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C H A P T E R 6 Alkynes
7. (a) An alkyne (A) with molecular formula C5H8 reacts with neither sodamide nor with ammoniacal
cuprous chloride. Assign structural formula to (A).
(b) A compound has a molecular formula, C4H6. When it is treated with excess hydrogen in the presence
of nickel catalyst, a new compound, C4H10, is formed. When it is treated with ammoniacal silver
nitrate, a white precipitate is formed. What is the structural formula of the given compound?
Ans: (a) (A) CH3CH2C CCH3 (2-pentyne).
(b) The given compound is 1-butyne.
8. Compound A, C4H6, gave the following tests:
(a) A + Br2/CCl4 → B (C4H6Br4)
(b) A + H2/Pt → C (C4H10)
(c) A + AgNO3 → D (precipitate)
What are the structures of A, B, C, D?
Ans: A = 1-butyne B = 1,1,2.2-tetrabromobutane
C = n-butane D = CH3CH2C C–Ag
+
9. An unknown compound (A) has the molecular formula, C5H8. Suggest a possible structure for (A)
from the following data:
281
(a) One mole of (A) takes up two moles of hydrogen.
(b) Compound (B) decolourizes a solution of Br2 in CCl4.
(c) Treatment of (A) with H2O in the presence of dilute sulphuric acid and mercuric sulphate yields
2-pentanone as the main product.
Ans: (A) = CH3CH2CH2C CH (1-pentyne)
10. Compound (A), C6H12, is treated with chlorine in the presence of carbon tetrachloride to give (B),
C6H12Cl1. Compound (B) is treated with alcoholic KOH, followed by NaNH2, resulting in the formation
of (C). Compound (C) is treated with hydrogen in the presence of nickel catalyst to give 2-methypentane.
Compound (C) does not react with sodamide or an ammoniacal solution of silver nitrate. Ozonolysis
of (A) gives two aldehydes, (D) and (E). Compounds (E) is identied as acetaldehyde. From this
information, deduce the structural formulas of (A), (B), (C), (D). Write equations for all reactions.
11. An unknown compound (C) has the molecular formula C5H8. Suggest a possible structure for (C)
from the data:
(a) One mole of (C) takes up two moles of bromine.
(b) (C) does not react with ammoniacal AgNO3.
(c) Ozonolysis of one mole of (C) yields two moles of formaldehyde and one mole of
2-oxopropanal.
(C) = CH2 C CH CH2 (2-methyl-1, 3-butadiene)
Ans:
282
PHARMACEUTICAL ORGANIC CHEMISTRY
12. A known compound (C) has the molecular formula, C5H8. Suggest a possible structure for (B) from
the following data:
(a) One mole of (B) takes up two moles of chlorine.
(b) Compound (B) decolourizes a solution of Br2 in CCl4.
(c) Compound (B) forms no precipitate with ammoniacal AgNO3.
(d) Treatment of (B) with H2O in the presence of dilute sulphuric acid and mercuric sulphate
catalyst yields an equimolar mixture of 2-pentanone and 3-pentanone.
(B) = CH3CH2C CCH3 (2-pentyne)
Ans:
MULTIPLE CHOICE QUESTIONS
1. The carbon atoms involved in the double bond of an alkene are
(a) sp Hybridized (b) sp2 Hybridized
(c) sp3 Hybridized (d) None of these
2. Which of the following compounds will show geometrical isomerism?
(a) Propene (b) 2-Butene
(c) Propyne (d) 2-Butyne
3. The major product of acid-catalysed dehydration of 3-pentanol is
(a) 1-Pentene (b) 2-Methyl-1-butene
(c) 2-Pentene (d) 3-Methyl-1-butene
4. Ethylene is obtain from ethyl bromide by
(a) Simple heating (b) Hydrolysis
(c) Dehydrohalogenation (d) Nucleophilic substitution
5. In the reaction of propane with HCl, H+ ion acts as the
(a) Electrophile (b) Carbonium ion
(c) Nucleophile (d) Carbanion
6. Propene reacts with bromine to form 1,2-dibromopropane. This is an example of
(a) Nucleophilic addition (b) Electrophilic addition
(c) Nucleophilic substitution (d) Electrophilic substitution
7. Baeyer’s reagent is
C H A P T E R 6 Alkynes
283
(a) Dilute KMnO4 (b) HCl + ZnCl
(c) Br2 in CCl4 (d) NH2NH
2
2
8. In the addition of HX to a double bond, the hydrogen goes to the carbon that already has more
hydrogens is a statement of
(a) Hund’s rules (b) Markownikoff’s rule
(c) Huckel rule (d) Saytzeff rule
9. Markovnikoff’s addition of HBr is not applicable to
(a) Propane (b) 1-Butene
(c) 1-Pentene (d) 2-Butene
10. In the reaction of CH3CH2CH CH2 with HCl, the H of the HCl will become attached to which
carbon
(a) C-1 (b) C-2
(c) C-3 (d) C-4
11. The disappearance of the purple colour of KMnO4 in its reaction with alkene is known as
(a) Markownikov test (b) Grignard test
(c) Baeyer’s test (d) Wurtz test
12. Which is of the following compounds will react most readily with bromine in CCl4?
(a) CH3CH2CH3 (b) (CH3)3CH
(c) CH3 CH CH2 (d) (CH3)4C
13. 2-Butene reacts with HBr to give
(a) 1-Bromobutane (b) 2,3-Dibromobutane
(c) 2-Bromobutane (d) 2,2-Dibromobutane
14. Which of the following reagents will react with propene?
(a) Hot alkaline KMnO4 (b) Sodium metal
(c) Cold dilute HNO3 (d) LiAlH
4
15. The propane reacts with HBr in the presence of a peroxide to give
(a) n-Propyl bromide (b) Ally bromide
(c) Isopropyl bromide (d) Vinyl bromide
284
PHARMACEUTICAL ORGANIC CHEMISTRY
16. Propane reacts with Cl2 in H2O to give
(a) 1-Chloro-2-propanol (b) 2-Chloro-2-propanol
(c) 1-Chloro-1-propanol (d) 2-Chloro-1-propanol
17. Catalytic hydrogenation of 3-methyl-1-butene gives
(a) Isobutane (b) 2,2-Dimethylbutane
(c) 2-Methylbutane (d) 2,3-Dimethylbutane
18. Which is of the following alkenes will give a mixture of acetone and acetaldehyde an ozonolysis?
(a) 1-Butene (b) 2-Methyl-2-butene
(c) 2-Butene (d) 2-Methylpropane
19. Combustion of an alkene with sufcient oxygen will produce
(a) Carbon dioxide and water (b) Carbon monoxide and water
(c) Only carbon dioxide (d) Only carbon monoxide
20. How many sigma bonds are there in CH
CH CH CH2?
2
(a) 3 (b) 6
(c) 9 (d) 12
ANSWERS
1. (b) 2. (b) 3. (c) 4. (c) 5. (a) 6. (b) 7. (a) 8. (b) 9. (d) 10. (a)
11. (c) 12. (c) 13. (c) 14. (a) 15. (a) 16. (a) 17. (c) 18. (b) 19. (a) 20. (c)
CYCLOALKANES
Chemistry ... is like the maid occupied
5
7
7.1 Introduction, 7.2 Nomenclature of Cycloalkanes, 7.3 Preparation of Cycloalkanes, 7.4 Physical Properties, 7.5 Chemical Properties,
7.6 Relative Stability of Cycloalkanes (Baeyer’s Strain Theory), 7.6.1 Advantages of Baeyer’s Strain Theory, 7.6.2 Limitations of Baeyer’s
Strain Theory, 7.7 Heat of Combustion and Stability of Cycloalkanes, 7.8 Sachse–Mohr Theory of Strainless Rings
7.8.1 Effects of Cyclohexane on Human Beings and Environment
7.1 INTRODUCTION
The carbocyclic or homocyclic compounds containing closed chains or ring of carbon atom and bearing
certain resemblance to aliphatic compounds in their properties are known as cycloalkanes. They vary from
aliphatic compounds in that they possess a cyclic structure.
Simple saturated cycloalkanes, also known as alicyclic hydrocarbons, have the general formula CnH
The general formula is isomeric with those of alkenes but cycloalkanes do not have any double bond but
they contain the ring of carbon atoms linked together by single bonds.
Cycloalkanes are classied roughly into four groups:
with daily civilisation; she is busy with
fertilisers, medicines, glass, insecticides...
for she dispenses the recipes.
–Jean Jacques
Chapter Outline
2n.
1. Small rings (cyclopropane, cyclobutane)
2. Common rings (cyclopentane, cyclohexane, cycloheptane)
3. Medium rings (from 8- to 12-membered)
4. Large rings (13-membered and higher)
The cycloalkanes having more than 20 carbon atoms are generally called cycloparafns.
7.2 NOMENCLATURE OF CYCLOALKANES
There are two systems for naming cycloalkanes:
1. Common system: Since cycloalkanes are made up of methylene groups joined together to form a
ring, they are known as polymethylenes in a trivial system. The number of carbon atoms forming the
ring is indicated by a Greek or Latin prefix such as tri, tetra, penta, etc., before the word methylene.
Thus, C
on.
is known as trimethylene, C4H8 as tetramethylene and C5H10 as pentamethylene and so
3H6
286
Trimethylene Tetramethylene Pentamethylene
Hexamethylene
CH
2
CH
2
H2C
CH
2
H2C
CH
2
H2C
CH
2
CH
2
H2C
CH
2
H2C
CH
2
C
H
2
CH
2
H2C
CH
2
H2C
or
CH
2
H2C
CH
2
CH
2
H2CCH
2
H2C
or
H2C
CH
2
CH
2
CH
2
H2C
or
or
H
2
C
H
2
C
CH
2
CH
2
CH
2
CH
2
PHarMaCeuTiCaL OrGaniC CHeMiSTry
The structures of these compounds are drawn as a regular polygon whose each corner depicts a
CH2 group.
2. IUPAC rules for nomenclature: The general molecular formula of cycloalkanes is CnH2n. These
are simplest cyclic molecules. These are named after their corresponding linear alkanes with the
prefix cyclo.
a. Determine the cycloalkane to use as parent chain: The parent chain is one with the
maximum amount of carbon atoms. if there are two cycloalkanes, then the cycloalkanes
with more carbons will be considered as the parent chain.
Cycloalkane Cycloalkyl Structure
Cyclopropane Cyclopropyl
Cyclobutane Cyclobutyl
Cyclopentane Cyclopentyl
Cyclohexane Cyclohexyl
Cycloheptane Cycloheptyl
Cyclooctane Cyclooctyl
C H a P T e r 7 Cycloalkanes
H3C
9
87
65
43
2
1
10
H
3
C
H
3
C
1-Ethyl-5-methylcyclohexane (1 + 5 = 6) 1-Ethyl-3-methylcyclohexane (1 + 3 = 4)
H
5C2
CH
3
1
6
5
4
3
2
H2C
5
CH
3
1
2
3
4
5
6
CH
3
Ethylcyclopentane 1,1-Dimethylcyclobutane
CH
3
CH
3
b. if the number of carbons in an alkyl straight chain is greater that in the cycloalkane, the alkyl
chain must be treated as the primary parent chain and cycloalkane as a substituent. Therefore
it must be named as a cycloalkyl. For example, the alkyl straight chain contains 10 carbons
while cyclopropane contains only 3 carbons. So cyclopropane is a substituent, and so it would
be named cyclopropyl-substituted alkane.
c. Find out if there is any other alkyl groups or functional group.
d. number the carbons of the cycloalkane in such a way that the carbons with functional groups
or alkyl groups get the lowest possible number. if many substituents are present, preference
should be given to carbon with a functional group.
numbering should be done in a way that sum of numbers assigned to substituents is the
lowest possible.
287
e. Substituents and functional groups must be written in the alphabetical order while naming the
cycloalkane.
288
Br
CH
3
Cl
1
5
43
2
Cl
12
43
CH
3
CH
3
1
5
463
Br2
F
CH
3
CH
3
6
1
2
3
4
5
PHarMaCeuTiCaL OrGaniC CHeMiSTry
2-Bromo-1-chloro-3-methylcyclopentane
f. a dash ‘–’ must be placed between numbers and name of the substituent while writing the
name of the cycloalkane. after the carbon number and the dash, the name of substituent can
follow. if parent chain contains only one substituent, indication of the number of the carbon
atoms with the substituent is not necessary.
1-Chlorocyclobutane or chlorocyclobutane is acceptable
g. always put commas between numbers, and dashes between numbers and names.
2-Bromo-1,1-dimethylcyclohexane
Always write in the alphabetical order. For example, ‘f’ of uoro comes rst alphabetically
than ‘m’ of methyl. Do not consider ‘d’ of di or ‘t’ of tri, etc.
h. in case of geometrical isomers, indicate so by placing cis- or trans- in front of the name of the
structure.
2-Fluoro-1,1-dimethylcyclohexane not 1,1-dimethyl-2-fluorocyclohexane
C H a P T e r 7 Cycloalkanes
CH
3
Cl
1
3
4
5
2
H2C + 2Na + 2NaBr
1,3-Dibromopropane Cyclopropane
Ethanol
∆
CH2Br
CH
2
Br
H2C
CH
2
CH
2
H2C
H
2
C
+ Zn + ZnBr
2
1,4-Dibromobutane Cyclobutane
Ethanol
∆
CH2Br
CH
2
Br
H2C
H
2
C
CH
2
CH
2
cis-1-Chloro-2-methylcyclopentane
Both chlorine and methyl group are pointed in the same direction on the axis of the
molecule and therefore is cis isomer.
after all the functional groups and substituents have been mentioned with their corresponding
numbers, the name of the cycloalkane can follow.
7.3 PREPARATION OF CYCLOALKANES
Following methods are commonly used for the preparation of cycloalkanes.
289
Freund’s Method from α, ω-dihalides: Dichloro or dibromoalkanes having terminal halogen
1.
atoms, when treated with sodium or zinc, yield the corresponding cycloalkanes. For example,
The method is an extension of Wurtz reaction and may be regarded as an internal Wurtz reaction.
The reaction is useful for the synthesis of three- to six-membered rings. With higher homologues,
the yields are poor due to occurrence of various side reactions.
2.
From barium or calcium salts of dibasic acids (Wislicenus method): On dry distillation of
barium or calcium salts of dicarboxylic acids, a cycloketone is obtained which can be reduced to
cycloalkane by Clemmenson’s reduction (Zn–Hg/HCl).
290
H C–CH–COO
22
—
H C–CH–COO
22
–
Ca
2+
Heat
CH CH
22
CO + CaCO
3
CH CH
22
—
Cyclopentanone
Reduce
Zn–Hg/HCl
CH CH
22
CH
2
CH CH
22
—
Cyclopentane
Calcium adipate
–
Propene Methylene
iodide
Zn–Cu couple
2-Butene
CH
2
Zn–Cu couple
CH3CH
CH
2
+ CH2I
2
CH3CH
CHCH
2
+ CH2I
2
CH
3
CH
2
CH
CH
2
CHCH
3
CH3CH
CH
2
CH
2
Diethyl adipate
H
2
C
H
2
C
CH
COOC
2H5
— C2H5OH
CH
2
H2C
H
2
C
COOC
2H5
C2H5ONa
∆
COOC2H
5
H2O
CH
COOH
CH
2
H2C
H
2
C
C O C O
PHarMaCeuTiCaL OrGaniC CHeMiSTry
This method is useful for the preparation of only, six- and seven-membered ring ketones
which can be further reduced to cycloalkanes. This method is not suitable for preparation of
cyclopropane or its derivatives.
3.
From alkenes: Derivatives of cycloalkanes are prepared by treating alkenes with CH2i2 in presence
of Zn–Cu couple or by diazomethane (CH2n2) in presence of uV light.
4. By Dieckmann cyclization: Cyclic β-keto esters are prepared by heating esters of dicarboxylic
acids in the presence of sodium ethoxide. it undergoes intramolecular Claisen condensation. These
β-keto esters on hydrolysis and subsequent heating gives cyclic ketones, which are reduced by
Zn–Hg/HCl to give cycloalkanes: