This document provides an overview of organic chemistry. It discusses how organic chemistry originated from distinguishing between compounds from living versus non-living sources. It then defines organic chemistry as the study of carbon compounds, as carbon can form diverse structures through its ability to form four strong covalent bonds. The document proceeds to discuss atomic structure, bonding theory including hybridization, and provides examples of sp3 and sp2 hybridization in molecules like ethane and ethene.

1. Bonding, Structure & Reactivity
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UNIT 1
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2. 1.1. Historical Background & Definition of Organic Chemistry
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What is organic chemistry?
Why should you study organic chemistry?
➢ The foundations of organic chemistry date from the mid-1700s, when chemistry was
evolving from an alchemist’s art into a modern science.
✓ At that time, unexplainable differences were noted between substances obtained from living
sources (‘‘organic’’ substances) and those obtained from minerals (‘‘inorganic’’ substances).
• Following this, the term organic chemistry soon come to mean the chemistry of compounds
found in living organisms.
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➢ In fact, every living organism is made of organic chemicals.
✓ proteins that make up your hair, skin and muscles, DNA that controls your genetic heritage,
foods, medicines and so on are some examples.
✓ Wohler’s synthesis of urea (1828), began to undermine vitalism
➢ But today, Chemistry is unified and the same principles explain the behaviors of all
substances regardless of origin or complexity.
➢ The only distinguishing characteristic of organic chemicals is that all contain the element
carbon.
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➢ Then today, Organic chemistry is defined as the study of carbon compounds.
➢ In fact, more than 30 million presently known chemical cpds, do more than 99% of them
contain carbon mainly.
Why? -because carbon is special
Why is carbon special?
➢ The answers to this question come from carbon’s electronic structure and its consequent
position in the periodic table.
✓ Carbon is a group 4A element.
• Can share four valence electrons
• Can form four strong covalent bonds.

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✓Carbon atoms can bond to one another, forming long chains & rings.
✓Carbon, alone of all elements, is able to form an immense diversity of
compounds from the simple (methane,CH4, with one carbon atom) to DNA,
which can have more than 100 hundred million carbons
Note: Not all carbon compounds are derived from living organisms.
✓Medicines, dyes, polymers, food additives, pesticides, and a host of other
substances are now prepared in the laboratory.

6. 1.2. Atomic Structure of Carbon, Covalent Bond & Hybridization
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➢ An atom consists of a dense, positively charged nucleus surrounded at a relatively
large distance by negatively charged electrons.
✓ The nucleus consists of subatomic particles called neutrons, which are electrically
neutral, and protons, which are positively charged.
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➢ An atomic orbital
✓ represents the region of space around a nucleus where an electron spends most (90-
95%) of its time.
➢ There are four different kinds of orbitals, denoted s, p, d & f, each with a different
shape.
✓ Of the four, s and p orbitals are the most common in organic and biological chemistry.
✓ The s orbitals are spherical, with the nucleus at their center.
✓ The p orbitals are dumbbell-shaped.
• The two lobes of the p orbitals are separated by a region of zero electron density
called a node.
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➢The orbitals in an atom are organized into different layers or electron shells of
successively larger size and energy.
➢Different shells contain different numbers and kinds of orbitals, and each
orbital within a shell can be occupied by two electrons.
✓ The first shell contains only a single s orbital, denoted 1s, and thus holds only two
electrons.
✓ The second shell contains one 2s orbital and three 2p orbitals and thus holds a total of 8
electrons.
✓ The third shell contains one 3s orbital, three 3p orbitals, and five 3d orbitals, for a total
capacity of 18 electrons.
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 The three different p orbitals within a given shell are oriented in space along
mutually perpendicular directions, denoted px, py & pz.
➢ Carbon bonds to other atoms, not by gaining or losing electrons, but by sharing them.
 Such a shared-electron bond is called a covalent bond.
➢ The number of covalent bonds an atom forms depends on how many additional
valence electrons it needs to reach a noble-gas configuration.

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 Hydrogen has one valence electron (1s) and needs one more to reach the helium
configuration (1s²), so it forms one bond.
 Carbon has four valence electrons (2s² 2p²) and needs four more to reach the neon
configuration (2s² 2p6), so it forms four bonds.
 Nitrogen has five valence electrons (2s² 2p³), needs three more, and forms three
bonds.
 Oxygen has six valence electrons (2s² 2p4), needs two more, and forms two bonds.
 Halogens (F) have seven valence electrons, need one more, and form one bond.
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➢ Valence electrons that are not used for bonding are called lone-pair electrons or nonbonding
electrons.
 For instance, the nitrogen atom in ammonia shares six valence electrons in three covalent
bonds and has its remaining two valence electrons in a nonbonding lone pair.
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How does electron sharing lead to bonding between atoms?
➢ Two models have been developed to describe covalent bonding:
1. Valence bond theory
2. Molecular orbital theory
➢ Valence bond theory is the more easily visualized of the two.
➢ According to valence bond theory,
▪ A covalent bond forms when two atoms approach each other closely and a singly occupied
orbital on one atom overlaps a singly occupied orbital on the other atom.
✓The electrons are now paired in the overlapping orbitals and are attracted to the nuclei of
both atoms, thus bonding the atoms together.
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For example,
➢ In the H2 molecule, the H-H bond results from the overlap of two singly occupied
hydrogen 1s orbitals.
✓ Such bonds, which are formed by the head-on overlap of two atomic orbitals along a line
drawn between the nuclei, are called sigma (σ) bonds.
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➢ The bonding in the hydrogen molecule is fairly straightforward, but the situation is
more complicated in organic molecules with tetravalent carbon atoms.
➢ Carbon uses four valence electrons (2s² 2p²) from two kinds of orbitals (s and p) for
bonding.
➢ The s orbital and p orbitals on the carbon atom can combine mathematically or
hybridize to form equivalent atomic orbitals.
➢ The carbon atom can form three types of hybrid orbitals for bonding.
1. sp³ hybrid orbitals
2. sp² hybrid orbitals
3. sp hybrid orbitals
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15. A. sp³ Hybrid Orbitals
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➢ It is an atomic orbitals formed by the combination or hybridization of the one 2s and three
2p orbitals.
For instance,
❖ Ethane (H3C-CH3)
✓ The s orbital and three p orbitals on the carbon atom of ethane hybridize to form four equivalent sp³
hybrid orbitals
✓ The two carbon atoms bond to each other by σ overlap of an sp³ hybrid orbital from each. The
remaining three sp³ hybrid orbitals of each carbon overlap with the 1s orbitals of three hydrogens to
form the six C-H bonds.
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