As you might suspect, b-carotene, C40H56, precursor to

Which alkenes show cis,trans isomerism? For each alkene that does, draw the trans isomer. (a) 2- Pentene (b) 2-Methyl-2-pentene (c) 3-Methyl-2-pentene

Name each alkene and specify its confi guration by the E,Z system. (a) Cl (b) (c) Br Cl

Write the IUPAC name of each cycloalkene. (a) (b) (c)

Draw structural formulas for the other two stereoisomers of 2,4-heptadiene.

(10E,12Z)-10,12-hexadecadien-1-ol is a sex pheromone of the silkworm. Draw a structural formula for this compound.

Predict all approximate bond angles about each highlighted carbon atom. To make these predictions, use valence-shell electron-pair repulsion (VSEPR) (Section 1.4). O (a) (b) CH2OH (c) C OH (d) (e) HC C CH CH2

For each highlighted carbon atom in Problem 5.9, identify which atomic orbitals are used to form each s bond and which are used to form each p bond.

The structure of 1,2-propadiene (allene) is shown to the right. (a) Predict all approximate bond angles in this molecule. (b) State the orbital hybridization of each carbon. (c) Explain the three-dimensional geometry of allene in terms of the orbitals used.

Following are lengths for a series of C!C single bonds. Propose an explanation for the differences in bond lengths. CH2 C CH2 1,2-Propadiene (Allene) Length of C!C Structure Single Bond (pm) CH3!CH3 153.7 CH2"CH!CH3 151.0 CH2"CH!CH"CH2 146.5 HC#C!CH3 145.9

Draw structural formulas for these alkenes. (a) trans-2-Methyl-3-hexene (b) 2-Methyl-2-hexene (c) 2- Methyl-1-butene (d) 3-Ethyl-3-methyl-1-pentene (e) 2,3-Dimethyl-2-butene (f) cis-2-Pentene (g) (Z)-1- Chloropropene (h) 3-Methylcyclohexene (i) 1-Isopropyl-4-methylcyclohexene ( j) (E)-2,6-Dimethyl-2,6- octadiene (k) 3-Cyclopropyl-1-propene (l) Cyclopropylethene (m) 2-Chloropropene (n) Tetrachloroethylene (o) 1-Chlorocyclohexen

Name these alkenes and cycloalkenes. Cl (a) (b) (c) (d) (e) (f) (g) (h) Cl Cl Cl FF FF

Arrange the following groups in order of increasing priority. (a) !CH3 !H !Br !CH2CH3 (b) !OCH3 !CH(CH3)2 !B(CH2CH3)2 !H (c) !CH3 !CH2OH !CH2NH2 !CH2Br

Assign an E or Z confi guration to these dicarboxylic acids, each of which is an intermediate in the tricarboxylic acid cycle. Under each is its common name. (a) CC H CH2COOH COOH (b) HOOC C H COOH C H Fumaric acid Aconitic acid HOOC

Name and draw structural formulas for all alkenes with the molecular formula C5H10. As you draw these alkenes, remember that cis and trans isomers are different compounds and must be counted separately.

For each molecule that shows cis,trans isomerism, draw the cis isomer. (a) (b) (c) (d) CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3

b-Ocimene, a triene found in the fragrance of cotton blossoms and several essential oils, has the IUPAC name (Z)-3,7-dimethyl-1,3,6-octatriene. Draw a structural formula for b-ocimene

Draw the structural formula for at least one bromoalkene with the molecular formula C5H9Br that shows: (a) Neither E,Z isomerism nor chirality. (b) E,Z isomerism but not chirality. (c) Chirality but not E,Z isomerism. (d) Both chirality and E,Z isomerism.

Following are structural formulas and common names for four molecules that contain both a carbon- carbon double bond and another functional group. Give each an IUPAC name. Acrylic acid O O CH2"CHCOH Acrolein O CH2"CHCH Methyl vinyl ketone O CH3CCH"CH2 H3C (a) (b) (c) (d) Crotonic acid C C COH H H

Trans-cyclooctene has been resolved, and its enantiomers are stable at room temperature. Trans- cyclononene has also been resolved, but it racemizes with a half-life of 4 min at 0C. How can racemization of this cycloalkene take place without breaking any bonds? Why does trans- cyclononene racemize under these conditions but trans- cyclooctene does not? You will fi nd it especially helpful to examine the molecular models of these cycloalkenes

Which alkenes exist as pairs of cis,trans isomers? For each that does, draw the trans isomer. (a) CH2"CHBr (b) CH3CH"CHBr (c) BrCH"CHBr (d) (CH3)2C"CHCH3 (e) (CH3)2CHCH"CHCH3

Four stereoisomers exist for 3-penten-2-ol. 3-Penten-2-ol OH CH39CH"CH9CH9CH3 (a) Explain how these four stereoisomers arise. (b) Draw the stereoisomer having the E confi guration about the carbon-carbon double bond and the R confi guration at the chiral center.

Measure the CH3, CH3 distance in the energy-minimized model of cis-2-butene, and the CH3, H distance in the energy-minimized model of trans-2-butene. In which isomer is the nonbonded interaction strain greater?

Measure the C"C!C bond angles in the energy-minimized models of the cis and trans isomers of 2,2,5,5-tetramethyl-3-hexene. In which case is the deviation from VSEPR predictions greater?

Measure the C!C!C and C!C!H bond angles in the energy-minimized model of cyclohexene and compare them with those predicted by VSEPR. Explain any differences

Measure the C!C!C and C!C!H bond angles in the energy-minimized models of cis and trans isomers of cyclooctene. Compare these values with those predicted by VSEPR. In which isomer are deviations from VSEPR predictions greater?

Show that the structural formula of vitamin A (Section 5.4) can be divided into four isoprene units bonded head-to-tail and cross-linked at one point to form the six- membered ring

Following is the structural formula of lycopene, C40H56, a deep-red compound that is partially responsible for the red color of ripe fruits, especially tomatoes. Approximately 20 mg of lycopene can be isolated from 1 kg of ripe tomatoes. Lycopene is an important antioxidant that may help prevent oxidative damage in atherosclerosis. CH3 CH3 H3C CH3 CH3 CH3 CH3 H3C H3C CH3 Lycopene (a) Show that lycopene is a terpene, that is, its carbon skeleton can be divided into two sets of four isoprene units with the units in each set joined head-to-tail. (b) How many of the carbon-carbon double bonds in lycopene have the possibility for cis,trans isomerism? Of these, which are trans and which are cis?

As you might suspect, b-carotene, C40H56, precursor to vitamin A, was fi rst isolated from carrots. Dilute solutions of b-carotene are yellow, hence its use as a food coloring. In plants, it is almost always present in combination with chlorophyll to assist in the harvesting of the energy of sunlight and to protect the plant against reactive species produced in photosynthesis. As tree leaves die in the fall, the green of their chlorophyll molecules is replaced by the yellows and reds of carotene and carotene- related molecules. Compare the carbon skeletons of b-carotene and lycopene. What are the similarities? What are the differences? H3C CH3 CH3 CH3 CH3 CH3 CH3 H3C H3C CH3 b-Carotene

Calculate the index of hydrogen defi ciency for b-carotene and lycopene.

a-Santonin, isolated from the fl ower heads of certain species of Artemisia, is an anthelmintic (meaning against intestinal worms). This terpene is used in oral doses of 60 mg to rid the body of roundworms such as Ascaris lumbricoides. It has been estimated that over one third of the worlds population is infested with these slender, thread-like parasites. O CH3 CH3 O CH3 O a-Santonin (a) Locate the three isoprene units in santonin, and show how the carbon skeleton of farnesol might be coiled and then cross-linked to give santonin. Two different coiling patterns of the carbon skeleton of farnesol can lead to santonin. Try to fi nd them both. (b) Label all chiral centers in santonin. How many stereoisomers are possible for this molecule? (c) Calculate the index of hydrogen defi ciency for santonin

Pyrethrin II and pyrethrosin are two natural products isolated from plants of the chrysanthemum family. Pyrethrin II is a natural insecticide and is marketed as such. (a) Label all chiral centers in each molecule and all carbon-carbon double bonds about which there is the possibility for cis,trans isomerism. (b) State the number of stereoisomers possible for each molecule (c) Show that the bicyclic ring system of pyrethrosin is composed of three isoprene units. (d) Calculate the index of hydrogen defi ciency for each of these natural products. O O O O H CH3 CH2 C C C H H H H3COC H3C CH3 CH3 H H CH3 CH3 CCH3 CH2 O O H O O O Pyrethrin II Pyrethrosin

Limonene is one of the most common inexpensive fragrances. Two isomers of limonene can be isolated from natural sources. They are shown below. The one on the left has the odor of lemons, and the one on the right has the odor of oranges. Limonene (a) What kind of isomers are they? (b) Are E,Z isomers possible in limonene? (c) Why do these two isomers smell different?

Napetalactone is the active ingredient of catnip. It is isolated as an oil from the plant nepata cataria. Show that it is a terpene, that is, that its carbon skeleton can be divided into isoprene units. Is the molecule chiral? How many stereoisomers are possible? Nepetalactone (oil of catnip) H3C O O CH3

Bromine adds to cis and trans-2-butene to give different diastereomers of 2,3-dibromobutane. What does this say about the mode of addition of bromine to this alkene? Br2 H H3C CH3 Br Br H Br2 H H3C H Br Br CH3