CBSE Class 11 Chemistry Hydrocarbon Summary

HYDROCARBON SUMMARY

HYDROCARBON ARE COMPOSED OF HYDROGEN AND CARBON

TYPES— 1) SATURATED 2) UNSATURATED

SATURATED ---- ALKANE (CnH2n+2) SINGLE BONDED (C-C)

UNSATURATED ---- ALKENE (CnH2n) DOUBLE BONDED (C=C)

ALKYNE (CnH2n-2) TRIPLE BONDED (C-C)

ALKANE

HALOGENATION REACTIONS

Features of Halogenations:-

(i) The reactivity of Halogens:- F2 > Cl2 > Br2 > I2.

(ii) The rate of replacement of Hydrogens of alkanes is: 3° > 2° > 1°

(iii) Mechanism:- halogenations reaction take place by free radical mechanism. The reaction proceeds in the following steps: Initiation (i) Chain initiation step:- (ii) Chain Propagation step:- (iii) Chain Termination

SULPHONATION:-

Replacement of hydrogen atom of alkane by –SO3H group, this method is also called dehydrogenation or hydroforming similarly, heptane gives toluene, n-Octane give o-xylene and 2, methyl heptane give m-xylene.

AEROMATISATION

Thermal decomposition or Pyrolysis or cracking or Fragmentation: - when higher alkanes are heated at high temp (about 700-800k) in the presence of alumina or silica catalysts, the alkanes break down to lower alkanes and alkenes.

CONFORMATIONAL ISOMERISM: The different molecular arrangements arising as a result of rotation around carbon carbon single bonds are called conformational isomers or rotational isomers and the phenomenon is called conformational isomerism. Numerous possible arrangements of ethane are possible. Two extreme conformations are known. These are eclipsed conformation and staggered conformation. 

Alkenes

Unsaturated hydrocarbon which have double bond. General molecular formula CnH2n C–C bond 1.34 A0 , sp 2 hybridization When we treated Alkene with chlorine, oily products are obtained. So Alkenes are also known as Olefins. (Greek olefin meaning oil forming). These Show chain, positional and geometrical isomerism

Addition reaction of HBr to unsymmetrical alkenes takes place according to Markovnikov Rule

Markownikov rule:- negative part of the addendum (adding molecule) gets attached to that carbon atom which possesses lesser number of hydrogen atoms Peroxide effect or Kharasch (Anti Markownikoff’s addition):- In 1933 Kharasch and Mayo observed that when HBr is added to an unsymmetrical double bond in the presence of organic peroxide, the reaction take places opposite to the Markovnikov rule.

Noted :- peroxide effect is applicable only to HBr and not to HF, HCl and HI. Addition of HF, HCl and HI takes place according to Markovnikov’s rule even in the presence of peroxide The alkaline potassium permanganate solution is known as Baeyer’s reagent. It has bright pink colour. It oxidizes alkenes to glycols which is colour less. This reaction is used as a test for the presence of double bond in a molecule. This is also known as Baeyer test Bromine water test and Baeyer’s test are used to detect the presence of double bond while ozonolysis is used to detect the position of double bond.

AEROMATI HYDROCARBON

Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non-benzenoids.

The orbital overlapping picture benzene- All the six carbon atoms in benzene are sp 2 hybridized and these hybrid orbitals form sigma bonds 

The six π electrons are delocalized and can move freely about the six carbon nuclei. The delocalized π electron cloud is attracted more strongly by the nuclei of the carbon atoms than the electron cloud localized between two carbon atoms. Therefore, presence of delocalized π electrons in benzene makes it more stable .

Aromaticity:- The compounds that follow the following features are to be considered aromatic. (i) Planarity (ii) Complete delocalisation of the π electrons in the ring (iii) Presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .). This is often referred to as Hückel Rule.

Arenes are characterized by electrophilic substitution reactions proceed via the following three steps: (a) Generation of the eletrophile (b) Formation of carbocation intermediate (c) Removal of proton from the carbocation intermediate

CARCINOGENICITY AND TOXICITY-Benzene and polynuclear hydrocarbons containing more than two benzene rings fused together are toxic and said to possess cancer producing (carcinogenic) property.

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