CHAPTER 4 HW SOLUTIONS: ALKANES

Transcription

1 CAPTER W SOLUTONS: ALKANES CATEGORZATONS. assify each carbon atom pointed to below as,, or.. Draw an alkane that contains only: and carbon atoms carbon atoms and carbon atoms and carbon atoms or or or or EAT OF COMBUSTON DATA. The combustion data for propane and cyclohexane is shown in the table below. Explain why it is incorrect to say (based on the data) that cyclohexane is.8 kcal/mol ( ) higher in energy than propane. D comb (kcal/mol) + 5 O à CO + O O à CO + O -9. Although the full combustion of cyclohexane gives off.8 kcal/mol more heat energy than propane, it s incorrect to attribute all of this to differences in energy between cyclohexane and propane. The two alkanes have different numbers of C- bonds being broken, different amounts of O being broken, and they form different amounts of CO and water in the combustion reactions. The D values represent the changes in energy considering all of the bonds that are broken and formed, and it s incorrect to pick out one part of the process (e.g. the alkane part) and attribute the entire energy difference to just that. Page

2 . Use the heat of combustion data below to determine which organic compound is lower in energy, and by how much. Support your calculation with a diagram. D comb (kcal/mol) + 5/ O à 8 CO + 9 O / O à 8 CO + 9 O -0.0 The only difference between these reactions is the organic part, so the D values can be used to measure the relative energies of the organic compounds..5 O O The reactions finish on the same energetic line since the products are the same. Reaction releases 0. kcal/mol, which is a larger amount than 0.0, so it must start off at a higher energy. This means that the alkane of reaction is. kcal/mol higher in energy than the alkane in reaction ( =. kcal/mol). -0. kcal/mol 8 CO + 9 O -0.0 kcal/mol ALKANE AND ALKYL ALDE NOMENCLATURE 5. (Multiple Choice) Which is the correct UPAC name for each compound? -methylbutane,-methylbutane,-dimethylbutane -dimethylbutane -bromo--chlorobutane -bromo--chlorobutane -chloro--bromobutane -chloro--bromopentane,-dimethyl--iodocyclopentane -iodo-,-dimethylcyclopentane -iodo-,-dimethylcyclopentane,-dimethyl--iodocyclopentane Page

3 . Give the UPAC name for each compound. Name,-dimethyloctane iodocyclohexane -chloro--methyloctane Name -ethyl--propyl cyclopentane,,-tribromobutane,,7,8-tetramethylnonane F Name -t-butyl--fluoro cycloheptane OR -(,-dimethylethyl)-- fluorocycloheptane,-dichloro-5-methyl hexane sopropylcyclobutane OR (-methylethyl)cyclobutane Name -iodo--propylheptane (-ethylpropyl)cyclohexane -chloro--cyclopropyl cyclooctane 7. Each compound below is named incorrectly. Give the correct name for each. 9 -propyl-5-ethylheptane (eptane isn t the longest chain.) -ethyl--methylnonane -t-butylhexane (Pick the chain w/ most branches) -ethyl-,-dimethylhexane Page

4 NEWMAN PROJECTONS 8. Determine which Newman projection in each set is lower in energy. Explain your answers. a. C electronic repulsions. C LEFT conformation is lower in energy because it is staggered while the right one is eclipsed. Staggered is lower in energy because the - and C - distances are longer (groups on the front are further apart from groups on the back), thus minimizing b. C C RGT conformation is lower in energy because it is anti while the left one is gauche. Anti is lower in energy because it puts the largest groups (methyl and isopropyl) furthest apart, lowering electronic repulsions. c. C C RGT conformation is lower in energy. The most important repulsion to minimize is between the two large isopropyl groups (methyl group is smaller). aving those groups opposite (anti to each other) is lower in energy than having them gauche. 9. Draw the following Newman projections of: exane, looking down the C -C bond exane, down the C -C bond exane, down the C -C bond,-dimethylheptane, down the C -C bond C C C C C C C C C C C C C C C 5 C C C C 0. Draw the line structure of each compound, shown below in a Newman projection. C 5 C C C C C (hidden is #) C 5 (hidden is #5) Page

5 . Consider the compounds -bromopropane and ethylene glycol (OC C O). a. Draw the anti and gauche conformations of both compounds. Anti -bromopropane Gauche -bromopropane Anti ethylene glycol Gauche ethylene glycol C O O O C O b. Explain why the anti conformation of -bromopropane is lower in energy than the gauche conformation. The two large groups ( and C, which are larger than all the s) have the lowest energy when they are farther apart (minimized electron repulsions). n the anti conformation they are 80 from each other, which is as far apart as they can be. n the gauche conformation, they are only 0 apart. c. Ethylene glycol is unusual in that the gauche conformation is lower in energy than the anti conformation. Offer an explanation. O O When the two O groups are gauche, they are close enough together to form an intramolecular (within a molecule) hydrogen bond. This is a stabilizing interaction, which compensates for any electronic repulsion these groups may experience. RNG STRAN. t is approximated that each C group in a linear alkane has a D comb of -57. kcal/mol. Cyclooctane has an experimental!d comb of -9. kcal/mol. a. Calculate the amount of ring strain in cyclooctane. The expected comb is 8 (-57.) = -59. kcal/mol Experimental Actual = = 0.0 kcal/mol This means the actual cyclooctane releases 0.0 kcal/mol MORE energy than predicted, or has 0.0 kcal/mol of ring strain. b. Compared to other ring systems, would you classify this amount of strain as high, moderate or low? kcal/mol = high; kcal/mol = lowish (5 + 7 ring), so this is probably moderate. Page 5

6 . Describe two sources of ring strain in cyclobutane. Non-ideal bond angles (90 instead of 09.5 ) leading to bent bonds with poor orbital overlap. C Eclipsing interactions of the groups on the ring (in this case atoms), which repel and increase the energy. The ring is too small to twist and allow the bonds to be staggered. C. Explain why,-dimethylcyclopropane has more ring strain than ethylcyclopropane. int: draw Newman projections. Newman projections of cyclopropane structures are completely eclipsed due to the constraints form the small ring. C n,-dimethylcyclopropane (left) there are two bad C - C eclipsing interactions, which increase its energy. n ethylcyclopropane, there is only one bad interaction (ethyl-) to increase its energy. Ethyl groups aren t much larger than methyl groups (see their A-values), so the one with the two bad interactions is higher in energy than the one with only one bad eclipsing interaction. C C C C 5. Which should release more energy with combustion, cyclopentane or methylcyclobutane? Explain. nclude a diagram with your answer. 7.5 O O Methylcyclobutane should release more energy. Both compounds have the same molecular formula but methylcyclobutane has more ring strain. This means cyclobutane would start at a higher energy and release more energy when burned to produce 5 CO + 5 O. 5 CO + 5 O Page

7 CAR CONFORMATONS. Practice drawing chair conformations: nstructions Drawing of chair a. Draw in the equatorial C- bond (using the correct angle) on the carbon atom labeled #. parallel to bolded bonds b. Draw all axial and equatorial C- bonds for the front carbon atoms of both chairs. Draw the equatorial C- bonds using the correct angles. c. Draw all axial and equatorial C- bonds for the back carbon atoms of both chairs. d. Draw,-dibromocyclohexane with both bromine atoms axial. e. Draw,-dimethoxycyclohexane with one group axial and one group equatorial. ( Methoxy is OC ) C O OC f. Draw an isopropyl group equatorial on the dot. Then perform a ring flip and draw the second chair conformation beside it. 5 5 g. Draw a fluoro group axial on the dot. Draw the second chair conformation. F 5 5 F h. Draw a tert-butyl group axial on the left dot and a methyl group equatorial on the right dot. Draw the second chair conformation. 5 C C 5 i. Draw an iodo group equatorial on the dot. Draw the second chair conformation. 5 5 Page 7

8 AXAL AND EQUATORAL POSTONS 7. Consider ethylcyclohexane. a. Draw the Newman projections of axial-ethylcyclohexane and equatorial-ethylcyclohexane. C C C C C C C C Axial Equatorial b. Why is axial-ethylcyclohexane higher in energy than equatorial-ethylcyclohexane? Axial-ethylcyclohexane has a gauche interaction between the C C group and two of the C groups in the ring (carbons +, you can only see one in the Newman projection that s drawn). When the C C group is equatorial these interactions are anti and further apart. Thus there are more electronic repulsions (increased energy) in the axial conformation. OR n the axial position, the ethyl group experiences electronic repulsions with the other axial groups (the on carbons +), which increases its energy. t is not as close to any other groups when equatorial. 8. The two chair conformations of Compound M are equal in energy, while the two chair conformations of Compound N are different energy. Explain. M n either conformation of M, you have two groups axial and two equatorial. That makes these conformations equal in energy. N The two conformations of N are different energy C C because different groups are axial. n one C conformation two s are axial and in the other two C C are axial. and C have different sizes (different A values) so repel the other axial groups to different extents. 9. Explain why a tert-butyl group has a larger A value than an ethyl group. Tert-butyl groups are gigantic: no matter how they rotate C C they have huge repulsive interactions with the s across C the ring (diaxial interactions). Ethyl groups don t have a much higher A-value than methyl groups: they can rotate away so that only s (and not the rest of the ethyl group) experience diaxial interactions. C C Page 8

9 CAR TO CAR NTERCONVERSONS ( RNG FLPS ) 0. For each compound, perform a ring flip and draw the second chair conformation. Then determine which chair is lower in energy. (Use a table from lecture for A values.) C C C C C C OC OC C C Both groups are equatorial A =.7 A = 0.75 smaller group (smaller A) is axial. A = 0. A =.5 Both groups are equatorial OC O O OC A = =.50 A = =. Larger total A means bigger, worse axial (by a little) Page 9

10 . n each set, determine which isomer is lower in energy, and explain your answer. (Remember that cyclohexane rings are not flat!) a. Cyclopentane rings are quasi flat, at least it s OK to think of them like that. Trans (right) is lower energy because the s are on opposite sides, so don t repel as much. C Trans (right) is lower in energy. n its lowest energy b. C C C conformation, both ethyl groups are equatorial (far right conformation). C C C C Note: the trans compound favors the di-equatorial conformation, since that s lower energy. Therefore, it makes sense to say the trans compound is lower energy than cis, because its main conformation is lower in energy than any cis conformation. CS C C C C C C C C C C C C C C C C *Main conformation TRANS c. C C C C C C C C CS TRANS Cis (left) is lower in energy. n its lowest energy conformation (bottom left) both groups are equatorial. C C *Main conformation C C C C C C C C d. C C C C C (left structure) C (right structure) e. C C C C C C C C C (left structure) C C C (right structure) Page 0

11 STEREOSOMERS. Define and give an example of: a. Constitutional somers Compounds that are isomers (same molecular formula, but are different) because their atoms are connected in a different order. O O O OC b. Stereoisomers someric compounds whose atoms are connected in the same order, but are different in space (-dimensions).. Convert each chair into a line structure (hexagon), using dashed and wedged bonds. O O O O C C O O O O. Are the following statements concerning the molecules below true or false? iefly explain each answer. up up up up a. Both molecules are cis. True Both s are pointed up in each. b. A cis isomer always has the groups axial-axial or equatorial-equatorial. False The example shows this isn t true. The left structure has axial-axial groups, but the right structure has axial-equatorial groups, and yet both are cis. Page

12 5. Give the UPAC name for each compound, including cis/trans designations. C C Name trans-,- dimethylcyclobutane cis--chloro-- isopropylcycloheptane cis--bromo-- isopropylcyclohexane C C C C C Name trans--ethyl-- methylcyclohexane trans-,- dichlorocyclohexane cis--t-butyl-- ethylcyclohexane C F Name cis--iodo-- methylcyclohexane trans--fluoro-- propylcyclohexane. assify the relationship between each pair as constitutional isomers, stereoisomers, identical, or not isomers. Pair O O O O Relationship Stereoisomers (cis/trans) Stereoisomers (left = cis, right = trans) Pair Relationship Constitutional isomers (both C ) Not isomers : C and C O O Pair C C Relationship dentical Constitutional isomers (, vs.,) Page