Question

General formula of an alkyne is :

• A. \(\text{C}_\text{n}\text{H}_{2\text{n}+2}\)
• B. \(\text{C}_\text{n}\text{H}_{2\text{n}-2}\)
• C. \(\text{C}_\text{n}\text{H}_{2\text{n}}\)
• D. \(\text{C}_\text{n}\text{H}_{2\text{n}+3}\)

Hint:

Remember the general formulas for acyclic hydrocarbons:
{Alkane} (all single bonds, suffix "-ane"): \(\text{C}_\text{n}\text{H}_{2\text{n}+2}\)
{Alkene} (one double bond, suffix "-ene"): \(\text{C}_\text{n}\text{H}_{2\text{n}}\) (remove 2 H's from alkane)
{Alkyne} (one triple bond, suffix "-yne"): \(\text{C}_\text{n}\text{H}_{2\text{n}-2}\) (remove 4 H's from alkane, or 2 H's from alkene) The question asks for alkyne.

Answer 1

The Correct Option is B

Concept: Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon. They are classified into alkanes, alkenes, alkynes, and aromatic hydrocarbons based on the types of bonds between carbon atoms. Each class has a general formula. Step 1: Recall the general formulas for different types of acyclic hydrocarbons
Alkanes: These are saturated hydrocarbons containing only single bonds between carbon atoms. Their general formula is \(\text{C}_\text{n}\text{H}_{2\text{n}+2}\) (where \(n \geq 1\)). Example: Methane (\(\text{CH}_4\), n=1), Ethane (\(\text{C}_2\text{H}_6\), n=2).
Alkenes: These are unsaturated hydrocarbons containing at least one carbon-carbon double bond (\(C=C\)). For acyclic alkenes with one double bond, their general formula is \(\text{C}_\text{n}\text{H}_{2\text{n}}\) (where \(n \geq 2\)). Example: Ethene (\(\text{C}_2\text{H}_4\), n=2), Propene (\(\text{C}_3\text{H}_6\), n=3).
Alkynes: These are unsaturated hydrocarbons containing at least one carbon-carbon triple bond (\(C \equiv C\)). For acyclic alkynes with one triple bond, their general formula is \(\text{C}_\text{n}\text{H}_{2\text{n}-2}\) (where \(n \geq 2\)). Example: Ethyne (Acetylene, \(\text{C}_2\text{H}_2\), n=2), Propyne (\(\text{C}_3\text{H}_4\), n=3). Step 2: Identify the general formula for an alkyne Based on the definitions, the general formula for an alkyne (with one triple bond and no rings) is \(\text{C}_\text{n}\text{H}_{2\text{n}-2}\). Step 3: Analyzing the options
(1) \(\text{C}_\text{n}\text{H}_{2\text{n}+2}\): This is the general formula for Alkanes.
(2) \(\text{C}_\text{n}\text{H}_{2\text{n}-2}\): This is the general formula for Alkynes (with one triple bond). Correct.
(3) \(\text{C}_\text{n}\text{H}_{2\text{n}}\): This is the general formula for Alkenes (with one double bond) or Cycloalkanes.
(4) \(\text{C}_\text{n}\text{H}_{2\text{n}+3}\): This formula does not correspond to a stable neutral hydrocarbon series. (For example, if n=1, \(CH_5\), which is not standard). Therefore, the general formula of an alkyne is \(\text{C}_\text{n}\text{H}_{2\text{n}-2}\).