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Shapes and Structures of Organic Molecules HYBRIDISATION 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. What is Hybridization? 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. Rules for Observing the Type of Hybridisation The following rules are observed to understand the type of hybridisation in a compound. 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 3) sp – Hybridisation • In such hybridisation one s- and three p-orbitals are 3 mixed to form four sp – 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 . sp3 HYBRIDISATION If we superimpose one s and three p atomic orbitals we get 4sp3 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 HYBRIDISATION If we superimpose one s and two p atomic orbitals we get 3sp2 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 2 Consider two sp 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 (H2O), which has an angle of about 105°. A water molecule has two pairs of bonded electrons and two unshared lone pairs. Tetrahedral: Tetra- signifies four, and -hedral relates to a face of a solid, so "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) is a tetrahedral molecule. Octahedral: Octa- signifies eight, and -hedral relates to a face of a solid, so "octahedral" means "having eight faces". The bond angle is 90 degrees. For example, sulfur hexafluoride (SF6) is an octahedral molecule. Trigonal pyramidal: A trigonal pyramidal molecule has a 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 Ideal bond Electron bonded angle Lone domains to Shape (example's Example Image pairs (Steric central bond number) atom angle) 2 0 2 linear 180° CO2 Atoms Ideal bond Electron bonded angle Lone domains to Shape (example's Example Image pairs (Steric central bond number) atom angle) trigonal 3 0 3 120° BF3 planar 2 1 3 angular 120° (119°) SO2 4 0 4 tetrahedral 109.5° CH4 trigonal 3 1 4 109.5 (107.8°) NH3 pyramidal 109.5° 2 2 4 angular [10][11] H2O (104.48°) trigonal 5 0 5 90°, 120° PCl5 bipyramidal ax–ax 180° (173.1°), 4 1 5 seesaw eq–eq 120° SF4 (101.6°), ax–eq 90° 90° (87.5°), 3 2 5 T-shaped ClF3 180° (175°) 2 3 5 linear 180° XeF2 Atoms Ideal bond Electron bonded angle Lone domains to Shape (example's Example Image pairs (Steric central bond number) atom angle) 6 0 6 octahedral 90°, 180° SF6 square 5 1 6 90° (84.8°) BrF5 pyramidal square 4 2 6 90°, 180° XeF4 planar 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 syn n-Butane syn n-Butane according to dihedral Newman projection sawhorse projection angle .
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