Alkane, Alkene And Conjugated Dienes

Introduction

Organic chemistry deals with the number of hydrocarbons. Some of the major carbohydrates are alkane, alkene and conjugated dienes. These are the fundamental compounds composed only carbon and hydrogen. Alkanes are also known as the saturated hydrocarbons and utilized as fuel. Alkenes contain at least carbon-carbon double bond and used in plastic, agriculture and pharmaceuticals. Conjugated dienes contain alternate single and double bond and mostly used as intermediates in organic synthesis. In this article we will study structure and properties of alkane, alkene and conjugated dienes.

Alkanes

Alkanes are the saturated organic compounds which consist of single bonded carbon and hydrogen atoms. That means all atoms share only one pair of electrons with each other. The general formula for alkanes is C_nH_2n. Where, n is the number of atoms of carbon in their chemical structure. The simplest alkane is the methane, in which four hydrogen atoms are bonded together to carbon atom via single bonds. So, the four valance electrons of carbon will bond with one valance electron of each hydrogen atom. Some examples of alkanes are, methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂), etc.

Preparation of alkanes

Alkanes can be synthesized by using unsaturated hydrocarbon, from alkyl halides and from carboxylic acids.

From unsaturated hydrocarbons (hydrogenation)

Alkanes can be prepared from unsaturated hydrocarbons (alkene and alkyne) by the process hydrogenation. In this reaction dihydrogen gas is added to unsaturated hydrocarbons in presence of finely divided catalyst like nickel, palladium or platinum. For example, ethane is prepared by using ethene and dihydrogen gas in the presence of platinum at room temperature.

H2C=CH2 + H-H –> H3C-CH3

From alkyl halides

There are two methods by using alkyl halides.

Reduction reaction: Alkanes can be prepared from alkyl halides (except fluorides) by reaction with zinc and dilute hydrochloric acid. For example, methane can be prepared by using methyl chloride and dihydrogen gas in the presence of dilute hydrochloric acid.

H3C-Cl + H2 — > CH4 + HCl

Wurtz reaction: In this reaction alkyl halides are treated with sodium metal in the presence of ether solution. This reaction produces high yield of alkanes with even number of carbon atoms. For example, ethane is produced by using methyl bromide with sodium metal in presence of ether solution.

H3C-Br + 2Na + BrCH3 –> H3C-CH3 + 2NaBr

From carboxylic acids

There are two methods of alkane preparation from carboxylic acids.

Decarboxylation: In this reaction alkanes can be prepared by using carboxylic acids with the removal of carbon dioxide. This process is known as decarboxylation. This reaction produces alkane with a carbon atom lesser than that present in carboxylic acid. For example, methane is prepared by reacting sodium acetate with sodium hydroxide.

CH3COONa + NaOH — > CH4 + Na2CO3

Kolbe’s reaction: In this reaction alkane produced through electrolysis of sodium or potassium salt of carboxylic acid. For example,

2CH3COONa + 2H2O — > CH3 – CH3 + 2CO2 + H2 + 2NaOH

Properties of alkane

Alkanes are saturated hydrocarbons which are colourless and odourless. As they possess weak Van Der Walls forces, the first four member of alkanes (C1 to C4) are gases and C5 to C17 are liquids and those containing more than 18 carbon atoms are solids at 298 K.

Alkenes

Alkenes are the unsaturated hydrocarbons containing at least one carbon-carbon double bond. Alkenes are also known as olefins. First member of alkene family, ethene is very important in the chemical industry. Alkenes are more reactive than the alkanes due to presence of carbon-carbon double bond. The chemical reactivity of alkenes is affected by the number of alkyl groups attached to the C=C structural unit. This phenomenon is called as degree of substitution. Alkenes are classified according to the number of substitutions as follows.

Type of alkene	Structure
Monosubstituted	RCH=CH2
Disubstituted	RCH=CHR
Trisubstituted	R2C=CHR
Tetrasubstituted	R2C=CR2

Synthesis of alkenes

There are various methods of preparation of alkenes. Some of the important laboratory methods are described below.

Dehydration of alcohols

Alcohols on dehydration produces alkenes. There are two methods of dehydration of alcohols,

By passing the vapours of an alcohol over heated alumina, and
By heating alcohol with concentrated mineral acid, such as concentrated H2SO4 or concentrated H3PO4.

By passing the vapours of an alcohol over heated alumina (Al2O3), at high temperature, 350⁰C. the general reaction for this method id given below.

RCH2-CH2OH — > RCH=CH2

The order of dehydration of alcohols is, tertiary > secondary > primary.

Secondary and tertiary alcohols are best dehydrated by dilute sulfuric acid.

Another method of is by heating the alcohols with concentrated sulfuric acid at 180⁰C. the general reaction is given below.

C2H5OH — > H2C=CH2

Some other dehydrating agents like phosphoric acid and anhydrous zinc chloride may also be used.

Hydrogenation of alkynes

Alkenes can be easily obtained by hydrogenation of alkynes. An alkyne on controlled hydrogenation with hydrogen in the presence of nickel or palladium at 200⁰C give a corresponding alkene. The general reaction is given below.

CnH2n-2 + H2 — > Cn H2n

From dehydrogenation of haloalkanes

Alkenes can be obtained from alkyl halides (haloalkanes). These haloalkanes are usually bromo and iodo and less commonly, chloro derivatives. Haloalkanes on heating with alcoholic KOH loses one molecule of hydrogen halide to give alkene. The general reaction for this synthesis is given as below.

RCH2-CH2R + KOH — > RCH2=CH2R + KX + H2O

If two alkenes may be formed due to dehydrohalogenation of a haloalkane, the one which is most substituted is the main product. The order of reactivity of haloalkanes in dehydrohalogenation is, tertiary > secondary > primary.

Properties of alkenes

Alkenes contain at least one carbon-carbon double bond. This carbon-carbon double bond changes the physical properties of alkenes. At room temperature, alkenes exist in all three states of matter i.e. solid, liquid and gases. Melting and boiling points of alkenes are similar to that of alkanes, however, isomers of cis isomers of alkene have lower melting point than the trans isomers. Alkenes display a week dipole-dipole interactions due to the electron attracting sp² carbon. Alkenes are widely used to produce plastics, medicines and other useful materials.

Conjugated dienes

A diene is a hydrocarbon chain that has two double bonds that may or may not be adjacent to each other. The arrangements of these double bonds can have varying affects on the compound reactivity and stability. Conjugated dienes are two double bonds separated by a single bond,

CH3-CH2-CH=CH-CH=CH-CH2-CH3 (3,5-octadiene)

Nonconjugated (isolated) dienes are two double bonds which are separated by more than one single bond,

CH3-CH=CH-CH2-CH=CH=CH3 (2,5-heptadiene)

Stability of conjugated dienes

Conjugated dienes are more stable than non-conjugated dienes (both isolated and cumulated). This more stability is due to delocalization of charge through resonance and hybridization energy. This can also explain why alkyl radicals are more stable are much more stable than the secondary or even tertiary hydrocarbons. This happens due to the positioning of pi orbitals and ability for overlap to occur to strengthen the single bond between two double bonds. This overlapping of pi orbitals is known as the resonance.

Synthesis of dienes

Dehydrohalogenation of dihalides

Dienes can be prepared by dehydrohalogenation of dihalides by successive elimination reactions.

XCH2-CH2-CH2-CH2X — > H2C=CH-CH=CH-CH3

Dehydration of diols

Dienes can be prepared by dehydration of diols by two successive elimination reactions.

OH-CH2-CH2-CH2-CH2-OH — > H2C=CH-CH-CH=CH2

Other methods of preparation of dienes are,

Butadiene is prepared in industry by thermal cracking of butane.
Dicyclopentadiene is prepared in industries from coal tar.

Properties of dienes

In case of dienes, the position adjacent to a carbon-carbon double bond is acidic. Dienes acidic character is due to the extra stability of allyl ion due to resonance. This effect involved to create more stable alkenes. Dienes are widely used as chelating ligands in organometallic chemistry. In some cases, they serve as placeholder ligands, being removed during a catalytic cycle.

Summary

Alkanes are hydrocarbons consisting of single bonds between carbon atoms. They have general formula CnH2n+2 and are commonly found in petroleum and natural gas. Alkanes exhibit properties such as low reactivity and are used as fuels and lubricants. Alkenes are hydrocarbons containing at least one carbon-carbon double bond, known as unsaturated hydrocarbons. They have general formula CnH2n and exhibit higher reactivity compared to alkanes due to presence of pi bonds. Alkenes are important in organic synthesis and are used in the production of plastics, solvents, and other chemicals. Conjugated dienes are a subclass of dienes where there are two or more double bonds separated by a single bond, leading to a delocalization of pi electrons across the molecule. This conjugation imparts unique properties to conjugated dienes, making them useful in various applications such as in the synthesis of polymers, pharmaceuticals and natural products. Conjugated dienes exhibit different reactivity patterns compared to simple alkenes due to their extended pi system.

Alkane, Alkene and Conjugated Dienes