CHEM1010 Study Guide - Final Guide: Phenyl Group, Cyclohexane Conformation, Cyclopentane

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Organic Chemistry Summary
Organic chemistry involves the chemistry molecules containing carbon and a few other elements -
hydrogen, oxygen and nitrogen, and sometimes sulfur, phosphorus and halogens (fluorine, chlorine,
bromine and iodine).
If we want to understand how molecules react, we must begin by knowing where the outer valence
shell electrons are in the molecule.
We need to know:
1. Which ones are involved in bonds
2. Which are non-bonding
3. If electrons are more strongly associated with particular atoms in a molecule rather than
others
Once we know this, we can predict and explain how and why molecules react.
One of the main things electrons are involved is bonding: holding atoms together to form molecules
and compounds. Covalent bonding two electrons are shared equally between two atoms to form
one bond. Ionic bonding - atoms lose or gain electrons to form oppositely charged ions which attract
each other.
The number of ods a tpial orgai ato ake depends on the octet rule and the number of
valence electrons they have.
Hydrogen is often seen as H+ and
Lithium, Sodium, Beryllium and
Magnesium prefer to lose these
electrons instead of making 1 or 2
bonds to fill their outer shell.
Common rules, hydrogen will make 1
bond, oxygen will make 2 bonds,
nitrogen will make 3 bonds and
carbon will make 4 bonds.
However, in caffeine nitrogen has 3 bonds because nitrogen also has a lone pair of
electrons that completes the octet.
Most bonds in organic molecules are covalent, sharing electrons more or less equally.
In chemistry, the bonds where sharing between atoms is less equal are more
interesting because this is where molecules are most likely to react. Electronegativity
is a easure of the fore of a atos attraction for electrons that it shares in a
chemical bond with another atom.
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e.g. H3C-Mg-Br in methyl magnesium bromide the bond between Mg and C is polar covalent
Electronegativites: C = 2.5, Mg = 1.5
2.5 -1.5 = 1 therefore polar covalent
- Lewis Structures
Conventions:
Each atom is represented by its elemental symbol
Only the valence electrons appear
A line joining two elemental symbols represents one pair of electrons that is shared between
two atoms
Dots placed next to an elemental symbol represents nonbonding electrons on that atom
There are two important issues:
1. How many electrons should / can each atom have in its outer shell? How many bonds does
each atom make?
Beause ere oe ro do o the periodi tale for eleets like “ulfur ad Phosphorus their
valence electrons are further out fro uleus. Beause of this theres ore spae for ods to
occur i.e. they can expand their valence shells. e.g. in H2SO4 sulfur has 6 bonds instead of the two it
should be able to make.
When an atom donates or receives both electrons required to make a bond, they are said to be
formally charged.
e.g. for the ion AlCl4-, how do you know which atom the negative charge resides on?
Formal charge = No. of valence e- in the neutral unbonded atom (all unshared e- + (all shared e-)/2)
Al: 3 (0 + 8/2) = -1
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Cl: 7 (6+2/2) = 0
2. What is the arrangement of atoms in the molecule?
i. Write down all of the atoms in their correct relative positions and lightly sketch in which
atoms are bonded to each other
C2H6O CH3CH2OH
ii. Count all valence electrons - remember to adjust for charge (add for negative charges,
subtract for positive charges)
2 x C = 8, 6 x H = 6, 1 x O = 6 = 20
iii. Put in 2 electrons for each bond
iv. How many valence electrons are left to be distributed?
v. Starting with the most electronegative eleets, oplete the atos alee shells
until all the electrons are used up.
vi. Determine formal charges for all the atoms
There should never be identical charges on neighbouring atoms. If there are, move electrons from
negatively charged atoms to positive ones. If + and atoms appear side-by-side and the positive
ato is apale aeptig aother pair of eletros i its alee shell, eutralise the harges 
placing a pair of electrons from the negative atom between the two atoms thus making another
bond.
vii. Rewrite the structure by putting in a line for each shared pair of electrons between
atoms (bonds) and leave dots or pairs of dots (lone pairs) for unshared electrons.
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Document Summary

Organic chemistry involves the chemistry molecules containing carbon and a few other elements - hydrogen, oxygen and nitrogen, and sometimes sulfur, phosphorus and halogens (fluorine, chlorine, bromine and iodine). If we want to understand how molecules react, we must begin by knowing where the outer valence shell electrons are in the molecule. We need to know: which ones are involved in bonds, which are non-bonding. If electrons are more strongly associated with particular atoms in a molecule rather than others. Once we know this, we can predict and explain how and why molecules react. One of the main things electrons are involved is bonding: holding atoms together to form molecules and compounds. Covalent bonding two electrons are shared equally between two atoms to form one bond. Ionic bonding - atoms lose or gain electrons to form oppositely charged ions which attract each other.

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