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Aliphatic Hydrocarbon
Organic compounds composed of only carbon and hydrogen are called hydrocarbons. Hydrocarbons are two types (1) Aliphatic Hydrocarbon (Alkanes, Alkenes and Alkynes). (2) Aromatic Hydrocarbon (Arenes) (1) Sources of aliphatic hydrocarbon Mineral oil or crude oil, petroleum [Petra rock; oleum oil] is the dark colour oily liquid with offensive odour found at various depths in many regions below the surface of the earth. It is generally found under the rocks of earth’s crust and often floats over salted water. (2) Composition (i) Alkanes : found 30 to 70% contain upto 40 carbon atom. Alkanes are mostly straight chain but some are branched chain isomers. (ii) Cycloalkanes : Found 16 to 64% cycloalkanes present in petroleum are; cyclohexane, methyl cyclopentane etc. cycloalkanes rich oil is called asphaltic oil. (iii) Aromatic hydrocarbon : found 8 to 15% compound present in petroleum are; Benzene, Toluene, Xylene, Naphthalene etc. (iv) Sulphur, nitrogen and oxygen compound : Sulphur compound present to the extent of 6% include mercaptans [R-SH] and sulphides [R-S-R]. The
unpleasant smell of petroleum is due to sulphur compounds. Nitrogenous compounds are pyridines, quinolines and pyrroles. Oxygen compounds present in petroleum are. Alcohols, Phenols and resins. Compounds like chlorophyll, haemin are also present in it. (v) Natural gas : It is a mixture of Methane (80%), Ethane (13%), Propane (3%), Butane (1%), Vapours of low boiling pentanes and hexanes (0.5%) and Nitrogen (1.3%). L.P.G. Contain butanes and pentanes and used as cooking gas. It is highly inflammable. This contain, methane, nitrogen and ethane. (vi) C.N.G. : When natural gas compressed at very high pressure is called compressed natural gas (CNG). Natural gas has octane rating of 130 it consists, mainly of methane and may contain, small amount of ethane and propane. (3) Theories of origin of petroleum : Theories must explain the following characteristics associated with petroleum, Its association with brine (sodium chloride solution). The presence of nitrogen and sulphur compounds in it. The presence of chlorophyll and haemin in it. Its optically active nature. Three important theories are as follows. (i) Mendeleeff’s carbide theory or inorganic theory
Hydrocarbon
Chapter
(ii) Engler’s theory or organic theory (iii) Modern theory (4) Mining of petroleum : Petroleum deposits occurs at varying depth at different places ranging from 500 to 15000 feet. This is brought to the surface by artificial drilling.
(5) Petroleum refining : Separation of useful fractions by fractional distillation is called petroleum refining.
Table : 24.
Fraction Boiling range (oC) Approximate composition
Uses
Uncondensed gases Upto room temperature
C 1 – C 4 Fuel gases: refrigerants; production of carbon black, hydrogen; synthesis of organic chemicals. Crude naphtha on refractionation yields,
30 – 150 o^ C 5 – C 10
(i) Petroleum ether 30 – 70 o^ C 5 – C 6 Solvent (ii) Petrol or gasoline 70 – 120 o^ C 6 – C 8 Motor fuel; drycleaning; petrol gas. (iii) Benzene derivatives 120 – 150 o^ C 8 – C 10 Solvent; drycleaning Kerosene oil 150 – 250 o^ C 11 – C 16 Fuel; illuminant; oil gas Heavy oil 250 – 400 o^ C 15 – C 18 As fuel for diesel engines; converted to gasoline by cracking. Refractionation gives, (i) Gas oil, (ii) Fuel oil, (iii) Diesel oil Residual oil on fractionation by vacuum distillation gives,
Above 400o^ C 17 – C 40
(i) Lubricating oil C 17 – C 20 Lubrication (ii) Paraffin wax C 20 – C 30 Candles; boot polish; wax paper; etc (iii) Vaseline C 20 – C 30 Toilets; ointments; lubrication. (iv) Pitch C 30 – C 40 Paints, road surfacing Petroleum coke (on redistilling tar)
As fuel.
(6) Purification (i) Treatment with concentrated sulphuric acid : The gasoline or kerosene oil fraction is shaken with sulphuric acid to remove aromatic compounds like thiophene and other sulphur compound with impart offensive odour to gasoline and kerosene and also make them corrosive. (ii) Doctor sweetening process :
Mercaptan^2 RSH^ Na^2 PbO^2 S Disulphide RSSR s PbS ^2 NaOH (iii) Treatment with adsorbents : Various fractions are passed over adsorbents like alumina, silica or clay etc, when the undesirable compounds get adsorbed. (7) Artificial method for manufacture of Petrol or gasoline (i) Cracking, (ii) Synthesis
(called as AK- 33 - X) is used in developed countries as antiknocking compound. (4) Other methods of improving octane number of hydrocarbon. (i) Isomerisation [Reforming] : By passing an
alkane over AlCl 3 at 200 oC.
(OctaneIsopentanenumber^90 )
2 3
3 200 3 (OctanePentanenumber^ 62)
3 2 2 2 3
HCH
CH
CH CHCHCHCH AlCl oC CH CHC
(ii) Alkylation :
(OctaneIso-octanenumber 100)
3 3
3
3
Isobutylene^32
3 3 Isobutane
3
3
3 2
2 4
CH
CHCH
CH
CH
CHCCH
CH
CCH
CH
CH
CH CH CH HSO
(iii) Aromatisation :
2 Toluene
3 500
/ 3 (Heptane^2 ) 5 3 4
2 3 H
CH
CH CH CH Pt Al o (^) C O
The octane no. of petrol can thus be improved. By increasing the proportion of branched chain or cyclic alkanes. By addition of aromatic hydrocarbons Benzene, Toluene and Xylene (BTX). By addition of methanol or ethanol.
By additon of tetraethyl lead( C 2 H 5 ) 4 Pb
(5) Cetane number : It is used for grading the diesel oils.
CH 3 ( CH 2 ) 14 CH 3 Cetane cetane no. =
The cetane number of a diesel oil is the percentage of cetane (hexadecane) by volume in a
mixture of cetane and -methyl naphthalene which
has the same ignition property as the fuel oil under consideration. (6) Flash point : The lowest temperature at which an oil gives sufficient vapours to form an explosive mixture with air is referred to as flash point of the oil.
The flash point in India is fixed at 44 oC , in France it is fixed at 35o C , and in England at 22.8o C. The flash point of an oil is usually determined by means of “ Abel’s apparatus ”. Chemists have prepared some hydrocarbons with octane number even less than zero (e.g., n - nonane has octane number – 45) as well as hydrocarbon with octane number greater than 100 (e.g., 2, 2, 3 trimethyl- butane. has octane number of 124). (7) Petrochemicals : All such chemicals which are derived from petroleum or natural gas called petrochemicals. Some chemicals which are obtained from petroleum are summarised in table :
Table : 24.
Hydrocarbons Compounds derived Methane Methyl chloride, chloroform, methanol, formaldehyde, formic acid, freon, hydrogen for synthesis of ammonia. Ethane Ethyl chloride, ethyl bromide, acetic acid, acetaldehyde, ethylene, ethyl acetate, nitroethane, acetic anhydride. Ethylene Ethanol, ethylene oxide, glycol, vinyl chloride, glyoxal, polyethene, styrene, butadiene, acetic acid. Propane Propanol, propionic acid, isopropyl ether, acetone, nitromethane, nitroethane, nitropropane. Propylene Glycerol, allyl alcohol, isopropyl alcohol, acrolein, nitroglycerine, dodecylbenzene, cumene, bakelite. Hexane Benzene, DDT, gammexane. Heptane Toluene Cycloalkanes Benzene, toluene, xylenes, adipic acid. Benzene Ethyl benzene, styrene, phenol, BHC (insecticide), adipic acid, nylon, cyclohexane, ABS detergents. Toluene Benzoic acid, TNT benzaldehyde, saccharin, chloramine-T, benzyl chloride, benzal chloride.
Alkanes [Paraffines]
“Alkanes are saturated hydrocarbon containing only carbon-carbon single bond in their molecules.” Alkanes are less reactive so called paraffins; because under normal conditions alkanes do not react with acids, bases, oxidising agents and reducing agent.
General formula : Cn H 2 n 2
Examples are CH 4 , C 2 H 6 , C 3 H 8 ,
(1) General Methods of preparation (i) By catalytic hydrogenation of alkenes and alkynes (Sabatie and sanderen’s reaction)
CH 3
- Methyl naphthalene
Cetane no. = 0
Alkene^2 ^2 heat n Alkane^2 n ^2 Cn Hn H Ni CH ;
2 2 ^2 2 heat n Alkane 2 n 2
Ni
CnAlkyne Hn H CH
Methane is not prepared by this method (ii) Birch reduction : R CH CH 2 ^12 .. NaCH ^ / 3 NH OH ^3 R CH 2 CH 3 (iii) From alkyl halide (a) By reduction : RX H 2 Zn^ / HCl RH HX (b) With hydrogen in presence of pt/pd : RX H 2 Pd^ orPt . RH HX (c) With HI in presence of Red phosphorus : Purpose ofRed istoremove 2 intheformof^23
RBr 2 HIP RHI HBr IPI
(iv) By Zn-Cu couple :
2 CH 3 CH 2 OH Zn-Cu Zn couple Cu ^ ( CH Zinc 3 CH ethoxide 2 O ) 2 Zn 2 H
RX 2 H RH HX
(v) Wurtz reaction : Alky l R^ X halide^2 Na Alky l X halide R Dry ^^ ether R Alkane R^ ^2 NaX
R Br or RI preferred in this reaction. The net
result in this reaction is the formation of even no. of carbon atoms in molecules. (vi) Frankland’s reaction : 2 RX Zn R R ZnX 2 (vii) Corey-house synthesis
CH 3 CH 2 Cl 2 ^1 .. CuI Li^ ( CH 3 CH 2 ) 2 LiCu CH 3 ^ CH ^2 Cl
CH 3 CH 2 CH 2 CH 3
Reaction is suitable for odd number of Alkanes. (viii) From Grignard reagent (a) By action of acidic ‘ H ’ : RMgX HOH Water Alkane RH Mg ( OH ) X Alkylhalidemagnesium
(b) By reaction with alkyl halide : R X R MgX R R MgX 2 (ix) From carboxylic acids (a) Laboratory method [Decarboxylation reaction or Duma reaction]
R COONa NaOH CaOheat ^ R Alkane H Na 2 CO 3
NaOH and CaO is in the ratio of 3 : 1. (Sodalime) (b) Kolbe’s synthesis :
O Na
O
R C
O ^ Na
O
R C
At anode [Oxidation] :
2 || 22 || O 2 R 2 CO 2
O
R C
O
R C O e
2 R^ R R (alkane) At cathode [Reduction] : 2 Na 2 e 2 Na ^2 H^ 2 ^ O 2 NaOH H 2 ( ) Both ionic and free radical mechanism are involved in this reaction. (c) Reduction of carboxylic acid : CH Acetic (^) 3 COOH acid 6 HI Re duction p ^ CH Ethane 3 CH 3 2 H 2 O 3 I 2 (x) By reduction of alcohols, aldehyde, ketones or acid derivatives 150 Red Methane^422 (MethylMethanolalcohol)
CH 3 OH 2 HI o PC CH HO I
150 Red Ethane^2622 Acetaldehy(Ethanal)de CH (^) 3 CHO 4 HI o P (^) C CH HO 2 I
150 Red^3 Propane^2 (PropanoneAcetone^ )
CH 3 COCH 3 4 HI o P C CHCHCH HO 2 I
200 Red Ethane^3 (Ethanoy lAcety lchloridechloride)
Cl HI CH CH HO HCl I
O
CH C o P C
200 Red Ethane^3 (EthanamidAcetamidee)^
NH HI CH CH HO NH I
O
CH C o (^) PC
Aldehyde and ketones when reduced with
amalgamated zinc and conc. HCl also yield alkanes.
Clemmenson reduction : CH CHO H ZnHClHg CH Ethane 3 CH 3 H 2 O Acetaldehy(Ethanal)de
3 ^4 ^
CH COCH H ZnHClHg CH 3 Propane CH 2 CH 3 H 2 O
(PropanoneAcetone^ )
3 3 ^4 ^
Aldehydes and ketones ( C O )can be reduced to hydrocarbon in presence of excess of hydrazine and sodium alkoxide on heating. Wolff-kishner reduction :
2 22218025 , 2 CH 2
R
R
C NNH
R
R
C O
R
R
C N
CHONa HO
HNNH
^ ^ o^
(xi) Hydroboration of alkenes (a) On treatment with acetic acid
R CH Alkene CH 2 B^^2 H ^6 ( R CH Trialky l b 2 orane CH 2 ) 3 B CH 3 COOH
Electrolysis Ionization
Methane CH^^4 1000 C C^2 H^2
o
C Ethane (^) 2 H (^6) Cr 2 O^5003 AlC 2 O 3 CH Ethy lene 2 CH 2 H 2 o
C 3 H 8 C 2 H 4 CH 4 or C 3 H 6 H 2
This reaction is of great importance to petroleum industry. (v) Isomerisation :
Isobutane
3 (^3) - Butane (^223200) , 35 3 3
CH
CH CHn CHCH AlClo (^) (^) C HClatm CH CHCH
(vi) Aromatisation :
(vii) Step up reaction
(a) Reaction with CH 2 N 2 (Diazo methane) :
R CH 2 H CH 2 N 2 hv ^ R CH 2 CH 2 H
(b) Reaction with CHCl 3 / NaOH :
R CH 2 H CHCl : CCl^^3 / 2 OH R CH 2 CHCl 2
(c) Reaction with
O
CH C
(^2) : 2 / / 2 3 2 || R CH H CHCH CCO R CH CH
O (viii) HCN formation : 2 CH (^) 4 N^^2 / electric arc 2 HCN 3 H 2 or CH (^) 4 NH 3 700 Al^2 O (^) o (^) C^3 HCN 3 H 2 (ix) Chloro sulphonation/Reaction with SO 2 + Cl 2
CH (^) 3 CH 2 CH 3 SO 2 Cl 2 u. v light
CH 3 CH 2 CH 2 SO 2 Cl HCl
This reaction is known as reed’s reaction. This is used in the commercial formation of detergent. (x) Action of steam : CH (^) 4 H 2 O Ni 800 / Al o^2 (^) C ^ O ^3 CO 3 H 2
Individual members of alkanes
(1) Methane : Known as marsh gas. (i) Industrial method of preparation : Mathane gas is obtained on a large scale from natural gas by liquefaction. It can also be obtained by the application of following methods, (a) From carbon monoxide : A mixture of carbonmonoxide and hydrogen is passed over a catalyst containing nickel and carbon at 250 oC when methane is formed. CO 3 H 2 250 Ni (^) oC (^) C CH 4 H 2 O (b) Bacterial decomposition of cellulose material present in sewage water : This method is being used in England for production of methane.
( C 6 Cellulose H 10 O 5 ) n nH 2 O 3 nCH 4 3 nCO 2
(c) Synthesis : By striking an electric arc between carbon electrodes in an atmosphere of hydrogen at 1200o C , methane is formed. C 2 H 2 1200 C CH 4 o By passing a mixture of hydrogen sulphide and carbon disulphide vapour through red hot copper, methane is formed. CS (^) 2 2 H 2 S 8 Cu High temperatur e CH 4 4 Cu 2 S (ii) Physical properties (a) It is a colourless, odourless, tasteless and non-poisonous gas. (b) It is lighter than air. Its density at NTP is 0. g/L. (c) It is slightly soluble in water but is fairly soluble in ether, alcohol and acetone. (d) Its melting point is ^182.^5 oC and boiling point is 161. 5 oC. (iii) Uses (a) In the manufacture of compounds like methyl alcohol, formaldehyde, methyl chloride, chloroform, carbon tetrachloride, etc. (b) In the manufacture of hydrogen, used for making ammonia. (c) In the preparation of carbon black which is used for making printing ink, black paints and as a filler in rubber vulcanisation.
2 - Methyl pentane heat
AlCl ^^3 HCl
2, Dimethyl butane
C atm
CrO AlO 600^ o^ / 15 ^2 3 /^ ^2 ^3 + 4 H 2 CH (^2) Benzene
CH 2
H 2 C H 2 C n-Hexane
CH 3 CH 3
C
CrO AlO 600^ o
^2 3 /^ ^2 ^3 ^ H ^2
n- Heptane
Methyl cyclo Hexane
CH 3
Toluen e
CH 3
(d) As a fuel and illuminant. (2) Ethane (i) Methods of preparation (a) Laboratory method of preparation :
C Ethyl 2 H iodide 5 I 2 H ZnC^ 2 CuH 5 coupleOH C Ethane 2 H 6 HI
(b) Industrial method of preparation : CH Ethy lene(ethene) (^) 2 CH 2 H (^2300) CCH Ethane 3 CH 3 Ni o
(ii) Physical properties (a) It is a colourless, odourless, tasteless and non-poisonous gas. (b) It is very slightly soluble in water but fairly soluble in alcohol, acetone, ether, etc. (c) Its density at NTP is 1.34 g/L (d) It boils at – 89 o C. Its melting point is – 172 o C. (iii) Uses (a) As a fuel. (b) For making hexachloroethane which is an artificial camphor. (3) Interconversion of Alkanes Ascent of alkane series, (i) Methane to ethane : Heatwith inether Ethane^3 3 Wurtzreaction Methane^4 CH UVCl ^^2 CHCl (^) Na CH CH (ii) Butane from ethane : Heatwith inether^2 Butane^5 Wurtzreaction (Ethaneexcess)^2 6 Ethyl^2 chloride^5
C H UVCl ^^2 CHCl Na CH CH
Descent of alkane series : Use of decarboxylation reaction is made. It is a multistep conversion. Ethane to methane Acetaldehy^3 de
[] Ethyl^2 alcohol^5
. (excess)Ethane^2 6 Ethyl^2 chloride^5
C H ClUV ^^2 CHCl Aq KOH CHOH O CHCHO
heat Methane^4
/ Acetic^3 acid Sodium^3 acetate
[^ O ]^ CH COOH NaOH CHCOONa NaOH CaO CH
Higheralkane ClUV ^2 halideAlkyl KOH Aq .^ Alcohol [ O ] Aldehyde [ O ] Acid NaOH Sodiumtheacidsaltof NaOH (^) heat/^ CaO Loweralkane
Alkenes
These are the acyclic hydrocarbon in which carbon-carbon contain double bond. These are also known as olefins, because lower alkene react with halogens to form oily substances. General formula is
Cn H 2 n. Examples, C 2 H 4 , C 3 H 6 , C 4 H 8.
(1) Preparation methods (i) From Alkynes :
R
H
C
H
R C C R H Pd BaSO R C |
. 4 2 Lindlar'sCataly st
Poison’s catalyst such as BaSO 4 , CaCO 3 are
used to stop the reaction after the formation of alkene. (ii) From mono halides :
H
H
C
H
H AlcKOH R C
H
X
C
H
H
R C HX
Alkene
If we use alc. NaOH in place of KOH then
trans product is formed in majority because of its stability. According to saytzeff rule. (iii) From dihalides (a) From Gem dihalides
If we take two different types of gemdihalides then we get three different types of alkenes. Above reaction is used in the formation of symmetrical alkenes only. (b) From vicinal dihalides :
300 2
| H ZnX
H
C
H
H Zndust R C
H
X
C
H
X
R C o (^) C
Alkene is not formed from 1, 3 dihalides. Cycloalkanes are formed by dehalogenation of it. |^2 ^2 |^ H^2 ^ Zn^ dust X
H CH C
X
C
2 2
2 HC CH
CH ZnX 2
(iv) By action of NaI on vicinal dihalide :
Br C
Br C | |
acetone NaI
I C
I C | |
I 2 C C
(v) From alcohols [Laboratory method] :
CH Ethyl 3 CH alcohol 2 OH H^2 SO^ 4434 orK H^3 PO ^4 CH Ethene 2 CH 2 H 2 O
(vi) Kolbe’s reaction :
CO H KOH
CH
CH
HO
CHCOOK
CHCOOK
Ethene^2
Electroly sis^2 2 Potassium^2 succinate
2
(vii) From esters [Pyrolysis of ester] :
2 2
3 .
Glasswool 450 2 2
3 | | (^2) CH CH
CH COOH
CH CH
CH CO O H
liq N
o
R – CH + + CH – R 2 ZnX 2 R – CH = CH – R
X Zn X X Zn X
vic dihalide unstable alkene
(v) Birch reduction : This reaction is believed to proceed via anionic free radical mechanism.
R CH CH 2 Na e ^ R CH CH 2 Et O ^ H R CH CH 3
Na e^ R CH CH 3 Et . O H R CH 2 CH 3 (vi) Halogenation
CH CH CH Cl o C ClCH CH CH HCl
or 3 - Chloro-Allylchloride^1 - propene
(^3) Propene 2 2 500 2 2 If NBS [N-bromo succinimide] is a reagent used for the specific purpose of brominating alkenes at the allylic position.
In presence of polar medium alkene form vicinal dihalide with halogen.
Vicinaldihalide
H
X X
HH
H X X R C C
H H
R C C CCl ^
Reactivity of halogen is F 2 Cl 2 Br 2 I 2
(vii) Reaction with HX [Hydrohalogenation]
alkene Alky lhalide
X
C
H
C C HX C
According to markownikoff’s rule and kharasch effect.
H
H H
BrH
CH CH CH HBr CH C C
According to Anti Markownikoff rule (Based on F.R.M.) CH 3 CH CH 2 HBr Peroxide
(major)
3 (minor)
3
| | H
H H
H Br
H CH C C
H H
Br H
CH C C
(viii) Reaction with hypohalous acids : CH Ethy lene (^) 2 CH 2 HOCl CH Ethy lene 2 OH chlorohy dr. CH 2 Cl in
In case of unsymmetrical alkenes markownikoff rule is followed. (ix) Reaction with sulphuric acid :
CH Ethylene 2 CH 2 H HSO 4 Ethyl CH hydrogen 3 CH 2 HSO sulphate 4
CH (^) 3 CH 2 HSO 4 CH 2 CH 2 H 2 SO 4 This reaction is used in the seperation of alkene from a gaseous mixture of alkanes and alkenes. (x) Reaction with nitrosyl chloride NO C Cl
C C NOCl C
( NOCl is called
Tillden reagent) If hydrogen is attached to the carbon atom of product, the product changes to more stable oxime. H
NO
C
Cl
C
Oxime
C NOH
Cl
C
C
C
Cl
C
NO
NOCl C C
C
C C
(Blue colour)
(xi) Oxidation : With alkaline KMnO 4 [Bayer’s
reagent] : This reaction is used as a test of unsaturation.
gly col
| | | | [ ] | | 4 H
H H
HO OH
H O H OH R C C
H H R C C Alk^ KMnOOH
With acidic KMnO 4 :
O H CO HO
O
H O R C
H
C
H
R C KMnOacidic 2 2
[ ]
(xii) Hydroxylation (a) Using per oxy acid :
Trans(racemic)
3
3
,
2 - Butene
3
3
22
CH
CH
HO C H
H C OH
CH
CH
H C
H C HorOHCOHCOOHH
(b) Hydroxylation by OsO 4 :
| CC | OsO 4 NaHSO 4 I
CH 2 – CH = CH 2 +
Br
CH 2 – CO
CH 2 – CO
N – H
Succinimid e Allyl bromide
CH 3 CH=CH 2 +
CH 2 – CO
CH 2 – CO
N – Br Propene^ NBS
H
HO
OH
H
R
R
R H
H^ R
Trans
If per benzoic acid or peroxy acetic acid is used then oxirane are formed. or C^ CHH COCO HH R ^ H O OH
CH
OH
R CH CH R^65333 R CH | |^2
[Oxirane]
R
O
R CH CH
(xiii) Combustion :
C n H 2 n ^32 n^ O 2 nCO 2 nH 2 O
They burn with luminous flame and form explosive mixture with air or oxygen. (xiv) Ozonolysis
I
C C O ^3
Application of ozonolysis : This process is quite useful to locate the position of double bond in an alkene molecule. The double bond is obtained by Joining the carbon atoms. of the two carbonyl compounds. (xv) Oxy – mercuration demercuration : With mercuric acetate (in THF), followed by reduction with
NaBH 4 / NaOH is also an example of hydration of alkene
according to markownikoff’s rule. ( CH (^) 3,3- 3 ) 3 dimethyl C CH - 1 - butene CH 2 ( CH Mercuric 3 COO acetate ) 2 Hg
3 , 3 Dimethy l 2 butanol
/ 33 3 3
CH
OH
Hg CH C CH OCOCH
CH C CH CH NaBHTHFNaOH
(xvi) Epoxidation
(a) By O 2 / Ag :
O
CH (^) 2 CH 2 ^12 O 2 Ag CH 2 CH 2 (b) Epoxidation by performic acid or perbenzoic acid :
2 2 2 CH 2
O
CH CH CH
3 2 3 2
|| CH
O
CH CH CH HCOOH CH CH
O (xvii) Hydroboration
3 R CH CH 2 BH 3 ( R CH Trialky l bora 2 CH ne 2 ) 3 B H^2 O^2 /^ OH
R CH 2 CH 2 OH B ( OH ) 3
(Anti markownikoff’s rule) (xviii) Hydroformylation :
H
C O
H
C H
H
H
R CH CH CO H CoHCO R C
2 2 ( )^4
If CO H 2 O is taken then respective acid is
formed.
COOH
R CH CH CO HO CoHCO R CH CH (^2) | 2
2 2 ()^4
(xix) Addition of formaldehyde
H 2 C O H [ H 2 C O H H 2 C OH ]
H
RCHCH 2 R CH CH 2 CH 2 OH HOH
1 , 3 diol
/ 2 2
OH
CH CH
OH
HCHOH R CH
(xx) Polymerisation
n
highpressure
TraceO Catalyst
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
H
C
H
o
1500 / 2
If in polymerisation zeigler- natta catalyst
[( R ) 3 Al TiCl 4 ] is used then polymerisation is known as
zeigler-natta polymerisation. (xxi) Isomerisation :
CH 3 CH 2 CH 2 CH CH 2
CH 3 CH 2 CH CH CH 3
The mechanism proceeds via carbocation. (xxii) Addition of HNO 3 : CH (^) Ethene 2 CH 2 HO NO 2 CH 22 OH - Nitroethan. CH (^2) ol NO 2 (xxiii) Addition of Acetyl chloride : CH (^) Ethene 2 CH 2 CH 3 COCl CH 4 2 - Chlorobuta ClCH 2 COCH none- 2 3 (4) Uses (i) For the manufacture of polythene – a plastic material; (ii) For artificial ripening of fruits; (iii) As a general anaesthetic; (iv) As a starting material for a large number of compounds such as glycol, ethyl halides, ethyl alcohol, ethylene oxide, etc; (v) For making poisonous mustard gas (War gas); (vi) For making ethylene-oxygen flame.
Alkynes
O O
C C C + C
O O
O
Ozonide
H^^2 O / IIH / Zn ZnO
O || C H – O – O – |
AlCl 3
H
2
C CH 2
O
R –
CH
Cyclic acetal
C
H
2
O
Explanation for the acidic character : It explained by sp hybridisation. We know that an
electron in s orbital is more tightly held than in a p -
orbital. In sp hybridisation s - character is more (50%)
as compared to sp^2 (33%) or sp^3 (25%), due to large
s^ - character the carbon atom is quite electronegative. (ii) Reaction with formaldehyde 2 2 2 / ^3 | | 2 LiNH OH
CH
OH
HC CH CHO CH C C
CH CH CHOH
OH
CH | (^) 2 2 [Trans-product ]
(4) Chemical properties of acetylene Oxidative–Hydroboration : Alkynes react with
BH 3 (in THF) and finally converted into carbonyl
compounds. 3 CH (^) (^3) Propy ne C CH BH ^^3 / THF ( CH 3 CH CH ) 3 B HOH^2 O ^2
CH 3 CH CHOH CH (Propanal) 3 CH 2 CHO
Tautomeris es
or 3 3
4
2 4 CH
O
HgSOH^ SO CH C (Acetone) Thus it is useful for preparing aldehyde from terminal alkyne. Reduction of Alkyne : Alkynes add on hydrogen in presence of suitable catalysts like finely divided Ni, Pd. CH CH H 2 Ni^ CH 2 CH 2 H Ni 2 CH 3 CH 3 If the triple bond is not present at the end of the carbon chain of the molecule, the alkene formed may be cis and trans depending upon the choice of reducing agents.
With Na / NH 3 or Li / NH 3 in (liquid ammonia)
trans alkene is almost an exclusive product while catalytic reduction at alkyne affords mainly cis alkenes.
42 ^ / ^3
.(/Lindlarcat/aly stquinoline)
Li NH Pd BaSO
H cis
R C C R
H
R
C C
H
R
Trans^ R
H
C C
H
R
Red hot tube
Chloroprene
NH 3 S/H 2 S H^ 40% 2 SO 480 /1 %HgSO o C 4
Hg2+, 80 o C
with CH 3 COOH
CH Acety lene CH
C Benzene 6 H 6
C (^) 4 H Py rrole 5 N H 2
Thiophene C^4 H^4 S
CH 3 CHO
Acetic^3 anhy dride
3
Acetaldehy^3 de CHCO O
CH CHO CHCO
Cl 2 CHCl^2 CHCl 2
(Westron)
CHCl 2 CHCl 2
(Westroso l)
Alc. KOH
AlCl 3
ClAsCl 2^ CHCl CHAsCl 2
Lewisite
(Cadet and Busen reaction)
Hg +
CH 3 COOH
HC (^) Sol. CNa Acety lide XR HC Higheralky nes C R
CH (^) 2 Vinyl CHOOCCH acetate 3
60 oC
Hg 2+ /HCl CH Viny l (^) 2 chloride CHCl
Ba(CN) 2
HCN CH Viny l (^) 2 cy anide CHCN
Cuprene
Hexa C chloro^2 Cl ethane^6 (Artifici camphor)al
Lindlar’s Catalyst Ethy lene C^2 H^4 ( Cis ) Ni
300° C C Ethane^2 H^6
Na
NH 4 Cl
Cu 2 Cl 2 , HCl Cu 2 O
Cataly st Lindlar
C C – C C Li / NH ^3
Trans
Cis
Degree of unsaturation : The number of degree of unsaturation in a hydrocarbon is given by
2
2 n 1 2 n 2 , Where n 1 is the number of carbon
atoms; n 2 is the number of hydrogen atoms.
For example in C 6 H 12 , the degree of unsaturation
is^2 ^6 22 ^12 1 Tests of unsaturation
(a) Baeyer’s reagent : It is 1% KMnO 4 solution
containing sodium carbonate. It has pink colour. An aqueous solution of the compound, a few drops of Baeyer’s reagent are added, the pink colour of the solution disappears. The decolourisation of pink colour indicates the presence of unsaturation in the compound. Alkene without any hydrogen atom on the
carbon forming the double bond
R
R
C C
R
R
don't
show this test. (b) Bromine- carbon tetrachloride test : The compound is dissolved in carbon tetrachloride or chloroform and then a few drops of 5% bromine solution in carbon tetrachloride are added to it, the colour of bromine disappears. It indicates the presence of unsaturation. This test also fails in the case of alkene of the
R
R
C C
R
R
(5) Uses (i) Acetylene is used as an illuminant. (ii) It is used for the production of oxy-acetylene flame. The temperature of the flame is above 3000 oC. Is is employed for cutting and welding of metals. (iii) Acetylene is used for artificial ripening of fruits. (iv) It is used as a general anaesthetic under the name naracylene. (v) Acetylene has synthetic applications. It serves as a starting material for the manufacture of a large variety of substances. (vi) On electrical decomposition acetylene produces finely divided carbon and hydrogen. Hydrogen is used in airships. C (^) 2 H 2 2 C H 2 (6) Interconversion
(i) Conversion of ethane into ethene : (Alkane into alkene) Ethene^2
. Ethane^3 3 Ethyl brom^25 ide
CH CH^2 CHBr CH CH
KOH
Alc hv
Br ^
(ii) Ethene into ethane : (Alkene into alkane) Ethene^2 2 ,^300 Ethane^3
CH CH^2 CH CH
Ni C H o (iii) Ethane into ethyne (acetylene) : i.e., alkane into alkyne 4 2 2 Ethene^2 3 2. Ethane^3 3 CCl
Br KOH
Alc hv CH CH Br ^ CHCHBr CH CH
or Ethyne
.
CH 1, 2 2 - Dibromoeth Br CH ane 2 Br NaNH 2 CH CH
Alc^ KOH
(iv) Ethyne into ethane : (Alkyne into alkane) Ethy ne , (^300) Ethene^2 2 , (^300) Ethane^3
CH CH^2 CH CH^2 CH CH
Ni C
H Ni C
H o o (v) Ethene into propene : Ascending in alkene series Reduction [] (Ethy lPropanecy anide)^ nitrile
CH Ethene 2 CH 2 HI ^ CH Iodoethane 3 CH 2 I KCN CH 3 CH 2 CN H
13 - Aminopropa^22 ne^231 Propanol^22
2
CH CHCH NH CHCH CHOH
HNO
CH 3 CH Propene CH 2 KOHAlc .^ CH 1 3 Bromopropa CH 2 CH ne 2 Br ^ PBr 3
or (^3) Propane 2 3 ( ) Ethene^2 2 Iodoethane^32 CH CH HI ^ CHCHI LiCH 3 ^2 Cu CHCHCH
(^3) Propene 2 . 1 - Chloro^3 propane^22
2 CH CHCHCl CHCH CH
KOH
Alc hv
Cl ^
or CH (^) 2 CH 2 HI ^ CH 3 CH 2 I CH 3 I / Na CH (^3) Propane CH 2 CH 3
(^3) Propene 2 . 1 - Chloro^3 propane^22
2 CH CHCHCl CHCH CH
KOH
Alc hv
Cl ^
(vi) Propene into ethene : Descending an alkene series 4
3 2 [] Ethanal^3
/ (^3) Propene 2 LiAlH
CH CH CH O^ H O CHCHO H
Ethanol^3 2170 Ethene^2
CH CHOH^2 4 CH CH
C HSO (^) o (vii) Acetylene into propyne (methyl acetylene) : (Ascent) Acety lene Monosodiumacety lide Propy ne 3 CH CH Na ^ CH CNa CH ^3 I CH C CH
(viii) Propyne into acetylene : (Descent)
CH Propy ne 3 C CH Lindlar' scataly st CH 3 Propy lene CH CH 2 O^3 / H^2 O
Acety lene
. Ethy lidenechloride CH Acetaldehy (^) 3 CHO de PCl ^^5 CH 3 CHCl 2 KOH Alc CH CH
(ix) 1 - Butyne into 2-pentyne : (Ascent)
Ammonical Cu 2 Cl 2 – – Red precipitate
(Red)
CCu
CCu CuCl NHOH CH
CH
+ 2NH 4 Cl + 2H 2 O Ammonical silver nitrate
C Ag
C Ag AgNO NHOH CH
CH
|| | 2 3 2 4 ||| + 2NH 4 Cl + 2H 2 O Cycloalkane (1) Methods of preparation (i) From dihalogen compounds (Freund reaction) :
(ii) From alkenes :
Methy l cy clopropane^2
(^3) Propene 2 22 alloy 3 2
CH
CH CH CH CHI Zn^ Cu CH CH CH
(iii) From Aromatic compounds
(2) Physical properties (i) First two members are gases, next three members are liquids and higher ones are solids. (ii) They are insoluble in water but soluble in alcohol and ether. (iii) Their boiling points show a gradual increase with increase of molecular mass. Their boiling points are higher than those of isomeric alkenes or corresponding alkanes. (iv) Their density increase gradually with increase of molecular mass. (3) Chemical properties : Cycloalkanes behave both like alkenes and alkanes in their chemical properties. All cycloalkanes undergo substitution reaction with halogen in the presence of light (like alkane). All cycloalkane (lower members) undergo
addition reaction (ex. Addition of H 2 , HX , X 2 ). Further
the tendency of forming addition compounds decreases with increase in size of ring cyclopropane >
Cyclobutane > Cyclopentane. Relative ring opening of ring is explained by Baeyer strain theory. (i) Addition in spiro cycloalkane : If two cycloalkane fused with one another then addition take place in small ring
Because small ring is more unstable than large ring Higher cycloalkanes do not give addition due to more stability. (ii) Free radical substitution with Cl 2 Cl HCl CH
Cl CH CH CH
CH CH hv ^ Chlorocy cl^2 opropane
2 2 Cy clopropa^2 ne
2 2
(iii) Addition reaction
(iv) Oxidation
Cycloalkene
+ 3 H 2 under pres Ni ,^200 o sure C Benzene Cyclohexane
+ H 2
Spiro compound
H 2 C – CH 2
CH 2
Cyclopropane
BrH 2 C – CH 2 – CH 2 Br
Propane
CH 3 – CH 2 – CH 2 Br
CH 3 – CH 2 – CH 2 OH
CH 3 – CH 2 – CH 3
1 - Propanol
1 - Bromopropane
1, 3-Dibromopropane
Br 2
HBr
(i) Conc. H 2 SO 4 (ii) H 2 O
(CCl 4 ) dark
H 2 , Ni 80 oC
CH 2 CH 2 COOH
4 CH 2 CH 2 COOH
Alk KMnO Adipic acid
+ 5 [O]
H 2 C
H 2 C
CH 2
CH 2
Cyclohexane
CH 2
CH 2
CH 2
CH 2 Cl
CH 2 Cl
+2 Na 1, Dichloropropane
heat H 2 C (^) CH 2
CH 2
Cyclopropan e
+2 NaCl
Carbocyclic compounds with double bonds in the ring are called cycloalkenes. Some of the common cycloalkenes are
Cycloalkenes can be easily obtained by Diels- Alder reaction. These compounds undergo the electrophilic addition reactions which are characteristic of alkenes, while the ring remains intact. Cycloalkenes decolourise the purple colour of dilute
cold KMnO 4 or red colour of bromine in carbon
tetrachloride.
Dienes
These are hydrocarbon with two carbon-carbon double bonds. Dienes are of three types (1) Conjugated dienes : Double bonds are seperated by one single bond.
Ex : CH 2 CH CH CH 2 (1, 3-butadiene)
(2) Cumulative dienes : Double bonds are adjacent to each other.
Ex : CH 2 C CH 2 Propadiene [allene]
(3) Isolated or Non-conjugated : Double bonds are separated by more than one single bond.
Ex : CH 2 CH CH 2 CH CH 2 (1, 4 pentadiene)
The general formula is Cn H 2 n 2. The
predominant member of this class is 1, 3-butadiene. (1) Method of preparation (i) From acetylene :
4
2 4
2 2
2 Vinylacetylene 2 Pd / BaSO
H NHCl
HC CH Cu^ Cl HC C CH CH
CH (^) 2 1, CH 3 - Butadiene CH CH 2 (ii) From 1 , 4 - dichlorobutane :
(^2) 1, 3 - Butadiene 2 . 1,4^2 - Dichlorobu^2 tane^22
CH CH CH CH
Cl
CHCH CH
Cl
CH Alc^ KOH
(iii) From 1,4- butanediol :
(^2) 1, (^4) - Butanediol (^222) heat (^2) 1, 3 - Butadiene 2
| | 24 CH CH CH CH
OH
CHCH CH
OH
CH H^ SO
(iv) From butane : (^6002) 1, 3 - Butadiene^2
Catalyst
CH 3 CH n - Butane 2 CH 2 CH 3 o C CH CH CH CH
( Cr 2 O 3 used as catalyst.) (v) From cyclohexene :
CH (^) 2 1, CH 3 - Butadiene CH CH 2 CH (^2) Ethene CH 2 (2) Physical property : 1,3-butadiene is a gas. (3) Chemical properties (i) Addition of halogens :
(ii) Addition of halogen acids :
(iii) Addition of water :
(iv) Polymerisation :
nCH 2 1, 3 CHCH - Butadiene CH 2 Peroxide [ CH 2 CH Buna rubber CHCH 2 ] n
Diels-alder reaction :
Cyclobuten e
Cyclopenten e Cyclohexen e
1, 4- Cyclohexadiene
1
6
2 3
4
5
+ Br 2 Cyclopentene Br
Br
1, 2-Dibromo cyclopentane +O + H 2 O Cyclopent- 1 - ene (^) Cyclopentane 1,2- diol
OH
OH
KMnO 4 (aq.)
O
O O
(Cyclohexene)
+ O 3 H^2 O^ CH 2
CH 2
CHO
CH 2
CH 2
CHO
CH 2 BrCHBrCH=CH 2 Addition) predominates (62%)^ 3,4-Dibromo-^1 - butene (1, 2- in non-ionising solvent (hexane) CH 2 BrCH=CH.CH 2 Br Addition) predominates (70%)^ 1,4-Dibromo-^2 - butene (1, 4- in an ionising solvent (acetic acid)
CH 2 = CHCH = CH 2 + Br 2 1, 3-Butadiene CCl
4
CH 2 =CH–CH=CH 2 +HBr
CH 3 CHBrCH=CH 2 3 -^ (1, 2Bromo-Addition)- 1 - butene (Major yield at low temp.) CH 3 – CH=CH–CH 2 Br 1 -^ (1, 4Bromo-Addition)- 2 - butene (Major yield at high temp.)
CH 2 =CH–CH=CH 2 +H 2 O
CH 3 CHOHCH=CH 2
But- 3 - en- 2 - ol
CH 3 CH=CHCH 2 OH But – 2 - en- 1 - ol
HC
HC
CH 2
CH 2
CH 2
CH 2
Cyclohexene (Adduct)
200 o C
H – C
H – C
CH 2
CH 2
CH 2
CH 2
1, 3-Butadiene
Ethene (Dienophil e)
Similarly cyclolpentadienyl anion or tropylium ion
are also aromatic because of containing 6 electrons
( n =1).
Hetrocyclic compounds also have 6 electrons ( n
Molecules do not satisfy huckel rule are not aromatic.
(4) Antiaromaticity : Planar cyclic conjugated species, less stable than the corresponding acyclic unsaturated species are called antiaromatic. Molecular orbital calculations have shown that such compounds
have 4 n electrons. In fact such cyclic compounds
which have 4 n electrons are called antiaromatic compounds and this characteristic is called antiaromaticity. Example : 1,3-Cyclobutadiene, It is extremely
unstable antiaromatic compound because it has^4 n
electrons ( n 1 )and it is less stable than 1,3 butadiene by about 83.6 KJ mol –^1.
4 n 4 ;
n 44 1 Thus, cyclobutanediene shows two equivalent
contributing structures and it has n 1.
Benzene ( C 6 H 6 )
Benzene is the first member of arenes. It was first discovered by Faraday (1825) from whale oil. Mitscherllich (1833) obtained it by distillating benzoic acid with lime. Hofmann (1845) obtained it from coal tar, which is still a commercial source of benzene. (1) Structure of benzene : Benzene has a special structure, which is although unsaturated even then it generally behave as a saturated compound. (i) Kekule's structure : According to Kekule, in benzene 6-carbon atoms placed at corner of hexagon and bonded with hydrogen and double bond present at alternate position. (a) Evidence in favour of Kekule's structure Benzene combines with 3 molecules of hydrogen or three molecules of chlorine. It also combines with 3 molecules of ozone to form triozonide. These reactions confirm the presence of three double bonds. Studies on magnetic rotation and molecular refraction show the presence of three double bonds and a conjugated system. The synthesis of benzene from three molecule of acetylene also favour's Kekule's structure.
3 CH CH
Benzene gives cyclohexane by reduction with hydrogen.
C (^) 6 H 6 3 H 2 O Ni
(b) Objections against Kekule's formula Unusual stability of benzene. According to Kekule, two ortho disubstituted products are possible. But in practice only one ortho disubstituted product is known. Heat of hydrogenation of benzene is 49. kcal/mole, whereas theoretical value of heat of hydrogenation of benzene is 85.8 kcal/mole. It means resonance energy is 36 kcal/mole.
N
Pyrrole^ H
. O.
Furan
. S.
Thiophen e
Pyridine^ N
Cyclopentadien e 4 electrons
Cyclopentadienyl cation 4 electrons
H
Cyclooctatetraene 8 electrons
H .
Cyclopropenyl anion 4 electrons
Tropyllium ion 6 electrons ( n= 1)
H
H
H H
H
H
H
Cyclopropenyl cation ( n = 0)
Cyclopentadienyl anion 6 electrons ( n= 1)
H H
H H
H
Cyclopentadienyl 4 electrons
Cyclopropenyl anion 4 electrons
Cycloctatetraene 8 electrons Cycloheptatrienyl anion 8 electrons
Cyclohexane
C C bond length in benzene are equal,
(although it contains 3 double bonds and 3 single bonds) and are 1.39 Å. Kekule explained this objection by proposing that double bonds in benzene ring were continuously oscillating between two adjacent positions.
(2) Methods of preparation of benzene (i) Laboratory method :
(ii) From benzene derivatives (a) From phenol :
(b) From chlorobenzene :
(c) By first preparing grignard reagent of chlorobenzene and then hydrolysed
Cl C HCl Mg ^ CHMgCl H O CH Mg OH Chlorobenz^6 5 ene dryether Pheny l^6 chloridemagnesium^5 Benzene^66
2
(d) From benzene sulphonic acid :
(e) From benzene diazonium chloride :
(f) From acetylene :
Cyclic polymerisation takes place in this reaction. (g) Aromatisation : Benzene^662 at high^500 pressure
C 6 H 14 2 3 /^23 CH 4 H
C
CrO AlO n Hexane ^ ^ ^ (3) Properties of benzene (i) Physical properties (a) Benzene is a colourless, mobile and volatile liquid. It's boiling point is 80° C and freezing point is 5.5° C. It has characteristic odour. (b) It is highly inflammable and burns with sooty flame. (c) It is lighter than water. It's specific gravity at 20°C is 0.8788. (d) It is immiscible with water but miscible with organic solvents such as alcohol and ether. (e) Benzene itself is a good solvent. Fats, resins, rubber, etc. dissolve in it. (f) It is a non-polar compound and its dipole moment is zero. (g) It is an extremely poisonous substance. Inhalation of vapours or absorption through skin has a toxic effect. (ii) Chemical properties : Due to the presence of
electron clouds above and below the plane benzene
ring, the ring serves as a source of electrons and is easily attacked by electrophiles (Electron loving reagents). Hence electrophilic substitution reaction are the characteristic reactions of aromatic compounds. Substitution reactions in benzene are prefered rather than addition are due to the fact that in the former reactions resonance stabilised benzene ring system is retained while the addition reactions lead to the destruction of benzene ring. Principal reactions of benzene can be studied under three heads, (a) Addition reactions (b) Substitution reactions (c) Oxidation reactions (a) Addition reactions : In which benzene behaves like unsaturated hydrocarbon. Addition of hydrogen : Benzene reacts with hydrogen in the presence of nickel (or platinum) as catalyst at 150°C under pressure to form cyclohexane.
COONa
Sodium benzoate
CaO
+ NaOH heat
Benzene
Cl
Chlorobenzen e
Benzene
+ (^) 2 H Ni- NaOHAl^ alloy + HCl
OH
Phenol Benzene
+ Zn distill + ZnO
N 2 Cl
- 2 H (^) NaOHSnCl^2 Benzene
+N 2 +HCl
+ HC
HC
HC
CH
CH
+^ HC
Three molecules of acetylene
Benzene
red hot 1500 tube- 2000°C
Benzene sulphonic acid
SO 3 H
Benzene
150°- 200°C HCl,pressu re
+ HOH +H 2 SO 4
Steam
Benzene (^) Cyclohexane
3 H (^2) 150°C,press Ni ure
+ or C 6 H 12
