Methods of preparation of alkanes alkenes and alkynes

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ALKANES

Organic compounds having only two elements, carbon and hydrogen are called hydrocarbons. They are divided into two main classes, aliphatic and aromatic.

The Alkane family

General formula: CnH2n+2

First member is Methane (CH4)

Also known as “Marsh gas”, chief constituent of natural gas (~97%)

Hybridisation sp3

Structure: tetrahederal

Bond angle: 109.5o

Intermolecular force of attraction: van der waal type

Preperation of alkanes

1. Wurtz reaction (for symmetrical alkanes)

When only one alkyl halide is taken

When two different types of alkyl halides are taken then we get a mixture of products.

This is because the two alky halides not only react with each other but with themselves as well. So, we cannot prepare an alkane with odd no. of carbon atoms by this method.

1. Reduction of alkyl halides

1. With hydrogen in presence of transition metal catalyst

1. With HI and red phosphorus

That’s why Grignard reagent is always protected from moisture because it is very reactive and immediately gets converted to alkane.

In Place of RMgX, RLi can also be used.

1. Using Lithiumdialkylcuprate (Corey-House synthesis)

A molecule of CO2 is removed from carboxylic acid. That’s why this reaction is called decarboxylation reaction.

CaO keeps NaOH dry because NaOH is hygroscopic (absorbs moisture).

1. Kolbe’s electrolysis (gives symmetrical alkane): An aq. Solution of sodium/potassium salt of a carboxylic acid is electrolysed to give alkane at anode.

Physical properties of alkanes

1. Non-polar molecules which are held by weak Van der Waal’s forces of attraction. These forces depend upon size (surface area) of the molecule. C1-C4 are gases, C5 –C17 are liquids and rest are solids.

2. They have low boiling points which inc. With inc. In chain length. Boiling point decreases with branching.

3. Melting point of alkanes with even number of carbon atoms is higher than that of next alkane with odd no. of carbon because melting point is a symmetry dependent property.

1. They are soluble in non-polar solvents like ether, benzene etc.

2. Alkanes are ligher than water. Their density increases With increases in no. of carbon atoms.

Chemical properties of alkanes

As they are saturated hydrocarbons, they are very less reactive and inert but they undergo some substitution reactions.

1. Halogenation: Replacement of one/more hydrogen by halogen atoms. (Details of mechanism have been discussed in free radical chemistry.)

Bromination is less rapid than chlorination.

Iodination requires the presence of an oxidizing agent because HI reverses this reaction.

In more complex alkanes, the abstraction of each different kind of H atom gives a different isomeric product 1o, 2o, 3o.

Three factors determine the relative yields of the isomeric product:

(1) Probability factor. This factor is based on the number of each kind of H atom in the molecule. For example, in CH3CH2CH2CH3, there are six equivalent 1º H’s and four equivalent 2º H’s. The odds on abstracting a 1º H are thus 6 to 4, or 3 to 2.

(2) Reactivity of H.: The order of reactivity of H is 3º > 2º > 1º.

(3) Reactivity of X.: The more reactive Cl. is less selective and more influenced by the probability factor. The less reactive Br. is more selective and less influenced by the probability factor. According to the reactivity-selectivity principle, if the attacking species is more reactive, it will be less selective, and the yields will be closer to those expected from the probability factor.

         In absence of air, they undergo pyrolytic cracking.

In presence of air, alkanes are oxidized to carbon dioxide and water with evolution of large amount of heat. That is why they are used as fuels.

1. Controlled oxidation: On heating with regulated supply of air at high pressure and in presence of suitable catalyst, alkanes give variety of products depending upon the condition.

                        Tertiary alkanes can be oxidised to corresponding alcohols using KMnO4.

1. Isomerisation: n-Alkanes on heating with anhyd. Aluminium chloride in presence of HCl isomerise to branched chain isomers.

1. Aromatization/reforming: n-alkanes having 6 or more carbons on heating at high temperature and pressure of 10-20 atm in presence of transition metal oxides get cyclised.

1. Methane reacts with steam at 1273 K in presence of Ni to give CO and H2.

Unsaturated hydrocarbons containing C=C having general formula CnH2n. They are also known as Olefins.

Nomenclature

1. The longest continuous chain should include both the carbon atoms of the double bond

2. The suffix used for double bond is –ene.

3. The chain is numbered from the end which gives lower no. to the first carbon containing double bond.

1. If there are two or more double bonds, it is named as adiene or atriene.

Isomerism

Alkanes show chain and position isomerism.

1. Chain Isomerism: Alkenes higher than ethane and propene show this isomerism.

1. Position isomerism: These isomers differ in position of double bond.

1. Geometrical isomerism: discussed in isomerism section

Preperation of alkenes

1. From alkyl halides: Dehydrohalogenation

This reaction is β−elimination reaction.

Ease of dehydrohalogenetion

Iodine > bromine > chlorine

Tertiary > Secondary > Primary

Saytzeff rule: In case of dehydrohalogenation, that alkene is the preferred product which has more no. of alkyl groups attached to the double bonded carbon atom.

1. From dihalogen derivative (Vicinal dihalides)

1. From alkynes: Pd-C, H2 (Lindlar’s catalyst) gives Cis alkene while Na/liquid Ammonia (Birch reduction) gives Trans alkene.

1. From alcohols: Dehydration of alcohols (β−ellimnation reaction)

Problem: Dehydration of: (a) CH3CH2CH2CH2OH yields mainly CH3CH=CHCH3 rather than CH3CH2CH=CH2, (b) (CH3)3CCHOHCH3 yields mainly (CH3)2C=C(CH3)2.

Ans: (a) The carbocation (R+) formed in this reaction is 1° and rearranges to a more stable 2° R2CH+ by a hydride shift (indicates as ~H:; the H migrates with its bonding pair of electrons).

(b)  The 2° R2CH+ formed undergoes a methide shift (~:CH3) to the more stable 3° R3C+.

1. By electrolysis of Sodium/potassium salts of dicarboxylic acids (Kolbe’s reaction)

Physical Properties

First 4 members are gases. C5-C18 are liquids and higher members are solids. They are colourless and odourless.

Melting points of alkenes are higher than alkanes because p electrons of double bond are polarizable. As a result, intermolecular forces of attraction are stronger. M.P. & B.P. inc. With inc. In the molecular mass but they do not show any regular trend. Trans-alkenes have higher m.p and lower b.p. than Cis-alkenes.

Dipole moments: Alkenes are weakly polar because p-electrons of double bomds are easily polarisable. So, their dipole moments are higher compared to alkanes.

(More is discussed in geometrical isomerism.)

They are soluble in non-polar solvents.

Chemical Properties

As they have π bond present in them, they are quite reactive compared to alkanes because π electrons are weakly held and more exposed. They have a tendency to undergo electrophilic addition reaction.

The electrophilic addition takes place in 2 steps via the formation of carbocation intermediate.

Addition of Br2 to alkenes

It takes place via two mechanisms, one is given to explain the formation of cis product and other to explain trans.

.Alkynes

Alkynes are unsaturated hydrocarbons containing carbon-carbon triple bond having general formula as CnH2n-2. First member is Ethyne C2H2.

Nomenclature

1. They are named as derivative of corresponding alkanes replacing –ane by –yne.

2. The longest continuous chain should include both the carbon atoms of the triple bond.

3. The suffix used for triple bond is –yne.

4. The chain is numbered from the end which gives lower no. to the first carbon containing triple bond.

Just like Alkenes, they show chain and position isomerism.

As they are linear molecules (bond angle 180o), they do not show geometrical isomerism.

Preperation

1. By action of water on calcium carbide

1. By dehydrohalogenation of vicinal dihalides

1. By the action of zinc on tetrahalogen derivatives of alkanes

1. Higher alkynes can be prepared by heating sodium salt of acetylene with alkyl halide.

1. By electrolysis of aq. Solution of potassium salt of fumaric acid.

1. By dehalogenation of haloform by heating with silver powder.

1. Acetylene is prepared by passing hydrogen through electric arc discharge between Carbon electrodes.

Chemical reactions

They are more reactive than alkanes and alkynes due to the presence of two π bonds. They undergo addition and oxidation reactions readily. One important property of alkynes is the acidic nature of the hydrogen attached to the triple bonded carbon (because hydrogen is attached to sp hybridised carbon).

Reactivity of halogens: Chlorine > Bromine > Iodine

Iodination stops after one step resulting in trans product.

1. Addition of halogen acids

1. Addition of hydrogen is stereoselective depending upon the conditions used. (Already    discussed in preparation of alkenes)

1. Reaction with Boron hydride

With dialkylacetylenes, the products of hydrolysis and oxidation are cis-alkenes and ketones, respectively.

        Acetylene/ethylene produces polyacetylene/polyethylene on polymerisation.

         On passing through red hot iron tube at 873K, ethylene polymerises to form benzene.

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