Question detail
Why does an alkene with n carbon atoms have two fewer hydrogen atoms than the corresponding alkane?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Reactions of alkenes and alcohols (chemistry only)
Question
- A. The carbon-carbon double bond reduces the number of hydrogens needed
- B. Alkenes contain oxygen atoms instead of hydrogen atoms
- C. Alkenes have no carbon-carbon bonds
- D. The molecule loses all hydrogen during cracking
Answer
The correct option is The carbon-carbon double bond reduces the number of hydrogens needed. The double bond means two carbon atoms bond to each other twice, so the molecule has two fewer hydrogens than the matching alkane.
Explanation
The correct option is The carbon-carbon double bond reduces the number of hydrogens needed. This explains unsaturation using structure. A corresponding alkane has only single carbon-carbon bonds and follows CnH2n+2. An alkene with one C=C bond follows CnH2n, so it has two fewer hydrogen atoms. The difference is not caused by oxygen atoms or by losing all hydrogen.
Common mistake
Understanding Unsaturation
Students often state that alkenes are unsaturated because they have a double bond, without mentioning the hydrogen atom difference.
Emphasize that alkenes are unsaturated specifically because they contain two fewer hydrogen atoms than the corresponding alkane.
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