Department of Chemistry

Anugrah Memorial College, Gaya

Class- B.Sc. 1 (Hons) Subject- Organic Chemistry Paper- 1C Unit- Shapes and Structures of Organic molecules Teacher – Dr. Nidhi Tripathi Assistant Professor

Shapes and Structures of Organic molecules

HYBRIDISATION

The formation of bonds is no less than the act of courtship. Atoms come closer, attract to each other and gradually lose a little part of themselves to the other atoms. In chemistry, the study of bonding, that is, Hybridization is of prime importance. What happens to the atoms during bonding? What happens to the atomic orbitals? The answer lies in the concept of Hybridisation. Let us see!

Introducing Hybridisation

All elements around us, behave in strange yet surprising ways. The electronic configuration of these elements, along with their properties, is a unique concept to study and observe. Owing to the uniqueness of such properties and uses of an element, we are able to derive many practical applications of such elements.

When it comes to the elements around us, we can observe a variety of physical properties that these elements display. The study of hybridization and how it allows the combination of various molecules in an interesting way is a very important study in science.

Understanding the properties of hybridisation lets us dive into the realms of science in a way that is hard to grasp in one go but excellent to study once we get to know more about it. Let us get to know more about the process of hybridization, which will help us understand the properties of different elements.

Scientist Pauling introduced the revolutionary concept of hybridization in the year 1931. He described it as the redistribution of the energy of orbitals of individual atoms to give new orbitals of equivalent energy and named the process as hybridisation. In this process, the new orbitals come into existence and named as the hybrid orbitals.

What is Hybridization?

The following rules are observed to understand the type of hybridisation in a compound.

Rules for Observing the Type of Hybridisation

Calculate the total number of valence electrons.

Calculate the number of duplex or octet OR

Number of lone pairs of electrons

Number of used orbital = Number of duplex or octet + Number of lone pairs of electrons

If there is no lone pair of electrons then the geometry of orbitals and molecule is different.

The following are the types of hybridisation:

1) sp – Hybridisation

• In such hybridisation one s- and one p-orbital are mixed to form two sp – hybrid orbitals, having a linear structure with bond angle 180 degrees. For example in the formation of BeCl2, first be atom comes in excited state 2s12p1, then hybridized to form two sp – hybrid orbitals. These hybrid orbitals overlap with the two p-orbitals of two chlorine atoms to form BeCl2

2) sp2 – Hybridisation

In such hybridisation one s- and to p-orbitals are mixed form three sp2– hybrid orbitals, having a planar triangular structure with bond angle 120 degrees.

3) sp3

• In such hybridisation one s- and three p-orbitals are mixed to form four sp

– Hybridisation

3–

hybrid orbitals having a tetrahedral structure with bond angle 109 degrees 28′, that is, 109.5 degrees.

HYBRIDISATION OF CARBON

The ground state electronic configuration of carbon is . The electronic configuration of carbon in

excited state is

sp

. 3

If we superimpose one s and three p atomic orbitals we get 4sp

HYBRIDISATION

3 hybrid orbitals.

Each hybrid orbital contains single electron, has 25% s character and 75% p character. They are directed towards the four corners of a regular tetrahedron with the carbon located in the centre. The angle between any two sp3 hybrid orbitals is 109º 28' (109.5º).

These hybrid orbitals can overlap with four s atomic orbitals provided by four hydrogen atoms to form methane molecule.

sp2

If we superimpose one s and two p atomic orbitals we get 3sp

HYBRIDISATION

2

hybrid orbitals

Each sp2 hybrid orbital has 33% s character and 67% p character. They lie in the same plane with their axis directed towards the corner of an equilateral triangle and are 120º apart from each other. The unhybridized pz atomic orbital is perpendicular to the plane of sp2 hybrid orbitals.

BONDING IN ETHYLENE

Consider two sp2 hybridised carbon atoms approaching to

each other and four hydrogen atoms which provide four s atomic orbitals

sp HYBRIDISATION

If we superimpose one s and one p atomic orbitals we get 2sp hybrid orbitals.

Each sp hybrid orbital has 50% s character and 50% p character. They are diagonally present with their axis forming an angle of 180º. The unhybridized 2py and 2pz

atomic orbitals are perpendicular to each other and perpendicular to hybrid orbitals also.

BONDING IN ACETYLENE

Department of Chemistry

Anugrah Memorial College, Gaya

Class- B.Sc. 1 (Hons) Subject- Organic Chemistry Paper- 1C Unit- Shapes and Structures of Organic molecules Topic- Bond Angle Teacher – Dr. Nidhi Tripathi Assistant Professor

Shapes and Structures of Organic molecules

Molecules, by definition, are most often held together with covalent bonds involving single, double, and/or triple bonds, where a "bond" is a shared pair of electrons

Molecular geometries can be specified in terms of bond lengths, bond angles and torsional angles. The bond length is defined to be the average distance between the nuclei of two atoms bonded together in any given molecule.

A bond angle is the angle between two bonds originating from the same atom in a covalent species. It is the angle formed between three atoms across at least two bonds.

eg. 1:

eg. 2:

In simple terms, a bond angle is the geometric angle between two adjacent bonds. Some common shapes of simple molecules include:

Linear: In a linear model, atoms are connected in a straight line. The bond angles are set at 180°. For example, carbon dioxide and nitric oxide have a linear molecular shape.

Trigonal planar: Molecules with the trigonal planar shape are somewhat triangular and in one plane (flat). Consequently, the bond angles are set at 120°. For example, boron trifluoride.

Angular: Angular molecules (also called bent or V-shaped) have a non-linear shape. For example, water (H2

Tetrahedral: Tetra- signifies four, and -hedral relates to a face of a solid, so "

O), which has an angle of about 105°. A water molecule has two pairs of bonded electrons and two unshared lone pairs.

tetrahedral" literally means "having four faces". This shape is found when there are four bonds

all on one central atom, with no extra unshared electron pairs. In accordance with the VSEPR (valence-shell electron pair repulsion theory), the bond angles between the electron bonds are 109.47°. For example, methane (CH4

Octahedral: Octa- signifies eight, and -hedral relates to a face of a solid, so "

) is a tetrahedral molecule.

octahedral" means "having eight faces". The bond angle is 90 degrees. For example, sulfur hexafluoride (SF6

Trigonal pyramidal: A trigonal pyramidal molecule has a

) is an octahedral molecule.

pyramid-like shape with a triangular base. Unlike the linear and trigonal planar shapes but similar to the tetrahedral orientation, pyramidal shapes require three dimensions in order to fully separate the electrons. Here, there are only three pairs of bonded electrons, leaving one unshared lone pair. Lone pair – bond pair repulsions change the bond angle from the tetrahedral angle to a slightly lower value. For example, ammonia (NH3

).

The bond angles in the table below are ideal angles from the simple VSEPR theory, followed by the actual angle for the example given in the following column where this differs.

Atoms bonded

to central atom

Lone pairs

Electron domains (Steric

number) Shape

Ideal bond angle

(example's bond angle)

Example Image

2 0 2 linear 180° CO2

Atoms bonded

to central atom

Lone pairs

Electron domains (Steric

number) Shape

Ideal bond angle

(example's bond angle)

Example Image

3 0 3 trigonal planar

120° BF3

2 1 3 angular 120° (119°) SO2

4 0 4 tetrahedral 109.5° CH4

3 1 4 trigonal pyramidal

109.5 (107.8°) NH3

2 2 4 angular

109.5° (104.48°)[10][11] H2 O

5 0 5 trigonal bipyramidal

90°, 120° PCl5

4 1 5 seesaw

ax–ax 180° (173.1°),

eq–eq 120° (101.6°),

ax–eq 90°

SF4

3 2 5 T-shaped

90° (87.5°), 180° (175°) ClF3

2 3 5 linear 180° XeF2

Atoms bonded

to central atom

Lone pairs

Electron domains (Steric

number) Shape

Ideal bond angle

(example's bond angle)

Example Image

6 0 6 octahedral 90°, 180° SF6

5 1 6 square pyramidal

90° (84.8°) BrF5

4 2 6 square planar

90°, 180° XeF4

Torsional Angle/ Dihedral angle

For four atoms bonded together in a chain, the torsional angle or dihedral angle is the angle between the plane formed by the first three atoms and the plane formed by the last three atoms.

A dihedral angle is the angle between two intersecting planes. It is the angle between planes through two sets of three atoms, having two atoms in common.

A torsion angle is defined as a particular example of a dihedral angle, describing the geometric relation of two parts of a molecule joined by a chemical bond. Every set of three not-colinear atoms of a molecule defines a plane. When two such planes intersect (i.e., a set of four consecutively-bonded atoms), the angle between them is a dihedral angle.

Configuration names according to dihedral angle

syn n-Butane Newman projection

syn n-Butane sawhorse projection