L35-Hybridization

1-1

Specific • By the end of this Lecture Objectives the student, should be able to: • Define the general concepts of The hybridization. Hybridization• List the important characteristics of hybridization. • List the types of hybridization (sp, sp2s,p3). • Distinguish between (sp, sp2s,p3) • Classify Sp, Sp2, and Sp3 –molecules. • Apply the hybridization on the molecules Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the description of atomic bonding properties. very useful in the explanation of the shape of molecular orbitals for molecules bonding. (i) The number of hybridized orbitals formed is equal to the number of orbitals hybridized.

(ii) The hybridized orbitals are always equivalent in energy and shape. (iii) The hybrid orbitals are more effective in forming stable bonds than the pure atomic orbitals.

(iv) The hybrid orbitals are directed in space (preferred directions to have stable arrangements) .Therefore, the type of hybridization gives the geometry of the molecule. Depending upon the different combinations of - and three -orbitals, three types of hybridizations are know

1- ( -hybridized). 2- ( -hybridized). 3- ( -hybridized). In this case, one - and three -orbitals hybridize to form four hybrid orbitals.

These four -hybrid orbitals are oriented in a tetrahedral arrangement. The common example of molecule involving - hybridisation is methane (CH4). Therefore, CH4 has tetrahedral geometry and HCH bond angle is 109.5o. For e.g., in the formation of CH4 molecule, one C-H bond will be formed by the overlapping of 2s-orbital of C and 1s-orbital of H . whereas the other three C- H bonds will be formed by the overlapping of 2p- orbitals of C and 1s-orbital of H. Therefore, all the bonds will not be equivalent.

Methane The carbon atom is sp3 hybridized to obtain tetrahedral geometry. 3 All bond angles Hsp H H are equal at 109.50. C H 109.50

E ______p ______Promote e- Mix orbitals ______sp3 __ s __ Hybridization of Atomic Orbitals 1s + 3P

s + 3 p Combination of 4 sp3 orbital's

Each sp orbital possesses 25% s character and 75% p character 4 sp3 orbitals Sigma Bond (s) – covalent bond that is symmetric about the bond axis.

1-14 hybrid orbitals – sp, sp2, or sp3

formation of s bond In this case, one - and two -orbitals hybridize to form three hybrid orbitals. These three hybrid orbitals are oriented in a trigonal planar arrangement, 120 angles

Take, for example, ethene (C2H4). Ethene has a double bond between the carbons. In sp2 hybridization the 2s orbital is mixed with only two of the three available 2p orbitals two carbon atoms form a σ- bond by overlapping two sp2 - orbitals and each carbon atom forms two covalent bonds with hydrogen by s–sp2 overlap all with 120° angles. The π-bond between the carbon atoms perpendicular to the molecular plane is formed by 2p–2p overlap. The hydrogen-carbon bonds are all of equal strength and length, which agrees with experimental data. Ethelene

The carbon atom is sp2 hybridized to obtain sp2 sp2 H H trigonal planar C C 116.6o geometry H H 121.7o

E ______p ______p Promote e- Mix orbitals ______sp2 hydrid __ s __ Hybridization of Atomic Orbitals 1s + 2P

s + 2 p Combination of 3 sp2 orbitals

Each sp orbital possesses 33% s character and 67% 2 3 sp orbitals p character Orbital Picture of Ethelene

1-25 This involves the mixing of one - and one - orbital forming two -hybrid orbitals.

The two hybrid orbitals are oriented in a linear arrangement and bond angle is 180°. in (ethyne) (C2H2) consists of sp–sp overlap between the two carbon atoms forming a σ- bond and two additional π bonds formed by p–p overlap. Each carbon also bonds to hydrogen in a sigma s–sp overlap at 180° In this model, the 2s orbital mixes with only one of the three p-orbitals resulting in two sp orbitals and two remaining unchanged p orbitals. Hybridization of Atomic Orbitals 1s + 1p a single atom mix to form new, hybrid orbitals. Acetylene These hybrid orbitals have characteristics of sp sp H C C H both s and p orbitals. 1 2

180o

E ______p promote e- ______Mix orbitals __ __ p __ __ sp hydrid __ s __ Hybridization of Atomic Orbitals 1s + 1P

s + p = sp + sp

Remember, there are two p orbital's

leftover and these would be z located on the y and z axes.

Each sp orbital possesses 50% s x y character and 50% p character. Combination of 2 sp orbitals Orbital Picture of Acetylene In this case, one - and two -orbitals hybridize to form three hybrid orbitals. These three hybrid orbitals are oriented in a trigonal planar arrangement, 120 angles

Ethelene

The carbon atom is sp2 hybridized to obtain sp2 sp2 H H trigonal planar C C 116.6o geometry H H 121.7o

E ______p ______p Promote e- Mix orbitals ______sp2 hydrid __ s __ Hybridization of Atomic Orbitals 1s + 2P

s + 2 p Combination of 3 sp2 orbitals

3 sp2 orbitals Orbital Picture of Ethelene

1-39 s bond

remaining p orbitals from sp or sp2

p bond hinders rotation Planar molecule (each carbon is about the carbon-to-carbon trigonal planar) with p cloud above bond and below the plane

formation of p bond 1s

First, review the shapes of the hydrogen-like orbitals. 2s

First, review the shapes of the hydrogen-like orbitals. 2px

First, review the shapes of the hydrogen-like orbitals. 2py

First, review the shapes of the hydrogen-like orbitals. 2pz

First, review the shapes of the hydrogen-like orbitals. 2s + 2px + 2py + 2pz

Here are all of the n=2 level orbitals. The problem: these do not point directly towards the surrounding atoms (e.g. for tetrahedral, trigonal planar, or linear molecules) so it is not easy to imagine adding these to make bonds. Ethane H H

C C H H H H

For example, consider the bonding in ethane. Each carbon atom has a tetrahedral geometry, but the orbitals s, px, py, and pz do not have a tetrahedral geometry. s + px + py + pz

To to solve the problem of orbitals pointing in the wrong direction, we will hybridize: combine all four of them and get four new orbitals … sp3 + sp3 + sp3 + sp3

Because we combined the s orbital and all three p orbitals, we call these new orbitals “sp3 orbitals”. There are four of them, each pointing towards a corner of a tetrahedron, exactly where we want them.

Each of the C-H bonds in ethane, CH3CH3, can be described as the combination of a carbon sp3 and a hydrogen 1s orbital. These are cylindrically symmetrical, and are called sigma bonds (s) Copyright © 2003 Charles B. Abrams C1sp3 + C2sp3

The resulting combination is cylindrically symmetrical, and is therefore called a sigma (s) bond. Because it is cylindrically symmetrical, this bond can rotate without changing the overlap between the two sp3 orbitals Copyright © 2003 Charles B. Abrams The next few slides show that rotating around the C-C bond in ethane does not change the overlap of the two C sp3 orbitals, and therefore does not change the bond in any way. Copyright © 2003 Charles B. Abrams (Rotated 30 degrees)

Consider the bonding in ethene (also known as ethylene). The carbon atoms have a triangular planar geometry.

Here again are the four n=2 orbitals. These orbitals are not in a triangular planar arrangment.. However, they can be hybridized by combining the s orbital with only two of the p orbitals.

These hybrid orbitals are called sp2 orbitals. The complete set of orbitals available for bonding now includes three sp2 orbitals and the p orbital which was not involved in the hybridization.

Each of the C-H bonds ethene, CH2=CH2, can be described as the combination of a carbon sp2 and a hydrogen 1s orbital. These are cylindrically symmetrical sigma bonds (s)

There are two bonds between the carbon atoms. One of these can be described as the combination of an sp2 orbital on one carbon with an sp2 orbital on the other carbon. Copyright © 2003 Charles B. Abrams s=C1sp2 + C2sp2

The resulting combination is cylindrically symmetrical, and is therefore called a sigma (s) bond. Before we can decide if this bond can rotate, we must consider the other bond in ethene.

The second bond between the carbon atoms can be described as the combination of the p orbital on one carbon with the p orbital on the other carbon. These two p orbitals are parallel and therefore have good overlap.

The resulting combination is not cylindrically symmetrical; instead it has a plane of symmetry. It is called a pi (p) bond. The p orbitals can only overlap if they are parallel. This bond can not rotate.

(Rotated 90 degrees.) The p orbital on carbon 2 now does not overlap with the p orbital on carbon 1. Because the p orbital now points in a different direction, it is labeled with a different Cartesian coordinate. No pi bond can form.

The carbon-carbon double bond in ethene can be described as one sigma and one pi bond. The pi bond prevents the double bond from rotating. All of the atoms (H and C) are in one plane, so this is a planar molecule.

H C C H

Now consider the bonding in ethyne (also called acetylene).

Here again are the four n=2 orbitals. The geometry around the carbons in acetylene is linear. There is a way to hybridize these so they point in a line. We combine the s orbital with only one of the p orbitals.

These hybrid orbitals are called sp orbitals. The complete set of orbitals available for bonding now includes these two sp orbitals and the two p orbital which were not involved in the hybridization. Copyright © 2003 Charles B. Abrams C1sp + C2sp

One bond between the carbons is described as the combination of the carbon 1 sp orbital plus the carbon 2 sp orbital.

This is called a sigma bond, as in the previous examples.

There are now two sets of p orbitals which can be combined. The py orbitals on each carbon can combine to form one pi bond…

This is one of the pi bonds in ethyne.

The other set of p orbitals can also combine to form a second pi bond.

Here is another pi bond in ethyne.

p s

The triple bond in ethyne (acetylene) is described as one sigma bond and two pi bonds.

Bond length and bond strength

MOLECULAR GEOMETRY

1-85