CCEA ADVANCED SUBSIDIARY

CHEMISTRY

 

MODULE 2

 

2.1        Isomerism in Organic Compounds

 

Structural isomerism for aliphatic compounds containing up to six carbon atoms, to include branched structures and position of the C=C double bonds in alkenes. (Cyclic compounds excluded).

 

Isomerism (greek isos, equal; meros, parts) occurs when there are two or more compounds (called isomers) with the same molecular formula but different arrangements of their atoms.  Isomers have different physical and chemical properties but the differences may be great or small depending on the type of isomerism.

There are two main classes of isomerism;

(i)                            structural isomerism

(ii)                         stereoisomerism

which are themselves sub-divided.

 

ISOMERISM

 

 


structural isomerism                                     stereoisomerism

 

 


chain            positional     functional group        geometric        optical

isomerism     isomerism     isomerism                  isomerism         isomerism

                                      [Module 4.5]                                  [Module 4.5]

 

Structural isomerism

Structural isomers are molecules with the same molecular formula but with different structural arrangements of the atoms.

 

·        Chain isomers

This occurs where isomers have different arrangements of the carbon chain.  There is only one alkane corresponding to each of the formulae CH4, C2H6 and C3H8.

 

 

 

For butane C4H10 two arrangements are possible.

      H   H   H  H                                     H      H       H

                │  │ │  │                   │   │    │


H—C—C—C—C—H                                     H—C      C        C—H

   │  │ │  │                   │   │    │

      H   H  H   H                                     H H—C—H  H

                                                                                │

                                                                                H

        butane                                          methylpropane

       b.pt. 273K                                         b.pt. 261K

 

As the number of carbon atoms increases  the number of  possible isomers increases rapidly.

 

Formula

Number of structural isomers

C5H12

3

C6H14

5

C7H16

9

C10H22

75

C20H42

366,319

C30H62

Over 4000 million

 

 

·        Positional isomers

 This occurs when isomers have the same carbon skeleton but the functional group is in different positions in the molecule.

     H   H   H                                           H      H       H

                │  │ │                     │   │    │


H—C—C—C—OH                              H—C      C       C—H

   │  │ │                      │   │    │

      H   H  H                                           H     OH     H

   propan-1-ol                                          propan-2-ol

 

Cl                                   Cl                                   Cl

│   Cl              │                  │

                                                           

                                                        Cl                            │

                                                                                       Cl

1,2-dichlorobenzene            1,3-dichlorobenzene      1,4-dichlorobenzene

 

 

 

Exercise 1

(a)      Draw and name the five structural isomers of C6H14.

 

(b)             Write the structural formulae of all the alcohols of molecular formula C4H10O.  Name the isomers.

 

 

Stereoisomerism : cis-trans isomers for compounds containing one C=C bond, the energy barrier to rotation in these compounds.

 

Stereoisomerism

Stereoisomers have identical molecular formulae, and the atoms are linked together in the same order, but have different relative positions in space.

The two types of stereoisomerism are

(i)                            Geometric (or cis-trans ) isomerism

(ii)                         Optical isomerism [See Module 4.5]

 

·        Geometric isomers

Geometric or cis-trans isomers exist because the π bond of the C=C bond prevents free rotation .

 

H                     H                                Cl                    H

C=C                                                    C=C

Cl                    Cl                               H                     Cl

cis-1,2-dichloroethane                         trans-1,2-dichloroethane

 

 

          H                     H                 HOOC                   H

          C=C                                          C=C

  HOOC                      COOH               H                     COOH

cis-butenedioic acid                            trans-butenedioic acid

 

Cis-trans isomerism can also occur in inorganic complexes about a single bond with square planar or octahedral structures.

 

e.g. diaminedichloro platinum (II)

 

Cl                          NH3             Cl                          NH3

 


                   Pt                                             Pt

 

          NH3                       Cl                Cl                          NH3

                   trans                                           cis

      (non-polar)                                   (polar)

 

tetraaminedichloro cobalt (II)

 Cl                                            NH3

 


H3N                       NH3         H3N                          NH3

 


                   Co                                            Co

 

          NH3                       NH3             NH3                       Cl

 

                    Cl                                            Cl

trans isomer                              cis isomer

 

 

Exercise 2

1.  Draw and name all the structural isomers, including geometric isomers, of C4H8.