The number of organic compounds, which are present around us and in our environment, is infinite. The large numbers are credited to the property of catenation as exhibited by carbon. Carbon can make four strong bonds; which, in turn, results in the formation of chain and ring structures. As the name indicates, hydrocarbons are organic compounds of hydrogen and carbon. Although the hydrocarbons are composed of only carbon and hydrogen, they occur in varied forms. This diversity can be attributed to the fact that hydrocarbons are present in the form of linear compounds, branched-chain structures, and ringed forms. The most common and the simplest hydrocarbon is methane. Hydrocarbons are versatile in their utility and are also present in crude oil, coal, natural gas, and other sources of energy; naturally.
Nonetheless, hydrocarbons never miss a chance to play a significant role in our daily life. The fuels that we use as sources of energy like CNG and LPG are nothing but a mixture of hydrocarbons only. Besides the uses mentioned above, the hydrocarbons render other great advantages as well. Let’s discuss the hydrocarbons in detail.
Classification of Hydrocarbons
The hydrocarbons can be classified into three types; depending upon the types of carbon-carbon bonds present;
I. Saturated Hydrocarbons: The saturated hydrocarbons contain only carbon-carbon and carbon-hydrogen single bonds. They include open-chain as well as closed-ring structures. Alkanes and cycloalkanes are examples of saturated hydrocarbon; which are formed when different carbon atoms join to form an open-chain or a ringed structure.
II. Unsaturated Hydrocarbons: The unsaturated hydrocarbons contain multiple bonds; carbon makes double or triple bonds with other carbon atoms. The unsaturated hydrocarbons are of two types; alkene (containing double bonds) and alkynes (containing triple bonds).
III. Aromatic Hydrocarbon: The aromatic hydrocarbons are cyclic compounds. The aromatic compounds may contain one or more benzene rings.
There is also a homologous series of hydrocarbons. In the homologous series, a group of chemically similar compounds have a general formula.
As mentioned above, alkanes are saturated hydrocarbons and contain only single carbon-carbon bond. Methane is the simplest and the most common saturated hydrocarbon.
Replacing one of the hydrogens of methane with another methyl (-CH3) group leads to the formation of another alkane, known as ethane.
Alkanes fail to react with acids, bases, and other reagents, therefore, they are also known as ‘Paraffins.’ According to the homologous series, the general formula of alkanes is CnH2n+2.
Isomerism of Alkanes
The simplest alkanes, which are methane, ethane, propane, have only one structure.
The next alkane with four carbons (C4H10) occurs either as a continuous chain or branched chain as follows:
Similarly, the structure of C5H12 can be arranged in the following three ways:
As the branching of a structure increases, hence, the boiling point decreases. Therefore, neopentane (b.p. 282.5 K) has the lowest boiling point, and pentane (b.p. 309 K) has the highest boiling point. The boiling point of isopentane (b.p. 301 K) is intermediate between neopentane and pentane.
Structural Isomers: All the different structures of the same compound have different properties. The difference in properties is because of different structures; therefore, they are called structural isomers.
Chain Isomers: The structural isomers which have different configurations concerning the carbon chains are called chain isomers. C4H10 has two chain isomers whereas C5H12 has three chain isomers.
Preparation of Alkanes
1. By Catalytic Hydrogenation of Unsaturated Hydrocarbons (Sabatier and Senderen’s Method)
- Hydrogenation is the process whereby dihydrogen gas gets added to the alkenes and alkynes to form alkanes.
- The process occurs in the presence of finely divided platinum, palladium, and nickel. These metals act as catalysts.
- The dihydrogen gas is adsorbed on the surface of metals; which leads to the activation of hydrogen-hydrogen bonds.
- Nickel is used for carrying out the hydrogenation at high temperature and pressure. Palladium and platinum are capable of catalysing the reaction at room temperature.
2. From Alkyl Halides
- Zinc and hydrochloric acid (dil.) reduce alkyl halides (exception: fluorine) to yield alkanes.
- Wurtz Reaction: Higher alkanes are also obtained by treating sodium metal with alkyl halides in the presence of dry ether. This reaction is important because it is used to increase the length of the carbon chain (only in even numbers), also known as the ascent of the series.
3. By Soda-Lime Decarboxylation (from carboxylic acids)
- Alkanes are also obtained by heating sodium salts of carboxylic acids with soda-lime. This reaction which eliminates carbon-dioxide from the carboxylic acid is known as decarboxylation. The carbon dioxide is removed in the form of sodium carbonate, Na2CO3.
- This reaction is employed to decrease the length of the carbon chain, also known as the descent of the series.
4. Kolbe’s Electrolysis
- Another method to get alkanes is Kolbe’s electrolysis; in which an aqueous solution of sodium or potassium salt of a carboxylic acid undergoes electrolysis.
- The alkanes obtained contain even number of carbon atoms. However, methane cannot be prepared by Kolbe’s electrolysis.
Physical Properties of Alkanes
- The first four members (C1 to C4) are colourless gases whereas the next thirteen members are (C5 to C17) are colourless liquids and the higher members are colourless solids. The probable explanation to this phenomena lies in the fact that with the increase in the molecular mass, the strength of the van der Waals force also increases; which, in turn, leads to a solid character.
- With an increase in the molecular mass of straight chain alkanes, the boiling point of the alkanes also increases. On an average, each -CH2 group increases the boiling point by 20-30K. The increase in the molecular mass increases the size of the molecule which, in turn, leads to an increase in the surface area of the molecule. Thus, the van der Waal forces of attraction increase. Therefore, the boiling point of straight chain isomers is more than the branched chain isomers.
- The melting point of the alkanes shows irregular variation with the increase in the molecular size. The alkanes with the even number of carbon atoms have a higher melting point as compared to the next lower alkane with the odd number of carbon atoms. This is known as the Alternation Effect. The alkanes with the even number of carbon atoms have higher melting points because such alkanes fit well into the crystal lattice since they are more symmetrical. Henceforth, more energy is required to break them.
- Alkanes are non-polar in nature; therefore, they are soluble in only non-polar solvents and insoluble in polar solvents like water.
Chemical Properties of Alkanes
All alkanes are chemically inert; because of the presence of saturation (C-C bond). Nonetheless, they undergo the following reactions:
1. Substitution Reactions: In substitution reactions, the halogens, nitro group, and sulphonic acid group replace one or more hydrogens of the alkanes.
- The halogenation of the alkanes is the process which involves the replacement of one or more H atoms of an alkane with the corresponding number of halogen atoms. Halogenation occurs at high temperatures (573-773 K) or in the presence of diffused sunlight or ultraviolet light. The rate of reaction of alkanes with halogens is F2 > Cl2 > Br2 > I2.
- The reaction in which the H atom of an alkane is substituted by a nitro group (-NO2) is called nitration. Hexanes and higher alkanes can be nitrated very easily.
- The reaction in which H atoms of alkanes are replaced by sulphonic group (-SO3H) is called sulphonation.
2. Isomerization: The process of conversion of one isomer into another is called isomerization.
3. Aromatization: The conversion of the aliphatic compound into aromatic compounds is known as aromatization. The alkanes with six carbon atoms or more are heated up to 773K at 10-20 atmospheric pressure. The reaction takes place in the presence of oxides of vanadium, molybdenum or chromium supported over alumina. The alkanes get dehydrogenated and cyclise to yield benzene or its homologues.
4. Oxidation of Alkanes
- Combustion (Complete Oxidation): Alkanes on complete combustion result in the formation of CO2 and H2O.
- Controlled Oxidation: When alkanes are heated at high temperature and pressure in the presence of appropriate catalysts, a variety of oxidation products are formed.
5. The Action of Steam: Methane in the presence of nickel catalyst at 1273K reacts with steam to yield carbon-dioxide and dihydrogen as the end product.
6. Pyrolysis: The higher alkanes when heated to the higher temperature disintegrate into lower alkanes, alkenes, etc.
The unsaturated hydrocarbons containing carbon-carbon double bond and having general formula CnH2n are called alkenes. These are also called ‘Olefins‘; because lower gaseous members form oily products when treated with chlorine. The double bond in alkenes is known as the ethylenic bond or olefinic bond.
Isomerism of Alkenes
Alkenes display structural as well as geometrical isomerism.
- Structural Isomerism: Simple alkenes like ethene (C2H4) and propene (C3H6) have only one structure but higher alkenes usually have more than one structural isomers; for example, there are three structural isomers for butene (C4H8).
- Geometric Isomerism: This is a type of stereoisomerism. The geometric isomers are compounds with the same number of atoms, and types of bonds and atoms but have different geometries for the atoms.
Preparation of Alkenes
1. By the Controlled Hydrogenation of the Alkynes: Alkenes are formed by partial reduction of alkynes with the calculated amount of dihydrogen. The hydrogenation reaction takes place in the presence of Lindlar’s catalyst; which is deactivated with poisons like sulphur compounds or quinoline. The palladised charcoal is often referred to as Lindlar’s catalyst.
2. By dehydrohalogenation of Alkyl Halides: Dehydrohalogenation of alkyl halides, heated at the high temperature with alcoholic potash, leads to the elimination of a molecule of halogen acid and forms alkenes. This is also known as an elimination reaction.
3. By Dehalogenation of Vicinal Dihalides: Vicinal dihalides are the ones in which two halogen atoms are attached to two adjacent carbon atoms. Dehalogenation occurs when vicinal dihalides are treated with zinc metal.
Physical Properties of Alkenes
- Alkenes with two to four carbon atoms are gases, those containing five to fifteen are liquids, and higher alkenes are solids.
- They are insoluble in water but soluble in organic solvents.
Chemical Properties of Alkenes
In alkenes, the C=C bond is made up of a stable σ-bond and a reactive π-bond. As π-bonds can be easily broken, therefore, alkenes undergo addition reactions.
1. Addition of Dihydrogen: Alkanes are formed by the addition of dihydrogen to alkenes. The reaction takes place in the presence of finely divided nickel, platinum, and palladium.
2. Addition of Halogens: Vicinal Dihalides are formed when halogens are added to alkenes.
3. Addition of Hydrogen Halides: Alkyl halides form when hydrogen halides (HCl, HBr, HI) react with alkenes. The reactivity of hydrogen halides with alkenes is as follows; HI > HBr > HCl.
4. Addition of Sulphuric Acid: Alkyl hydrogen sulfate is formed when the cold concentrated sulphuric acid reacts with alkenes.
The compounds containing carbon-carbon triple bonds are called alkynes. The general formula of alkynes is CnH2n-2.
Isomerism of Alkynes
Ethyne and propyne have only one possible structure. There are two structures for butyne;
Preparation of Alkynes
1. From Calcium Carbide: Under laboratory conditions, ethyne is prepared by the reaction of calcium carbide with water. This is also known as Wohler’s Reaction.
2. From Dibromo Derivatives of Alkanes: Dehydrohalogenation occurs when vicinal dihalides or dibromo derivatives react with alcoholic potassium hydroxide. An elimination reaction yield alkenyl halide; which when treated with sodamide results in the formation of an alkyne.
Physical Properties of Alkynes
- The first three members (C2-C4) are gases, the next eight members (C5-C12) are liquids, and the next higher members are solids.
- They are all colourless and odourless with the exception of acetylene.
- The melting and boiling points of alkynes increase regularly with an increase in the molecular weights. The branched chain alkynes have lower boiling points than normal chain alkynes. The boiling and melting points are higher in alkynes than for alkenes and alkanes due to the greater polarity of bonds in alkynes.
- Alkynes are practically insoluble in water but soluble in organic solvents like alcohol, acetone, benzene, and carbon tetrachloride.
Chemical Properties of Alkynes
The carbon-carbon triple bond in alkynes consists of one strong σ-bond and two weak π-bonds. As the π electrons are loosely held by two carbon nuclei; therefore, alkynes undergo electrophilic addition reactions. However, they are less reactive towards electrophilic addition reactions than alkynes.
1. Acidic character of Alkynes: When sodium metal and sodamide react with ethyne; sodium acetylide and dihydrogen gas are formed. The hydrogen atoms in ethyne are liberated more easily as protons compared to those in ethane and ethene. Therefore, hydrogen bonds attached to the carbon atoms in ethyne are acidic in nature.
2. Addition Reactions: Alkynes can easily add up dihydrogen, halogen, halogen halides, etc. A vinylic cation is formed and the formation of the addition product depends upon the stability of the vinylic cation.
- Addition of dihydrogen:
- Addition of halogen:
- Addition of halogen halides:
- Addition of water: Alkynes are insoluble in water. However, water can react with alkynes in the presence of warm mercuric sulphate and dilute sulphuric acid at 333 K.
Applications of Hydrocarbons
- Natural Gas and Fuels: Most of the natural fuel sources that we utilise in our day-to-day life are hydrocarbons only.
- Plastics: The plastic substances that we use in our daily life are the long chain of monomers formed from petrochemicals. The petrochemicals are hydrocarbons with different chemical composition.
- Paraffin: The waxes employed in the formation of candles; and used for medical and industrial purposes contain hydrocarbons.
- Asphalt: Asphalt when heated forms tar; which is a key industrial ingredient and used extensively in the construction of the roads.
- Pharmaceutics: Hydrocarbons are employed in the manufacturing of drugs.