Fundamentals in Organic Chemistry - CHEM 209 PDF

Fundamentals in Organic Chemistry - CHEM 209 Essential Organic Chemistry By Paula Yurkanis Bruice, 3rd Ed. Chapter 3 An Introduction to Or...

Fundamentals in Organic Chemistry - CHEM 209 Essential Organic Chemistry By Paula Yurkanis Bruice, 3rd Ed. Chapter 3 An Introduction to Organic Compounds Nomenclature, Physical Properties, and Representation of Structure Dr. Mohamed Mady [email protected] © 2017 Pearson Education, Ltd.  Chapter 3 An Introduction to Organic Compounds Nomenclature, Physical Properties, and Structure Paula Yurkanis Bruice University of California, Santa Barbara © 2017 Pearson Education, Ltd.  Lecture Outline  Sections covered in chapter 3 © 2017 Pearson Education, Ltd.  Forms of Carbon Diamond Graphite Fullerene © 2017 Pearson Education, Ltd.  Hydrocarbons Hydrocarbon is an organic compound consisting of hydrogen and carbon found in crude oil, natural gas, and coal. © 2017 Pearson Education, Ltd.  Organic Compound Classification © 2017 Pearson Education, Ltd.  Functional Groups  Functional group (group of atoms)- a collection of atoms at a site that have a characteristic behavior in all molecules where it occurs.  The group reacts in a typical way, generally independent of the rest of the molecule.  For example, the double bonds in simple and complex alkenes react with bromine in the same way. © 2017 Pearson Education, Ltd.  Functional Groups with Multiple Carbon–Carbon Bonds  Alkenes have a C-C Double bond  Alkynes have a C-C Triple bond  Arenes have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring (aromatic compounds) © 2017 Pearson Education, Ltd.  Functional Groups with Carbon Singly Bonded to an Electronegative Atom All these compounds have a carbon atom singly bonded to an electronegative atom—halogen, oxygen, nitrogen, or sulfur In all cases, the bonds are polar, with the carbon atom bearing a partial positive charge δ+ and the electronegative atom bearing a partial negative charge δ-. © 2017 Pearson Education, Ltd.  Functional Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups) The carbonyl group, CO (pronounced car-bo-neel) The carbonyl carbon atom bears a partial positive charge δ+, and the oxygen bears a partial negative charge δ-. © 2017 Pearson Education, Ltd.  Surveyof Functional Groups © 2017 Pearson Education, Ltd.  Survey of Functional Groups © 2017 Pearson Education, Ltd.  Problems Solution : The functional groups in this molecule are ester, tertiary amine, and carbon-carbon double bond. The molecular formula of the compound is C8H13NO2. © 2017 Pearson Education, Ltd.  Problems Identify the functional groups in each of the following molecules: Problem A. Ibuprofen, a pain reliever: Solution : An illustration shows a benzene with C1 bonded to a methine group bonded to a carboxylic acid group and methyl group. C4 is bonded to a 3-carbon chain with C2 of the chain bonded to a methyl group Problem Capsaicin, the pungent substance in chili peppers: Solution : Ether, alcohol, aromatic ring, amide, C=C bond © 2017 Pearson Education, Ltd.  Alkanes & Alkyl Groups: Isomers  Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups)  Connecting carbons can lead to large or small molecules  The formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is n  Alkanes are saturated hydrocarbons with hydrogen (no more can be added  They are also called aliphatic compounds © 2017 Pearson Education, Ltd.  Naming Straight Chain Alkanes  Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes.  Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes © 2017 Pearson Education, Ltd.  Alkane Isomers  CH4 = methane, C2H6 = ethane, C3H8 = propane  The molecular formula of an alkane with more than three carbons can give more than one structure (isomers)  C4 (butane) = n-butane and isobutane  C5 (pentane) = n-pentane, 2-methylbutane, and 2,2- dimethylpropane © 2017 Pearson Education, Ltd.  Constitutional (skeletal isomers, Structural) Isomers  Isomers that differ in how their atoms are arranged in chains are called constitutional isomers.  Compounds other than alkanes can be constitutional isomers of one another.  They must have the same molecular formula to be isomers. © 2017 Pearson Education, Ltd.  Drawing Alkane Hydrocarbons: Condensed Structures of Alkanes  We can represent an alkane in a brief form or in many types of extended form.  A condensed structure does not show bonds but lists atoms, such as  CH3CH2CH2CH3 (butane) suffix -ane  CH3(CH2)2CH3 (butane) Structural formulas © 2017 Pearson Education, Ltd.  Depicting Structures of Organic Compounds Ethyl acetate is an organic molecule with: O HH – 3D drawing: H H H C C H C O C H H – Condensed structural formula CH3CO2CH2CH3 O – Line angle drawing [zigzag] O © 2017 Pearson Education, Ltd.  Naming Straight Chain Alkanes © 2017 Pearson Education, Ltd.  Alkyl Groups (Alkane-H)  Alkyl group – remove one H from an alkane (a part of a structure)  General abbreviation “R” (is used to indicate any alkyl group)  Name: replace -ane ending of alkane with –yl ending  -CH3 is “methyl” (from methane)  -CH2CH3 is “ethyl” from ethane The symbol R is used here and throughout this text to represent a generalized alkyl group. © 2017 Pearson Education, Ltd.  Alkyl Substituents Removing a hydrogen from an alkane results in an alkyl substituent. Replace “ane” of alkane with “yl.” © 2017 Pearson Education, Ltd.  Common Names © 2017 Pearson Education, Ltd.  Propyl and Isopropyl © 2017 Pearson Education, Ltd.  Alkyl Groups (Alkane-H) Prefixes: sec- (for secondary) tert- (for tertiary) used for the C4 alkyl groups © 2017 Pearson Education, Ltd.  Alkyl Groups (Continued) © 2017 Pearson Education, Ltd.  Primary, Secondary, and Tertiary Carbons A primary carbon is bonded to one carbon. A secondary carbon is bonded to two carbons. A tertiary carbon is bonded to three carbons. Primary hydrogens are attached to primary carbons. Secondary hydrogens are attached to secondary carbons. Tertiary hydrogens are attached to tertiary carbons. © 2017 Pearson Education, Ltd.  Types of Alkyl Groups  Classified by the connection site  a carbon at the end of a chain (primary alkyl group)  a carbon in the middle of a chain (secondary alkyl group)  a carbon with three carbons attached to it (tertiary alkyl group) The symbol R is used here and throughout this text to represent a generalized alkyl group. © 2017 Pearson Education, Ltd.  Types of Carbon Atoms When discussing structural formulas, it is often useful to distinguish different groups of carbon atoms by their structural characteristics. A primary carbon (1º) is one that is bonded to no more than one other carbon atom. A secondary carbon (2º) is bonded to two other carbon atoms, and tertiary (3º) and quaternary (4º) carbon atoms are bonded respectively to three and four other carbons. © 2017 Pearson Education, Ltd.  “Iso” Notice in the following structures that whenever the prefix “iso” is used, the iso Structural unit is at one end of the molecule, and any group replacing hydrogen is at the other end: Iso is at one end, and the group replacing the H is at the other end. © 2017 Pearson Education, Ltd.  Alkyl Group Names © 2017 Pearson Education, Ltd.  Group work Propose structures for two isomers with the formula C2H7N Strategy We know that carbon forms four bonds, nitrogen forms three, and hydrogen forms one. Write down the carbon atoms first, and then use trial and error plus intuition to put the pieces together. Solution There are two isomeric structures. One has the connection C—C—N , and the other has the connection C—N—C. © 2017 Pearson Education, Ltd.  Problems © 2017 Pearson Education, Ltd.  Naming Alkanes IUPAC International Union of Pure & Applied Chemistry STEP 1 STEP 2 STEP 3 STEP 4 Find the parent Number the atoms in Identify and number the Write the name as a hydrocarbon. the main chain. substituents. single word. © 2017 Pearson Education, Ltd.  IUPAC Names  Find the longest continuous carbon chain.  Number the carbons, starting from the end closest to the first branch. (The carbons in the parent hydrocarbon are numbered in the direction that gives the substituent as low a number as possible).  Name the groups attached to the chain (alkyls), using the carbon number as the locator.  Alphabetize substituents.  Use di-, tri-, etc., for multiples of the same substituent. © 2017 Pearson Education, Ltd.  Naming Branched-Chain Alkanes  Compounds are given systematic names by a process that uses  Follows specific rules  STEP 1 Find the parent hydrocarbon. (a) longest continuous carbon chain in the molecule (b) If two chains of equal length are present, choose the one with the larger number of branch points as the parent © 2017 Pearson Education, Ltd.  Naming Alkanes (Continued) STEP 2 Number the atoms in the main chain.  Carbons in that main chain are numbered in sequence The carbons in the parent hydrocarbon are numbered in the direction that gives the substituent as low a number as possible. The first branch occurs at C3 in the proper system of numbering but at C4 in the improper system. © 2017 Pearson Education, Ltd.  Naming Alkanes (Continued) STEP 3 Identify and number the substituents.  Substituents are identified and numbered Assign a number, called a locant, to each substituent to specify its point of attachment to the parent chain. Two substituents on the same carbon, assign them both the same number. 3-Ethyl-4,7-dimethylnonane 4-Ethyl-2,4-dimethylhexane © 2017 Pearson Education, Ltd.  Naming Alkanes (Continued) STEP 4 Write the name as a single word. Use hyphens to separate the various prefixes Use commas to separate numbers. Alphabetize substituents (different side chains). If two or more identical side chains are present, use the appropriate multiplier prefixes di-, tri-, tetra-, and so forth. Don’t use these prefixes for alphabetizing. © 2017 Pearson Education, Ltd.  Naming Alkanes  For historical reasons, non-systematic common names (simple branched-chain alkyls) are numbered in sequence 3-Isopropyl-2-methylhexane © 2017 Pearson Education, Ltd.  Common versus Systematic Nomenclature Common names never have numbers. Only systematic names have numbers. © 2017 Pearson Education, Ltd.  List Substituents in Alphabetical Order The correct name is the one that contains the lowest number. © 2017 Pearson Education, Ltd.  Multiple Substituents The chain is numbered in the direction that puts the lowest number in the name. Substituents are listed in alphabetical order. (di, tri, tetra, sec, tert are not alphabetized) © 2017 Pearson Education, Ltd.  When Both Have the Same Lowest Number When both names have the same lowest number, go for the next lowest number. © 2017 Pearson Education, Ltd.  When Both Have the Same Numbers When the same numbers are obtained in both directions, the first group listed gets the lower number. © 2017 Pearson Education, Ltd.  Worked Example What is the IUPAC name for the following alkane? Strategy Find the longest continuous carbon chain in the molecule, and use that as the parent name. This molecule has a chain of eight carbons—octane—with two methyl substituents. (You have to turn corners to see it.) Numbering from the end nearer the first methyl substituent indicates that the methyls are at C2 and C6. Solution © 2017 Pearson Education, Ltd.  Worked Example Converting a Chemical Name into a Structure Draw the structure of 3-isopropyl-2-methylhexane. Strategy Look at the parent name (hexane), and draw its carbon structure. Next, find the substituents (3-isopropyl and 2-methyl), and place them on the proper carbons. Finally, add hydrogens to complete the structure. Solution © 2017 Pearson Education, Ltd.  Problems 1. Give IUPAC names for the following compounds: 2. Give the IUPAC name for the following hydrocarbon, and convert the drawing into a skeletal structure Solution An illustration shows a ball-and-stick model wherein grey balls represent carbon atoms and white balls represent hydrogen atoms. A central methine group is bonded to a methyl group at the top. On the left, the methine group is bonded to a carbon that is bonded to two methyl group at the bottom and a methine group at the top; the methine group is further bonded to a methyl group. On the right, the central methine group is bonded to another methine group that is bonded to a methyl group at the bottom and a methine group on the right that is further bonded to a methyl group. © 2017 Pearson Education, Ltd.  Problems © 2017 Pearson Education, Ltd.  Cycloalkanes  We have discussed open-chained compounds up to this point  Many organic compounds contain rings of carbon atoms e.g. - Prostaglandins - Steroids © 2017 Pearson Education, Ltd.  Cycloalkanes (alicyclic, aliphatic cyclic)  Cycloalkanes are saturated cyclic hydrocarbons  Have the general formula (CnH2n) In skeletal drawings, they are represented by polygons. © 2017 Pearson Education, Ltd.  Naming Cycloalkanes STEP 1. Find the parent. # of carbons in the ring, STEP 2. Number the substituents and write the name. Start at a point of attachment and number around the ring 1. Monosubstituted Cycloalkanes: A number is not needed. 2. Two or more substituents Cycloalkanes: Begin numbering at the group that has alphabetical priority and proceed around the ring so as to give the second substituent the lowest number. © 2017 Pearson Education, Ltd.  Two Substituents with the Same Low Number If more than one name has the same low number, choose the name with the next lowest number. © 2017 Pearson Education, Ltd.  Worked Example Give IUPAC names for the following cycloalkanes a) b) © 2017 Pearson Education, Ltd.  Worked Example Solution: 1-Isopropyl-2-methylcyclohexane a) 4-Bromo-1-tert-butyl- b) 2-methylcycloheptane © 2017 Pearson Education, Ltd.  Problems © 2017 Pearson Education, Ltd.  Organohalide  An organic compound containing at least one carbon-halogen bond (C-X); X (F, Cl, Br, I) replaces H  Can contain many C-X bonds  Properties and some uses: Fire-resistant solvents, Refrigerants and Pharmaceuticals and precursors  Reactions involving organohalides are less frequently encountered than other organic compounds, but reactions such as nucleophilic substitutions/eliminations that they undergo will be encountered.  Alkyl halide chemistry is a model for mechanistically similar but more complex reactions. © 2017 Pearson Education, Ltd.  Naming Alkyl Halides Common versus Systematic Nomenclature © 2017 Pearson Education, Ltd.  Naming Alkyl Halides 1. Name the longest chain (parent chain), begin at the end nearer the substituent having its name first in the alphabet (Contains double or triple bond if present). 2. number from end nearest any substituent (alkyl or halogen), begin at the end nearer the substituent having its name first in the alphabet. 3. Write the name. List all substituents in alphabetical order, and use one of the prefixes di-, tri-and so forth if more than one of the same substituent is present. © 2017 Pearson Education, Ltd.  Naming Alkyl Halides Give the IUPAC names of the following alkyl halides 2,3-dichloro-4-methylhexane © 2017 Pearson Education, Ltd.  Nomenclature of Alkyl Halides © 2017 Pearson Education, Ltd.  Problems © 2017 Pearson Education, Ltd.  Classification of Alkyl Halides Alkyl halides are molecules in which a halogen is bonded to an sp3 hybridized carbon atom. locate the carbon that is connected to the halogen and count how many carbon atoms are connected to it: © 2017 Pearson Education, Ltd.  Classification of Alcohols Alcohols are classified in the same way. © 2017 Pearson Education, Ltd.  Classification of Amines Amines are classified based on how many alkyl groups are bonded to the nitrogen. The common name of an amine consists of the names of all the alkyl groups bonded to the nitrogen, in alphabetical order, followed by “amine.” © 2017 Pearson Education, Ltd.  Classification of Amines The classification depends on how many groups (R) are bonded to the N. primary amine = one group bonded to the N secondary amine = two groups bonded to the N tertiary amine = three groups bonded to the N © 2017 Pearson Education, Ltd.  The Structures of Alkyl Halides Alkyl halide: 1. The C-X bond of an alkyl halide is formed from the overlap of an sp3 orbital of carbon with an sp3 orbital of the halogen. 2. Thus, the C-X bond becomes longer and weaker as the size of the halogen increases. © 2017 Pearson Education, Ltd.  The Structures of Alcohols & Ethers Alcohol: An alcohol is structurally like water with one H replaced by an R. Ethers An ether is structurally like water with alkyl groups in place of both hydrogens. © 2017 Pearson Education, Ltd.  The Structures of Amines Amines The N in an amine has the same geometry as the N in ammonia. © 2017 Pearson Education, Ltd.  Noncovalent Interactions Attractive forces (Intermolecular attractions) Noncovalent interactions are molecular forces that don't involve electron sharing, such as van der Waals forces, hydrogen bonding, and dipole-dipole interactions. These weaker forces affect the physical properties of organic compounds and contribute to the structure, stability, and function of molecules, particularly in biological systems. The greater the area of contact, the stronger the van der Waals forces and the © 2017 Pearson Education, Ltd. greater the amount of energy needed to overcome them.  Boiling points of Alkanes van der Waals forces © 2017 Pearson Education, Ltd.  Boiling points of Alkanes Branching Lowers the Boiling Point © 2017 Pearson Education, Ltd.  Boiling points of Ethers Dipole–Dipole Interactions Van der Waals Van der Waals Dipole-dipole interactions © 2017 Pearson Education, Ltd.  Boiling points of Alkyl Halides Dipole–Dipole Interactions © 2017 Pearson Education, Ltd.  Boiling points of Alcohols Hydrogen Bonds Hydrogen bond: It is a special kind of dipole–dipole interaction that occurs between a hydrogen that is attached to an oxygen, nitrogen, or fluorine and a lone pair of oxygen, nitrogen, or fluorine in another molecule. © 2017 Pearson Education, Ltd.  Boiling points of Amines Hydrogen Bonds Hydrogen bonds are stronger in primary amines than in secondary amines because primary amines have stronger dipole–dipole interactions. Tertiary amines cannot form hydrogen bonds with each other. © 2017 Pearson Education, Ltd.  Solubility of Alkanes Polar compounds dissolve in polar solvents nonpolar compounds dissolve in nonpolar solvents. Solvation: The interaction between solvent molecules and solute molecules (molecules dissolved in a solvent) © 2017 Pearson Education, Ltd.  Rotation Occurs About Carbon-Carbon Single Bonds Conformations of Ethane  Stereochemistry concerned with the 3-dimensional aspects of molecules  σ bonds are cylindrically symmetrical  Rotation is possible around C-C bonds in open-chain molecules, affording constantly changing geometric relationships between the hydrogens on one carbon and those on the other. Conformations of ethane - Newman projection Organic Chemistry Animations Introduction: (chemtube3d.com) © 2017 Pearson Education, Ltd. https://www.chemtube3d.com/main-page/  Conformers (conformational isomers)  Conformation- Different positional arrangement of atoms resulting from bond rotation  Conformers: Molecules that have different arrangements  Called conformational isomers, or rotational isomers  Can be represented in 2 ways Sawhorse projection It views the C-C bond from an oblique angle and indicates spatial relationships by showing all the C-H bonds. © 2017 Pearson Education, Ltd.  Conformers A Newman projection is a drawing that helps visualize the 3-dimensional structure of a molecule. This projection most commonly sights down a carbon-carbon bond, making it a very useful way to visualize the stereochemistry of alkanes. A Newman projection visualizes the conformation of a chemical bond from front to back The front atom represented by the intersection of three lines (a dot). The front atom is called proximal. The back atom as a circle, and it is It views the C-C bond directly called distal. end-on and represents the two carbon atoms by a circle. © 2017 Pearson Education, Ltd.  Conformers  We do not observe perfectly free rotation  There is a barrier to rotation, and some conformers are more stable than others  Staggered (lowest-energy)- most stable: all 6 C- H bonds are as far away as possible  Eclipsed (Highest-energy)- least stable: all 6 C-H bonds are as close as possible to each other  What is true for ethane is also true for higher alkanes, staggered arrangements are more favorable. https://www.chemtube3d.com/stethanenewman/ © 2017 Pearson Education, Ltd.  Conformers https://www.chemtube3d.com/sym-ethanestaggered/ https://www.chemtube3d.com/symethaneecld3h/ What is true for ethane is also true for propane, butane, and all higher alkanes. The most favored conformation for any alkane is the one in which all bonds have staggered arrangements © 2017 Pearson Education, Ltd.  A Staggered Conformer is More Stable Than an Eclipsed Conformer Figure 3.5. The potential energies of all the conformers of ethane obtained in one complete 360° rotation about the C¬C bond. Notice that staggered conformers are at energy minima, whereas eclipsed conformers are at energy maxima. A molecule’s conformation changes from staggered to eclipsed millions of times per second at room temperature. As a result, the conformers cannot be separated from each other. © 2017 Pearson Education, Ltd.  Rotation Can Occur About the Three Carbon–Carbon Bonds in Butane Newman projection for conformers rotation about the C1-C2 bond in butane © 2017 Pearson Education, Ltd.  Rotation About C-2—C-3 in Butane D is more stable (has lower energy) Anti conformer B and F are called gauche (“goesh”) conformers. Steric strain is repulsion between the electron clouds of atoms or groups. © 2017 Pearson Education, Ltd.  Worked Example Sight along the C1-C2 bond of 1-chloropropane and draw Newman projections of the most stable and least stable conformations The most stable conformation of a substituted alkane is generally a staggered one in which large groups are as far away from one another as possible. The least stable conformation is generally an eclipsed one in which large groups are as close as possible. Problem 2.13: Looking along the C2—C3 bond of butane, there are two different staggered conformations and two different eclipsed conformations. Draw them. © 2017 Pearson Education, Ltd.  Bond Angles in Planar Cyclic Alkanes Ideal a sp3 carbon © 2017 Pearson Education, Ltd.  Angle Strain tetrahedral bond angle = 109.5° bond angle < 109.5° Angle strain results from poor orbital–orbital overlap because bonds have to deviate from the ideal (109.5°) bond angle. © 2017 Pearson Education, Ltd.  Stability of Cycloalkanes: Ring Strain Angle strain: is the increase in potential energy of a molecule due to bond angles deviating from the ideal values. For example, deviate from the ideal 109.5°tetrahedral value in sp3 tetrahedral geometry Torsional strain: (eclipsing strain) is the increase in potential energy of a molecule due to repulsion between electrons in bonds that do not share an atom) Steric strain - Caused due to repulsive interactions when atoms approach each other too closely © 2017 Pearson Education, Ltd.  Conformations of Some Cycloalkanes  Cycloalkanes have much less freedom of motion. Cyclopropane, for example, must be a rigid, planar molecule. No rotation around a C-C bond can take place in cyclopropane without breaking open the ring. The deviation of bond angles from the normal 109.5°tetrahedral value causes an angle strain in the molecule that raises its energy and makes it more reactive than unstrained alkanes. © 2017 Pearson Education, Ltd.  Conformations of Cycloalkanes Cyclopropane Most strained of all rings due to angle strain caused by its C–C–C bond angles of 60° Has considerable torsional strain Has bent bonds C–H bonds are eclipsed Can form stereochemical isomers, or stereoisomers, and can be isolated. © 2017 Pearson Education, Ltd.  Conformations of Cyclohexane  Substituted cyclohexanes occur widely in nature  The cyclohexane ring is free of angle strain and torsional strain.  Three-dimensional shape called a chair conformation.  In the chair conformation, All bond angles are around 109.5° and all adjacent bonds are staggered.  The conformation has alternating atoms in a common plane and tetrahedral angles between all carbons. © 2017 Pearson Education, Ltd.  How to Draw Cyclohexane © 2017 Pearson Education, Ltd.  Drawing the Chair Conformer Draw 2 parallel lines. Connect ends with a V. Notice that the chair has 3 sets of parallel lines. © 2017 Pearson Education, Ltd.  Axial and Equatorial Bonds Notice that each equatorial bond is parallel to the two ring bonds one black bond away. © 2017 Pearson Education, Ltd.  How to Draw Cyclohexane Step 1 Draw two parallel lines, slanted downward and slightly offset from each other. This means that four of the cyclohexane carbons lie in a plane. Step 2 Place the topmost carbon atom above and to the right of the plane of the other four, and connect bonds. Step 3 Place the bottom most carbon atom below and to the left of the plane of the middle four, and connect the bonds. Note that the bonds to the bottom most carbon atom are parallel to the bonds of the top most carbon. lower bond is in front (heavily shaded) and the Remember upper bond is in back. © 2017 Pearson Education, Ltd.  Axial and Equatorial Bonds in Cyclohexane  The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions  Chair cyclohexane has six axial hydrogens vertical to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring ( they point outward from the ring because the bond angles are greater than 90°). (up/downward slant)  Each carbon atom has one axial and one equatorial position, and each side of the ring has three axial and three equatorial positions in an alternating arrangement. © 2017 Pearson Education, Ltd.  Drawing the Axial and Equatorial Hydrogens Problem: Draw two chair structures for methylcyclohexane, one with the methyl group axial and one with the methyl group equatorial. © 2017 Pearson Education, Ltd.  Conformational Mobility of Cyclohexane  Chair conformations easily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip.  This interconversion occurs rapidly at room temperature, the individual axial and equatorial isomers can’t be isolated. © 2017 Pearson Education, Ltd.  Ring Flip Cyclohexane interconverts between two chair conformers. Equatorial bonds become axial and axial bonds become equatorial. © 2017 Pearson Education, Ltd.  Conformers of Monosubstituted Cyclohexanes The chair conformer with the methyl substituent in an equatorial position is the more stable of the two conformers because a substituent has more room and, therefore, fewer steric interactions when it is in an equatorial position © 2017 Pearson Education, Ltd.  Conformers of Monosubstituted Cyclohexanes methyl group in an 1,3-Diaxial equatorial position Interactions extends into space, away from the rest of the molecule © 2017 Pearson Education, Ltd.  Conformers of Disubstituted Cyclohexanes Cis and Trans Isomers Geometric isomers: Cis–trans isomers © 2017 Pearson Education, Ltd.  Conformers of Disubstituted Cyclohexanes Each Isomer Has Two Chair Conformers © 2017 Pearson Education, Ltd.  Worked Example Draw 1,1-dimethylcyclohexane in a chair conformation, indicating which methyl group in your drawing is axial and which is equatorial. Strategy Draw a chair cyclohexane ring, and then put two methyl groups on the same carbon. The methyl group in the rough plane of the ring is equatorial, and the one directly above or below the ring is axial. Solution © 2017 Pearson Education, Ltd.  Worked Example Strategy : To solve this kind of problem, we need to determine whether the two substituents are on the same side of the ring (cis) or on opposite sides of the ring (trans). If the bonds bearing the substituents are both pointing upward or both pointing downward, then the compound is the cis isomer; if one bond is pointing upward and the other downward, then the compound is the trans isomer. Because the conformer in question has both methyl groups attached to downward pointing bonds, it is the cis isomer © 2017 Pearson Education, Ltd.  Summary and Key Words 1. A functional group is an atom or group of atoms within a larger molecule that has a characteristic chemical reactivity. 2. Alkanes are a class of saturated hydrocarbons having the general formula CnH2n+2. They contain no functional groups, are chemically rather inert, and can be either straight-chain or branched 3. Alkanes are named systematically by a series of IUPAC rules of nomenclature. 4. Isomers—compounds that have the same chemical formula but different structures—exist for all but the simplest alkanes. Compounds such as butane and isobutane, which have the same formula but differ in the way their atoms are connected, are called constitutional isomers. 5. Alkanes can therefore adopt any of a large number of rapidly interconverting conformations. A staggered conformation is more stable than an eclipsed conformation. © 2017 Pearson Education, Ltd.  Summary and Key Words 6. Cycloalkanes contain rings of carbon atoms and have the general formula CnH2n. Because complete rotation around C-C bonds is not possible in cycloalkanes, conformational mobility is reduced and disubstituted cycloalkanes can exist as cis– trans stereoisomers. In a cis isomer, both substituents are on the same side of the ring, whereas in a trans isomer, the substituents are on opposite sides of the ring. 7. Cyclohexanes are the most common of all rings because of their wide occurrence in nature. Cyclohexane exists in a puckered, strain-free chair conformation in which all bond angles are near 109° and all neighboring C-H bonds are staggered. 8. Chair cyclohexane has two kinds of bonding positions: axial and equatorial. Axial bonds are directed up and down, parallel to the ring axis; equatorial bonds lie in a belt around the ring equator. 9. Chair cyclohexanes can undergo a ring-flip that interconverts axial and equatorial positions. Substituents on the ring are more stable in the equatorial than in the axial position. © 2017 Pearson Education, Ltd.