Carbon and its Compounds

Electron dot structure of CO₂:

Carbon has 4 valence electrons and oxygen has 6 valence electrons. In CO₂, carbon forms two double bonds with two oxygen atoms. Each oxygen shares 2 electrons with carbon.

(Each line represents a shared pair of electrons; O=C=O shows two double bonds)

Sulphur exists as S₈ in which eight sulphur atoms are joined together in a ring. Each sulphur atom has 6 valence electrons and forms 2 single bonds with its two neighbouring sulphur atoms, while 4 electrons remain as 2 lone pairs.

The ring structure: S–S–S–S–S–S–S–S–S (ring of 8 S atoms, each bonded to two others)

Molecular formula: S₈

Three structural isomers are possible for pentane (C₅H₁₂):

(i) n-Pentane (straight chain): CH₃–CH₂–CH₂–CH₂–CH₃

(ii) Isopentane (2-methylbutane): CH₃–CH(CH₃)–CH₂–CH₃

(iii) Neopentane (2,2-dimethylpropane): C(CH₃)₄ — a central carbon with four methyl groups

The two features of carbon that give rise to a large number of compounds are as follows: (i) Catenation − It is the ability to form bonds with other atoms of carbon. (ii) Tetravalency − With the valency of four, carbon is capable of bonding with four other atoms.

The formula for cyclopentane is C₅H₁₀.

It is a ring of 5 carbon atoms, each bonded to 2 hydrogen atoms:

Each carbon in the ring has 2 hydrogen atoms. Total: 5 × CH₂ = C₅H₁₀

(i) Ethanoic acid (CH₃COOH):

(ii) Bromopentane (C₅H₁₁Br): Yes, structural isomers are possible for bromopentane. The bromine can be attached to C1, C2, or C3 giving different isomers. One example (1-bromopentane):

(iii) Butanone (CH₃COCH₂CH₃):

(iv) Hexanal (C₆H₁₂O):

(i) Bromoethane (ii) Methanal (formaldehyde) (iii) Hexyne

Since the conversion of ethanol to ethanoic acid involves the addition of oxygen to ethanol, it is an oxidation reaction.

When ethyne is burnt in air, it gives a sooty flame. This is due to incomplete combustion caused by limited supply of air. However, if ethyne is burnt with oxygen, it gives a clean flame with temperature 3000°C because of complete combustion. This oxy-acetylene flame is used for welding. It is not possible to attain such a high temperature without mixing oxygen. This is the reason why a mixture of ethyne and air is not used.

We can distinguish between an alcohol and a carboxylic acid on the basis of their reaction with carbonates and hydrogen carbonates. Acid reacts with carbonate and hydrogen carbonate to evolve CO2 gas that turns lime water milky. Alcohols, on the other hand, do not react with carbonates and hydrogen carbonates.

Detergents are ammonium or sulphonate salts of long chain carboxylic acids. Unlike soap, they do not react with calcium and magnesium ions present in hard water to form scum. They give a good amount of lather irrespective of whether the water is hard or soft. This means that detergents can be used in both soft and hard water. Therefore, it cannot be used to check whether the water is hard or not.

A soap molecule has two parts namely hydrophobic and hydrophilic. With the help of these, it attaches to the grease or dirt particle and forms a cluster called micelle. These micelles remain suspended as a colloid. To remove these micelles (entrapping the dirt), it is necessary to agitate clothes.

(b) Ethane has 7 covalent bonds.

(c) The functional group of butanone is ketone.

(b) While cooking, if the bottom of the vessel is getting blackened on the outside, then it means that the fuel is not burning completely.

Carbon can neither lose four of its electrons nor gain four electrons as both the processes require extra amount of energy and would make the system unstable. Therefore, it completes its octet by sharing its four electrons with other carbon atoms or with atoms of other elements. The bonds that are formed by sharing electrons are known as covalent bonds. In covalent bonding, both the atoms share the valence electrons, i.e., the shared electrons belong to the valence shells of both the atoms. Here, carbon requires 4 electrons to complete its octet, while each hydrogen atom requires one electron to complete its duplet. Also, chlorine requires an electron to complete the octet. Therefore, all of these share the electrons and as a result, carbon forms 3 bonds with hydrogen and one with chlorine.

(a) Ethanoic acid (CH₃COOH): C has 4 valence electrons; the –COOH group has C double-bonded to O and single-bonded to OH.

(b) H₂S: Sulphur (6 valence e⁻) shares one electron each with two H atoms (1 valence e⁻ each). Two lone pairs remain on S.

(c) Propanone (CH₃COCH₃): Central carbon is double-bonded to oxygen (ketone group), single-bonded to two CH₃ groups.

(d) F₂: Each fluorine has 7 valence electrons; they share one electron each to form a single covalent bond, with 3 lone pairs each.

A homologous series is a series of carbon compounds that have different numbers of carbon atoms but contain the same functional group. For example, methane, ethane, propane, butane, etc. are all part of the alkane homologous series. The general formula of this series is CnH2n+2. Methane CH4 Ethane CH3CH3 Propane CH3CH2CH3 Butane CH3CH2CH2CH3 It can be noticed that there is a difference of −CH2 unit between each successive compound.

• Ethanol is a liquid at room temperature with a pleasant odour while ethanoic acid has vinegar-like smell. The melting point of ethanoic acid is 17°C. This is below room temperature and hence, it freezes during winters. • Ethanoic acid reacts with metal carbonates and metal hydrogencarbonates to form salt, water, and carbon dioxide gas while ethanol does not react with them. For example,

Ethanol does NOT react with sodium carbonate.

A soap is a sodium or potassium salt of long chain fatty acids. It has one polar end and one non-polar end. The polar end is hydrophilic in nature i.e., this end is attracted towards water. The non-polar end is hydrophobic but lipophilic, i.e., it is attracted towards hydrocarbons. When soap is added to water, soap molecules arrange themselves in a cluster to keep the non-polar portion out of water such that the non-polar ends are in the interior of the cluster and the polar ends are on the surface of the cluster. Since the dirt present on clothes is organic in nature and insoluble in water, the hydrophobic ends of the clusters attach themselves to the dirt. This cluster formation in which the dirt is entrapped is the micelle. Micelle formation does not occur in alcohol because the alkyl chain of soap becomes soluble in alcohol.

Most of the carbon compounds give a lot of heat and light when burnt in air. Saturated hydrocarbons burn with a clean flame and no smoke is produced. The carbon compounds, used as a fuel, have high calorific values. Therefore, carbon and its compounds are used as fuels for most applications.

Soap does not work properly when the water is hard. A soap is a sodium or potassium salt of long chain fatty acids. Hard water contains salts of calcium and magnesium. When soap is added to hard water, calcium and magnesium ions present in water displace sodium or potassium ions from the soap molecules forming an insoluble substance called scum. A lot of soap is wasted in the process.

Since soap is basic in nature, it will turn red litmus blue. However, the colour of blue litmus will remain blue.

Hydrogenation is the process of addition of hydrogen. Unsaturated hydrocarbons are added with hydrogen in the presence of palladium and nickel catalysts to give saturated hydrocarbons. This reaction is applied in the hydrogenation of vegetables oils, which contain long chains of unsaturated carbons.

Unsaturated hydrocarbons undergo addition reactions. Being unsaturated hydrocarbons, C3H6 and C2H2undergo addition reactions.

Butter contains saturated fats. Therefore, it cannot be hydrogenated. On the other hand, oil has unsaturated fats. That is why it can be hydrogenated to saturated fats (solids).

Cleansing action of soaps: The dirt present on clothes is organic in nature and insoluble in water. Therefore, it cannot be removed by only washing with water. When soap is dissolved in water, its hydrophobic ends attach themselves to the dirt and remove it from the cloth. Then, the molecules of soap arrange themselves in micelle formation and trap the dirt at the centre of the cluster. These micelles remain suspended in the water. Hence, the dust particles are easily rinsed away by water.