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LLEEW WIISS S STTRRU UC CTTU UR REESS,, V VSSE EP PR R AAN ND DM MO OLLEEC CU ULLA AR RM MO OD DEELLIIN NG G Introduction and Purpose: Molecular compounds (and polyatomic ions) are formed when atoms with similar electronegativities covalently bond with one another. In most cases, each atom will form the requisite number of bonds in order to achieve a “stable” octet of electrons in its outer valence shell (the Octet Rule). These molecules (or ions) will take on specific, stable molecular geometries in a manner consistent with valence shell electron pair repulsion theory (VSEPR). VSEPR states that pairs of electrons (bonding or non-bonding (lone) pairs) in an atom’s valence shell will mutually repel each other and thereby move away from each other as much as possible in order to minimize that repulsion and produce the most stable (lowest energy) geometric arrangement around a given atom. This lab has a number of objectives: · to give students additional experience with drawing correct Lewis structures · to apply valence shell electron pair repulsion (VSEPR) theory for predicting correct molecular geometries · to predict the correct hybridizations, molecular polarities and resonance forms of various molecules · to construct and manipulate molecular models in order to better visualize various types of molecular isomers (structural, geometric and rotational) Materials: molecular modeling kits or Styrofoam spheres and toothpicks Bond Polarity and Molecular Polarity Bond Polarity • No electronegativity difference between two atoms in a bond leads to a pure non-polar covalent bond. • A small electronegativity difference leads to a polar covalent bond. • A large electronegativity difference leads to an ionic bond. Polar bonds and polar molecules In a simple molecule like HCl, if the bond is polar, so also is the whole molecule. What about more complicated molecules? In CCl4, each bond is polar with the more electronegative chlorines with partial negative charges (denoted as δ–) and carbon with a partial positive charge (δ+). COP=CON The molecule as a whole, however, isn't polar. The center of positivity (COP) is located on the carbon. And while the individual chlorines are partially negative, given the geometry of the molecule, the center of negativity (CON) is also on the carbon. With the COP and the CON at the same point of the molecule (COP=CON), the molecule is nonpolar. By contrast, CHCl3 is polar. COP The hydrogen at the top of the molecule is less electronegative than carbon and so is slightly positive. This means that the COP is between the C and the H. However, the CON lies between under the C and at the center of the three chlorines. The COP≠CON so the molecule is polar. CON Part A — Ring Structures Benzene, C6H6, consists of a ring of six carbon atoms. Draw the Lewis structure for benzene and construct a model of this molecule. Questions: 1. What is the geometry at each carbon atom? 2. Is benzene capable of exhibiting resonance? Why or why not? 3. Would the overall molecule be polar or non-polar? Part B —Glycine– An Amino Acid Amino acids are a class of biologically important organic molecules. These molecules contain an amine group, NH2, a Bronsted-Lowry base (H+ acceptor) and a carboxylic acid group, COOH, a Bronsted-Lowry acid (H+ donor). The structure of the carboxylic acid unit is similar to the structure and reactivity of the carbonic acid molecule. The term amino acid usually refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic side-chain. In an α-amino acid, the amine group is covalently bonded the carbon atom (the αcarbon) immediately adjacent to the carboxylic acid unit. Glycine, the simplest α-amino acid, has the following formula, H2NCH2COOH. Draw the complete Lewis structure and construct a model for glycine. [Hint: The structure of the carboxylic acid unit, –COOH, is similar in structure and reactivity to the carbonic acid molecule, H2CO3] Questions: 1. What is the geometry around each carbon atom? Around the nitrogen atom? 2. What is the hybridization for each carbon? For the nitrogen? 3. Can this molecule exhibit resonance? Explain. 4. Is the molecule polar or non-polar? Explain. Because amino acids have both amine and carboxylic acid functional groups, these molecules can act as both an acid and a base at the same time. At a particular pH (known as the isoelectric point), the amine group will become protonated (having accepted a proton from the carboxylic acid functionality). This form of the amino acid, with a protonated amine group and a deprotonated carboxylate group, is known a zwitterion. Draw the Lewis structure for the zwitterionic form of glycine, H3NCH2COO. Questions: 1. What is the geometry around the nitrogen atom? 2. What is formal charge on the nitrogen? 3. What is the formal charge of each of the two oxygens? Part C — Multiple Bonds, Geometries and Hybridizations A hydrocarbon with the formula C5H6 has the following “skeleton” structure, showing only the sigma bonds and without any regard for the actual geometry at each carbon: H C C H H H C C C H H Determine the number of electrons that should be in this molecule and draw the complete and correct Lewis structure for this compound. Construct a molecular model with the geometry at each carbon as determined using VSEPR. Questions: 1. What is the geometry at each carbon atom? 2. What is the hybridization at each carbon atom? Part D — Geometric Isomers Draw the Lewis structure(s) and construct the model(s) for C2H2Cl2 Questions: 1. How many different Lewis structures are possible? Draw and construct the models for each. 2. What is the geometry at each carbon? 3. Which molecules are polar? Part E — Rotational and Structural Isomers Draw Lewis structures and make models for each of the following: a. C2H6 b. C2H4Cl2 For (b), you should draw and construct both structural isomers. Questions: Examine the model for C2H6 carefully. Note that the central C–C bond can undergo “free” rotation (these are called rotational isomers — “rotamers”). Looking along the central C–C bond, it is possible to produce two rotamers, an eclipsed form and a staggered form. Staggered Eclipsed 1. Using VSEPR, what would be the best possible alignment of the bonds in each structure? Explain your reasoning. Now examine the rotamers for the isomeric (structural) form of C2H4 Cl2 in which one chlorine is on each carbon atom. 2. Using the concept of electronic cloud repulsion, known as steric interaction, rank the four rotamers depicted from highest energy (least stable) to lowest energy? Cl Cl Cl Cl Syn Eclipsed (1) Cl Cl Anti Cl Eclipsed (2) Cl Highest energy Lowest energy Part F — Structural or Functional Isomers Draw two different Lewis structures (structural or functional isomers — molecules with the same chemical formulas but with different bond combinations that create different organic “functional groups”) for C2 H4 O Questions: 1. Describe the geometry around the carbons in each. 2. Describe the hybridization around the carbons in each. 3. Draw the “electron cloud” structure for each. Part G — Expanded Octets and Other Exceptions to the Octet Rule Draw the Lewis structures and construct models for: a. BCl3 b. PCl5 c. SF6 d. SF4 e. IF5 Questions: 1. What are the geometries and hybridizations of each?