Document Sample

                Dr Nahed Elsayed
     Learning Objectives
Chapter six concerns alcohols and phenols and by the end of this chapter the student

 know the difference in structure of alcohols and phenols
 Know the types of alcohols
 Know the different classes of monohydroxy alcohols
 Know how to name alcohols and phenols
 Know the physical properties (solubility, boiling and melting points) of alcohols and
  phenols and factors affecting them.
Know how hydrogen bonds are formed and its effect on solubility and boiling points of
 a compound.
Know how to differentiate between alcohols and phenols using NaOH
 know the different methods that can be used to prepare alcohols and phenols.
 Know the chemical reactions of these compounds ( some reactions are review, others
  are extensions of the chemistry that will be discussed on chapters 8, 9 & 10
   Structure Of Alcohols and Phenols
 Alcohols are the family of compounds that contain one or more hydroxyl (-OH)
groups. The OH group is bound to a carbon atom.
They can be considered both derivatives of hydrocarbons (by replacing a hydrogen
atom with a hydroxyl group -OH) and water (H2O) derivatives (the result of the
substitution of a hydrogen atom by an organic radical).
 Thus the chemical formula of alcohols is ROH or ArCH2OH

 On the other hand Phenol’s chemical formula is C6H5OH, it has the OH group
directly attached to the benzene ring.

                  OH                                   OH

                 Phenol          Benzyl alcohol    Cyclohexanol
               Types Of Alcohols
Monohydroxyls: containing one hydroxyl group (for example ethanol C2H5OH)
Dihydroxyls (glycols): containing two hydroxyl groups connected by different
carbon atoms for example: Ethylene glycol CH2OH-CH2OH.
Polyhydroxyls: containing more than two hydroxyl groups on different carbon atoms
(for example: 1,2,3-propanetriol CH2OH-CHOH-CH2OH).

Classification of Monohydroxyl Alcohols
The mono hydroxyl alcohols can be classified into three types according to the type of
the carbon atom connected to the hydroxyl group:
- primary alcohols
- secondary alcohols
- tertiary alcohols

                            Primary 1°     Secondary 2°      Tertiary 3°
                                                  R'                     R'
      H3C     OH        R      CH2     OH     R   CH     OH       R       C      OH
         Carbinol           Primary alcohol   Secondary alcohol
                                                                      Tertiary alcohol


 Methanol CH3OH, ethanol CH3-CH2-OH, allyl alcohol CH2=CHCH2OH are primary

 2-Propanol CH3-CH(OH)-CH3 is a secondary alcohol

 2-Methyl-2-propanol CH3-C(CH3)OH-CH3 is a tertiary alcohol.

Common Nomenclature Of Alcohols
 Alcohols are named as alkyl alcohols i.e. name the alkyl group and follow it by the
word alcohol
                   CH3OH                   CH3CH2OH                 CH2=CHCH2OH
Common names       Methyl alcohol          Ethyl alcohol            Allyl alcohol

                H3C    C    CH3

Common names Isopropyl alcohol          Cyclopentyl alcohol      Methylcyclohexyl

The alcohols containing 2 hydroxyl groups (diols) connected to two different carbons
are called glycols for example: CH2OH-CH2OH is called ethylene gylcol
                                CH3-CH(OH)-CH2(OH) is called propylene gylcol 6
       IUPAC Nomenclature Of Alcohols
 Find the longest chain of C atoms containing the O-H group; to obtain the root name
of the parent alkane

 Replace the e ending by ol suffix in the basic name

 Number the chain starting from the end nearer to the O-H group and add a locator
number for OH group just before the ol suffix or before the full name

e.g. CH3CH(OH)CH2CH3 is named 2 -butanol or butan-2-ol

 Identify the substituents, allocate them numbers, then list them in alphabetical order.


 CH3 - CH(CH3) - CH2 - CH2 - CH(OH) - CH3        is named 5-methylhexan-2-ol
 6      5          4     3      2      1                  or 5-methyl-2-hexanol
 If a molecule contains both an OH group and a c=c or c=c bond, the name should
include suffixes indicate presence of both OH group and the unsaturated groups. The
OH group takes precedence over the double or triple bonds in getting the lower

                  5        4   3   2           1
                   4- Methyl-3-penten-2-ol
 If the parent hydrocarbon contain two hydroxyl groups, the suffix diol is added to
  the name; the suffix triol is added when there are three OH groups. In each case
  the relative positions of OH groups must be identified.

                                              OH                     OH

                    HO           OH                            OH         OH

           IUPAC                       Propane-1,2-diol     Propane-1,2,3-triol
                                      or 1,2-Propanediol   or 1,2,3,-Propantriol
           Common   Ethylene glycol    Propylene glycol     Glycerol or Glycerin

                Nomenclature Of Phenols
 Phenols are generally named as derivatives of the simplest member of the family,
The ortho, meta, para system is used in common names.
 While the numbering system is employed in IUPAC names and in this case
numbering of the ring begins at the hydroxyl-substituted carbon and proceeds in the
direction of the next substituted carbon that possesses the lower number.
Some phenols have common names as shown in the following examples

      OH             OH                      OH                  OH               OH
                           NO2       O2N           NO2                       Cl         Cl
                                                                             Cl         Cl
                     Br                      No2                 NH2              Cl
    Phenol   4-Bromo-2-nitrophenol    2,4,6-Trinitrophenol                    2,3,4,5,6-Pentachlorophenol
                           OH                         OH                        OH

  IUPAC: 2-Methyl-phenol                      3-Methyl-phenol           4-Methyl-phenol
  Common:   o-Cresol                            m-Cresol                   p-Cresol

             OH                          OH                       OH                  OH
                   OH                                                                      OH

                                               OH                                          OH
IUPAC:  2-Hydroxyphenol               3-Hydroxyphenol        4-Hydroxyphenol         2,3-Dihydroxyphenol
      or 1,2-Benzenediol
Common:    Catechol                      Resorcinol              Hydroquinone              Pyrogallol
          Physical Properties of Alcohols
1) Boiling Points of Alcohols and Phenols
 Phenols have higher boiling and melting points than corresponding aliphatic alcohols
(due to they form stronger H-bonds with themselves).
                           OH                OH

                        Phenol          Cyclohexanol

A hydrogen bond is the attractive interaction of a hydrogen atom with an
electronegative atom, like nitrogen, oxygen or fluorine. The hydrogen must be
covalently bonded to another electronegative atom to create the bond. These bonds
can occur between molecules (intermolecularly), or within different parts of a
single molecule (intramolecularly).
 Intermolecular hydrogen bonding is responsible for the high boiling point of water
(100 °C).
 Alcohols have higher boiling points than alkanes of similar mass this is due to the
presence of inter-molecular hydrogen bonding that connect molecules together
thus more energy is required to separate them.

                                     hydrogen bonding

                          Mr. Wt                     bp / °C
Propane C3H8              44                        42
Ethanol C2H5OH            46                        78
 The boiling points increases with increase in molecular weights
 Boiling point is higher for straight chain isomers i.e. branching decreases the
boiling points

         Butan-1-ol           Butan-2-ol                 2-Methylpropan-2-ol
        CH3CH2CH2CH2OH        CH3CH2CH(OH)CH3
Bp/°C       118                      100                     83

 Boiling point increased as number of OH groups in a molecule increase
 Thus triols > diol> monohydroxy alcohols
2) Solubility Of Alcohols and Phenols
Low molecular mass alcohols (4 or less C’s) are miscible with water due to
hydrogen bonding between the two molecules
 While heavier alcohols are less miscible i.e. as the number of carbon atoms
increase the miscibility decrease.

 Phenol itself is crystalline solid moderately soluble in water (H-bonds), other
phenols are not very soluble.
As the number of OH groups present in a molecule increase the miscibility increase.
Thus the order of solubility is as follows triols > diols > monohydroxyl alcohols.
 Also benzene diols (catechol, resorcinol and hydroquinone) are more soluble than
phenol which in turn more soluble than benzene.
3) Acidity and Acidic Properties of Alcohols and Phenols
Acidity refers to the ease with which a compound donates a hydrogen ion.
 In alcohols and phenols, the most acidic hydrogen is the one attached to the oxygen
and its acidity depends on the presence of "electron withdrawing" groups ( such as
COOH, CHO, CN, NO2) or electronegative elements (X) elsewhere in the molecule

 Alcohols are very weak acids (due to difference in electro negativity) thus they can
lose H+ with very reactive metals but not with bases

                     
       R      O      H                            RO      +     H+

        ROH + Na                             RO    Na+ + 1/2 H2

 phenol is slightly acidic, thus phenol molecule has weak tendency to lose the H+ ion
from the hydroxyl group, resulting in the highly water-soluble phenolate anion C6H5O−,
called phenoxide anion.

 Compared to aliphatic alcohols, phenols shows much higher acidity (about 1
million times more acidic) thus they can react with bases.
N.B. Phenols are less acidic than carboxylic acids.

 Thus NaOH (base) can be used to differentiate between alcohols and phenols


                         + NaOH                         No reaction


            OH                                      O- Na+

                      + NaOH                                 + H2O

 One explanation for the increased acidity over alcohols is resonance stabilization of
the phenoxide anion by the aromatic ring. In this way, the negative charge on oxygen is
shared by the ortho and para carbon atoms.

     OH                 O               O                                     O


  Resonance structures of phenoxide anion

 Introduction of electron withdrawing groups such as NO2, CHO, COOH, SO3H, or
CN i.e. all m-directors and halogens on the benzene ring increases the acidity of
phenols, While introduction of electron releasing groups (e.g. OR, R, NH2) i.e. all o,p-
directors except for halogens decrease the acidity of phenols compared to
unsubstituted phenol as shown in the following examples.
OH        OH           OH             OH                     OH

      <          <          <                    <

          OCH3                        Cl
                                                       H3C         O

OH        OH         OH         OH                            OH

                                           NO2   O2N                   NO2

      <          <          <                <

CH3                  NO2        NO2                           NO2
        Preparation of Alcohols
1- From Alkenes

                     1) B2H6           H3C                       OH

               CH2   2) H2O2 / NaOH      Anti-Markovnikov's product

                        H2O / H2SO4                        OH


                                             Markovnikov's product

                         KMnO4 / OH


                      1) RCO3H                            OH

                      2) H2O2 / NaOH

2- From alkyl halide by nucleophilic substitution

3-    By reduction of aldehydes, ketones and carboxylic
acids using metal hydrides
                                1) LiAlH4 or NaBH4
    Aldehyde R       C                                 R       CH2-OH Primary alc.
                                2) H3O
                                  1) LiAlH4 or NaBH4
             R           R'                                R     C-OH Secondary alc.
                                  2) H3O
                     O                                           H
     Acid R                   1) LiAlH4 or NaBH4
                 C                                     R       CH2-OH Primary alc.
                              2) H3O
4- By nucleophilic addition of Grignard reagent          to
aldehydes, ketones and esters
 Addition of RMgX to formaldehyde gives 1◦ alc.
 Addition of RMgX to any other aldehyde gives 2◦ alc.
 Addition of RMgX to ketones and esters give 3◦ alc.
       Preparation of Phenols
1- Via hydrolysis of Diazonium salts
         N2+ Cl-                     OH
                  H2SO4 / H2O
                                            + N2

2- Via fusion of sodium hydroxide with benzene-sulfonates
     SO3H                          O- Na+             OH
           NaOH / 350 °
                      C                     H3O+

3- From alkyl halide:
             Cl                       O- Na+          OH
                     NaOH / 350°
                               C               H3O+
                     300 atm
    Reaction of Alcohols and Phenols
1) Salt Formation By Reaction With Active Metals

   2R         OH     + 2 Na                 2R      ONa   + H2

         Alcohol                             Sodium alkoxide

   H3C        OH      + 2 Na                2 CH3   ONa   + H2
    Methanol                             Sodium methoxide

         OH                                        ONa

                   + 2 Na      or NaOH

                                           Sodium phenoxide
2) Elimination Of Water (Dehydration)
Reagent/catalyst                       conc. sulphuric acid (H2SO4)
Conditions                              reflux at 180°C
Product                                alkene

                           CH3   H2SO4 or H3PO4
                                 180 °C- H2O
               OH                                  1- Butene          2-Butene
                                                   Minor                 Major

While dehydration of alcohols at lower temperature will give ethers
3) Ester Formation
 Carboxylic acids react with alcohols in presence of strong acid catalyst (e.g.
  conc. H2SO4) to produce esters
              O                                  O
        R           +   R'OH                 R        + H2O

              OH                                 OR

               COOH                                        COOCH3
                        + CH3OH                                     + H2O

4)   Alkyl Halides Formation
                  R OH    + HX                         R      X    + H2O

                  R OH     + SOX2                      R      X    + SO2 + HCl

                  R OH    + PX3                        R      X   + HOPX2

                  R OH + PX    5                      R       X   + HOPX4
                       5) Oxidation Of Alcohols
                  Alcohols can be oxidised depending on their class

Oxidation is used to differentiate between primary, secondary and tertiary alcohols
The usual reagent is acidified potassium dichromate (VI) K2Cr2O7

Primary         Easily oxidised to aldehydes and then to carboxylic acids.
e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l)
              ethanol                        ethanal

           it is essential to distil off the aldehyde before it gets oxidised to the acid

                   CH3CHO(l) + [O]        ——>      CH3COOH(l)
                    ethanal                       ethanoic acid

Secondary            Easily oxidised to ketones

Tertiary             Not oxidised under normal conditions.
                     They do break down with very vigorous oxidation

                       PRIMARY 1°       SECONDARY 2°        TERTIARY 3°
       Why 1° and 2° alcohols are easily oxidised and 3° alcohols are not
 For oxidation to take place easily you must have two hydrogen atoms on adjacent
C and O atoms.

               H    H
  1°     R     C    O     +    [O]               R    C    O    +    H2O
               H                                      H
               H    H
  2°     R     C    O     +    [O]               R    C    O    +    H 2O
               R                                      R

             This is possible in 1° and 2° alcohols but not in 3° alcohols.

               R    H
  3°     R     C    O     +    [O]
               R                                                              28
              OH                                                  O
                    H2 Cr2O7 or Na2CrO7 / H

                    or KMnO4 / heat

Cyclohexanol                                          Cyclohexanone
Secondary alcohol                                     Ketone

    OH                                               O                               OH
              H2Cr2O7 or Na2CrO7 / H                                  KMnO4 / heat

              or KMnO4 / heat

                                                     O                               OH         29
     Phenol                                   [1,4]Benzoquinone                  Hydroquinone
Reactions Of Aromatic Ring In Phenols
                         Br               Br
            Br2 / H2O


                               OH                       OH
           Br2 / CCl4                         +


                                OH                          OH
            dil HNO3

                                         OH                 NO2
                              O2N                 NO2
           conc. HNO3


                                     OH                           OH
           conc. H2SO4                        SO3H


Shared By: