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

Chapter 2 Alkanes

Basic Organic Nomenclature by Dave Woodcock at Okanagan University College
[requires MDL Chime; use Netscape]
http://www.molecularmodels.ca/nomenclature/nom1.htm
Organic Chemistry OnLine by Paul R.Young at the University of Illinois at Chicago
Alkanes and Cycloalkanes: Structure, Bonding, and Nomenclature tutorials

  [requires MDL Chime; use Netscape]
 http://www.chem.uic.edu/web1/OCOL-II/WIN/ALKANE/ALKANE.HTM
Exploring Alkanes by Gary Trammell and Srinivas Vuppuluri at the University of Illinois at Springfield
http://people.uis.edu/gtram1/organic/alkanesmenu.htm
Carey PowerPoint slides for chapter 2 [2.1 to 2.7, hydrocarbon classes, functional groups, alkanes from C1 to C5, higher n-alkanes] from Columbia University can be seen at
http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/02_01_07.html
Carey PowerPoint slides for chapter 2 [2.8 to 2.12, IUPAC nomenclature for alkanes and cycloalkanes] from Columbia University can be seen at
 http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/02_08_12.html
Carey PowerPoint slides for chapter 2 [2.13 to 2.16, sources, physical properties, chemical properties, redox in organic chem] from Columbia University can be seen at
 http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/02_13_16.html
A set of  PowerPoint slides on alkanes and cycloalkanes in PDF format
by Paul R. Young of the University of Illinois at Chicago can be seen at
 http://www.chem.uic.edu/web1/PDF/CH2.PDF
Even though it was written for a course at a lower level, there's lots of good stuff on hydrocarbons, alkanes, cycloalkanes, and functional groups on my CHEM 30B "Alkanes" page
 http://www.mpcfaculty.net/ron_rinehart/30B/alkanes.htm

Chapter 2.  Alkanes. 

To see a large number of alkane and cycloalkane Chime structures
[
requires MDL Chime; use Netscape]
,
go to Dave Woodcock's site at OUC [Okanagan University College]: 
 http://www.molecularmodels.ca/molecule/Alkanes.htm
[new window, close when done]

and particularly relevant to this section is his "Molecular Fragments: Alkane Isomers" page at
http://www.molecularmodels.ca/molecule/molfrag.html
  [new window, close when done]

I. Introduction. 
            A.  Classes of Organic Compounds 
                        1.  ~7,000,000 organic compounds.
                        2.  Organic compounds can be characterized by Functional Groups:  the structural unit
                                    responsible for a given molecule's chemical behavior.
                        3.  Functional Groups:  See Tables 2.1, 2.2 or click here

            B.  Types of Carbon Atoms in Organic Species: 
                       
  1o (primary) carbons.      C bonded to 1 C           (typically, a -CH3 group).
                       
  2o (secondary) carbons.   C bonded to 2 C's        (e.g., a –CH2- group).
                       
  3o (tertiary) carbons.      C bonded to 3 C's         (typically, a CH group).
                       
  4o (quaternary) carbons.  C bonded to 4 C's.
                                    The molecule shown below has six 1o C, three 2o C, two 3o C, and one 4o C. 

 

There are four different types of 1o C (methyl groups, CH3-), three different types of 2o C (methylene groups, -CH2-), and two different types of 3o C (methinyl groups, >CH-) in this molecule. These distinctions will be important when we discuss the free-radical halogenation of alkanes [Chapter 4], and also when we consider 1H-NMR and 13C-NMR spectra [Chapter 13].

II.  Classes of Hydrocarbons. 
            A.  Hydrocarbons. 
 
                        1.  Compounds composed of carbon and hydrogen.
                        2.  Can be aliphatic or aromatic (determined by their structure and bonding.) 
            B.  Aliphatic Hydrocarbons.  
                        1.   Aliphatic hydrocarbons are broken into the following categories: 
                                    a)  Alkanes.            sp3 hybrids.   -C-
                                    b)  Cycloalkanes
    sp3 hybrids.   -C-
                                    c)  Alkenes.            sp2 hybrids.   C=C
                                    d)  Alkynes.            sp hybrids.    C≡C 
            C.  Aromatic Hydrocarbons.
                        1. Most common example:  benzene.      

 III.  Alkanes.
            A. Bonding. 
                        1.  sp3 hybridization.
                        2.  Sigma bonds only.
                        3.  109.5o C<  bond angles.
                        4.  C-H bond length in CH4 =  109 pm.
                        5.  C-C bond length in C2H6 = 153 pm.
                                    C-H bond length in C2H6 = 111 pm.
                        6.  Acyclic alkanes have the formula CnH2n+2 

            B.  Nomenclature of Alkanes.   IUPAC Rules. 

You will find a trip to Dave Woodcock's nomenclature site at  OUC very worthwhile -- 
[requires MDL Chime; use Netscape]
 http://www.molecularmodels.ca/nomenclature/nom1.htm 

IUPAC Nomenclature from ACD Labs
http://www.acdlabs.com/iupac/nomenclature/

Official IUPAC Nomenclature Home Page by G.P. Moss at Queen Mary University of London
http://www.chem.qmw.ac.uk/iupac/

Nomenclature Shortcuts by Yorke E. Rhoades at Harvard University
 http://www.courses.fas.harvard.edu/~chem30/handouts/Handout00.pdf

IUPAC nomenclature of hydrocarbons by Linda M. Sweeting at Towson University http://www.towson.edu/~sweeting/enrich/iupachc.pdf

IUPAC nomenclature of simple organic compounds by Linda M. Sweeting at Towson University http://www.towson.edu/~sweeting/enrich/iupac.pdf
covers more than we need to know at present, but will be very useful in the near future!

                        1.         The names of the first ten [or even twenty -- see below] alkanes should be memorized: 

methane

CH4

hexane

C6H14 or CH3(CH2)4CH3

ethane

C2H6 or CH3CH3

heptane

C7H16 or CH3(CH2)5CH3

propane

C3H8 or CH3CH2CH3

octane

C8H18 or CH3(CH2)6CH3

butane

C4H10 or CH3CH2CH2CH3

nonane

C9H20 or CH3(CH2)7CH3

pentane

C5H12 or CH3(CH2)3CH3

decane

C10H22 or CH3(CH2)8CH3

                          2. Alkyl groups are substituent groups which are attached to other atoms or parent chains. 

                                    a)  They are named by dropping -ane from the name of the corresponding alkane and
                                                adding -yl.
                                    b)   Examples.
                                                -CH3                   methyl
                                                -CH2CH3             ethyl
                                                -CH2CH2CH3      propyl 

                        3.  IUPAC Rules:  International Union of Pure and Applied Chemistry. 
                                    a)  Select the longest continuous carbon chain as the "parent" structure and then
                                                consider the attached groups. 

                                                                   

                                                            methylpropane                           methylbutane 

                                    b)  Where necessary indicate by number the carbon to which the group is attached.
                                                This number is called a locant
                                                i)  In numbering the parent carbon chain start from the end closest to
                                                      the first substituent. 

                                                            

                                                            3-methylpentane                                    2-methylpentane 

                                    c)  If the same alkyl group occurs more than once on the parent chain then indicate  
                                           this by the use of the appropriate prefix (di-, tri-, tetra-, etc).
                                                i)  Indicate by number the position of  each group. 
                                    d)  If there are several different groups attached to the parent structure, then            
                                            list them in the name of the compound in alphabetical order. 
                                                i)  The prefixes di-, tri-, etc are ignored when alphabetizing. 

                            

                             2,2,4-trimethyloctane                                     3,3,5-triethyl-4-methyloctane 

                                       

e)  Notice from the above examples that locant numbers are separated by commas; 
a number and a letter are separated by dashes. 

                                    f)  When two chains are of equal length, choose the chain with the greater number
                                          of substituents. 
                        4.  Nomenclature of Cycloalkanes
                                    a)  Cycloalkanes are named by adding the prefix "cyclo" to the parent stem. 
                                    b)  Notice the chemical formula of a cycloalkane is CnH2n

To see Dave Woodcock's Chime structures of cycloalkanes and other related compounds
[requires MDL Chime; use Netscape], go to
http://www.molecularmodels.ca/molecule/Cyclic_(non-aromatic).htm

                        5.  Name the following compounds.                                                             

                                    a)           or   

                                                                                                   4,6-diethyl-3-methyloctane

                                                             

                                    b)           or  

                                                                                                                           

                                                4-ethyl-3,5-dimethylheptane

                                     c)             or   

                                                 

                                                5,5-diethyldecane

 

                                    d)            or 

                                                 2,3,6,6-tetramethyloctane

           

                                    e)             or   

                                                 1-ethyl-2-methyl-4-propylcyclopentane

                                               

                                    f)                     or       

                                                 1,1-diethyl-3,3-dimethylcyclobutane

                         6.  Draw structures for the following compounds: 
                                    a)  4-ethyl-3,5-dimethylheptane 
                                    b)  4,6-diethyl-3-methyloctane 
                                    c)  1,1,3-tripropylcyclohexane 
                        7.  IUPAC rules for complex alkyl groups. 
                                    a)  What difficulty arises in naming the following compound? 

                                                             

                                                i)  Such alkyl groups are named with IUPAC rules or in some cases common                                                           names. 

                                    b)  With IUPAC rules the substituent group is named by taking the longest carbon
                                         chain in the group as the parent alkyl group and listing attached alkyl groups
                                         with corresponding locants. 
                                                i)   The above alkyl group name is written in parentheses.
                                                ii)  In numbering the parent chain of the alkyl group, the carbon of the
                                                     substituent group attached to the main parent is carbon 1 of the
                                                     substituent alkyl group.
                                                iii)  Now name the compound in part (a). 
                                                Answer:  5-isobutylnonane or 5-(2-methylpropyl)nonane 

                        8.  Common names of some complex alkyl groups. 

                                    a)  Alkyl groups with (CH3)2CH- (on the otherwise unbranched group) whose point
                                         of attachment to the parent is at the opposite end is named by adding the
                                         prefix "iso" to the name of the alkyl group containing all carbons. 

structure




 




 




 

common name

isopropyl

isobutyl

isopentyl or isoamyl

IUPAC name

(1-methylethyl)

(2-methylpropyl)

(3-methylbutyl)

                                     b)         Groups with 4 carbons or more in a straight chain but with the point of
                                                attachment to the parent at the second carbon gets the prefix "sec-".
 

structure




 




 




 

common name

sec-butyl

sec-pentyl (or sec-amyl)

sec-hexyl

IUPAC name

(1-methylpropyl)

(1-methylbutyl)

(1-methylpentyl)

                                     c)   The group -C(CH3)3 is known as the tert-butyl (or t-butyl) group.  
                                                i)   Name this using IUPAC rules:   (1,1-dimethylethyl) 
                                                ii)  Adding an extra carbon to this results in the t-pentyl (or t-amyl)
group:   
                                                                         
                (1,1-dimethylpropyl)
                                    d)  Some other important common names: 

structure




 




 

common name

neopentyl (NOT neoamyl)

neohexyl

IUPAC name

(2,2-dimethylpropyl)

(2,2-dimethylbutyl)

                        9.         Name the following compounds. 

                                    a)                         b)      

                         Answers: 

                        a) 1,1-dimethyl-2-(1,2-dimethylpropyl)cyclopentane         

                        b) 7-sec-butyl-5-ethyl-3,3-dimethyldecane 
                                a better name is 5-ethyl-3,3,8-trimethyl-7-propyldecane -- at C-7, the "main" chain continues
                                 upward. Why? Given a choice such as this, select the main chain to include as many of the
                                substituents as possible.

            C.  Isomers. 
                        1. As the formula of an alkane increases in number of carbon atoms, the number of
                            possible constitutional isomers grows.

# of C

name

# of constitutional

isomers

condensed formula of

‘normal’ isomer

1

methane

1

CH4

2

ethane

1

CH3CH3

3

propane

1

CH3CH2CH3

4

butane

2

CH3CH2CH2CH3

5

pentane

3

CH3(CH2)3CH3

6

hexane

5

CH3(CH2)4CH3

7

heptane

9

 

8

octane

18

 

9

nonane

35

 

10

decane

75

 

11

undecane

 

 

12

dodecane

 

 

13

tridecane

 

 

14

tetradecane

 

 

15

pentadecane

4347

 

16

hexadecane

 

 

17

heptadecane

 

 

18

octadecane

 

 

19

nonadecane

 

 

20

eicosane

366,319

 

22

docosane

 

 

30

triacontane

 

 

40

tetracontane

6.249…x 1012

 

100

hectane

 

CH3(CH2)98CH3

n

gen. alkane

 

CH3(CH2)n-2CH3

                         2. Draw all possible constitutional isomers for alkanes of the formula:   
                                    i)    C5H12
    ii)     C6H14    iii)   C4H8 

            D.  Physical Properties. 
                        1.  Intermolecular vs intramolecular forces of attraction.     
                        2.  Recall 3 major forces of attraction: 
                                    a)  Hydrogen bonding 
                                    b)  Dipole-dipole 
                                    c)  London (dispersion) forces:  induced dipole-induced dipole 
                        3.  Intermolecular force of attraction between alkanes:  London forces. 
                                    a)  These weak forces result in low boiling points for alkanes. 

                                                CH3CH­2OH            CH3CH­2F        CH3CH­2CH3

                                                       78oC                 -32oC                -42o

                        4.  Boiling Point vs Branching.  More branching results in less contact points between
                                molecules:  less intermolecular forces of  attraction and a lower boiling point.

                                                          

                                                 36oC                         28oC                  9o

                        5.   Boiling Point vs Molecular Weight.  The boiling points of alkanes increase with    
                              increasing molecular weight:  higher alkanes have a greater number of atoms which can
                              attract to each other via
London forces.   
                                    a)  C1-C4 are all gases are room temperature and 1 atmosphere pressure. 

                                                CH4               CH3CH­3        CH3CH­2CH3
                                               -160oC             -89oC                -42oC

                        6.  Melting Point vs Branching.  Greater symmetry  results in a better fit in the crystal
                             lattice (pack together better) and a higher  melting point:

                                                         
                                                -129oC                 -160.5oC           -16.6oC       

                        7.  Solubility in Water. 
                                    a)  "Like dissolves like."  
                                    b)  Alkanes (nonpolar) and water (polar) do not mix. 

For greater appreciation of the importance of the role of intermolecular attractive forces
in determining many of the physical properties of compounds, see my page on
Physical Properties and Intermolecular Forces
at
http://www.mpcfaculty.net/ron_rinehart/12A/molforce.htm

Colby College's Shockwave tutorial on Physical Properties is at
 http://www.colby.edu/chemistry/OChem/DEMOS/physprops.html

            E.  Chemical Properties of Alkanes. 
                        1.  Alkanes are relatively unreactive. 
                        2.  Combustion.      (Table 2.5)                                   

Dr. Ron's magic formula for balancing combustion equations for any HYDROCARBON

since C + O2 à CO2  and  2H2 + O2 à  2H2O,  it can readily be shown that

CxHy + (x + ¼·y)O2  à  xCO2 + ½yH2O
 

Combustion from the Virtual Textbook of Organic Chemistry by William Reusch at Michigan State U
http://www.cem.msu.edu/~reusch/VirtualText/funcrx1.htm#combust

                                    CnH2n+2  +  O2  à H2O  +  CO2  +  energy 
                                    CH4  +  2O2 
à   2H2O +  CO2  +  890 kJ 
                                    C7H16 + 11 O2   
à   8H2O  +  7CO2 + 4817 kJ 
                                    a)  Add ~653 kJ/mole for each -CH2
                                    b)  Heat of combustion can be used as an indicator of relative stabilities of isomeric alkanes. 
                                    c)  The isomer with the lowest heat of  combustion is the most stable.
                                    d)  Branching decreases the heat of combustion.  The more compact shape
                                          maximizes intramolecular forces of  attraction which stabilizes (and
                                          therefore decreases the potential energy of) the molecule.

                                         C8H18  +  25/2 O2  à  8CO2  +  9H2O

            CH3(CH2)6CH3

                                   

                                     (CH3)2CH(CH2)4CH3 

       5471 kJ     

                                                                         (CH3)3C(CH2)3CH3

                                        5466 kJ

                                                                                                      (CH3)3CC(CH3)3

                                                                              5458 kJ      

 

                                                                                                               5452 kJ

 

 

                                                                                                     8CO2 +  9H2O                                   

e)  Notice the same product in each case:    The difference in the heats of
combustion  must be due to a difference in potential energy of the isomeric
forms of C8H18

                                                i)  (Compare the 8CO2 + 9H2O to four identical bricks at the same ground
                                                    level.  Which released the most energy upon falling?  It depends
                                                    upon the height at which the brick was released. 

IV. Oxidation-Reduction in Organic Chemistry. 

If you're really rusty on the subject, see my PowerPoint review of oxidation-reduction (redox) reactions at
 http://www.mpcfaculty.net/ron_rinehart/12A/redox_files/frame.htm

For a summary of the basic take-home message, see the redox table

A. The concepts of oxidation and reduction are important in understanding organic chemistry
 and biochemistry.

            B.  Oxidation.           

                        1.  Oxidation is the loss of electrons.  (OIL)
                        2.   Oxidation of carbon is the decrease of bonds to H atoms:
                                                C2H6 
à  C2H4  +  H2
                        3.  Oxidation of carbon is the increase of bonds to O atoms:
                                                 C  +  O2 
à  CO2
                                                H3C-OH 
à   H2C=O 
            C.  Reduction.     
                        1. Reduction is the gain of electrons.  (RIG)
                        2.  Reduction of carbon is the increase of bonds to H atoms:
                                       C2H4  +  H2 
à  C2H6
                        3. Reduction of carbon is the decrease of bonds to O atoms:    
                                (CH3)2C=O   +  H2 
à   (CH3)2CHOH

Many thanks to Rod Oka of MPC for generously sharing his "Lecture Companion" outline, reproduced here in edited form by permission, with web references and other goodies added by me.
Structures drawn using CS ChemOffice ChemDraw™ and MDL IsisDraw™

updated 9/15/07