Functional Groups

 

Functional Groups

 

Definition

The functional groups are centers of chemical reactivity. Compounds having a similar functional group have undergone similar reactions. The presence of functional groups enables the systematization of organic compounds into different classes. The functional groups determine the way the molecule works both chemically and biologically.

 

An atom or group of atoms whose presence confers on organic molecule characteristics properties unique to that group is called Functional Group. It is the site of chemical reactivity of organic compounds.

OR

An atom or group of atoms which is present within the organic molecule and is responsible for its chemical behaviour and characteristic properties is called Functional Group.

 

A functional group is an atom or group of atoms, double or triple bonds whose presence imparts specific properties to organic compounds. They are chemically functional part of organic molecule. The hydrocarbon part i.e. alkyl group of the molecule is usually inert, the reactivity is due to functional group. Hence it is called active part. The functional group determines the basic chemistry of an organic compound. It gives specific and characteristic properties to an organic molecule, while the remainder hydrocarbon part (alkyl group) of the molecule has an effect on its physical properties.

 

For Example

(i)  In alcohols (R–OH),   the hydroxyl group (–OH) is functional group.

(ii) In alkyl halide (R–X), the halide group (–X) is functional group.

 

Characteristics of functional groups

 

1.         Each functional group defines an organic family.

2.         Each functional group undergoes characteristic reaction.

3.         Function group helps us to name the organic compounds.

4.         All the compounds with the same functional group belong to the same class.

5.         Molecules can contain more than one functional group called poly-functional compounds.

6.         One functional group can modify the properties of other functional groups.

 

Types of Functional groups

1.         Non-terminating FG; (without C-containing FG)

2.         Terminating FG; (C-containing FG)

 

Common and Important Functional Groups

Only alkanes are devoid of any functional group. All other families possess a characteristic functional group:

1. Olefinic Double Bond

Olefinic double bond (i.e. carbon-carbon double bond) is the functional group of alkenes or olefins or alkylenes. 

e.g.

 

(i)        Ethene or Ethylene         (C₂H₄)    ;    H₂C = CH₂

(ii)       Propene or Propylene   (C₃H₆)    ;    CH₃ – CH = CH₂

(iii)       butenes or Butylene       (C₄H₈)    ;    (3 isomers or 4 isomers including stereoisomers) 

2. Acetylenic Triple Bond

Acetylenic triple bond (i.e. carbon-carbon triple bond) is the functional group of alkynes or acetylenes. 

e.g.

(i) Ethyne or AcetyleneC₂H₂   ;    HCºCH                   

(ii)Propyne/Allylene/methyl acetyleneC₃H₄ ;CH₃–CºCH              

(iii) but-1-yne or ethyl ethyne or ethyl acetylene C₄H₆ ;    

CH₃–CH₂–CºCH     

(iv) but-2-yne or crotonylene  or dimethyl acetylene  C₄H₆ ;  

  CH₃–CºC– CH₃ (2 isomers)

3. Amino Group

Amino group is the functional group of amine. They are of 3 types:

 

(i)Primary amines ; (–NH₂)   ; e.g.   CH₃–NH₂, CH₃–CH₂–NH₂

(ii) Secondary amines;(>NH) ; e.g.   (CH₃)₂.NH (dimethyl amine)

(iii) Tertiary amine ;   (àN)   ; e.g.   (CH₃)₃.N

 

There is a little difference in the way amines are classified! Unlike the previous cases, the amines are classified based on the number of carbons connected to the nitrogen. Another difference with the amines is that the nitrogen can have four groups connected by using the lone pair and getting a positive formal charge. These are called quaternary ammonium salts.

4. Ether Linkage

The linkage of two carbon atoms through an oxygen atom is called an Ether linkage (C – O – C). It is the functional group of ether family. e.g.

 

(i)        CH₃–O–CH₃       ; (dimethyl ether/Methoxymethane)                   

(ii)       C₂H₅–O–CH₃     ; (ethyl methyl ether/Methoxyethane)                

(iii)      C₂H₅–O–C₂H₅    ; (diethyl ether/Ethoxyethane)       

5. Epoxide Group

In this group, two carbon atoms are directly bonded to each other through an oxygen atom. It is the functional group of alkanepoxides or alkyl oxirane or alkylene oxide.

                        

 

6.         Thiol or mercapto (–SH)

It is the functional group of mercaptan or thiol or alkanethiol. Its type formula is RSH.

 

(i)        CH₃–SH       ;   Methanethiol                         

(ii)       C₂H₅–SH     ;   ethanethiol                             

 

7.         Sulphonic acid group

It is the functional group of sulphonic acids. Its type formula is R–SO₃H.

 

(i)        CH₃–SO₃H         ; methane Sulphonic acid                        

(ii)       C₂H₅–SO₃H       ; ethane Sulphonic acid                            

    

8. Halide Group

Halide group (–X) is the functional group of two series of organic compounds i.e. alkyl halide (R–X) and aryl halide (C₆H₅–X) where X represents Cl, Br and I.

 

(a) Alkyl Halides

It is the series of organic compounds in which halogen atom is attached directly to aliphatic (open chain) carbon atoms. They are mono-halogen derivative of alkane. 

There are 3 types of alkyl halides:

 

(i)  primary (1°) alkyl halides ; (–CH₂–X) e.g.CH₃–Cl, CH₃–CH₂–Cl 

(ii)secondary (2°) alkyl halides ; (>CH–X) e.g. (CH₃)₂ – CH – Cl 

(iii) tertiary (3°) alkyl halides  ; (àC–X) e.g. (CH₃)₃.C– Cl  (3°–butyl chloride)

 

(b) Aryl Halides

It is the series of organic compounds in which –X atom is directly attached to carbon atom of benzene ring. e.g.