• The great majority of organic compounds have C—C and C—H bonds. These are strong, non-polar bonds. They provide a non-reactive framework for the molecule.

• A functional group is an atom or a group of atoms other than a C—C or C—H bond. It is the reactive part of the molecule and important to determining its physical properties.

Heteroatoms— nonmetal atoms other than carbon or hydrogen.

Bonds—the most common bonds occur in C—C and C—O double bonds.

Bigger groupings of atoms, e.g., -CO2H, -C6H5

Metals

Functional Groups

Functional Groups Heteroatoms have lone pairs and create electron-deficient

sites on carbon.

Bonds are relatively weak and easily broken in chemical reactions. A bond makes a molecule a base and a nucleophile.

Functional GroupsHydrocarbons are compounds made up of only the elements carbon and hydrogen.

Functional Groups

Functional Groups

Functional Groups and Physical Properties: Intermolecular Forces

Types of intermolecular forces (noncovalent interactions) (nonbonded interactions)

• van der Waals (London) forces

• dipole-dipole interactions

•hydrogen bonds

•ionic bonds

As the polarity of an organic molecule increases, so does the strength of its intermolecular forces.

Functional Groups and Intermolecular Forces

Physical Properties—Boiling Point

• boiling point = temp at which liquid molecules are converted into gas.

• energy is needed to overcome the attractive forces in the much more densely packed liquid state.

• The stronger the intermolecular forces, the higher the boiling point.

• For compounds with approximately the same molecular weight:

Example Questions:

• For alkanes of the same molecular weight, increased branching translates to a lower bp.

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• 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.

Physical Properties—Melting Point

Introduction to Organic Molecules and Functional Groups

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

Physical Properties—Melting Point

Introduction to Organic Molecules and Functional Groups

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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.

Physical Properties—Melting Point

Introduction to Organic Molecules and Functional Groups

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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.

Physical Properties—Solubility

Introduction to Organic Molecules and Functional Groups

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• “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.

• Most 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.

Physical Properties—Solubility

Introduction to Organic Molecules and Functional Groups

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• Rule of thumb: 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.

Introduction to Organic Molecules and Functional Groups

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

Physical Properties—Solubility

Introduction to Organic Molecules and Functional Groups

Figure 3.4Dissolving an ioniccompound 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.

Physical Properties—Solubility

Introduction to Organic Molecules and Functional Groups

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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.

Physical Properties—Solubility

Introduction to Organic Molecules and Functional Groups

Figure 3.5Solubility summary