What is the general formula of alkenes?
$C_n H_2n$
To determine the general formula for alkenes, we need to understand the structure of these hydrocarbons. Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. Because of this double bond, the hydrogen content is less than that of alkanes. The general formula for alkenes is expressed as C_nH_{2n}.
Here's how we can derive this:
- Consider the simplest alkene, ethylene (also known as ethene), which has the formula C_2H_4. It fits the general formula since n = 2, leading to C_2H_{2 \times 2} = C_2H_4.
- Now consider increasing the number of carbon atoms. For propylene (propene), n = 3, which leads to C_3H_{2 \times 3} = C_3H_6.
- The pattern holds for any alkene since the double bond reduces the number of hydrogen atoms compared to alkanes, which have the formula C_nH_{2n+2}.
Therefore, the correct answer is C_nH_{2n}. This formula accounts for the double bond that reduces the number of hydrogen atoms in alkenes by 2 compared to their saturated counterparts (alkanes).
Let's rule out the other options:
- C_nH_n: This formula would not satisfy hydrocarbons with a double bond.
- C_nH_{2n+2}: This is the general formula for alkanes, not alkenes.
- C_nH_{2n-2}: This formula fits alkynes, which contain a triple bond, not alkenes.
This logical examination ensures that C_nH_{2n} is the definitive formula for alkenes.
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