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TAMU CHEM 227 - Cycloalkanes
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CHEM 227 1st Edition Lecture 5Outline of Last Lecture I. 3.1 Functional GroupsII. 3.2 Alkanes and Alkane IsomersIII. 3.3 Alkyl GroupsIV. 3.4 Naming AlkanesV. 3.5 Properties of AlkanesVI. 3.6 Conformations of EthaneVII. 3.7 Conformations of other AlkanesOutline of Current Lecture I. Cycloalkanes general overviewII. 4.1 Naming cycloalkanesIII. 4.2 Cis-trans isomerism in cycloalkanesIV. 4.3 Stability of cycloalkanes: ring strainV. 4.4 Conformations of cycloalkanesVI. 4.5 Conformations of cyclohexaneVII. 4.6 Axial and Equatorial bonds in cyclohexaneVIII. 4.7 Conformations of monosubstitutedcyclohexanesIX. 4.8 Conformations of disubstitutedcyclohexanesCurrent Lecture- Cycloalkanes/alicyclic compounds: consist of rings of –CH2 unitso Saturated monocyclic compuoundso Have the general formula (CnH2n)4.1 Naming CycloalkanesThese 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.Naming:1. find the parent: count the number of carbons in the ring and number in largest substituent. If number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane. If the number of carbon atoms in the largest substituent is greater than the number in the ring, the compound is named as a cycloalkyl-substituted alkane.2. Number the substituents, and write the name: start with a point of attachment of a substituent as attached to carbon 1 and number the ring so that the second substituent has as low a number as possible.- when two or more groups could potentially receive the same number are present, number them by alphabetical priority, ignoring prefixes such as di- and tri-, etc.- if halogens are present, treat them just like alkyl groups4.2 Cis-Trans Isomerism in cycloalkanes- Cycloalkanes are less flexible than open-chain alkanes- Much less conformational freedom in cycloalkanes- Because of their cyclic structure, cycloalkanes have 2 faces as viewed edge-on : “top” face and “bottom” face- substituted cycloalkanes show isomerism cis /transo substituents on the same face: ciso substituents on opposite faces: trans4.3 Stability of cycloalkanes: ring strain- Rings larger than 3 atoms are not flat- Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain- Rings larger than 6 have many more possible conformations and are difficult to analyze- Angle strain: the strain induced in a molecule when bond anles are forced to deviate from the ideal 109º tetrahedral value- Baeyer (1885): since carbon prefers to have bond angles of approximately 109°, 5 and 6 membered rings are the most common.4.4 Conformations of cycloalkanes- Cyclopropane- 3-membered ring must have planar structure- Symmetrical with C–C–C bond angles of 60°- Requires that sp3 based bonds are bent (and weakened)- All C-H bonds are eclipsed4.5 Conformations of Cyclohexane- Cyclohexane- free of angle strain and torsional strain All C’s have the ideal 109.5 degrees angles- Chair conformation: a visual of cyclohexane that allows us to see all substituents and their location/orientation on the ringDrawing Chair Conformations4.6 Axial and Equatorial Bonds in Cyclohexane- Two types of positions for substituents in cyclohexaneo Axial and Equatorialo Chair cyclohexane has six axial(red) hydrogens perpendicular to thering and six equatorial (blue) near the plane of the ringo Each carbon atom in cyclohexane has one axial and one equatorial hydrogeno Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangemento Ring flip: Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions Axial substituents become equatorial and vice versa However, all substituents stay on the same side of the ring4.7 Conformations of monosubstituted cycloalkanes- Cyclohexanes are more stable when their larger substituents are in the equatorial position- Cyclohexane ring rapidly flips between the two possible chair conformations at room temp.- Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions- This is because of 1,3-diaxial interactions that occur when substituents are in the axial position on the ring- Hydrogen atoms of the axial methyl group at C1 are too close to the axial hydrogens three carbons away on C3 and C5, resulting in 7.6 kJ/mol of steric strain- Different substituents have different strain due to 1,3-diaxial interactions with hydrogen- Each one of these energy values is multiplied by 2 because of the 2 hydrogens each substituent interacts with4.8 Conformations of disubstituedcyclohexanes- There are two isomers of 1,2-dimethylcyclohexane. cis and trans: - In the cis isomer: o Consider the sum of all interactions (1,3diaxial and gauche) both conformations are equal in energy, there is no preference.- In the trans isomer:o One trans conformation has both methyl groups equatorial : only a gauche interaction and no 1,3-diaxial interactions.o The ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactions o trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial


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TAMU CHEM 227 - Cycloalkanes

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