Notes-Part-2-Class-11-Science-Chemistry-Chapter-15-Hydrocarbons-Maharashtra Board


Maharashtra State Board-Class-11-Science-Chemistry-Chapter -15

Notes Part-2

Topics to be Learn : Part-1

  • Alkanes
  • Alkenes

Topics to be Learn : Part-2

  • Alkynes
  • Aromatic Hydrocarbons

Alkynes :

Alkynes are aliphatic unsaturated hydrocarbons containing at least one triple bond (C≡C). Their general formula is CnH2n-2

For example, Ethyne H—C≡C—H

Lower alkynes :

Isomerism in alkynes :

Alkynes show position isomerism which is a type of structural isomerism.

For example, But-1-yne CH3—CH2—C≡CH

But-2-yne CH3—C≡C—CH3

Compounds have the same molecular formula (C4H6) and the same carbon skeleton but the double bonds are located at different positions.

Know This :

  • Simplest alkyne is ethyne which is known as acetylene.
  • The carbon-carbon triple bond is a functional group.
  • The aliphatic unsaturated hydrocarbons containing two or three carbon-carbon triple bonds are called alkadiynes and alkatriynes, respectively.
  • Cicutoxin from poisonous plants 'water hemlock'.


Preparation of alkynes :

(A) Industrial sources :

(a) Ethyne is industrially prepared by controlled, high temperature partial oxidation of methane.

6CH4 + 2O2  \(\underrightarrow{1773K}\) 2H-C≡C-H + 2CO2 + 10H2                    .

(b) From calcium carbide : Industrially the alkyne ethyne is prepared by reaction of

calcium carbide with water.

CaC2 + 2H2O  →   C2H2 + Ca(OH)2

(B) Methods of preparation of alkynes :

(i) By dehydrohalogenation of vicinal dihalides : Removal of H and X from adjacent carbon atoms is called dehydrohalogenation.

  • Vicinal dihalides react with alcoholic solution of potassium hydroxide to form alkenyl halide which on further treating with sodamide forms alkyne.

Examples :

(a) When 1, 2-Dibromoethane (ethylene dibromide) is boiled with alcoholic caustic potash (KOH), it forms vinyl bromide (bromoethene). On heating vinyl bromide with sodamide (NaNH2)5 ethyne is obtained.

Note : The yield of acetylene is poor as the intermediate compound, vinyl bromide is less reactive. Hence stronger bases such as NaNH2, KNH2 are used in the second step.

(b) When 1, 2-dichloropropane is treated with alcoholic caustic potash (KOH), it forms 1-ch1oropropene. On heating 1-chloropropene with sodamide (NaNH2) propyne is obtained.

Note : The yield of propyne is poor as the intermediate compound, 1-chloropropene is less reactive. Hence stronger bases such as NaNH2(Sodamide) is used in the second step.

(ii) From terminal alkynes :

  • Terminal alkynes are the compounds in which hydrogen atom is directly attached to triply bonded carbon atom.
  • In this method a smaller terminal alkyne first reacts with a very strong base like lithium amide to form metal acetylide (Lithium amide is easier to handle than sodamide).
  • Higher alkynes are obtained by reacting metal acetylides (alkyn-1-yl lithium) with primary alkyl halides.

Examples :

(a) When ethyne is treated with lithium amide, an intermediate ethynylithium is obtained. Further ethynylithium reacts with bromoethane forms but-1-yne.

(b) When propyne is treated with lithium amide, an intermediate prop-1-yn-1-yl lithium is obtained further prop-1-yn-1-yl lithium reacts with bromoethane forms pent-2-yne

Physical properties of alkynes :

The physical propeties of alkynes are similar to those of alkanes and alkenes.

  • Similar to alkanes and alkenes, lower alkynes namely first three members are gases, the next eight members are liquids and higher hamologues are solids.
  • Except acetylene, all alkynes are odourless. Acetylene has characteristic smell.
  • Alkynes have slightly higher boiling points than corresponding alkanes and alkenes. The melting point and boiling point increase with increase in molecular mass.
  • They are insoluble in water, less dense than water, but are soluble in organic solvents like benzene, ether, etc.

 Chemical properties of alkynes :

(1) Acidity of alkynes :

  • Sodium amide and lithium amide are strong bases. They react with H atom of ethyne to form sodium acetylide with the liberation of hydrogen gas.
  • Hydrogen atoms in ethyne are attached to the sp hybridised carbon atom. Due to maximum percentage of s-character (50%), the sp hybridised orbitals of carbon atoms in ethyne molecules have highest electronegativity.
  • Thus in ethyne, hydrogen atoms can be easily removed as protons, this gives acidic character to ethyne. Hence, hydrogen atoms of ethyne attached to triply bonded carbon atom are acidic in nature.

The relative acidity of alkanes, alkenes and alkynes follows the order

H−C≡C−H > H2C=CH2 > H3C−CH3

Know This : Acidic alkynes react with certain heavy metal ions like Ag+ and Cu+ and form insoluble acetylides. On addition of acidic alkyne to solution of AgNO3 in alcohol form a precipitate which indicates that the hydrogen atom is attached to triply bonded carbon. This reaction is used to differentiate terminal alkynes and non-terminal alkynes.

 (2) Addition of dihydrogen :

When acetylene is heated with hydrogen gas in the presence of raney nickel as a catalyst, ethane is formed by hydrogenation.

(3) Addition of halogens :

(a) Ethyne reacts with chlorine in the presence of inert solvent such as CCl4 to give acetylene tetrachloride (1,1,2,2—Tetrach1oroethane)

(b) Ethyne reacts with liquid bromine in the presence of inert solvent of CCl forms 1,1,2 2 Tetrabromoethane. Red-brown colour of solution of bromine in CCl4 disappears.

(4) Addition of hydrogen halides :

Hydrohalogenation of alkyne : Hydrogen halides (HCl, HBr and HI) add to alkynes across carbon-carbon triple bond in two steps to form geminal dihalides (in which two halogen atoms are attached to the same carbon atom).The addition of HX in both the steps takes place according to Markovnikov's rule.

Examples :

(i) Addition of HI to acetylene: Acetylene reacts with hydrogen iodide (HI) to form vinyl iodide (Iodoethene) which on further reaction forms 1,1-Diiodoethane.

(ii) Addition of HBr to acetylene : Acetylene reacts with hydrogen bromide (HBr) to form Bromoethene which on further reaction forms 1,1-Dibromoethane.

(iii) Addition of HBr to propyne : Propyne when passed through HBr to form 2-bromopropene which on further reaction forms 2,2-Dibromopropane.

The order of reactivity of hydrogen halides is HI > HBr > HCl

(5) Addition of water :

Alkynes react with water in presence of 40% sulphuric acid and 1% mercuric sulphate to form aldehydes or ketones i.e. carbonyl compounds.

Examples :

(1) On passing acetylene through warm 40 % H2SO4 in the presence of 1 % HgSO4, vinyl alcohol is obtained which tautomerises and forms acetaldehyde (ethanal). It is a hydration reaction.

(2) On passing propyne through warm 40% H2SO4 in the presence of 1% HgSO4, vinyl alcohol is obtained which tantomerises and forms propanone

Uses of acetylene :

  • Ethyne (acetylene) is used in preparation of Ethanal (acetaldehyde), Propanone (acetone), ethanoic acid (acetic acid).
  • It is used in the manufacture of polymers, synthetic rubber, synthetic fibre, plastic etc.
  • For artificial ripening of fruits.
  • In oxy-acetylene ( mixture of oxygen and acetylene) flame for welding and cutting of metals.

Aromatic Hydrocarbons :

Aromatic compounds : Benzene and all compounds that have structures and chemical properties resembling benzene are classified as aromatic compounds.

Examples :

Benzene :

The molecular formula of benzene is C6H6. Benzene is parent compound of most of the aromatic compounds.

  • Aryl group : Aromatic hydrocarbons are called arenes. Aryl group is derived from arenes and represented as, Ar.
  • Phenyl group : C6H5— is called phenyl group. Benzene was originally called phene and the phenyl group is derived from benzene.
  • Coal-tar and petroleum are the two large-scale sources of benzene
  • It is a colourless liquid having characteristic odour. Its boiling point is 353K.
Difference between aromatic and aliphatic compounds :

Difference between aromatic and aliphatic compounds :

Aromatic compounds Aliphatic compounds
1. Aromatic compounds contain higher percentage of carbon. 1. Aliphatic compounds contain lower percentage of carbon.
2. They burn with sooty flame. 2. They burn with non-sooty flame.
3. They are cyclic compounds with alternate single and double bonds. 3. They are open chain compounds.
4. They are not attacked by normal oxidizing and reducing agents. 4. They are easily attacked by oxidizing and reducing agents.
5. They do not undergo addition reactions easily. They do not decolourise dilute alkaline aqueous KMnO4 and Br2 in CCl4, though double bonds appear in their structure. 5. Unsaturated aliphatic compounds undergo addition reactions easily. They decolourise dilute aqueous alkaline KMnO4 and Br2 in CCl4.
6. They prefer substitution reactions. 6. The saturated aliphatic compounds give substitution reactions.


Structure of benzene :

  • Molcular formula of benzene, C6H6, indicates the high degree of unsaturation.
  • Open chain structure not possible : Open chain or cyclic structure having double and triple bonds can be written for C6H6. But benzene does not behave like alkenes or alkynes. This indicates that benzene cannot have the open chain structure.
Distinguish between the reactivity of alkenes and benzene :

Distinguish between the reactivity of alkenes and benzene :

Reaction Alkene Benzene
With dil. alka. KMnO4 Decolourisation of purple colour of KMnO4 No decolourisation
With Br2 in CCl4 Decolourisation of red brown colour of bromine No decolourisation
With H2O in acidic medium Addition of H2O molecule No reaction


Evidence of cyclic structure :

When benzene is treated with bromine in FeBr3, it gives monosubstituted bromobenzene (C6H5Br)

C6H6 + Br2 \(\underrightarrow{FeBr_3}\)  C6H5Br + HBr

This indicates that all the six hydrogen atoms in benzene are identical. This is possible only if benzene has cyclic structure of six carbon atoms attached to one hydrogen atom each.

Benzene on catalytic hydrogenation gives cyclohexane.

C6H6 + 3H2 \(\underrightarrow{Ni}\) C6H12

This confirms the cyclic hydrogenation gives cyclohexane.

Kekulé structure of benzene :

Molecular formula of benzene is C6H6 having a cyclic planar ring of six carbon atoms with alternate single and double bonds. All the six carbon atoms are sp2 hybridisecl and hydrogen atom is attached to each carbon atom.

According to Kekulé structure there are two possible isomeric 1, 2-dibromobenzenus, in one of the isomers, the bromine atoms would be attached to the doubly bonded carbon atoms whereas in the other, they would be attached to single bonded carbons.

However, benzene was found to form only one ortho-disubstituted benzene. This problem was overcome by Kekule' by suggesting the concept of oscillating nature of double bonds in benzene as given below.

Even with this modification, Kekule' structure of benzene failed to explain unusual stability and preference to substitution reactions rather than addition reactions, which was later explained by resonance.

Stability of benzene:

Resonating structures of benzene : Resonating structures are equivalent structures which can be written with identical positions of atoms but different arrangements of electrons.

Structure of benzene is a hybrid of various resonating structures. Two structures given by Kekulé are main contributing structures (fig A and B).

The real structure is a hybrid of these structures which is represented by inserting a circle or a dotted circle in the hexagon (Fig C).

The circle represents the six π electrons which are delocalized over the six carbon atoms of the ring. A double headed arrow between the resonance structures used to represent resonance phenomenon.

Stability of benzene : For benzene, the stability due to resonance is so high that π -bonds of the molecule resist breaking. This explains lack of reactivity of benzene towards addition.

All six carbon atoms in benzene are sp2 hybridised. Two sp2 hybrid orbitals of carbons overlap and form carbon-carbon sigma (σ) bond and the remaining third sp2 hybrid orbital of each carbon overlaps with s orbital of a hydrogen atom to form six C-H sigma bonds.

The unhybrid p orbitals of carbon atoms overlap laterally forming π bonds. There are two possibilities of forming three π bonds by overlap of p orbitals of C1-C2, C3-C4, C5-C6 or C2-C3, C4-C5, C6-C1, respectively, as shown in below Fig. both are equally probable. According to resonance theory these are two resonance structures of benzene.

Molecular orbital theory of benzene : According to molecular orbital (MO) theory the six p-orbitals of six carbons give rise to six molecular orbitals of benzene. Shape of the most stable MO is as shown in following fig.

  • Three of these π molecular orbitals lie above and the other below those of free carbon atom energies.
  • The six electrons of the p-orbitals cover all the six carbon atoms and are said to be delocalized. Delocalization of π electrons results in stability of benzene molecule.
Remember : In benzene ,

  • All carbon and hydrogen atoms lie in the same plane.
  • Six sigma (σ ) bonds lie in the same plane.
  • All bond angles are 1200

 Bond parameters of benzene :

  • Benzene has molecular formula C6H6
  • X-ray diffraction data indicate that all C-C bond lengths in benzene are equal (139 pm) which is an intermediate between C-C (154 pm) and C=C bond (133pm).

  • This is due to resonance phenomenon in benzene. Thus, absence of pure double bond in benzene, addition reactions are not possible under normal conditions.

Aromatic character (Huckel Rule) :

  • The property is common to all aromatic compounds and is referred to as aromaticity or aromatic character.
  • The aromatic character of benzene is correlated to its structure.
  • Aromaticity is due to extensive cyclic delocalization of p-electrons in planar ring structures.

The three rules of aromaticity are useful in predicting whether a particular compound is aromatic or non—aromatic.

  • Aromatic compounds are cyclic and planar (all atoms in ring are sp2)
  • Each atom in aromatic ring has a p-orbital. The p-orbitals must be parallel so that continuous overlap is possible around the ring.
  • Huckel Rule : The cyclic molecular orbital formed by overlap of p-orbitals must contain (4n+2) p-electrons, where n = integer 0, 1, 2, 3, etc.

Applying Huckel Rule to the Benzene :

Benzene : It is cyclic and planar. It has three double bonds and six π electrons. It has a p orbital on each carbon of the hexagonal ring. Hence a continuous overlap above and below the ring is possible.

This compound is aromatic, 4n + 2 = Number of π electrons.

4n + 2 = 6, ∴ 4n = 6 - 2 = 4

n = 4/4 = 1, Here 'n' comes out to be an integer.

Hence benzene is aromatic.

Applying Huckel Rule to the Naphthalene :

Naphthalene : It is cyclic and planar. It has 5 double bonds and 10 π electrons. It has p-orbital on each carbon atom of the ring. Hence, a continuous overlap around the ring is possible. This is in accordance with Huckel rule.

4n + 2 = Number of p electrons

4n + 2 = 10, ∴ 4n = 10 - 2 = 8

n = 8/4 = 2, Here 'n' comes out to be an integer.

Hence napthalene is aromatic.

Applying Huckel Rule to the Pyridine :

Pyridine : Pyridine has three double bonds and 6 π electrons. The six p orbital containing six electrons form delocalized p molecular orbital. The unused sp2 hybrid orbital of nitrogen containing two non-bonding electrons is as it is.

4n + 2 = Number of p electrons

4n + 2 = 6, ∴ 4n = 6 - 2 = 4

n = 4/4 = 1, Here 'n' comes out to be an integer.

Hence pyridine is aromatic.

Applying Huckel Rule to the Cycloheptatriene :

It is cyclic and planar. It has three double bonds and 6 π electrons. But one of the carbons is saturated (sp3 hybridized) and does not possess a p orbital. Hence a

continuous overlap around the ring is not possible. Therefore, it is non- aromatic.

Preparation of aromatic compounds

(A) Industrial source of aromatic compounds :

Coal tar and petroleum are major sources of aromatic compounds.

(B) Methods of preparation of benzene

(i) From ethyne (By trimerization) : Ethyne when passed through a red hot iron tube at 873 K undergoes trimerization to form benzene.

(ii) From sodium benzoate : (By decarboxylation) : When anhydrous sodium benzoate is heated with soda-lime it gives benzene.

(iii) From phenol (By reduction) : When vapours of phenol are passed over heated zinc dust, it gives benzene.

Physical properties of benzene :

  • Benzene is colourless liquid.
  • Its boiling point is 353 K and melting point is 278.5 K. ,
  • It is insoluble in water. It forms upper layer when mixed with water.
  • It is soluble in alcohol, ether and chloroform.
  • Benzene vapours are highly toxic which on inhalation lead to unconsciousness.

Chemical properties of benzene :

Aromatic compounds are characterised by electrophilic substitution reactions. However, they undergo addition and oxidation reactions under special conditions. Some reactions of benzene are :

(1) Addition reactions :

(i) Addition of chlorine : Benzene when treated with chlorine in presence of bright sunlight or UV light, adds up three molecules of chlorine to give benzene hexachloride .

γ - isomer of benzene hexachloride is called gammexane or lindane which is used as insecticide.

(ii) Addition of hydrogen : When a mixture of benzene and hydrogen gas is passed over heated catalyst nickel at 453 K to 473 K, cylohexane is formed.

(iii) Addition of ozone : When benzene is treated with ozone in presence of an inert solvent carbon tetrachloride, benzene triozonide is formed which is then decomposed by zinc dust and water to give glyoxal.

(2) Substitution reactions : Benzene shows electrophilic substitution reactions, in which one or more hydrogen atoms of benzene ring are replaced by electrophilic groups like -Cl, -Br, -NO2, -SO3H, -R (alkyl group) , -COR (Acyl group) etc.

(i) Halogenation : In this reaction, hydrogen atom of benzene ring is replaced by halogen atom.

Examples :

Chlorination : Chlorine reacts with benzene in dark in the presence of iron or ferric chloride or anhydrous aluminium chloride or red phosphorous as catalyst to give chlorobenze .

Electrophile : Cl+, Chloronium ion

Formation of the electrophile : Cl—Cl + FeCl3 → Cl+ + [FeCl4]

Bromination of benzene is similar to chlorination : Bromine reacts with benzene in dark in the presence of FeBr3, bromobenzene is obtained.

Electrophile : Br+

Formation of the electrophile : Br—Br + FeBr3 → Br + + [FeBr4]

Iodination of benzene is not possible as it is reversible process. With excess of chlorine, benzene gives hexachlorobenzene.

(ii) Nitration : When benzene is heated with a mixture of concentrated nitric acid and concentrated sulfuric acid (nitrating mixture) at about 313 K to 333 K , it gives nitrobenzene.

Electrophile : NO2+ , nitronium ion.

Formation of the electrophile : HO—NO2 + 2H2SO4 ⇌ 2HSO4 + H3O+ + NO2+

(iii) Sulfonation : When benzene is heated with fuming sulfuric acid (oleum) at 373 K, it gives benzene sulfonic acid.

Electrophile : SO3, free sulfur trioxide

Formation of the electrophile : 2H2SO4  →  H3O+ + HSO4 + SO3

(iv) Friedel-Craft's alkylation reaction : When benzene is treated with an alkyl halide like methyl chloride in the presence of anhydrous aluminium chloride, it gives toluene.

The reaction is used to extend the chain outside the benzene ring.

Electrophile : R+

Formation of the electrophile : R—Cl + AlCl3 → R+ + AlCl4

(v) Friedel-Craft's acylation reaction : When benzene is heated with an acyl halide or acid anhydride in the presence of anhydrous aluminium chloride, it gives corresponding acyl benzene.

Electrophile : R—C+=O acylium ion

Formation of the electrophile : R—COCl + AlCl3 → R—C+=O + AlCl4

(vi) Combustion : When benzene is heated in air, it burns with sooty flame forming carbon dioxide and water.

C6H6 + 15/2 O2 → 6CO2 + 3H2O

Directive influence of a functional group in monosubstituted benzene :

In benzene, all hydrogen atoms are equivalent. Therefore, only one product is possible when it undergoes electrophilic substitution reactions.

Monosubstituted benzene :

Positions of carbon atoms in mono substituted benzene :

  • The positions 2 and 6 are equivalent and give ortho (o-) products.
  • The position 3 and 5 are equivalent and give meta (m-) products.
  • The position 4 is unique and and gives para (p-) product.
  • Now in benzene, five positions are available for electrophilic substitution.

Positions of carbon atoms in monosubstituted benzene which is subjected to electrophilic substitution :

When monosubstituted benzene is subjected to further electrophilic substitution, the second substituent i.e. electrophile or incoming group (E) can occupy any of these positions and give three disubstituted products. But these products are not formed in equal amounts.

Two types of behavior :

Ortho- and para - products or metaproducts :

The groups which direct the incoming group to ortho and para positions are called ortho and para directing groups.

Ortho and para directive influence of –OH group : The resonance theory clearly explains why certain substituents are ortho/para or meta directing.

It is clear from the above resonance structures that the ortho and para positions have a greater electron density than the meta positions. Therefore, -OH group activates the benzene ring for the attack of second substituent E at these electron -rich centres.

Resonating structures of chlorobenzene :

Meta directing groups : The groups which direct the incoming group to meta positions are called meta directing groups. All meta directing groups have positive charge on the atom which is directly attached to an aromatic ring.

Metadirective influence of -NO2 group can be explained by resonance theory : Meta directing group like nitro groups withdraws electrons from the aromatic ring by resonance, making the ring electro-deficient. Therefore, meta groups are ring deactivating groups.

Due to — I effect, — NO2 group reduces electron density in benzene ring on ortho and para positions compare to that of meta position.

Therefore, the attack of incoming group becomes difficult at ortho and para positions. Incoming group can attack electron rich meta positions more easily resulting meta substitution.

 Carcinogenicity and Toxicity :

  • Polycyclic aromatic molecules with more than two fused benzene rings, such as benzene, are poisonous and carcinogenic (cause cancer).
  • An abundance of polycyclic aromatic compounds are created when organic materials such as coal, tobacco, and petroleum are not burned completely.
  • In the human body, benzopyrene is transformed into an epoxy diol while benzene is oxidized to an epoxide. These substances interact with DNA through a variety of metabolic processes, which can result in cancer.
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