EMISTRY 104 elp Sheet #3 Organic (Part II): ISOMERS hapters 2.9 (structural isomers), 6.5 (geometric isomers), 6.11 (constitutional isomers), 7.2f (optical isomers) Do the topics appropriate for your lecture Prepared by Dr. Tony Jacob http://www.chem.wisc.edu/areas/clc (Resource page) Nuggets: Isomers (The instructor will clarify which isomers are covered.) ISOMERS Molecules with the same formula but different connectivities or different arrangements in space; (molecules that have different formulas, e.g., 6 12 and 6 14, are not isomers) I. STRUTURAL ISOMERS (also called onstitutional Isomers): same formula, different bonding/connectivities; e.g., 2 different structural isomers of 4 10 : butane = and 2-methylpropane = II. STEREOISOMERS: same formula, same bonding/connectivities but different spatial arrangements (positions) of the atoms A. ONORMATIONAL ISOMERS: isomers differ because of a rotation through a σ-bond; e.g., versus versions of butane shown as a Newman Projection (as done in lab) and dash-wedge: 3 3 3 3 Newman Projections can also be drawn: 3 3 3 Dash-Wedge Diagrams B. GEOMETRI ISOMERS ALKENES: cis versus trans configuration Does the molecule have geometric isomers? riteria: 1. Does it contain a = (or a double bond between other atoms) 2. On each of the =, there must be 2 different groups (one of these groups maybe a pair e - ). Both criteria must be true for a molecule to have cis/trans isomers. The molecule below does not have cis/trans isomers. 3 3 3 2 no cis or trans {Above molecule does have a double bond and meets the first criteria. The left atom of the = bond does have 2 different groups (it has a 3 and a 2 3 group) but the right atom of the = has two atoms (same group; needs different groups on each of the =).}
GEOMETRI ISOMERS (continued): cis versus trans configuration If molecule has geometric isomers, is it cis or trans? Molecule below has a double bond (satisfies the first criteria) and each of the = has two different groups (each has a and a 3 group; satisfies the second criteria) 1. Draw a line through = bond 2. ind the 2 groups that are the same on the 2 of the = and circle them; if they re on the same side of the line cis isomer; different sides trans isomer 3 3 3 3 trans cis GEOMETRI ISOMERS YLI STRUTURES (as discussed in lab) Does the molecule have geometric isomers? riteria: 1. Is the molecule a ring in which atoms can move to cis or trans positions compared to the plane of the ring? cis trans. ENANTIOMER ISOMERS Enantiomers are chiral (pronounced ki-ral) molecules (i.e., they have a chiral center). riteria: To be a chiral center, the atom, usually must have 4 different groups attached to it. (Note: there are exceptions to this usually not covered in hem 104.) If a single atom has 4 different groups attached, the molecule will be chiral have a non-superimposable mirror image the two mirror images are called enantiomer pairs (pronounced ee-nan-tee-oh-mer) The enantiomers (mirror image molecules) will be optically active (interacts with plane-polarized light) have the same physical properties (boiling point, D, etc.) rotate plane-polarized light in different directions (clockwise and counterclockwise), and may differ in their chemical reactivity You must check every atom in the molecule; if any atom has 4 different groups the molecule is chiral. mirror * hiral (denoted with an asterisk, *) has: (not shown), 3, 2 3, and ( 3 ) 2 attached (4 different groups) enantiomers - mirror images not a chiral molecule chiral molecule
Summary hemicals (same chemical formula?) No Not Isomers 3 8 5 12 Yes Isomers When comparing two molecules, do they have the same bonding/connectivity? 4 10 = butane 4 10 = 2-methylpropane No Structural Isomers (also called constitutional isomers) Yes Stereoisomers onformational Isomers (e.g., and conformations) 3 3 3 3 Newman Projections Stereoisomers Geometric Isomers (contains a double bond or a ring; cis/trans) cis trans Enantiomers (nearly always contains a with 4 different groups attached; nonsuperimposible mirror images) 3 O O 3 3 3 3 3 cis trans Dash-Wedge Diagrams A methodology to draw all isomers of a given molecular formula is on the last two pages of this elp Sheet. 1. Draw and name all 3 structural isomers for 5 12. Use line notation. (Naming the isomers helps to prevent drawing duplicate structures.) 2. Draw and name all 8 structural isomers for 5 11. Use line notation. (Naming the isomers helps to prevent drawing duplicate structures.) 3. Draw and name all 5 structural isomers for 3 6. Use line notation. (Naming the isomers helps to prevent drawing duplicate structures.) 4. Draw and name all 6 structural and geometric alkene isomers (cis/trans) with the formula 5 10 ; do not include cycloalkanes. (Naming the isomers helps to prevent drawing duplicate structures.)
5. Which molecule is chiral? Select answer e if none are chiral. a. O b. c. O d. e. none are chiral 6. Identify which of the following molecules have optical isomers; there may be more than one molecule. O O a. I b. 3 2 3 c. 3 2 2 2 2 3 d. 3 2 3 e. 3 2 3 7. Which molecule is a structural isomer of trans-2-butene? a. b. c. d. e. 8. Draw and name all 8 alcohol isomers that can be derived from 5 11 O. (Naming the isomers helps to prevent drawing duplicate structures.) ANSWERS 1. pentane; 2-methylbutane; 2,2-dimethylpropane 2. 1-chloropentane; 2-chloropentane; 3-chloropentane ; 1-chloro-2-methylbutane; 1-chloro-3-methylbutane; 2-chloro-2-methylbutane; 2-chloro-3-methylbutane; 1-chloro-2,2-dimethylpropane 3. 1-bromo-1-chloropropane; 1-bromo-2-chloropropane; 1-bromo-3-chloropropane; 2-bromo-1-chloropropane; 2-bromo-2-chloropropane
4. 1-pentene; cis-2-pentene; trans-2-pentene; 2-methyl-1-butene; 3-methyl-1-butene; 2-methyl-2-butene * 5. c {need to find a atom with 4 different groups attached to it; the chiral is denoted with an * 6. a, d, and e have optical isomers O 7. b {trans-2-butene has this structure: and a formula: 4 8 ; a has a formula = 4 10 isomers must have the same formula; b has the same formula and a different structure; c is the cis version of trans-2-butene so it is a geometric isomer not a structural isomer; d has a formula = 5 10 isomers must have the same formula; e has a formula = 5 8 isomers must have the same formula} 8. O O 1-pentanol 2-pentanol O 3-pentanol O O 2-methyl-1-butanol O 3-methyl-1-butanol O 2-methyl-2-butanol 3-methyl-2-butanol O 2,2-dimethyl-1-propanol
Ex: Draw all structural isomers with the formula 5 11. ow to systematically draw structural isomers 1. Determine whether the molecule is an alkane, alkene, etc. Since occupies one site on a atom, it is similar to a atom. ence, the formula, 5 11 is analogous to 5 12 which corresponds to a n 2n+2 formula alkane so 5 11 is also an alkane. 2. Start with the longest chain; in this example, a 5- chain: 3. Place the atom at the far left end of the chain: and name the molecule: 1-chloropentane. 4. Take the for a walk down the chain naming each new molecule: 2-chloropentane 3-chloropentane named incorrectly: 4-chloropentane appears to be a new isomer duplicate; correctly named: 2-chloropentane named incorrectly: 5-chloropentane appears to be a new isomer duplicate; correctly named: 1-chloropentane 5. Once you have identified all 5- chain molecules reduce the chain to a 4- chain. 6. There are now 2 groups to work with: a and a 3 group. The 3 group cannot be placed on either end of the chain otherwise the chain becomes a 5- chain again which is already done. The should be placed on the far left end of the chain. name the molecule: 1-chloro-2-methylbutane 7. Take the for a walk down the 4- chain naming each new molecule: 2-chloro-2-methylbutane 2-chloro-3-methylbutane 1-chloro-3-methylbutane 8. Move the 3 group to the next position on the chain; this position is still in position 2 (reading right to left) so all the next molecules will be duplicates. ence, all 4- chain isomers have been identified and now reduce the chain to a 3- chain. 9. Note: There are now 3 groups to work on: a and two 3 groups. The 3 groups cannot be placed on either end of the chain otherwise the chain becomes a 4- chain again which is already done so they must go on the middle ; the is placed on the far left end of the chain. name the molecule: 1-chloro-2,2-dimethylpropane 10. Take the for a walk down the 3- chain naming each new molecule: 2-chloro-2,2-dimethylpropane Note: this molecule is NOT allowed since the middle atom has 5 bonds! named incorrectly: 3-chloro-2,2-dimethylpropane appears to be a new isomer duplicate; correctly named: 1-chloro-2,2-dimethylpropane 11. inally check to see if the 2 3 groups can be combined into a 2- ethyl group and start the at the far left end of the chain: 5- chains: 4- chains: 3- chains: named incorrectly: 1-chloro-2-ethylpropane appears to be a new isomer duplicate; correctly named: 1-chloro-2-methylbutane 1-chloropentane 2-chloropentane 3-chloropentane 1-chloro-2-methylbutane 2-chloro-2-methylbutane 2-chloro-3-methylbutane 1-chloro-3-methylbutane 1-chloro-2,2-dimethylpropane
ow to systematically draw structural AND geometric isomers Ex: Draw all structural and geometric isomers with the formula 4 7 ; don t include cyclic compounds. 1. Determine whether the molecule is an alkane, alkene, etc. Since occupies one site on a atom, it is similar to a atom. ence, the formula, 4 7 is analogous to 4 8 which corresponds to a n 2n formula alkene so 4 7 is also an alkene. 2. Start with the longest chain; in this example, a 4- chain: 3. Place the atom and the double bond at the far left end of the chain: and name the molecule: 1-chloro-1-butene. 4. heck for cis/trans isomers; this molecule has cis/trans isomers; draw the other isomer by flipping one pair of atoms on one of the = double; rename as trans-1-chloro-1-butene 5. name the molecule: cis-1-chloro-1-butene 6. Take the for a walk down the chain naming each new molecule and checking for cis/trans isomers: 2-chloro-1-butene (no cis/trans isomers) 3-chloro-1-butene (no cis/trans isomers) 4-chloro-1-butene (no cis/trans isomers) 7. ing the back to the far left side of the chain and move the double bond. Take the for a walk down the chain naming each new molecule and checking for cis/trans isomers: 1-chloro-2-butene; has cis/trans: rename as trans-1-chloro-2-butene cis-1-chloro-2-butene 2-chloro-2-butene; has cis/trans: rename as trans-2-chloro-2-butene cis-2-chloro-2-butene named incorrectly: 3-chloro-2-butene appears to be a new isomer duplicate; correctly named: trans-2-chloro-2-butene named incorrectly: 4-chloro-2-butene appears to be a new isomer duplicate; correctly named: trans-1-chloro-2-butene 8. Once you have identified all 4- chain molecules reduce the chain to a 3- chain. 9. There are 3 groups to work with:, 3 group, and a double bond. The 3 group cannot be placed on either end of the chain otherwise it becomes a 4- chain and must be placed on position 2; the is placed on the far left end of the chain; the double bond is placed in position 1. name the molecule: 1-chloro-2-methyl-1-propene (no cis/trans isomers) 10. Take the for a walk down the 3- chain naming each new molecule: 2-chloro-2-methyl-1-propene Note: this molecule is NOT allowed since the middle atom has 5 bonds! 3-chloro-2-methyl-1-propene (no cis/trans isomers) 11. Move the double bond to the next position and bring the back to the far left position: named incorrectly: 1-chloro-2-methyl-2-propene appears to be a new isomer duplicate; correctly named: 3-chloro-2-methyl-1- propene 12. The next two positions will yield duplicate isomers. The 3 cannot be moved from the second position without increasing the chain length, and a 2- chain is not possible. Done! 4- chains: trans-1-chloro-1-butene cis-1-chloro-1-butene 2-chloro-1-butene 3-chloro-1-butene 4-chloro-1-butene trans-1-chloro-2-butene cis-1-chloro-2-butene trans-2-chloro-2-butene cis-2-chloro-2-butene 3- chains: 1-chloro-2-methyl-1-propene 3-chloro-2-methyl-1-propene