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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
H
Cl
CH
3
ClH
HOOC
C
C
C
C
H COOH
HHOOC
CC
H
C
H
3
C
OH
CH
3
ClH
CC
H
CC
CH
3
HC2H
5
C h a P T E r 3 Stereochemistry
REVISION QUESTIONS
1. What is optical isomerism? Classify the different types of isomerism giving an example of each.
2. Differentiate between structural isomerism and stereoisomerism giving two examples of each.
3. Write all the possible structural isomers having molecular formula C4h10O and give IuPaC names to
these isomers.
4. What is tautomerism? Explain tautomerism taking acetoacetic ester as an example.
5. What is optical isomerism? ‘Chirality is the essential condition for optical activity.’ Justify the
statement.
6. What are enantiomers? Write down their main characteristics.
7. What do you mean by resolution? Describe various methods to resolve a racemic modication.
8. Write an account of geometrical isomerism. Explain giving example of maleic acid and fumaric
acid.
9. What do you understand by absolute conguration and its specication? Give detail of the sequence
rules and illustrate your answer with examples.
10. What do you understand by E and Z system of nomenclature? Give E and Z conguration of the
following compounds:
181
(a)
(c)
(b)
(d)
11. Explain the term racemization. In what respect does racemic tartaric acid differ from meso-tartaric
acid? Write a brief account of the methods available for the resolution of racemic compounds.
12. What are R and S congurations? State and illustrate the sequence rules.
13. What do you understand by hindered rotation about a carbon–carbon double bond? Explain geometrical
isomerism shown by an aldoxime.
14. Give an account of isomerism exhibited by lactic acid. What is meant by resolution? Discuss at least
two methods for resolving a racemic compound.
15. how many optically active forms corresponding to the following formulae are possible?
(a) OhC–ChOh–ChOh–Ch2Oh (b) Ch3–ChOh–ChOh–Ch
(c) Ch3–ChO–ChBr–Ch3 (d) Ch3–ChOh–ChOh
3
Cl
I
HCSO
3
H
H
CH
3
Cl CB
r
OH
CH
3
HOOC CH
COOH
CH
3
H2NCH
CHO
CH
2
OH
HO CHO
C(CH3)
3
CH
3
HCCl
182
PharMaCEuTICal OrGanIC ChEMISTry
16. Write short notes on:
(a) Cause of optical isomerism (b) resolution of a racemic mixture
(c) E and Z methods of conguration determination of geometrical isomers
17. Dene and illustrate the term geometric isomerism. Taking examples of maleic and fumaric acids,
explain how you will distinguish between cis- and trans-isomers.
18. Write short notes on:
(a) Fisher formulae (b) Sawhorse formulae (c) newman formulae
19. What do you understand by the R, S, E and Z notations? What is the advantage of using these notations?
Write the structures of the stereoisomers of tartaric acid? What is the difference between meso and
racemic varieties?
20. (a) Optical activity of a molecule is linked with the presence of asymmetric carbon atom. Justify the
statement with a suitable example.
(b) assign R and S conguration on the following compounds:
(i)
(iii)
(ii)
(iv)
21. (a) an acid having molecular formulae C3h6O3 is optically active. What is its structure and name of
the compound? Write structure of its enantiomers.
(b) Distinguish between meso and racemic forms of tartaric acid.
22. (a) Give various cases of specication of conguration when optical isomers are represented by
cross formula.
(b) Explain racemization of optical isomers with suitable example.
23. (a) What are enantiomers? Write down their main characteristics.
(b) a compound C4h10O shows optical activity. Write down the possible stereoisomers.
(c) assign R and S conguration of the following:
(i)
(ii)
H3C Br
ClH
CC
H3CCH
2
CH (CH3)
2
H
CH
3
CC
Br
H
I CCl
H3C
CC
C6H
5
(CH3)2CH C
2H5
H3C
CC
C6H
5
(CH3)2CH C
2H5
C h a P T E r 3 Stereochemistry
24. (a) Taking example of maleic and fumeric acids, explain how will you distinguish between cis- and
trans-isomers.
(b) What is meant by resolution? How will you resolve (±) lactic acid by a chemical method?
(c) Write E and Z congurations of the following structures:
(i)
(ii)
183
25. (a) What do you understand by rectus and Sinister system of designation of chiral centres? State
and illustrate the sequences rules.
(b) State and explain the necessary and sufcient conditions for a compound to show geometrical
isomerism. Give any two methods by which geometrical isomers can be distinguished.
(c) Complete the statement:
a molecule may not be chiral even though it may contain more than one ...................................
26. (a) What is optical activity? Give the stereoisomers of tartaric acid. how do you account for the lack
of optical activity in meso form and racemic form?
(b) Which of the following compound can exhibit optical activity?
(i) Ch3Ch (Oh) Ch2Ch3 (ii) Ch3–Ch2COOh
(iii) Ch2Cl–Ch2Br
27. (a) Write a note on racemic modication.
(b) Distinguish between the terms conguration and conformation.
(c) What do you mean by the symbol R and S?
(d) assign the configuration (R or S) to the following:
28. (a) Draw the isomeric structure of the following and specify each as E and Z.
(b) Dene the term enantiomers, diastereomers and meso compounds giving examples for each one
29. (a) assign E and Z specication of conguration to the following compounds:
(i) 1-Chloropropene (ii) 1-Chloro-2-methyl-butane
of them.
(i)
(ii)
CHO
HOH
HOH
CH
2
OH
HOH
Et
ClMe
SO
3
CC
Ph
PhMe
Me
CCCC
Ph
N
N
Ph
OH
N
C
PhH
184
(b) What is the necessary and sufcient condition for a compound to show enantiomerism?
(c) Dene the term racemization.
30. (a) Explain necessary and sufcient condition for optical activity.
(b) Explain the term ‘meso form’ with a suitable example.
(c) How can you determine conguration of geometrical isomers by
(iii) Converting into optical isomers
31. Draw the conguration and specify the R and S enantiomers of 2-chloropentane.
32. arrange the isomeric butyl groups in decreasing order of priority.
33. Assign the absolute conguration R or S to each chiral centre in the following compound:
(a)
PharMaCEuTICal OrGanIC ChEMISTry
(i) Formation of cyclic compounds (ii) Dipole moment
34. assign E or Z conguration to the following:
(a)
(c)
Ans: (a) E (b) E
(c) E (d) Z
(b)
(d)
C h a P T E r 3 Stereochemistry
MULTIPLE CHOICE QUESTIONS
1. Two compounds have the same composition and also have the same atoms attached to the same
atoms, although with different orientations in space. These compounds are
(a) Identical (b) Position isomers
(c) Structural isomers (d) have same functional group
2. The compounds Ch3Ch2OCh2Ch3 and Ch3OCh2Ch2Ch3 are
(a) Enantiomers (b) Conformational isomers
(c) Metamers (d) Optical isomers
3. how many structural isomers are possible for C4h9Br?
(a) 2 (b) 3
(c) 4 (d) 5
4. alkenes show geometrical isomerism due to
(a) asymmetry (b) rotation around a single bond
(c) resonance (d) restricted rotation around a double bond
185
5. Which of the following compounds exhibit geometrical isomerism?
(a) 1-Pentene (b) 2-Methyl-2-pentene
(c) 2-Pentene (d) 2-Methyl-2-butene
6. Geometrical isomerism is shown by
(a) lactic acid (b) Maleic acid
(b) 1-Butene (d) 1,1-Dichloroethylene
7. a molecule is said to be chiral
(a) If it contains plane of symmetry (b) If it contains centre of symmetry
(c) If it cannot be superimposed on its mirror image (d) If it can be superimposed on its mirror
image
8. Which of the statements is false regarding chiral compounds?
(a) rotate the plane of polarized light (b) have cis and trans isomers
(c) Exist as enantiomers (d) Can be detected with a polarimeter
9. an optically active compound
(a) Must contain at least four carbons
(b) When in solution rotate the plane of polarized light
(c) In solution always give a negative reading in polarimeter
(d) all of the above
186
PharMaCEuTICal OrGanIC ChEMISTry
10. Plane polarized light is affected by
(a) Identical molecules (b) all polymers
(c) Chiral molecules (d) all biomolecules
11. It is possible to distinguish between optical isomers
(a) By using chemical tests (b) By mass spectrometry
(b) By Ir spectroscopy (d) By polarimetry
12. Optical isomers that are mirror images are called
(a) Tautomers (b) Diastereomers
(c) Enantiomers (d) Metamers
13. a meso compound
(a) Is an achiral molecule which contains (b) Contains a plane of symmetry or
chiral carbons a centre of symmetry
(c) Is optically inactive (d) Is characterized by all of the above
14. What is the possible number of optical isomers for a compound containing n dissimilar asymmetric
carbon atoms?
(a) n2 (b) 2
n
(c) n 1 1 (d) n 1 2
15. meso-Tartaric acid is
(a) Sometimes optically active (b) always optically active
(c) Sometimes optically inactive (d) always optically inactive
16. Which of the following isomeric compounds show optical isomerism?
(a) 1-aminopentane (b) 2-aminopentane
(c) 3-Aminopentane (d) 2,2-Dimethylpropylamine
17. 2-Butanol is optically active because it contains
(a) an asymmetric carbon (b) a centre of symmetry
(c) a hydroxyl group (d) a plane of symmetry
18. Which of the following represents a racemic mixture?
(a) 75% (R)-2-butanol, 25% (S)-2-butanol (b) 25% (R)-2-butanol, 75% (S)-2-butanol
(c) 50%(R)-2-butanol, 50% (S)-2-butanol (d) none of the above
19. Consider (R)– and (S)-2-butanol. Which physical property distinguishes the two compounds?
(a) Melting point (b) Solubility in common solvents
(c) rotation of plane polarized light (d) Infrared spectrum
Me Me
HH
CC
H
Me
Me
H
CC
Ph
COOH
H
H
CC
Ph COOH
HH
CC
Me
H
Me
H
CC
Me H
HMe
CC
Me H
Me H
CC
HOOC H
H COOH
CC
C h a P T E r 3 Stereochemistry
20. The number of congurational isomers of the compound HOCH2(ChOh)3Ch2Oh is
(a) 8 (b) 2
(c) 6 (d) 4
21. The number of geometrical isomers of the compound PhCh 5 Ch – Ch 5 Ch – COOh is
(a) 1 (b) 2
(c) 6 (d) 4
22. The number of optically active isomers of hOCh2(ChOh)4ChO is
(a) 4 (b) 8
(c) 16 (d) 24
187
23. Which of the following will give meso form with Br2?
(a)
(c)
(b)
(d)
24. Which of the following will form a (±)-mixture with Br2?
(a)
(c)
(b)
(d)
ANSWERS
1. (d) 2. (c) 3. (c) 4. (d) 5. (c) 6. (b) 7. (c) 8. (b) 9. (b) 10. (c) 11. (d) 12. (a)
13. (d) 14. (b) 15. (d) 16. (b) 17. (a) 18. (c) 19. (c) 20. (d) 21. (d) 22. (c) 23. (b) 24. (a)
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Chapter Outline
4.1 Introduction, 4.2 Structure of Alkanes, 4.3 Structural Isomerism, 4.4 General Methods of Preparation of Alkanes,
4.5 Physical Properties of Alkanes, 4.6 Chemical Properties of Alkanes, 4.7 Mechanism of Halogenation
4.1 INTRODUCTION
Organic chemistry is the study of hydrocarbons and their derivatives. The derivatives are prepared by
replacing one or more hydrogen atoms with suitable atoms and groups from the corresponding hydrocarbons.
Thus hydrocarbons are the parents of organic compounds. The studies on the structures, methods of
preparation and properties of hydrocarbons, being the basic units of organic compounds are very essential
for further studies in this important branch of chemistry. Hydrocarbons are of mainly two types: saturated
and unsaturated.
H single
bonds only, whereas unsaturated hydrocarbons have carbon–carbon double bonds ( H
bonds in them. In this chapter, we will take up the study of saturated hydrocarbons, namely, alkanes.
Alkanes are the open chain saturated hydrocarbons, in which the carbon atoms are linked by single
parafns earlier because of their lower activity (Latin: parum—
little, afnis–.
nH
2n+2
where n is an integer (1, 2, 3, 4 … and so on). They can also
be represented by RH.
4.2 STRUCTURE OF ALKANES
In alkanes, all the carbon atoms linked through single covalent
bonds. They undergo sp 3 (tetrahedral) hybridization. Let us
two members, namely methane and ethane.
1. Orbital structure of methane: In the formation of methane,
each sp
3
hybrid orbital of carbon overlaps with the 1s orbitals
H bonds as shown in Fig. 4.1.
H bond length is 1.10 Å
(or 110 pm) and H H bond angle is 109°28'.
Learning Plus
Alessandro Volta discovered and isolated
methane gas between 1776 and 1778 when he
was studying marsh gas from Lake Maggiore.
ALKANES
4
There is excitement, adventure and challenge, and there
can be great art in organic synthesis.
–R.B. Woodward
H
H
H
H
C
109°28
1.10A
°
C
H
H
H
H
1.10 Å
109°28′
sp
3
sp
3
sp
3
sp
3
H
H
H
H
σ
σ bond
bond
H
H
H
C
C
CH
H
H
H
Ethane Propane
CHH
H
H
H
H
C
CHH
H
H
H
C
H
H
H
C
190190
H bond in plane of the page and
H bonds, projecting behind the plane of the page.
(a) Orbital structure of methane
(b) 3-D representation
Figure 4.1 Spatial and graphical formula of methane.
It is called spatial formula or 3-D representation. The two-dimensional representation of methane molecule
is shown in Fig. 4.1(c). It is called graphic or 2-D representation. For common purpose, such representation
is always used. In a similar manner, ethane and propane are depicted as follows:
2. Orbital structure of ethane (structure of carbon–carbon single bond): In the formation of ethane:
a. One sp 3
CH3 CH
(c) 2-D representation
3
CH
CH2 CH
3
3