chemistry

41. The difference
between the energy
of most stable
contributing
structure and the
energy of actual
molecule.

45. The difference in energy between the resonance hybrid
and the canonical form of lowest energy is called the
resonance energy.

58. Why is RCO2H more acidic than ROH?
Increased resonance stabilization of the conjugated base

59. Hyperconjugation
σ-conjugation or no-bond resonance
Hyperconjugation is the stabilising interaction that results from
the interaction of the electrons in a σ-bond (usually C-H or C-C)
with an adjacent empty or partially filled p-orbital
or a π-orbital to give an extended molecular orbital
that increases the stability of the system.

62. Stability of alkenes:
A general rule is that, the stability of alkenes increases with increase in the number of
alkyl groups (containing hydrogens) on the double bond. It is due to increase in the
number of contributing no bond resonance structures.
For example, 2-butene is more stable than 1-butene. This is because in 2-butene, there
are six hydrogens involved in hyperconjugation whereas there are only two hydrogens
involved in case of 1-butene. Hence the contributing structures in 2-butene are more
and is more stable than 1-butene.

64. Due to hyperconjugation, in the ethyl carbocation, the net
charge of +1 is shared by a total of four atoms, one carbon
atom and three hydrogen atoms.
Stability of carbocation:

67. Inductive effect
When a covalent bond is formed between atoms of different electronegativity,
the electron density is more towards the more electronegative atoms of the
bond, such a shift in electron density is called as inductive effect.

68. • It arises due to electronegativity difference between two atoms forming a sigma bond.
• It is transmitted through the sigma bonds. No pi bonds are involved.
• The magnitude of inductive effect decreases while moving away from the groups
causing it and is not that much significant beyond the 3rd carbon atom.
• It is a permanent effect. It may create permanent dipole in the molecule.
• In general, the inductive effect is relatively weak and is overshadowed by other
electronic effects like resonance effect, hyperconjugation etc.
• It influences the chemical and physical properties of compounds.

69. TYPES OF INDUCTIVE EFFECT
The inductive effect is divided into two types based on the electron withdrawing or electron releasing nature
of atom/group inducing it.

71. The electron withdrawing groups showing negative inductive effect (-I) reduce the negative
charge on the conjugate base (like carboxylate ion) by pulling the electron density and thus
by stabilizing it. Hence the acidic strength increases when -I groups are present.
On the contrary, the +I groups decrease the acidic strength as they destabilize the conjugate
base of acid (anionic) by releasing electron density which in turn increases the negative
charge on the anion and thus destabilizing it.
E.g.
i) The acidic strength increases with increase in the number of electron withdrawing
Fluorine atoms as shown below.
CH3COOH < CH2FCOOH < CHF2COOH < CF3COOH
ii) Formic acid is stronger acid than acetic acid since the –CH3 group destabilizes the
carboxylate ion.

74. The p-nitrophenol is stronger acid than phenol since the -NO2 group is a -I group that
withdraws electron density.
Whereas, the para-cresol is weaker acid than phenol, since the -CH3 group shows positive inductive effect (+I).
For example, the order of acidic strength following phenols is:

75. The electron donating groups like alkyl groups increase the basic strength of amines whereas
the electron withdrawing groups like aryl groups decrease the basic nature.
Therefore alkyl amines are stronger Lewis bases than ammonia, whereas aryl amines are
weaker than ammonia.
Thus the order of basic strength of alkyl and aryl amines with respect to ammonia is:
CH3NH2 > NH3 > C6H5NH2

76. The strength of basicity of amines depends on two factors i.e.,
the availability of the lone pair and the stability of the
conjugate acid formed in an aqueous solution.

77. Steric effects
Steric effects arise from the spatial arrangement of atoms.
When atoms come close together there is a rise in the energy of the
molecule.
Steric effects are nonbonding interactions that influence the shape
(conformation) and reactivity of ions and molecules.
Steric effect essentially arises because of the fact that each atom of
molecule occupies some space. If two atoms are brought closer than
their vanderwaal’s radii, their electron clouds repel each other. Thus
such process in energetically unfavorable.

78. Steric hindrance
Steric hindrance is a consequence of steric effects. Steric hindrance
is the slowing of chemical reactions due to steric bulk.

79. Applications Of Steric Effect
Stability Of Two Geometric Isomers:
Trans alkenes are more stable because of a less steric hindrance

80. Reactivity:

81. Steric Effects on Solvation:
The presence of three alkyl groups
sharply diminishes the ability of the
solvent to stabilize the corresponding
ammonium ion.

82. Planarity is the main condition for resonance that means resonance can occur only
when all the atom involved in resonance lie in the same plane or nearly in the same
plane.
Any change in structure which destroys planarity of molecule will restrict or inhibit
resonance, this phenomenon is known as steric inhibition of resonance.
Steric Inhibition of Resonance:
CH3NH2 > NH3 > C6H5NH2
In case of N,N,2,6 tetramethyl aniline having two bulky methyl
group in ortho position of the benzene ring , the NMe2 group
can not remain in the same plane. That is why the p-electron
on N-atom cannot delocalized through pi orbital in para
position.

83. Tautomerism
Tautomerism is a phenomenon where a single chemical compound
tends to exist in two or more interconvertible structures that are
different in terms of the relative position of one atomic nucleus which is
generally the hydrogen.
The two structures are called tautomers and they also exist in dynamic
equilibrium.

89. Tautomerism basically happens in the presence of a catalyst.
•Acid-catalyst, First the protonation occurs, cation will be delocalized. Then,
deprotonation will occur.
•For base catalysts,
deprotonation is the
first step.
Here, instead of
cation delocalization,
anion delocalization
occurs and finally
protonation to the
different position of
the anion.

92. Tautomerism in Non-Carbonyl Compounds