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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
C H a P T e r 7 Cycloalkanes
H
H
H
H
H
H
H
H
H
H
H
H
2
C
H
2
C
H
Eclipsing
interactions
Flagpole
hydrogen
BoatChair
Flip this
end of the
chair up
H
H
H
H
C
C
H
H
H
H
H
2
All bonds staggered
Look down
these two
bonds
Equatorial bonds
Axial bonds
H
H
CH
3
H
H
CH
3
1,3-Diaxial
interactions
Equatorial methyl Axial methyl
Note: Even cyclobutane and cyclopentane are found in puckered (nonplanar) conformations.
Learning Plus
Substituents prefer to take equatorial positions
over axial positions when they are placed on the
cyclohexane ring. For example, when the methyl
group acts as a substituent, if it occupies the axial
position, there is steric hindrance between it and
the axial hydrogens three carbons away. These
repulsive effects are called 1,3-diaxial interac-
tions. So it prefers to take equatorial position.
301
7.8.1 Effects of Cyclohexane on Human Beings and Environment
The effects of cyclohexane on human health and the environment depend on the extent cyclohexane is
present and the length and frequency of exposure. Such effects also depend on the health of a person or the
condition of the environment when exposure occurs.
PH arM a C euT i CaL OrG aniC CH eM iSTry
302
OH
3
+
3KMnO
4
+
4H
2
O
3
+ 2MnO
2
+ 2KOH
Purple
solution
OH
Brown precipitate
Human health effects associated with breathing or taking
in smaller amounts of cyclohexane over long periods of time
are not known. But cyclohexane adversely affects the human
nervous system if a person breathes large amounts of it for
short periods of time. its effects range from headaches to
Learning Plus
Approximately 90% of cyclohexane is used in making
nylon fibre and nylon molding resin, and the rest of it
is used in solvents for paint, resins and plasticizers.
anaesthesia, tremors and convulsions. it can damage the
eyes if on contact.
REVISION QUESTIONS
1. What are cycloalkanes? How are they prepared? Describe important reactions of cycloalkanes.
2. (a) How can cyclopropane be prepared?
(b) How does cyclopropane react with
(i) Conc. HBr (ii) H
(iii) Cl
/uV light (iv) Br2/CCl4 in dark
2
/ni at 80°C
2
(c) The C–C bonds in cyclopropane are weaker than the C–C bonds in propane. explain this fact
on the basis of molecular orbital theory.
3. explain why
(a) Cyclohexane is more stable than cyclopropane
(b) Cyclobutane is more stable than cyclopropane
(c) Cyclohexyne cannot exist
4. How will you distinguish between
(a) Cyclohexane and cyclohexene
(b) Cyclopentane and 2-pentene
Ans: (a) Cyclohexene decolourizes the purple colour of dilute cold KMnO
solution. Brown precipitate
4
of MnO2 is also formed in the reaction. Cyclohexane does not react with this reagent.
(b) Use dilute cold KMnO4 or bromine in carbon tetrachloride.
5. Discuss the stability of cycloalkanes on the basis of Baeyer’s strain theory and its later
developments.
C H a P T e r 7 Cycloalkanes
Conc. HBr
NaCN
H
2
O/H
+
CH3CH2CH2Br CH3CH2CH2CN CH3CH2CH2COOH
Cyclopropane
Butanoic acid
Cl
2
KOH
Cl
UV
Alcohol/∆
Cyclohexane Chlorocyclohexane
Cyclohexene
OH OH
3H2/Ni
3H
2
/Ni
H2SO
4
Phenol Cyclohexanol Cyclohexene Cyclohexane
∆∆∆
6. Discuss the stability of cycloalkanes on the basis of molecular orbital theory.
7. explain
(a) Baeyer’s strain theory
(b) Sachse–Mohr theory of strainless rings
(c) Stability of cycloalkanes
(d) Dieckmann reaction
8. How will you synthesize
(a) 1-Bromopropane from 1,3-dibromopropane
(b) butanoic acid from cyclopropane
(c) cyclohexane
(d) cyclohexane from phenol
(e) cyclopentane from diethyl adipate
Ans: (a) Steps are: (I) Zn/heat and (II) Conc. HBr
(b) Following steps are involved:
303
(c) Following steps are involved:
(d) Following steps are involved:
9. Write a note on conformations of cyclohexane and its derivatives.
304
PHarMaCeuTiCaL OrGaniC CHeMiSTry
10. a hydrocarbon, C4H8, neither decolourized bromine in carbon tetrachloride nor reacted with HBr.
When heated to 200ºC with hydrogen in the presence of a nickel catalyst, a new hydrocarbon,
C4H10, was formed. What was the original hydrocarbon?
Ans: Cyclobutane.
MULTIPLE CHOICE QUESTIONS
1. The most stable conformation of cyclohexane is the
(a) Haworth form (b) Boat form
(c) newman form (d) Chair form
2. Cycloalkanes have the same molecular formula as
(a) alkanes (b) alkenes
(c) alkynes (d) Cycloalkenes
3. Which of the following statements is false about cyclohexane?
(a) it is a saturated cyclic hydrocarbon.
(b) all C – C – C bond angles are 109°28′.
(c) it is very unstable, strained compound.
(d) it can exist in two conformations which are designated as the boat form and the chair form.
4. Which of the following compounds will give cyclopropane on treatment with sodium in dry
ether?
(a) 1,3-Dibromopropane (b) 1,1-Dibromopropane
(c) 1,2-Dibromopropane (d) 2,2-Dibromopropane
5. The bond angle between carbon atoms in cyclohexane is
(a) 60° (b) 90°
(c) 90° (d) 120°
6. Which of the following cycloalkanes is most reactive?
(a) Cyclopropane (b) Cyclohexane
(c) Cyclobutane (d) Cycloheptane
7. Which of the cycloalkanes is not expected to have ring strain?
(a) Cyclopropane (b) Cyclobutane
(c) Cycloheptane (d) none of these
C H a P T e r 7 Cycloalkanes
8. Which of the following compounds will react most readily with concentrated sulphuric acid?
(a) ethane (b) Cyclohexane
(c) Propane (d) Cyclohexene
9. What percentage of cyclohexane molecule is estimated to be in the boat form at any given time?
(a) Over 99% (b) Between 90 and 99%
(c) approximately 50% (d) Less than 1%
10. a compound of formula C6H12 does not react with concentrated sulphuric acid. The compound
could be
(a) alkane (b) Cycloalkane
(c) alkene (d) Cycloalkene
11. Cyclohexanol can be converted into cyclohexene by heating with
(a) Zn(Hg) and HCl (b) Concentrated H2SO
4
(c) SOCl2 (d) H2 and ni
12. Which of the following molecules will decolourize bromine in carbon tetrachloride most readily?
305
(a) 1,2-Dimethylcyclopropane (b) Cyclopentane
(c) 1,2-Dimethylcyclobutane (d) Cyclohexane
13. Cyclobutane reacts with hydrogen in the presence of nickel catalyst at 200ºC to give
(a) Butane (b) 1-Butene
(c) 2-Butene (d) none of these
14. Cyclopropane reacts with concentrated HBr to give
(a) 1-Bromopropane (b) Bromocyclopropane
(c) 2-Bromopropane (d) 1,2-Dibromopropane
ANSWERS
1. (d) 2. (b) 3. (c) 4. (a) 5. (c) 6. (a) 7. (c)
8. (d) 9. (d) 10. (b) 11. (b) 12. (a) 13. (a) 14. (a)
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1,4-Pentadiene 1,5-Hexadiene
CH
2
CH
2
CH
2
CH CH CH
2
CH
2
CH
2
CH CHCH
2
1,2-Propadiene or Allene
1,2-Butadiene or Methylallene
CH
3
CH
2
CH CCH
2
CH
2
C
DIENES
Vitamin A(Retinol)
5
8
I never see what has been done; I only
see what remains to be done.
–Madam Marie Curie
Chapter Outline
8.1 Introduction, 8.2 Nomenclature, 8.3 Methods of Preparation of Conjugated Dienes, 8.4 Relative Stability of Conjugated Dienes,
8.4.1 Resonance Structure of Conjugated Dienes, 8.4.2 Orbital Picture of Conjugated Dienes, 8.5 Chemical Properties of Conjugated Dienes,
8.5.1 Electrophilic Addition Reactions, 8.5.2 Free Radical Addition to Conjugated Dienes, 8.5.3 Diels–Alder Reaction, 8.5.4 Reduction,
8.1 INTRODUCTION
Compounds having two carbon–carbon double bonds are called alkadienes or di-olens. They are represented
by the general formula C
two double bonds in the carbon chain. These are as follows:
1. Isolated dienes
2. Cumulated dienes
3. Conjugated dienes
8.5.5 Polymerization, 8.5.6 Physiological Effect of 1,3-Butadiene
. Dienes are further divided into three classes depending upon the position of
nH2n–2
1. Isolated dienes: The dienes in which two double bonds are separated by more than one single bond
are called isolated dienes, for example
2. Cumulated dienes: The dienes in which two double bonds are at adjacent positions are known as
cumulated dienes. These compounds are also known as allenes or cumulenes, for example
308
1,3-Butadiene 2,4-Hexadiene
CH
2
CH
2
CH CH CH
3
CH CH
3
CHCH CH
1,2-Propadiene or Allene
1,2-Butadiene or Methylallene
CH
2
CH
2
C
CH3CH CH
2
C
1,4-Pentadiene 1,3-Pentadiene
CH
2
CH
2
CH CHCH
2
CH
3
CH
2
CH CH CH
Heat
Acid
Butane-1,4-diol
1,3-Butadiene
CH
2
CH2CH
2
OH
CH
2
OH
CH2CH CH CH
2
PHarmaCeuTICal OrganIC CHemIsTry
3. Conjugated dienes: The dienes in which two double bonds are separated by a single bond are
known as conjugated dienes, for example
These are the most important members of the diene series.
8.2 NOMENCLATURE
With the exception of cumulated dienes which are commonly
REMEMBER
C C > C C
The double bond is given priority over a triple
bond only when there is a tie.
named as alkyl derivatives of allene CH
dienes are named in the same way as alkenes, except that the
word diene is used at the end with two numbers indicating the
position of two double bonds.
C CH2, the other
2
8.3 METHODS OF PREPARATION OF CONJUGATED DIENES
methods used for the preparation of dienes are similar to those used for alkenes.
1. By cracking of alkanes: When n-butane is cracked at a temperature 550–590ºC in the presence of
chromic oxide (catalyst) on an alumina support, 1,3-butadiene is formed.
CH2 CH2 CH3 CH2 CH CH CH2 + H
CH
3
550—590°C, D
Cr2O3/Al2O
2. By dehydration of dihydric alcohols: When dihydric alcohols are heated in the presence of an
acid, dienes are formed.
3. Pyrolysis of cycloalkenes (retro Diels–Alder reaction): 1,3-Butadiene is produced by passing
vapours of cyclohexene over heated nichrome (ni–Cr–Fe) alloy. This reaction is called retro
Diels–Alder reaction as it is the reversal of Diels–alder reaction.
3
2
C H a P T e r 8 Dienes
1,4-Pentadiene
CH
2
CH
2
CH CHCH
2
1,5-Hexadiene
CH
2
CH
2
CH
2
CH CHCH
2
1,3-Butadiene
CH
2
CH
2
CH CH
CH
3
CH
2
CH CH CH
trans-1,3-Pentadiene
Most
stable
Least
stable
Conjugated diene > Isolated diene > Cumulated diene
8.4 RELATIVE STABILITY OF CONJUGATED DIENES
We nd a surprising difference in the nature of stabilities of different dienes because of change in the position
of double bonds in the carbon chain. Their relative stabilities can be best explained on the basis of heat of
hydrogenation data. It has been found that heat of hydrogenation of alkenes having similar structures is constant.
The heat of hydrogenation for monosubstituted alkenes (rCH CH2), disubstituted alkenes (r2C CH2 or
rCH CHr) and trisubstituted alkenes (r2C CHr) is very close to 30, 28 and 27 kcal mole, respectively.
The change in heat of hydrogenation of alkenes is attributed to the structural change. since each molecule
of diene has two double bonds, we might expect heat of hydrogenation to be double than what it has been
calculated for an individual double bond. This is found to be correct in nonconjugated dienes. For example,
309
Nonconjugated dienes
Conjugated dienes
Expected heat of
hydrogenation
2 × 30 = 60 kcal/mole 60.8 kcal/mole
2 × 30 = 60 kcal/mole 60.5 kcal/mole
2 × 30 = 60 kcal/mole 57 kcal/mole
2 × 30 = 60 kcal/mole 54 kcal/mole
Observed heat of
hydrogenation
as shown above, the observed
REMEMBER
Relative stabilities of dienes:
heat of hydrogenation for a
nonconjugated diene is almost the
same as the calculated value. The
heat of hydrogenation is slightly
lower in the case of conjugated
dienes. The heat of hydrogenation
of trans-1,3-pentadiene (a conjugated diene) is 54 kcal/mole whereas that of 1,4-pentadiene (a nonconjugated
diene) is 61 kcal/mole. Both consume two moles of hydrogen producing the same product, n-
pentane. since
1,3-pentadiene evolves less energy than 1,4-pentadiene it means, 1,3-pentadiene is more stable than 1,4pentadiene, which is a nonconjugated diene. Because of the stability of conjugated dienes, they are the
preferred products of elimination reactions. The stability of conjugated dienes can be best explained as
being due to delocalization of p-electrons.
310
. .
CH
2
CH
2
CH CH
⊕
Θ
CH
2
CH
2
CH CH
1234
(A) (B)
(C) (D)
. .
⊕⊕
Θ
Θ
CH
2
CH
2
CH CH CH
2
CH
2
CH CH
2CH
2
CHCH2CH3 + 2H
2
2CH3CH2CH2CH3 (2 × 30.3) ∆H
−60.6 kcal/mole
1-Butene
CH3CH2CH2CH3 ∆H
−57.0 kcal/mole
Difference = 3.5 kcal/mole
CH
2
CH2 + 2H
2
CH CH
PHarmaCeuTICal OrganIC CHemIsTry
8.4.1 Resonance Structure of Conjugated Dienes
as we have already seen, 1,3-butadiene and other conjugated dienes are more stable than isolated dienes.
We know that resonance confers extra stability to a molecule. 1,3-Butadiene can be regarded as a resonance
hybrid of the following canonical structures (resonating structures):
an inspection of structures (B), (C) and (D) indicates that C2–C3 bond has some double bond character
and C
the observed bond length of C
bond length of C
and C3–C4 bonds should have some single bond character. This is found to be so in practice as
1–C2
and C3–C4 bonds is 1.35 Å (as against 1.34 Å for carbon–carbon double bond).
1–C2
bond is 1.47 Å (as against 1.54 Å for carbon–carbon single bond) and
2–C3
Thus 1,3-butadiene is stable to the extent of 3.5 kcal/mole because of resonance:
In other words, the resonance energy of 1,3-butadiene is 3.5 kcal/mole.
It has been observed recently that resonance contribution in case of 1,3-butadiene is negligible and stability
to the extent of 3.5 kcal per mole is due to hybridization of C
is more stable than the former. similarly, the observed difference in bond length may also be attributed to
hybridization.
8.4.2 Orbital Picture of Conjugated Dienes
The orbital picture of conjugated dienes affords a better way of understanding the nature of bonds in
conjugated dienes. For this purpose, let us consider 1,3-butadiene. each of the four carbon atoms of 1,3butadiene use three sp2 hybrid orbitals for the formation of sigma bonds and an unhybridized p orbital
for the formation of p
functions p
while p
Out of the two binding orbitals p
higher because of one node. similarly, energies of p3* and p4* also increase with the number of nodes.
, p2, p3*; and p4*; out of these p1and p2, called the bonding orbitals, contain two electrons each,
1
* and p4* orbitals, called anti-bonding orbitals, remain vacant in the ground state of the molecule.
3
3
3
bond from C
2–C3
sp
sp
–C
to C
2
sp
bond. The lCaO of these four p orbitals give four molecular orbitals having wave
is of the lowest energy as it has no node and energy of p2 is somewhat
1
–C
2
sp
, which