Chapter 18: Hydrocarbon and its Compounds
Compounds around us are of two types: compounds obtained from minerals (non-carbonic / inorganic compounds) and compounds obtained from plants and animals (carbonic / organic compounds).
All compounds of carbon are called organic compounds, except its oxides, carbonates, bicarbonates, and carbides (for example CO2, CO, HCO3-, CO3^2- are NOT organic compounds).
Organic compounds made up of only carbon and hydrogen are called hydrocarbons.
1. Hydrocarbon
What is a Hydrocarbon?
A hydrocarbon is an organic compound composed only of carbon (C) and hydrogen (H) atoms.
Substances that burn (combustible) and are obtained from plants and animals are organic compounds. Examples: firewood, kerosene, charcoal, spirit.
Body substances like protein, hormone, carbohydrate, fat, enzyme, and protoplasm are also organic compounds.
Common sources of hydrocarbons: petroleum, natural gas, coal, and biogas.
Very Important for SEE: Petroleum is the main source of hydrocarbons. Hydrocarbons are classified as saturated and unsaturated on the basis of the type of covalent bond between carbon atoms.
2. Saturated Hydrocarbon (Alkane)
Definition
Hydrocarbons having only a single covalent bond between carbon atoms are called saturated hydrocarbons.
- They are very stable and less reactive, so they are also called paraffins.
- They are also called alkanes.
- General formula: CnH2n+2, where n = number of carbon atoms (n = 1, 2, 3, 4 ...).
Ball-and-stick molecular models of methane (CH4), ethane (C2H6), and propane (C3H8).
Common Alkanes
| Name | Molecular formula | Condensed formula |
|---|---|---|
| Methane | CH4 | CH4 |
| Ethane | C2H6 | H3C-CH3 |
| Propane | C3H8 | H3C-CH2-CH3 |
| Butane | C4H10 | H3C-(CH2)2-CH3 |
| Pentane | C5H12 | H3C-(CH2)3-CH3 |
SEE Focus: In the structural formula, every carbon atom forms 4 single bonds — either to hydrogen or to another carbon.
3. Unsaturated Hydrocarbon
Definition
Hydrocarbons having a double or triple covalent bond between carbon atoms are called unsaturated hydrocarbons.
- They are less stable and highly reactive, so they are also called olefins.
- They are of two types: alkene and alkyne.
Alkene (Double Bond)
Hydrocarbons having a double covalent bond (C=C) between carbon atoms are called alkenes. Example: ethene, propene, butene.
General formula: CnH2n, where n = number of carbon atoms.
| Name | Molecular formula | Condensed formula |
|---|---|---|
| Ethene | C2H4 | H2C=CH2 |
| Propene | C3H6 | H3C-CH=CH2 |
| Butene | C4H8 | H3C-CH2-CH=CH2 |
Alkyne (Triple Bond)
Hydrocarbons having a triple covalent bond (C≡C) between carbon atoms are called alkynes. Example: ethyne, propyne, butyne.
General formula: CnH2n-2, where n = number of carbon atoms.
| Name | Molecular formula | Condensed formula |
|---|---|---|
| Ethyne | C2H2 | HC≡CH |
| Propyne | C3H4 | H3C-C≡CH |
| Butyne | C4H6 | H3C-CH2-C≡CH |
Comparison of single, double, and triple covalent bonds in alkane, alkene, and alkyne.
Hydrogenation
The process of converting an unsaturated compound into a saturated compound by adding hydrogen is called hydrogenation. Vanaspati ghee is made from vegetable oil by hydrogenation; unsaturated fatty acids change into saturated fatty acids.
Equation: H2C=CH2 (Alkene) + H2 → C2H6 (Alkane)
Difference Between Saturated and Unsaturated Hydrocarbon
| Saturated Hydrocarbon | Unsaturated Hydrocarbon |
|---|---|
| Has only a single covalent bond between carbon atoms. | Has a double or triple covalent bond between carbon atoms. |
| Stable and less reactive. | Unstable and highly reactive. |
| Called alkane, general formula CnH2n+2. Example: CH4, C2H6, C3H8. | Called alkene (CnH2n) or alkyne (CnH2n-2). Example: C2H4, C3H4. |
4. Homologous Series
A series of compounds that can be represented by the same general formula is called a homologous series. Each member of the series is called a homologue.
- Adjacent homologues differ by a -CH2- group.
- The molecular weight of adjacent members differs by 14.
| Name | Molecular Formula |
|---|---|
| Methanol | CH3OH |
| Ethanol | CH3CH2OH |
| Propanol | CH3CH2CH2OH |
5. Alkyl Radical
The group of atoms formed by removing one hydrogen atom from an alkane molecule is called an alkyl radical.
General formula: CnH2n+1. Examples: -CH3 (methyl), -C2H5 (ethyl).
6. Functional Group
An atom or group of atoms that determines the structure and chemical reactivity of a group of organic compounds is called a functional group.
- Functional groups attach to alkyl groups to form different families of organic compounds.
- Examples of functional groups: -O- (ether), -CHO (aldehyde), -COOH (carboxylic acid), -OH (alcohol).
7. Nomenclature of Hydrocarbons (IUPAC)
IUPAC stands for International Union of Pure and Applied Chemistry. It is a system that gives every organic and inorganic compound one uniform name, avoiding confusion.
Steps of Common Nomenclature
- 1Find the word root based on the number of carbon atoms.
- 2Add the suffix -ane (single bond), -ene (double bond), or -yne (triple bond) according to the bond type.
| No. of carbon atoms | Word root | Notation |
|---|---|---|
| C-1 | Meth | C1 |
| C-2 | Eth | C2 |
| C-3 | Prop | C3 |
| C-4 | But | C4 |
| C-5 | Pent | C5 |
| C-6 | Hex | C6 |
| C-7 | Hept | C7 |
| C-8 | Oct | C8 |
| C-9 | Non | C9 |
| C-10 | Dec | C10 |
Example: CH3-CH3 has 2 carbon atoms, so the word root is 'eth'. There is only one bond between the carbons, so the suffix is '-ane'. Name = eth + ane = ethane.
8. Important Hydrocarbons and Their Uses
Methane (CH4)
Methane is found in marshy places, so it is also called marsh gas. It is found above mineral oil, and also in biogas (gobar gas) and sewage gas. It is colourless, odourless, and tasteless. It is insoluble in water but soluble in organic solvents like ether and alcohol.
- Used for cooking as gobar gas / biogas and as an industrial fuel.
- Used to prepare carbon black (printing ink, shoe polish, paint).
- Used to prepare chloroform, carbon tetrachloride, methyl alcohol, and formaldehyde.
- Used in the industrial preparation of hydrogen gas.
Ethane (C2H6)
Ethane is a saturated hydrocarbon with two carbon atoms joined by a single covalent bond. It is found with methane in natural gas, coal gas, and petroleum. It is colourless, odourless, tasteless, insoluble in water, and soluble in organic solvents.
- Produces heat on combustion, so it is used in metal welding.
- Used to prepare organic compounds such as ethyl chloride and nitroethane.
Propane (C3H8)
Propane is a saturated hydrocarbon with three carbon atoms. It is found in natural gas and petroleum mines, and is colourless and odourless.
- Highly flammable, so it is used as a fuel.
- Used to make different organic compounds.
- Used as a coolant in petroleum industries.
Butane (C4H10)
Butane is a saturated hydrocarbon with four carbon atoms. It is found in natural gas and petroleum mines, and is colourless and odourless.
- Used as a raw material for making synthetic rubber.
- Mixed with methane in LPG (Liquefied Petroleum Gas), used as fuel. It changes easily to liquid under pressure.
Uses of hydrocarbons as fuel: a biogas (gobar gas) plant and an LPG cylinder.
9. Alcohol
Alcohols are compounds formed by replacing one or more hydrogen atoms of an alkane with the -OH (hydroxyl) group. Hydroxyl (-OH) is the functional group of alcohols.
General formula: CnH2n+1OH.
| Type | Number of -OH groups |
|---|---|
| Monohydric alcohol | 1 |
| Dihydric alcohol | 2 |
| Trihydric alcohol | 3 |
Methyl Alcohol (Methanol) — CH3OH
Methanol is a monohydric alcohol. Structure: H3C-OH (one carbon bonded to 3 hydrogens and one -OH group).
- Used to prepare methylated spirit.
- Used as a solvent for paints and varnishes.
- Used to prepare perfumes, paints, medicines, and synthetic clothes.
- Used to manufacture formaldehyde.
- Used in dry cleaning.
Ethyl Alcohol (Ethanol) — C2H5OH
Ethanol is also a monohydric alcohol. Structure: H3C-CH2-OH.
- Used to prepare drinks like whisky, wine, and beer.
- Used to sterilize syringes and wounds in hospitals and health posts.
- Used to preserve biological specimens in laboratories.
- Used in thermometers.
- Used in manufacturing polyethylene, terylene, soaps, paints, and dyes.
- Used as an organic solvent.
Containers of methanol (methyl alcohol) and ethanol (ethyl alcohol) used in industry and medicine.
Glycol (Dihydric Alcohol)
Glycol is a dihydric alcohol; each carbon of the hydrocarbon carries one hydroxyl group. A common example is ethane diol (IUPAC: ethane-1,2-diol), molecular formula CH2OH-CH2OH. Glycols are used in industry and food products.
Glycerol (Trihydric Alcohol)
Glycerol is formed by replacing three hydrogen atoms of propane with three hydroxyl groups. It is a trihydric alcohol, also called glycerine. Its name comes from a Greek word meaning 'sweet'. It is a colourless, sweet, viscous liquid, soluble in water and alcohol, but insoluble in ether.
IUPAC name: propane-1,2,3-triol. Molecular formula: C3H5(OH)3.
- Protects skin by trapping moisture and preventing dryness.
- Used as a sweetening agent in food.
- Used to make good quality soap, lotion, cosmetics, and shaving creams.
- Keeps tobacco moist; prevents fruits and food from drying and decaying.
Everyday uses of glycerol: skin lotion, soap, and cosmetics.
Common Mistakes in SEE
- Do not confuse alkane (single bond, -ane), alkene (double bond, -ene), and alkyne (triple bond, -yne).
- Do not mix up the general formulas: alkane CnH2n+2, alkene CnH2n, alkyne CnH2n-2.
- Do not confuse monohydric (1 -OH), dihydric (2 -OH), and trihydric (3 -OH) alcohols.
- Remember CO2, CO, HCO3-, and CO3^2- are NOT organic compounds even though they contain carbon.
- Always write molecular formula correctly with the right subscript numbers.
Quick Revision
- Hydrocarbon = compound of only carbon and hydrogen.
- Saturated hydrocarbon (alkane) = single bond, CnH2n+2, stable.
- Unsaturated hydrocarbon = double bond (alkene, CnH2n) or triple bond (alkyne, CnH2n-2), reactive.
- Homologous series = same general formula, adjacent members differ by CH2 (mass difference 14).
- Alkyl radical = alkane minus one H, formula CnH2n+1.
- Functional group decides the chemical family, e.g. -OH for alcohol, -COOH for carboxylic acid.
- IUPAC name = word root (based on carbon number) + suffix (-ane/-ene/-yne).
- Methanol, ethanol = monohydric alcohols; glycol = dihydric; glycerol = trihydric.
- Hydrogenation turns unsaturated fat/oil into saturated fat (e.g. Vanaspati ghee).