Carbon and Its Allotropes – NDA Chemistry Notes

Carbon is one of the most important elements in all of chemistry. It is the backbone of all living things. It forms more compounds than any other element, over 10 million carbon compounds are known. Yet carbon itself can exist in several completely different physical forms. The same element (all carbon atoms) can be the hardest known natural substance or one of the best-known lubricants. This is because of allotropy.

An allotrope is a different physical form of the same element, with different physical properties but the same chemical composition.

NDA tests this chapter very heavily. The most tested topics are: which statements about graphite are correct and which are NOT, whether graphite has sp² or sp³ hybridisation, what holds graphite layers together, which is the most stable allotrope, what is the purest form of carbon, water gas vs syngas, hexane and sooty flames, and H₂ moles to saturate benzene. Fly ash and coal as non-allotropes are also directly tested.

1. Why Carbon is Special

Carbon has two unique properties that make it form so many compounds.

1. Catenation: Carbon atoms can bond with other carbon atoms. They can form long chains, branched chains, and rings. No other element does this as extensively as carbon.

2. Tetravalency: Carbon has four valence electrons. It can form four bonds: with other carbon atoms, with hydrogen, oxygen, nitrogen, and many other elements.

Carbon forms the highest number of compounds of any element.

Carbon can form more compounds than any other element. The reason is catenation (ability to bond with other carbon atoms), NOT simply because it has multiple allotropes. [NDA 2009-I]

2. Allotropes of Carbon

An allotrope is a different structural form of the same element. Carbon has several allotropes, each with dramatically different properties.

Important point for NDA: Nitrogen does NOT have an allotrope. It exists only as N₂. [NDA 2012-II] Coal is NOT a pure allotrope of carbon. It is an impure form containing carbon along with other substances. [NDA 2023-I] Fly ash is NOT an allotrope of carbon. [NDA 2021-I] Charcoal and coke are NOT allotropes. They are amorphous (non-crystalline) impure forms of carbon.

Diamond

Diamond is a crystalline allotrope of carbon. It has the most remarkable properties of any natural material.

Structure:

• Each carbon atom is bonded to four other carbon atoms by strong covalent bonds
• The bonds are arranged in a tetrahedral pattern
• This creates a three-dimensional, rigid, giant covalent network that extends in all directions
• This structure is why diamond is incredibly hard
• Diamond is isomorphous with crystalline silicon. Both have the same type of structure [NDA 2022-I]

Property	Detail
Hardness	Hardest natural substance known, 10 on Mohs scale
Electrical conductivity	Very poor conductor, no free electrons
Thermal conductivity	Excellent, best thermal conductor of any natural material
Appearance	Transparent, extremely high refractive index
Melting point	Very high (~3550°C)
Chemical nature	Burns to produce CO₂ when heated strongly
Hybridisation	sp³ (all four bonds in tetrahedral arrangement)

Diamond is a bad conductor of electricity. This is directly tested in NDA. [NDA 2011-I, NDA 2012-II]

Diamond burns to produce carbon dioxide when heated in oxygen. [NDA 2009-I]

Diamond has a tetrahedral structure. [NDA 2019-II]

Diamond is the hardest natural substance. [NDA 2023-I] (Next to diamond, graphite is NOT the second hardest. Graphite is actually very soft.)

Uses of diamond:

• Cutting glass, marble, and very hard materials (drill bits, cutting wheels)
• Drilling of rocks and oil wells
• Jewellery: high refractive index creates brilliant sparkle
• Abrasive for polishing and grinding

What is NOT correct about diamond:

• Diamond is a good conductor of electricity: FALSE [NDA 2011-I]
• Diamond is an allotrope of silicon: FALSE (it is an allotrope of CARBON) [NDA 2009-I]
• Diamond is thermodynamically the most stable allotrope of carbon: FALSE (graphite is the most stable) [NDA 2014-I]
• Diamond can be used as a lubricant: FALSE
• Diamond is the second hardest known substance: FALSE (it is the FIRST/hardest, not second) [NDA 2023-I]

Graphite

Graphite is the most common crystalline allotrope of carbon and has properties completely opposite to diamond.

Structure:

• Each carbon atom is bonded to three other carbon atoms in a flat hexagonal ring pattern
• This creates flat layers of hexagonal rings (like a honeycomb)
• Within each layer, bonds are strong covalent bonds (sigma bonds)
• Carbon undergoes sp² hybridisation in graphite, NOT sp³ [NDA 2015-II, NDA 2018-I]
• The fourth electron on each carbon is delocalised. It can move freely across the whole layer
• Between layers: only weak van der Waals forces hold the layers together, NOT carbon-carbon single bonds [NDA 2018-II]

Property	Detail
Hardness	Very soft, layers slide over each other easily
Electrical conductivity	Good conductor, delocalised electrons move freely within layers
Thermal conductivity	Good (within layers)
Appearance	Black, shiny, greasy feel
Melting point	Very high (about 3600°C)
Chemical stability	Most stable allotrope of carbon
Hybridisation	sp² (three bonds per carbon, one electron delocalised)

Graphite is the most stable allotrope of carbon. [NDA 2014-I, NDA 2010-II]

Graphite is a good conductor of electricity because of delocalised electrons. [NDA 2012-II]

Graphite is a better electrical conductor than diamond.

Graphite NOT correct statements (Asked in Various NDA):

• Graphite has sp³ hybridisation: FALSE (it has sp²) [NDA 2018-I]
• Graphite layers are held together by carbon-carbon single bonds: FALSE (van der Waals forces) [NDA 2018-II]
• Graphite is the second hardest substance after diamond: FALSE (graphite is very soft) [NDA 2023-I]
• Graphite is the highest grade of coal: FALSE (graphite is a pure crystalline element; coal is a fossil fuel mixture) [NDA 2010-II]

Uses of graphite:

• Pencils: graphite layers slide off onto paper when you write
• Lubricant: used as dry lubricant in machines; layers slide over each other easily
• Electrodes in electrolytic cells: good conductor AND chemically inert to most substances [NDA 2015-I]
• Moderator in nuclear reactors: slows down fast neutrons
• Crucibles: high melting point, heat resistant

Graphite is used in electrolytic cells because it is a good conductor of electricity AND it is chemically inert. It remains intact even in harsh electrolytic conditions. [NDA 2015-I]

Fullerene (C₆₀)

Fullerene was discovered in 1985 by Curl, Kroto, and Smalley (Nobel Prize 1996).

Structure:

• Cage-like spherical molecules made entirely of carbon
• The most common fullerene is C₆₀ (Buckminsterfullerene, shaped like a soccer ball)
• Contains both five-membered rings AND six-membered rings
• NOT only six-membered rings [NDA 2014-I]

Fullerene is the purest form of carbon. [NDA 2013-I, NDA 2018-I]

Fullerenes are cage-like molecules. [NDA 2014-I]

Fullerene is one of the allotropes of carbon. [NDA 2023-I]

Uses of fullerene:

• Drug delivery systems (hollow cage can carry drug molecules inside)
• Lubricants and superconductors
• Research in nanotechnology

Graphene

Graphene is a single layer of graphite, one atom thick.

Properties:

• Thinnest material known, one atom thick [NDA 2024-I]
• Almost completely transparent, absorbs only 2.3% of light
• Extremely strong, stronger than steel
• Highly conducting, excellent conductor of electricity and heat
• Graphene is NOT a wide band-gap semiconductor. It is actually a conductor/semimetal

Graphene is the thinnest and strongest material. [NDA 2024-I]

Property	Diamond	Graphite	Fullerene (C₆₀)	Graphene
Structure	3D tetrahedral network	Layered hexagonal	Cage-like spherical	Single hexagonal layer
Hybridisation	sp³	sp²	sp²	sp²
Hardness	Hardest natural substance	Very soft	Moderate	Strongest material
Electrical conductivity	Very poor (insulator)	Good conductor	Semiconductor	Excellent conductor
Bond between units	C-C covalent (all directions)	Covalent within layers; van der Waals between layers	C-C covalent	C-C covalent within layer
Most stable?	No	YES	No	—
Purest form?	No	No	YES	—

3. What is NOT an Allotrope of Carbon

NDA specifically tests which substances are NOT allotropes of carbon:

• Coal: NOT a pure allotrope. Coal is a fossil fuel, a complex mixture of carbon compounds with various impurities. [NDA 2023-I]
• Fly ash: NOT an allotrope of carbon. Fly ash is fine particles produced from burning coal in power plants. [NDA 2021-I]
• Charcoal: NOT a crystalline allotrope. Charcoal is amorphous carbon, no regular structure. Sometimes called amorphous allotrope.
• Coke: NOT a crystalline allotrope. Coke is nearly pure amorphous carbon used in iron extraction.
• Carbon black: amorphous form, not a crystalline allotrope.
• Nitrogen: has NO allotrope at all. [NDA 2012-II]

4. Important Gaseous Fuels: Carbon-Based

Water Gas: Water gas is a mixture of carbon monoxide (CO) and hydrogen (H₂).

Made by passing steam over hot coke (carbon): C(s) + H₂O(g) → CO(g) + H₂(g) (at high temperature).

Water gas = CO + H₂. [NDA 2014-I, NDA 2016-I, NDA 2018-II, NDA 2024-I]

Water gas is also called syngas, synthesis gas. [NDA 2018-II]

Syngas = CO + H₂. [NDA 2018-II]

Syngas is used in chemical synthesis, particularly in making methanol and ammonia.

Producer Gas: Producer gas is a mixture of carbon monoxide (CO) and nitrogen (N₂).

Made by passing air through red-hot coke.

Producer gas is NOT the same as water gas. [NDA 2016-I]

Coal Gas: A mixture of hydrogen, methane, and carbon monoxide, obtained by destructive distillation of coal.

Biogas: Biogas is mainly methane (CH₄), approximately 55–65%. Produced by anaerobic decomposition of organic waste (cow dung, crop residue). [NDA 2013-II, NDA 2019-II]

Gas	Composition	Made by
Water gas	CO + H₂	Steam + hot coke
Producer gas	CO + N₂	Air + hot coke
Syngas	CO + H₂ (same as water gas)	Steam reforming of natural gas OR water gas method
Biogas	Mainly CH₄	Anaerobic digestion of organic matter

5. Sooty Flame : Saturated vs Unsaturated Hydrocarbons

When carbon compounds burn in insufficient air, they produce a yellow, sooty flame with black smoke. This happens because carbon is not fully oxidised. Small carbon particles (soot) appear.

Unsaturated hydrocarbons (containing C=C or C≡C bonds) have a higher carbon-to-hydrogen ratio. They are more likely to produce sooty flames.

Saturated hydrocarbons (only C-C single bonds) have a lower carbon-to-hydrogen ratio. They burn more cleanly: blue flame, less soot.

Hexane is a saturated hydrocarbon (C₆H₁₄). It does NOT produce a sooty flame. [NDA 2016-I]

Benzene (C₆H₆), naphthalene (C₁₀H₈), and anthracene (C₁₄H₁₀) are all aromatic or unsaturated. They produce sooty flames.

Sooty flame = unsaturated/aromatic hydrocarbons.

6. Hydrogenation of Benzene

Benzene (C₆H₆) is an aromatic compound with a ring structure containing alternating double bonds (actually delocalised electrons).

To saturate benzene (convert all its double bonds to single bonds), hydrogen (H₂) must be added across each double bond.

Benzene has 3 double bonds in its ring.

To saturate one mole of benzene, 3 moles of H₂ are needed: C₆H₆ + 3H₂ → C₆H₁₂ (cyclohexane). [NDA 2016-I]

7. Carbon Monoxide (CO)

Carbon monoxide is a colourless, odourless, highly toxic gas.

Why CO is toxic: CO binds to haemoglobin approximately 250 times more strongly than oxygen, forming stable carboxyhaemoglobin. This blocks oxygen transport to body tissues. [NDA 2018-I]

CO is a neutral oxide. It does not react with water to form an acid or base. [NDA 2025-I]

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