Name. Optical Isomers

Transcription

1 Name KEY Lab Day Optical Isomers Introduction: Stereoisomers are compounds that have the same structural formulas, but differ in their spatial arrangements. Two major types of stereoisomers are geometric isomers (cis-trans) and optical isomers (the ability to rotate plane-polarized light). In this lab, you will construct and study models of optical isomers to observe the spatial structures and the criteria for compounds which have this property. Definitions: hiral (asymmetric) carbon atom: A carbon atom that is bonded to four different atoms or groups of atoms. Superimposable molecules: When one molecule is laid on top of another molecule and all the atoms of both molecules coincide exactly, the molecules are identical. Optical activity: Molecules that have the ability to rotate planpolarized light. Enantiomers: Non-superimposable molecules that appear as mirror images of one another (right hand vs. left hand). Diastereomers: stereoisomers of the same compound, but are not enantiomers (opposites). hiral molecule: A molecule that is not superimposable on its mirror image. Meso compounds: Stereoisomers that contain chiral carbon atoms and are superimposable on their own mirror images. Projection formula: The three-dimensional and projection formulas for 1-bromo-1-chloroethane, 3l, are shown below: 1 1 l l 3 3 Three-dimensional Projection formula

2 2 arbon 1 is chiral with four different groups attached to it: 3,, l, and. In the three-dimensional formula the bonds from carbon to and l are coming out of the plane of the paper (like a hug) toward you. Likewise, in the projection formula the horizontal bonds to the and l are coming out of the plane towards you. In the three-dimensional formula the bonds from carbon to and 3 are projecting behind the plane of the paper away from you. Likewise, in the projection formula the vertical bonds to the and 3 are going behind the plane away from you. Experimental: Obtain a Model Kit containing a collection of plastic sticks and balls. The black balls with four holes drilled at angles of approximately 109.5º represent chiral carbons. The white balls represent hydrogen. The green balls represent chlorine. The red balls represent bromine or the O group. The blue balls will be used to represent the 3 group. 1-bromo-1-chloroethane, 3l can be represented then as shown below: red l white black green 3 Projection formula blue Model Kit representation Activities: A. onstruct two models of 1-bromo-1-chloroethane. heck to see if the two models are superimposable on one another. If they are superimposable, swap the position of the and l on one of the models. You should now have two models that are not superimposable. Draw these molecules so that they appear as mirror images. Put the on the top position and the 3 on the bottom, and the and l on the horizontal positions. Draw the projection formulas below (assume the chiral carbon is located where the lines cross).

3 3 Since these two molecules are not superimposable and are mirror images, they must be different compounds. These molecules are opposites or enantiomers. B. onstruct two models of lactic acid. arbon 2 is chiral OO O 1 Use a yellow ball to represent the OO group heck to see if the two models are superimposable on one another. If they are superimposable, swap the position of the and O on carbon 2 on one of the models. You should now have two models that are not superimposable. Draw these molecules so that they appear as mirror images. Put the OO on the top position and the 3 on the bottom, and the and O on the horizontal positions. Draw the projection formulas below (assume the chiral carbon is located where the lines cross).

4 onstruct two models of the compound 3l3 arbon 2 and 3 are both chiral. Position the two 3 groups on the top and bottom vertical positions. Now arrange the l on carbon 2 and the on carbon 3 so that both groups are on your right on one model and both are on your left on the other model. Draw the projection formulas below (assume the chiral carbon is located where the lines cross) onstruct two more models of the compound 3l3 Position the two 3 groups on the top and bottom vertical positions. Now arrange the l on carbon 2 on the right and the on carbon 3 on the left on one model, and the l on carbon 2 on the left and the on carbon 3 on the right on the other model. Draw the projection formulas below (assume the chiral carbon is located where the lines cross).

5 5 You now have two pairs of mirror images in these four structures. All four of these isomers are optically active. If you compare either of the first two pair with either of the second pair, you will notice although they are stereoisomers of the same compound, they are not mirror images of each other. Instead they are diastereomers D. onstruct four models of the compound 3ll3 Take two of these models and position the two 3 groups on the top and bottom vertical positions. Now arrange the l on carbon 2 on the right and the l on carbon 3 on the left on one model, and the l on carbon 2 on the left and the l on carbon 3 on the right on the other model. Draw the projection formulas below (assume the chiral carbon is located where the lines cross). Take the other two models and position the two 3 groups on the top and bottom vertical positions. Now arrange the l on carbon 2 on the right and the l on carbon 3 on the right on one model, and the l on carbon 2 on the left and the l on carbon 3 on the left on the other model. Draw the projection formula below (assume the chiral carbon is located where the lines cross).

6 6 Did you notice anything about these last pair? The last pair of models constructed are superimposable. The molecule is symmetrical about a horizontal plane passing through its center, between carbon 2 and carbon 3. The molecule shows no optical activity. Stereoisomers that contain chiral carbon atoms and are superimposable on their own mirror images are meso compounds E. Draw all projection formulas for 3l Put the 3 on the top and the l of carbon 3 on the bottom vertical positions.

7 7 F. Draw all projection formulas for 22 Use a blue or yellow ball to represent the two 2 groups and put the groups on the top and bottom vertical positions. ircle the two identical molecules. same

8 8 Question 1. For an organic compound to show optical activity, it must have at least one chiral carbon atom. Place a check mark next to the following compounds that would show optical activity. li 3l2 3l 32(3)23 32(3)223 3l2(3)3 2ll2l 2l2 * l I * l * l 3 * Draw expanded structural formulas for these condensed formulas before checking for chiral carbon atoms Question 2. The maximum number of stereoisomers for a chiral compound is given by the formula 2 n, where n is the number of chiral carbon atoms in the compound. If n = 0, the compound is not chiral. What is the maximum number of stereoisomers for the following compounds: 2l2l 0 3ll3 2=l3 2 2llOO 2 Draw expanded structural formulas for these condensed formulas before checking for chiral carbon atoms. Question 3. onstruct the following compounds using a blue ball for 3, a white ball for, a red ball for, and a yellow ball for O. Are the compounds mirror images or are they the same molecule? 1 2 l * l * l l 3 l * * l l O O 3 O ircle the correct answer: O 3 Mirror images Same molecule