Functional Groups The great majority of organic compounds

Functional Groups • 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. , -CO 2 H, -C 6 H 5 § Metals

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 Groups Hydrocarbons 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 • hydrogen bonds • dipole-dipole interactions • ionic bonds

Functional Groups and Intermolecular Forces As the polarity of an organic molecule increases, so does the strength of its 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.

Introduction to Organic Molecules and Functional Groups Physical Properties—Melting Point • 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. 12

Introduction to Organic Molecules and Functional Groups Physical Properties—Melting Point • The trend in melting points of pentane, butanal, and 1 -butanol parallels the trend observed in their boiling points. 13

Introduction to Organic Molecules and Functional Groups Physical Properties—Melting Point • 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 CH 3 group dangling from a four-carbon chain, does not. Thus, neopentane has a much higher melting point. 14

Introduction to Organic Molecules and Functional Groups Physical Properties—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. 15

Introduction to Organic Molecules and Functional Groups Physical Properties—Solubility • “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 (CCl 4) and hexane [CH 3(CH 2)4 CH 3], or weakly polar, like diethyl ether (CH 3 CH 2 OCH 2 CH 3). • Most ionic compounds are soluble in water, but insoluble in organic solvents. 16

Introduction to Organic Molecules and Functional Groups • 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. 17

Introduction to Organic Molecules and Functional Groups Physical Properties—Solubility • 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 H 2 O 18

Introduction to Organic Molecules and Functional Groups Physical Properties—Solubility • 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. 19

Introduction to Organic Molecules and Functional Groups Physical Properties—Solubility • The nonpolar part of a molecule that is not attracted to H 2 O is said to be hydrophobic. • The polar part of a molecule that can hydrogen bond to H 2 O 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 20