IIT-JEE CHEMISTRY (Hydrocarbons) THEORY ALKANE 1. General introduction (a) These are the hydrocarbons in which carbon-Carbon contains single bond. (b) These are also called as ’Paraffins’ (Parum + Affinis i.e., less reactive). (c) General formula is C n H 2n+2 . (d) Hybridisation state of carbon is sp 3 . (e) Geometry of carbon is tetrahedral. (f) Bond angle is 109 o 28 ’ (g) C – C bond energy is 80 kcal/mole, while C – H bond, bond energy is 97 kcal/mole. (h) The molecules of alkanes are angular, so carbon chains in these molecules are zig-zag type (not straight), which may be branched or unbranched as shown below. Methods of preparation of alkanes: From alkene: (i). By catalytic hydrogenation of unsaturated hydrocarbon: Addition of H 2 on alkene and alkyne takes place in cis manner in the presence of Pt or Ni or Pd to give alkane. eg., (i) (ii) (Branched) (Branched) (Unbranched) C = C R R D D D D H R H R Meso + H2 R D C = C R R D D + H2 D D H R H D H R H R
Transcript
IIT-JEE CHEMISTRY(Hydrocarbons)
THEORY
ALKANE1. General introduction
(a) These are the hydrocarbons in which carbon-Carbon contains single bond.(b) These are also called as ’Paraffins’ (Parum + Affinis i.e., less reactive).(c) General formula is CnH2n+2.(d) Hybridisation state of carbon is sp3.(e) Geometry of carbon is tetrahedral.(f) Bond angle is 109o 28’
(g) C – C bond energy is 80 kcal/mole, while C – H bond, bond energy is 97 kcal/mole.(h) The molecules of alkanes are angular, so carbon chains in these molecules are zig-zag type (not straight), which may be branched or unbranched as shown below.
Methods of preparation of alkanes:From alkene:(i). By catalytic hydrogenation of unsaturated hydrocarbon: Addition of H2 on alkene and alkyne takes place in
cis manner in the presence of Pt or Ni or Pd to give alkane. eg.,
(i)
(ii)
Note: This reaction is exothermic and heat of hydrogenation depend upon no. of ‘’ hydrogen.
(ii). Wurtz’s Reaction: When alkyl halide reacts with sodium in presence of dry ether then we get higher alkane. Mechanism of the reaction is based on ionic and free radical both.
(iii).Frankland’s reaction/method: If alkyl halide is treated with Zn dust in closed tube then higher symmetrical alkanes will be formed.
2RX + Zn R R + ZnX2
(Branched) (Branched) (Unbranched)
C = C
R R
D D
D
D
H
R H
R
Meso
+ H2
R D
C = C
R
R
D
D+ H2
D
D
H
R H
D
H
R H
R
(iv). Corey- House synthesis:
(v). From Grignard’s reagent:(a) By action of acidic ‘H’: When GR reacts with acidic hydrogen compounds, then respective alkanes are formed.
Acidic hydrogen compounds are H– O– H, R – OH, C6H5 – O – H,
(b) By reaction with alkyl halide: eg., R X + R’MgX R R’+ MgX2
(vi). By reduction of aldehydes, ketone, alcohol and carboxylic acids: When these are reduced with HI in presence of red phosphorus alkanes are formed having the same no. of carbon atom as in parent.
Clemmensen reduction:
Reaction is called “Clemmensen reduction”(vii). From sodium salt of carboxylic acid: When sodium salt of carboxylic acid is heated with soda lime, then we
get a less carbon alkane. The reaction is called decarboxylation.
Ex. 1 The no. of isomeric sodium salt that is required to obtain isopentane.(a) 4 (b) 3 (c) 2 (d) 1
N – H, R – NH
H
H
HX
OHCl
OCH3
Ex. 2 In Wurtz reaction if we take CH3Cl and C2H5Cl then product will be(a) Propane + Ethane (b) Propane(c) Propane + Ethane + Butane + Ethane + CH4 (d) Propane + Butane
Ex. 3 In Wurtz’s reaction if we take ethyl chloride and isopropyl chloride, the no. of alkene will be.(a) 1 (b) 2 (c) 3 (d) 4
Ex. 4 , The ratio of molecular weight of A and B is
(a) 1 (b) 2 (c) 3 (d) 4Ex. 5 Which of the following is wrong
(a)
(b)
(c)
(d)
Ex. 6 Which of the following method can not be used for preparation of CH3 CH3.
(a) (b)
(c) (d)
Ex. 7. Which of the following is not suitable for preparation of propane?
(a) (b)
(c) (d)
Physical Properties:(a) Alkanes are colourless, odourless and tasteless(b) These are insoluble in water and soluble in organic solvents(c) Physical state:
Alkanes
(d) Alkanes are lighter than water, so it floats over water.(e) Boiling point: as no of carbon atom increases in normal alkane boiling point increases in case of isomeric alkane more branching lesser will be the B.P. (f) Melting point: alternation effect is shown here.
H Ni2 / C = C
C2H5
CH3 CH3
C2H5
CH3
C2H5
C2H5CH3
H
H
Ex. 8 Which of the following has maximum melting point(a) C4H10 (b) C5H12 (c) C6H14 (d) C7H16
Ex.9 Which of the following has maximum boiling point(a) C4H10 (b) C5H12 (c) C6H14 (d) C7H16
Chemical Properties:(i) Halogenations: Alkanes form alkyl halides with halogen
R – H + X – X R – X + HXIn this reaction the reactivity order of halogen is
F2 > Cl2 > Br2 > I2
Fluorine can react in dark. Cl2 and Br2 require light energy. I2 does not show any reaction at room temperature, but on heating it shown iodination.(a) When chlorine reacts with methane in presence of benzoyl peroxide the product is CCl4, because in presence
of benzoyl peroxide chlorine does not require light energy.(b) When methane reacts with chlorine with excess O2, the reaction is stopped because oxygen reduces the
reactivity of alkyl group and changes it into peroxide radical.(c) The reaction is reversible with iodine because by product HI is a strong reducing agent.
CH4 + I2 CH3I + HIIodination of methane is done in presence of oxidizing agents such as HNO3/HIO3/HgO which neutralizes HI.
(ii). Nitration: When alkane is heated with nitrating mixture at 250oC, then nitro alkanes are formed.Nitrating mixture: (i) (Conc. HNO3 + Conc. H2SO4) at 250oC (ii) [HNO3 vapour at 400o -500oC]
(iii). Chlorosulphonation / reaction with SO2 and Cl2: The reaction is also called as Reed’s reaction. When propane reacts with SO2 and Cl2 in presence of ultraviolet light then propyl sulphonyl chlorides are formed.
(iv). Isomerisation: lower alkanes are not isomerised but butane or higher number of alkanes if heated with aluminium chloride at high temperature then they convert into stable isomers by the rearrangement reaction. Isomerisation is also held by heating alkane with –[AlX3 + HX ; X =Cl, Br, I or Al2(SO4)3 + H2SO4] at 200oC.
(v). Pyrolysis: (Thermal decomposition or cracking)According to the name, break down of a compound due to temperature is known as Pyrolysis. When alkane is heated at high temperature then it forms unsaturated hydrocarbon alkene and lower hydrocarbon alkane, The reaction is based on free radical mechanism.
(vi). Aromatization: When six carbon or more than six carbon alkane is heated with alumina (Al 2O3) and oxides of chromium or molybdenum or V2O5 at 600oC, then by the dehydrogenation and cyclization reaction respective aromatic hydrocarbons are formed.
CH2
CH2
CH2
CH2
CH3
CH3
n-hexane
Al2O3 + CrO3/MoO3
600oC CHCH
CH
Benzene
CH
CH
CH+ H2
If we take n-heptane
Ex 10. No. of required O2 mole for complete combustion of one mole of propane(a) 7 (b) 5 (c) 16 (d) 10
Ex 11. How much volume of air will be needed for complete combustion of 10 lit. of ethane(a) 135 lit. (b) 35 lit (c) 175 lit (d) 205 lit
Ex 12. In nitration of propane the main product will be(a) 1-nitro propane (b) 2-nitropropane(c) 1 & 2 (d) 1-nitro propane + 2-nitro propane + nitro methane + nitro ethane
Fractional distillation of PetroleumDistillation Boiling Point Comment
Natural gas Up to 303 K C1- C2 alkanes; used primarily in industry and as a fuel.
Light Petroleum and Ligroin Below 293 – 393 KC5 –C7 compounds; large amount are heated to crack them to small alkenes, a major feed stock for chemical industry.
Gasoline 373 – 473 KC7- C10 compounds; contain large fraction of straight chain alkanes, which tend to detonate or “knock” when burned; heating over catalyst break bonds, which reform to produce branched alkanes, which burn more smoothly.
Kerosene 473 – 573 K C12 – C18 compounds; used as jet fuel and diesel fuel.
Gas oil and Lubrication oil Above 573 KLarge alkanes; used as heating and lubricating oils; larger amount used to produce gasoline by heating over a catalyst in a process called cracking.
Residue Non-volatile asphalt and bitumen
Knocking and Octane-Number: Alkanes vary significantly in their quality as motor fuels. Branched-chain alkanes are better motor fuels than unbranched ones. The quality of a motor-fuel relates to its rate of ignition in an internal combustion engine. A sharp metallic rattling sound is produced in an internal combustion engine during premature ignition called “Knocking”. There is considerable loss of energy during “knocking” and there are also chances of engine damage. The tendency of knocking decreases in the order:Straight chain alkane > branched chain alkane > alkenes > cycloalkanes > aromatic hydrocarbonsThe knocking quality of an automobile fuel is measured in terms of the so-called “Octane-number”. Hydrocarbons
(a) 2, 2, 4-trimethylpentane (isooctane) is free from knocks in highest compression motors and has ‘octane number’ of 100 and
(b) Heptane is found to knock badly and has ‘octane number’ of zero.
“The octane number of a gasoline is defined as the percentage of isooctane present in a mixture of isooctane and heptane, when the mixture has the same knocking efficiency in the engine as the gasoline under study”. The higher the octane-number, the better the gasoline. All fuels are graded on the basis of their octane-number ranging from 0-100. Gasoline used in automobile should have an octane-number of 80 or higher, while gasoline used in aeroplanes has an octane-number 100 or even higher. Gasoline has octane-number of 80 when it is as good as a mixture of 80% isooctane and 20% heptane.
CH2
CH2
CH2
CH2
CH3
CH2 – CH3
n-heptane
CHCH
CH
Toluene
CH
CH
CH+ H2
CH3
A (octane number = 100) B (octane number = 0)
The modern cracking methods of producing gasoline not only increase its yield but also give better quality of fuel having higher octane number. During cracking there is also isomerisation, polymerization, reforming etc, which result in giving branched chains hydrocarbons of higher octane number. Knocking can be reduced by adding antiknock compounds to petroleum. TEL (Tetraethyl lead) which is a source of the free radical (C 2H5) converts the straight chain alkanes to branched chain alkanes. But due to toxic effects of lead and other environmental problems, the use of TEL has been prohibited. Today, tert-butyl methyl ether (MTBE, (CH 3)3C-O –CH3) is the major additive used for improvement of octane-number; Methanol has an octane rating of 116, and along with ethanol is being used as motor fuel.
Octane numbers of some hydrocarbons are:Hydrocarbons Octane-Number
Heptane 0Butane 89Propane 96Isooctane 100Ethane 101Methane 122Triptane 125Natural gas 130
Cetane-Number: Cetane-number is the scale to decide quality of diesel fuel.(a) Hexadecane, C16H34, (cetane) ignites rapidly and has been given a rating of 100.(b) 1-Metyl naphthalene ignites badly and is given a rating of zero.
The cetane number is defined as the percentage of cetane by volume in a mixture of (A) and (B) which has same ignition quality as the sample fuel under study.
Flash point: Safety of oils in different countries is decided by Flash point. This point requires that only those oils should be used which do not give enough vapours at a certain minimum temperature. This minimum temperature is called flash point or the ignition temperature and is defined as the “minimum temperature at which an oil gives off enough vapours to form a momentary flash of light when a naked flame is brought near its surface”. The flash point fixed in cold countries is low, while in hot countries it is high.
CH3 (CH2)14CH3
(A)Cetane number = 100
CH3
(B)Cetane number = 0
ALKENEGeneral introduction
(a) These are the acyclic hydrocarbons in which carbon-carbon contains double bond.(b) These are also known as olefins (i.e, oleum, oil + fines, forming) because lower alkene react with halogens to form oily substances.(c) General formula is CnH2n.(d) Hybridisation of unsaturated ‘C’ atoms are sp2.
Methods of preparation of alkenes
(i) From alkynes:
Poison of catalyst such as BaSO4, CaCO3 are used to stop the reaction after the formation of alkene, otherwise alkanes are formed.(a) The reaction takes place at the surface of Pd, that is why it is cis addition and the product is cis form eg.,
(b) Alkyne can be reduced to trans alkene by using Na + NH3, or Li AlH4
Terminal alkynes are not reduced by the Na – NH3.(ii) From mono halides- When mono halide react with alcoholic KOH or NaOH then respective alkenes are formed
(a)
(b)
(iii) From dihalides:(a) From gem dihalides: When gem dihalide is heated with Na in ether then higher alkenes are formed.
Conclusion: If we take two different types of gemdihalides then we obtain three different types of alkenes.
C = C
R
H H
R’
H / Pd2BaSO4
R C C R
C = C
R
H
H
R
NaNH liq.3
R C C R
C = C
CH3
H
H
CH3Trans
NaOH Alc.3 2 3
|Cl
CH CH CH CH
2Znx2
x Zn x
R CH + CH R R CH CH R
x Zn x
3 3 3 32ZnCl2
Cl Zn Cl
CH CH + CH CH CH CH CH CH
Cl Zn Cl
Note: The above reaction is used in the formation of symm. Alkenes only, because if we take two different types of halides then mixture of alkenes is obtained so the yield of an individual alkene is reduced and it is improper to separate each alkene from the mixture because the difference of boiling points in alkenes is very less.(b) From vicinal dihalides: When vicinal dihalides are heated with Zn dust, alkene of same no. of carbon is obtained.
Note: Alkene is not formed from 1, 3 dihalides. Cycloalkanes are formed by dehalogenation of it. For e.g.,
Stereoselectivity: (i) De-halogenation of vic-dihalide is Trans elimination.(ii) (+) or (–) di halide gives cis alkene
(iii) Meso-di halide gives Trans alkene
(iv) From Alcohols: When alcohol is heated with conc. H2SO4 at about 160oC, alkenes are formed after dehydration.
C
C C
2 2 2| | X X
CH CH CH Zn dust
Zn dust C = C
CH3
H H
CH3
Br
CH3
CH3
H
H
Br
Zn dust C = C
CH3
H
H
CH3
Br
CH3
CH3H
H
Br
(v) Kolbe’s synthesis: When aqueous solution of K or Na succinate is electrolyzed, ethylene is released at anode.
At Anode:
At Cathode: 2K+ + 2e– 2K : 2K + 2H2O 2KOH + H2
Note: If we use methyl succinic acid as reactant then propylene is formed.
(a) cis2- Butene (b) Trans -2- butene (c) depend upon reactant (d) Recemic mixture
Ex.15. Here A is-
(a) (Ph2)C = CH –CH3 (b) (c) (d)
CH3|
3 2|
CH3(Minor) Negligible
CH C CH CH
H2SO4 Conc. 160oC
Ex. 16 If we heat ethylidene chloride with Na/ether or Zn dust then products are(a) CH3- CH = CH –CH3 (b) CH3-CH=CH2 (c) both (a) and (b) (d) none
Ex. 17 In the above reaction if we take methylene chloride and isopropylidene chloride then products are
(a) (b) CH2 = CH2 (c) (d) all of the above
Physical properties:
(a) Alkenes are colourless and odourless.(b) These are insoluble in water and soluble in organic solvents.(c) Physical state, C1 – C4 gas : C5 – C16 Liquid > C16 solid wax(e) The melting points of cis isomers are lower then trans isomers because cis isomer is less symmetrical than trans. Thus trans packs more tightly in the crystal lattice and hence has a higher melting point.(f) The boiling point of cis isomers are higher than trans isomers because ci-alkenes has greater polarity (dipole moment) than trans one.(g) These are lighter than water.
Chemical reactions:
(i) Reaction with Hydrogen-
Mechanism: The reaction takes place at the surface of Ni, therefore the addition is cis addition. e.g.,
Note: Hydrogenation (catalytic hydrogentation) of alkene is a cis addition and is an exothermic reaction. thus the heat evolved decreases with increasing stability of alkene. Remember, stability of alkene depends upon hyper conjugation and type of geometrical isomerism
(ii) Reduction of alkene via hydroboration(a) Alkene can be converted in to alkane by hydroboration followed by protolysis
This reaction is also represented as
(b) Alkene can be converted into alkane by hydroboration followed by treatment with AgNO3 + NaOH. This method gives coupling.
(iii) Halogenation: In presence of polar medium alkene form vicinal dihalide with halogen.
CH3
CH3 (Meso)
D
D
H
HH2/Ni
C = CCH3
D
CH3
D
CH3
CH3
H D
DH
CH3
CH3
DH
HD+
C = CCH3
DCH3
D
H2/Ni
AgNO3/NaOH
Order of reactivity of halogens is: F2 >Cl2 > Br2 > I2.Mechanism: It is an electrophilic addition by molecular attack in which the addition takes place in trans manner.
That is why:
Remember: In suitable condition(i) Trans addition on cis alkene gives ()(ii) Trans addition on trans alkene give meso, similarly;
(iv) Reaction with HX (Hydrohalogenation):
Markovnikoff’s Rule: When an unsaturated unsymmetrical hydrocarbon reacts with HX then halogen goes on that
unsaturated carbon which has minimum number of hydrogen atom. Mechanism of the reaction is based on
.
Mechanism:
(v) Anti Markovnikoff’s Prnciple/ Kharasch Effect/ peroxide Effect:
C = C Br–Br C C C C C C Br–
–Br–
Br
BrBr
Br
+ Br
CH3
CH3
H BrH
CH3
CH3
H
H+
C = C
CH3
H
CH3
HCCl4
Br2
Cis
Br
Br
Br
Br2
CCl4
Br
Br
H
H
CH3
CH3
Meso
C
C
CH3
CH3
H
H
Trans
CH3 CH3
CCl4
Cl2 Cl
Cl
CH3
CH3
Optically active
C = C + HX C C
H
XAlkene Alkyl halide
H H X | | | X
3 2 3 3 2Less stable Minor
X | X
3 3 3 3more stable Major
CH CH CH CH CH CH CH
CH CH CH CH CH CH
CH3 – CH = CH2
H+
H+
To understand anti Markovnikoff’s principle let us consider the following reactions.
(Markovnikoff’s rule)
(Anti Markovnikoff’s rule)
It is based on free radical mechanism.Note: It is interesting to note that anti Markovnikov addition in the presence of peroxide is not applicable for HCl and
HI(i). In the case of H – Cl, the step
, endothermic (as H. = + 12.6 KJ mol-1)
(ii) In the case of H–I, the step
, endothermic (as H = + 46 KJ mol-1)
But in the case of HBr both of the steps are exothermic, which results spontaneous reaction.(vi) Reaction with Hypohalous Acid:
(a) When chlorine water or bromine water is used.
(b) When aq. solution of HOCl is added in the presence of strong acid
(vii) Reaction with H2SO4:
(a) With Conc. H2SO4.
(b) With dil H2SO4.
Cl
Cl Cl | | CH CH R H OH 2 2
2 2H O2 |
H O:| H
Cl O H Cl CH CH R CH CH R
Cl |
2| OH
CH CH R -H+
Cl OH | | Zn dust
CH COOH3HalohydrineC C C = C
Mechanism:
Since hydration proceeds via carbo cation intermediate therefore rearrangement is always probable
(viii) Oxidation: Oxidation is completed by the following ways.(a) With Acidic KMnO4/ Hot KMnO4
(b) With alkaline KMnO4/Bayer’s reagent:
H H2 3 3 3:O H | |
| OHO H H| (Classical carbocation)H
R CH CH R CH CH R CH CH R CH CH
CH CH CH3 3 3 | | | H
3 3 3 3 :O H| |CH 3 H
CH C CH CH CH C CH CH
CH CH CH CH3 3 3 3| | | |H
3 3 3 3| | | |
OH H H H O |
H
CH C C CH CH C C CH
Similarly:
(c) Hydroxylation by OsO4:
(ix) Ozonolysis : This is the two step reaction(i) Ozonide formation (ii) decomposition of ozonide (reductive hydrolysis) e.g.,
Remember: (i) Ozonolysis gives oxidative cleavage of alkene to form two carbonyl group for each . Whether it is in acyclic or cyclic or in aromatic compound e.g.,:
(a)
(b)
(c)
C
C
H
HR
R O–O
O O
MnC
C
H
HR
RO
O
MnO–
O
OH
OH
H
H
R
R
(meso)
H2O/H+
C
C
R
HR
H
KMnO4/OH–
H
OH
HO
H
R
R
()
OH
H
H
HO
R
R
+
C
C
H
HR
R OO
O O
OsC
C
H
HR
RO
O
OsO
O
H+/H2OC
C
H
HR
ROH
OH
C
C
H
RH
R
+ OsO4 OH
H
HO
HO
H
R
R()
C = CO3
IH2O /H+/Zn
IIC + C
O O
ZnO +
O
OO
CC
O+O
O+O
OzonolysisTrans
Ozonolysis
O
O
(ii) Reductive hydrolysis of ozonide is necessary for the formation of carbonyl compound by using Zn. Other wise carbonyl compounds may oxidise to carboxyl group.
(x) Epoxidation by O2/Ag
(a)
(b)
(xi) Epoxidation by performic acid or perbenzoic acid
(a)
(b)
It is observed that is an excellent reagent for epoxidation
Epoxidation is usually stereospecific, e.g.:
(i) (ii)
(c) Oxidation by SeO2 : ‘’ hydrogen is converted into ‘OH’ group eg:
(xii) Hydroboration: Alkene with borane hydride form an important compound called trialkyl borane
(a) Trialkyl borane is an important compound because it gives respective alkane on acidic hydrolysis.
(b) It gives respective alcohol on alkaine hydrolysis
2 2HCl
2 2 2 3 3
2 2
R CH CH H O H
R CH CH B H O H 3R CH CH B(OH)
R CH CH H O H
2 2OH
2 2 2 2 3
2 2
R CH CH H O OH
R CH CH B H O OH 3RCH CH OH B(OH)
R CH CH H O OH
R CH CH R
O O
O
H2O/H+
2R C H + H2O2Oxi. agent
O
R C OH + H2O
O
C O O H
O
CH2 = CH2 + ½ O2
O
CH2 CH2
CH2 CH = CH2 + ½ O2
O
CH3 CH2 CH2
HC O O H
O
CH2 = CH2 + ½ O2Ag
O
CH2 CH2
CH2 CH = CH2 + ½ O2Ag
O
CH3 CH2 CH2
R CH = CH R
O
R CH CH RCF3COOOH
CF3CO3H
O
H H
R R
Cis
R
R H
H
C
C
Cis
CF3CO3H
O
H R
R H
Trans
R
H R
H
C
C
Trans
(c) It gives 1o amine on alkaline hydrolysis with chloramine
(xiii) Hydroformylation/Reaction with CO and H2:
Note: - If CO + H2O is taken then respective acid is formed
The above reaction is also called as ‘Oxo reaction’ or ‘Carbonylation’.(xiv) Substitution reaction:
(a) Except ethane other higher alkene having allyl hydrogen when treated with chlorine or bromine ‘’ H is substituted.
(b) allylic bromination can be easily done by NBS (N-bromo succinamide)
(xv)Isomerisation: Alkenes isomerizes when heated at high temperature or at lower temperature in the presence of various catalysts as AlCl3 e.g.,
(a)
(b)
The mechanism proceeds through carbocation
Ex. 18. What would be the product when 2-pentene reacts with HBr-(a) 2- bromo pentane (b) 3- bormo pentane (c) both a and b (d) 1-bromo pentane
Ex. 19. What would be the product when propene reacts with chlorine in presence of CCl4.
Ex 21. Cis- diol can be prepared by hydrolysis of following(a) Manganate ester (b) osmium ester (c) both of these (d) oxyrane
(xvi) Reaction with HCCl3/NaOH: Cyclic compound is obtained
(a)
(b)Mechanism:
Note: The above reactions are examples of addition of carbon on . Similar types of reactions take place by other carbon producing reactants as CH2N2, CH2CO etc. eg,.
(i)
(ii)
(iii)
(xvii) Alkylation Isobutene reacts with isobutane in the presence of H2SO4
Mechanism:
CH2 == CH2 HCCl3
NaOHCH2 CH2
C
Cl Cl
HCClCl
ClCH3 CH== CH CH3
NaOHCH3 CH3
Cl Cl
Cl CHCl
Cl
–OH–H2O
Cl C–
Cl
ClCl C Cl C === C C C
Dichloro carbon
C
Cl Cl
CH2
CH2
C
Cl Cl
Important points: (a) Ethene react with sulphur monochloride (S2Cl2) to form poisonous mustard gas (, - dichloro di ethyl sulphide). Gas is used as a war gas. (Used in Ist world war)
(b) Ethene used as an anaesthesia.
ALKYNE
General introduction(i) These are the acyclic hydrocarbons which contains carbon-carbon triple bond are called alkyne.(ii) Hybridisation state of triply bonded carbon in alkyne is sp or also called as diagonal hybridisation.(iii) Geometry of carbon is linear in alkynes.(iv) Bond angle in alkyne is 180o. (v) Their general formula is CnH2n – 2.
Methods of preparation(i) From Gem dihalides (Dehydrohalogenation):
(ii) From Vicinal Dihalides (Dehydrohalogenation)
(iii) From tetrahaloalkanes (Dehalogenation)
(iv) From Kolbe’s synthesis:
At Anode:
At Cathode: 2K+ + 2e- 2K : 2K + 2H2O 2KOH + H2
(v) Laboratory method of preparation of acetylene:(a) In laboratory acetylene is prepared by hydrolysis of calcium carbide.
(b) It can also be prepared from CHCl3 with Ag dust.
(vi) From alkynes: (To form higher alkynes)
-6AgClunstable
Cl Cl
H C Cl 6Ag Cl C H H C C H
Cl Cl
C H OH CH OH
Ca ||| ||| Ca
H OH CH OHC
2+
O O|| ||
H C C OK H C C O
H C C OK H C C O|| ||O O
Potassium Malaete
|| || 2K
(a) With Na: When acetylene or 1-alkyne react with Na in presence of liq NH3 then an intermediate compound sodium acetylide or sodium alkynide is formed which gives higher alkyne with alkyl halide.
(b) With GR: When acetyline or 1-alkyne react with GR then alkane and unsaturated GR is formed which further react with alkyl halide and form higher alkyne.
Physical Properties:(i) Alkynes are colourless, odurless and tasteless.(ii) Lower alkynes are partially soluble in H2O. (It is due to its polarisibility)(iii) Higher alkynes are insoluble in water due to more% of covalent character.(iv) completely soluble in organic solvents.(v) Upto C4 alkynes are gaseous. C5-C11 are liquid C12 and above are solids.(vi) Pure acetylene is odourless and impure acetylene has odour like garlic. It is due to impurities of arsene (AsH 3) and Phosphine (PH3).(vii). Acetylene and 1-alkyne are acidic in nature. It is due to greater electronegativity of sp hybridised ‘C’.(viii) Acetylene has two acidic hydrogen atoms. It can neutralize two equivalents of base at the same time. So it is also called as dibasic acid. But the base should be very stronger as –NH2 or –CH3 etc.
Chemical properties: Like alkene, alkyne also shows electrophilic addition reaction but alkynes are less reactive towards electrophilic addition than alkene. Of course alkyne is more reactive towards hydrogenations as it is expected due to greater unsaturacy. Lower reactivity of alkyne towards electrophilic addition probably caused by greatr activation energy due to less stable intermediate.
Chemical reactions:(i) With hydrogen:
The above reaction is called as Sabatier Senderson’s reaction.(a) Hydrogenation in the presence of lindelar’s catalyst: Addition up to alkene takes place in cis manner.
(i)
(ii)
(b) Hydrogenation by Na + NH3 (liq). Addition upto alkene takes place in trans manner
(c) Hydrogenation by LiAH4: Addition up to alkene takes place in trans manner by LiAlH4 also.
(d) Reduction with the help of B2H6: alkyne is first reacted with B2H6 and is followed by acidic hydrolysis, cis alkene is obtained.
H
H
H
H
Pd BaSO42H C C H H C C
CH3
H
CH3
H
H
R
R
H
H
R
R
H
LiAlH4R C C R C = C
H
R
H
R
1. B H2 6
2. H O / H2R C C R C = C
(ii) Halogenation: In presence of Lewis acid as a catalyst alkyne form tetrahaloderivative with halogen.
(iii) Reaction with HX/Hydrohalogenation: Alkyne form gem dihalide with HX because reaction follows markownikoff’s Ist and 2nd rule both.
(e) Reaction with Hypochlorous acid or Chlorine water: Hypochlorsous acid is broken into OH Cl and ions and give addition.
According to markownikoff’s rule
(i)
(ii)
(g) Addition of alcohol
(h) Addition of carboxylic acid: In the presence of Hg2+ unioxylation of carboxylic acid takes place.
(i) Hydroboration: Carboxyl compound is obtained by hydroboration followed by treatment with
(j) Addition of HCN: Addition takes place in the presence of CuCl
(k) Addition of AsCl3: Lewisite is obtained
O H O| ||B H H O2 6 2 2
2H BH2R C CH C = C R CH CH R CH C H
H
H
R
BH2
| |ClCl As Cl AsClCl 2Lewisite
R C C H R C C H
(l) Oxidation:(a) With acidic or alkaline KMnO4 alkyne break into two parts from triply bonded carbon and every part
forms respective acid.
Exception: Acetylene forms oxalic acid with alkaline KMnO4 exceptionally.
(b) Oxidation with Ozone:
(m) Acidic nature of 1-Alkyne or Acetylene: In 1-alkyne or acetylene, the H which is linked with sp hybridised carbon is called as acidic or active H. It can easily be substituted by metal or alkaline species. Hence 1- alkyne or acetylene, are acidic in nature. e.g.,(i) Reaction with Na:
Note: Where this alkynide is treated with alkyl halide higher alkyne is obtained.
(ii) Reaction with ammonical silver nitrate solution:
(iii) Reaction with ammonical cuprous chloride solution:
O
HOH, Zn dust3
| | || ||O O O O
Ozonide
R C C H O R C C H R C C H
(n) Polymerisation Reaction: Alkyne mainly shows adition polymerization reactions.(i) Dimerisation and Cyclysation(a) Dimerisation: Two mole acetylene reacts with Cu2Cl2 and NH4Cl and forms vinyl acetylene.
Note: If acetylene would be in excess then product would be divinyl acetylene and the reaction is called trimerisation.
(b) Trimerisation: If three mole of acetylene is passed into red hot iron or Cu or quartz tube, then a cyclic trimer is formed which is called benzene.
Important: Mesitylene can also be obtained from acetone by condensation polymerization.(c) Tetramerisation: According to the name four moles of acetylene are heted with nickel tetra cyanide, then
acetylene forms a cyclic tetramer cycloocta tetraene.
(o) Reaction with Formaldehyde: 1-Alkyne in the presence of copper react with methanal to form alkynol.
(p) Isomerisation: (i) When 1- alkyne is treated with alcoholic KOH 2-alkyne is formed.
(ii) When 2- alkyne is treated with sodamide then it is converted into 1- alkyne.
CH
CH
CH
CH
CH
CH
Red hot Fe/ Cu/Quartz
Co(CO)8 +Br2
(Octa carbonyl cobalt with bromine)
Benzene
CH
CH
CH
CH
CH
CH
C – CH3
HC
CH3
|C
CHCH3 – C
CH
CH3
|C
CH CH
CH
C–CH3CH3 – C
Mesitylene ( having 3- 1o, 3- 2o and 3- 3o carbons )
CH
CH
CH
CH
CH CH
CH CH
CH = CH
CH = CH
CHCH
CH CH
[Ni(CN)4]-2
Cyclo octa tetraene (Non aromatic)
BENZENEStructure of Benzene and aromaticity(i) Structure of Benzene: following reactions are important in the determination of the structure of C6H6.
(a)
(b)
(c)
(d)
(e)
On the basis of (I) and (II) reactions KAKULE in 1865 suggested that benzene is equilibrium mixture of cylcohexatrienes [(I) and (II)] as follows
But reaction (iii), (iv) and (v) can not be explained by KAKULE’S structure.(ii) The M.O. structure of Benzene: Accirdubg ti tgus tgeirtm akk if tge sux carvib atins ub vebzebe rubg are ub so 2
hybridised state. By overlapping of hybrind orbitals these six carbon atoms form a planer hexagonal ring. Now each of ‘p’ orbital on the six carbon atoms can overlap on either side with adjacent ‘p’ orbitals. These result a molecular orbital which is actually made of two continuous rings, one ring above and another below the plane of hexagon.
(iv) Aromaticity: Benzene and other organic compound which resemble benzene in certain character characteristics properties are called aromatic compound. These characteristic properties constitute what is commonly known as aromatic character or aromaticity. Such important properties are summarized below.(a) Unusual stability (b) Substitution rather than addition reaction.(c) Resistant to oxidation (d) C = C bond length value(e) Ring current.
Note: According to modern concepts aromaticity can now be defined as the ability to sustain an induced current. In general aromatic compounds are those which are(i) Cyclic (ii) Planer(iii) Containing a total of (4n +2) delocalized electrons where n may be 0, 1, 2, 3,….. Third condition is well known as Huckel rule.
Examples: (i) (ii) (iii)
4n + 2 = 6 ; n = 1 Aromatic 4n + 2 = 10 ; n = 2 Aromatic 4n + 2 = 14 ; n = 3 Aromatic
(iv) (v) (vi)
H /26 6 Pt
C H
(I) and (II)
Sidewiseoverlaping
Delocalisationof electrons
M.O.
sp2
4n + 2 = 8 ; n = 3/2 4n + 2 = 4 ; n = 1/2 4n + 2 = 2 ; n = 0Not aromatic Not aromatic but Anti aromatic Aromatic(because it is
planar)
(vii) (viii) (ix)
4n + 2 = 4 ; n = 1/2 4n + 2 = 4 ; n = 1/2 4n + 2 = 6 ; n = 1It is antiaromatic (4n system) not aromatic (But antiaromatic) Aromatic
(x) (xi) (xii)
4n + 2 = 6 ; n = 1 4n + 2 = 6 ; n = 1 4n + 2 = 6 ; n = 1 Aromatic Aromatic Aromatic
(xiii) (xiv) (xv)
4n + 2 = 6 ; n = 1 4n + 2 = 6 ; n = 1 4n + 2 = 6 ; n = 1 Aromatic It is not aromatic but antiaromatic Also a planar compound
(xvi) (xvii) (xviii)
(Tropone), this compound is aromatic 4n + 2 = 10; n = 2 (Azulene) 4n + 2=18; n = 4 Aromatic [18] Anulene it is an aromatic.
Methods of Preparation of Benzene:(i) From sodium salt of benzoic acid: By using soda-lime (decarboxylation):-
(ii) From benzene sulphonic acid: Hydrolysis
(iii) From benzene diazonium chloride: With SnCl2 + NaOH or absolute ethyl alcohol or H3PO2.
(iv) From Phenol: With Zn dust (Reduction)
CaOHeat
NaOH
C ONa
O
Sodium benzoate Benzene
SO3Ho150 200 C
HCl, Pressure HOH + H2SO4
BenzeneBenzene sulphonic acid
N2Cl
+ C2H5OH + N2 + HCl + CH3CHO
OH
Phenol
+ Zn (dust) + ZnO
Benzene
O O
sp2
NHsp2
NN
sp2
sp2N
(v) From Grigneard-Reagent:
(vi) From Acetylene:
Physical properties:(i) Benzene is colourless volatile liquid. It has characteristic smell.(ii) Its boiling point is 80o C and freezing point is 5.5oC.(iii) It is highly inflammable and burns with sooty flame.(iv) It is lighter than water. Its specific gravity at 20oC is 0.8788.(v) It is immiscible with water but miscible with organic solvents such as alcohols(vi) It is a non-polar compound and its dipole moment is zero. Glyoxal(g) It is extremely poisonous substance. Inhalation of vapours or absorption though skin has a toxic effect.
Chemical properties: Chemical properties of benzene are classified as(i) Addition reactions
(a) Reaction with hydrogen (Reduction):
(b) Addition with halogen:
Note: (i) The product is called benzene hexachloride (BHC).(ii) It’s commercial name is Lindane or 666.
(c) Addition with Ozone:
CH
CH
CH
CH
CH
CH
Red hot Cu tube
500oC
Benzene
CH
CH
CH
CH
CH
CH
CH2
CH2
CH2
CH2
CH3
CH3
CrO3 + Al2O3
500oC+ 4H2
Benzene
(C6H6Cl6)
+ 3Cl – Cl
H Cl
C
CC
C
CC
H Cl
Cl
Cl
Cl
Cl
H
H
H
H
+ 3O3
O
O O
O
O
O
O
O
O
(ii) Electrophilic substitution: Benzene undergoes electrophilic substitution reactions because it is an electron rich system due to delocalized 6 electrons. Benzene ring shows bimolecular type of substitution reaction due to the formation of intermediate (half bond structure). So reactions of aromatic compounds are called electrophilic bimolecular substitution reaction. Examples of such reactions are halogenation, nitration, sulphonation and Friedel Crafts Reaction (FCR).
Step (1)
Step (2)
Step (3)
(a) Halogenation:
(b) Nitration:
Note: (i) If nitrobenzene is further nitrated m-dinitrobenzene is formed at 90oC.
(ii) On heating benzene with fuming nitric acid and conc. H2SO4, 1, 3, 5-trinitro benzene is formed, which is used as a powerful explosive.
(c) Sulphonation: Benzene forms benzene sulphonic acid with hot concentrated sulphuric acid while with fuming sulphuric acid or oleum (conc. H2SO4 + SO3) at high temperature, m- benzene disulphonic acid is formed.
So for sulphonation, it must be hot and conc. H2SO4 and the attacking species is SO3.
(d) Friedel-Craft’s Reactions: (i) Alkylation: Attachment of alkyl group in the benzene.
(ii) Acylation (Reation with acid chloride or acid anhydride): Aromatic ketones are formed.
(iii) Drawbacks of Friedel-Craft’s reactions: Friedel-Craft’s reactions has the following drawbacks
(a) Mono alkyl derivative readily undergoes further alkylation to produce poly-substituted derivatives.
(b) It is easy to introduce methyl, ethyl or isopropyl group but usually difficult to introduce n-propyl or n-
butyl group as isomeric change may occur.
(e) Formylation (Reaction with CO and HCl): Introduction of –CHO group in a compound is called formylation. Formylating agent is CO and HCl
The electrophile, is first formed.
Note: The reaction is called ‘Gattermann Koch aldehyde Synthesis’.
(f) Formation of benzoic acid:
(g) Formation of benzoyl chloride:
(h) Formation of benzamide:
HR
4 AlX
R
Alkylbenzene
+ AlX3 + HX
anhydrous AlCl36 6C H RCOCl
R – C = O
O+ HCl
C HAlCl3CO HCl
H
Benzaldehyde
AlCl / CO3 2
Benzoic acid
COOH
AlCl / COCl3 2
COCl
Benzoyl chloride
COCl
AlCl / ClCONH3 2
Benzamide
CONH2
AlCl 336 6 3 2 2 6 5
3
CHC H CH CH CH Cl C H CH HCl
CH
(i) Formation of benzophenone:
(j) Formation of benzyl chloride:
(k) Formation of Acetoxy mercury benzene:
(l) Gattermann-aldehyde reaction:
. The electropile, is first
formed.
:
(iii) Oxidation Reactions: Benzene is a stable compound. It is not attacked by ordinary oxidizing agents. Strong oxidizing agents convert benzene slowly into carbon dioxide and water on heating. Being inflammable liquid it burns in air with smoky flames (combustion)
(a)
(b)
(c)
Ex. 18 Which of the following is used as a solvent in Friedel Craft’s reaction(a) Aniline (b) Nitrobenzene (c) Toluene (d) benzene
Ex. 19 Which of the following triad of group activates the benzene ring and directs the electrophile to o- and p- position for substitution.
(a) –NO2, -CHO, -COOH (b) –OH, , –CH3
(c) –OH, –SO2OH, –NO2 (d) –NH2, –CHO, –SO2OH
Ex. 20
Functional groups Y, –CH = CH2 and X respectively are …… and ….. directing.(a)Meta, ortho-pare and ortho –para (b) Meta, meta and meta
AlCl / COCl3 2
C H CO6 5
Benzophenone
CO
HCHO / HCl
ZnCl2
Benzyl chloride
CH2Cl
(CH COO) Hg3 2
Acetoxy mercury benzene
HgOOCCH3
+ 15 O2 12CO2 + 6H2O2
V O2 56 5o300 C
C H OH
CH=CH2
(c) Meta, ortho-para and meta (d) Al the three ortho-para
TOLUENEIntroduction
(i) When a methyl group is directly attached with benzene ring, the compound is called Toluene.(ii) Toluene is a higher homologue to benzene(ii) It can be obtained by the light oil obtained from distillation of ‘Coal-tar’.(iv) Toluene is the compound, which is more reactive than benzene in chemical reactions.
Methods of preparation:(i) From Toluic acid: When sodium salt of o-, m- or p- toluic acid is heated with sodalime forms toluene.
ii)From Cresol: o-, m- or p- cresol is heated with zinc dust to form toluene.
(iii) From toluene sulphonic acid: When o-, m-, or p- toluene sulphonic acid is heated strongly with water, toluene is formed.
(iv) From diazonium salt of toluene: o-, m- or p- toluene diazonium chlorides are reduced by ethanol and toluene is obtained.
(v) From Friedel-Craft’s Reaction: Methyl chloride reacts with benzene in the presence of anhy, AlCl3 and forms toluene
(vi) From Wurtz-fitting Reaction: A mixture of aryl and alkyl halide reacts in ether solution with sodium and
(vii) From Grignard’s reagent: Phyenyl magnesiumiodine reacts with pure methyl iodide and forms toluene.
Physical properties:(i) Colourless liquid with benzene like smell(ii) B.P. 111oC(iii) Lighter than water and insoluble in it but soluble in organic solvents.
Chemical Properties: Toluene mainly gives four types of reactions:(i) Addition reactions (ii) Ring substitution reactions
CH3COONa + NaOH CH3
CH3 OH + Zn CH3 + ZnO
CH3SO3H + HOH CH3
+H2SO4
CH3N = N – Cl + C2H5OH CH3 +CH3CHO + HCl + N2
Dry ether3Br 2Na Br CH CH3 + 2NaBr
Bromo methaneBromobenzene
3MgI I CH CH3 + MgI2
(iii) Side chain substitution reaction (iv) oxidation reactionAddition reactions: Toluene on catalytic hydrogenations forms methyl cyclohexane:-
(i) Ring substitution reactions: Like benzene, toluene also gives electrophilic substitution reactions. Methyl group is o- and p- directing. So substituent is placed at o- and p- positions. Halogentation, nitration and sulphonation are as shown below.(a) Halogenation:
(b) Nitration:
Note: Trinitrotoluene is a powerful explosive(c) Sulphonation:
(d) Alkylation:
(ii) Side chain substitution reaction
(iii) Oxidation reaction:
CH3
+ Cl2
CH3
ClCH3
Clo-chloro toluenep-chloro toluene
+
CH3
NO2
CH3 CH3
NO2NO2
NO2
O2NCH3
2,4,6- trinitrotoluene (T.N.T)
+ Conc. (HNO3 + H2SO4) +
o-nitro toluene
p-nitro toluene
Nitration
CH3 CH3
SO3HCH3
SO3H
+ Conc. H2SO4 +
o-toluenesulphonic acid p-toluene
sulphonic acid
CH3 CH3
R
CH3
R
+ RX +
(a)
(b) Etard reaction:
(c)
(d)
(e)
Ex. 21 Chlorination of toluene in the presence of light and heat followed by treatment with aqueous NaOH gives-
