Chapter 3
Introduction to Organic Molecules
and Functional Groups
Organic Chemistry, Second Edition
Janice Gorzynski Smith
University of Hawai’i
Prepared by Rabi Ann Musah
State University of New York at Albany
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
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Functional Groups
• A functional group is an atom or a group of atoms with
characteristic chemical and physical properties. It is the
reactive part of the molecule.
• Most organic compounds have C—C and C—H bonds.
However, many organic molecules possess other
structural features:
 Heteroatoms—atoms other than carbon or hydrogen.
  Bonds—the most common  bonds occur in C—C
and C—O double bonds.
 These structural features distinguish one organic
molecule from another. They determine a molecule’s
geometry, physical properties, and reactivity, and
comprise what is called a functional group.
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Functional Groups
• Heteroatoms and  bonds confer reactivity on a particular
molecule.
Heteroatoms have lone pairs and create electrondeficient sites on carbon.
 Bonds are easily broken in chemical reactions. A 
bond makes a molecule a base and a nucleophile.
Don’t think that the C—C and C—H bonds are unimportant.
They form the carbon backbone or skeleton to which 3 the
functional group is attached.
Functional Groups
• Ethane: This molecule has only C—C and C—H bonds, so it has
no functional group. Ethane has no polar bonds, no lone pairs,
and no  bonds, so it has no reactive sites. Consequently, ethane
and molecules like it are very unreactive.
• Ethanol: This molecule has an OH group attached to its
backbone. This functional group is called a hydroxy group.
Ethanol has lone pairs and polar bonds that make it reactive with
a variety of reagents.
The hydroxy group makes the properties of ethanol very
different from the properties of ethane.
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Hydrocarbons
Hydrocarbons are compounds made up of only the elements
carbon and hydrogen. They may be aliphatic or aromatic.
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Aromatic hydrocarbons
• Aromatic hydrocarbons are so named because many of the
earliest known aromatic compounds had strong characteristic
odors.
• The simplest aromatic hydrocarbon is benzene. The sixmembered ring and three  bonds of benzene comprise a single
functional group.
• When a benzene ring is bonded to another group, it is called a
phenyl group.
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Examples of Molecules Containing C-Z  Bonds
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Compounds Containing the C=O Group
Compounds Containing the C=O Group:
• This group is called a “carbonyl group”.
• The polar C—O bond makes the carbonyl carbon an
electrophile, while the lone pairs on O allow it to react
as a nucleophile and base.
• The carbonyl group also contains a  bond that is more
easily broken than a C—O  bond.
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Molecules Containing the C=O Group
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A functional group determines all of the
following properties of a molecule
 Bonding and shape
 Type and strength of intermolecular forces
 Physical properties
 Nomenclature
 Chemical reactivity
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Intermolecular Forces
• Intermolecular forces are interactions that exist between
molecules. Functional groups determine the type and
strength of these interactions.
• There are several types of intermolecular interactions.
• Ionic compounds contain
oppositely charged particles
held together by extremely
strong electrostatic interactions. These ionic interactions are much stronger
than the intermolecular forces
present between covalent
molecules.
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Intermolecular Forces
• Covalent compounds
molecules.
are
composed
of
discrete
• The nature of the forces between molecules depends on
the functional group present. There are three different
types of interactions, shown below in order of
increasing strength:
 van der Waals forces
 dipole-dipole interactions
 hydrogen bonding
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Intermolecular Forces—van der Waals Forces
• van der Waals forces are also known as London forces.
• They are weak interactions caused by momentary changes in
electron density in a molecule.
• They are the only attractive forces present in nonpolar
compounds.
Even though CH4 has no
net dipole, at any one
instant its electron density
may not be completely
symmetrical, resulting in a
temporary dipole. This can
induce a temporary dipole
in another molecule. The
weak interaction of these
temporary dipoles
constitutes van der Waals
forces.
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Intermolecular Forces—van der Waals Forces
• All compounds exhibit van der Waals forces.
• The surface area of a molecule determines the strength of the
van der Waals interactions between molecules. The larger the
surface area, the larger the attractive force between two
molecules, and the stronger the intermolecular forces.
Figure 3.1 Surface area and van der Waals forces
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Intermolecular Forces—van der Waals Forces
• van der Waals forces are also affected by polarizability.
• Polarizability is a measure of how the electron cloud around an
atom responds to changes in its electronic environment.
Larger atoms, like iodine,
which have more loosely
held valence electrons,
are more polarizable than
smaller atoms like
fluorine, which have more
tightly held electrons.
Thus, two F2 molecules
have little attractive force
between them since the
electrons are tightly held
and temporary dipoles
are difficult to induce.
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Intermolecular Forces—Dipole-Dipole Interactions
• Dipole—dipole interactions are the attractive forces between
the permanent dipoles of two polar molecules.
• Consider acetone (below). The dipoles in adjacent molecules
align so that the partial positive and partial negative charges
are in close proximity. These attractive forces caused by
permanent dipoles are much stronger than weak van der Waals
forces.
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Intermolecular Forces—Hydrogen Bonding
• Hydrogen bonding typically occurs when a hydrogen
atom bonded to O, N, or F, is electrostatically attracted
to a lone pair of electrons on an O, N, or F atom in
another molecule.
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Intermolecular Forces—Hydrogen Bonding
Note: as the polarity of an organic molecule increases, so
does the strength of its intermolecular forces.
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Physical Properties—Boiling Point
• The boiling point of a compound is the temperature at which
liquid molecules are converted into gas.
• In boiling, energy is needed to overcome the attractive forces
in the more ordered liquid state.
• The stronger the intermolecular forces, the
boiling point.
higher the
• For compounds with approximately the same molecular
weight:
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Physical Properties—Boiling Point
Consider the example below. Note that the relative
strength of the intermolecular forces increases from
pentane to butanal to 1-butanol. The boiling points of
these compounds increase in the same order.
For two compounds with similar functional groups:
• The larger the surface area, the higher the boiling point.
• The more polarizable the atoms, the higher the boiling
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point.
Physical Properties—Boiling Point
Consider the examples below which illustrate the effect of
size and polarizability on boiling points.
Figure 3.2 Effect of surface area and polarizability on boiling point
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Physical Properties—Boiling Point
Liquids having different boiling points can be separated in the
laboratory using a distillation apparatus, shown in Figure 3.3.
Figure 3.3
Schematic of a distillation
apparatus
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Physical Properties—Melting Point
• The melting point is the temperature at which a solid is
converted to its liquid phase.
• In melting, energy is needed to overcome the attractive
forces in the more ordered crystalline solid.
• The stronger the intermolecular forces, the higher the
melting point.
• Given the same functional group, the more symmetrical
the compound, the higher the melting point.
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Physical Properties—Melting Point
• Because ionic compounds are held together by
extremely strong interactions, they have very high
melting points.
• With covalent molecules, the melting point depends
upon the identity of the functional group. For
compounds of approximately the same molecular
weight:
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Physical Properties—Melting Point
• The trend in melting points of pentane, butanal, and 1butanol parallels the trend observed in their boiling
points.
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Physical Properties—Melting Point
• Symmetry also plays a role in determining the melting points of
compounds having the same functional group and similar
molecular weights, but very different shapes.
• A compact symmetrical molecule like neopentane packs well into
a crystalline lattice whereas isopentane, which has a CH3 group
dangling from a four-carbon chain, does not. Thus, neopentane
has a much higher melting point.
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Physical Properties—Solubility
• Solubility is the extent to which a compound, called a
solute, dissolves in a liquid, called a solvent.
In dissolving a compound, the energy needed to break up the
interactions between the molecules or ions of the solute
comes from new interactions between the solute and the29
solvent.
Physical Properties—Solubility
• Compounds dissolve in solvents having similar kinds of
intermolecular forces.
• “Like dissolves like.”
• Polar compounds dissolve in polar solvents. Nonpolar or
weakly polar compounds dissolve in nonpolar or weakly
polar solvents.
• Water and organic solvents are two different kinds of
solvents. Water is very polar and is capable of hydrogen
bonding with a solute. Many organic solvents are either
nonpolar, like carbon tetrachloride (CCl4) and hexane
[CH3(CH2)4CH3], or weakly polar, like diethyl ether
(CH3CH2OCH2CH3).
• Most ionic compounds are soluble in water, but insoluble
in organic solvents.
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Physical Properties—Solubility
• An organic compound is water soluble only if it contains
one polar functional group capable of hydrogen bonding
with the solvent for every five C atoms it contains. For
example, compare the solubility of butane and acetone in
H2O and CCl4.
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Physical Properties—Solubility
• Since butane and acetone are both organic compounds
having a C—C and C—H backbone, they are soluble in
the organic solvent CCl4. Butane, which is nonpolar, is
insoluble in H2O. Acetone is soluble in H2O because it
contains only three C atoms and its O atom can
hydrogen bond with an H atom of H2O.
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Physical Properties—Solubility
• To dissolve an ionic compound, the strong ion-ion
interactions must be replaced by many weaker ion-dipole
interactions.
Figure 3.4
Dissolving an ionic
compound in H2O
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Physical Properties—Solubility
• The size of an organic molecule with a polar functional group
determines its water solubility. A low molecular weight alcohol
like ethanol is water soluble since it has a small carbon
skeleton of  five C atoms, compared to the size of its polar
OH group. Cholesterol has 27 carbon atoms and only one OH
group. Its carbon skeleton is too large for the OH group to
solubilize by hydrogen bonding, so cholesterol is insoluble in
water.
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Figure 3.5 Solubility summary
• The nonpolar part of a molecule that is not attracted to H2O is
said to be hydrophobic.
• The polar part of a molecule that can hydrogen bond to H2O is
said to be hydrophilic.
• In cholesterol, for example, the hydroxy group is hydrophilic,
whereas the carbon skeleton is hydrophobic.
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Solubility
Application—Vitamins
• Vitamins are organic compounds needed in small
amounts for normal cell function.
• Most cannot be synthesized in our bodies, and must be
obtained from the diet.
• Most are identified by a letter, such as A, C, D, E and K.
There are several different B vitamins, so a subscript is
added to distinguish them. Examples are B1, B2, and B12.
• Vitamins are fat soluble (they dissolve in organic media)
or water soluble.
• Vitamins A and C illustrate the differences between fatsoluble and water-soluble vitamins.
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Solubility
Application—Vitamins
• Vitamin A, or retinol, may be obtained directly from the
diet. In addition, -carotene, the orange pigment found in
many plants including carrots, is readily converted into
vitamin A in our bodies. Vitamin A is an essential
component of the vision receptors in our eyes. It also
helps to maintain the health of mucous membranes and
skin. Vitamin A is fat insoluble.
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Solubility
Application—Vitamins
• Vitamin C, or ascorbic acid, is important in the formation
of collagen
• It is obtained from eating citrus fruits.
• Vitamin C deficiency results in scurvy
• It is heavily hydroxylated, which makes it capable of
hydrogen bonding, and thus, water soluble.
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Solubility
Application—Soap
Figure 3.6
Dissolving soap in water
Soap:
Soap molecules
have two distinct
parts—a
hydrophilic
portion composed
of ions called the
polar head, and a
hydrophobic
carbon chain of
nonpolar C—C
and C—H bonds,
called the
nonpolar tail.
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Solubility
Application—The Cell Membrane
Figure 3.7
The cell
membrane
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Solubility
Application—The Cell Membrane
Transport Across a Cell Membrane:
• Polar molecules and ions are transported across cell membranes
encapsulated within molecules called ionophores.
• Ionophores are organic molecules that complex cations. They have a
hydrophobic exterior that makes them soluble in the nonpolar interior
of the cell membrane, and a central cavity with several oxygens
whose lone pairs complex with a given ion.
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Solubility
Application—The Cell Membrane
Figure 3.8
Transport of ions
across a
cell membrane
By binding an ion on one side of a lipid bilayer (where the
concentration of the ion is high) and releasing it on the other
side of the bilayer (where the concentration of the ion is low),
an ionophore transport an ion across the membrane. 42
Solubility
Application—The Cell Membrane
Several synthetic ionophores have also been prepared, including
one group called crown ethers.
Crown ethers are cyclic ethers containing several oxygen atoms
that bind specific cations depending on the size of their cavity.
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Influence of Functional Groups on Reactivity
Recall that:
• Functional groups create reactive sites in molecules.
• Electron-rich sites react with electron poor sites.
All functional groups contain a heteroatom, a  bond or
both, and these features create electron-deficient (or
electrophilic) sites and electron-rich (or nucleophilic)
sites in a molecule. Molecules react at these sites.
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Influence of Functional Groups on Reactivity
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Influence of Functional Groups on Reactivity
• An electron-deficient carbon reacts with a
nucleophile, symbolized as :Nu¯.
• An electron-rich carbon reacts with an electrophile,
symbolized as E+.
For example, alkenes contain an electron rich double
bond, and so they react with electrophiles E+.
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Influence of Functional Groups on Reactivity
On the other hand, alkyl halides possess an
electrophilic carbon atom, so they react with electronrich nucleophiles.
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Biomolecules
• Biomolecules are organic compounds found in biological
systems.
• Many are relatively small with molecular weights of less than
1000 g/mol.
• There are four main families of small molecule biomolecules:
Simple sugars—combine to form complex carbohydrates
like starch
Nucleotides—are the building blocks of DNA
Amino acids—join together to form proteins
Fatty acids—are the building blocks of triacylglycerols,
lipids that are stored as fat droplets in adipose tissue.
• Biomolecules often have several functional groups.
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Figure 3.9
Simple and complex biomolecules
Simple biomolecules are the building blocks of more complex
biomolecules, which then compose important cellular 49
structures.
Figure 3.9 continued
Simple and complex biomolecules
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