Question

1. an unknown compound, believed to be a hydrocarbon, showed the following behavior: it burned with a yellow sooty flame; no heat or color appeared when sulfuric acid was added; permanganate solution remained purple; the red color of bromine solution was lost only after a catalyst was added. from the compounds below, circle the one that fits the observations. write equations for the tests that work.

ch₃ch₂ch₂ch₂ch₃ hexagon - no double bonds hexagon - with double bonds h - c≡c - ch₂ch₂ch₃

Explanation:

Step1: Analyze combustion behavior

Burning with a yellow - sooty flame indicates a high carbon - to - hydrogen ratio. Alkanes burn with a clean blue flame, cycloalkanes burn with a less sooty flame compared to aromatic compounds. Aromatic compounds burn with a yellow sooty flame due to high carbon content.

Step2: Analyze reaction with sulfuric acid

No heat or color change with sulfuric acid rules out compounds that react vigorously with sulfuric acid like alkenes and alkynes which can undergo addition reactions or dehydration reactions.

Step3: Analyze reaction with permanganate

The permanganate solution remaining purple indicates the absence of alkenes and alkynes as they would decolorize permanganate solution through oxidation reactions.

Step4: Analyze reaction with bromine

The red color of bromine solution being lost only after a catalyst was added indicates an addition reaction that requires a catalyst. Aromatic compounds undergo electrophilic aromatic substitution with bromine in the presence of a catalyst (e.g., $FeBr_3$).

The compound is benzene (the third structure in the given set of compounds).

The reaction of benzene with bromine in the presence of a catalyst ($FeBr_3$) is an electrophilic aromatic substitution reaction:
$$C_6H_6 + Br_2\xrightarrow{FeBr_3}C_6H_5Br + HBr$$

Answer:

The compound is the benzene ring (the third structure in the given list)