Carbon and its Compounds — NCERT Solutions, AI Tutor & Practice

Understand: Carbon has an atomic number of $6$, so its electronic configuration is $2, 4$. It has four electrons in its outermost shell.

Analyze: Carbon cannot easily lose $4$ electrons (would need huge energy to form $C^{4+}$) nor gain $4$ electrons (hard to hold $10$ electrons with only $6$ protons). So it achieves a stable octet by sharing electrons, forming covalent bonds.

Two special properties allow a huge number of compounds:

1. Catenation — carbon atoms link with other carbon atoms to form long chains, branched chains and rings.
2. Tetravalency — with four valence electrons, carbon can bond with four other atoms (of carbon, hydrogen, oxygen, nitrogen, sulphur, etc.).

Conclude: Because C–C bonds are strong and stable, and carbon can bond with many elements in different arrangements, carbon forms millions of compounds.

Definition: A covalent bond is a chemical bond formed by the mutual sharing of electrons between two atoms so that both attain a stable noble-gas configuration.

Formation of CH₄:

• Carbon has $4$ valence electrons and needs $4$ more to complete its octet.
• Each hydrogen has $1$ electron and needs $1$ more to complete its duplet.

So one carbon shares its $4$ electrons with $4$ hydrogen atoms, each sharing forms one shared pair (single bond).

Electron dot structure: $H : \overset{\displaystyle H}{\underset{\displaystyle H}{\overset{..}{\underset{..}{C}}}} : H$

Carbon forms four single covalent bonds, giving the formula $CH_4$. Each bond is one shared pair of electrons.

Ethane ($C_2H_6$): Each carbon bonds to $3$ hydrogens and to the other carbon by a single bond.

Structural formula: $H_3C-CH_3$

Electron sharing: one shared pair between the two carbons, three shared pairs (C–H) on each carbon. Total bonds satisfy tetravalency of both carbons.

Ethene ($C_2H_4$): Each carbon bonds to $2$ hydrogens and to the other carbon by a double bond.

Structural formula: $H_2C=CH_2$

Electron sharing: two shared pairs between the carbon atoms (double bond) and two C–H single bonds per carbon.

Both carbons in each molecule have four bonds, satisfying tetravalency.

Saturated hydrocarbons: Hydrocarbons in which all carbon–carbon bonds are single bonds. Example: ethane, $C_2H_6$. These are called alkanes.

Unsaturated hydrocarbons: Hydrocarbons that contain at least one carbon–carbon double or triple bond. Example: ethene, $C_2H_4$ (alkene) or ethyne, $C_2H_2$ (alkyne).

Experimental distinction:

1. Bromine water test: Unsaturated hydrocarbons decolourise bromine water (orange → colourless) due to an addition reaction. Saturated hydrocarbons do not.
2. Combustion: Saturated hydrocarbons burn with a clean blue flame; unsaturated hydrocarbons burn with a yellow, sooty (smoky) flame because of higher carbon content.

Homologous series: A series of organic compounds having the same general formula and the same functional group, in which each successive member differs from the previous by a –CH₂– unit (i.e., by $CH_2$, mass $14$ u).

Alcohols contain the functional group –OH (hydroxyl).

| Name | Formula | Structure | |------|---------|-----------| | Methanol | $CH_3OH$ | $CH_3-OH$ | | Ethanol | $C_2H_5OH$ | $CH_3-CH_2-OH$ | | Propanol | $C_3H_7OH$ | $CH_3-CH_2-CH_2-OH$ |

Each member differs from the next by a $-CH_2-$ group. They show similar chemical properties because of the common $-OH$ group but a gradual change in physical properties (melting/boiling point) with increasing molar mass.

(a) Combustion: Ethanol burns completely in air, giving carbon dioxide, water and releasing heat and light (blue flame): $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O + \text{heat}$

(b) Reaction with sodium: Ethanol reacts with sodium metal to give sodium ethoxide and hydrogen gas (shows –OH is slightly acidic in behaviour toward Na): $2C_2H_5OH + 2Na \rightarrow 2C_2H_5ONa + H_2\uparrow$ (Sodium ethoxide $=$ $C_2H_5ONa$.)

(c) Dehydration: On heating with excess concentrated sulphuric acid at about $443\text{ K}$ ($170^\circ C$), ethanol loses a water molecule to form ethene. Here $H_2SO_4$ acts as a dehydrating agent: $C_2H_5OH \xrightarrow[443\text{ K}]{\text{conc. } H_2SO_4} C_2H_4 + H_2O$

Esterification: The reaction of a carboxylic acid with an alcohol in the presence of a small amount of concentrated sulphuric acid (catalyst) to form a sweet-smelling ester and water.

Reaction: $CH_3COOH + C_2H_5OH \xrightarrow{\text{conc. } H_2SO_4} CH_3COOC_2H_5 + H_2O$

The product $CH_3COOC_2H_5$ is ethyl ethanoate (ethyl acetate).

Properties of esters:

1. They have a pleasant, sweet (fruity) smell and are used in perfumes and flavouring agents.
2. On reaction with a base (like NaOH), an ester undergoes saponification, breaking back into the alcohol and the sodium salt of the acid (this is the basis of soap making): $CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH$

Structure of soap: A soap molecule has two parts:

• A hydrophilic (water-loving) ionic head (–COO⁻ Na⁺).
• A hydrophobic (water-hating), oil-loving long hydrocarbon tail.

Cleansing action:

1. Dirt is usually oily/greasy and does not dissolve in water.
2. The hydrophobic tails of soap molecules attach to the oily dirt, while the hydrophilic heads point outward into the water.
3. This forms tiny clusters called micelles, with the oil trapped inside and the ionic heads on the surface.
4. The micelles remain suspended in water and are washed away with rinsing, carrying the dirt away.

Soaps in hard water: Hard water contains calcium and magnesium ions. These react with soap to form an insoluble precipitate (scum): $2C_{17}H_{35}COONa + Ca^{2+} \rightarrow (C_{17}H_{35}COO)_2Ca\downarrow + 2Na^+$ Much soap is wasted forming scum instead of cleaning, so soaps do not lather or clean effectively in hard water.

(a) Ethanoic acid is a weak acid: Ethanoic acid ($CH_3COOH$) does not ionise completely in water. Only a small fraction of its molecules dissociate to give $H^+$ (as $H_3O^+$) ions: $CH_3COOH \rightleftharpoons CH_3COO^- + H^+$ Since the concentration of $H^+$ ions produced is low, it is a weak acid (unlike HCl, which ionises completely and is a strong acid). This is why vinegar has a mild acidity.

(b) Carbon compounds are poor conductors: Carbon compounds are formed by covalent bonds, which are made by sharing of electrons. They do not produce free ions or free electrons in solution or in the molten state. Since there are no charged particles to carry current, carbon compounds are generally poor conductors of electricity and have low melting and boiling points.

Isomers of $C_4H_{10}$: Isomers are compounds with the same molecular formula but different structures. $C_4H_{10}$ has two isomers:

1. n-Butane (straight chain): $CH_3-CH_2-CH_2-CH_3$

2. Isobutane / 2-methylpropane (branched): $CH_3-\underset{\displaystyle CH_3}{\underset{|}{CH}}-CH_3$

–CHO group with two carbons: The –CHO group is the aldehyde functional group. A two-carbon aldehyde is ethanal (acetaldehyde): $CH_3-CHO$

Here one carbon is in the methyl group and one carbon is in the –CHO group (total two carbons).

Conversion: When ethanol is heated with an oxidising agent, it is oxidised to ethanoic acid: $CH_3CH_2OH \xrightarrow[\text{oxidising agent}]{} CH_3COOH$

Oxidising agents used:

• Alkaline potassium permanganate ($KMnO_4$), or
• Acidified potassium dichromate ($K_2Cr_2O_7$).

Why it is called oxidation: In this reaction the alcohol group is converted to a carboxylic acid group by the addition of oxygen (removal of hydrogen). Addition of oxygen to a substance is oxidation, so the reaction is an oxidation reaction.

Because these substances add oxygen to other compounds, potassium permanganate and potassium dichromate are called oxidising agents.

(Observation: The purple colour of $KMnO_4$ disappears as it is used up in oxidising the ethanol.)

Functional group: A functional group is an atom or group of atoms present in a molecule that determines the chemical properties of the compound. It replaces one or more hydrogen atoms of a hydrocarbon and gives the compound its characteristic reactions.

| Compound | Functional group | Name of group | Class | |----------|------------------|---------------|-------| | $CH_3COOH$ | $-COOH$ | Carboxyl | Carboxylic acid | | $C_2H_5OH$ | $-OH$ | Hydroxyl | Alcohol | | $CH_3CHO$ | $-CHO$ | Aldehyde | Aldehyde | | $CH_3COCH_3$ | $\displaystyle{>}C{=}O$ | Ketone (carbonyl) | Ketone |

Compounds with the same functional group show similar chemical behaviour, which is the basis of a homologous series.