Introduction to
Organic Molecules
And
Functional Groups
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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:
ƒ Heteroatoms—atoms other than carbon or hydrogen.
ƒ π Bonds—the most common π bonds occur in C—C
and C—O double bonds.
ƒ 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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• Heteroatoms and π bonds confer reactivity on a
particular molecule.
Heteroatoms have lone pairs and create electrondeficient sites on carbon.
π Bonds are easily broken in chemical reactions. A π
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 the
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functional group is attached.
• 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 π 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 are compounds made up of only the
elements carbon and hydrogen. They may be aliphatic or
aromatic.
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• 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 π 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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Introduction to Organic Molecules and Functional Groups
Functional Groups
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Examples of Molecules Containing C-Z σ Bonds
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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 π bond that is more
easily broken than a C—O σ bond.
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Molecules Containing the C=O Functional Group
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It should be noted that the importance of a functional
group cannot be overstated.
A functional group determines all of the following
properties of a molecule:
ƒ Bonding and shape
ƒ Type and strength of intermolecular forces
ƒ Physical properties
ƒ Nomenclature
ƒ 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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• Covalent compounds are composed of discrete
molecules.
• 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:
ƒ van der Waals forces
ƒ dipole-dipole interactions
ƒ hydrogen bonding
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• 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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• 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.
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• 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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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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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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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 higher the
boiling point.
• For compounds with approximately the same molecular
weight:
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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
point.
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Consider the examples below which illustrate the effect of
size and polarizability on boiling points.
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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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• 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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• The trend in melting points of pentane, butanal, and
1-butanol parallels the trend observed in their
boiling points.
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• 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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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 the solvent.
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• 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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• 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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• 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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• To dissolve an ionic compound, the strong ion-ion
interactions must be replaced by many weaker ion-dipole
interactions.
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• 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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• 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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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 π 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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• An electron-deficient carbon
nucleophile, symbolized as :Nu¯.
reacts
with
a
• 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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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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