3. Cyclic compounds The presence of the ring in all but very large ring cyclic molecules...

3. Cyclic compounds The presence of the ring in all but very large ring cyclic molecules prevents full rotation of the ring atoms. For this reason, stereoisomerism may also occur in cyclic molecules. a) Prepare a model of cyclohexane, C6H12. Draw the condensed formula. b) Build a model of methylcyclohexane (C7H14) by replacing one of the hydrogens of cyclohexane with a methyl group. Draw the skeletal formula for methylcyclohexane. 2 c) How many different isomers exist for methylcyclohexane (CyH34)? d) Start with the model of methylcyclohexane built in 4b), substitute different hydrogen atoms on the carbon atoms adjacent to the methyl group of methylcyclohexane with a methyl group to form isomers of 1,2-dimethylcyclohexane. How many different isomers exist for 1,2-dimethylcyclohexane? Prepare models and draw skeletal formulas and provide IUPAC names for all isomers.

B. Diastereomers: Conformational isomers are molecules with the same structural formula but with different shapes due to rotations about one or more sigma bonds. Cis-trans isomers are a subtype of diastereomers that differ in the spatial position around a bond with restricted rotation and widely occur in alkenes and cyclic compounds 2. Alkenes a) Prepare a model and draw the Lewis structure for propene. b) How many different isomers exist for propene (C3H6)? c) Start with the model of propene built in 3a), substituting different hydrogen atoms of propene with a methyl group to form isomers of butene, CaHs. How many different isomers exist for butene (CH)? Prepare models and draw the Lewis structures for each of the isomers. d) Two of isomers of butene comprise a pair of stereoisomers. Draw the skeletal structures and include IUPAC names for the cis and trans isomers. Recall in the cis isomer both methyl groups are on the same side of the double bond; in the trans isomer they are on opposite sides.

A. Enantiomers: Certain substances have the unique property of rotating the plane of plane-polarized light. Such light rotation is detectable with the aid of a polarimeter. In order for a molecule to be optically active it must be chiral. Chiral objects lack a plane of symmetry and are non-superimposable on their mirror images. A sp. hybridized carbon atom can fulfill these requirements if all four of its substituents are different. 1. Methane a) Prepare a methane molecule and then substitute two of its hydrogens with two different atoms (with different color) to get the model of CH,XY. Is there a plane of symmetry cutting through the X, Y, and Catoms? b) Build the model for CHXYZ by substituting one of the hydrogens in CH,XY with another different substituent (a third atom with a different color) Does this molecule have a plane of symmetry? c) Place the model in front of you and prepare another model exactly like the first one, with all the substituents pointing in exactly the same directions. These models are superimposable. Now, exchange the places of any two substituents on one of the models. Are the models superimposable now? d) Set the models beside each other with their hydrogen atoms pointing upward. (Include the picture of models when they are set aside). As you rotate one of the models, you will find a position where the two appear as reflections in an imaginary mirror between them.Draw dash-and-wedge formulae of the two models from this perspective

d) Set the models beside each other with their hydrogen atoms pointing upward. (Include the picture of models when they are set aside). As you rotate one of the models, you will find a position where the two appear as reflections in an imaginary mirror between them.Draw dash-and-wedge formulae of the two models from this perspective. The two models represent enantiomers, non-superimposable mirror image isomers. Enantiomers have identical physical properties except for the direction in which they rotate plane-polarized light. The enantiomer rotating the plane of light clockwise is the dextrorotatory enantiomer, which is identified by (+) before its IUPAC name. The levorotatory enantiomer rotates the plane of light counter-clockwise 1 and has the symbol (-) before its name. (Note: an older system using (d) and (1) designations is occasionally encountered.) This property can only be determined experimentally. e) Draw Lewis structures for the following molecules. Circle any structures capable of existing as enantiomers. Mark (*) the chiral carbon if it is present. Recall, a carbon is chiral only if it is surrounded by four different substituents. i) (CH3)2CHCHCICH3 ii) CH3CHCICH2CH2CH3 iii) CH3CH2CHCICH2CH3 iv) CH3CH2CH(CH3)CH,CI v) (CH3)3CCHC vi) CH3CH2CCI(CH3)2