Non-covalent Intermolecular Interactions

Review Guide #8 Chapters 8 and 9 I. Nomenclature You should understand the nomenclature rules discussed in class for (alcohols, ethers and sulfur containing compounds). II. Non-covalent Intermolecular Interactions A. Hydrogen Bonding 1. A good hydrogen bond donor has an H atom bonded to a very electronegative atom (O, N, or F). We are primarily concerned with molecules that have an O–H or an N–H bond. Alcohols, carboxylic acids, and amides were used as examples of good hydrogen bond donors. 2. A good hydrogen bond acceptor has an electronegative element with lone pairs. Examples mentioned in class were: water, alcohols, amines, carbonyls, and amides. 3. A hydrogen bond consists of a small covalent interaction combined with a particularly strong dipole-dipole interaction that arises due to the small size of H and the large bond dipole moment when H is bonded to a highly electronegative element. You should be able to schematically represent the hydrogen bonding interactions between molecules that can form them. B. Electrostatic Forces (attractive or repulsive) 1. Strong electrostatic attractions occur between ions of opposite charges. That attraction can by used to achieve ion separation and ion solvation that stabilize ions in solution. The ability of a solvent to separate ions is measured by its dielectric constant ε. 2. Repulsive electrostatic interactions are forces that occur between ions of the same charges exemplified by the action of soap. A polar head group is solvated by water and a hydrocarbon tail is not. The soap molecules can form aggregates called micelles whereby the polar head groups are surrounded by water and the nonpolar tails surround nonpolar dirt or grease within the interior of the micelle. The dirt or grease is thereby lifted away from the surface and carried into the solution. C. Dipole Interactions 1. Temporary dipoles are created when electron clouds around a molecule become momentarily deformed to create a temporary dipole. That action can cause the electron cloud of a nearby molecule to form a complementary induced dipole. Those van der Waals attractions must be overcome to vaporize a liquid. Molecules or atoms that are small and very electronegative are not very polarizable whereas larger molecules or atoms with lower electronegativity are more polarizable. Liquids having more polarizable molecules (unbranched) will generally have higher boiling points than those less polarizable molecules (branched). 2. Permanent dipole-dipole interactions are apparent in some molecules such as ethers that have higher boiling points than alkanes of similar shape and molecular mass. The higher boiling points result from greater dipole attractions between molecules with permanent dipoles. III. Organic Solvents Organic solvents can be categorized according to the properties mentioned in lecture: A. Bronsted properties: Protic solvents are good hydrogen bond donors. Aprotic solvents are not capable of acting as hydrogen bond donors. B. Lewis properties: Donor solvents have an atom with lone pair electrons and are capable of acting as Lewis bases. Non-donor solvents do not have lone pair electrons and are not capable of acting as Lewis bases. C. Polar properties: The dielectric constant ε is a measure of a solvent’s ability to separate charges. Although the dielectric constant is a property that has no direct relationship to an easily determined molecular property, it is often (but not always) true that polar molecules (with a large molecular dipole moment) have large dielectric constants. Highly polar solvents are effective at dissolving ionic substances, because they lower the energy required to separate the ions. Polar or non-polar solvents can be either protic or aprotic. III. Organic Solvents C. Polar properties: 1. Polar solvents have high dielectric constants ε > 15. All of them are donors with lone pair electrons and thereby act as good Lewis bases. They are either protic or aprotic. a. Polar, protic solvents provide excellent anion (-) and cation (+) solvation. Examples of polar, protic solvents include water, alcohols and formamide. b. Polar, aprotic solvents provide excellent cation by no anion solvation. Anions in these solvents are highly nucleophilic. The four widely used polar, aprotic solvents are DMF, DMSO, HMPA and acetonitrile. 2. Nonpolar solvents have low dielectric constants ε < 15. They can be either protic or aprotic. a. Protic, non-polar solvents provide moderate cation and moderate anion solvation. Examples of protic, non-polar solvents include acetic acid and tert-butanol. b. Aprotic, non-polar solvents can be donors or non-donors. (1.) Donor, aprotic, non-polar solvents provide moderate cation and no anion solvation. Examples of donor, aprotic, non-polar solvents include diethyl ether and THF. (2.) Non-donor, aprotic, non-polar solvents provide no cation or anion solvation. Examples of non-donor, aprotic, non-polar solvents are hexane, pentane, dichloromethane, chloroform and benzene. D. The acid-base properties of the solvent must be considered when choosing the solvent for a reaction. 1. The strongest acid that can exist in any solvent is the conjugate acid of that solvent. Any stronger acid will simply protonate the solvent. For example, the strongest acid that can exist in methanol, CH3OH, is protonated methanol, CH3OH2 +. 2. The strongest base that can exist in any solvent is the conjugate base of that solvent. Any stronger base will simply deprotonate the solvent. For example, the strongest base that can exist in methanol, CH3OH, is the methoxide anion, CH3O–. IV. Substitution and Elimination Reaction Rate Laws You should be able to recognize when a reaction is a substitution reaction or an elimination reaction, and know the distinction between an a-elimination and a b-elimination. (a-elimination will come later). Some important concepts from chemical kinetics apply in substitution and elimination reactions. You should understand the following: definition of reaction rate; rate laws (cannot be deduced from reaction stoichiometry), rate constants, and how they are derived; overall order and reactant order; unimolecular vs. bimolecular rate laws; and the relationship of rate constant to activation energy. A. Substitution Reaction Rate Laws 1. Primary alkyl halides undergo SN2 reactions. The reaction is overall 2nd order with the rate dependent upon the concentration of the nucleophile and the alkyl halide. 2. Tertiary alkyl halides undergo SN1 reactions whereby the reaction is overall 1st order. The rate is dependent upon the concentration of the alkyl halide only. 3. Secondary alky halides can undergo SN1, SN2 or both kinds of reactions depending upon the solvent. B. b-Elimination Reaction Rate Laws 1. Primary, secondary and tertiary alkyl halides can undergo E2 reactions. The reaction is second order thereby the rate is dependent upon the concentration of the base and alkyl halide. 2. Secondary and tertiary alkyl halides under E1 reactions depending upon the solvent. The reaction is first order so the rate is only dependent upon the concentration of the alkyl halide.