H 3 C. staggered H 2 C

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1 EMISTRY 104 elp Sheet #3 Organic-Part II: ISOMERS (Text: h 2: 2.9, h 6: 6.11, 6.5, h 7: 7.2f) Do topics appropriate for your lecture Prepared by Dr. Tony Jacob (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 = ONSTITUTIONAL ISOMERS: same formula, different bonding/connectivities; Example 1: Draw the two different structural/constitutional isomers of Answer 1: butane = and 2-methylpropane = II. STEREOISOMERS: same formula, same bonding/connectivities but different spatial arrangements (positions) of the atoms A. onformational Isomers: isomers differ because of a rotation through a s-bond; e.g., versus versions of butane shown as a Newman Projection and dash-wedge: Note: onformational isomers and Newman Projections are sometimes presented in lab and often not during whole-class session; they are not emphasized Newman Projections can also be drawn: B. GEOMETRI ISOMERS - versus configuration 1. ALKENES: 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 / isomers. The molecule below does not have / isomers Dash-Wedge Diagrams no or {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 =).}

2 GEOMETRI ISOMERS (continued): versus configuration If molecule has geometric isomers, is it or? 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 of the = and circle them; if they re on the same side of the line isomer; different sides isomer GEOMETRI ISOMERS - versus configuration 2. yclic Structures: Does the molecule have geometric isomers? Note: yclic Geometric Isomers is sometimes presented in lab and often not during wholeclass session; they are not emphasized. riteria: 1. Is the molecule a ring in which atoms or groups can move to or positions relative to the plane of the ring?. OPTIAL ISOMERS/ENANTIOMER PAIRS Enantiomers are chiral (pronounced kī-ral) molecules (i.e., they have a chiral center). riteria: To have a chiral center, an atom, often, 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 en-nan-tee-oh-mer) The enantiomer pairs (mirror image molecules) will be optically active, i.e., interacts with light have the same physical properties; i.e., same boiling points, D, etc. may differ in their chemical reactivity rotate plane-polarized light in different directions (clockwise and counterclockwise) Must check every atom; if a single atom has 4 different groups attached the molecule is chiral. Example 2: Is 2,3,4-trimethylpentane chiral? Answer 2: No here s why mirror enantiomers - mirror images not a chiral molecule these 5 atoms have more than one atom attached these 2 atoms have two 3 groups attached (circled) this atom has two ( 3 ) 2 groups attached (circled) Example 3: Is 2,3-dimethylpentane chiral? Answer 3: Yes here s why * chiral molecule these 6 atoms have more than one atom or 3 group attached this atom has 4 different groups attached and is a chiral center; chiral centers are denoted with an asterisk (*); the 4 different groups attached are:, 3, 2 3, ( 3 ) 2 (circled)

3 Isomer Summary hemicals (same chemical formula?) No Not Isomers Yes Isomers When comparing two molecules, do they have the same bonding/connectivity? No Structural/onstitutional Isomers Yes Stereoisomers see below 4 10 = butane 4 10 = 2-methylpropane onformational Isomers (e.g., and conformations; not emphasized) Stereoisomers Geometric Isomers (contains a double bond or a ring; /) Enantiomers (nearly always contains a with 4 different groups attached; nonsuperimposible mirror images) 3 Newman Projections (not emphasized) Alkenes 3 O O Dash-Wedge Diagrams 3 yclic (not emphasized) A methodology to draw all structural or structural+geometric 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 Use line notation. (orrectly naming the isomers helps to prevent drawing duplicate structures.) 2. Draw and name all 8 structural isomers for Use line notation. (orrectly naming the isomers helps to prevent drawing duplicate structures.) 3. Draw and name all 5 structural isomers for 3 6. Use line notation. (orrectly naming the isomers helps to prevent drawing duplicate structures.) 4. Draw and name all 6 structural and geometric alkene isomers (/) with the formula 5 10 ; do not include cycloalkanes. (orrectly 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

4 6. Identify which of the following molecules have chiral centers; there may be more than one molecule. O O a. I b c d e. 7. Which molecule is a structural isomer of -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; -2-pentene; -2-pentene; 2-methyl-1-butene; 3-methyl-1-butene; 2-methyl-2-butene

5 O 5. c {need to find a atom with 4 different groups attached to it; the chiral is denoted with an * ; note the 4 different groups attached to that atom:, 3, 2 O, 2 ( 3 ) 2 } 6. a, d, and e have optical isomers 7. b {-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 version of -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 2-methyl-2-butanol O 3-methyl-2-butanol O 2,2-dimethyl-1-propanol

6 Ex: Draw all structural isomers with the formula 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

7 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 / isomers; this molecule has / isomers; draw the other isomer by flipping one pair of atoms on one of the = double; rename as -1-chloro-1-butene 5. name the molecule: -1-chloro-1-butene 6. Take the for a walk down the chain naming each new molecule and checking for / isomers: 2-chloro-1-butene (no / isomers) 3-chloro-1-butene (no / isomers) 4-chloro-1-butene (no / 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 / isomers: 1-chloro-2-butene; has /: rename as -1-chloro-2-butene -1-chloro-2-butene 2-chloro-2-butene; has /: rename as -2-chloro-2-butene -2-chloro-2-butene named incorrectly: 3-chloro-2-butene appears to be a new isomer duplicate; correctly named: -2-chloro-2-butene named incorrectly: 4-chloro-2-butene appears to be a new isomer duplicate; correctly named: -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 / 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 / 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: -1-chloro-1-butene -1-chloro-1-butene 2-chloro-1-butene 3-chloro-1-butene 4-chloro-1-butene -1-chloro-2-butene -1-chloro-2-butene -2-chloro-2-butene -2-chloro-2-butene 3- chains: 1-chloro-2-methyl-1-propene 3-chloro-2-methyl-1-propene