Hydrocarbon and its Compounds

The compounds around us are sorted into two big groups:

• Inorganic (non-carbonic) compounds: at first, the compounds obtained from minerals: salt, water, lime, sand, metal ores.
• Organic (carbonic) compounds: at first, the compounds obtained from living things, plants and animals: sugar, oil, ghee, starch, protein, alcohol.

From this activity we can conclude that combustible substances are organic, and that they all contain carbon and hydrogen. Kerosene in village lamps, firewood for cooking, spirit in the laboratory burner, charcoal used to melt metals for making utensils: all of them contain carbon and hydrogen, and all of them came from plants or animals.

The substances in your body, such as proteins, hormones, carbohydrates, fats, enzymes and the living jelly of your cells (protoplasm), are all organic compounds.

What an organic compound is

Today, organic compounds are the compounds in which carbon atoms are joined by covalent bonds to other carbon atoms or to hydrogen atoms. Besides hydrogen, carbon often bonds with oxygen, nitrogen, the halogens (chlorine, bromine…), sulphur and phosphorus; a few organic compounds even contain metals.

A few small carbon compounds are counted as inorganic, because they behave like minerals: the oxides of carbon (CO₂, CO), the carbonates (CO₃²⁻, as in limestone), the hydrogencarbonates or bicarbonates (HCO₃⁻, as in baking soda) and the carbides (such as calcium carbide).

Examples of organic compounds: methane, ethane, ethene, acetylene, methanol, chloroform, urea, insulin, proteins, oils. The branch of chemistry that studies them is called organic chemistry (carbonic chemistry).

Organic compounds made of only carbon and hydrogen are called hydrocarbons. Their main source is petroleum (crude oil) and natural gas, made over millions of years from the remains of tiny sea creatures buried under the sea floor.

Hydrocarbons are sorted into two groups by the kind of bond between their carbon atoms: saturated and unsaturated.

Want more? Build water, ammonia, methane and many more molecules from atoms in the Molecule Kitchen game.

Saturated hydrocarbons: the alkanes

Hydrocarbons in which the carbon atoms are joined only by single covalent bonds are called saturated hydrocarbons. “Saturated” means full: each carbon already holds as many hydrogen atoms as it possibly can. They are very stable and react little, so they are also called paraffins (from Latin parum affinis, “little affinity”, little desire to react). Their family name is the alkanes.

The general formula of the alkanes is

$$\text{C}_n\text{H}_{2n+2}$$

where n is the number of carbon atoms (n = 1, 2, 3, 4…). So an alkane with 5 carbons has 2 × 5 + 2 = 12 hydrogens: C₅H₁₂.

Name	Molecular formula	Condensed formula
methane	CH₄	CH₄
ethane	C₂H₆	CH₃CH₃
propane	C₃H₈	CH₃CH₂CH₃
butane	C₄H₁₀	CH₃(CH₂)₂CH₃
pentane	C₅H₁₂	CH₃(CH₂)₃CH₃

The molecular formula only counts the atoms. The structural formula draws every atom and every bond. The condensed formula is a short way of writing the structure, carbon by carbon, in one line.

Unsaturated hydrocarbons: alkenes and alkynes

Hydrocarbons with at least one double or triple covalent bond between carbon atoms are called unsaturated hydrocarbons. Their carbons could hold more hydrogen: the extra bonds can open up and take in more atoms. That makes them less stable and much more reactive than alkanes. They form two families: the alkenes and the alkynes.

The alkenes also have an old name, the olefins, from olefiant gas, “oil-making gas”, the old name for ethene, because ethene and chlorine make an oily liquid. The alkynes have their own old name: the acetylenes.

Alkenes

Hydrocarbons with one double covalent bond between two carbon atoms are called alkenes: ethene, propene, butene and so on. Their general formula is

$$\text{C}_n\text{H}_{2n}$$

Name	Molecular formula	Condensed formula
ethene (ethylene)	C₂H₄	CH₂=CH₂
propene	C₃H₆	CH₂=CH–CH₃
butene	C₄H₈	CH₂=CH–CH₂–CH₃

Alkynes

Hydrocarbons with one triple covalent bond between two carbon atoms are called alkynes: ethyne, propyne, butyne and so on. Their general formula is

$$\text{C}_n\text{H}_{2n-2}$$

Name	Molecular formula	Condensed formula
ethyne (acetylene)	C₂H₂	CH≡CH
propyne	C₃H₄	CH≡C–CH₃
butyne	C₄H₆	CH≡C–CH₂–CH₃

Hydrogenation: from oil to vanaspati ghee

Because of their double bond, alkenes can add hydrogen. Heated with hydrogen gas and a nickel catalyst, ethene becomes ethane:

$$\underset{\text{alkene}}{\text{CH}_2\text{=CH}_2} + \text{H}_2 \xrightarrow[\text{about 200 °C}]{\text{Ni}} \underset{\text{alkane}}{\text{CH}_3\text{–CH}_3}$$

Adding hydrogen in this way is called hydrogenation. It is how vanaspati ghee (vegetable ghee, dalda) is made: liquid vegetable oils contain unsaturated fatty acids, with C=C double bonds that make them bend and stay liquid. Hydrogenation turns some of them into saturated fatty acids, which are straight, pack together, and make a solid fat.

Saturated and unsaturated hydrocarbons compared

Saturated hydrocarbons	Unsaturated hydrocarbons
Only single covalent bonds between carbon atoms.	At least one double or triple covalent bond between carbon atoms.
Stable; react little.	Less stable; very reactive.
Called alkanes (paraffins), CₙH₂ₙ₊₂, e.g. CH₄, C₂H₆, C₃H₈.	Alkenes, CₙH₂ₙ (e.g. C₂H₄, C₃H₆), and alkynes, CₙH₂ₙ₋₂ (e.g. C₂H₂, C₃H₄).
Can’t add hydrogen.	Add hydrogen (and other atoms) across the double or triple bond.
Burn with a clean blue flame.	Burn with a smoky, sooty flame.

Look at the alkanes again: CH₄, C₂H₆, C₃H₈, C₄H₁₀. Each one is the one before it plus one CH₂ (one more carbon with its two hydrogens). A family like this is called a homologous series.

A homologous series is a series of organic compounds that can be written with the same general formula, in which each member differs from the next by a –CH₂– group. Each member is called a homolog. Since CH₂ has a mass of 12 + 2 × 1 = 14, the molecular masses of neighboring members differ by 14.

Name	Molecular formula	Molecular mass
methanol	CH₃OH	32
ethanol	CH₃CH₂OH	46
propanol	CH₃CH₂CH₂OH	60

The members of a homologous series:

1. have the same general formula (CₙH₂ₙ₊₂ for alkanes, CₙH₂ₙ₊₁OH for alcohols);
2. differ by CH₂, and by 14 in molecular mass;
3. have the same functional group (below), so they have similar chemical properties, and can be made by similar methods;
4. show a gradual change in physical properties: the bigger the molecule, the higher its boiling point.

Alkane	Boiling point	At room temperature
methane, CH₄	−162 °C	gas
ethane, C₂H₆	−89 °C	gas
propane, C₃H₈	−42 °C	gas
butane, C₄H₁₀	−0.5 °C	gas
pentane, C₅H₁₂	36 °C	liquid
octane, C₈H₁₈	126 °C	liquid (in petrol)

Take one hydrogen atom away from an alkane and what is left has one free bond, ready to join something else. This group of atoms is called an alkyl radical (or alkyl group). Its general formula is

$$\text{C}_n\text{H}_{2n+1}\text{–}$$

Alkane	Alkyl radical	Name
methane, CH₄	CH₃–	methyl
ethane, C₂H₆	C₂H₅– (CH₃CH₂–)	ethyl
propane, C₃H₈	C₃H₇–	propyl

The name changes -ane to -yl. The free bond is written as a dash, CH₃– (or as a dot for its unpaired electron, CH₃•). Join a methyl group to –OH and you get CH₃OH, methyl alcohol.

The methyl group in CH₃OH, CH₃Cl and CH₃COOH is the same; what makes these three behave so differently is the group stuck to it. An atom or group of atoms that decides the structure and chemical properties of a family of organic compounds is called a functional group. Functional groups join onto alkyl groups to make the different families of organic compounds.

Functional group	Family	Ends in	Example
–OH (hydroxyl)	alcohols	-ol	CH₃OH, methanol
–CHO (aldehyde)	aldehydes	-al	HCHO, methanal (formaldehyde)
–COOH (carboxyl)	carboxylic acids	-oic acid	CH₃COOH, ethanoic acid (acetic acid, in vinegar)
–O– (ether)	ethers	ether	CH₃–O–CH₃, dimethyl ether
C=C	alkenes	-ene	CH₂=CH₂, ethene
C≡C	alkynes	-yne	CH≡CH, ethyne

Ethene is called ethylene in old books and olefiant gas in older ones; ethyne is acetylene; methanol is wood spirit. With so many names, mistakes are easy. So chemists agreed on one system, made by the IUPAC: the International Union of Pure and Applied Chemistry. In the IUPAC system each compound has one name, the same everywhere in the world, and the name tells you the structure.

To name a straight-chain hydrocarbon:

1. Count the carbon atoms in the chain and find the word root:

Carbon atoms	Word root	Carbon atoms	Word root
1	meth-	6	hex-
2	eth-	7	hept-
3	prop-	8	oct-
4	but-	9	non-
5	pent-	10	dec-

2. Add the suffix for the bonds between the carbon atoms: -ane for only single bonds, -ene for a double bond, -yne for a triple bond.

Methane, CH₄

Methane bubbles up from the mud of marshes and paddy fields, where dead plants rot without air, so it is also called marsh gas. It is found above deposits of petroleum and in natural gas (which is mostly methane), in gobar gas (biogas) and in sewage gas. It is a colorless, odorless, tasteless gas. It doesn’t dissolve in water but dissolves in organic solvents such as ether and alcohol.

Uses of methane:

1. As a fuel: biogas for cooking, and natural gas in industries and power stations.
2. To make carbon black (fine soot), used in printing ink, shoe polish, paint and car tyres.
3. To make chloroform, carbon tetrachloride, methyl alcohol and formaldehyde.
4. To make hydrogen gas in industry (most of the world’s hydrogen comes from methane and steam).

It burns with a clean blue flame:

$$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{heat}$$

Ethane, C₂H₆

Ethane is a saturated hydrocarbon of two carbon atoms joined by a single bond. It is found with methane in natural gas, coal gas and petroleum wells. Like methane, it is colorless, odorless and tasteless, insoluble in water, and soluble in organic solvents such as ether and alcohol.

Uses of ethane:

1. It gives out heat when it burns, so it is used as a fuel.
2. To make other carbon compounds, such as ethyl chloride and nitroethane. Its biggest use today is to make ethene, the raw material of polythene.

Propane, C₃H₈

Propane is a saturated hydrocarbon with three carbon atoms. It is found in natural gas and petroleum wells. It is a colorless and odorless gas, insoluble in water but soluble in organic solvents.

Uses of propane:

1. It burns easily and gives a lot of heat, so it is used as a fuel (in LPG, below).
2. To make other carbon compounds.
3. As a coolant in the petroleum industry (and, as “R-290”, in some modern air conditioners and fridges).

Butane, C₄H₁₀

Butane is a saturated hydrocarbon with four carbon atoms, found in natural gas and petroleum wells. It is a colorless, odorless gas, insoluble in water but soluble in organic solvents.

Uses of butane:

1. As a raw material for synthetic rubber.
2. As a fuel in LPG, liquefied petroleum gas. Butane turns into a liquid easily when squeezed (its boiling point is about 0 °C), so a lot of it fits in a cylinder.

LPG is a mixture of butane and propane, mostly butane in warm countries like Nepal and India. (Methane can’t be liquefied by pressure alone at room temperature, which is why natural gas is sold as compressed gas, CNG, not in LPG cylinders.)

Ethyne (acetylene), C₂H₂

You will meet ethyne in the exercises. It is made by adding water to calcium carbide; in the old “carbide lamps” of miners, a drip of water on calcium carbide made ethyne that burned with a bright flame. Burned with pure oxygen, ethyne gives one of the hottest flames there is, about 3,100 °C, hot enough to cut and weld steel (oxy-acetylene welding).

$$\text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_2 + \text{Ca(OH)}_2$$

Alcohols are the compounds formed when one or more hydrogen atoms of an alkane are replaced by –OH groups. The –OH group, the hydroxyl group, is the functional group of the alcohols. Alcohols with one –OH have the general formula

$$\text{C}_n\text{H}_{2n+1}\text{OH}$$

They are sorted by how many –OH groups they have:

• Monohydric alcohols: one –OH (methanol, ethanol).
• Dihydric alcohols: two –OH groups (glycol).
• Trihydric alcohols: three –OH groups (glycerol).

Methyl alcohol (methanol), CH₃OH

Methyl alcohol is a monohydric alcohol; its IUPAC name is methanol. Its molecular formula is CH₃OH; its structural formula is drawn in Figure 18.6.

Uses of methyl alcohol:

1. To make methylated spirit (ethanol with methanol added so that nobody drinks it).
2. As a solvent for paints and varnishes.
3. To make perfumes, paints, medicines and synthetic fabrics.
4. To make formaldehyde (methanal), used in plastics and to preserve specimens (as formalin).
5. In dry cleaning (as a solvent).

Ethyl alcohol (ethanol), C₂H₅OH

Ethyl alcohol is also a monohydric alcohol; its IUPAC name is ethanol. Its molecular formula is C₂H₅OH (also written CH₃CH₂OH).

Ethanol is made by fermentation: yeast, a tiny fungus, feeds on sugar without air and turns it into ethanol and carbon dioxide. This is how jaand (rice beer) and raksi are made; raksi is then distilled to make it stronger.

$$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$$

Uses of ethyl alcohol:

1. In alcoholic drinks such as beer, wine and whisky.
2. To sterilize syringes, skin before an injection, and wounds, in hospitals and health posts. Hand sanitizers are 60–80% alcohol: oddly, a 70% mixture kills germs better than pure alcohol, because a little water helps it get into the germ.
3. To preserve biological specimens in laboratories.
4. In thermometers (the red-colored ones): ethanol freezes only at −114 °C, so it works in places too cold for mercury.
5. To make polyethylene, terylene, soaps, paints and dyes.
6. As an organic solvent, and as a fuel in spirit lamps (and, mixed into petrol, in cars in many countries).

Glycol (ethane-1,2-diol)

Glycol is a dihydric alcohol: two –OH groups, one on each carbon atom of ethane. The common glycol is ethylene glycol, whose IUPAC name is ethane-1,2-diol (the 1,2 says the –OH groups are on carbons 1 and 2). Its formula is CH₂OH–CH₂OH, or C₂H₄(OH)₂. Glycols are used in industry and in some foods, but not this one: ethylene glycol is poisonous (the glycol in foods and medicines is a different one, propylene glycol, propane-1,2-diol). Ethylene glycol is the antifreeze put into car radiators in cold countries, and a raw material of polyester (terylene) cloth and plastic bottles.

Glycerol (propane-1,2,3-triol)

When three hydrogen atoms of propane, one on each carbon, are replaced by three –OH groups, we get glycerol, also called glycerine. It is a trihydric alcohol. Its name comes from the Greek word glykys, “sweet”. It is a colorless, sweet, thick (viscous) liquid; it dissolves in water and alcohol but not in ether. Its IUPAC name is propane-1,2,3-triol, and its molecular formula is C₃H₅(OH)₃.

Uses of glycerol:

1. To protect the skin: it draws in and holds moisture, so it stops the skin drying and cracking (in winter creams, and in plain glycerine sold at the chemist).
2. As a sweetener in food.
3. In good-quality soaps, lotions, cosmetics and shaving creams.
4. To keep tobacco moist, and to stop dried fruits and foods from drying out and spoiling.

1. Choose the best option

2. Write the differences between

3. Give reasons

4. Answer the following questions