Chapter 1 – Electronic Structure and Covalent Bonding

Section 1.6 – 1.14

Why do atoms form covalent bonds? The answer is STABILITY

Consider the Hydrogen molecule, H2.

As the two orbitals start to overlap to form the covalent bond, energy is released (and stability increases) because the electron in each atom is attracted to its own nucleus and to the positive charge of nucleus of other atom

Bond Strength is given by the standard bond dissociation energy (ΔHo or BDE). It is defined as the energy required breaking a covalent bond in a hemolytic cleavage.

The greater the electron density in the region of orbital overlap, the stronger the bond

Two special names for covalent bonds of Organic molecules

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Sigma (σ) bonds

Created when “head on” overlap occurs of orbital

Pi (π) bonds

Created when “side on” overlaps occurs of orbital (p orbitals)

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Pi bonds are usually weaker than sigma bon. From the perspective of quantum mechanics, this bond's weakness is explained by significantly less overlap between the component p-orbitals due to their parallel orientation. This is contrasted by sigma bond which form bonding orbitals directly between the nucleus of the bonding atoms, resulting in greater overlap and a strong sigma bond.

The Hybridization Model for Atoms in Molecules

If the four hydrogen atoms in a methane molecule (CH4) were bound to the three 2p orbitals and the 2s orbital of the carbon atom, what would be the angle about H-C-H?

The following molecules provide examples of all three basic shapes found in organic chemistry. In these drawings, a simple line indicates a bond in the plan of the paper, a wedged line indicates a bond coming out in front of the page and a dashed line indicated a bond projecting behind the page

Hybridization describes the mixing of atomic orbitals to form special orbital for bonding. In organic chemistry, our orbital mixtures will be simple combinations of valence electrons in the 2s and 2p orbital on a single carbon atom. We will mix these orbitals three ways to generate the three common shape of organic chemistry: linear (2s + 2p), trigonal planar (2s+2p+2p) and tetrahedral (2s+2p+2p+2p)

The sp3 Hybridization

-Mixing 2s and all 2p atomic orbital

-Four sp3 hybridized orbitals equal in size, energy and shape

-Responsible for sigma bond ( single bond)

-Tetrahedral shape

-Electrons in sp3 are held more tightly than electrons in 2p orbital, but less tightly in 2s orbital

Consider methane (CH4)

3-D representation of methane (CH4)

Bonding to O and N

Like Carbon, O and N can participate in single bond and multiple bonds compose of σ and π

*Note: the lone pair or non bonding e- pair occupies space just as bonded atom

E.g Indicate the hybridization at carbon and oxygen, the angle at H-C-O and C-O-H. Then draw a bonding picture forthe formation of sigma in CH3OH typo

The sp2 Hybridization

-Mixing 2s and two 2p atomic orbitals

-Three sp2hybridized orbitals equal in size, energy and shape

-Responsible for σ bond ( single bond)

-One π bond (double bond)

-Trigonal planar shape

Consider ethane (C2H4)

3-D representation of ethane (C2H4)

E.g Assign hybridization for all carbon atoms and identify the angle at C-C-C. Then draw bonding pictures for the σ and π framework.

The sp Hybridization

-Mixing 2s and one 2p atomic orbitals

-Two sp2 hybridized orbitals equal in size, energy and shape

-one σ bond ( single bond)

-Two π bond (triple bond)

-Linear

Consider ethyne (C2H2)

3-D representation of ethyne (C2H2).

Hybridization for radical carbon and carbon with charges

E.g Predict the hybridization, geometry, and bond angle for the carbon and nitrogen atoms in acetonitrile (). Draw a bonding picture for the formation of pi bond(s)

E.g Predict the hybridization, geometry, and bond angle for the carbon and oxygen atoms in acetaldehyde (CH3CHO)

E.g Predict the hybridization, geometry, and bond angle for the carbon and nitrogen for

E.g Show a bonding picture (both sigma and pi framework) for CH3NH2

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