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Organic Chemistry, Smith Fifth Edition, modified by Dr. Hahn

Polarity of MoleculesUse the following procedure to determine if a moleculehas a net dipole: (little horses)• Use electronegativity differences to identify all of the polar

bonds and the directions of the bond dipoles.• Determine the geometry around individual atoms by

counting groups (VSEPRT), and decide if individual dipolescancel or reinforce each other in space.

Figure 1.13 Electrostatic potential plot of CH3Cl End class 8/18 F

Polar MoleculesA polar molecule has either one polar bond, or two ormore bond dipoles that reinforce each other. An exampleis water:

• It is a bent molecule

• Two dipoles reinforce

• It has a net dipole, making it a polar molecule

Nonpolar MoleculesA nonpolar molecule has either no polar bonds, or two ormore bond dipoles that cancel. An example is carbon dioxide:

• It is a linear molecule

• Two dipoles are equal and opposite in direction

• Two dipoles cancel

• It is a nonpolar molecule with no net dipole

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Organic Chemistry, Fifth EditionJanice Gorzynski Smith

University of Hawai’i

Chapter 3Modified by Dr. Juliet Hahn

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Functional Groups

• A functional group is an atom or a group of atoms with characteristic chemical and physical properties.

• Most organic molecules contain a carbon backbone consisting of C-C and C-H bonds to which functional groups are attached.

• Structural features of a functional group include:• Heteroatoms—atoms other than carbon or

hydrogen. • π Bonds most commonly occur in C-C and C-O

double bonds.

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• Functional groups distinguish one organic molecule fromanother.

• They determine a molecule’s:• geometry• physical properties• reactivity

Functional Groups

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• Heteroatoms and π bonds confer reactivity on a particularmolecule.

• Heteroatoms have lone pairs and create electron-deficient sites on carbon.

• A π bond makes a molecule a base and a nucleophile,and is easily broken in chemical reactions.

Reactivity of Functional Groups

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Parts of a Functional Group

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• This molecule has only C—C and C—H σ bonds.

• It contains no polar bonds, lone pairs, or π bonds.

• Therefore, ethane has no reactive sites (functional groups).

• Consequently, ethane and molecules like it (alkanes) arevery unreactive.

Ethane, a Molecule with No Functional Group

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• This molecule has an OH (called a hydroxy group) attached toits backbone.

• Compounds containing an OH group are called alcohols.• The hydroxy group makes the properties of ethanol very

different from the properties of ethane.• Ethanol has lone pairs and polar bonds that make it reactive.• Other molecules with hydroxy groups will have similar

properties to ethanol.

Ethanol

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• Hydrocarbons are compounds made up of only the elementscarbon and hydrogen.

• They may be aliphatic (ex. alkanes, alkenes, alkynes) or aromatic.

Hydrocarbons

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• Aliphatic hydrocarbons have three subgroups.

• Alkanes have only C—C σ bonds and no functional

group.

• Alkenes have a C—C double bond.

• Alkynes have a C—C triple bond.

Aliphatic Hydrocarbons

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• Aromatic hydrocarbons are so named because many ofthe 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, found in most aromatic compounds.

Aromatic Hydrocarbons

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• Several types of functional groups contain C-Z σbonds.

• The electronegative heteroatom Z creates a polar bond, making carbon electron deficient.

Functional Groups with Carbon-Heteroatom (C-Z) σ bonds

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Functional Groups with C-Z σ bonds

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• This group is called a “carbonyl group”.

• The polar C-O bond makes the carbonyl carbon anelectrophile, while the lone pairs on O allow it to reactas a nucleophile and base.

• The carbonyl group also contains a π bond that is moreeasily broken than a C-O σ bond.

Functional Groups with C=O Group

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A functional group determines all of thefollowing properties of a molecule:

• bonding and shape

• type and strength of intermolecularforces

• physical properties

• nomenclature

• chemical reactivity

Importance of Functional Groups

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Molecules can Contain Several Functional Groups

• Each of these molecules have several differentfunctional groups

• These molecules would also have several different types of reactivity

End class 8/21