Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
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 5 Alkenes
OH OH
3CH
3CH2CH2CH2
CH
1-Hexene (purple)
(colourless) (brown ppt)
CH2 + 2MnO2 + 2KOH
CH2 + 2KMnO4 + 4H2O
CH3CH2CH2CH2CH
CCl
4
1-Hexene (red) (colourless)
BrBr
CH
3CH2CH2CH2
CH CH
2
CH3CH2CH2CH2CH
CH
2
+ Br
2
(ii) Reaction with Br2/CCl4: 1-Hexene will decolourize the red colour of the solutions of Br2 in
CCl4 immediately. n-Hexene will react only if the mixture is subjected to high temperature
or UV light.
4 solution (purple) or
Br2 in CCl4 solution (red). Propene will decolourize both the solutions; propene does not react.
6. Write a note on:
(a) Markownikoff’s rule
251
(b) Peroxide effect
7. (a) What is hydroboration? Give an example.
(b) Give the general mechanism of electrophilic addition reactions.
8. Give the mechanism of:
(a) Addition of bromine to ethylene
(b) Addition of HBr to ethylene
(c) Addition of HBr to propene
(d) Addition of HBr to propene in the presence of a peroxide
9. How will you synthesize:
(a) Propylene from n-propyl bromide? (b) n-Propyl bromide from propylene?
(c) Ethylene from ethyl alcohol? (d) Ethyl alcohol from ethylene?
Hint:
10. How will you synthesize:
(a) Ethylene from ethane?
(b) Propane from isopropyl alcohol?
(c) n-Propyl bromide from isopropyl bromine?
peroxide, (c) concentrated H24/heat and (d) H2+ heat.
252
Conc. H2SO
4
H2/Ni
CH3CH2CH
3
Isopropyl alcohol Propane
∆
OH
CH
3
CH CH
3
CH3CH CH
2
Isopropyl bromide
alcohol, ∆
peroxide
n-Propyl bromide
KOH HBr
CH3CH2CH2CH
3
Br
CH
3
CH CH
3
CH
2
CH3CH
Hint: 2
(b) The following steps are involved:
(c)
11. (a) Write the structure of the alkene, which on ozonolysis, gives 2-butanone and
2-methylpropanal.
(b) A hydrocarbon reacts with conc. H24 to form an alkyl hydrogen sulphate, which on hydrolysis,
yields tert-butyl alcohol. What is the structure of isobutylene?
(a) The alkene is 2,4-dimethyl-3-hexene.
Ans:
(b) The hydrocarbon is isobutylene.
12. A hydrocarbon (A) adds one mole of hydrogen in the presence of a platinum catalyst to form
n4, a single carboxylic acid containing
three carbon atoms is isolated. Give the structure and name of (A).
Ans:
The hydrocarbon (A) is 3-hexene.
13. A primary alkyl halide (A), C4H9
Compound (B) reacted with HBr to give an isomer of (A), (C). When (A) was reacted with sodium it
gave compound (D), C8H18, which was different than the compound produced when n-butyl bromide
Ans: The compounds are:
(a) = 1-Bromo-2-methylpropane (b) = 2-Methylpropene
(c) = 2-Bromo-2-methylpropane (d) = 2,5-Dimethylhexene
14. A hydrocarbon of formula C6H12 colourless bromine solution, dissolves in concentrated sulphuric
acid, yields 2-methyl-1-pentane on hydrogenation, and on ozonolysis gives formaldehyde and
3-methylbutanal. What is the structure of the hydrocarbon?
Ans:
The hydrocarbon is 4-methyl-1-pentene.
15. The hydrocarbon (A) is 1-butene.
C H A P T E R 5 Alkenes
MULTIPLE CHOICE QUESTIONS
1. The carbon atoms involved in the double bond of an alkene are
(a) sp hybridized (b) sp2 hybridized
sp3 hybridized (d) none of these
(c)
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 obtained from ethyl bromide by
253
+ ion acts as the
(a) Electrophile (b) Carbonium ion
6. Propene reacts with bromine to form 1,2-dibromopropane. This is an example of
7. Baeyer’s reagent is
4 (b) HCl + ZnCl
(c) Br2 in CCl4 2
2
2
hydrogens is a statement of
(a) Hund’s rules (b) Markownikoff’s rule
9. Markownikoff’s addition of HBr is not applicable to
(a) Propane (b) 1-Butene
(c) 1-Pentene (d) 2-Butene
254
3CH2CH=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
4 in its reaction with alkene is known as
(a) Markownikoff’s 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) CH3CH=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?
4
3
4
15. The propane reacts with HBr in the presence of a peroxide to give
(a) n-Propyl bromide (b) Allyl bromide
16. Propane reacts with Cl2 in H2
(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
(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
(a) Carbon dioxide and water (b) Carbon monoxide and water
C H A P T E R 5 Alkenes
20. How many sigma bonds are there in CH2 = CH – CH = CH2?
(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)
255
“This page intentionally left blank"
ALKYNES
H
C
C
H
σ
π
π
Fill the brain with high thoughts, highest ideals, place
5
6
6.1 Introduction, 6.2 Structures of Alkynes, 6.3 Isomerism in Alkynes, 6.4 Nomenclature of Alkynes (Acetylenes), 6.5 Classification of
Alkynes, 6.6 Commercial Methods of Preparation of Acetylene, 6.7 General Methods of Preparation of Alkynes, 6.8 Physical Properties of
Alkynes, 6.9 Chemical Properties of Alkynes, 6.10 Low Reactivity of Alkynes Towards Electrophilic Addition Reactions, 6.11 Addition Reac-
tions, 6.12 Addition of Hydrogen, 6.13 Electrophilic Additions, 6.14 Nucleophilic Addition Reactions, 6.15 Acidity of Acetylenic Hydrogen,
6.16 Oxidation Reaction of Alkynes, 6.17 Polymerization, 6.18 Distinction Between Alkane, Alkene and Alkene
6.1 INTRODUCTION
These are the open chain unsaturated hydrocarbons characterized by the presence of a triple bond (C C)
in its molecule. These are called unsaturated hydrocarbons because they have four hydrogen atoms less
than the corresponding alkanes. They have the general formula CnH
acetylene (HC CH) and the family as a whole is referred to as acetylenes. The triple bond in alkynes is
referred to as acetylenic or alkynic linkage. Compounds with two triple bonds are known as alkadiynes and
with three triple bonds are known as alkatriyne.
them day and night before you, and out of that will
come great work.
–Swami Vivekananda
Chapter Outline
. The rst member of the alkyne is
2n–2
6.2 STRUCTURE OF ALKYNES (CARBON–CARBON TRIPLE BOND)
Let us illustrate the structure of alkynes by taking an example of acetylene, the rst member of alkynes. The
carbon–carbon triple bond is the distinguishing feature of the alkyne structure. The carbon atoms linked by
triple covalent bond undergo sp (diagonal) hybridization.
In the formation of acetylene,
1. one sp hybrid orbital of one carbon atom overlaps axially with the other carbon atom to form a
C sigma (s) bond;
C
2. other sp hybrid orbital of each carbon atom over laps with the 1s orbital of hydrogen atom to form a
C H sigma bond;
3. the two unhybridized p orbitals of one carbon atom overlap sideways to a small extent with the similar
p orbital of other carbon atom to form two p bonds in planes at right angle to each other;
4. if the electron cloud of one p bond lies above and below the line joining the carbon nuclei, the electron
cloud of the other p bond lies in front and at the back of the line. However, the electron clouds of
the two p bonds merge to form a single electron cloud, which is cylindrically symmetrical about the
internuclear axis (Fig. 6.1).
258
σ Bond
σ Bond
1s
sp
sp
H
2p
y
2p
y
2p
x
2p
x
H
σ Bond
π Bond
sp sp
HH
σσσ
π
C C
C
C
H
°
°
°
H
C
1.06 A
1.20 A
1.06 A
180°
C
PHARMACEUTICAL ORGANIC CHEMISTRY
Figure 6.1 Orbital structure of acetylene.
5. Thus carbon–carbon triple bond consists of one strong s bond and two weak p bonds.
Figure 6.2 Parameters of acetylene.
Various parameters of the acetylene molecule are as follows (Fig. 6.2):
1.
Bond angle: The angle between H C C is 180o, i.e. it is a linear molecule.
Bond length: The bond length of C C is 1.2 Å or 120 pm and of C H is 1.06 Å or 106 pm.
2.
3. Bond strength: The bond strength of C C is 198 kcal. It is stronger than C C of ethylene (163 kcal)
or the C C of ethane (88 kcal).
4. The carbon–carbon triple bond length (C C) in acetylene is 1.20 Å, which is shorter as compared
with 1.34 Å in ethene and 1.54 Å in ethane. This is because of the smaller effective size of sp hybrid
orbitals with more s-character and the sideways overlapping of p orbitals which brings the atoms closer
and thus shortens the bond length.
C H bond is shorter than both C H and C H bonds.
5.
Due to the smaller effective size of sp orbitals, the C H distance of 1.06 Å in acetylene is also shorter than
C H distance in ethene (1.09 Å), which in turn is shorter than C H distance in ethane (1.10 Å).
6.3 ISOMERISM IN ALKYNES
Alkynes do not exhibit geometrical isomerism.
molecules have, therefore, a linear structure. There is no question of any different arrangement of atoms in
space. Hence alkynes do not exhibit geometrical isomerism.
It may be noted that in alkynes, the carbon atoms are linked to each other by triple (C C) bond. The
C H A P T E R 6 Alkynes
1-Pentyne
3-Methy-1-butyne
and
CHCH
3CH2CH2
C
CH
3
CHCH3 CH C
32 22
CH CH CCHand CH CH CH CH
1-Butyne 1, 3-Butadiene
6.3.1 Structural Isomerism in Alkynes
Alkynes show three types of structural isomerism:
1. Chain isomerism: It is due to different number of carbon atoms in the chain, for example,
2. Functional isomerism: They show functional isomerism with alkadienes (i.e. having same
molecular formula but different functional group).
For example:
3. Position isomerism: It is due to the different position of triple bond in the carbon chain, for
example,
259
CH3 CH2 C CH and CH3 C C CH
1-Butyne 2-Butyne
6.4 NOMENCLATURE OF ALKYNES (ACETYLENES)
1. Common system: According to this system the rst member of the homologous series (general
formula CnH
) is acetylene (C2H2). All other members are named as alkyl derivatives of acetylene,
2n–2
e.g. C3H4 [CH3C CH] is named as methylacetylene.
2. IUPAC system
a. The longest carbon chain containing the carbon carbon triple bond (C C) is selected as the
parent alkyne.
b. The sufx ane of the alkane is replaced by yne. If triple bond occurs twice or thrice in the
parent chain, the alkyne is called diyne or triyne, respectively.
c. The longest chain is numbered from that end which gives the lowest number to the carbon
atom of the triple bond and written just before the sufx yne. If while numbering the chain,
the triple bond gets the same number from either side, the carbon chain is numbered in such
a manner that the substituent gets the lowest possible number.
d. In case there are more triple bonds in the compound, lowest sum rule should be followed.
e. The name and position of other groups (substituents) is indicated by prexes.
3
260
CH
3
CH3CH CHC
Propyne 3-Methyl-1-butyne1-Butyne
CH
3
CH2CH; CH3CH2CCH;
CH
3
CH3CH CCH
PHARMACEUTICAL ORGANIC CHEMISTRY
The common and IUPAC names of rst few alkynes are given below:
No. of C
atoms
2
3
4
5
Molecular
formula
C2H
2
C3H
4
C4H
6
C5H
8
Structure Common name IUPAC name
HC CH Acetylene Ethyne
H3C C CH
(i) CH3 CH2 C CH
(ii) CH3 C C CH
3
(i) CH3 CH2 CH2 C CH
(ii) CH3 CH2 C C CH
3
(iii)
Methylacetylene or
Alkylene
Ethylacetylene
or α-Crotonylene
Dimethylacetylene
or β-Crotylene
Propyne
1-Butyne
2-Butyne
n-Propylacetylene 1- Pentyne
Ethylmethylacetylene 2- Pentyne
Isopropylacetylene 3-Methyl-l-butyne
f. If the compound contains both C C and C C bonds, lowest sum rule is followed. However,
if the sum turns out to be same from either end of the chain, the preference is given to C C
bond while numbering. However, compound is always written as a derivative of alkynes.
6 5 4 3 2 1 1 2 3 4 5
For example:
HC
C — CH CH — C CH3 and HC C — CH CH — CH
Hex-3-ene-1,5-diyne Pent-3-en-1-yne
3
6.5 CLASSIFICATION OF ALKYNES
Alkynes are mainly classied into two categories: (1) terminal and (2) nonterminal alkynes.
1. Terminal alkynes: Such alkynes which contain the triple bond at one end of the carbon chain, i.e.
alkynes in which one of the triply bonded carbon atoms is attached to a hydrogen atom (i.e.
C H) are called terminal alkynes. Thus,
The hydrogen atom bonded with one of the triply bonded carbon atom is called acetylenic
hydrogen.
C