CEB1013 ORGANIC CHEMISTRY
THE BASIC:
Electron Configuration, Chemical
Structures & Formal Charge
Course Learning Outcome:
CLO1: Describe the electronic structure, bonding and shape of the various
functional groups and extrapolate these to describe the origins of reactivity
of organic compounds
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LEARNING OUTCOME
By end of this lecture, student should be able to
1. Write electron configuration for any given atom.
2. Draw chemical structures of organic compounds and its derivatives
using several methods.
3. Calculate formal charges of each atom in the chemical compounds,
particularly organic compounds.
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PERIODIC TABLE
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ELECTRON CONFIGURATION
How the electron configures themselves in electron cloud?
Bohr diagrams indicate how many electrons fill each principal shell
4
ELECTRON CONFIGURATION (C
ONT.)
How the electron configures themselves in electron cloud?
5
ISOTOPES – THE “ TWIN’
▪ Each of two or more forms of the same element that contain equal numbers of
protons but different numbers of neutrons in their nuclei, and hence differ in
relative atomic mass but not in chemical properties; in particular, a radioactive
form of an element.
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VALENCE ELECTRONS
▪ The outermost electrons.
▪ The presence of valence electrons can determine the element's chemical
properties and whether it may bond with other elements.
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CHEMICAL BONDS
What
does
the
have to do with
electron configuration
chemical bonds?
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IONIC BONDS
THE OCTET RULE
▪ This rule states that the elements lose or gain electrons in order to achieve
noble gas configurations.
▪ Atoms tend to combine in such a way that they have eight electrons in their
valence shells, giving them the same electron configuration as (the nearest)
Noble gas.
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IONIC BONDS (C
ONT.)
“The Giver”
▪ To reach noble gas configuration, sodium has to either gain 7 electrons OR lost
one electron.
▪ It is easier for sodium to release one electron than to add 7 electrons to its
orbital.
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IONIC BONDS (C
ONT.)
“The Receiver”
?
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IONIC BONDS (C
ONT.)
▪ Sodium and fluorine undergoing ionic bonds to form sodium fluoride.
▪ Sodium loses its outer electron to give it a stable electron configuration, and
this electron enters the fluorine atom exothermically.
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IONIC BONDS (C
ONT.)
ELECTRONEGATIVITY
▪ Electronegativity is a measure of the ability of an atom to attract electrons – the
higher the electronegativity, the higher its ability to attract electrons.
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COVALENT BONDS
Does carbon give OR receive electrons?
Carbon atoms have the ability to form huge network…
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COVALENT BONDS (C
ONT.)
How the electron configures themselves in electron cloud?
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COVALENT BONDS (C
ONT.)
▪ Carbon like to “share” electrons – known as covalent bond.
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COVALENT BONDS (C
ONT.)
▪ Covalent bonds form by sharing of electrons between atoms of similar
electronegativities to achieve the configuration of noble gas.
1. Duplet rule, only for covalent bond involving hydrogen.
2. Octet rule, for (almost) other atoms.
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EXCEPTION TO OCTET RULE
▪ Molecules containing atoms of Group 3A elements, particularly boron and
aluminum
B
Boron trifluoride
: Cl
Al
: Cl :
:
:
:F:
: Cl :
: :
: :
:F
:
:
: F:
6 electrons in the
valence shells of boron
and aluminum
Aluminum chloride
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EXCEPTION TO OCTET RULE (C
ONT.)
▪ Molecules Atoms of third-period elements have 3d orbitals and may expand their
valence shells to contain more than 8 electrons
▪ Phosphorus may have up to 10
:
Cl :
:
H- O-P- O-H
Cl :
: Cl
:
P
:O:
: :
O-H
:
Trimethylphosphine
: Cl
: : : :
CH3
: : : :
:
CH3 -P- CH3
: Cl :
Phosphorus
pentachloride
Phosphoric
acid
▪ sulfur, another third-period element, forms compounds in which its valence shell
contains 8, 10, or 12 electrons
H-O- S-O-H
:
CH 3 -S-CH 3
:O :
Hydrogen
sulfide
Dimethyl
sulfoxide
Sulfuric
acid
Did you
know?
:
:
:
:
H-S- H
:
: O:
:
: O:
Compounds that do NOT follow
octet rule are very reactive. It will
react to form more stable
compound that follow octet rule
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LEWIS STRUCTURE - ATOM
Lewis dot structure: The symbol of an element represents the nucleus and all inner
shell electrons dots represent valence electrons
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LEWIS STRUCTURE - BOND
Did you
know?
Atoms bond together so that each atom acquires an electron configuration the
same as that of the noble gas nearest it in atomic number
Hydrogen do NOT follow octet rule
due to its small size (and its orbital).
Instead of octet, it goes duplet.
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LEWIS STRUCTURE – BOND (C
ONT.)
The number of shared pairs
▪ one shared pair forms a single bond
▪ two shared pairs form a double bond
▪ three shared pairs form a triple bond
Did you know?
In neutral molecules
▪ hydrogen has one bond
▪ carbon has 4 bonds and no lone pairs
▪ nitrogen has 3 bonds and 1 lone pair
▪ oxygen has 2 bonds and 2 lone pairs
▪ halogens have 1 bond and 3 lone pairs
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LEWIS STRUCTURE – BOND (C
ONT.)
For more complicated molecules and molecular ions, it is helpful to follow the
step-by-step procedure outlined here:
1. Determine the total number of valence (outer shell) electrons. For cations,
subtract one electron for each positive charge. For anions, add one electron
for each negative charge.
2. Draw a skeleton structure of the molecule or ion, arranging the atoms around
a central atom. (Generally, the least electronegative element should be
placed in the center.) Connect each atom to the central atom with a single
bond (one electron pair).
3. Distribute the remaining electrons as lone pairs on the terminal atoms
(except hydrogen), completing an octet around each atom.
4. Place all remaining electrons on the central atom.
5. Rearrange the electrons of the outer atoms to make multiple bonds with the
central atom in order to obtain octets wherever possible.
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STRUCTURAL FORMULA OF ORGANIC COMPOUNDS
Condensed
VS
Skeletal
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CONDENSED STRUCTURES
▪ Condensed structures are most often used for compounds having a chain of
atoms bonded together, rather than a ring.
▪ The following conventions are used:
1. All of the atoms are drawn in, but the two-electron bond lines are generally
omitted.
2. Atoms are usually drawn next to the atoms to which they are bonded.
3. Parentheses are used around similar groups bonded to the same atom.
4. Lone pairs are omitted.
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CONDENSED STRUCTURES (C
ONT.)
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CONDENSED STRUCTURES (C
ONT.)
▪ Condensed structures containing heteroatom
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SKELETAL STRUCTURES
▪ Skeletal structures are used for organic compounds containing both rings and
chains of atoms.
▪ Three important rules are used to draw them:
1. Assume there is a carbon atom at the junction of any two lines or at the end
of any line.
2. Assume there are enough hydrogens around each carbon to make it
tetravalent.
3. Draw in all heteroatoms and the hydrogens directly bonded to them.
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SKELETAL STRUCTURES (C
ONT.)
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SKELETAL STRUCTURES (C
ONT.)
Take care in interpreting skeletal structures for positively and negatively charged
carbon atoms, because both the hydrogen atoms and the lone pairs are omitted.
Keep in the mind the following
▪ A charge on a carbon atom takes the place of one hydrogen atom.
▪ The charge determines the number of lone pairs. Negatively charged carbon
atoms have one lone pair and positively charged carbon atoms have none.
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SKELETAL STRUCTURES (C
ONT.)
Skeletal structures often leave out lone pairs on heteroatoms, but don't forget
about them.
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CLASS ACTIVITY 1.1
1. Convert each skeletal structure to a Lewis structure.
e.
h.
f.
i.
j.
g.
k.
2. Write the condensed structures for compounds 1a to 1k.
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CLASS ACTIVITY 1.1
2. Convert each molecule into a skeletal structure
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FORMAL CHARGES
▪ Formal charge: the charge on an atom in a molecule or a polyatomic ion
▪ To derive formal charge
1. Write a correct Lewis structure for the molecule or ion
2. Assign each atom all its unshared (nonbonding) electrons and one-half its
shared (bonding) electrons
3. Compare this number with the number of valence electrons in the neutral,
unbonded atom
Or FC = Ve – (B+D)
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FORMAL CHARGES (C
ONT.)
▪ Formal Charge Observed with Common Bonding Patterns for C, N, and O
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CLASS ACTIVITY 1.2
Calculate the formal charge for each structure.
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WHAT HAVE YOU LEARNED?
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