Organic Chemistry Sixth Edition - Chapter 4 PDF

Organic Chemistry Sixth Edition Chapter 4 1  Alkanes General concepts Nomenclature As discussed, naming will not be emphasized for this course; but the following slides give a general overview. This will allow us to use names as needed in...

Organic Chemistry Sixth Edition Chapter 4 1  Alkanes General concepts Nomenclature As discussed, naming will not be emphasized for this course; but the following slides give a general overview. This will allow us to use names as needed in future topics and emphasizes why nomenclature is needed for specificity of location, type of molecule, functional group, etc. Properties Conformations Reactions 2 Aliphatic hydrocarbons containing carbon and hydrogen All have sigma (single) bonds Straight chains are named acyclic alkanes general formula of CnH2n+2 Cycloalkanes are joined in a ring 3  Tetrahedral Geometry of Carbon All C atoms in an alkane are surrounded by four groups, making them sp3 hybridized and tetrahedral, with all bond angles of 109.5 degrees. 3-D representations as ball-and-stick models Lewis structures show “wedges” in and out of the plane Note the # of C & H 4  Drawing Propane CH3CH2CH3, propane, has a molecular formula C3H8. C atom has two bonds in the plane (solid lines), one bond in front (on a wedge) and one bond behind the plane (on a dashed line). (also skeletal and Ball & Stick models) 5  Drawing Pentane five-carbon alkane CH3CH2CH2CH2CH3 molecular formula C5H12. Each of the following representations has five carbons in a row and represents pentane, not isomers of pentane. 6  Straight chain alkanes The suffix “-ane” identifies a molecule as an alkane. Table 4.1 Summary: Straight-Chain Alkanes; each molecule is one CH2 larger (methylene group) Number of Number of Number Molecular Name (n- Number of C Molecular Name (n- constitutio constitutional of C formula alkane) atoms formula alkane) nal isomers atoms isomers 1 CH4 methane — 9 C9H20 nonane 35 2 C2H6 ethane — 10 C10C22 decane 75 3 C3H8 propane — 11 C11C24 undecane 159 4 C4H10 butane 2 12 C12H26 dodecane 355 5 C5H12 pentane 3 13 C13C28 tridecane 802 6 C6C12 Hexane 5 14 C14H30 tetradecane 1858 7 C7C14 heptane 9 15 C15H32 pentadecane 4347 8 C8H18 octane 18 20 C20H42 icosane 366,319 7  Constitutional Isomers There are two different ways to arrange four carbons, giving two compounds with molecular formula C4H10, named butane and isobutane. Butane and isobutane are constitutional isomers—two different compounds with the same molecular formula. Constitutional isomers (also called structural isomers) differ in the way the atoms are connected to each other. 8  Cycloalkanes Cycloalkanes contain carbons joined in one or more rings. Because their general formula is CnH2n, they have two fewer H atoms than an acyclic alkane with the same number of carbons. 9  Cycloalkanes Cycloalkanes have molecular formula CnH2n and contain carbon atoms arranged in a ring. Simple cycloalkanes are named by adding the prefix cyclo- to the name of the acyclic alkane having the same number of carbons. 10  Nomenclature (general) The name of every organic molecule has 3 parts: 1. The parent name indicates the number of carbons in the longest continuous chain. 2. The suffix indicates what functional group is present. 3. The prefix tells us the identity, location, and number of substituents attached to the carbon chain. Rules for naming are based on the International Union of Pure and Applied Chemistry system: IUPAC 11  Labeling Carbons and Hydrogen by location (important as these position will be used in future topics with functional groups) Carbon attached to one Carbon is a 1o carbon; hydrogens are 1o Carbon attached to two Carbons is a 2o carbon; hydrogens are 2o Carbon attached to 3 Carbons is a 3o carbon; hydrogens are 3o 12  Naming Substituents -Alkyl Groups Carbon substituents bonded to a long carbon chain are called alkyl groups. An alkyl group is formed by removing one H atom from an alkane. To name an alkyl group, change the –ane ending of the parent alkane to –yl. methane (CH4) becomes methyl (CH3—) ethane (CH3CH3) becomes ethyl (CH3CH2—). Propyl, butyl, 13  Three Carbon Alkyl Groups starts the process of naming from different carbon locations………. three- or four-carbon alkyl groups: parent hydrocarbons have more than one type of hydrogen atom. Ex. propane has different H atoms removal of each of these H atoms forms a different alkyl group with a different name, propyl or isopropyl. 14  Naming Four Carbon Alkyl Groups…more complicated two different butane isomers which yield four possible alkyl groups containing four carbon atoms. 15  Completing Structure Names Note that by understanding the name, we can; 1. Know the parent chain (straight chain or cyclic); 2. The position of substituents and/or functionality; 3. Distinguish isomers from each other 16  Common Names of Polycyclic Molecules Some organic compounds are identified using common names that do not follow the IUPAC system of nomenclature. names were in place before the IUPAC system was adopted, and are still widely used. names may be descriptive of shape and structure, like those below: Figure 4.4 17  Properties of Alkanes nonpolar C—C and C—H bonds. They only exhibit weak van der Waals forces. This affects solubility and boiling point and melting point characteristics of alkanes. Solubility of alkanes Alkanes are soluble in organic solvents. Alkanes are insoluble in water. 18  Boiling and Melting Points of Alkanes Alkanes have low bp’s an mp’s compared to more polar compounds of comparable size. Bp and mp increase as the number of carbons increases due to increased surface area. Same discussion as before on forces of attraction 19  Boiling and Melting Points of Alkanes bp of isomers decreases with branching due to decreased surface area. Mp increases with increased symmetry. 20  Conformations of Acyclic Alkanes Conformations are different arrangements of atoms that are interconverted by rotation about single bonds. 21  Eclipsed and Staggered Conformations Names are given to two different conformations. In the eclipsed conformation, the C—H bonds on one carbon are directly aligned with the C—H bonds on the adjacent carbon. In the staggered conformation, the C—H bonds on one carbon bisect the H—C—H bond angle on the adjacent carbon. 22  Conformations and Dihedral Angle Rotating the atoms on one carbon by 60 degrees converts an eclipsed conformation into a staggered conformation, and vice versa. The angle that separates a bond on one atom from a bond on an adjacent atom is called a dihedral angle. ethane in the staggered conformation, the dihedral angle for the C—H bonds is 60 degrees; for eclipsed ethane, it is 0 degree. From: http://guweb2.gonzaga.edu/faculty/cro nk/CHEM440pub/dihedral.html Dihedral angle 23  Newman Projections End-on representations for conformations are commonly drawn using a convention called a Newman projection. HOW TO Draw a Newman Projection: Step Look directly down the C—C bond (end-on), and draw a circle with a dot in the center to represent the carbons of the C—C bond. 24  Completing a Newman Projection Step 2. Draw in the bonds. Draw the bonds on the front C as three lines meeting at the center of the circle. Draw the bonds on the back C as three lines coming out of the edge of the circle. Step 3. Add the atoms on each bond. 25  Newman Projections - Ethane Figure 4.6 26  Conformations of Ethane The staggered and eclipsed conformations of ethane interconvert at room temperature. The staggered conformations are more stable (lower in energy) than the eclipsed conformations. Electron-electron repulsion between bonds in the eclipsed conformation increases its energy compared with the staggered conformation, where the bonding electrons are farther apart. 27  Torsional Energy of Ethane The energy difference between staggered and eclipsed conformers is called torsional energy. The difference in energy between staggered and eclipsed conformers is ~3 kcal/mol, with each eclipsed C—H bond contributing 1 kcal/mol. Torsional strain is an increase in energy caused by eclipsing interactions. Because of rotation about the bond Figure 4.8 Look at red ball position 28  Newman Projections - Propane Figure 4.7 Remember: newman projections are always along a C – C bond 29  Newman Projections - Butane An energy minimum and maximum occur every 60 degrees as the conformation changes from staggered to eclipsed. Conformations that are neither staggered nor eclipsed are intermediate in energy. Butane and higher molecular weight alkanes have several C—C bonds, all capable of rotation. Figure 4.9 Six different conformations of butane 30  Anti and Gauche Conformations A staggered conformation with two larger groups 180 degrees from each other is called anti. A staggered conformation with two larger groups 60 degrees from each other is called gauche. The staggered conformations are lower in energy than the eclipsed conformations. 31  Steric Strain The relative energies of the individual staggered conformations depend on their steric strain. Steric strain is an increase in energy resulting when non-bonded atoms are forced too close to one another. Gauche conformations are generally higher in energy than anti conformations because of steric strain. Steric strain caused by two eclipsed CH3 groups 32  Conformation and Energy of Butane Figure 4.10 Note: 1 shows the methyl groups furthest away from each other…lower energy, etc. 33  Barrier to Rotation Table 4.3 Summary: Torsional and Steric strain Energies in Acyclic Alkanes Energy Increase Energy Increase Type of interaction KJ/mol Kcal/mol H,H eclipsing 4.0 1.0 H,CH3 eclipsing 6.0 1.4 CH3,CH3 eclipsing 11 2..6 gauche CH3 groups 3.8 0.9 The energy difference between the lowest and highest energy conformations is called a barrier to rotation. All this means is that energy is needed to cause the rotation……. 34  Zigzag Skeletal Structures Since the lowest energy conformation has all bonds staggered and all large groups anti, alkanes are often drawn in zigzag skeletal structures to indicate this. 35  Newman Projection Summary 1. Look at one C-C bond 2. Eclipsed vs staggered conformation 3. Dihedral angle changes due to rotation (reference one group on each carbon 4. Molecules with larger groups than H can have different staggered conformations 1. Anti 180o apart 2. Gauche 60o apart 5. Steric strain causes energy changes/difference in staggered forms 36  Cycloalkanes: added strain torsional strain and steric strain, the conformations of cycloalkanes are also affected by angle strain. Angle strain: bond angles deviate from tetrahedral angle of 109.5o Cycloalkanes are “puckered” to reduce strain (approach 109.5o, except cyclopropanes. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Alkanes/Properties_of_Alkanes/Cycloalkanes/Ring_Strain_and_the_Stru cture_of_Cycloalkanes 37 Strain in Cyclopropane: Flat: 60o https://chem.libretexts.org/Courses/Athabasca_University/Chemistry_350%3A_Organic_Chemistry_I/Chapter_0 4%3A_Organic_Compounds%3A_Cycloalkanes_and_their_Stereochemistry/4.3_Stability_of_Cycloalkanes_- _Ring_Strain#:~:text=Eclipsing%20(torsional)%20strain%20exists%20when,enough%20to%20close%20the%2 0ring. https://www.masterorganicchemistry.com/2014/04/03/cycloalkanes-ring-strain-in-cyclopropane-and-cyclobutane/ 38  Three to Ten Carbon Cycloalkanes Figure 4.11 Cycloalkanes distort their shapes to alleviate angle and torsional strain. 39  Cyclohexane cyclohexane adopts a puckered “chair” conformation: more stable than any other possible conformation. 40  Chair Conformation chair conformation is stable eliminates angle strain (all C—C—C angles are 109.5 degrees); and torsional strain (all hydrogens on adjacent C atoms are staggered). Figure 4.12 In cyclohexane, three C atoms pucker up and three C atoms pucker down, alternating around the ring. 41  Axial and Equatorial Positions Each C in cyclohexane has two different kinds of hydrogens: (1) Axial hydrogens are located above and below the ring (along a perpendicular axis). (2) Equatorial hydrogens are located in the plane of the ring (around the equator). 42  HOW TO Draw the Chair Form of Cyclohexane The bottom 3 C’s come out of the page, and bonds to them are sometimes in bold. 43  HOW TO Draw the Chair Form of Cyclohexane From chapter The axial H is down on a down C, so the equatorial H must be up. The axial H is up on an up C, so the equatorial H must be down. Basically, H’s on each opposite C are as far away from each other as possible 44  Conformational Change – Ring-Flipping Cyclohexanes undergo a conformational change called “ring-flipping.” Axial and equatorial H atoms are interconverted during a ring-flip; they change positions Notice the position of the red and blue balls 45  Conformational Change – Ring-Flipping of Chair two possible chair conformations. Equatorial vs Axial position equatorial position has more room than the axial position, larger substituents are more stable in the equatorial position. Figure 4.13 46  Chair Conformations and Energy The two chair conformations of cyclohexane are different, so they are not equally stable. Larger axial substituents create destabilizing (and thus unfavorable) 1,3- diaxial interactions. In methylcyclohexane, Conformation B is less stable than Conformation A. 47  Preference of Equatorial Position in Substituted Cyclohexanes Three dimensional representations for the two chair conformations of methylcyclohexane. Figure 4.15 48  Preference of Equatorial Position in Substituted Cyclohexanes The larger the substituent on the six-membered ring, the higher the percentage of the equatorial conformation at equilibrium. Figure 4.16 49  Disubstituted Cycloalkanes There are two different 1,2-dimethylcyclopentanes—one having two CH3 groups on the same side of the ring and one having them on opposite sides of the ring. A and B are stereoisomers. 50  Cis and Trans Stereoisomers Stereoisomers are isomers that differ only in the way the atoms are oriented in space. The prefixes cis and trans are used to distinguish these isomers. The cis isomer has two groups on the same side of the ring. The trans isomer has two groups on opposite sides of the ring. Same side of the ring opposite sides of the ring 51  Disubstituted Cycloalkanes 1,4-dimethylcyclo-hexane, cis and trans stereoisomers. Each of these stereoisomers has two possible chair conformations. 52 Trans Disubstituted Cycloalkanes 53  Cis Disubstituted Cycloalkanes Figure 4.17 Note the positions of the methyl groups 54  Can Alkanes undergo reactions What can be done to C – C bonds and C-H bonds? 2 basic reactions: - oxidation reactions - combustion 55  Oxidation and Reduction Reactions Oxidation results in an increase in the number of C—Z bonds. (carbon loses e density to the Z atom which is more electronegative) Oxidation results in a decrease in the number of C—H bonds. Reduction results in a decrease in the number of C—Z bonds. (carbon gains e density because of the loss of the Z atom Reduction results in an increase in the number of C—H bonds. Figure 4.17 56  Combustion of Alkanes Alkanes undergo combustion — in the presence of oxygen they form carbon dioxide and water. example of an oxidation-reduction reaction. Every C—H and C—C bond in the starting material is converted to a C—O bond in the product. 57 End Chapt 4 HW: 4.37a; 4.44; 4.45b;4.52a; 4.54d; 4.57a; 4.60a; 58