University of Cincinnati Cincinnati,OH 45221
Which Organic Molecule Should I Pick? Robert Perkins Kwantlen College Surrey, BC, Canada V3T 5H8 One of the difficulties encountered by students facing organic chemistry for the first time is the vast number of possible compounds for a given molecular formula. There is quite oRen more than one correct answer to a given question. Every student needs plenty of practice in drawing the structures of organic compounds and providing their IUF'AC names. At the same time, i t also is desirable for them to be able to relate the molecular structure of a compound with its chemical reactivity The followingtwo questions will give the student a chance to practice those skills. Question 1 There are many possible compounds that have the same molecular formula C5H100.Provide the structures (and IUPAC names) of compounds that constitute: (a) a pair of saturated positional isomers (b) a pair of unsaturated positional isomers
a pair of saturated skeletal isomers (dl a pair of unsaturated skeletal isomers (el a pair of saturated functional isomers (fl a pair of unsaturated functional isomers (g) a pair of saturated geometric isomers (h) a pair of unsaturated geometric isomers (i) a pair of saturated optical isomers (i)a pair of unsaturated optical isomers (c)
Acceptable Solution Saturated compounds contain only sigma bonds. Unsaturated compounds must contain a t least one pi bond. (a) Positional isomers have the same functional group present with the same chain length.
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Journal of Chemical Education
1-ethyl-1-cyclopropanol Bethyl-1-cyciopropanol (b) Positional isomers have the same functional group Present with the same chain length.
,'-I"' 0
2-pentanone 3-pentanone (c) Skeletal isomers have the same functional group present with a different chain length.
SOHL
*
cyciopentanol 2-ethyl-1-cyclopropanoi (dl Skeletal isomers have the same functional group present with a different chain length.
+H
2-methylbutanal
2.2-dimethylpropanal
(el Functional isomers have different functional erouns.
(el Does not react with any of H2/Pt,Na, or KMnO,
Acceptable Solution (a) Reacts with Na, but not with KMn04. cyclopentanol
methoxycyclobutane
(flFunctional isomers have different functional groups. OH I
I-methyl-I-cyclobutanol
f-i''
Euidence 1. must be an alcohol to react with Na 2. must be a saturated 3" alcohol for no reaction with KMn04
4-penten-2-01 3-pentanone (g) Geometric isomers have different bonding topologies.
cisl-methoxy-2-butme transl-methoxy-2-butene (h) Geometric isomers have different bonding topologies. OH
OH
(b) Reacts with KMnO,, but not with Na
4-methoxy-I-butene
pentanal
Euidence 1. cannot be an alcohol because no reaction with Na 2. contains either an aldehyde or an alkene to react with
KMnO, (c) Reacts with H2/Pt and Na tmn42-ethyl-I-cyclopropanol cis2-ethyl-I-cyclopropanol
(i) Optical isomers are non-superimposable mirror images of each other.
4-penten-2-01 both are I-cyclopropyl-$-ethanol ti) Optical isomers a r e non-superimposable mirror images of each other. H
H
both are 1-pentenQ-01 Question 2 There are many possible compounds that have the same molecular formula C5Hs0. Provide t h e structure of a compound (having molecular formula CSH,oO)that will satisfy the various requirements below. Indicate what each reactionlack of reaction tells you about the possible functional groups present. For example, if a compound gave a positive Brady's test, it would contain either the aldehyde or ketone functional group. If it also gave a negative Tollen's test, then the compound would have to be a ketone. Note: The structures of the products are not required. (a) Reacts with Na, but not with KMnO, (b) Reacts with KMn04,but not with Na (cl Reacts with H2/Ptand Na (dl Reacts with H2/Pt, hut not with KMnO,
Evidence 1. must be an alcohol to react with Na 2. must be an alkene to react with H2/Pt
(dl Reacts with H21Pt, but not with KMn0,
3-pentanone Evidence 1. must contain a double bond to react with H 2 m 2. must be a ketone as no reaction with KMn0,
(el Does not react with any of H2/Pt, Na, or KMnOa
ethoxycyclopropane Evidence 1. the compaund cannot be an alcohol, nor contain any double
honds 2. it must be a cyclic compound containing an ether
Volume 72 Number 2
February 1995
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