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UH CHEM 3331 - Cycloalkanes
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CHEM 3331 1nd Edition Lecture 7 Outline of Last Lecture I. Conformational analysisII. Newman projection formula for ethaneIII. Newman projection formula for propaneIV. Newman projection formula for butaneOutline of Current Lecture I. CycloalkanesII. NomenclatureIII. StabilityIV. Conformational analysisCurrent LectureI. CycloalkanesThe formula for cycloalkanes is CnH(2n). Cycloalkanes are unsaturated and create a ring of carbons with hydrogens bonded to each carbon. II. NomenclatureThey follow the same basic rules as naming alkanes except you add cyclo-alkane to the end instead of an alkane. However, the substituent is always in place one and we number substituents so they have the lowest number possible. (see examples below). Cycloalkanes also have stereoisomers, but only with 2 substituents and you place cis- or trans- in front of the name if applicable. (see examples below).These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.III. StabilitySince the alkanes are made into rings they don’t possess their ideal sp3 angle. This causes what is called ring strain. Cyclopropane has an angle of 60˚, cyclobutane has 90˚, cyclopentane has 108˚, cyclohexane has 120˚. Since these angles are not 109.5˚, the molecules will move to other planes to get closer to the ideal 109.5˚. The least stable compounds have higher energy. (CH2)n + 3/2 n O2  nCO2 + nH2O + ECyclopropane has ΔE of 166, cyclobutane= 163, cyclopentane= 158.7, cyclohexane= 157 (in Kcal/mol). We also have the non-strain reference point which is from a normal alkane. This ΔE is 157 Kcal/mol. From this we determine the strain. For cyclopropane= 9, cyclobutane= 6 cyclopentane= 1.7, and cyclohexane= 0.IV. Conformational analysisCyclopropane cannot do much moving to keep its hydrogens from being eclipsed.Cyclobutane:Cyclopropane: Cyclohexane: this is called the chair conformation. It also possesses axial and equatorial hydrogens. The blue are equatorial and the red are axial. Theaxial and equatorial hydrogen switch when flipped. From equatorial to axial (CH3) costs 1.8 Kcal/mol. For F= 0.2, CH3= 1.8, and C(CH3)3=


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UH CHEM 3331 - Cycloalkanes

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