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CH 320M (28)
Chapter 2

CH 320M Chapter 2 Textbook Notes

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University of Texas at Austin
CH 320M
John A.Colapret

Alkanes and Cycloalkanes Thursday, May 16, 2013 12:36 AM Hydrocarbon- compound composed of only carbon and hydrogen Alkanes - saturated hydrocarbons(only C-C singlebonds) Aliphatic hydrocarbons- physical properties of the higher membersof this class resemblethose of the long carbon-chain moleculesin animal fats and plant oils [refer to alkanes] Unsaturatedhydrocarbon- contains one or more C-C double bond, triplebond or benzene ring 2.1 The Structure of Alkanes Friday, May17, 2013 2:21 PM Line-angle formula - an abbreviated way to draw the structural formulas in which vertices and line endings represent carbons Alkanes have the general molecular formula: Chapter 2 Page 2 2.2 Constitutional Isomerism in Alkanes Friday, May 17, 2013 2:27 PM Constitutionalisomers - compounds that have the same molecularformula but different structural formulas • Differ in the kinds of bonds they have or connectivity of their atoms • Example: butane and 2-methylpropane Examples Friday, May 17,2013 2:31 PM Chapter 2 Page 4 2.3 Nomenclatureof Alkanesand the IUPAC System Friday, May 17,2013 2:33 PM RULESOF NAMING ALKANES: IUPAC (International Union of Pure and Applied Chemistry) - established a set of 1. Name for an alkane with unbranched chain of C atoms consists of a prefix showing the # of rules to name every organic compound C atoms in the chain and the ending -ane 2. For branch-chain alkanes, selectthe LONGEST chain of C atoms as the parent chain --> its name becomes the root name 3. Give each substituent on the parent chain a name and a number a. The # shows the C atom of the parent chain to which the substituent is bonded b. Use a hyphen to connect the # to the name i. 4. If there's one substituent, number the parent chain from the end that gives the substituent the lower number i. 5. If there are 2< identical substituents, number the parent chain from the end that gives the Alkyl group - substituent group derived from an alkane by the removal of a hydrogen lower number to the substituent encountered first. atom • Replaces suffix -ane with -yl a. The number of times the substituent occurs is indicated by di-, tri-, tetra-, penta-, Example: hexa- and so on i. Use comma to separate position numbers ii. 6. If there are 2< different substituents, list them in alphabetical order and number the chain from the end that gives the lower number to the substituent encountered first a. If there are different substituents in equivalent positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number i. 7. The prefixes di-, tri-, tetra- and so on are not includedin alphabetizing i. 8. When there're 2< parent chains of identical length, choose the parent chain with the greater number of substituents i. Chapter 2 Page 5 Example Friday, May 17, 2013 3:55 PM Chapter 2 Page 6 Common Names Friday, May 17, 2013 3:55 PM Iso- = one end of an otherwiseunbranched chain terminates in a (C3 2 CH-group Neo- = it terminatesin -C(C3 3 The IUPAC System - A General System of Nomenclatures Tuesday, May 21,2013 4:08 PM Nameof compoundconsists of: prefix,infix(a modifyingelement insertedinto a word),and a suffix 1. PREFIX= # of C atomsin the parentchain 2. INFIX = nature of the C-Cbonds in the parentchain a. 3. SUFFIX= classof compound to which the substance belongs a. Chapter 2 Page 8 Classification of Carbon and Hydrogen Atoms Tuesday, May 21, 2013 4:12 PM ClassifyC atom as primary, secondary, tertiary or quaternary H atoms can also be classified! 2.4 Cycloalkanes Tuesday, May 21, 2013 4:17 PM Cyclic hydrocarbon- a hydrocarbon that contains carbon atoms joined to form a ring Cycloalkane - when all carbons of the ring are saturated Structure and Nomenclature Tuesday, May 21, 2013 4:18 PM Cyclopentanes- five-memberedrings Cyclohexanes - six-memberedrings CYCLOALKANESCONTAINS FEWER H ATOMS! Naming a cycloalkane: 1. Prefixthe name of the corresponding open-chain alkane with cyclo- a. Nameeach substituent on the ring i. If there's only 1 substituent on the ring, there's no need to giveit a number ii. If there are 2 substituents, number the ring by beginningwith the substituent of lower alphabetical order iii. If there are 3< substituents, number the ring to givethe substituents the lowest set of numbers and list them in alphabetical order Example Tuesday, May 21, 2013 4:39 PM Bicycloalkanes Tuesday, May 21, 2013 4:40 PM Bicycloalkane - an alkane that contains two rings that share 2 C atoms in common • Bridgehead carbons - the shared carbon atoms • Bridge - the carbon chain connecting them • General formula: C n 2n-2 • Examples Tuesday, May21, 2013 4:56PM Chapter 2 Page 14 2.5 Conformations of Alkanes and Cycloalkanes Tuesday, May 21, 2013 4:57 PM Strain - measure of the energystored in a compound due to a structural distortion • Intramolecularstrain has 3 types ○ Torsional strain ○ Steric strain ○ Anglestrain The interconversionbetween the strained and relaxedstructures results form collisionsand thermal motions! Alkanes Tuesday, May 21, 2013 5:16 PM Conformation - any 3D arrangementof atoms that results from rotation about singlebonds Staggered conformation - the atoms or groups on one carbon are as far apart as possible from atoms groups on an adjacent carbon • Lowest energy= most stable Newman projection - a way to viewa molecule by looking along a C-C singlebond to help evaluatethe relativeorientations of attached groups Eclipsed conformation- the atoms or groups on one carbon are as close as possible to the atoms or groups on an adjacent carbon • Highestenergy = least stable Conformationalisomers - different conformations Dihedral angle - anglecreated by two intersecting planes(each plane definedby 3 atoms); has the symbol θ • Eclipsed has θ = 0° in ethane • Staggered has θ = 60° in ethane Torsional strain - strain that ariseswhen nonbonded atoms separated by 3 bonds are forced from a staggeredconformation to an eclipsed conformation; also called eclipsed-interaction strain Anti conformation- a conformation about a singlebond in which 2 groups on adjacent carbons lieat a dihedral angle of 180° Gauche conformation- a conformation about a singlebond of an alkane in which two groups on adjacent carbons lieat a dihedral angle of 60 Angle strain - the strain that arises when a bond angleis either compressed or expanded compared to its optimal value Steric strain - the strain that arises when nonbonded atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii would allow • Alsocalled nonbondedinteraction strain or can der waals strain Example Saturday, August 31, 2013 6:31 PM Cycloalkanes Saturday,August31, 2013 6:32 PM Small ring strain - a strain associated with ring sizes below six that arises from nonoptimal bond angles • Due to the C-C-C bond angles not reaching the optimal tetrahedral angle of 109.5 i. Cyclopropane a.
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