CHM102 HYBRIDIZATION

CHM 102 Organic Chemistry

STRUCTURAL THEORY: HYBRIDIZATION AND BONDING

INTRODUCTION

organic chemistry is like a jungle. It looks complex, but with a guide, it is navigable. Our guide to understanding organic chemistry, lies in our ability to visualize the molecules we study. There must be principles we can apply, to aid our visualization. This principles are contained in the STRUCTURAL THEORY. This theory is the framework of ideas of how molecules are put together. It talks about the order or arrangement of the components of molecules, the electrons and bonds involved, as well as the shapes and sizes of the resulting molecules. Molecules are often represented by pictures or models. Considering the vast number of organic molecules, we can surely say that the structural theory is light. Today we are gonna talk about the concept of HYBRIDIZATION, with respect to the orbitals involved, and how we can apply the structural Theory to aid our understanding of this topic. Enjoy!

ATOMIC ORBITALS

A positively charged nucleus is surrounded by electrons at different energy levels called SHELLS. The shells are not physical boundaries, but they are more like regions. Each energy level has a particular amount of energy, and electrons can be promoted (or demoted) to higher energy levels, when they get 'excited'. The lowest energy level is called the GROUND STATE.

'The region in space where an electron is likely to be found, is called an ORBITAL. There are different kinds of orbitals of different shapes and sizes. The particular kind of orbital an electron occupies, depends on the energy content of the election (I.e how much energy it has). The ORBITALS consist of electrons arranged in space such that they appear CLOUD like. An Orbital has no physical boundary.

'S' Orbital:- This orbital is a SPHERICAL shaped orbital, with a spherical cloud of electrons, surrounding the nucleus, that is at the centre of the sphere. The S orbitals vary depending on the atom involved, the lowest being the 1S and 2S ORBITALS respectively. The 1S orbital is at the lowest energy level. The 2S orbital is next in line, at the next higher energy level. It is naturally larger than the 1S orbital.

'P' Orbital:- The next orbitals are called the P orbitals. The P orbitals have three sub-orbitals (Px, Py and Pz). They all have the same energy. Each P-suborbital is DUMBBELL SHAPED. The axis of each P-suborbital is perpendicular to the other two. The lowest P-orbital is the 2P orbital. It has 2Px, 2Py, and 2Pz sub-orbitals.

MOLECULAR (BOND) ORBITALS

The Pauli exclusion principle states that 'only two electrons can occupy the same atomic orbital, and to do so the two electrons must have opposite spins'. Hund's rule of maximum multiplicity makes it known to us, that electrons enter an orbital singly first, before pairing up. This means some orbitals may end up with unpaired electrons. Isolated atoms may then contain these orbitals. When two of such atoms come together, there will be interactions. If a covalent bond forms, then these unpaired electrons can be said to be found in 'localised molecular orbitals' . Remember, that covalent bonding involves sharing of electron pairs, such that both electrons can revolve round the nuclei of both atoms. Using this idea, the region in space where these electrons revolve, is called a BOND ORBITAL.

The two electrons that occupy a bond orbital, must have opposite spins (paired) and they belong to both nuclei. To simplify this analogy, we make two assumptions

(a) two electrons are in between two nuclei;

(b) The shapes of these bond orbitals are related in a simple way to the atomic orbitals.

Bond formation, causes overlapping of the involved orbitals. For a bond to form, the two atoms must be brought close enough for their orbital to overlap. Overlapping is kind of merging together. Imagine this, two bubbles merging to become one larger bubble. That's what we mean. Taking hydrogen for example, an atom has a 1S orbital. When a molecule of hydrogen is formed, the S orbitals overlap, to form a bond (fig 1). The bond orbital is 'sausage' shaped. Bond orbitals having this shape are called SIGMA BOND ORBITALS, and bonds of this kind are called SIGMA BONDS.

HYBRIDIZATION

Hybridization is the process of mixing two or more different things together, to form something new. Let's consider BeCl2 beryllium chloride. Be has no unpaired electrons. How do we account for its covalent bond? (Be molecules are mostly covalent in nature) . This brings us to these PRINCIPLES of hybridization.

1) Promotion of electrons usually occurs

2) hybrid orbitals are formed by mixing two or more orbitals

SP- hybridization

Let's take a look at the electronic configuration of beryllium. Be.....1S² 2S². First we promote one 2S electron to a higher energy level—2Px. So we can have 2 unpaired electrons (1S² 2S¹ 2Px¹). Next, we HYBRIDIZE The 2S and 2P orbitals. The mixing of these orbitals, results in the formation of TWO SP-hybrid orbitals (1+1=2 they are called SP because its one 'S' orbital and one 'P' orbital that come together). The shape is given in fig 2 below. The chlorine atoms form bonds with these orbitals.The arrangement of the hybrids is such that the maximum bond strength is achieved. This occurs when they are allowed to get as far away from each other as possible (maximum repulsion). This allows high stability. The angle formed between the hybrids is thus 180° (extreme left and right). This is known as The BOND ANGLE of SP hybridized bonds.

'SP²'-Hybridization

Let's consider another molecule, BF3 Boron trifloride. Here, Boron has only one unpaired electron, in the 2Px suborbital. B.....1S² 2S² 2Px¹.. First we promote one 2S electron to a higher energy level orbital — 2Py. Then we have three unpaired electrons, (1S² 2S¹ 2Px¹ 2Py¹). We can then hybridize these orbitals. This results In the formation of three SP²-Hybrid orbitals. (1+1+1=3. They are called SP² because we combine 1 'S' orbital and 2 'P' orbitals).The fluorine atoms form bonds with these hybrid orbitals. Similarly, the arrangement is such that the highest bond strength is achieved. This allows for high stability (maximum repulsion), and the resulting bond angle is 120°. The resulting molecule is TRIGONAL IN SHAPE.

'SP³'-Hybridization

Let's consider yet another molecule, methane CH4. Carbon has two unpaired electrons. C...1S² 2S² 2Px¹ 2Py¹. First we promote one 2S electron to a higher energy level— 2Pz. Then we have four unpaired electrons. (1S² 2S¹ 2Px¹ 2Py¹ 2Pz¹). We can then hybridize these orbitals. This results in the formation of four SP³-Hybrid orbitals. (1+3=4. they are called SP³ because its one 'S' orbital and three 'P' orbitals that come together) The hydrogen atoms form bonds with these hybrid orbitals. Here as well, the arrangement is such that the highest bond strength is achieved. This allows for high stability (maximum repulsion), and the resulting bond angle is 109.5° (similar to a rectangular based pyramid or TETRAHEDRON). The resulting molecule is therefore TETRAHEDRAL IN SHAPE.

Hybridization occurs in covalent compounds, of which organic chemistry compounds are not exempted. Hope this material was useful. Got any questions? Send us an e-mail. Click search to find topical keywords. Got materials you'd like to share or articles you'd like to submit? Send us an e-mail.

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