bond depends on the angular relation between atoms in
polyatomic molecules.
Early concepts in covalent bonding arose from this kind of image of the molecule of
methane. Covalent bonding is implied in the Lewis structure that indicates sharing of
electrons between atoms.
2. Write a note on Resonance
Many bonding situations can be described with more than one valid Lewis Dot Structure
(for example, ozone, O3). In an LDS diagram of O3, the center atom will have a single
bond with one atom and a double bond with the other. The LDS diagram cannot tell us
which atom has the double bond; the first and second adjoining atoms have equal chances
of having the double bond. These two possible structures are called resonance structures.
In reality, the structure of ozone is a resonance hybrid between its two possible
resonance structures. Instead of having one double bond and one single bond, there are
actually two 1.5 bonds with approximately three electrons in each at all times.
A special resonance case is exhibited in aromatic rings of atoms (for example, benzene).
Aromatic rings are composed of atoms arranged in a circle (held together by covalent
bonds) that may alternate between single and double bonds according to their LDS. In
actuality, the electrons tend to be disambiguously and evenly spaced within the ring.
Electron sharing in aromatic structures is often represented with a ring inside the circle of
atoms.
Ethylene
Ethylene (IUPAC name: ethene) is the chemical compound with the formula C2H4. It is
the simplest alkene. Because it contains a carbon-carbon double bond, ethylene is called
an unsaturated hydrocarbon or an olefin. It is extremely important in industry and also
has a role in biology as a hormone. Ethylene is the most produced organic compound in
the world; global production of ethylene exceeded 107 million metric tonnes in 2005. To
meet the ever increasing demand for ethylene, sharp increases in production facilities
have been added globally, particularly in the Gulf countries.
Structure
This hydrocarbon has four hydrogen atoms bound to a pair of carbon atoms that are
connected by a double bond. All six atoms that comprise ethylene are coplanar. The H-CH angle is 119°, close to the 120° for ideal sp² hybridized carbon. The molecule is also
relatively rigid: rotation about the C-C bond is a high energy process that requires
breaking the π-bond, while retaining the σ-bond between the carbon atoms.
The double bond is a region of high electron density, and most reactions occur at this
double bond position.
Uses
Approximately 80% of ethylene used in the United States and Europe is used to create
ethylene oxide, ethylene dichloride, and polyethylene. In smaller quantities, ethylene is
used as an anesthetic agent (in an 85% ethylene/15% oxygen ratio), to hasten fruit
ripening, as well as a welding gas.
Polyethylenes of various density and melt flow account for more than 50% of world
ethylene demand. The primary use of polyethylene is in film applications for packaging,
carrier bags and trash liners. Other applications include injection moulding, pipe
extrusion, wire and cable sheathing and insulation, as well as extrusion coating of paper
and cardboard.
Ethylene derivatives include: ethylene oxide, styrene monomer (via ethyl benzene) and
linear higher olefins.
Ethylene oxide is a key raw material in the production of surfactants and detergents. It is
also used to manufacture ethylene glycols, which are in turn used in packaging and
textiles, and to make glycol ether solvents.
Styrene monomer is used principally in polystyrene for packaging and insulation, as well
as in styrene butadiene rubber for tires and footwear.
Linear higher olefins are used as base materials for the manufacture of detergents,
plasticisers, synthetic lubricants and additives, but also as co-monomers in the production
of polyethylenes.
Ethylene is a key component in Levinstein sulfur mustard, a chemical weapon agent.
3. Write a note on Polar effect
The Polar effect or electronic effect in chemistry is the effect exerted by a substituent on
modifying electrostatic forces operating on a nearby reaction center. The main
contributors to the polar effect are the inductive effect, mesomeric effect and the throughspace electronic field effect.
An electron withdrawing group or EWG draws electrons away from a reaction center.
When this center is an electron rich carbanion or an alkoxide anion the presence of the
substituent has a stabilizing effect.
Examples of electron withdrawing groups are

halogens (F, Cl);

nitriles CN;

carboxylic acids COOH;

carbonyls CO.

nitro groups NO2
An electron releasing group or ERG (otherwise called electron donating groups or
EDG) releases electrons into a reaction center and as such stabilizes electron deficient
carbocations.
Examples of electron releasing groups are

alkyl groups;

alcohol groups;

amino groups.
The total substituent effect is the combination of the polar effect and the combined steric
effects.
In electrophilic aromatic substitution and nucleophilic aromatic substitution substituents
are divided into activating groups and deactivating groups where the direction of
activation or deactivation is also taken into account.
Interested in contributing to Wikipedia?
4. Explain about Inductive effect
The inductive effect in chemistry is an experimentally observable effect of the
transmission of charge through a chain of atoms in a molecule by electrostatic induction.
The net polar effect exerted by a substituent is a combination of this inductive effect and
the mesomeric effect.
The electron cloud in a σ-bond between two unlike atoms is not uniform and is slightly
displaced towards the more electronegative of the two atoms. This causes a permanent
state of bond polarization, where the more electronegative atom has a slight negative
charge(δ-) and the other atom has a slight positive charge(δ+).
If the electronegative atom is then joined to a chain of atoms, usually carbon, the positive
charge is relayed to the other atoms in the chain. This is the electron-withdrawing
inductive effect, also known as the -I effect.
Some groups, such as the alkyl group are less electron-withdrawing than hydrogen and
are therefore considered as electron-releasing. This is electron releasing character is
indicated by the +I effect.
As the induced change in polarity is less than the original polarity, the inductive effect
rapidly dies out, and is significant only over a short distance. The inductive effect is
permanent but feeble, as it involves the shift of strongly held σ-bond electrons, and other
stronger factors may overshadow this effect.
The inductive effect may be caused by some molecules also. Relative inductive effects
have been experimentally measured with reference to hydrogen.
Inductive effects can be measured through the Hammett equation.
Applications

Aliphatic carboxylic acids. The strength of a carboxylic acid depends on the
extent of its ionization: the more ionized it is, the stronger it is. As an acid
becomes stronger, the numerical value of its pKa drops. In aliphatic acids, the
electron-releasing inductive effect of the methyl group increases the electron
density on oxygen and thus hinders the breaking of the O-H bond, which
consequently reduces the ionization. Greater ionization in formic acid when
compared to acetic acid makes formic acid (pKa=3.75) stronger than acetic acid
(pKa=4.76). Monochloroacetic acid (pKa=2.82), though, is stronger than formic
acid, since the electron-withdrawing effect of chlorine promotes ionization.

Aromatic carboxylic acids. In benzoic acid, the carbon atoms which are present in
the ring are sp2 hybridised.As a result, benzoic acid(pKa=4.20) is a stronger acid
than cyclohexane carboxylic acid(pKa=4.87). Also, electron-withdrawing groups
substituted at the ortho and para positions, enhance the acid strength.

Dioic acids. Since the carboxyl group is itself an electron-withdrawing group, the
dioic acids are, in general, stronger than their monocarboxyl analogues.

In the so-called Baker-Nathan effect the observed order in electron-releasing alkyl
substituents is apparently reversed.
5. Explain about Electromeric effect
A molecular polarizability effect occurring by an intramolecular electron
displacement (sometimes called the ‘conjugative mechanism’ and, previously,
the ‘tautomeric mechanism’) characterized by the substitution of
one electron pair for another within the same atomic octet of electrons. It
can be indicated by curved arrows symbolizing the displacement of electron
pairs, which represents the hypothetical electron shift
The term has been deemed obsolescent or even obsolete. It has long been custom to use
phrases such as
‘enhanced substituent resonance effect’ which imply the operation of the
electromeric effect, without using the term, and various modern theoretical
treatments parametrize the response of substituents to ‘electronic demand’,
which amounts to considering the electromeric effect together with the
inductomeric effect.
Help us provide free content to the world by donating today!
6. Write a note on Steric effects
The steric effect of tri-(tert-butyl)amine makes electrophilic reactions, like forming the
tetraalkylammonium cation, difficult. It is difficult for electrophiles to get close enough
to allow attack by the lone pair of the nitrogen (nitrogen is shown in blue)
Steric effects arise from the fact that each atom within a molecule occupies a certain
amount of space. If atoms are brought too close together, there is an associated cost in
energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect
the molecule's preferred shape (conformation) and reactivity.
Types of steric effects
Steric hindrance or steric resistance occurs when the size of groups within a molecule
prevents chemical reactions that are observed in related smaller molecules. Although
steric hindrance is sometimes a problem, it can also be a very useful tool, and is often
exploited by chemists to change the reactivity pattern of a molecule by stopping
unwanted side-reactions (steric protection). Steric hindrance between adjacent groups
can also restrict torsional bond angles. However, hyperconjugation has been suggested as
an explanation for the preference of the staggered conformation of ethane because the
steric hindrance of the small hydrogen atom is far too small. [1] [2].
Regioselective dimethoxytritylation of the primary 5'-hydroxyl group of thymidine in the
presence of a free secondary 3'-hydroxy group as a result of steric hindrance due to the
dimethoxytrityl group and the ribose ring (Py = pyridine).[3]
Steric shielding occurs when a charged group on a molecule is seemingly weakened or
spatially shielded by less charged (or oppositely charged) atoms, including counterions in
solution (Debye shielding). In some cases, for an atom to interact with sterically shielded
atoms, it would have to approach from a vicinity where there is less shielding, thus
controlling where and from what direction a molecular interaction can take place.
Steric attraction occurs when molecules have shapes or geometries that are optimized
for interaction with one another. In these cases molecules will react with each other most
often in specific arrangements.
Chain crossing — A random coil can't change from one conformation to a closely
related shape by a small displacement if it would require one polymer chain to pass
through another, or through itself.
Steric effects vs. electronic effects
The structure, properties, and reactivity of a molecule is dependent on straight forward
bonding interactions including covalent bonds, ionic bonds, hydrogen bonds and lesser
forms of bonding. This bonding supplies a basic molecular skeleton that is modified by
repulsive forces. These repulsive forces include the steric interactions described above.
Basic bonding and steric are at times insufficient to explain many structures, properties,
and reactivity. Thus steric effects are often contrasted and complemented by electronic
effects implying the influence of effects such as induction, conjunction, orbital symmetry,
electrostatic interactions, and spin state. There are more esoteric electronic effects but
these are among the most important when considering structure and chemical reactivity.
7. Explain about Hyperconjugation
Hyperconjugation in organic chemistry is the stabilizing interaction that results from the
interaction of the electrons in a sigma bond (usually C-H or C-C) with an adjacent empty
(or partially filled) non-bonding p-orbital or antibonding π orbital or filled π orbital to
give an extended molecular orbital that increases the stability of the system.[1] Only
electrons in bonds that are β to the positively charged carbon can stabilize a carbocation
by hyperconjugation.
Applications
Hyperconjugation can be used for rationalizing a variety of other chemical phenomena,
including the anomeric effect, the gauche effect, the rotational barrier of ethane, the betasilicon effect, the vibrational frequency of exocyclic carbonyl groups, and the relative
stability of substituted carbocations. Hyperconjugation is proposed by quantum
mechanical modeling to be the correct explanation for the preference of the staggered
conformation rather than the old textbook notion of steric hindrance.[3][4]
Rotational barrier of ethane
An instance where hyperconjugation may be overlooked as a possible chemical
explanation is in rationalizing the rotational barrier of ethane. It had been accepted as
early as the 1930’s that the staggered conformations of ethane were more stable than the
eclipsed. Wilson had proven that the energy barrier between any pair of eclipsed and
staggered conformations was approximately 3 kcal/mol, and the generally accepted
rationale for this was the unfavorable steric interactions between hydrogen atoms.
The hyperconjugative effects delocalize charge and stabilize the molecule. Further, it is
the vicinal hyperconjugative effects that keep the molecule in the staggered
conformation[3]. Thanks to this work, the following model of the stabilization of the
staggered conformation of ethane is now more accepted:
Hyperconjugation can also explain several other phenomena whose explanations may
also not be as intuitive as that for the rotational barrier of ethane[5]. One such example is
the explanations for certain Lewis structures. The Lewis structure for an ammonium ion
indicates a positive charge on the nitrogen atom. In reality, however, the hydrogens are
more electropositive than is nitrogen, and thus are the actual carriers of the positive
charge. We know this intuitively because bases remove the protons as opposed to the
nitrogen atom.
Effect on chemical properties
Hyperconjugation has an effect on several chemical properties Bond length:
Hyperconjugation is suggested as a key factor in shortening of sigma bonds (σ bonds) in
such systems. For example, the single C-C bonds in 1,3-butadiene and methylacetylene
are approximately 1.46 angstrom in length, much less than the value (1.54 angstrom)
found in saturated hydrocarbons. This is due mainly to hyperconjugation that gives
partial double-bond character of the bond.
1. Dipole moments: The large increase in dipole moment of 1,1,1-trichloroethane as
compared with chloroform can be attributed to hyperconjugated structures.
2. The heat of formation of such molecules are greater than sum of their bond
energies; and the heats of hydrogenation per double bond are less than the heat of
hydrogenation of ethylene.
3. Stability of carbocations:
(CH3)3C+ > (CH3)2CH+ > (CH3)CH2+ > CH3+
The C-C σ bond adjacent to the cation is free to rotate, and as it does so, the three C-H σ
bonds of the methyl group in turn undergoes the stabilization interactions.