CSUMB
ESSP 311 Organic Chemistry I
Ronald W. Rinehart, Ph.D.

Chapter 3 Conformations of Alkanes and Cycloalkanes

Chapter 3.  Conformations of Alkanes and Cycloalkanes. 

I.   Conformational Analysis. 
        A.   Conformations:  Various spatial arrangements of a  molecule produced by rotations about single bonds.
                        1.   Conformations influence chemical and physical properties. 
                        2.   Analysis of a molecules in terms of its conformations help to explain how
                                    molecules interact with each other. 
                        3.   Types of Notation

                        4.   Important Terms. 
                                    a)   Eclipsed. 
                                    b)   Staggered. 
                                                i)   Anti. 
                                                ii)   Gauche.    
                                    c)   Dihedral angle (torsion angle). 
                                    d)   Torsional strain. 

II.   Conformations of Various Molecules. 
            A.   Ethane (See figure 3.4). 
                        1.   Eclipsed:  Dihedral angle = 0^o 
                        2.   Gauche:  Dihedral angle = 60^o 
                        3.   Anti:  Dihedral angle = 180^o 
                        4.   At room temperature there is rapid rotation about the C-C bond. 
                        5.   Staggered is more stable than eclipsed due to the maximum separation of bonded electron pairs
                                (not van der Waals repulsion of H's): Torsional Strain. 
            B.   Butane (See figure 3.7). 
                        1.   Due to the size of the methyl groups there exists both torsional strain and van der Waals strain (steric hindrance.)
            C.   Higher Alkanes. 
                        1.   Zigzag arrangement of carbon skeleton. 
                                    a)   This allows C atoms to be anti to each other:  all bonds are staggered. 
                        2.   For liquids and gases:  there is rapid interconversion at room temperature. 
                        3.   In crystals:  all anti in conformation.   
                                    a)   This is stable and allows for the packing of molecules in crystals.
            D.   Cyclohexane.       (See figure 3.15).  see more details here
                        1.   Cyclohexane is nonplanar.  Why? 
                        2.   Chair vs Boat conformation. 
                                    a)   The chair form is 6.4 kcal/mole more stable than boat.   
                                    b)   While the different conformations of cyclohexane are in rapid equilibrium,
                                             the chair form is greatly favored.
                                            i)   Only .1%-.2% of a sample would be in the higher energy skew boat
                                                 or boat conformations at any given time. 
                        3.   Boat. 
                                    a)   Eclipsed bonds on four carbons of the boat:  Significant torsional strain. 
                                    b)   van der Waals repulsions due to "flagpole" hydrogens. 
                                    c)   Bond angles ~111^o:  Little angle strain. 
                        4.   Twist or skew boat.  
                                    a)   Less stable than chair but .6 kcal/mole more stable than boat.   
                        5.   Chair. 
                                    a)   Bond angles ~111^o:  Little angle strain. 
                                    b)   All bonds are staggered: free of torsional strain. 
                                    c)   In the chair conformation H's (or substituted groups) can be axial or
                                                equatorial; up or down. 
                                    d)   When the chair "flips", what was axial becomes equatorial; 
                                            what was equatorial becomes axial. 
                                    e)   For methylcyclohexane 95% of the molecules would have the methyl group
                                             in the equatorial position; 5% in the axial position.  (van der Waals repulsion.) 
                                    f)   For tert-butylcyclohexane 99.99% of the molecules would have the t-butyl group
                                            in the equatorial position; .01% in the axial position.  

            E.   Disubstituted Cyclohexanes. 
                        1.   cis-1,2-dimethylcyclohexane vs trans-1,2-dimethylcyclohexane. 
                                    a)   Heat of combustion for cis isomer: 5223 kJ/mole 
                                    b)   Heat of combustion for trans isomer: 5217 kJ/mole 
                                    c)   Trans isomer is 6 kJ/mole more stable than cis isomer.  Why? 

                        2.   As a homework exercise you should analyze the relative stabilities of the cis and trans
                                    isomers for 1,3-dimethylcyclohexane and for 1,4-dimethylcyclohexane. 

III.   Polycyclic Compounds. 
            A.   Types of Polycyclic Compounds. 
                        1.   Spirocyclic.   
                                a)      A single carbon atom is common to two rings. 

                        2.   Polycyclic Compounds. 
                                    a)   Two or more atoms common to more than one ring. 
                                    b)   Compounds can be bicyclic, tricyclic, tetracyclic, etc.                       

            B.   Nomenclature of Bicylic Compounds. 
                        1.   Count the number of carbon atoms in the ring system. 
                                    a)   Use the parent name of the corresponding alkane. 
                                    b)   Use the prefix "bicyclo". 
                                    c)   Write the number of atoms in each of the bridges in brackets, placed in
                                                descending order. 
                                                i)   The bracketed numbers are placed between the parent name and the
                                                            prefix bicyclo. 
                                    d)   The numbering of the bicyclo compound used to locate substituent groups starts
                                            at a bridgehead position and counts out along the longest bridge, and then
                                          continues through the next longest.
                                                i)   Assign the number 1 to the bridgehead which results in the lowest locant.
                                     e)   Examples: 

                                    f)   Name the following bicyclic compounds:
 

                         i)   bicyclo[4.2.0]octane                          ii)   bicyclo[2.1.1]hexane 
                        iii)   bicyclo[3.2.1]octane                         iv)   bicyclo[4.1.1]octane     
                        v)   bicyclo[2.2.2]octane                         vi)   4-methyl-1-isopropyl-bicyclo[3.1.0]hexane 

IV.   Heterocyclic Compounds. 
            A.   Introduction. 
                        1.   Heteroatom:  An atom other than carbon. 
                        2.   Heterocyclic compounds:  Cyclic organic compounds which have an atom other than
                                    carbon in the ring.
            B.   Examples. 
                        1.   Some important heterocycles [only the common names are given]:                       

Many thanks to Rod Oka of MPC for generously sharing his "Lecture Companion" outline, reproduced here by permission with web references and other goodies added by me.
Structures redrawn using ACD Labs ChemSketch™ and MDL IsisDraw™

 updated 9/15/07