STRUCTURAL ISOMERISM (Organic Chemistry)
Introduction:
Berzelius introduced the term Isomer (Gr. Isos=Same, Mers
= parts). Chemical compounds that have identical chemical
formulae but differ in properties and the arrangement of
atoms in the molecule are called isomers. This phenomenon
is known as Isomerism. Isomers exhibit different physical (ii) Functional Isomerism: This is due to difference in the
and chemical properties. functional groups
Example:n butyl alcohol and diethyl ether, both have the C2 H5 OH CH3 OCH3
same formula C4 H10 O. Only butyl alcohol releases H2 with Na ethyl alcohol dimethyl ether
metal, but not ether. Such compounds are isomers. CH3 COOH HCOOCH3
Acetic acid Methyl format
There are two main types of Isomerism. CH3 CH2 CHO CH3 COCH3
Propionaldehyde Acetone.
(iii) Position Isomerism: Isomers that have the same
parent chain and functional group and which differ in the
(i) Structural or Constitutional Isomerism; (It is due to position of the functional group are called position isomers.
difference in the arrangement of atoms within the molecule.)
Example: Ethanol CH3 CH2 OH and dimethyl ether
CH3 OCH3 have different connectivities
inspite of having the same formula C2 H6 O.
(ii) Stereo Isomerism or Space Isomerism: (It is due to
different spatial arrangement of some atoms and groups.)
Example:But-2-ene also exists in two forms, differing in the
arrangement of atoms and groups around doubly-bonded
carbons.
(iv) Metamerism: These are isomers which are having the
Types of Structural Isomerism: same functional group and differ in the alkyl or aryl groups
present around the functional group.
(i) Chain Isomerism: Chain isomers have different carbon
skeletons. Particularly, they have different parent chains.
Among chain isomers, possessing a functional group, the
functional group present in them is same and the position of (v) Ring Chain Isomerism : Out of two isomers, if one has
it in their parent chains also is same. open chain structure and the other has ring structure-they
are ring chain isomers.
Examples :-
(vi) Tautomerism (Tauto = Same, Mers = Parts): It is due
to the presence of a mobile atom in the molecule and the
same substance behaves in such a way as if it is a mixture of
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two or more compounds.
Further we have,
(a) Dyad System : When the mobile atom oscillates between
two adjacent atoms
e.g. H − C ≡ N ⇌ H − N ≡ C
Hydrocyanic acid Iso-hydrocyanic acid
(b) Triad System: When the mobile atom oscillates between
atoms one position ahead eg.
Conditions for Exhibiting Tautomerism
The compound should have strong withdrawing group like
, −NO2 or C ≡ N, with at least one H on saturated α
carbon. Compounds having H atom on a saturated carbon
Aceto acetic ester reacts with HCN, NH2 OH, C6 H5 NHNH2 which is separated from the withdrawing group by alternate
showing the properties of a ketone and also reacts with 𝛑 bonds also exhibit tautomerism.
CH3 COCl , PCl 5 , Na showing the properties of OH group.
Examples of Compounds Exhibiting Keto-enol
It gives colour change with 1%FeCl 3 a characteristic
Tautomerism
1. Nitro alkanes, with 𝛼H can exhibit tautomerism.
There exists an equilibrium between the two forms
which is dynamic in nature.
also exhibits tautomerism but no
Here the H migrates to 2nd atom and called triad system
2. Amides exhibit tautomerism.
Triad system containing nitrogen
3. Amines exhibit tautomerism.
It dissolves in NaOH on account of acid form. Tautomeri c 4. Cyanides exhibit tautomerism
form which is less stable is called labile form. CH3 − C ≡ N ⇌ CH2 = C = NH
Resonance and Tautomerism
Resonating structures differ only in the position of 𝝅 bonds,
not any atom. They cannot be separated as they are
hypothetical as real structure is a hybrid of them. Tautomers
can be separated and exist in equilibrium.
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STRUCTURAL ISOMERISM (Organic Chemistry)
EXERCISE – I metamers
Q.1 The compounds C2 H5 OC2 H5 and CH3 OCH2 CH2 CH3
are
(A) Chain isomers (B) Position isomers
(C) Metamers (D) Conformational isomers
metamers
Q2. The number of primary, secondary and tertiary
amines possible with the molecular formula C3 H9 N
respectively.
(A) 1,2,2 (B) 1, 2, 1
(C) 2, 1, 1 (D) 3,0,1 functional isomers
(A) TFTF (B) FTTF
Q3. How many benzenoid aromatic isomers shown by (C) TTFT (D) TFFT
C7 H7 Cl ?
(A) 4 (B) 3 Q8. Following compounds are:
(C) 5 (D) 6
Q4. Molecular formula C3 H6 Br2 can have:
(A) Two gem dibromide
(B) Three vic dibromide (A) Functional isomer (B) Chain isomer
(C) Two tert. -dibromo alkane (C) Metamer (D) Position isomer
(D) Two sec.-dibromo alkane Q9. The type of isomerism observed in urea molecule is
Q5. Following compounds are: (A) Chain (B) Position
(C) Metamers (D) Functional
Q10. How many minimum no. of C-atoms are required for
position isomer in alkene?
(A) 6 (B) 4
(C) 3 (D) 5
(A) Position isomer (B) Chain isomer
(C) Metamers (D) Functional isomer Q.11 Which of the following cannot be written in an
isomeric form?
Q6. Which of the following is incorrect relation: (A) CH3 − CH(OH) − CH2 − CH3
(B) CH3 − CHO
(C) CH2 = CH − Cl
(A) identical (D) Cl − CH2 CH2 − Cl
Q12. Given compound shows which type of isomerism
(B) positional isomers
(C) positional isomers
(A) Chain isomerism
(B) Positional isomerism
(C) Metamerism
(D) Functional group isomerism
(D) homologues
Q.13 Which of the following involves Diad system for prototropy :
(A) CH3 COCH3 (B) CH3 CH2 NO2
Q7. Select whether given relationship is true or false?
(C) (D) HCN
Q.14 How many structural isomers of C5 H10 are possible.
functional isomer (A) 10 (B) 11
(C) 12 (D) 13
Q.15 The number of isomers of dibromo derivative of an
alkene (molar mass 186 g mol −1) is
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(A) 2 (B) 3
(C) 4 (D) 6
Q.16 How many structural isomeric primary amines are
possible for the formula C4 H11 N? Q.22 Decreasing order of enol content of the following
(A) 2 (B) 3 compounds in liquid phase is:
(C) 4 (D) 5
Q.17 For the given compound, choose the incorrect option?
(A) 2 > 1 > 3 > 4 (B) 1 > 2 > 3 > 4
(A) On treatment with NaOD /D 2 O for long time, it has (C) 4 > 3 > 2 > 1 (D) 3 > 1 > 2 > 4
7 deuterium in its enol form
(B) It has all types of permanent electronic Q.23 Decreasing order of enol content of the following
displacement effect present. compound in liquid phase
(C) It has IUPAC name "Pent-2-en-4-one."
(D)None of the above.
Q.18 Among the following the compounds having the
highest enol content:
(A) CH3 CHO
(A) a > b > c > d (B) c > b > a > d
(B) CH3 COCH 3
(C) c > b > d > a (D) b > c > a > d
(C)
(D) CH3 − CO − CH2 − CO2 CH3
Q.19 Given interconversion takes place in
(A) Acidic medium (B) Basic medium
(C) Both (D) None
Q.20 (I) isomerizes to (II) on addition on small amount of
base then structure of (II) is
(A) (B)
(C) (D)
Q.21 Major product (P) obtained is:
(A) (B)
(C) (D)
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EXERCISE – II
Q1. Which of the following statements is (are) not correct?
(A) Metamerism belongs to the category of structural (C) (D)
isomerism
(B) Tautomeric structures are the resonating structures of Q6. In which of the following pairs first will have higher
a molecule enol content than second:
(C) Keto form is always more stable than the enol form
(D) Both B and C
Q2. Which compound can show tautomerism:
(A) (B)
(C) (D) Acetyl acetone
Q7. Among these tautomers, correct stability order is:
Q3. Tautomerism form of this compound is(are):
(A) I > II > III (B) III > II > I
(C) II > I > III (D) II > III > I
Q8. Most stable tautomer of following compound is:
(A) (B)
(A)
(C) (D)
(B)
Q4. Which of the following is not the correct relationship
(C)
(D)
(A) II & IV are metamer
(B) I & II are functional isomer Q9. Which of the following can show tautomerism.
(C) I & III are chain isomer
(D) I and IV are positional isomer
Q5. Which of the following compounds have higher enolic (A) (B)
content than Keto content:
(A) (B)
(C) (D)
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Q10. The isomerism observed in cyclo alkanes is(are): (C) position isomerism (D) Both B and C
(A) metamerism (B) chain isomerism
Q11. Only two isomeric monochloro derivatives structure are possible for:
(A) n-butane (B) 2, 4-dimethyl pentane (C) benzene (D) 2-methyl propane
Q12. An organic compound with molecular formula C2 H5 NO contains doubly linked atoms. It can shows:
(A) chain isomerism (B) Functional Isomerism (C) tautomerism (D) positional isomerism
Q.13 Which of the following is/are correct matchings?
(A) − Metamers
(B) CH3 − CH2 − C ≡ CH and CH3 − C ≡ C − CH3 − Position isomers
(C) CH3 CH2 CH2 NH2 and − Tautomers
(D) CH3 CH2 OH and ( CH3 ) 2 O − Functional isomers
Q.14 Which of the following compounds can show tautomerism?
(A) (B)
(C) (D)
Q.15 Match the Column I with Column II:
Column I (Pair) Column II (Relation)
(A) (P) Chain isomers
(B) (Q) Positional isomers
(C) (R) Metamers
(D) (S) Tautomers
Q.16 Match the column:
Column I Column II
(A) (P) Tautomers
(B) (Q) Structural isomers
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(C) (R) Position isomers
(D) (S) Atleast one of the two structures is enol
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EXERCISE – III
Q1. How many benzenoid isomer are possible for (A)
molecular formula of cresol?
Q2. Find out the total number of cyclic structural
isomers of C6 H12 .
Q3. Calculate the number of Benzenoid isomers possible (B)
for C6 H3 ClBrI.
Q4. Calculate the total number of structural isomers of
3∘ -amines for the molecular formula C6 H15 N are?
(C)
Q5. How many cyclopentane structural isomers are
possible for C7 H14 .
Q6. Mention the specific type of isomerism exhibited by
each of the following pairs:
(a) 1,2-dichloro ethane and 1,1-dichloro ethane (D)
(b) Propanoic acid and methyl acetate
(c) Methyl acetate and ethyl formate
(d) o-Nitrophenol and P-nitrophenol
(e) Anisole and o-cresol
(f) Phenol and Cyclohexa-2,4-dien-1-one
(E)
Q7. In each of the following pairs which is more stable:
(A) Q9. In each of the following pairs which will have less
enol content:
(B)
(C)
Q10. In each of the following pairs which will have less
enol content:
(D)
Q11. In each of the following sets of compounds write the
decreasing order of % enol content.
(E)
Q8. In each of the following pairs which is more stable:
Q12. In each of the following sets of compounds write the
decreasing order of % enol content.
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(C) (D)
(E)
Q18.
Q13. In each of the following sets of compounds write the (A), (B) and (C) are structural isomers and
decreasing order of % enol content. isomerization is effectively carried out by trace of
base. Give structure of (B) and (C) and also write
base catalysed mechanism for this interconversion.
Q19. Calculate the total number of open chain isomeric
carbonyl compounds of molecular formula C5 H8 O .
Q.14 Q20. Which of the following compounds will show the
maximum 'enol' content?
Among these give ease of enolization. (A) CH3 COCH2 COCH3
(B) CH3 COCH3
Q.15 % enol content of acetylacetone in following
(C) CH3 COCH2 COOC 2 H5
solvents is found as:
(D) CH3 COCH 2 CONH2
Solvent % enol content
H2 O 15
Liquid state 76
hexane 92
gas phase 92
Explain the observation.
Q16.
Explain the observation.
Q17. Decreasing order of enol content of the following.
(along with proper explanation).
(A) (B)
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ANSWER KEY
EXERCISE – I
1. (C) 2. (C) 3. (A) 4. (A) 5. (A) 6. (B) 7. (C) 8. (C) 9. (D) 10. (B) 11. (C) 12. (C)
13. (D) 14. (A) 15. (B) 16. (C) 17. (C) 18. (C) 19. (C) 20. (C) 21. (C) 22. (B) 23. (B)
EXERCISE – II
1. (D) 2. (A, C, D) 3. (A, B, D) 4. (A, D) 5. (B, D) 6. (A, C) 7. (C) 8. (A) 9. (A, B, C)
10. (D) 11. (A, D) 12. (C) 13. (B, D) 14. (A, B, C, D)
15. (A) R, (B) Q, (C) S, (D) Q
16. (A) P, Q, S (B) Q, S (C) Q, R, S, (D) Q, S
EXERCISE – III
1. (5) 2. (12) 3. (10) 4. (7) 5. (4) 6. (a) Positional (b) Functional (c) Metamerism (d) Positional (e) Functional
(f) Tautomerim 7. (a) 2; (b) 2; (c) 1; (d) 1; (e) 1 8. (a) 2; (b) 2; (c) 1; (d) 2; (e) 1
9. (a) 2; (b) 1; (c) 2; (d) 1; (e) 2 10. (a) 2; (b) 2 11. (a) 3>1>2; (b) 4>2>1>3; 12. (i) 4 > 1 > 3 > 2; (ii) 3 > 1 > 4 > 2
13. 3 > 4 > 2 > 1 14. 3>1>2
17.
(a) Tightly on stableketo due to repulsion between α − CO groups has 100% enol.
(b) Active ' H ' atom / Acidic 'H' atom so has more enolic content (enol stabilise by resonance & Intra molecular H-bonding)
(c) Enolic contents decreases with introduction of e − donator group which causes repulsion in enolic form.
(d) Due to ester group acidic structure of active H decreases
& C = C of enol undergoese cross resonance.
(e) Lowest enolic content because is more stable than Bond
18.
19. (8)
20. (A)
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