Chapter 19
Organic Chemistry
and Hydrocarbons
(part 1 Intro material)
Hein * Best * Pattison * Arena
1
Organic Chemistry
• Historically, organic chemicals were defined as those
that came from living sources. These chemicals
could only originate from a “vital force”.
• Friedrich Wohler then synthesized urea from carbon
dioxide and ammonia thus disputing the vital force
theory
• Organic chemistry is now defined as the branch of
chemistry dealing with carbon compounds (ie, no
vital force needed!)
– Certain carbon compounds are not organic: oxides of
carbon, metal carbides, and metal carbonates.
2
• Carbon, in combination with hydrogen, oxygen, nitrogen,
and sulfur, forms the basis for millions of organic
compounds.
• Carbon is unique in that it can bond to itself in lengthy
chains (a process called catenation).
•
Organic molecules make up the majority of:
1.
2.
3.
4.
5.
6.
7.
8.
Cosmetics and perfumes
Nutrients (carbs, fats, proteins)
Fuels
Fabrics
Pharmaceutical products
Soaps and detergents
Paints, varnishes, dyes
Many more!
3
The Carbon Atom:
Bonding and Shape
4
• The carbon atom is central to all organic
compounds.
• A carbon atom usually forms four covalent
bonds.
– Single bonds
• Ex: H3C-CH3 in ethane
– Double bonds
• Ex: H2C=CH2 in ethene
– Triple bonds
• Ex: HCCH in ethyne (acetylene)
5
• An organic compound is classified as
saturated if it contains only single bonds.
– Ex: H3C-CH3 in ethane
• An organic compound is classified as
unsaturated if the molecules possess one or
more multiple carbon-carbon bonds.
– Ex: H2C=CH2 in ethene
– Ex: HCCH in ethyne (acetylene)
6
The Shapes of CarbonContaining Compounds
• Valence Shell Electron Pair Repulsion
theory (VSEPR)
– VSEPR theory states that electron pairs
repel each other because they have like
charges.
– The electron pairs will try to spread out
as far as possible around an atom.
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Figure 19.1 Shows the tetrahedral
structure of carbon: (a) a regular
tetrahedron; (b) a carbon atom with
tetrahedral bonds; (c) a carbon atom
within a regular tetrahedron; (d) a
methane molecule, CH4
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Figure 19.2 (a) Lewis structure for C2H4. (b)
Shape of a molecule with a carbon-carbon double
bond. The hydrogens and carbon form the vertices
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of a triangle. The bond angles are 120°.
In a triple bond, the carbon has only two
regions for shared electrons. To be
placed as far apart as possible, a linear
arrangement is required.
180°
H:C:::C:H H C C H
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Molecular Models
• Models are often used in organic
chemistry to illustrate molecules.
Figure 19.3 shows molecular models
of methane: (a) ball-and-stick model;
(b) spacefilling model
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Figure 19.4 Models of ethane shows
(a) Lewis structure; (b) bond-line
drawing; (c) ball-and-stick model; (d)
spacefilling model
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Structural Formulas
• In a structural formula, the
arrangement of atoms within the
molecule is clearly shown.
• These structural formulas are often
shortened into condensed structural
formulas.
– e.g. CH3CH2CH3 & CH3-CH2-CH3
13
Classes of
Organic
Compounds
14
Classifying Organic Compounds
Functional Groups
• Organic compounds with similar
structures are grouped into classes as
shown in Table 19.1.
• The members of each class of compounds
contain a characteristic atom or group of
atoms called a functional group. These
are shown in red in the structural
formulas on the table.
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Functional Groups
• A functional group:
– is part of a larger molecule.
– is composed of an atom or group of
atoms that have characteristic chemical
behavior.
– Chemically, a given functional group
behaves in nearly the same way in every
molecule it’s part of.
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Hydrocarbons
• Hydrocarbons are compounds that are
composed entirely of carbon and
hydrogen atoms bonded to each other
by covalent bonds.
• Fossil fuels – natural gas, petroleum,
and coal – are the principal sources of
hydrocarbons.
17
Hydrocarbons
• Hydrocarbons are classified into
two major categories, aliphatic and
aromatic.
– aliphatic (do not contain benzene ring)
• alkanes, alkenes, alkynes, cycloalkanes
– aromatic (contain a benzene ring)
– See figure 19.5
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Saturated
Hydrocarbons:
Alkanes
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The alkanes, also known as paraffins or saturated
hydrocarbons, are straight- or branched-chain
hydrocarbons with only single covalent bonds
between the carbon atoms.
CH3CH2CH2CH2CH3
straight-carbon chain
(CH3)2CHCH2CH2CH3
branched chain of carbon atoms
20
Homologous Series
• When each member of a series differs
from the next member by a CH2 group,
the series is called a homologous
series.
• The members of a homologous series
are similar in structure, but have a
regular difference in formula.
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Homologous Series
CnH2n + 2
general formula for open-chain
alkanes
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Carbon Bonding in Alkanes
Figure 19.6 Schematic hybridization of 2s22px12pz0
orbitals of carbon to form four sp3 electron orbitals.
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Figure 19.7 The tetrahedral nature of sp3 orbitals:
(a) a single sp3-hybridized orbital;
(b) four sp3-hybridized orbitals in a tetrahedral
arrangement; (c) sp3 and s orbitals overlapping to form
C-H bonds in methane.
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Sigma () Bond
• A C-C and C-H single bond result from the
overlap of sp3 orbitals.
• This type of bond is called a sigma ()
bond.
• A sigma bond exists if the electron cloud
formed by the pair of bonding electrons lies
on a straight line drawn between the nuclei
of the bonded atoms.
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The valence bonds or points of attachment
may be represented in structural formulas
by a corresponding number of dashes
attached to each atom:
C
N
O
H
Cl
S
F
Br
I
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• Alkane molecules are non-polar.
– Carbon-carbon bonds are non-polar, since
they are between like atoms.
– Carbon-hydrogen bonds are only slightly
polar.
– The bonds in an alkane are symmetrically
directed toward the corners of a tetrahedron.
– In virtue of their low polarity, alkane
molecules have relatively low boiling points
compared with other organic compounds of
similar molar mass.
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Figure 19.8 Ball-and-stick models illustrating
structural formulas of methane, ethane and
propane.
29
Writing the Correct Structural Formula
for Propane, C3H8
An alkane contains only single bonds.
H
H
H
H
C
C
C
H
H
H
H
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Isomerism
It is possible to write two structural formulas
H C4H10:
corresponding to the molecular formula
H
H
C
H
C
H
C
H
C
H
H
H
C
H
C
C
C
H
H
H
H
H
H
H
H
H
H
butane or n-butane
bp = -0.5 C
mp = -138.3 C
isobutane
bp = -11.7 C
mp = -159.5 C
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Isomerism
• The phenomenon of two or more
compounds having the same molecular
formula but different structural
arrangements of their atoms is called
isomerism.
• The various individual compounds are
called isomers.
– e.g. There are two isomers of C4H10.
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Figure 19.8 Ball-and-stick models illustrating
structural formulas of butane and isobutane.
33
Isomers are compounds that
have the same molecular
formula, but different structural
formulas.
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Example for Drawing
Isomers
35
There are three isomers of pentane, C4H12. Write
structural formulas and condensed structural formulas
for these isomers.
1. Write the carbon skeleton of the compound containing the
longest continuous carbon chain.
C-C-C-C-C
2. Complete the structure by attaching hydrogen atoms
around each carbon.
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
CH3CH2CH2CH2CH3
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Isomers of Pentane, C5H12 (continued)
• For
the next isomer, start by writing a four-carbon chain and
attach the fifth carbon atom to either of the middle carbon
atoms – do not use the end ones. Add 12 hydrogens to
complete the structure.
H
H
H
H
H
H
C
H
C
C
C
C
H
H
H
H
CH3
H
CH3CH2CHCH3 CH3CH2CH(CH3)2
•This is the only four-chain compound that can be
written.
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Isomers of Pentane, C5H12 (continued)
H
H
For the third isomer, start with a three-carbon
chain, attach the other carbon atoms to the
central carbon atom, and complete the structure
by adding 12 hydrogen atoms:
H
H
H
C
H
C
C
C
H
C
H
H
H
CH3
H
CH3CCH3
No more isomers of
C5H12 can be written.
CH3
H
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