Form 5 Chemistry – ORGANIC CHEMISTRY 2

EXPLANATION ON COMPARISON OF ACIDIC STRENGTH  IN ALCOHOLS AND PHENOLS
Phenol is stronger acid than alcohol due to following reason
1.   Easier of releasing hydrogen proton
2.   Stability of phenoxide ion formed after releasing hydrogen proton

1.   EASIER OF RELEASING HYDROGEN ATOM
Lone pair electrons in oxygen of  groups in phenol are localized they are involved in mesomerism (+M) i.e they are delocalized.

The delocalization of lone pair electron tends to form +ve charge  in oxygen atom of group thus making easier to releasing oxygen proton.

There is no such occasion in alcohol

2.   STABILITY OF PHENOXIDE ION FORMED AFTER RELEASING HYDROGEN PROTON

Consider

From  above mechanism in phenoxide ion, it clearly understood negative charged electrons in oxygen atom of phenoxide is delocalized thus stabilizing the negative charge formed releasing hydrogen proton i.e ( it can exist on its own with combining with hydrogen proton).  There is no such occusion alcoxide ion RO^– formed in alcohol after releasing hydrogen proton.

ACIDIC THAN ALCOHOL

A. a) REACTION WITH ALKALINE SOLUTION

With alkaline solution like  NaOH phenol tend to react with them

Example with phenol form sodium phenoxide.

But

There is no such reaction with alcohol indicating that alcohol has basic character and hence phenol is more acidic than alcohol

Alcohol+ alkaline solution No reaction

Note

The reaction is very important in separating phenol from other class of alcohol.

Qn. Without using the distillation explain how would you separate the following pair of organic compounds

i. Phenol and 1- hexanol

ii. Phenol and benzene

B.
b) REACTION WITH SODIUM CARBONATE

Acids react with carbonate to form carbon dioxide gas phenol like other acids react with sodium carbonate to evolve a gas.

 Phenol like other acid react with sodium carbonate evolve a gas which turns lime water milky.

 But
No reaction

c) REACTION WITH CARBOXYLIC ACID

Alcohol react with carboxylic acid in presence of sulphuric to form ester and water.
i.e.

This verify that alcohol has basic character in this case phenol it does not react with carboxylic acid either in presence or in absence of suplhuric acid. This verify that phenol some alcohol character and hence phenol has higher strength than alcohol.

CHEMICAL REACTION OF ALCOHOL

Ø  Chemical reaction of alcohol can be classified into the following

a.  Reaction which involve replacement of whole OH^– group

b.  Reactions which involve replacement of H from  OH^– group

c.  Reactions which involve   OH^– group and β- hydrogen

d.  Oxidation

A.   REACTION WHICH INVOLVE REPLACEMENT OF HYDROGEN ATOM

Ø  Under this heading alcohol react with strong alkaline  metals like Li, Na, K to form alkoxide and hydrogen gas is

Ø  Generally

Example

Ø  The main reaction under this heading is halogenations of alcohol.

Examples

involved in the above reaction produce denser white fumes with ammonia so that reaction is used as test of presence of  group in the compound.

 

         B.  REACTION WHICH INVOLVE  OH^–  GROUP AND B- HYDROGEN.

Under this heading alcohol undergo elimination reaction to from alkene.

Example

       The production of either in above reaction is more favoured when   is dilute and cold.

C. OXIDATION

Ø  With weak oxidation agent like   primary alcohol tend to form aldehyde.

Example

With strong oxidizing agent like   carboxylic acid is formed.

Example

                         Orange                           Green

-With either weak oxidizing or strong oxiding agent secondary
alcohol from Ketone.

Generally

NOTE

Tertiary alcohol remit oxidation since there is no hydrogen to be removed.

        IODOFORM TEST

Ø  This is the chemical test for presence of terminal methyl group which is directly bonded to carbon with OH^– group or (to carbonyl group in the case of carbonyl compounds) by giving yellow ppt of trichloromethane ( Iodoform) CHI₃.

Ø  When compounds with terminal methyl group bonded to carbon with OH  group is heated with 1₂ in presence of  NaOH they give yellow ppt of Iodoform.

Example

CHEMICAL TEST TO DISTINGUISH BETWEEN PRIMARY, SECONDARY AND TERTIARY ALCOHOLS.

 – Three classes of alcohol can be distinguished by using Lucas reagent.

– Lucas reagent is the mixture of concentrated HCl and ZnCl mixed at equal proportion.

– With Lucas reagent at room temperature, primary alcohol do not form cloudness or turbidity ( insoluble substance) at all.

– Secondary alcohol tends to form turbidity ( insoluble substance which is chloroalkane within 5 minutes).

– Tertiary alcohol, tend to form turbidity immediately.

Example

Qn. Gave chemical test to distinguish between

 i. Ethanol and propanol ( I test)

            ii.  Butan – 1-ol and butan – 2- ol (2 test)

iii.  Butan -2- ol and 2- methyl propan – 2-ol ( 2 test apart from using Lucas reagent)

ANS

i.  Ethanol gives positive Iodoform test while propanol give negative Iodoform test.

    ii.   1^st TEST
With Lucas reagent at room temperature butan -2-ol give turbidity ( cloudness) within 5 minutes while butan -1-ol  do not give turbidity at all.

         2^nd TEST ( Iodoform Test)
           Butan -2- ol  give positive iodoform test while but-1-ol give negative iodoform test.

    iii.1^st TEST
When butan -2- ol  is mixed with solution the orange colour of   is changed to green while 2- methylpropan- 2 – ol do not show any change.

2^nd TEST
      Butan decolourise  KMnO₄  with black ppt appearing at the bottom of the beaker which is deep purple while 2-methylpropan do not.