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Villanova CHM 2201 - How to Calculate the Index of Hydrogen Deficiency

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1 How to Calculate the "Index of Hydrogen Deficiency" (also known as the "Units of Unsaturation") Given a Molecular Formula Overview: Frequently, a great deal can be learned about an unknown substance simply from knowledge of its molecular formula. This information is based on the following general molecular formulas. acyclic alkanecycloalkane or alkenecyclic alkene, diene, or alkyne,for exampleCnH2n+2CnH2nCnH2n-2Difference of 2 hydrogensDifference of 2 hydrogens Notice that each time a ring or a ! bond is introduced into a molecule, the number of hydrogens in the molecular formula is reduced by two. For every triple bond (two ! bonds) introduced into a molecule, the number of hydrogens in the molecular formula is reduced by four. When the molecular formula for a compound contains non-carbon or non-hydrogen elements, the ratio of carbon to hydrogen may change. Following are three simple rules that may be used to predict how this ratio will change. 1. To convert the formula of an open-chain, saturated hydrocarbon to a formula containing Group V elements (N, P, As, Sb, Bi), one additional hydrogen must be added to the molecular formula for each such Group V element present. In the following examples, each formula is correct for a two-carbon acyclic, saturated compound. C2H6, C2H7N, C2H8N2, C2H9N3 2. To convert the formula of an open-chain, saturated hydrocarbon to a formula containing Group VI elements (O, S, Se, Te), no change in the number of hydrogens is required. In the following examples, each formula is correct for a two-carbon acyclic, saturated compound. C2H6, C2H6O, C2H6O2, C2H6O3 3. To convert the formula of an open-chain, saturated hydrocarbon to a formula containing Group VII elements (F, Cl, Br, I), one hydrogen must be subtracted from the molecular formula for each Group VII element present. In the following examples, each formula is correct for a two-carbon acyclic, saturated compound. C2H6, C2H5F, C2H4F2, C2H3F3 CHM 2201 Organic Chemistry Lab IFall 2008Villanova University Department of Chemistry2""" The"index of hydrogen deficiency"&sometimes"calle d"the"unsaturation index0"is"the"number"of"the"!"bonds"and/or"rings"a"molecule"contains.""I t"is"determined"from"an"e:amination"of "the"molecular"formula"on"an"unkno<n" substance"and"from"a"comparison"of" th at" formula" <ith" a" formula" for" a" corresponding" acyclic," saturated" compound."" The"difference" in" the" numbers" of" hydrogens" bet<een" these" f ormulas," <hen" divided" by" 2,"gives"the" inde:"of"hydrogen"deficiency."" The" inde:" of" h ydrogen"d eficiency"can" be" very"useful" in" structure" determination"problems.""To reiterate: A great deal of information can be obtained about a molecule just from the molecular formula."""" For"e:ample,"a"compound"<ith"an"inde:"of"one" must"have"one"double"bond"or"one"ring,"but"it"cannot"have"both"structural"features.""A"co mpound"<ith"an"inde:"of"two"could"have"a"triple"bond,"or"it"could"have"t<o"double"bonds,"or"t<o"rings,"or"one"of"each.""BenDene"c ontains"one"rin g"and"three"Edouble"bondsF"and"thus"has" an"inde:"of"hydrogen"deficiency"of"four.""Any"substance"<ith"an"inde:"of"four"or"more"can"possibly"cont ain"an"arene"ringG"a"substance"<ith"an"inde:"of"less"than"four"cannot"contain"such"a"ring.""""To"determine"the"inde:"of"hydrogen"deficiency"for"a"unkno<n"com pound"given"its"molecular"formula,"apply"the"follo<ing"steps.""1." Determine"th e" formula"for" the" saturated,"acyclic"hydrocarbon"containing"the"same"number"of"carbon"atoms"of"the"unkno<n"substance.""2." Correct" this" formula" for" the" nonKhydrocarbon" elements" present" in" the" unkno<n.""Add" one" hy drogen" atom" for"each"Group" V" element"present"&i.e.," N0," and" subtract"one"hydrogen"atom"for"each"Group"VII"element"&i.e.,"a"halogen0"present.""3." Compare" t his" fo rmula"<ith" the" molecular" formula" of" the" unkno<n." " Determine"the"number"of"hydrogens"by"<h ich"the"t<o"formulas"differ.""4." Divide" the" difference" in" the" number" of" hydrogens" by" two" to" obtain" the" inde:" of"hydrogen"deficiency.""The"eQuals"the"sum"of"the"number"of"!"bonds"and/or"r ings"in"the"structural"formula"of"the"unkno<n"substance.""&From"Introduction to Spectroscopy,"by"Pavia,"Lampman,"and"KriD0""CHM 2201 Organic Chemistry Lab IFall 2008Villanova University Department of


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