IUPAC Nomenclature of Unsaturated Hydrocarbon, Alkene and Alkyne as a substituent, Practice Problems & FAQs

There are various applications and uses of unsaturated hydrocarbons, and we mostly use unsaturated hydrocarbons in our daily lives. We mostly use plastics, such as bags, polythene, grocery bags, etc. All these contain unsaturated hydrocarbons. The major sources of these hydrocarbons are petroleum and coal. These are also used in cooking as fuel.

There are millions of unsaturated hydrocarbons present, so to ease the classification and analysis of these compounds, a systematic naming procedure is followed, which is given by IUPAC and known as IUPAC nomenclature.

TABLE OF CONTENT

Unsaturated Hydrocarbons
Rules to assign IUPAC naming to unsaturated hydrocarbons
Alkene/Alkynes as a substituent group
Practice Problems
Frequently asked questions (FAQs)

Unsaturated Hydrocarbons

Unsaturated hydrocarbons are the organic compounds that contain at least one double or triple bond between two adjacent carbon atoms. It may be cyclic or acyclic and depending on the type of bonding, these are classified into two types.

1. Alkenes : Open chain unsaturated hydrocarbons, containing carbon-carbon double bonds, are known as alkenes.

Examples of alkenes : Ethene (CH₂=CH₂) , Propene (CH₂= CH-CH₃ )

General formula of Alkenes is C_nH_2n.

The primary suffix of alkenes is ‘ene’.

2. Alkynes : Open chain unsaturated hydrocarbons, containing carbon-carbon triple bonds, are known as alkynes.

Examples of alkynes : Ethyne

General formula of alkynes is C_nH_2n-2.

The primary suffix of alkynes is ‘yne’.

Rules to assign IUPAC naming

There are several rules to be followed to give naming to unsaturated hydrocarbons.

Rule 1 : Parent Chain selection

If a hydrocarbon contains multiple bonds, then the longest carbon chain which contains the maximum number of multiple bonds will be considered as parent or principal chain whether it violates the longest chain selection rule.

Examples :

Possibility 1	Possibility 2

Incorrect	Correct
This selection for the chain is wrong because the double bond must be included in the chain.	This selection is correct as the double bond is a part of the parent chain.

Rule 2 : Assigning Locants

Out of multiple bonds and substituents, multiple bond is given higher preference and If the multiple bond gets the same number from either direction, then the correct name will be the one that contains the lowest substituent number.

If both the multiple bonds and substituents get the same numbers from either direction, then the numbering must be done in such a way that substituents get the numbers in accordance with the alphabetical order.
If double bond and triple bond get the same number, then double bond is given higher priority over triple bond.

Example :

Chain	Explanation
	Both ene and yne are at the terminal, and according to the alphabetical order, ene is given priority. So, the numbering in red is wrong and that in green is correct.
	The numbering will be done where the double bond comes first. Therefore, the green numbering is correct and the red one is wrong.
	Double bond is at the same position from both the ends. In green numbering, branching is starting from 2nd and in red it is from 4th. So Green numbering is correct.

Rule 3 : Presence of identical multiple bonds more than once

If there is more than one identical double or triple bond present in the parent chain then di, tri, tetra etc are used in the primary suffix.

Extra ‘a’ is added to the word root, if the primary suffix contains di, tri etc.

Example :

Chain

IUPAC name

CH2=CH-CH=CH₂

Buta-1,3-diene

CHC-CH₂-CH₂-CCH

Hexa-1,5-diyne

Alkene/Alkynes as a substituent group :

If a hydrogen atom is removed from the alkane, alkene, and alkyne, the hydrocarbons obtained are known as alkyl, alkenyl, and alkynyl, respectively.

Compound	IUPAC name
	Ethenyl
	Prop-1-enyl
	Prop-2-enyl
	Ethynyl
	Prop-1-ynyl

Practice Problems

Q 1 : Write the IUPAC name of the following compound:

Solution :

Step 1: Choose the longest carbon chain and start numbering

The parent chain has 6 carbons. On numbering from both sides, both the triple bonds get the same positions. However, on numbering from left to right, the branched chain (substituent)

gets the lower number. Hence, the numbering has to be done from left to right (green numbering).

Step 2: Determine the position of the substituent

While numbering, the methyl group position is at 3. So, the name of this compound is

3-Methylhexa-1,5-diyne.

Q 2 : Write the IUPAC name of the following compound:

Solution :

In the given compound, the parent chain is of eight carbon atoms. The substituent is prop-2-enyl

(or allyl) at the fifth position. Therefore, the IUPAC name of the compound is 5-(prop-2-enyl)octa-1,3,7-triene.

Q 3 What is the general formula for unsaturated hydrocarbons which contains only one carbon-carbon double bond?

Answer: (B)

Q 4 What is the IUPAC name of
?

A) 2-methylbut-1-yne-3-ene
B) 1-methylbut-1-yne-3-ene
C) 2-methylbut-1-ene-3-yne
D) 1-methylbut-1-ene-3-yne

Answer : (C)

Q 5 What is the IUPAC name of CH3-CH3-CH2-CCH?

A) Pentane
B) Pentyne
C) Pentylene
D) Pentene

Answer : (B)

Q 6 What is the structure of 2-methylpent-2-ene ?

A)

B)

C)

D)

Answer : (B)

Frequently asked questions (FAQs)

Question 1. What is the difference between saturated and unsaturated hydrocarbons?
Answer : When organic compounds only contain carbon and hydrogen and all the carbon-carbon bonds are single covalent bonds, then those organic compounds are classified as Saturated hydrocarbons. Examples : Methane, Ethane etc.

When organic compounds consist of only carbon and hydrogen and contain at least one carbon-carbon, multiple bonds are classified as Unsaturated hydrocarbons. Examples : Ethene, Propyne etc.

Question 2. What are the real life examples of unsaturated hydrocarbons ?
Answer : Unsaturated hydrocarbons are used in most of the fuels which are used to produce energy. These are also used in plastics, fibers.

Question 3. What is the primary source of unsaturated hydrocarbons ?
Answer : Petroleum and Coal are the major source of unsaturated hydrocarbons.

Question 4. Is it possible to convert unsaturated hydrocarbons to saturated hydrocarbons ?
Answer : Yes, by Hydrogenation (reduction) of unsaturated hydrocarbons, multiple bonds of unsaturated hydrocarbons reduce to single covalent bonds to form saturated hydrocarbons.

Question 5. What is the physical state of unsaturated hydrocarbons ?
Answer : At room temperature, Unaturated hydrocarbons upto 4 carbons are gaseous in nature and from 5 to 18 carbon containing hydrocarbons are present in liquid state and even more carbon containing are solid in nature.

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