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Organic Chemistry, Second Edition

Janice Gorzynski SmithUniversity of Hawai’i

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 3Lecture Outline

Prepared by Rabi Ann MusahState University of New York at Albany

Introduction to Organic Molecules and Functional Groups

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3.1 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.π Bonds—the most common π 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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• Heteroatoms and π bonds confer reactivity on a particular molecule.

Heteroatoms have lone pairs and create electron-deficient 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 functional group isattached.

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• 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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3.1 Overview of Functional Groups

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 six-membered 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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Functional Groups

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3.2B Compounds Containing C-Z σ Bonds

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3.2C 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 andbase.

• The carbonyl group also contains a π bond that is more easily broken than a C—O σ 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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3.3 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 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:

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 surfacearea, 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 F2molecules have little attractive force between them since the electrons are tightly held and temporary dipoles are difficult to induce.

van der Waals Forces

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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 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.

Hydrogen Bonding

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Note: as the polarity of an organic molecule increases, so does the strength of its intermolecular forces.

Hydrogen Bonding

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3.4 Physical Properties3.4A 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.

Figure 3.2 Effect of surface area and polarizability on boiling point

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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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• 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.

3.4B 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 approximatelythe 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 molecularweights, but very different shapes.

• A compact symmetrical molecule like neopentane packs well into acrystalline 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 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.

3.4C Solubility

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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.

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.

Figure 3.5 Solubility summary

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3.5 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. Thereare 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 fat-soluble and water-soluble vitamins.

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3.5A Vitamin A Vitamin A, or retinol, may be obtained directly from the diet. In addition, β-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 water insoluble.

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• 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.

3.5B Vitamin C

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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.

3.6 Application of Solubility: SoapFigure 3.6 Dissolving soap in water

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3.7 Application: 3.7A The Cell Membrane

Figure 3.7The cell membrane

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• 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.

3.7B Transport Across a Cell Membrane

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Figure 3.8Transport of ions across a

cell membrane

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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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3.8 Functional Group and ReactivityRecall 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 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 carbonatom, so they react with electron-rich nucleophiles.

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3.9 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 starchNucleotides—are the building blocks of DNAAmino acids—join together to form proteinsFatty 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

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Figure 3.9 continued Simple and complex biomolecules