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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
Alkanes
298 K
Benzoyl peroxide Benzoate free radical
O
C
6H5
OC C C2H
5
O
O
°
O
2C
6H5
OC
°
C
6H5
+ CO
2
Phenyl free radical
°
O
C
6H5
OC
°
C
+ Cl – Cl C2H5Cl + C°l
2H5
211
Chlorine free radical
CH4 + C°l
C°H
+ HCl, C°H3 + Cl
3
b. Effect of organic peroxide:
R – O – O – R 2RO
298 K
or,
°
C
+ Cl – Cl C6H5 – Cl + C°l
6H5
This chlorine free radical propagates the chain reaction.
2
Alkoxy free radical
CH
°
Cl + C°l and so on.
3
3.
formed. It is due to repeating chain propagating steps.
4. Role of inhibitor: The substances which slow down or stop the reaction even if present in small
amount are known as inhibitors and the process is known as inhibition.
It has been observed that oxygen, if present in the reaction mixture, slows down the reaction. Oxygen
reacts with alkyl free radical to form less reactive alkyl peroxy free radical which is unable to propagate
the chain reaction.
°
R
+ O = O R – O – O
Alkyl free
radical
Alkyl peroxy
free radical
°
Thus oxygen acts as an inhibitor. After all the oxygen molecules present have combined with alkyl free
radical the reaction proceeds normally. The period of time during which inhibition lasts and after which the
reaction proceeds normally is known as inhibition period.
l
2
Diffused sunlight
or UV Cl
2
or heated 600 K
Methyl Iodide
CH
3
I + HI
CH
4
Methane
2O
2
Complete oxidation
CH3NO
2
+ H
2
O
Nitromethane
CO
2
+ 2H2O ∆H = −890 kJ
2CH3OH
Molybdenum
Oxide
HCHO + H2O
Formaldehyde
475/100 atm
Cu tube
750 K
HlO
3
HNO
3
Cl
2
Cl
2
Cl
2
CCl
4
CH3Cl CH2Cl
2
CHCl
3
Methylchloride
(Chloromethane)
Methylene chloride
(Dichloromethane)
Carbon tetrachloride
(Tetrachloromethane)
Chloroform
(Trichloromethane)
212
MEMORY FOCUS
REVISION QUESTIONS
1.
5H12 giving
2. 6H14
3.
4.
Explain the orbital structure of ethane.
5.
6.
8.
9.
10.
Discuss important methods of preparation of alkanes.
Explain the variation of melting points of alkanes with increase of their molecular weights.
Discuss important chemical properties of alkanes.
Alkanes
11.
merits/demerits.
How are alkanes prepared by the reduction of alkyl halides and decarboxylation of monocarboxylic
12.
Discuss the free radical mechanism in case of halogenation of alkanes.
13.
14.
15.
structure for the alkane.
16.
(a)
(c)
(c) Frankland method
18. How will you convert
(a) Methane to ethane (b) Ethane to propane and butane
213
(c) Ethyne to ethane (d) Methyl magnesium chloride to methane
(e) Propionic acid to ethane
(a) H
(b) 2C
(c) C
(d) H
(e) H
(f) H
(g) H
+ H2O ................ + ................
4C3
I + 2Na ................ + ................
2H3
+ O2 ................ + ................
2H6
CCOOK
3
COONa + NaOH
5C2
Mg Br + H2O ................ + ................
5C2
C C ≡ CH + 2H
3
Electrolysis
i
2
................................................................
................................................................
................................................................
(c) Methyl iodide with nascent hydrogen (d) Ethyl magnesium bromide with ammonia
214
MULTIPLE CHOICE QUESTIONS
(a) All bond angles are 109.5
o
(b) Each carbon is sp 3 hybridized
(c) The compound is combustible
(d) The compound undergoes polymerization to give polypropylene
2.
(a) Primary (b) econdary
(c) Tertiary (d) one of these
5H12 6H
5H10 6H
14
12
(a) 2 (b) 3
(c) 4 (d) 5
(a) Many alkenes are soluble in water
(b) All alkenes have a lower density than water
(d) All alkenes burn
(a) n-Octane (b) Isopentane
(c) n-Butane
(a) The reaction of iodomethane with sodium in dry ether
(b) The reaction of methanol with concentrated H24
(c) The reaction of sodium methanoate with soda-lime
(d) The reaction of sodium ethanoate with soda-lime
Alkanes
322
215
8H16 6H
8H18 6H
14
12
(a) Alkanes (b) Alcohols
(c) Alkenes (d) Alkynes
4 is an example of
(a) An electrophilic addition (b) A free-radical substitution
(c) A nucleophilic addition (d) An electrophilic substitutions
(a) hlorine (b) Bromine
(c) Iodine (d) Fluorine
(a) at a slower rate (b) at a faster rate
upon the source of alkane
(a) Bromomethane (b) Dibromomethane
(c) Tribromomethane (d) All of these
3H8 4 in the dark. The compound
could be
(a) A
(c) Alkene
3H8, produces how many moles of H2
(a) 2 (b) 3
(c) 4 (d) 5
216
(a) O
(b) It occurs without the generation of Intermediates
(c) Each step generates the reactive intermediate that causes the next step to occur
(d) The reaction allows long chains of halogenated alkanes to be formed
18. The thermal decomposition of alkanes in the absence of air is called
(a) ombustion (b) Oxidation
(c) racking (d) Hydrogenation
24
(a) n-Hexane (b) Diethyl ether
(c) I-Butene (d) Aniline
20. A tertiary carbon is bonded directly to
(a) 2 Hydrogens (b) 3 arbons
(c) 2 arbons (d) 4 arbons
ANSWERS
1. (d) 2. (a) 3. (b) 4. (b) 5. (a) 6. (a) (d) 8. (a) 9. (b) 10. (a)
11. (b) 12. (c) 13. (b) 14. (d) 15. (a) 16. (c) (c) 18. (c) 19. (a) 20. (b)
5.1 INTRODUCTION
1. These are open-chain unsaturated hydrocarbons characterized by the presence of a double bond
(C C) in their molecules.
2. They have the general formula C
nH2n
.
3. These are called unsaturated hydrocarbons because they have two hydrogen atoms less than the
corresponding alkanes.
4. They are also known as olens (Greek: olefiant, oil-forming) as the lower gaseous members of the
family forms oily products when treated with chlorine or bromine.
6. Compounds with two double bonds are known as alkadienes and with three double bonds are known as
alkatrienes.
5.2 STRUCTURE OF ALKENES (STRUCTURE OF CARBON–CARBON DOUBLE BOND)
alkenes, the carbon atom linked by double bond undergoes sp2 (trigonal) hybridization.
ALKENES
5
Chapter Outline
5.1 Introduction, 5.2 Structure of Alkenes (Structure of Carbon–Carbon Double Bond), 5.3 Nomenclature of Alkenes, 5.3.1 Common
System, 5.3.2 IUPAC System, 5.4 Isomerism in Alkenes, 5.5 Method of Preparation of Alkenes, 5.5.1 Dehydration of Alcohols,
5.5.2 Dehydrohalogenation of Alkyl Halides, 5.3 By Partial or Selective Reduction of Alkynes, 5.5.4 Electrolysis of Sodium or
Potassium Salts of Dibasic Acids (Kolbe’s Electrolytic Method), 5.5.5 Dehalogenation of Vicinal Dihalides, 5.6 Physical Properties of
Alkenes, 5.7 Chemical Properties of Alkenes, 5.7.1 Addition Reactions, 5.7.2 Addition of Halogens, 5.7.3 Addition of Halogen Acids,
5.7.4 Addition of Hydrogen Halides to Unsymmetrical Alkenes: Markownikoff's Rule, 5.7.5 Addition of Sulphuric Acid, 5.7.6 Addition of
Water (Hydration of Alkenes), 5.7.7 Halohydrin Formation: Addition of Hypohalous Acids, 5.7.8 Ozonolysis, 5.7.9 Hydroboration Oxidation,
5.7.10 Hydroxylation, 5.7.11 Hydroxylation of Alkenes with Osmium Tetra Oxide, 5.7.12 Oxidation with Hot Alkaline KMnO
4
(Oxidative Cleavage of Alkenes)
Our experiments are not carried out to decide
whether we are right, but to gain new knowledge.
It is for knowledge’s sake that we plow and sow.
–R. Willstatter
π-bond
σ-bond
σ-bond
σ-bond
218
(a) (b)
σ-bondσ-bond
σ-bond
H
H
H
H
2P
z
2P
z
π-bond
sp
2
sp
2
sp2sp
2
sp
2
sp
2
H
H
H
H
CC
π
H
1.09A
1.34 A
121º
H
C
118º
HH
sp2 hybrid orbital of each carbon atom overlaps axially with other, forming a stronger carbon–
sp2 hybrid orbitals of each carbon atom lie in same plane, all the
six atoms (two carbon and four hydrogen atoms) lies in one plane.
The orbital structure of ethylene is show in Fig. 5.1.
2. Unhybridized 2pz orbitals of two carbon atoms overlap sideways to form a weaker bond.
Figure 5.1 Orbital structure of ethylene molecule. Two electron clouds of the bond.
Various parameter of the ethylene molecule are:
1. Bond strength: Carbon–carbon double bond = 598 kJ mol–1
Bond angles: H C H bond angle = 117.5°
2.
C
C
Bond length: Carbon–carbon double bond (C C) length = 1.34 Å or 134 pm
3.
H bond angle = 121°
Carbon hydrogen (C H) bond length = 1.09 Å or 109 pm
These parameters of ethylene can be depicted in Fig. 5.2.
Figure 5.2 Parameters of ethylene.
H bond pair of electrons is greater than two
C H bond pair of electrons, H C H bond angle gets distorted from 120° (expected value) to 118°.
As a result C C H bond angle gets increased from 120° (expected 120°) to 121°.
C bond is shorter than C C bond.
C
The carbon–carbon double bond length (C
compared to alkanes whose C C bond length is 1.54 Å. This is because the effective size of sp2 hybrid
orbital with more s-character is smaller than that of sp3 hybrid orbital. Moreover, the sideways overlapping
of p orbital brings the atom closer and thus shortens the bond length.
C) in ethylene is 1.34 Å (134 pm), which is shorter as
C H A P T E R 5 Alkenes
H
H
H
H
C
C
5.3 NOMENCLATURE OF ALKENES
5.3.1 Common System
of the corresponding alkane is substituted by ‘ylene’. For example:
nH
nH2n)
2n+2
CH4 , Methane CH2, Methylene (exists as a radical only)
C
C
C
C
, Ethane C2H4, Ethylene
2H6
, Propane C3H6, Propylene
3H8
, Butane C4H8, Butylene
4H10
, Pentane C5H10, Pentylene
5H12
(or amylene)
The position of double bond in case of isomeric alkenes is indicated by the Greek letter ene. For example:
CH2CH CH2CH3 1-butylene
CH3CH CH CH3 2-butylene
5.3.2 IUPAC System
219
1. The longest carbon chain containing the carbon–carbon double (C C) bond is selected as the parent
alkene.
chain, the alkene is called diene or triene, respectively.
3. The position of double bond or side chains indicated by numbers 1, 2, 3, etc.
4. The longest chain is numbered from that end, which gives the lowest number to the carbon atom of
bond gets the same number from either side, the carbon chain is numbered in such a manner that the
substituent gets the lowest number.
Molecular formula Structural formula IUPAC name
C
2H4
C
3H6
C
4H8
CH3CH CH
1. CH3CH2–CH CH
2. CH
CH CHCH
3
2
3
2
Ethene
Propene
1-Butene
2-Butene
Contd…
220
CH
3
C
CH
2
CH
3
322 2
CH CH CH CH CH
32 3
CH CH CH CHCH
4321
CH
3
CCH3CH
2
CH
2
1234
CH
3
CH CH CH
2
CH
3
4321
CH
3
C CH CH
3
CH
3
CH
3
2-Methylpropene1-Butene
CH CH2 CH
3
CH
2
C CH
3
CH
2
3 2 23 3
CH CH CH CH CH CH CH CH
1-Butene 2-Butene
Molecular formula Structural formula IUPAC name
3.
2-Methylpropene
C5H
10
1.
2.
1-Pentene
2-Pentene
3.
2-Methyl-1-butene
4.
3-Methyl-1-butene
5.
3-Methyl-2-butene
5.4 ISOMERISM IN ALKENES
Alkenes show the following types of structural isomerism:
1. Chain isomerism: This type of isomerism arises due to the difference in the structure of carbon chain.
For example:
2. Position isomerism: This type of isomerism arises from the difference in the position of the double
bond in the same chain. For example:
3. Geometrical isomerism: This type of space isomerism also known as cis–trans isomerism and is
already discussed in Chapter 3.