Identify the major product from the following reaction sequence
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Grignard reagents add alkyl groups to carbonyl compounds. Ketones give tertiary alcohols, which on acid-catalysed dehydration form alkenes as major products.
Step 1: Identify the Grignard reagent and carbonyl substrate. The first step involves a Grignard reagent, specifically $CH_3MgCl$ (methylmagnesium chloride), reacting with a ketone substrate. The exact ketone is shown in the image, but the principle is that $CH_3MgCl$ adds its $CH_3^-$ carbanion to the carbonyl carbon of the ketone. Step 2: Write the Grignard addition mechanism. The carbanion from $CH_3MgCl$ attacks the electrophilic carbonyl carbon ($C=O$) of the ketone via nucleophilic addition: \[ R_2C=O + CH_3MgCl \rightarrow R_2(CH_3)C-OMgCl \] Hydrolysis (acid workup) then gives the tertiary alcohol: \[ R_2(CH_3)C-OMgCl + H_3O^+ \rightarrow R_2(CH_3)C-OH \] Step 3: Identify the tertiary alcohol product. Since the starting ketone reacts with $CH_3MgCl$ and water workup is applied, a tertiary alcohol is formed (the two R groups from the ketone plus the $CH_3$ from Grignard, all on the same carbon bearing $-OH$). Step 4: Apply dehydration conditions to the tertiary alcohol. The next step in the reaction sequence involves dehydration of the tertiary alcohol. Tertiary alcohols dehydrate readily under acidic conditions (e.g., conc. $H_2SO_4$ or $H_3PO_4$) by the E1 mechanism, losing $-OH$ and a $\beta$-hydrogen to form an alkene (Zaitsev's rule: more substituted alkene is preferred). Step 5: Identify the dehydration product. The dehydration of the tertiary alcohol gives the more substituted alkene as the major product per Zaitsev's rule. The specific alkene structure (option 3) corresponds to the expected dehydration product from the Grignard-derived tertiary alcohol in the image-based question. Step 6: Confirm the overall reaction sequence and answer. Ketone $\xrightarrow{CH_3MgCl, \text{ then } H_3O^+}$ Tertiary alcohol $\xrightarrow{\text{dehydration}}$ Alkene (option 3). This is a classic two-step organic synthesis: nucleophilic addition of Grignard reagent followed by elimination. \[ \boxed{\text{Option 3: Dehydration alkene product}} \]
