Alkanes Chemistry Assignment Help




homework assignment help is most useful online help portal for the students that providing all Online Alkanes Chemistry assignment help Services.Alkanes (also known as paraffins or saturated hydrocarbons) are chemical compounds that consist only of the elements carbon (C) and hydrogen (H) (i.e., hydrocarbons), wherein these atoms are linked together exclusively by single bonds (i.e., they are saturated compounds). Alkanes belong to a homologous series of organic compounds in which the members differ by a constant relative molecular mass of 14.

Each carbon atom must have 4 bonds (either C-H or C-C bonds), and each hydrogen atom must be joined to a carbon atom (H-C bonds). A series of linked carbon atoms is known as the carbon skeleton or carbon backbone. In general, the number of carbon atoms is often used to define the size of the alkane (e.g., C2-alkane).

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Chemical Properties of Alkanes

Alkanes are saturated hydrocarbons with general formula CnH2n+2. Thus members of the homologous series of alkanes differ by unit of –CH2. All alkanes are named with a suffix –ane and first part of name is the number of carbon atoms in the chain. If 1Hydrogen is removed from the general formula of alkane it becomes CnH2n+1. These are called as Alkyl groups and are represented by R.

Alkanes are also called as ‘Paraffin’ because these compounds show very low reactivity towards most reagents. Major sources of alkanes are petroleum and natural gas. We restore different alkanes while fractional distillation of petroleum. Methane is one of the major components of natural gas.

Chemical Properties of Alkanes:-

Alkanes are saturated hydrocarbons so they are linked by single covalent bonds and these bonds are hard to break thus they are stable and so lesser chemical reactivity is there. The chemical properties of Alkanes are:

1) Combustion reactions:-

a) Combustion in excess of air:- On burning alkanes in excess of air carbon-dioxide and water vapor is formed with evolution of heat. Amount of heat evolved depends on the number of Carbon atoms. More the number of carbons in the hydrocarbon more are the heat evolved.

General equation:

CnH2n+2    +   (3n+1/2) O2 -------------> nCO2   + (n+1) H2O + Heat.

Example:-  CH4  +  2O2 -------------->  CO2 + 2H2O + Heat.

                 2C2H6 + 7O2 -----------> 4CO2 + 6H2O + Heat.

 b) Combustion in limited supply of air:-  When combustion takes place in limited supply of oxygen then carbon monoxide is produced instead of carbon dioxide. Carbon monoxide is a poisonous gas. Heat produced is also less.

Example:- 2CH4  +  3O2  -----------> 2CO  +  4H2O  +   Heat.

                2C2H6 + 5O2 ------------> 4CO + 6H2O +   Heat.

2) Controlled Oxidation:-

a) Hydrocarbon ---------> Alcohol. 

When 1volume of methane is mixed with 9volumes of oxygen is passed through Copper tube at 200°C, methyl alcohol is formed.

2CH4 + O2 –--------Cu(tube) 200°C --------> 2CH3OH.

b) Hydrocarbon ---------> Aldehyde.

When mixture of methane and oxygen is passed over heated molybdenum oxide (catalyst) formaldehyde is formed when the temperature range is 300°C- 500°C.

CH4 + O2 ----------MoO (350°-500°C) --------> HCHO + H2O.

Chemical Properties of Alkanes (some more Reactions) :-

1)      Cracking/ Pyrolysis: -

When alkanes are heated to high temperatures in absence of air, mixture of saturated and unsaturated hydrocarbons is formed. This can also be done at a lower temperature by use of catalyst such as silica or alumina. This type is called catalytic cracking.

Example:-  C2H6 ------(SiO2)-------->  C2H4  +  H2.

2) Substitution Reactions: -

Alkanes undergo substitution reactions on reaction with Halogens . In presence of UV light and excess halogen reaction is chain reaction and proceeds till all the hydrogen of hydrocarbons are replaced.

Example:-CH4 + Cl2---------> CH3Cl + HCl

                  CH3Cl + Cl2 -------> CH2Cl2 + HCl

                  CH2Cl2 + Cl2 ------> CHCl3 + HCl

                  CHCl3 + Cl2 --------> CCl4 + HCl.

IUPAC Naming Alkanes:--

Naming of organic chemical compound

The term naming means nomenclature.  The existence of a large number of organic compounds is responsible for evolving a definite systematic method of naming organic chemical compounds.  There are mainly two types of naming systems.

1. Trival system: In the early stages of development of organic chemistry, when a few organic chemical compounds were known, they were named on the basis of their history or source.  For example, HCOOH was called formic acid as it was obtained from red ants.  Similarly CH3COOH was called acetic acid, as it was derived from acetum.  These names are called common names or trival names.  Common names of alkanes use prefixes such as normal (n), iso- and neo.

2. IUPAC system:  Trival names do not have well defined structural basis.  A new system was established based on the structure of the compound.  The IUPAC name of a compound contains three parts namely root word, suffix and prefix.

IUPAC naming alkanes:

IUPAC (International Union of Pure and Applied Chemistry) systematized the naming of organic compounds in 1957 by making a general set of rules for the nomenclature of organic compounds.

This system of naming organic compounds enumerates that there are three parts to the name of an organic compound:-

  • The word root – this is the Greek names that represent the number of carbon atoms present in the longest carbon chain of a given alkane.

  • The Suffix – This word follows the word root and indicated the nature of the carbon-carbon bonds present in an organic compound

  • The Prefix – This word indicates the substituent, the alkyl group or the branched carbon chain attached to the main carbon chain, and the position of it on the carbon chain.

The following paragraph explains how to name alkanes according to IUPAC naming alkanes

Explanation of Iupac Naming Alkanes

The Word root

As explained above, the word root is a Greek word that denotes the number of carbon atoms present in the main (parent) chain of the Alkane. The following table illustrates the greekG words to be used :-

Number of carbon atoms

Word Root (Greek)

1

Meth

2

Eth

3

Prop

4

But

5

Pent

6

Hex

7

Hept

8

Oct

9

Non

10

Dec

  • The main (parent) chain of the alkane is that chain of continuous carbon atoms which is the longest for that alkane.

The Suffix

The suffix denotes the type of carbon-carbon bonds in the hydrocarbon. Since alkanes always have single covalent bonds in their carbon chain, they will always have the suffix of “-ane”.

In the case of Alkenes and Alkynes, there are two and three covalent bonds present in the carbon chain between any two carbon atoms. In that case, the suffixes will be “-ene” and “-yne” respectively.

The Prefix

The prefix indicates any other molecule attached to an alkane in the place of a regular Hydrogen atom. It also indicates the position number of the carbon atom on which the group is attached.

There can be three type of such molecules, a substituent, an alkyl, or a carbon chain. The position number of carbon atoms on which they are attached are indicated by the position number of that carbon atom in the parent chain.

Organic Chemistry Alkanes:--

Introduction to organic chemistry alkanes

Alkanes : (Methane and Ethane) Alkanes are saturated hydrocarbons having the general formula CnH2n+2 . In these compounds , carbon shows sp3 hybridisation and due to this it forms four single covalent bonds . Alkanes are less reactive and are called as paraffins . The IUPAC names of alkanes depend on the number of carbon atoms in the continuous carbon chain .

Methane

Methods of preparation :

Reduction of methyl halides : When methyl halides are reduced with reagents like Zn + CH3COOH  ,  Zn + NaOH  , HI + P or Zn - Cu couple in ethyl alcohol , methane is formed .

            CH3 - I   +  2(H)    img(Zn-Cu)^(ethylalcohol)       CH4 (methane)   +  HI

Decarboxylation : When sodium salt of a carboxylic acid is heated with sodalime , carbon-dioxide is eliminated and an alkane is formed . CO2 is removed from the carboxylic acid and this reaction is called as decarboxylation . Alkane obtained in this reaction contains one carbon less than the corresponding sodium salt of the carboxylic acid . Thus for example , methane is formed when sodium acetate is heated with sodalime . Soda-lime is a mixture of CaO and NaOH .

               CH3COONa   +  NaOH    img(Cao)   CH4  +  Na2CO3

Chemical Properties : Like other alkanes it undergoes substitution reactions . In these reactions one or more hydrogen atoms are substituted by different groups . The reactions take place at high temperatures or in the presence of light .

1) Nitration : At 400 to 470C methane reacts with nitric acid vapours and gives nitromethane .

             CH3 - H   +   HO - NO2    img(400to475^0C)    CH3 NO2   +   H2O

2) Action with steam : Methane is used as a source of synthetic gas . At 1000oC methane reacts with steam in the presence of nickel and gives hydrogen . This process is used for the industrial preparation of hydrogen .

                   CH4   +   H2O     img(Ni)^(1000^oC)`   CO    +   3H2  

3) Pyrolysis : Methane undergoes pyrolysis when heated to 1000oC and gives carbon and hydrogen . Carbon obtained in this reaction is called as Carbon black and is used in the manufacture of tyres , black printing ink and paints etc .

                   CH4       img(1000^oC)     C    +    2H2

4) Oxidation : At high pressure and in the presence of catalysts methane undergoes oxidation and gives methyl alcohol or formaldehyde .

                       2CH4   +   O2     img(Cu,250^oC)    2 CH3OH

                       CH4     +   O2        img(MoO3)^(350-500^oC)       HCHO    +   H2

when it reacts with nitrogen or ammonia hydrogen cyanide is formed .

                       2 CH4   +   N2   img   2HCN    +   3H2 

                         CH4   +    NH3    img   HCN   +  3H2

Uses

  • Methane is used as a domestic fuel .

  • It is used in the manufacture of rubber tyres , printer's ink etc .

  • It is used in the manufacture of methyl alcohol , formaldehyde , hydrogen and synthetic gas .

Ethane

Methods of preparation:-

Kolbe's electrolysis : Ethane is also obtained by the electrolysis of sodium or potassium acetate solution .

                                      2 CH3COOK   +   2H2O    img  C2H6  +   2CO2   +   2KOH    +   H2

Sabatier - Senderen's reaction : A large quantity of  ethane is prepared by the catalytic hydrogenation of ethylene and this reaction is called Sabatier - Senderen's reduction reaction .

                        H2C = CH2   +   H2     img(Ni)^(300^oC)       C2H6

Reduction of ethyl iodide : Ethane is prepared by the reduction of ethyl iodide by Zn-Cu couple in ethyl alcohol .

                        C2H5I   +   2H    img    C2H6   +   HI

Chemical Properties

1) Nitration : Ethane reacts with nitric acid (vapour) at 400oC and gives nitroethane .

                                C2H6    +   HNO3    img(450^oC)    C2H5NO2     +    H2O  

2) Pyrolysis : When ethane is heated in the absence of oxygen it decomposes giving ethylene and hydrogen .

                               C2H6   img   H2C = CH2     +    H2

3) Oxidation : Ethane burns in air or oxygen to form CO2 and water .

                               2C2H6    +   7O2    img  4CO2   +   6H2O   ;  `Delta`H   =  -3116.6 kJ.mol-1

Uses

  • Ethane is used as a fuel .

  • It is used in the preparation of choloroethanes .

  • It is used in the preparation of ethylene .


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