16.1 Hydrocarbons | Hydrocarbons | Chemistry 11

Hydrocarbons are organic compounds composed exclusively of hydrogen and carbon elements. They are the fundamental building blocks of organic chemistry and are broadly classified into two main categories: aliphatic and aromatic hydrocarbons.

Aliphatic Hydrocarbons: Derived from the Greek word for "fatty," this class includes open-chain hydrocarbons and closed-chain (cyclic) compounds that behave similarly to open-chain compounds.
Aromatic Hydrocarbons: Derived from the Greek word for "fragrant," this class includes benzene and compounds with properties similar to benzene.

Figure 16.1: Classification of hydrocarbons

16.1.1 Open Chain Hydrocarbons

Also known as acyclic hydrocarbons, these compounds feature carbon skeletons arranged in straight or branched chains. They do not form rings and have terminal carbon atoms.

1. Saturated Hydrocarbons

These hydrocarbons contain only single covalent bonds between carbon atoms. Each carbon atom is bonded to four other atoms via $s p^{3}$ hybrid orbitals, meaning no more atoms can be added — hence they are "saturated."

Key Features:
- All carbon-carbon bonds are single ( $C - C$ ).
- Each carbon atom is $s p^{3}$ hybridized with tetrahedral geometry (bond angle $\approx 109.5°$ ).
- General formula: $C_{n} H_{2 n + 2}$
- Can be straight-chain or branched-chain.
Example (Alkanes):
- Butane (straight chain): $C H_{3} - C H_{2} - C H_{2} - C H_{3}$
- 2-methylpropane (branched chain):

Figure 16.2: Structure of 2-methylpropane

2. Unsaturated Hydrocarbons

These hydrocarbons contain at least one multiple bond (double or triple) between carbon atoms. The carbon atoms involved in multiple bonds are bonded to fewer than four other atoms, so more atoms can be added through addition reactions.

Key Features:
- Contain at least one $C = C$ double bond or $C \equiv C$ triple bond.
- Include alkenes and alkynes.

a. Alkenes

Commonly known as olefins, alkenes are hydrocarbons that contain at least one carbon-carbon double bond.

Key Features:
- Contain at least one $C = C$ double bond.
- Carbon atoms in the double bond are $s p^{2}$ hybridized (bond angle $\approx 120°$ , trigonal planar).
- General formula: $C_{n} H_{2 n}$
- Alkenes with more than one double bond are called polyenes.

For properties and reactions of alkanes, see Reactivity of Alkanes→.

b. Alkynes

Commonly known as acetylenes, alkynes are hydrocarbons that contain at least one carbon-carbon triple bond.

Key Features:
- Contain at least one $C \equiv C$ triple bond.
- Carbon atoms in the triple bond are $s p$ hybridized (bond angle $= 180°$ , linear).
- General formula: $C_{n} H_{2 n - 2}$
- Alkynes with more than one triple bond are called polyynes.

16.1.2 Closed Chain Hydrocarbons

Also known as cyclic hydrocarbons, these compounds have carbon skeletons arranged in rings, with no terminal carbon atoms. Each carbon atom is bonded to at least two other carbon atoms.

1. Alicyclic Hydrocarbons

The term "alicyclic" is a blend of aliphatic and cyclic. These are aliphatic hydrocarbons that form rings of carbon atoms but do not possess aromatic character.

Key Features:
- Chemical properties are similar to open-chain aliphatic hydrocarbons.
- Have two fewer hydrogen atoms than the corresponding open-chain alkane (same formula as alkenes: $C_{n} H_{2 n}$ for cycloalkanes).
- Carbon atoms are $s p^{3}$ hybridized (in cycloalkanes).
Examples:

Figure 16.3: Examples of alicyclic hydrocarbons

2. Aromatic Hydrocarbons

These are cyclic hydrocarbons that contain at least one benzene ring in their molecular structure.

Key Features:
- The simplest example is benzene ( $C_{6} H_{6}$ ).
- The benzene ring is a planar hexagonal structure with all bond angles of $120°$ .
- All six carbon atoms in the benzene ring are $s p^{2}$ hybridized.
- The six $π$ -electrons are delocalized over the entire ring, giving benzene exceptional stability (aromaticity).
Examples:

Figure 16.4: Examples of aromatic hydrocarbons

Summary: Hybridization in Hydrocarbons

Hydrocarbon Class	Bond Type	Hybridization	Bond Angle	General Formula
Alkane	$C - C$ single	$s p^{3}$	$109.5°$	$C_{n} H_{2 n + 2}$
Alkene	$C = C$ double	$s p^{2}$	$120°$	$C_{n} H_{2 n}$
Alkyne	$C \equiv C$ triple	$s p$	$180°$	$C_{n} H_{2 n - 2}$
Cycloalkane	$C - C$ single (ring)	$s p^{3}$	$\approx 109.5°$	$C_{n} H_{2 n}$
Benzene (aromatic)	Delocalized	$s p^{2}$	$120°$	$C_{n} H_{2 n - 6}$

Concept Assessment Exercise 16.1

Q1. Differentiate between saturated and unsaturated compounds. Give examples.

Feature	Saturated Compounds	Unsaturated Compounds
Bond Type	Only $C - C$ single bonds	At least one $C = C$ or $C \equiv C$ bond
Hybridization	$s p^{3}$	$s p^{2}$ (alkenes) or $s p$ (alkynes)
Atom Capacity	Cannot add more atoms	Can add atoms via addition reactions
Example Class	Alkanes	Alkenes, Alkynes
Specific Example	Butane ( $C H_{3} C H_{2} C H_{2} C H_{3}$ )	Ethene ( $C H_{2} = C H_{2}$ ), Ethyne ( $CH \equiv CH$ )

Q2. Define aliphatic and aromatic compounds. Give two examples of each.

Aliphatic compounds are hydrocarbons with open-chain (straight or branched) structures, or cyclic structures that do not contain a benzene ring. They can be saturated or unsaturated.
- Examples: Butane ( $C_{4} H_{10}$ ), Cyclohexane ( $C_{6} H_{12}$ )
Aromatic compounds are cyclic hydrocarbons containing at least one benzene ring with delocalized $π$ -electrons, giving them special stability.
- Examples: Benzene ( $C_{6} H_{6}$ ), Toluene ( $C_{6} H_{5} C H_{3}$ )

Aliphatic Hydrocarbons: Derived from the Greek word for "fatty," this class includes open-chain hydrocarbons and closed-chain (cyclic) compounds that behave similarly to open-chain compounds.
Aromatic Hydrocarbons: Derived from the Greek word for "fragrant," this class includes benzene and compounds with properties similar to benzene.

16.1.1 Open Chain Hydrocarbons

Also known as acyclic hydrocarbons, these compounds feature carbon skeletons arranged in straight or branched chains. They do not form rings and have terminal carbon atoms.

1. Saturated Hydrocarbons

Key Features:
- All carbon-carbon bonds are single ( $C - C$ ).
- Each carbon atom is $s p^{3}$ hybridized with tetrahedral geometry (bond angle $\approx 109.5°$ ).
- General formula: $C_{n} H_{2 n + 2}$
- Can be straight-chain or branched-chain.
Example (Alkanes):
- Butane (straight chain): $C H_{3} - C H_{2} - C H_{2} - C H_{3}$
- 2-methylpropane (branched chain):

2. Unsaturated Hydrocarbons

Key Features:
- Contain at least one $C = C$ double bond or $C \equiv C$ triple bond.
- Include alkenes and alkynes.

a. Alkenes

Commonly known as olefins, alkenes are hydrocarbons that contain at least one carbon-carbon double bond.

Key Features:
- Contain at least one $C = C$ double bond.
- Carbon atoms in the double bond are $s p^{2}$ hybridized (bond angle $\approx 120°$ , trigonal planar).
- General formula: $C_{n} H_{2 n}$
- Alkenes with more than one double bond are called polyenes.

For properties and reactions of alkanes, see Reactivity of Alkanes→.

b. Alkynes

Commonly known as acetylenes, alkynes are hydrocarbons that contain at least one carbon-carbon triple bond.

Key Features:
- Contain at least one $C \equiv C$ triple bond.
- Carbon atoms in the triple bond are $s p$ hybridized (bond angle $= 180°$ , linear).
- General formula: $C_{n} H_{2 n - 2}$
- Alkynes with more than one triple bond are called polyynes.

16.1.2 Closed Chain Hydrocarbons

Also known as cyclic hydrocarbons, these compounds have carbon skeletons arranged in rings, with no terminal carbon atoms. Each carbon atom is bonded to at least two other carbon atoms.

1. Alicyclic Hydrocarbons

The term "alicyclic" is a blend of aliphatic and cyclic. These are aliphatic hydrocarbons that form rings of carbon atoms but do not possess aromatic character.

Key Features:
- Chemical properties are similar to open-chain aliphatic hydrocarbons.
- Have two fewer hydrogen atoms than the corresponding open-chain alkane (same formula as alkenes: $C_{n} H_{2 n}$ for cycloalkanes).
- Carbon atoms are $s p^{3}$ hybridized (in cycloalkanes).
Examples:

2. Aromatic Hydrocarbons

These are cyclic hydrocarbons that contain at least one benzene ring in their molecular structure.

Key Features:
- The simplest example is benzene ( $C_{6} H_{6}$ ).
- The benzene ring is a planar hexagonal structure with all bond angles of $120°$ .
- All six carbon atoms in the benzene ring are $s p^{2}$ hybridized.
- The six $π$ -electrons are delocalized over the entire ring, giving benzene exceptional stability (aromaticity).
Examples:

Summary: Hybridization in Hydrocarbons

Hydrocarbon Class	Bond Type	Hybridization	Bond Angle	General Formula
Alkane	$C - C$ single	$s p^{3}$	$109.5°$	$C_{n} H_{2 n + 2}$
Alkene	$C = C$ double	$s p^{2}$	$120°$	$C_{n} H_{2 n}$
Alkyne	$C \equiv C$ triple	$s p$	$180°$	$C_{n} H_{2 n - 2}$
Cycloalkane	$C - C$ single (ring)	$s p^{3}$	$\approx 109.5°$	$C_{n} H_{2 n}$
Benzene (aromatic)	Delocalized	$s p^{2}$	$120°$	$C_{n} H_{2 n - 6}$

Concept Assessment Exercise 16.1

Q1. Differentiate between saturated and unsaturated compounds. Give examples.

Feature	Saturated Compounds	Unsaturated Compounds
Bond Type	Only $C - C$ single bonds	At least one $C = C$ or $C \equiv C$ bond
Hybridization	$s p^{3}$	$s p^{2}$ (alkenes) or $s p$ (alkynes)
Atom Capacity	Cannot add more atoms	Can add atoms via addition reactions
Example Class	Alkanes	Alkenes, Alkynes
Specific Example	Butane ( $C H_{3} C H_{2} C H_{2} C H_{3}$ )	Ethene ( $C H_{2} = C H_{2}$ ), Ethyne ( $CH \equiv CH$ )

Q2. Define aliphatic and aromatic compounds. Give two examples of each.

Aliphatic compounds are hydrocarbons with open-chain (straight or branched) structures, or cyclic structures that do not contain a benzene ring. They can be saturated or unsaturated.
- Examples: Butane ( $C_{4} H_{10}$ ), Cyclohexane ( $C_{6} H_{12}$ )
Aromatic compounds are cyclic hydrocarbons containing at least one benzene ring with delocalized $π$ -electrons, giving them special stability.
- Examples: Benzene ( $C_{6} H_{6}$ ), Toluene ( $C_{6} H_{5} C H_{3}$ )