Question

question 19 (5 points)
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whats the general formula for an alkene if n is the number of carbon atoms?
a) c_nh_{2n + 4}
b) c_nh_{2n + 2}
c) c_nh_{n + 1}
d) c_nh_{2n}

Explanation:

Alkenes are hydrocarbons with at least one carbon - carbon double bond ($C = C$). The general formula for alkanes (saturated hydrocarbons with single bonds only) is $C_{n}H_{2n + 2}$. For alkenes, due to the presence of one double bond, two hydrogen atoms are less than in alkanes. So, if we start with the alkane formula and subtract 2 hydrogen atoms, we get the general formula for alkenes. Let's derive it:

• For alkanes: $C_{n}H_{2n+2}$
• For alkenes (with one double bond, which reduces the number of H atoms by 2 compared to alkanes): $C_{n}H_{(2n + 2)-2}=C_{n}H_{2n}$

Now let's analyze the options:

• Option A: $C_{n}H_{2n + 4}$ is not the formula for any common hydrocarbon series.
• Option B: $C_{n}H_{2n+2}$ is the formula for alkanes, not alkenes.
• Option C: $C_{n}H_{n + 1}$ does not match the formula for alkenes.
• Option D: $C_{n}H_{2n}$ is the correct general formula for alkenes.

Answer:

D. $C_{n}H_{2n}$