SPI Chemistry · Carbon and Its Compounds

Carbon Bonding and Hydrocarbons

Carbon shares its four valence electrons to make covalent bonds, links to itself in chains and rings, and so forms whole families of hydrocarbons.

All Carbon and Its Compounds notes

Why this matters

Seven questions in the bank, two EASY and five MODERATE, and two of them are from the 2022 paper (naming a ring of six carbons drawn in a figure). The others count the covalent bonds in an alkane, pick out graphite, find a compound with one single and one triple bond, and test the homologous series and isomers.

Concept 1 of 4: Covalent bonds and how to count them

Carbon has 4 valence electrons. Losing or gaining 4 would cost too much energy, so carbon shares them instead: each shared pair is one covalent bond. Carbon can also join other carbon atoms in long chains, which is why there are so many carbon compounds.

Definition

Carbon is tetravalent: it always makes 4 bonds.
Catenation is carbon's power to bond to other carbon atoms in chains, branches and rings.
A single bond shares one pair of electrons, a double bond two pairs, a triple bond three pairs.
To count the bonds in an alkane with $n$ carbons: $n - 1$ carbon-carbon bonds plus $2n + 2$ carbon-hydrogen bonds.
Covalent compounds have no ions, so they are mostly poor conductors with low melting and boiling points.

Covalent bonds in an alkane

\text{bonds in } \mathrm{C}_n\mathrm{H}_{2n+2} = (n - 1) + (2n + 2) = 3n + 1

nnumber of carbon atoms

Worked example

How many covalent bonds are there in a molecule of propane,

\mathrm{C_3H_8}

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The same idea in a practice question:

Practice questionModerate

Example 1 · Carbon and Its Compounds · Carbon Bonding and Hydrocarbons

Pentane has the molecular formula

\text{C}_5\text{H}_{12}

. It has

Counting only the carbon-hydrogen bonds

The number of hydrogens is not the number of bonds. An alkane with

n

carbons also has

n - 1

carbon-carbon bonds, so the total is

3n + 1

Thinking carbon forms ions

Carbon neither loses nor gains 4 electrons. It shares them, so its compounds are covalent and do not conduct electricity.

Concept 2 of 4: Allotropes of carbon

Diamond, graphite and fullerene are all pure carbon. They look and behave differently only because their atoms are joined in different patterns.

Definition

Allotropes are different forms of the same element.
In diamond each carbon bonds to 4 others in a rigid three-dimensional network: very hard, does not conduct.
In graphite each carbon bonds to 3 others in flat hexagonal layers stacked one above another. The layers slide, so graphite is soft and slippery, and the fourth electron of each carbon is free, so graphite conducts electricity.
Fullerenes are cages of carbon atoms. The best known, $\mathrm{C_{60}}$ (buckminsterfullerene), is shaped like a football.

Allotrope	Each carbon bonds to	Structure	Key property
Diamond	4 carbons	Rigid three-dimensional network	Hardest natural substance, does not conduct
Graphite	3 carbons	Hexagonal layers, one above another	Soft, slippery, conducts electricity Graphite is the only one of the three that conducts.
Fullerene	3 carbons	Closed cage, $\mathrm{C_{60}}$ is a football shape	Separate molecules, not one giant network

Same atoms, different arrangements: that is what allotropes means.

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The same idea in a practice question:

Practice questionEasy

Example 2 · Carbon and Its Compounds · Carbon Bonding and Hydrocarbons

Identify the allotrope of carbon formed by hexagonal arrays placed in layers one agove the other.

Calling diamond a conductor because it is carbon

Diamond does not conduct: every valence electron is held in a bond. Graphite conducts because one electron per carbon is free.

Concept 3 of 4: Alkanes, alkenes, alkynes and rings

Hydrocarbons contain only carbon and hydrogen. If every carbon-carbon bond is single, the compound is saturated. One double or triple bond makes it unsaturated. The name tells you which: -ane, -ene or -yne, with a prefix for the number of carbons.

Definition

Saturated: only single bonds (alkanes). Unsaturated: a double bond (alkenes) or a triple bond (alkynes).
Prefixes count the carbons: meth 1, eth 2, prop 3, but 4, pent 5, hex 6.
A cyclo compound has its carbons in a ring. Cyclohexane is a ring of 6 carbons with single bonds only.
Benzene, $\mathrm{C_6H_6}$ , is a ring of 6 carbons with alternate single and double bonds.

Family	General formula	Bonds	Example
Alkane	$\mathrm{C}_n\mathrm{H}_{2n+2}$	Single only	Methane $\mathrm{CH_4}$ , propane $\mathrm{C_3H_8}$
Alkene	$\mathrm{C}_n\mathrm{H}_{2n}$	One double bond	Ethene $\mathrm{C_2H_4}$ , propene $\mathrm{C_3H_6}$
Alkyne	$\mathrm{C}_n\mathrm{H}_{2n-2}$	One triple bond	Ethyne $\mathrm{C_2H_2}$ , propyne $\mathrm{C_3H_4}$
Cycloalkane	$\mathrm{C}_n\mathrm{H}_{2n}$	Ring of single bonds	Cyclohexane $\mathrm{C_6H_{12}}$ A ring has the same general formula as an alkene, but no double bond.
Cycloalkene	$\mathrm{C}_n\mathrm{H}_{2n-2}$	Ring with one double bond	Cyclohexene $\mathrm{C_6H_{10}}$
Benzene	$\mathrm{C_6H_6}$	Ring, alternate single and double bonds	Benzene

In a drawn ring, each corner is a carbon: count the double bonds to name it.

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The same idea in a real exam question:

SPI · 2022 · SPI Entrance Examination 2022 · Q97Moderate

Example 3 · Carbon and Its Compounds · Carbon Bonding and Hydrocarbons

Identify the compounds shown in figures given below

Taking a ring for an alkene

Cyclohexane,

\mathrm{C_6H_{12}}

, fits

\mathrm{C}_n\mathrm{H}_{2n}

but has only single bonds, because its carbons close into a ring. Look at the structure, not just the formula.

Counting the single bonds next to a triple bond

A triple bond does not make every other bond triple. Butyne has four carbons, so besides its triple bond it has two carbon-carbon single bonds. Propyne has exactly one.

Drill 2 more on alkanes, alkenes, alkynes and rings

Concept 4 of 4: Homologous series and isomers

Methane, ethane, propane and butane form a ladder: each step adds one

\mathrm{CH_2}

unit. Compounds on one ladder behave alike chemically. Isomers are the opposite idea: one formula, but the atoms joined in different ways.

Definition

A homologous series has the same general formula and the same functional group. Each member differs from the next by $\mathrm{CH_2}$ , which is 14 u.
Members have similar chemical properties. Their physical properties (melting and boiling points) change step by step as the chain grows.
Isomers have the same molecular formula but different structural formulae, so their properties differ.
Methane, ethane and propane have no isomers. Butane, $\mathrm{C_4H_{10}}$ , is the first alkane with isomers: a straight chain and a branched chain.

Mass step in a homologous series

M_{\text{next}} = M + 14 \text{ u} \qquad \mathrm{CH_2} = 12 + 2 \times 1 = 14 \text{ u}

Worked example

Ethanol,

\mathrm{C_2H_5OH}

, has a molecular mass of 46 u. Find the molecular mass of propanol, the next member of its series.

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The same idea in a practice question:

Practice questionEasy

Example 4 · Carbon and Its Compounds · Carbon Bonding and Hydrocarbons

Molecular weight of consecutive members of same series differs by ____ units :-

Saying isomers have the same structure

Isomers share the molecular formula only. Their structural formulae differ, and so do their properties.

Expecting the physical properties to stay the same

Down a homologous series the chemical properties stay similar, but the boiling point rises as the chain grows by each

\mathrm{CH_2}

Drill 1 more on homologous series and isomers

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Covalent bonds and how to count them
Covalent bonds in an alkane
$\text{bonds in } \mathrm{C}_n\mathrm{H}_{2n+2} = (n - 1) + (2n + 2) = 3n + 1$
Homologous series and isomers
Mass step in a homologous series
$M_{\text{next}} = M + 14 \text{ u} \qquad \mathrm{CH_2} = 12 + 2 \times 1 = 14 \text{ u}$

Reference tables (2)

Allotropes of carbon3 rows

Allotrope	Each carbon bonds to	Structure	Key property
Diamond	4 carbons	Rigid three-dimensional network	Hardest natural substance, does not conduct
Graphite	3 carbons	Hexagonal layers, one above another	Soft, slippery, conducts electricity Graphite is the only one of the three that conducts.
Fullerene	3 carbons	Closed cage, $\mathrm{C_{60}}$ is a football shape	Separate molecules, not one giant network

Same atoms, different arrangements: that is what allotropes means.

Alkanes, alkenes, alkynes and rings6 rows

Family	General formula	Bonds	Example
Alkane	$\mathrm{C}_n\mathrm{H}_{2n+2}$	Single only	Methane $\mathrm{CH_4}$ , propane $\mathrm{C_3H_8}$
Alkene	$\mathrm{C}_n\mathrm{H}_{2n}$	One double bond	Ethene $\mathrm{C_2H_4}$ , propene $\mathrm{C_3H_6}$
Alkyne	$\mathrm{C}_n\mathrm{H}_{2n-2}$	One triple bond	Ethyne $\mathrm{C_2H_2}$ , propyne $\mathrm{C_3H_4}$
Cycloalkane	$\mathrm{C}_n\mathrm{H}_{2n}$	Ring of single bonds	Cyclohexane $\mathrm{C_6H_{12}}$ A ring has the same general formula as an alkene, but no double bond.
Cycloalkene	$\mathrm{C}_n\mathrm{H}_{2n-2}$	Ring with one double bond	Cyclohexene $\mathrm{C_6H_{10}}$
Benzene	$\mathrm{C_6H_6}$	Ring, alternate single and double bonds	Benzene

In a drawn ring, each corner is a carbon: count the double bonds to name it.

Watch out for (7)

Counting only the carbon-hydrogen bonds
→ Covalent bonds and how to count them
Thinking carbon forms ions
→ Covalent bonds and how to count them
Calling diamond a conductor because it is carbon
→ Allotropes of carbon
Taking a ring for an alkene
→ Alkanes, alkenes, alkynes and rings
Counting the single bonds next to a triple bond
→ Alkanes, alkenes, alkynes and rings
Saying isomers have the same structure
→ Homologous series and isomers
Expecting the physical properties to stay the same
→ Homologous series and isomers

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