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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
C H A P T E R 1 Structure and Properties
180°
120°
109.5°
90°
120°
90°
90°
Table 1.4 Types of hybridization and molecular shapes
51
No. of effective
pairs
2
3
4
5
6
Arrangement of pairs Hybridization Shape
sp
Linear
2
sp
Trigonal plannar
3
sp
Tetrahedral
2
dsp
Trigonal bipyramidal
3
d2sp
Octahedral
MEMORY FOCUS
1.
sp 3d
Hybridization
(a) One s, three p and one d orbitals merge to give five new hybrid orbitals.
(b) Three orbitals lie in a plane at 120° and the two other at 90° to the plane, one lying above and
one below the plane.
(c) It gives a trigonal bipyramidal shape to the molecule.
(d) It is present in PCl
2
sp3d
2.
Hybridization
(a) One s, three p and two d orbitals merge to give six new orbitals.
(b) Four orbitals lie in plane at 90° angle to each other, one above and one below the plane.
(c) It gives octahedral shape to the molecule.
(d) It is present in SF
, PF5, etc.
5
, SeF6, TeF6, etc.
6
52
PHARMACEuTICAL ORgAnIC CHEMISTRy
2
3.
dsp
Hybridization
(a) One (n – 1)d, one ns and two np orbitals merge to give four hybrid orbitals.
(b) Four dsp
2
hybrid orbitals lie at 90° in a plane at the four corners of a square.
(c) It gives square planar shape to the molecule.
(d) It is present in [Ni(CN)4]2-, [Pt(NH3)4]2+, etc.
3
sp3d
4.
Hybridization
(a) One s three p and three d orbitals of the nth shell merge to give seven new orbitals.
(b) Five hybrid orbitals lie in a plane at 72° angle, one above and one below the plane perpendicular
to it.
(c) It gives pentagonal bipyramidal shape to the molecule.
(d) It is present in IF
.
7
NOTEWORTHY POINTS
Useful tips for type of hybridization and shapes of molecules
The structure of any molecule can be predicted on the basis of hybridization of the central atom, which
can be determined by the following formula:
1
H =
2
H =1[V + X – C + A]
2
No. of electrons in
3
valence shell of the atom
Value of H 2 3 4 5 6 7
hybridization sp sp
No. of monovalent
+
2
sp3 sp3d sp3d2 sp3d
atoms
Charge on
–
cation
+
Charge
on anion
3
4
Examples:
Type I. Central atom is surrounded by monovalent atoms only.
BCl
, BeF2, CCl4, PCl5, NH3, NCl3, H2O, OF2, IF7, CIF3, SF4, SF6, XeF2, XeF4, etc.
3
Structure and shape of PCl
5
P has five electrons in its outermost shell and there are five monovalent atoms around it.
3
H =1[5 + 5 – 0 + 0] = 5. Hybridization= sp
2
Type II. Central atom is surrounded by divalent atom only: CO
Hybridization in SO
H =1[6 + 0 – 0 + 0] = 3. Hybridization= sp2, shape triangular.
2
3
d, shape trigonal bipyramidal.
, CS2, SO2, SO3, XeO3.
2
It should be noted that divalent atoms are not counted.
Type III. Central atom is surrounded by monovalent as well as divalent atoms COCl2, XeO2F2, POCl3.
H =1[5 + 3 – 0 + 0] = 4
2
C H A P T E R 1 Structure and Properties
53
Hybridization in POCl
Hybridization =
Type IV. Hybridization in anions CO
Hybridization = CO
H =1[4 + 0 – 0 + 2] = 3. Hybridization - 3, shape: trigonal.
2
Type V. Hybridization in cations H
H =1[6 + 3 – 1 + 0] = 4. Hybridization - sp3, shape: tetrahedral.
2
3
3
sp
, shape tetrahedral.
2–
(charge = 2)
3
2–
, SO
3
O+, NH
3
2–
4
+
, PO
4
2–
4
, NO
–
, NO
2
–
.
3
1.14 INTERMOLECULAR FORCES
The forces present between the molecules of the substances are called intermolecular forces (inter means ‘between two or more’). These forces are present in all kinds of molecules. These weak attractive forces are present for all the states of matter and their structural and physical properties. These forces are present in even nonpolar substances such as H2, Cl2, CH4,etc.,sinceallthesesubstancescanbeliquied.Eventhe
inertgasessuchasHe,Ne,Ar,etcpossessintermolecularforcebecausetheycanalsobeliquied,andsome
weak forces must be operative among the molecules.
Theexistence of these forcesamong the nonpolar moleculesin their liquid andsolidstates was rst
proposed by the Dutch scientist J.D. van der Waals.
Types of interactions between various molecules which lead to intermolecular forces.
1. Dipole–dipole interaction
2. Ion–dipole
3. Ioninduced dipole
4. Induced dipole–induced dipole
5. Instantaneous dipole–instantaneous induced dipole forces (or dispersion forces)
It may be noted that the term van der Waals Forces refer to dipole–dipole, dipole–induced dipole and
dispersion forces.
1. Dipole–dipole interaction: These are second most strongest intermolecular forces. These forces exit
between permanent dipoles. The dipole tend to align with oppositely charged ends directed at each other.
As permanent dipoles exist only in polar compounds, so these interaction occur only in polar compounds,
e.g. acetone and dichloromethane. These forces weaken even more rapidly as the distance increases as
compared to an ion–dipole interaction.
54
Solid Liquid
+− +− +−
+−
+−
−+
+−
+−
−+
−+
−+
+−
+−
+−
+−
+− +− +− +−
−+
−+ −+ −+
Cl
−
Na
+
+
−
+
−
+−
+
−
−
+
−
+
−+
−
+
Polar
molecule
Polar
molecule
Molecule with
induced dipole
Nonpolar
molecule
+
−
+−
e
¯
e
¯
e
¯
e
¯
+
δ
+
δ¯
e
¯
e
¯
e
¯
e
¯
+
PHARMACEuTICAL ORgAnIC CHEMISTRy
These forces are also called Keesom forces asthesewererststudiedbyKeesom(1912).Hetermedthis
effect as orientation effect.
2. Ion–dipole forces: These are the strongest internuclear forces. These occur between the ions formed by
dissolving the ionic compound in polar solvents. The polar solvent is generally water but it can also include
organic salts dissolved in polar organic compound, e.g. [Et
n]Br in acetone. The negative end of the dipole
4
will be attracted to positive ions and vice versa.
The magnitude of the interaction between these dipoles and ions depends upon the ionic charges and size
of dipoles. These forces sharply decrease with increasing distance. Thus, they act over very short distances
only.
3. Ion-induced dipole interactions: These interactions occur between an ion and the induced dipole, for
example, dissolving benzene in acetone. This occurs when the ion approaches towards nonpolar molecule, the
symmetrical distribution of non-polar molecule gets distorted because the electrons on the nonpolar molecules
are attracted to the positive end of the dipole and repelled by the negative end of the dipole. The interaction
lasts as long as the polar molecule is near the nonpolar molecule. The formation of induced dipole depends
upon the charge on the ion and polarizability of the atom or molecule. As a result, these forces are weak forces
and effective only at short range.
C H A P T E R 1 Structure and Properties
4. Induced dipole–induced dipole: These forces are also called van der Waals forces or London dispersion
forces. These forces occur between nonpolar molecules such as noble gases. These forces occur when a
momentary imbalance in the electron density on an atom or molecule causes a dipole to be formed for an
instant (instantaneous dipole).
5. Instantaneous dipole–instantaneous induced dipole forces: Let us try to understand the formation of
instantaneous dipole. When two noble gas atoms are very close to each other, each atom is uncharged because
the electron distribution around the nucleus is symmetrical. However, due to motion of electrons, it is possible
that for the fraction of time, electron distribution is not symmetrical because of the distortion of the electron
cloud. As a result, a small temporary dipole is formed called induced dipole or instantaneous dipole.
The instant or induced dipole further distorts the electron cloud of neighbouring atom as shown below:
55
+
d
A B
Permanent dipole
(a polar molecule)
+
d
A B
Permanent dipole
(a polar molecule)
2
d
Nonpolar molecule
2
d
Induced dipole in a
nonpolar molecule
+ d2
d
Figure 1.29 Interaction between permanent dipole and induced dipole.
The attraction that occurs between induced dipole is called induced dipole–induced dipole interaction. As
the magnitude of charges developed is very small, these forces can operate over the shortest distance of all
forces and hence are weakest forces. The strength of these forces depends upon
a. size of molecule
b. no. of electrons present in the molecule and
c. molecular structure
The larger the size, the higher the number of electrons present in the molecule or an atom. More easily, the
atom can be polarized (i.e. easily undergo distortion of electron cloud) and greater will be the strength of these
forces. So, larger the size of the molecule or an atom, greater will be the strength of these forces and vice versa.
Mass (g/mol) Boiling point
H
2
D
2
T
2
2 20K
4 23K
6 25K
These forces affect the melting point and boiling point of the compounds. Larger will be the strength of
these forces, higher will be the melting and boiling points of the compounds.
56
Covalent
bond
. . . . . . .H F . . . . .H F . . . . . . .H F . . . . . .
Hydrogen
bond
PHARMACEuTICAL ORgAnIC CHEMISTRy
Compound Melting point Boiling point
CH
CF
CCl
4
4
4
-182°C -164°C
-150°C -130°C
-23°C 76°C
1.15 HYDROGEN BONDING
These are strongest intermolecular forces. These are present in compounds of hydrogen with strongly
electronegativeelementssuchasuorine,oxygenandnitrogen(suchasH2O, nH3, HF), etc. The shared pair
of electrons between the two atoms lies far away the hydrogen atom. Hence, the hydrogen atom acquires a
slight positive charge and other atom acquires a slight negative charge. It leads to the formation of dipole
(charge separation). This results in dipole–dipole interactions. As a result, two or more molecules associate
together to form a larger cluster of molecules. So, H–F exists as an associated molecule.
Hydrogen bond is represented by a dotted line while a solid line represents a covalent bond. In associated
molecules,hydrogen actsas abridge betweentwo stronglyelectronegative atoms,holding oneuorine
atom by a covalent bond and other by a hydrogen bond.
The attractive force which binds covalently bonded hydrogen atom of one molecule with electronegative
atom of another molecule of the same substance is known as the hydrogen bond.
Cause of hydrogen bonding: The formation of hydrogen bond is possible in the case of highly
electronegative atoms like F, O and n.
In such cases, the electron cloud is largely displaced towards the highly electronegative atom and as a result
hydrogen behaves as a bare proton, which attracts and gets attracted by the neighbouring electronegative
atom, resulting in the formation of associated molecule.
1.15.1 Factors Responsible for the Formation of Hydrogen Bond and Strength of Hydrogen Bond
The formation of hydrogen bond is based on the following two factors:
1. High electronegativity: The electronegativity of the atom to which hydrogen atom is linked should be
high. The more the electronegative atoms to which hydrogen is linked, greater is the polar character of
the bond and hence stronger is the hydrogen bond.
The electronegativity of F, O, n increases as F > O > n. As a result, the strength of hydrogen bond
increases accordingly.
C H A P T E R 1 Structure and Properties
29.0 kJ/mole
22.0 kJ/mole
17.0 kJ/mole
. . . . . . .H F. . . . .H
F. . . . . . .H
F. . . . . .H
. . . . . . .O H. . . . .O H. . . . . . .O H. . . . . .
. . . . . . .N H. . . . .N H. . . . . . .N H. . . . . .
F
F
F
F
HH
H
H
H
F
F
. . .
H
. . .
H
. . .
H
. . .
H
. . .
H
. . .
H
. . .
HHN
. . .
HN
. . .
N
2. Small size of the atom: The size of the atom attached to the hydrogen atom should be small. In case
the size of the electronegative atom is large, the bonding electrons will not be attracted effectively due
to screening effect of the intervening electrons. This reduces the polarity of the covalent bond and the
hydrogen bond may not be formed.
Chlorine and sulphur, which possess almost the same electronegativity as that of nitrogen do not form
hydrogen bonds.
Examples:
HCl and H2S do not form hydrogen bonds almost of the same electronegativity as that of nitrogen do not
form hydrogen bonds.
Examples of the hydrogen-bonded compounds:
a. Hydrogen uoride: In a hydrogen uoride molecule, hydrogen atom is linked to a strongly
electronegativeuorineatom.Inthesolidstate,hydrogenuoridemoleculesexistaslongzigzag
chains associated by hydrogen bonding as shown:
57
Eachhydrogenatomisclosertothecovalentlybondeduorineatomascomparedtotheuorineatom,
which is held by hydrogen bonding. On heating, the length of the chain becomes smaller and at temperature
justaboveitsboilingpoint,theassociated(HF)n becomes quite small. It can have any value up to 6.
In aqueous solution, hydrogen uoride on ionization results in biuoride ion HF
-
(F
…..H+ – F- )] and not as simple F- ion.
b. Ammonia: It is interesting to note that in ammonia molecule, four hydrogen bonds can be formed,
three hydrogen bonds through three hydrogen atoms and fourth with a lone pair of electrons on
nitrogen and forming giant molecules (Fig. 1.30).
Figure 1.30 Hydrogen bonding in ammonia.
-
[represented as
2
58
H
H
OO
O
H
H
H
H
RC
CR
O
. . . .
HO
. . . . .
O
OH
PHARMACEuTICAL ORgAnIC CHEMISTRy
c. Water: In the water molecule, the electronegative oxygen atom is bonded covalently to two hydrogen
atoms. Thus, oxygen atom being more electronegative acquires a partial negative charge while the
hydrogen attached to the oxygen atom acquires a partial positive charge. The molecules of water get
associated with one another through hydrogen bonds as shown below:
1.15.2 Consequences of Hydrogen Bonding
Hydrogen bonding inuences the physicalproperties of the compounds such as melting points boiling
points, solubility, etc., as described below:
1.
Association: Hydrogen bond links up two or more molecules of a compound to form bigger units.
These bigger units are known as associated molecules. For example, carboxylic acids associate due to
hydrogen bonding to exist as dimer even in the vapour state.
In aqueous solution, the molecules of these acids associate with water molecule rather than existing as
dimers.
Higher melting and boiling points: Compounds in which molecules have stronger intermolecular
2.
forces of attraction due to the presence of hydrogen bonding requires a higher energy to separate the
moleculesfromoneanother.Forexample,hydrogenuoride,waterandammoniahavehighmelting
and boiling points as compared to those of the hydrides of other elements of group 17, 16 and 15,
respectively.
C H A P T E R 1 Structure and Properties
Melting point (°C)
Molecular weight
100
50
0
–50
–100
–150
–200
AsH
2
PH
3
NH
3
H2Te
H
2
Se
H
2
S
H2O
HF
HI
HBr
HCl
SbH
3
Melting point (°C)
Molecular weight
AsH
2
H2Te
H
2
Se
H
2
S
H
2
O
100
50
0
–50
–100
–150
–200
HF
HI
HBr
HCl
NH
3
PH
3
SbH
3
H R H R
. . .H
O . . . .H O . . . .H O . . . .H O . . . .
Figure 1.31 Effects of hydrogen bonding on melting and boiling points.
In case of hydrogen halides, the melting and boiling points of acids decrease as we proceed from HI to
HBr to HCl. There is a sudden rise in the melting and boiling points of HF due to strong hydrogen bonding
between HF molecules. Similarly nH3 and H2O have high melting and boiling points than corresponding
hydrides of groups V and VI, respectively.
Out of propane, dimethyl either and ethyl alcohol of nearly the same molecular weight, ethyl alcohol
has the highest boiling point due to hydrogen bonding whereas is no hydrogen bonding is there in case of
propane and dimethyl either (Fig. 1.31).
59
CH3 – CH2 – CH3 CH3 – O – CH3 CH3 – CH2 – OH
Propane Dimethylether Ethylalcohol
Mol. wt. = 44 Mol. wt. = 46 Mol. wt. = 46
b.p. = – 45°C b.p. = – 25°C b.p. = 78°C
3. Solubility: The organic compounds which can form hydrogen bonds with the molecules of water are
soluble. For example, alcohols and carboxylic acids are soluble in water due to hydrogen bonding.
Benzene and CCl4, which do not form hydrogen bond with water, are insoluble.
4. Surface tension and viscosity: Hydrogen-bonded molecules have higher surface tension and viscosity.
Common examples are water and glycerol. There is extensive hydrogen bonding in glycerol (CH2OH.
CHOH.CH2OH), because of the presence of three –OH groups per molecule than hydrogen bonding in
alcohol.
ethyl alcohol containing one –OH group per molecule. That is why glycerol is more viscous than ethyl
60
o-Nitrophenol
o-Hydroxy benzaldehyde
(Salicylaldehyde)
O
N
O
O
H
O
O
C
H
H
PHARMACEuTICAL ORgAnIC CHEMISTRy
1.15.3 Types of Hydrogen Bonds
Hydrogen bonding is of the following two types:
1. Intermolecular hydrogen bond
2. Intramolecular hydrogen bond
1. Intermolecular hydrogen bond: The hydrogen bond formed between two or more molecules of
the same or different substances is known as intermolecular hydrogen bonding. Water, ammonia
andhydrogenuoridemoleculesassociateduetointermolecularhydrogenbonding.Solubilityof
alcohols in water is also an example of intermolecular hydrogen bonding.
Conditions for the formation of intramolecular hydrogen bonding
Example
2. Intramolecular hydrogen bond: When hydrogen bonding exists within the same molecule, then it
is called intramolecular hydrogen bonding. Intramolecular hydrogen bond results in the cyclization
of the molecule and prevents their association.
1. The molecule should contain two groups such that one group contains hydrogen atom linked to
highly electronegative atom and the other group should contain a highly electronegative atom
linked to a lesser electronegative atom.
2. The molecule should be planar.
3. The hydrogen bonding should lead to the formation of at least six membered ring including H atom.