Alkenes are represented with the general molecular formula

Analysis

The question asks for the general molecular formula of alkenes, which are a class of hydrocarbons characterized by the presence of at least one carbon-carbon double bond ($C=C$).

Concept Explanation

In organic chemistry, hydrocarbons are classified based on the type of bonds between their carbon atoms:

1. Alkanes: These are saturated hydrocarbons containing only single bonds. Their general formula is $C_nH_{2n+2}$.
2. Alkenes: These are unsaturated hydrocarbons containing at least one double bond. Because a double bond "uses up" two hydrogen atoms that would otherwise be attached to the carbons, the formula is reduced by two hydrogens compared to alkanes. The general formula is $C_nH_{2n}$.
3. Alkynes: These are unsaturated hydrocarbons containing at least one triple bond. Their general formula is $C_nH_{2n-2}$.
4. Alkanols (Alcohols): These are organic compounds containing a hydroxyl (-OH) group. Their general formula is $C_nH_{2n+1}OH$.

Step-by-Step Solution

• Step 1: Identify the homologous series. We are looking for Alkenes.
• Step 2: Recall the degree of unsaturation. Alkenes have one double bond, which removes two hydrogen atoms from the corresponding alkane ($C_nH_{2n+2} - 2H = C_nH_{2n}$).
• Step 3: Compare with the given options: • A: $C_nH_{2n}$ (Matches the alkene formula) • B: $C_nH_{2n-2}$ (This is the formula for Alkynes) • C: $C_nH_{2n+2}$ (This is the formula for Alkanes) • D: $C_nH_{2n+1}OH$ (This is the formula for Alkanols)

Conclusion

The general molecular formula for alkenes is $C_nH_{2n}$.

Exam Tip

To remember these easily, think of the hydrogen count:

• Alkanes (Single bond): $2n + 2$ (The "plus" gives you more)
• Alkenes (Double bond): $2n$ (The "middle" ground)
• Alkynes (Triple bond): $2n - 2$ (The "minus" takes away)