Question

26 multiple choice 2 points which of the alkyl chlorides listed below undergoes dehydrohalogenation in the presence of a strong base to give pent - 2 - ene as the only alkene product? 1 - chloro - 3 - methylbutane 3 - chloropentane 1 - chloropentane 2 - chloropentane 1 - chloro - 2 - methylbutane previous next

Explanation:

Step1: Understand dehydrohalogenation

Dehydrohalogenation of alkyl chlorides with strong base follows Zaitsev's rule (forms more substituted alkene).

Step2: Analyze 1 - chloro - 3 - methylbutane

Elimination gives alkenes like 2 - methyl - 1 - butene and 2 - methyl - 2 - butene (not pent - 2 - ene).

Step3: Analyze 3 - chloropentane

Elimination from 3 - chloropentane:
$$\text{CH}_3\text{CH}_2\text{CHClCH}_2\text{CH}_3+\text{Base}\to\text{CH}_3\text{CH}=\text{CHCH}_2\text{CH}_3$$ (pent - 2 - ene as only alkene product due to symmetric structure).

Step4: Analyze 1 - chloropentane

Elimination gives pent - 1 - ene (not pent - 2 - ene).

Step5: Analyze 2 - chloropentane

Elimination gives pent - 1 - ene and pent - 2 - ene (mixture).

Step6: Analyze 1 - chloro - 2 - methylbutane

Elimination gives alkenes like 3 - methyl - 1 - butene and 2 - methyl - 2 - butene (not pent - 2 - ene).

Answer:

B. 3 - chloropentane