Chemistry 142
Clark College
Chapter 20: Introduction to Organic Molecules
Reading: Ch. 20, sections 1 – 5, 7 – 13, excluding reactions, chirality. Review VSEPR shapes,
hybridization, Lewis structures, and electronegativites.
Suggested problems: 6, 13, 24, 27, 35, 37, 43, 46, 53, 55, 57, 68, 70, 73, 77, 81, 82.
Bonding patterns of the first-row, p-block elements
 Based on the number of valence electrons and the number of electrons needed for an octet.
In groups, draw good Lewis structures for the following molecules. It is not necessary to provide
resonance structures. Answer the questions for each element as well.
Carbon (as the central atom)
Chloromethane CH3Cl
Carbonate ion
How many valence electrons are in C?
How many bonds are needed to complete the octet for C?
Nitrogen (as the central atom)
Ammonia
Nitrite ion
How many valence electrons are in N?
How many bonds are needed to complete the octet for N?
Oxygen: H2O
Fluorine: HF
How many valence electrons are in O? F?
How many bonds are needed to complete the octet for O? F?
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Visualizing molecules
Consider propan-2-ol (or 2-propanol, or isopropanol), C3H8O. How can we represent this molecule?
Structural formulas
Condensed formulas
Line structures
Rules of line structures:
1. One line, one bond.
2. All vertices are Carbons, unless noted.
3. The ends of a line are also Carbons, unless noted.
4. Fulfill valencies with preferred bonding patterns: C makes 4
bonds, N makes three bonds an one lone pair, O makes two
bonds and two lone pairs.
5. When more bonds are needed, they are filled in with bonds to
hydrogen atoms.
Concept How many hydrogens are in the following molecules? Write the molecular formula for each.
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a) 11
b) 12
c) 13
a) 10
b) 11
c) 13
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Hydrocarbons: Carbons containing mainly carbon and hydrogen
Alkanes
CnH2n+2
Consider C4H10. How many different ways can it be drawn?
Structural (or Constitutional)
Isomers:
Compounds with the same molecular formula but different
connectivity, or structural formulas.
Problem Draw all structural isomers of C5H12. (there are 3 total)
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Cycloalkanes
CnH2n
What are some stable ring sizes?
Alkenes
CnH2n
Ethene, C2H4
Alkynes
CnH2n-2
Ethyne, C2H2
Can you rotate around a pi bond?
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Let’s consider 2–butene, CH3CH=CHCH3 or C4H8
Geometric Isomers
The atoms have the same connectivity but a different 3-dimensional shape.
Problem Draw all isomers of C4H8.
Aromatic Hydrocarbons and Benzene
H
H
H
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C
C
C
H
C
C
C
H
H
H
H
H
C
C
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C
C
H
C
C
H
H
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Chemistry 142
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Nomenclature (IUPAC system)
**Every unique compound has a unique name!**
The name of every organic compound can be broken down into parts, using a systematic method of
naming determined by the IUPAC. This system gets expanded on for every new type of functional
group or molecule. We will work from the insideout.
substituents
parent/root
suffix
functional group
Parent/root – determined by the longest continuous carbon chain in the molecule. See table 20.5 for a
list of the prefixes.
parent/
# of C’s
# of C’s
prefix
root
1
meth5
pent2
eth-
6
hex-
3
prop-
7
hept-
4
but-
8
oct-
Problem Determine the longest carbon chain in the following molecules.
Suffix – determined by the saturation of the carbon chain
-ane
-ene
-yne
Completely saturated, all C – C bonds are single bonds.
unsaturated, one C – C double bond.
unsaturated, one C – C triple bond.
Substituents – Groups that come off of the main chain. They get both a name and a number,
reflecting the group and its position on the chain.
•
•
Name the carbon substituents as alkyl groups, with a number in front giving the location of this
group. An alkyl group name typically consists of the prefix, based on the number of C’s in the
group, and the suffix –yl.
Halogens get a slightly different name: chlorine becomes ‘chloro’, bromine becomes ‘bromo’,
etc…
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Putting it all together
1. Determine the parent prefix and suffix from the longest carbon chain.
2. Number the carbon chain. You can number from either side, however certain groups have
priority!
• Non-carbon functional groups (alcohols, carboxylic acids, we’ll get to these!) have the
highest priority – should get the lowest # possible.
• Carbon functional groups (enes, ynes) are medium priority.
• If neither of the above are present, then the substituents should have the lowest set of
numbers possible.
3. Substituent names are listed in front of the parent prefix in alphabetical order, not numerical
order.
4. When two or more substituents are identical, they are listed together. Indicate this by the use of
the prefixes di-, tri-, tetra-, etc. Give each substituent its own number.
5. Use hyphens to separate numbers and letters, commas to separate numbers.
6. Practice!
Problem Using the IUPAC naming system, name the following compounds.
Br
F
Cl
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Functional Groups
Molecules with similar structure exhibit similar chemical reactivities, or functions.
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Examples
Br
OH
O
H
O
HO
Aromatic Compounds
Cl
Cl
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