Hydrocarbons

Chemistry project

What are hydrocarbons?

Compounds consisting of only hydrogen and carbons in it are known as hydrocarbons.

They are formed by either single ,double or tripe bonds so as to satisfy carbon’s tetravalent quality. It will be either a cyclic or a straight chain carbon compound.

Classification of

Organic chemistry

Classification of hydrocarbons

hydrocarbons

saturated unsaturated aromatic

Eg butane Eg butene Eg butyne

Saturated hydrocarbonsSaturated hydrocarbons contain carbon-carbon or carbon-hydrogen single bonds. If different carbon atoms are joined together to form open chain of carbon atoms with single bonds ,they are termed as alkanes. In organic compounds each carbon atoms are bonded with 4 hydrogen atoms so as to satisfy carbon’s tetravalent quality.

ethane

methane

PREPARATION

Alkanes can be prepared by the catalytic hydrogenation of unsaturated hydrocarbons in the presence of catalyst ‘Ni’ ,nickel, or ‘pt’ ,platinum at 200⁰C to 300⁰C.

FROM UNSATURATED HYDROCARBONS

•From alkyl halides

By Wurtz Reaction:When alkyl halides are heated with sodium metal in the presence of dry ethereal, alkanes are obtained (generally having double number of C-atoms than in alkyl halides). This reaction is known as Wurtz reaction and used for the preparation of symmetrical alkanes.

•By reduction

Alkyl halides (except fluorine) on reduction with zinc and dilute hydrochloric acid gives alkanes

CH3-cl+ H2 CH4 +HClChloromethane in presence of Zn,H+ gives methane and HCl

From carboxylic acids• By Kolbe’s electrolytic method

  Electrolysis of aqueous conc. solution of sodium or potassium salt of carboxylic acid gives alkanes.

     >>RCOONa—-Sodium salt      >>RCOOK—–Potassium salt

RCOONa——–> RCOO- + Na+

                                          anion             cation

By Heating Na-salt of Carboxylic acid

                When Na-salt of carboxylic acid is heated with soda lime (NaOH & CaO), alkane is obtained having one carbon less than salt by removal of a molecule of CO2. This reaction is also known as decarboxylation.

RCOONa + NaOH    R-H + Na2CO

Sodium ethanoate                         (methane)

This is the principle reaction for laboratory preparation of methane gas

Chemical properties1) Combustion reaction

They combust in oxygen to form carbon dioxide, water and heat. It is this heat which makes them good fuel sources.e.g.

C2H6 (g) + 7/2 O2(g)  → 2CO2(g) + 3H20(g) + heatC8H20 (g) + 13 O2(g) →   8CO2 (g) + 10H2O(g) + heat

2) Controlled oxidation alkanes on heating with a regulated supply of

dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.

2CH4 + O2 2CH3OH

3) Substitution reaction

They undergo substitution reactions with halogens in the presents of UV light.

CH4(g) + Cl2(g)   → CH4Cl(g) + HCl(g)

CH4(g) + 4Cl2(g) →   CCl4(g) + 4HCl(g)

Substitution reaction in cyclic compounds

4) Isomerisation n-alkanes on heating in the presence of anhydrous aluminum chloride and hydrogen chloride gas isomerise to branched chain alkanes

Unsaturated hydrocarbons

Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. Those with at least one carbon to carbon double bond are called alkenes and those with at least one carbon to carbon triple bond are called alkynes. These carbon compounds are bonded with the required number of hydrogen so as to fulfill the valency of carbon.

Trans-2-butene

H H H

H

H H H

2-butyne

H

preparationFrom alkyles alkyles on partial reduction with calculated amount of dihydrgen in the presence of palladised charcoal partially deactivated with poisons like sulphur compounds or quinoline give alkenes

From alkyl halides alkyl halides on heating with alcoholic potash eliminate one molecule of halogen acid to form alkenes. This reaction is known as DEHYDROHALOGENATION ,i.e., removal of halogen acid.

alkenes

From alcohols by acidic dehydration

In dehydration reactions, a molecule of water is eliminated from an alcohol molecule by heating the alcohol in the presence of a strong mineral acid. A double bond forms between the adjacent carbon atoms that lost the hydrogen ion and hydroxide group.

Dehalogenation. Vicinal dihalides, which are alkane molecules that contain two halogen atoms on adjacent carbon atoms, can form alkenes upon reaction with zinc.

From vicinal dihalides

Addition of dihydrogenAlkenes add up to one molecule of dihydrogen gas in the presence of finely divided nickel, palladium or platinum to form alkanes.

Chemical properties

Addition of halogensThe reaction with bromine (Br2)are used as a test for alkenes. Bromine solutions are brownish red. When we add a Br2 solution to an alkene, the color of the solution disappears because the alkenes reacts with the bromine:

Addition of waterThe reaction between an alkene and water to form an alcohol is called hydration, requires a catalyst—usually a strong acid, such as sulfuric acid (H2SO4):

Addition of sulphuric acid

Cold conc. Sulphuric acid adds to alkenes in accordance with markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction.

alkynespreparation

From calcium carbide

Ethyne (acetylene) preparation. Ethyne, which is commonly called acetylene, is the simplest alkyne. Historically, it was prepared by reacting calcium carbide with water.

From vicinal dihalidesThe loss of additional hydrogen and halogen atoms from the double‐bonded carbon atoms leads to alkyne formation. The halogen atoms may be located on the same carbon (ageminal dihalide) or on adjacent carbons (a vicinal dihalide).

Chemical propertiesAn alkyne reacts with hydrogen in the presence of catalyst (Pt or Ni) at 250°C, first forming alkenes and finally alkane.

Addition of hydrogen

Addition of HalogensAlkynes react with halogens (Cl2 or Br2) in the dark, forming dihaloalkenes first and finally tetrahaloalkanes. The reaction gets accelerated in the presence of light or halogen carriers.

Addition of Halogen Acids

Alkynes reacts with halogen acids according to the Markownikoff's rule i.e. the carbon atom carrying the least number of hydrogen atoms will have the negative part of the addendum attached to it.

Polymerization On heating, alkynes undergo polymerization in the presence of catalyst. The nature of products depends upon the conditions. For example,When ethyne (acetylene) is passed through a hot copper tube, it polymerizes to benzene.

OxidationOxidation of alkynes gives mono or dicarboxylic acids.

For example,Alkaline KMnO4 oxidizes ethyne to oxalic acid.

AROMATIC HYDROCARBONS

These hydrocarbons are also known s ‘arenes’ or most of such aromatic compounds are found to contain benzene ring, which are highly saturated but in most of aromatic compounds are found to unsaturated too .Aromatic compounds containing benzene rings are known as benzenoids and those not containing benzene rings are known as non-benzenoids.

preparationCyclic polymerization of ethyneEhthyne on passing through red hot iron tube at 873k undergoes

cyclic polymerization. Three molecule polymerize to form benzene ,which is the starting molecule for the preparation of derivatives of benzene, dyes, drugs and large number of other organic compounds. This is the best route for entering from aliphatic to aromatic compounds .

Decarboxylation of aromatic acidsSodium salt of benzenoic acid on heating with soda lime gives benzene.

Reduction of phenolPhenol is reduced to benzene by passing its vapor over heated zinc dust

Chemical properties Electrophilic substitution reactionsNitration

A nitro group is introduced into benzene ring when benzene is heated with a mixture of con. Nitric acid & con sulphuric acid.

Halogenations Arenes react with halogens in the presence of

a Lewis acid to yield haloarenes.

Addition reactions Under vigorous conditions ,i.e., at high

temperature and /or pressure in the presence of nickel catalyst , hydrogenation of benzene gives cyclohexane.

Combustion

When heated in air ,benzene burns with sooty flame producing CO2 and H2O

C6H6 + (15/2 )O2 6 CO2 +3 H2O

Thank you

BySreema

UmanathXi-- A