Organic Functional Groups

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Organic 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 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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Hydrocarbons are compounds made up of only the
elements carbon and hydrogen. They may be
aliphatic or aromatic.
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Aromatic Groups

• Aromatic hydrocarbons are so named because they
  have strong characteristic odors.
• The simplest aromatic hydrocarbon is benzene. The
  six-membered ring and three ? bonds of benzene
  comprise a single functional group.

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Functional Groups
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Functional Groups

• 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.
  Consequently, ethane and molecules like it are
  relatively 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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Alkyl Groups

• Alkyls are alkane side chains or alkane like
  substituents on an organic molecule
• Alkyls are nonpolar groups
• Alkyls
• Methyl -CH3 (Me-)
• Ethyl -CH2CH3 (Et-)
• n-butyl -(CH2)3CH3 (nBu-)

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Functional Groups on unsaturated aliphatic
hydrocarbons

• Vinylic substituents Allylic substituents
• Enol Allylic alcohol
• Enol ether Allylic ether
• Enamine

R Alkyl
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Functional Groups on Aromatic hydrocarbons

• Phenol
• Aniline

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Examples of Molecules Containing C-O ? Bonds
Polycyclic ether
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Carbonyls

• Compounds Containing the CO Group
• This group is called a carbonyl group.
• The carbonyl group contains a polar CO ? bond
  and a C-O ? bond that is more easily broken than
  a CO ? bond.

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Carbonyls (R)2CO
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Functional groups in Biomolecules

• Biomolecules are organic compounds found in
  biological systems.
• There are four main families of small molecule
  biomolecules
• amino acids, simple sugars, lipids and
  nucleotides
• Biomolecules often have several functional groups.

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Molecules Containing Multiple Functional Group
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Functional Groups
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 chemical reactivity type and strength
of intermolecular forces physical
properties nomenclature Bioactivity
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Reactivity with Unsaturated Functional Groups

• 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.

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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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Nucleophiles (Nu-) or (Nu) Lewis Bases can act
as Nucleophiles
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Alkene (Lewis base) reacting with an Electrophile
For example, alkenes contain a C-C double bond,
an electron-rich functional group with a
nucleophilic ? bond. Thus, alkenes react with
electrophiles E, but not with other electron
rich species like nucleophiles (e.g. OH or Br).
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Nucleophile Attacking an Electrophile
Alkyl halides possess an electrophilic carbon
atom, so they react with electron-rich
nucleophiles.
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Intermolecular Forces Organic Molecules
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Ionic Interactions Exist in Ionic
Compounds Ion-Ion Interaction

• 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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Intermolecular Forces Between Molecules

• Intermolecular forces are also referred to as
  noncovalent interactions or nonbonded
  interactions.

• 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 (London Dispersion)
• dipole-dipole interactions
• hydrogen bonding

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Intermolecular Forcesvan der Waals Forces or
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 constituents van der
Waals forces.
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van der Waals Forces-Surface Area

• 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-Polarizability

• 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.
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Intermolecular ForcesDipole-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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Intermolecular ForcesHydrogen 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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Summary of Intermolecular Forces
Note as the polarity of an organic molecule
increases, so does the strength of its
intermolecular forces.
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Physical Properties of Organic Molecules
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Quiz Review

• Tuesday 5-6 pm
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• You may come to one or all review sessions.
• Bring questions. (No Questions-No Review)
• If you can not make it to any of the review time
  then see me at my office hours.

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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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Comparing Boiling Points-molecules w/ different
functional groups
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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Comparing Boiling Points-molecules w/ different
functional groups
Consider the examples below which illustrate the
effect of size and polarizability on boiling
points.
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Separation of Molecules by Boiling Point
Liquids having different boiling points can be
separated in the laboratory using a distillation
apparatus, shown in Figure 3.4.
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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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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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Melting Point Trends

• The trend in melting points of pentane, butanal,
  and 1-butanol parallels the trend observed in
  their boiling points.

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The Symmetry of a Molecule Determines its Melting
Point

• 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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Solubility-comparing properties of solvents and
solutes

• 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 is very polar solvent (forms hydrogen bonds
  with solutes).
• Most ionic compounds are soluble in water, but
  insoluble in organic solvents.
• Many organic solvents are either nonpolar, like
  carbon tetrachloride (CCl4) and hexane
  CH3(CH2)4CH3, or weakly polar, like diethyl
  ether (CH3CH2OCH2CH3).

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Solubility-dissolving ionic compounds

• To dissolve an ionic compound, the strong ion-ion
  interactions must be replaced by many weaker
  ion-dipole interactions.

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Solubility-the five carbon rule for organic
compounds

• 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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Solubility-organic compounds which are soluble in
water

• Since butane and acetone are both organic
  compounds, 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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Organic Molecules which are not Water soluble

• 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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Solubility

• 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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ApplicationSoap
Soap Soap molecules have two distinct partsa
hydrophilic portion composed of ions called the
polar head, and a hydrophobic carbon chain of
nonpolar CC and CH bonds, called the nonpolar
tail.
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The Cell Membrane
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Ionophores

• 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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Transport Across a Cell Membrane
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Crown Ethers
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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Vitamins

• Vitamins are either fat or water soluble.
• Vitamin A - is fat soluble and water insoluble
• Contains one hydroxyl per 20 carbons
• Vitamine C - is Water soluble
• Contains 4 hydroxyl groups and a carbonyl group
  per 6 carbons.