Functional Groups

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Introduction to Organic Molecules and Functional
Groups
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 CC and CH bonds.
  However, many organic molecules possess other
  structural features
• Heteroatomsatoms other than carbon or hydrogen.
• ? Bondsthe most common ? bonds occur in CC and
  CO double bonds.
• These structural features distinguish one organic
  molecule from another. They determine a
  molecules geometry, physical properties, and
  reactivity, and comprise what is called a
  functional group.

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• Heteroatoms and ? bonds confer reactivity on a
  particular molecule.
• Heteroatoms have lone pairs and create
  electron-deficient sites on carbon.
• ? Bonds are easily broken in chemical reactions.
  A ? bond makes a molecule a base and a
  nucleophile.

Dont think that the CC and CH bonds are
unimportant. They form the carbon backbone or
skeleton to which the functional group is
attached.
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• Ethane This molecule has only CC and CH 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 are compounds made up of only the
elements carbon and hydrogen. They may be
aliphatic or aromatic.
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No Reaction
No Reaction
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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
  six-membered 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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• Compounds Containing the CO Group
• This group is called a carbonyl group.
• The polar CO 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 CO ? bond.

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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
• Bonding and shape
• Type and strength of intermolecular forces
• Physical properties
• Nomenclature
• Chemical reactivity

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Which of the following is an aldehyde?
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Which of the following is an ester?
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Which of the following is an amide?
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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 inter-actions. These ionic
  inter-actions 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
• van der Waals forces
• dipole-dipole interactions
• hydrogen bonding

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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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• 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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• 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

• Dipoledipole 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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What type of intermolecular forces are exhibited
by each molecule?
VDW
VDW and DD
VDW
VDW, DD and HB
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Physical PropertiesBoiling 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.
Figure 3.2 Effect of surface area
and polarizability on boiling point
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Which has the higher boiling point and why?
A has only VDW, while B has both VDW and DD
interactions
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A had VDW, DDD and H-bonding, while B lacks
H-bonding
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Both A and B only have VDW interactions, but B
has the higher bp b/c of a larger surface area.
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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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Which has the higher melting point and why?
B has stronger intermolecular forces (DD and
HBZ).
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Both only have VDW forces, so A has the higher mp
b/c it is more symmetrical. Closer packing means
higher mp.
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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 CC and CH 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.

Figure 3.4 Dissolving an ionic compound in H2O
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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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Which of the following are water soluable?
O atom, 5 or less Cs soluable
No O, N or F, nonpolar, not soluable
Has N, but more than 5 Cs, so not soluable
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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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• 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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On the other hand, alkyl halides possess an
electrophilic carbon atom, so they react with
electron-rich nucleophiles.
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Considering only electron density, will each
reaction occur?
Yes
E
Nu-
No
Nu-
Nu-
Yes
Nu-
E