Carbon And Its Compounds

The process which converts vegetable oil to vegetable ghee is called hydrogenation reaction.

Vegetable oil contains unsaturated carbon chains. When hydrogen is added to them in the presence of a catalyst (such as nickel, palladium) to form vegetable ghee which are saturated products.

Hydrogenation in general can be represented by the following reaction.

The general molecular formula of alkanes is C_nH_2n+2, where n is the number of carbon atoms present.

When n=1, the compound is CH₄.

When n=2, the compound is C₂H₆.

When n=3, the compound is C₃H₈ and so on.

Therefore the alkanes from the given set of hydrocarbons are CH₄, C₂H₆, C₃H₈.

Ethyne is an alkyne containing 2 carbon atoms which are connected by a triple bond. As carbon can have a maximum of 4 bonds around it (1 bond represents 2 shared electrons), the remaining one bond is satisfied by hydrogen atom.

Therefore, the electron dot structure looks like:

The structural formula can be drawn by replacing a pair of electrons by a short straight line or bond.

H—C≡C—H

A substance is a good conductor of electricity if it contains free electrons, so that its free flow would constitute an electric current. Covalent compounds are formed by the mutual sharing of electrons among atoms. In general, there are no free electrons in the compound which contribute to the conductivity, as the shared electrons are bound and attracted by the atoms. Therefore, covalent compounds are poor conductors of electricity.

The molecular formula of methanoic acid is HCOOH. Carbon should be surrounded by 4 bonds to attain stable electronic configuration. The valency of carbon (which is 4) is satisfied by one H, one –OH group, and one =O group attached to it.

Using this, one can draw the structure of methanoic acid:

Given that the compound decolourises bromine water. This means that the compound is unsaturated. i.e., it contains double/triple bonds.

The compound X is an alkene as it follows the general molecular formula of alkenes C_nH_2n for n=3. Thus the compound is propene. The structural formula of propene is:

CH₂=CH-CH₃

Ethane (C₂H₆) is an alkane with 2 carbon atoms. To obtain homologues, add a –CH₂ unit to the previous member of the series.

So, C₂H₆ + (–CH₂) → C₃H₈ (second member of the homologous series)

C₃H₈ + (–CH₂) → C₄H₁₀ (third member of the homologous series)

C₃H₈ is propane and C₄H₁₀ is butane.

(i) The prefix “eth-” corresponds to 2 carbon atoms, and the suffix “-ane” tells us that the compound is an alkane.

Ethane is a saturated hydrocarbon (alkane) containing 2 carbon atoms. The general molecular formula of alkanes is C_nH_2n+2, where n is the number of carbon atoms. For n=2, we get the molecular formula of ethane which is C₂H_2x2+2 or C₂H₆.

(ii) The prefix “eth-” corresponds to 2 carbon atoms, and the suffix “-oic acid” tells us that the compound is a carboxylic acid.

Ethanoic acid is a carboxylic acid containing 2 carbon atoms. One of the carbon atoms is included in the carboxylic acid functional group (-COOH or) and is attached to the carbon atom. The remaining valency is satisfied by hydrogen atoms.

Therefore, the molecular formula is C₂H₄O₂ (CH₃COOH).

Bromine water can be used to distinguish between ethane and ethane. Bromine water gets decolourised when it is added to unsaturated compounds due to the formation of addition product with the unsaturated compound. This is possible only when the carbon compound contains double/triple bonds which are later broken by the bromine molecules.

Saturated compounds do not decolourise bromine water as no reaction takes place.

When ethanol reacts with ethanoic acid, the product formed is ethyl ethanoate which is an ester. The reaction in which an alcohol and a carboxylic acid reacts to form an ester is called esterification reaction.

(a) Reaction with Na metal:

Ethanol reacts with sodium metal to liberate hydrogen gas.

2R-OH + 2Na → 2R-O^-Na⁺ + H₂↑

Iodoform Test:

Ethanol when treated with iodine solution and sodium hydroxide produces a yellow precipitate of iodoform (CHI₃).

CH₃CH₂OH + 4I₂ + 6NaOH → CHI₃ + HCOONa + 5NaI + 5H₂O

Esterification reaction:

Ethanol reacts with a carboxylic acid in the presence of an acid to produce sweet smelling compounds called esters. The reaction is called esterification reaction.

(b) Reaction with bases:

Ethanoic acid react with bases like NaOH to give salt and water. This is called neutralisation reaction.

Reaction with carbonates and bicarbonates:

Ethanoic acid reacts with metal carbonates and bicarbonates to produce brisk effervescence due to the evolution of carbon dioxide.

2CH₃COOH + Na₂CO₃→ 2CH₃COONa + CO₂↑ + H₂O

CH₃COOH + NaHCO₃→ CH₃COONa + CO₂↑ + H₂O

Esterification reaction:

Ethanoic acid reacts with an alcohol in the presence of an acid to produce sweet smelling compounds called esters. The reaction is called esterification reaction.

Carbon compounds exist in large numbers due to the following reasons:

(a) Carbon exhibits catenation to a very large extent. Catenation is the ability of an atom to form chains or bonds with itself. It can form long chains, branched chains or ring structures.

(b) Carbon has a valency of 4. It is satisfied by forming single, double or triple bonds with carbon or other elements such as oxygen, hydrogen, nitrogen etc. The bonds formed are strong and hence the compounds formed are stable.

(a) Soaps are basically sodium or potassium salts of long chain fatty acids. They are usually prepared using NaOH/KOH and fatty acid esters. The acidic or basic strength of soap depends upon the relative acidic/basic strength of the reactants used. In most cases, NaOH is used which is highly basic. Hence, soap solutions are generally basic in nature and turn red litmus blue.

(b) Temporary hardness is produced by the presence of bicarbonate ions of calcium, magnesium etc. Temporary hardness of water can be removed by heating. One can also use washing soda (sodium carbonate) to remove temporary hardness of water.

Soaps are sodium or potassium salts of long chain carboxylic acids. They contain two ends having different properties: the carboxylic acid part which is hydrophobic and the ionic end (Na⁺ or K⁺) which is hydrophilic as shown in the following figure.

Dirt present in clothes is a hydrocarbon. When soap is used in water, the soap molecules tend to surround the dirt such that they form a cluster in which the hydrophobic tails are in the interior of the cluster and the hydrophilic part are on the surface of the cluster (see figure below). The hydrophilic part dissolves in water and the hydrophobic part dissolves in the hydrocarbon (dirt). This formation or cluster is called micelle.

Agitating the water by hand or brush forces the hydrophilic end to move along the direction of flow of water, dragging the micelle out of the clothes and the dirt stuck to the micelle is rinsed away. In this way, soaps perform the cleansing action.

Silicon is an example of an element which forms covalent bond. It belongs to the group 14 of the periodic table. It has 4 outermost electrons like carbon. The electron dot structure of silicon will look like:

Similar to carbon, silicon can also form covalent molecules. 1 carbon and 4 hydrogen atoms form methane (CH₄). Similarly, one silicon and 4 hydrogen atoms form silane (SiH₄).

Ethanol (C₂H₅OH) is dehydrated in the presence of excess conc. sulphuric acid to produce ethene (C₂H₄).

Ethene upon hydrogenation in the presence of a catalyst like nickel or palladium yields ethane.

(i) The reactants are ethanol and oxygen, and ethanol undergoes complete oxidation to produce carbon dioxide and water, with the generation of heat and light.

C₂H₅OH + 3O₂→ 2CO₂ + 3H₂O +heat and light

(ii) Ethanol when heated with a dehydrating agent like conc. H₂SO₄ loses a water molecule to produce ethene.

(iii) The reactants are ethanoic acid and sodium bicarbonate. They react to produce the sodium salt of ethanoic acid, along with the formation of carbon dioxide and water.

Alkenes and alkynes generally undergo addition reaction(s) with hydrogen. For an addition reaction to occur there must be unsaturation present in the compound. i.e., the compound must contain double or triple bonds.

For an arbitrary alkene with formula C_nH_2n,

For example:

This reaction is also possible in alkynes:

For example:

Hydrogenation is used to reduce vegetable oils to vanaspati ghee. Vanaspati ghee is used as a substitute for butter containing animal fats which are harmful to our health and is cheaper than butter. Vegetable oils are unsaturated and they are hydrogenated in the presence of catalysts like nickel or palladium. Hydrogenation is the process of addition of hydrogen to unsaturated compounds to form saturated compounds. The chemical reaction can be represented by the equation:

The addition reaction is only exhibited by the unsaturated hydrocarbons because this reaction involves the breaking of multiple bonds presents between atoms to form saturated hydrocarbons.

Here, C₃H₆ is an alkene which is unsaturated and C₄H₁₀ is an alkane, which is saturated. Hence, C₃H₆ is more likely to undergo an addition reaction.

In addition reaction, the reactant breaks the multiple bonds present between atoms of the unsaturated compound. Then it later occupies the compound by forming a single bond between carbon and the reacting species.

e.g.

As one can see, hydrogen (reacting species) breaks the double bond between the carbon atoms and forms single bonds with them resulting in an alkane.

Ethanol undergoes dehydration process when it is heated in the presence of excess conc. sulphuric acid to form ethene (CH₂=CH₂).

Now ethene combines with hydrogen to form ethane (C₂H₆) which is a saturated hydrocarbon.

Given that,

C₂H₄O₂ (Acid) + Alcohol → X (in the presence of H₂SO₄)

The above reaction is esterification reaction.

Given that the alcohol upon oxidation with alkaline KMnO₄ gives the carboxylic acid C₂H₄O₂. This implies that the alcohol contains 2 carbons as well.

Therefore, the alcohol is ethanol and the carboxylic acid is ethanoic acid.

The condensed structural formula of alcohol used:

CH₃—CH₂—OH

The condensed structural formula of carboxylic acid used:

The compound X is then ethyl ethanoate,

where CH₃COOCH₂CH₃ is X.

Sodium ethanoate (CH₃COONa) can be formed from ethanoic acid in the following reactions:

Reaction with sodium carbonate:

Reaction with sodium bicarbonate:

Neutralisation Reaction:

Hydrocarbons upon complete combustion give carbon dioxide and water along with heat and/or light. Complete combustion of alkanes are represented by:

C_xH_y + (x+y/4)O₂→ xCO₂ + (y/2)H₂O

where ‘x’ is the number of carbon atoms and ‘y’ is the number of hydrogen atoms. The gas evolved is carbon dioxide. To test its presence, one can pass the gas through lime water (calcium hydroxide) turning it milky due to the formation of calcium carbonate.

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

To obtain the next higher homologue of a given entity or substance, one can add a –CH₂ unit to the given compound. For instance,

(a) To obtain the next higher of a homologue of C₃H₆,

C₃H₆ + (-CH₂ unit) → C₄H₈

i.e., the next higher homologue is butane.

(b) To obtain the next higher of a homologue of C₅H₈,

C₅H₈ + (-CH₂ unit) → C₆H₁₀

i.e., the next higher homologue is hexyne.

(a) Ethanol, when burnt in air, undergoes complete oxidation and produces carbon dioxide and water.

C₂H₅OH + 3O₂→ 2CO₂ + 3H₂O +heat and light

(b) Concentrated H₂SO₄ is a good dehydrating agent. When ethanol is heated with excess conc. H₂SO₄, one water molecule is removed from it to form ethene (C₂H₄).

(c) When sodium is dropped into ethanol, the solution colour changes to pink from colourless. This happens due to formation of sodium ethoxide.

The reaction is given below:

2C₂H₅OH + 2Na ⇒ 2C₂H₅0Na + H₂

Hydrocarbons are compounds which contain carbon and hydrogen only. The elements (carbon and hydrogen) form covalent bonds between them.

(i) Saturated hydrocarbons are represented by the molecular formula C_nH_2n+2, where ‘n’ is the number of carbon atoms. Only single bonds exist between the atoms in saturated hydrocarbons.

An example of a saturated hydrocarbon is methane (CH₄). Carbon can form a maximum of 4 bonds and all 4 are satisfied by hydrogen atoms.

Methane

(ii) Unsaturated hydrocarbons are hydrocarbons containing at least one double/triple bond. If the hydrocarbon is an alkene, then the general molecular formula is C_nH_2n, and if the hydrocarbon is an alkyne, then the molecular formula is C_nH_2n-2.

An example of alkene is propene (C₃H₆) in which n=3. One of the carbon-carbon bonds is a double bond. The other carbon-carbon bond is a single bond. The remaining valency of each carbon is satisfied by hydrogen atoms.

An example of an alkyne is ethyne (C₂H₂) in which n=2. Carbon can form 4 bonds with neighbouring atoms. Here, the carbon-carbon bond is a triple bond. So, the remaining 1 bond of each carbon is formed with a hydrogen atom.

H—C≡C—H

Hydrocarbons are compounds which contain carbon and hydrogen only. All alkanes, alkenes, and alkynes are examples of hydrocarbons.

As you can see ethane contains single bonds between C and H.

Ethene (alkene) contains one C=C bond (double bond).

Ethyne (alkyne) contains one C≡C bond (triple bond).

The reaction mentioned in the question is:

(i) Given that C is sweet smelling. Hence, C is an ester. Therefore the reaction mentioned is esterification reaction, where the reactants are alcohol and carboxylic acid.

i.e., A – alcohol, B – carboxylic acid (or vice versa)

(ii) R-OH represents an alcohol and R’-COOH represents a carboxylic acid, where R, R’ are different alkyl groups (such as methyl group (-CH₃), butyl group (C₄H₉) etc). Then, esterification reaction is given by:

(iii) Since esters are sweet smelling substances, they are used in making perfumes and as flavouring agents in foods.

(iv) Esters upon hydrolysis (water is one of the reactants) in the presence of an acid or base gives back the alcohol and the carboxylic acid from which the ester was formed. This reaction is called saponification reaction.

The hydrogenation reaction is the reaction in which an unsaturated carbon compound is converted into a saturated compound by the addition of hydrogen in the presence of a catalyst like nickel. The added hydrogen converts double/triple bonds present in the compound into single bonds.

The hydrogenation reaction is represented by:

The oxidation reaction is the process in which a substance reacts with oxygen to form various products. The substance gets bonded to electronegative atoms like oxygen once oxidation is complete.

Eg: CH₄ + 2O₂→ CO₂ + 2H₂O

Here, methane gets oxidized to carbon dioxide. One can see the formation of bond between carbon and oxygen.

The reaction in which one of the atoms of a compound is substituted by another is called substitution reaction. Consider the following example:

CH₄ + Cl₂→ CH₃Cl + HCl (in the presence of sunlight)

In this example, one of the hydrogen atoms of methane (CH₄) is replaced by a chlorine atom.

The reaction in which esters react in the presence of an alkali or an acid to give back the alcohol and the carboxylic acid which were used to produce the ester is called saponification reaction.

This is used in the preparation of soaps, hence the name “saponification”. It is considered as the reverse reaction of esterification.

Alcohols and carboxylic acids react in the presence of an acid catalyst to form sweet-smelling substances called esters. This reaction is called esterification reaction.

(a) – Esterification reaction

(b) – Addition reaction

(c) – Substitution reaction

(d) – Neutralisation reaction

(a) Here, an alcohol (CH₃OH) reacts with a carboxylic acid (CH₃COOH) reacts in the presence of an acid to form an ester (CH₃COOCH₃). This reaction is called esterification reaction.

(b) The alkene is converted to alkane by the addition of hydrogen. The reaction is called an addition reaction.

(c) One of the hydrogen atoms is replaced by chlorine in the presence of sunlight and hence the reaction is called substitution reaction.

(d) Here, an acid (CH₃COOH) reacts with a base (NaOH) to produce salt and water. This reaction is called a neutralisation reaction.

Structural isomers are those compounds which have the same molecular formula but different structural formula. The basic structure or skeleton of the carbon chain would be different in each of these compounds.

The minimum number of carbons required to form at least one branch is 4 (3 carbons in parent chain and 1 carbon in side chain). The possible skeletons using 4 carbon atoms are:

C—C—C—C

Using 3 carbons, one can only produce the following skeleton:

C—C—C

Hence, propane (containing 3 carbon atoms) cannot exhibit structural isomerism.

The possible structural isomers of butane (C₄H₁₀) are:

(a) n-butane:

(b) 2-methylpropane:

Preparation of ester:

Take a small quantity (for example, 1 ml) of ethanol (or any alcohol in general) and a small quantity of acetic acid (or any carboxylic acid) in a test tube. Add a few drops of concentrated sulphuric acid to the mixture.

Heat the test tube in a water bath for about five minutes as shown in the figure.

Then, pour the contents of the test tube into a beaker containing 20-50 ml of water. Smell the mixture in the beaker. One could notice a sweet fruity smell coming from the beaker. This smell is produced due to the formation of ester from alcohol and carboxylic acid. The reaction is called esterification reaction.

The reaction can be represented as: