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Title: Chemical Structure
Description: Well comprehensive notes on Chemical Structure
Description: Well comprehensive notes on Chemical Structure
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SCH 102
Chemical Structure
… structure determines properties …
Dr
...
The properties of a substance depend on
the atoms it contains and the way these atoms are
connected
...
Lewis structure,
II
...
Spatial arrangement of atoms in a molecule
(Stereochemistry)
Dr
...
It shows how valence
electrons are arranged among atoms in a molecule
...
In Lewis structures, the atomic symbol
represents the core; that is, the nucleus and all inner
shell electrons
...
Solomon Derese
79
SCH 102
There are three general rules for drawing Lewis
structures
...
Draw only the valence electrons
...
Give every second-row element no more than
eight electrons (octet rule)
...
Give each hydrogen two electrons (duet rule)
...
Dr
...
Skills in writing Lewis structures for
molecules is fundamental to the
understanding of organic chemistry
...
Solomon Derese
81
SCH 102
Let us draw the Lewis structure for HF
H has one valence electron and F has seven
...
Dr
...
The usual number of bonds and nonbonded
electron pairs of common neutral atoms are
summarized below
...
Solomon Derese
83
SCH 102
Steps of Drawing a Lewis Structure
Step 1: Arrange atoms next to each other that you
think are bonded together
...
For CH4
H
H
C
H
Dr
...
For CH5N:
4 atoms around C
3 atoms around N
H
H
Dr
...
• Count the number of valence electrons from all
atoms
...
• Add one electron for each negative charge
...
• This sum gives the total number of electrons that
must be used in drawing the Lewis structure
...
Solomon Derese
86
SCH 102
Step 3: Arrange the electrons around the atoms
...
• Use all remaining electrons to fill octets with lone
pairs
...
Step 4: Assign formal charges to all atoms
...
Solomon Derese
87
SCH 102
Example 1: Draw a Lewis structure for methane, CH4
...
H
H
Step 2: Count the total number of valence electrons
1C x 4e- = 4e4H x 1e- = 4eTotal = 8eDr
...
SCH 102
2e- s around H
H
H
C
H
Add a bond between
each C and H
H
H
H
C
H
H
8e- s around C
Adding four two-electron bonds around carbon uses
all eight valence electrons, and so there are no lone
pairs
...
Solomon Derese
89
SCH 102
To check whether a Lewis structure is valid, we must
answer YES to three questions:
• Have all the electrons been used?
• Is each H surrounded by two electrons?
• Is each second-row element surrounded by no
more than eight electrons?
The answer to all three questions is YES for methane,
so the Lewis structure drawn for CH4 is valid
...
Solomon Derese
90
SCH 102
Example 2: Draw a Lewis structure for methanol, a
compound with molecular formula
CH4O
...
Solomon Derese
91
SCH 102
Step 3: Add the bonds and lone pairs electrons
...
Add a bond between
each atoms
H
H
C
O
No octet
Add lone
H
pairs on O
H
Only 10 e- s used
Dr
...
O
...
Step 1: Arrange the atoms
H
C
C
H
H
H
Step 2: Count the total number of valence
electrons
2C x 4e- = 8e4H x 1e- = 4eTotal = 12eDr
...
No octet
...
Each C has 4 bonds
H
Dr
...
C
H
H
Move a lone
H
H
pair
C
C
H
H
H
94
SCH 102
Example 3: Draw a Lewis structure for each
compound C2H2
...
Solomon Derese
95
SCH 102
Step 3: Add the bonds and lone pairs electrons
...
Add lone
H C C
H
H C C
...
...
Solomon Derese
No octet
96
SCH 102
After placing all electrons in bonds and lone pairs, use
a lone pair to form a multiple bond if an atom does
not have an octet
...
The examples discussed above are for neutral
compounds
...
Dr
...
Formal charge is the charge assigned to individual
atoms in a Lewis structure
...
Solomon Derese
98
SCH 102
A species containing a positively charged carbon is
called a carbocation, and a species containing a
negatively charged carbon is called a carbanion
...
Carbocation
Dr
...
Formal charge = # of valence e− s − (# of nonbonding e− s +
# of bonding e- s)
For H, # of valence e− s = 1, # of nonbonding e− s = 0 and
number of bonding e− s = 2
Formal charge for H = 1 − (0 +
x2) = 0
For O, # of valence e− s = 6, # of nonbonding e− s = 2 and
number of bonding e− s = 6
Formal charge for O = 6 − (2 +
Dr
...
In this case the over all charge on the hydronium
ion is 0 + 0 + 0 + 1 = +1
...
Atom
# of valence 4e-s
C
4
N
5
O
6
Dr
...
For example, there are two acceptable
arrangements of atoms for the molecular formula
C2H6O
...
Dr
...
For example, ethanol has one C – C bond
and one O – H bond, whereas dimethyl ether has
two C – O bonds
...
Solomon Derese
103
Exceptions to the Octet Rule
SCH 102
Most of the common elements in organic
compounds—C, N, O, and the halogens—follow the
octet rule
...
Sometimes there are not enough electrons in an
atom to provide octet of electrons around the central
atom
This is the case for beryllium (Group 2A) and boron
(Group 3A)
...
Dr
...
There is
nothing we can do about this! There simply aren’t
enough electrons to form an octet
...
Because the Be and B atoms have less than an octet
of electrons, these molecules are highly reactive
...
Solomon Derese
105
SCH 102
Aluminum trichloride is an example of a compound in
which aluminum, the element immediately below
boron in Group 3A, has an incomplete valence shell
...
Solomon Derese
106
SCH 102
Another exception to the octet rule occurs with some
elements located in the third row and later in the
periodic table
...
For organic chemists, the two most common
elements in this category are phosphorus and sulfur,
which can have 10 or even 12 electrons around
them
...
Solomon Derese
107
SCH 102
8 e-s around S
10 e-s around S
Dr
...
Look at the acetate ion, CH3CO2- for instance
...
Experimental evidence however indicate the two
C-O bonds are equivalent
...
Solomon Derese
109
SCH 102
Both carbon–oxygen bonds in the acetate anion
are 127 pm in length, midway between the length
of a typical C-O single (135 pm) and a typical
C=O double bond (120 pm)
...
The Lewis structure does not depict this reality
...
Dr
...
Acetate anion
Resonance
is theory developed to reconcile
experimental data with the implications of Lewis
structures for compounds with two or more
structures
...
Solomon Derese
111
SCH 102
The individual structures are resonance contributors
to the structure of the acetate anion and it is pictured
as a resonance hybrid of these two structures
...
The actual properties of the acetate anion cannot be
represented by any one of the Lewis structures taken
alone, the experimental facts are represented by
drawing the two resonance contributors
...
Solomon Derese
112
SCH 102
The two resonance contributors taken together
indicate that each oxygen atom bears half of the
charge on an electron and that the C-O bonds are the
same length
...
Dr
...
The symbol ↔ does not mean that the two forms are
in equilibrium with each other
...
There is only one
structure for the acetate anion, which is a hybrid of
the tow structures drawn
...
Dr
...
Resonance occurs when electrons can flow through
neighboring p systems (double or triple bonds)
...
Lewis structures show electrons as being localized;
they either are shared between two atoms in a
covalent bond or are unshared electrons belonging to
a single atom
...
Solomon Derese
115
SCH 102
In reality, electrons distribute themselves in the way
that leads to their most stable arrangement
...
Writing the various Lewis formulas that contribute to
a resonance hybrid can be made easier by using
curved/curly/pushing arrows to keep track of
delocalized electrons
...
Solomon Derese
Tail
Head
Curved/Curly/Pushing arrow
116
Move an electron pair to O
SCH 102
Use this electron pair to form
a double bond
Curved arrows show the origin (tail) and destination
(head) of a pair of electrons
...
Dr
...
The nitrate and carbonate are
example of species for which a single resonance
structure are not satisfactory
...
Solomon Derese
118
SCH 102
A very important aspect of resonance structures is
that they have implications for the stability of the
molecule they represent
...
Structures for uncharged molecules may also have
resonance contributors
...
Benzene
All the C-C bonds are equivalent and due to resonance
benzene is a very stable molecule, more in SCH 202
...
Solomon Derese
119
SCH 102
The resonance contributors shown for the four
examples are equivalent to each other
...
For example two resonance
For example the two resonance structures drawn for
acetone anion are not equivalent
...
Solomon Derese
This resonance form has the
Negative charge on O
major contributor
120
SCH 102
The true structure of the acetone anion is more like that
of the form that places the negative charge on the
electronegative O atom rather than on C
...
These resonance contributors are thus more important
than and are known as major contributors
...
Dr
...
Resonance structures are written for compounds
that are not adequately described by a single Lewis
structure
...
Resonance contributors have the same connectivity
...
The electrons in
single bond are not involved
...
The nuclei of atoms in different resonance
contributors remain the same
...
Solomon Derese
122
SCH 102
4
...
When
resonance structures are identical, they are equally
important descriptions of the molecule
...
Dr
...
Resonance hybrids are more stable than any of the
Lewis structures used to describe them
...
In other words, resonance
leads to stability
...
Dr
...
Resonance forms obey normal rules of valency
...
10 e- s on this C
Not a valid resonance form
7
...
The real structure is a composite, or resonance
hybrid, of the different forms
...
Solomon Derese
125
SCH 102
Not all resonance structures contribute equally to a
resonance hybrid
...
The following preferences will help you to estimate
the relative importance of the various contributing
structures
...
Those that
follow the most preferences contribute the most to
the hybrid, and any structure that violates all four of
these preferences can be ignored
...
Solomon Derese
126
SCH 102
Preference I: Filled Valence Shells
Structures in which all atoms have filled valence shells
(completed octets) contribute more than those in
which one or more valence shells are unfilled
...
Solomon Derese
Lesser contribution:
C has only six electrons
in its valence shell
127
SCH 102
Preference 2: Maximum Number of Covalent Bonds
Structures with a greater number of covalent bonds
contribute more than those with fewer covalent
bonds
...
Solomon Derese
Lesser contribution:
seven covalent bonds
128
SCH 102
Preference 3: Least Separation of Unlike Charges
Structures that involve separation of unlike charges
contribute less than those that do not involve charge
separation because separation of charges costs
energy
...
Solomon Derese
Lesser Contribution:
separation of unlike
charges
129
SCH 102
Preference
Negative Charge on a More
Electronegative Atom
Structures that carry a negative charge on a more
electronegative atom contribute more than those
with the negative charge on a less electronegative
atom
...
Following are three
contributing structures for acetone:
Dr
...
while I contributes less because it involves separation of
charges and because carbon has an in complete octet
...
Structure III violates all
four preference rules and should not be drawn
...
Solomon Derese
131
Writing Organic Structures
SCH 102
In the structures we’ve been drawing until now, a
line between atoms has represented the two
electrons in a covalent bond
...
The most commonly used shorthand for drawing
chemical structures is skeletal structure
...
Solomon Derese
132
The rules for drawing skeletal structures
SCH 102
Rule 1
Carbon atoms aren’t shown
...
Rule 2
Hydrogen atoms bonded to carbon aren’t shown
...
Hydrogen atoms bonded to
atoms other than carbon are shown
...
Dr
...
Solomon Derese
134
Skeletal structure
SCH 102
becomes
H H Cl
H
C
H C C
C
H C
C
C C H
H
Br
HH H
Dr
Title: Chemical Structure
Description: Well comprehensive notes on Chemical Structure
Description: Well comprehensive notes on Chemical Structure