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

Chapter 4 Alcohols and Alkyl Halides

A set of  PowerPoint slides on reactions of alkyl halides 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/CH6.PDF
Organic Chemistry OnLine's page on alkyl halides 
by Paul R. Young of the University of Illinois at Chicago can be seen at 
http://www.chem.uic.edu/web1/OCOL-II/WIN/RX.HTM
Organic Chemistry OnLine's page on alcohols 
by Paul R. Young of the University of Illinois at Chicago can be seen at 
http://www.chem.uic.edu/web1/OCOL-II/WIN/ALCOHOLS.HTM
A set of  PowerPoint slides on radicals 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/CH10.PDF
Carey PowerPoint slides for chapter 4 from Columbia University
[4.1 to 4.3, nomenclature and classes of RX and ROH]

http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_01_03.html
Carey PowerPoint slides for chapter 4 from Columbia University
[4.4 to 4.5, bonding in and physical properties of RX and ROH]

http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_04_05.html
Carey PowerPoint slides for chapter 4 from Columbia University
 [4.6 to 4.7, acids and bases, mechanism of proton transfer]
http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_06_07.html
Carey PowerPoint slides for chapter 4 from Columbia University
[4.8 to 4.14, ROH
RX]
http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_08_14.html
Carey PowerPoint slides for chapter 4 from Columbia University
[4.15 to 4.19, halogenation of alkanes, free-radical reactions]
http://www.columbia.edu/itc/chemistry/c3045/client_edit/ppt/04_15_19.html
Colby College's Shockwave tutorial on Carbocation rearrangements
http://www.colby.edu/chemistry/OChem/DEMOS/rearrangements.html
Colby College's Shockwave tutorial on nucleophilic substitution
http://www.colby.edu/chemistry/OChem/DEMOS/Substitution.html
William Reusch of U Michigan has an excellent Virtual Textbook of Organic Chemistry
the section on Chemical Reactivity is at
http://www.cem.msu.edu/~reusch/VirtualText/react1.htm#rx1
Reaction Mechanisms at
http://www.cem.msu.edu/~reusch/VirtualText/react1.htm#rx4
Reaction Examples at
http://www.cem.msu.edu/~reusch/VirtualText/react2.htm#rx5
Alkyl halides by Gary Trammell and Srinivas Vuppuluri at the University of Illinois at Springfield
has info on structure, preparation and elimination and nucleophilic substitution
http://people.uis.edu/gtram1/organic/alkylHalidesmenu.htm 
Alcohols, ethers, and thiols by Gary Trammell and Srinivas Vuppuluri at the University of Illinois at Springfield
http://people.uis.edu/gtram1/organic/alcoholsmenu.htm 

Chapter 4:  Alcohols and Alkyl Halides 

I.   Mechanism in Organic Chemistry.   
            A.   Important Terms and Species
 
                        1.   Mechanism.   [there are several examples of mechanisms below on this page]
                                    a)   The sequence of elementary steps which describes how a chemical reaction occurs.
                                    b)   We will examine many reactions throughout this course.  It is best to learn
                                          and understand these reactions by the mechanisms in which they occur.
 
                        2.   Intermediate
                                    a)   A species that exists for some finite length of time having some stability.
                                    b)   It represents an energy minimum in the course of a reaction.
                                    c)   Carbocations and free radicals are two important examples of intermediates. 
                        3.   Transition State
                                    a)   A transient state on the path from one intermediate to another.
                                    b)   It corresponds to an energy maximum.
                                    c)   It is drawn in enclosed brackets in order to indicate its transient character.

                        4.   Free Radical.            
                                    a)   Species containing an unpaired electron.

                                     b)   The stability of free radicals increases from the methyl free radical to the
                                                tertiary free radical:


methyl   <

1o   <

2o   <

3o

least stable

 

 

most stable

                                    c)   sp2 hybridization. 

                        5.   Carbocation.
                                    a)   Carbon cations.
                                    b)   These are also referred to as carbonium ions and carbenium ions.
                                    c)   As with the free radicals, the stability of carbocations increases from the
                                                methyl to the tertiary carbocation.

methyl   <

1o   <

2o   <

3o

least stable

 

 

most stable

 

 

                                                 

                                    d)   sp2 hybridization. 
                                    e)   Carbocations and alkyl free radicals are stabilized by electron-releasing substituents:
                                                    alkyl groups release electrons.
 
                                                 i)   C-C sigma bonds are more polarizable than C-H bonds:
                                                       electrons in C-C bonds are drawn towards the positive charge
                                                       (or in free radical, the region of low electron density.) 

                                                ii)   Delocalization of C-H sigma bond of a substituent's carbon atom into
                                                       the empty p-orbital of a carbocation (or electron-deficient p-orbital of a free
                                                           radical) stabilizes the species ( this is known as hyperconjugation):

                         6.   Carbanion
                                    a)   Carbon anions; sp3 hybridization for alkyl carbanions. 
                        7.   Exercise.  Draw Lewis structures of an ethyl free radical, carbocation, and carbanion. 

free radical

carbocation

carbanion

 

The Curly Arrows tutorial by Mary Masson at the University of Aberdeen
is a great place to start learning about their use in the depiction of reaction mechanisms
http://www.abdn.ac.uk/curly-arrows/

II.   Alkyl Halides   
            A.   Structure of Alkyl Halides.
 
                        1.   R-X  where X = F, Cl, Br, I 
                        2.   Classes of Alkyl Halides.

a)   Primary (1o) Alkyl Halide:

b)   Secondary (2o) Alkyl Halide:

c)   Tertiary (3o) Alkyl Halide:

              B.   Bonding in Alkyl Halides. 
                        1.   The C-X bond is usually only sigma. 
                        2.   Pi bonds can exist in resonance structures and intermediates: 

                           3.   C-X bonds are polar.   

 

 

 

               C.   Nomenclature of Alkyl Halides.     
                        1.   Radicofunctional Nomenclature. 
                                    a)   Alkyl group/halide written as separate words.    
                                    b)   Alkyl group named by largest continuous carbon chain to which the halogen is
                                                 attached.       
                                    c)   Examples: 

CH3Br

CH3CH2CH2CH2F

methyl bromide

n-butyl fluoride

1-propylpentyl iodide

cyclopentyl chloride

                        2.   Substitutive Name.   
                                    a)   Treat the halogen as a substituent.

                                    b)   Use halo- for the name of the substituent halogen (fluoro-, chloro-, bromo-, iodo-).

                                    c)   Halogens and alkyl groups are given equal priority. 
                                    d)   Name the following compounds: 

2-chloro-5-methylheptane

2-bromo-1,1-dimethylcyclohexane

1-bromo-3-chloro-4-methylhexane

3,3-dibromo-4-methylheptane

Dave Woodcock has a large number of Chime structures of  halogen-containing molecules at
http://www.molecularmodels.ca/molecule/Halogen_containing.htm

                 D.   Physical Properties of Alkyl Halides.   
                        1.   Boiling Points.  Higher boiling points than alkanes of similar molecular weight. 
                                    a)   Boiling points increase with increasing molecular weight for both the alkyl
                                          group and the halide. 
                                    b)   Forms of attraction:       
                                                i)   London dispersion forces (induced dipole-induced dipole). 
                                                ii)   Dipole-induced dipole.                                                                 

                                                 iii)   Dipole-dipole.                                   

                                        c)   Polarizability of X 
                                                i)   I  >  Br  >  Cl  >  F 
                                                ii)   Boiling points:  
                                                            R-I  >  R-Br  >  R-Cl  >  R-F 

 

 

Boiling Points in oC

R

Formula

F

Cl

Br

I

Methyl

CH3X

-78

-24

3

42

Ethyl

CH3CH2X

-32

12

38

72

Propyl

CH3CH2CH2X

-3

47

71

103

Isopropyl

(CH3)2CHX

-11

35

59

90

Pentyl

CH3(CH2)3CH2X

65

108

129

157

Hexyl

CH3(CH2)4CH2X

92

134

155

180

                                                iii)   Usually increased # of X's increase the boiling point.   Why? 

CH3Cl

CH2Cl2

CHCl3

CCl4

-24oC

40oC

61oC

77oC

                                  Answer:   Due to an increase of induced dipole-induced dipole interactions.  

                                                iv)   Increase # of F's can lower boiling point.  Why is this trend observed? 

CH3CH3

CH3CH2F

CH3CHF2

CH3CF3

CF3CF3

-89oC

-32oC

-25oC

-47oC

-78oC

                         Answer:   Due to the low polarizability F: a decrease in induced dipoles.  The weak
                                    intermolecular force of attraction causes fluorinated hydrocarbons (fluorocarbons) to be
                                   low in friction:  Teflon coatings.                       

                        2.   Solubility of Alkyl Halides
                                    a)   Insoluble in water.  
                                    b)   Used to dissolve nonpolar organic compounds.  Common solvents:  CH2Cl2; CCl4
                        3.   Density of Alkyl Halides
                                    a)   R-F, R-Cl less dense than water
                                                [true for monohalo cmpds; polyhalo cmpds can be denser than H2O]. 
                                    b)   R-Br, R-I more dense than water. 
                                    c)   Solubility characteristics and difference in density with water: Extractions.  

             E.   Preparation of Alkyl Halides. 
                        1.   Halogenation of Alkanes

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

                                    a)   R-H  +  X2 Text Box: hn
	
    R-X  +  H-X 
                                                i)   X = Cl, Br
                                                ii)   I2 is unreactive
                                                iii)   F2 is explosive       
                                    b)   Relative rates of halogenation: 

X

3o  >

2o  >

1o

Cl

5.2

3.9

1

Br

1640

82

1

                                     c)   Examples (@25oC) :                                                                                       

CH3CH2CH3  +  Cl2     CH3CH2CH2Cl  (40%)   +  CH3CHClCH3  (60%) 
CH3CH2CH2CH3  +  Cl2  
     CH3CH2CH2CH2Cl  (28%)  +  CH3CH2CHClCH3  (72%) 
                            CH3CH2CH3  +  Br2 
   CH3CH2CH2Br (3%)  +  CH3CHBrCH3  (97%)                    

                                    d)   Reaction Mechanism.  Free Radical. 

X2    2X

Chain Initiation

X +  R-H    H-X  +  R
R
+  X2     R-X  +  X

Chain Propagation

2R    R-R
2X
    X-X
R. + X
  R-X

Chain Termination

                                    e)   For the reaction :        
                                                CH4  +  Cl2 
  CH3Cl  +  HCl 
                                                 Each Cl
repeats in the propagation cycle ~5000 times before termination. 
                                    f)   Why is bromination more selective than chlorination and why is the order of
                                                reactivity   3o > 2o > 1o  followed?

                                                (See The Hammond Postulate below.) 

                        2.   From Alcohols and Hydrogen Halides
                        3.   From Alcohols and Thionyl Chloride
                        4.   From Alcohols and Phosphorus Trihalides (PX3, X = Br, Cl) 

            F.   The Hammond Postulate. 
                        1.   The Hammond Postulate states that related species which are similar in energy are also
                                    similar in structure.   
                        2.   This general rule tells us something about the transition states in endothermic and
                                    exothermic reactions.
                        3.   The transition state is always the point of highest energy on the energy profile.
                                    a)   Its structure resembles either the reactants or the products, whichever
                                                one is higher in energy.
                                    b)   In an endothermic reaction the products are higher in energy, and the transition
                                                state is product-like.
                                    c)   In an exothermic reaction, the reactants are higher in energy and the transition
                                                state is reactant-like.
 

                         4.   Selectivity in the halogenation of alkanes can be explained by the Hammond Postulate. 
                                    Chlorination:    
                        CH3CH2CH3  +  Cl2 
  CH3CH2CH2Cl  (40%)  +  CH3CHClCH3  (60%)
                                    Bromination: 
                        CH3CH2CH3  +  Br2 
  CH3CH2CH2Br (3%)   +  CH3CHBrCH3  (97%)

                                    a)   In bromination the transition state is closer in energy to the free radical intermediate
                                                and therefore also closer to the free radical in structure: 
                                                i)   Factors which stabilize free radicals also stabilize the transition state. 
                                                ii)   For this reason there is a large difference in the reactivities for
                                                            1o, 2o, and 3o carbons.
 

                                    b)   In chlorination the transition state is closer in energy to the alkane reactant
                                           and therefore also closer to the alkane in structure: 
                                                i)   Factors which stabilize the free radical intermediate are not as important 
                                                   in the stabilization of the transition state leading to the
                                                     free radical formed (which in turn leads to the site of halogenation.) 
                                                ii)   The reactivities of 1o, 2o, and 3o carbons are therefore more similar in
                                                            the chlorination of alkanes resulting in less selectivity.

III.   Alcohols 
            A.   Structure
  
                        1.   R-OH

a)  1o Alcohols:     RCH2OH

b)  2o Alcohols:     R2CHOH

c)  3o Alcohols:     R3COH

            B.   Bonding        
                        1.   O bonded to C via a sigma bond. 
                        2.   A pi bond between C and O can exist via resonance. 
                        3.   The O-H bond is polar.                                                                            

                         

 

                                     a)   Hydrogen bonds can form in alcohols.       
                        4.   C-O-H bond angle ~109.5o  (sp3 hybrids). 

            C.   Nomenclature
                        1.   Some simple alcohols have common names which should be memorized.   

CH3OH

CH3CH2OH

(CH3)2CHOH

methyl alcohol

ethyl alcohol

isopropyl alcohol

wood alcohol

grain alcohol

rubbing alcohol

       

See my table of alcohols

Dave Woodcock's Chime structures of alcohols are at
http://www.molecularmodels.ca/molecule/Alcohols.htm

                           2.   IUPAC Rules.    
                              a)   The parent chain must contain the OH group.   
                              b)   The parent chain is named by replacing -ol for the final -e of the corresponding alkane.
                              c)   The parent chain is numbered so that the OH group is given the lowest possible number.                         3.   Name the following alcohols. 

2-butanol

1-butanol

3-heptanol

7-methyl-2-octanol

trans-2-ethylcyclopentanol

4,4-dimethylcyclohexanol

6,6-dimethyl-3-heptanol

4-ethyl-1-heptanol

5-sec-butyl-6-isopropyl-4-nonanol

 

Dave Woodcock's page on alcohol nomenclature is at
http://www.molecularmodels.ca/nomenclature/nom-720.htm

           
            D.  Physical Properties of Alcohols 
                        1.  Boiling Points
                                    a)   The boiling points of alcohols are higher than the boiling points of  alkanes and alkyl
                                                 halides of similar molecular weights.  bpAlc  >  bpRX  and bpRH.  Why?  
                                     b)   Example.

cmpd

CH3CH2CH3

CH3CH2F

CH3CH2OH

MW

44

48

46

bp

-42oC

-32oC

78oC

                                     Answer:  Alcohols can H-bond;  no H-bonds can form in RH or RX.                       

                        2.   Solubility.
                                    a)   Low MW alcohols ( < 7 C) are soluble in water:  H-bonds. 
                                    b)   High MW alcohols are less soluble since they are more hydrocarbon-like due to a                                           larger hydrocarbon segment. 
                                    c)   Alcohols can also dissolve in nonpolar compounds, for example MeOH in gasoline and paints. 
                        3.   Density
                                    a)   All liquid alcohols:  ~0.8 g/mL. 
            E.   Alcohols as Acids/Bases 
                        1.   Br
nsted-Lowry Definition:  
                                    a)   Acids-  Proton Donors 
                                    b)   Bases-  Proton Acceptors
                        2.   Alcohols as acids
                                    a)   ROH  +  :
  BH+  +   RO  Alkoxide Ion (Conjugate Base of ROH)
                                    b)   Alcohols:  Weaker acids than water (very similar). 
                                    c)   H2O  +  HA 
  H3O+  +  A
                                                            K =      [H3O+][A
] / [H2O][HA]
                                                            Ka =  K[H2O]  =   [H3O+][A
]/[HA]        
                                                                        pKa =  -logKa 

Acid

Ka

pKa

Conjugate Base

HI

1010

-10

I

HCl

107

-7

Cl

HF

3.5 x 10-4

3.5

F

HC2H3O2

1.8 x 10-5

4.7

C2H3O2

H2O

1.8 x 10-16

15.7

OH

MeOH

10-16

16

MeO

EtOH

10-16

16

EtO

iPrOH

10-17

17

RO

NH3

10-36

36

NH2

                                     d)   Important points from above:   
                                                i)   As Ka increases acid strength increases. 
                                                ii)   As pKa decreases acid strength increases. 
                                                iii)   pKa of alcohols ~  pKa of water. 
                                                iv)   Bases which can be used to effectively deprotonate alcohols must be the
                                                             conjugate bases of  acids which have a GREATER pKa than
                                                            the corresponding alcohol. 
                                                            ROH  +  NH2
    RO  +  NH3 

                                    e)   Alkoxides are important in organic reactions.  Synthesis of alkoxides: 
                                                i)   Normal preparation:  Reaction of Alcohols and Metals (M = Na, K). 
                                                            ROH  +  M 
  ROM+  +  H2 
                                                            2CH3OH + 2Na 
2CH3ONa+  +  H2 
                                                            Order of reactivity:   3o < 2o < 1o < CH3OH 
                                                            3o ROH least reactive:  usually use K in place of Na.   
                                                ii)   Reaction of Alcohols and NaH. 
                                                            ROH  +  NaH 
  RONa+  +  H2 
                                                            CH3OH  +  NaH 
CH3ONa+  + H2 

                                    f)   Alcohols as Bases:                                                                                                 

                                                 i)   Alkyloxonium ions are important in catalysis of alcohols in reactions. 

            F.   Reactions of Alcohols.  (Preparation of Alkyl Halides.) 
                        1.   Alcohols and Hydrogen Halides 
                                    a)   Overall reaction. ROH  +  HX 
  RX  +  H2O     
                                    b)   Examples.   Complete the following:                        

 

 

 

 

 

 

 

 

 

                                       c)   Some observations: 
                                                i)    Reactivity of Alcohol:   3o >  2o >  1o  <  MeOH  
                                                ii)   Reactivity of HX:    HI  >  HBr  >  HCl  >  HF 
                                                iii)   Addition of H2SO4 speeds reaction:  an acid catalyzed reaction. 
                                                iv)   A good method for the preparation of 3o alkyl chlorides. 
                                    d)   Mechanism: 
                                                i)   For 3o, most 2o ROH:  SN1.  Substitution Nucleophilic Unimolecular. 

Animated Chime-like movie of the SN1 mechanism by Jennifer Muzyka at Centre College
http://web.archive.org/web/20010725034401/http://web.centre.edu/~muzyka/organic/sn1/main.htm

Animated Chime-like movie of the SN1 mechanism [use Netscape!] from Mol4D at U Nijmegen
http://www.cmbi.kun.nl/wetche/organic/sn1/

 

Step 1.

Step 2.

Step 3.

             
                                                ii)   Why does the reaction rate follow the order 3o >  2o >  1o <  MeOH ?
                                                            See Figures 4.9 and 4.10 and invoke the Hammond Postulate. 
                                                iii)  Evidence for carbocation formation: Rearrangements. 

                                                 iv)   Show the mechanism for the above reaction.  (1,2-hydride shift).

Step 1.

Step 2.

 

           

                                                 

 

 

                       

     

                                                v)   For MeOH, 1o ROH:  SN2.    Substitution Nucleophilic Bimolecular. 

Animated Chime-like movie of the SN2 mechanism by Jennifer Muzyka at Centre College
http://web.archive.org/web/20010415184936/http://web.centre.edu/~muzyka/organic/sn2/main.htm

Animated reaction of ethanol with HBr by Richard C. Banks at Boise State U
http://chem.boisestate.edu/~rcbanks/organic/alcohol+hbr.gif

QuickTime Movie comments on factors affecting SN2 substitution by Brent Iverson at the University of Texas
http://web.archive.org/web/20030508150243/http://neon.cm.utexas.edu/CH610B/Iverson/reaction+movies/IVERSON/SN2HOME2.HTM
 http://neon.cm.utexas.edu/CH610B/Iverson/reaction%20movies/IVERSON/SN2HOME2.HTM

Animated Jmol [IE OK] and Chime-like [use Netscape!]  movie of the SN2 mechanism from Mol4D at U Nijmegen
http://www.cmbi.kun.nl/wetche/organic/sn2/

                                                vi)   The reason MeOH reacts faster than 1o alcohols: 
                                                               Less hindered to back-side attack by the nucleophile (X
).     

                        2.   Alcohols and Thionyl Chloride, SOCl2. 
                                    a)   Overall reaction. 
                                                ROH  +  SOCl2 
  RCl  +  SO2  +  HCl      
                                    b)   This reaction is mainly used to produce 1o and 2o alkyl chlorides. 
                                    c)   Examples.  Complete the equations: 

                                   

 

                                                               

   

 

 

                                     d)   Mechanism.     

                                                  i)   Backside attack by the Cl- on the chlorosulfite ester results in                                                             inversion of configuration.

           

                                                                              

                                    e)   The reaction is run in K2CO3 or pyridine.  WHY? 

                                    Answer:   The base scavenges HCl formed, preventing acid-catalyzed side reactions. 
                                                It also helps pull off  H+ from the intermediate. 

                        3.   From Alcohols and PX3.      X = Br, Cl, I 
                                    a)   Overall reaction. 
                                                3ROH  +  PX3 
   3RX  +  H3PO3 (run in ether)  
                                    b)   Examples: 

                                     c)   Observations:    
                                                i)   Works best with 1o and 2o ROH.
                                                ii)   Poor yields with 3o ROH.
                                                iii)   SN2 mechanism.

                                                iv)   1o alcohols react by the slower SN2 process.
                                                        PX3 increases the reaction rate by making -OH a better leaving group.
 
                                                v)   Often the best method to avoid rearrangement (No Rearrangement). 

                                                vi)   SOCl2 usually results in better yields than PCl3 in making alkyl chlorides.
                                    d)   Mechanism:       

                                     e)   Complete the equation and then write the mechanism: 

                                    CH3CH2CH2OH  +  PBr3                                                                               

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  MDL IsisDraw and CS ChemOffice ChemDraw; diagrams with MS EXCEL

updated 9/15/07