tert-Butylbenzene reacts with 1-chloro-2-methylpropane in the presence of anhydrous $\text{AlCl}_3$. The major product in this reaction is:
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In Friedel-Crafts alkylation, always check for carbocation rearrangements. Isobutyl and neopentyl halides almost always rearrange to give tert-butyl and tert-pentyl products, respectively.
Step 1: Name the reaction. tert-Butylbenzene reacting with an alkyl chloride and $\text{AlCl}_3$ is a Friedel-Crafts alkylation. The catalyst pulls off the chloride to make a carbocation, which then attacks the ring. The trick is to find out which carbocation actually attacks.
Step 2: Build the first carbocation. 1-chloro-2-methylpropane losing chloride first gives a primary carbocation, $(\text{CH}_3)_2\text{CHCH}_2^+$. Primary carbocations are very unstable and never survive long enough to react cleanly.
Step 3: Let the carbocation rearrange. An unstable primary carbocation quickly shifts a hydrogen from the neighbouring carbon (a 1,2-hydride shift). This turns it into the much more stable tertiary carbocation, $(\text{CH}_3)_3\text{C}^+$, the tert-butyl cation. So the real electrophile that attacks is the tert-butyl group, not the isobutyl group.
Step 4: Decide where it attaches. The ring already carries a bulky tert-butyl group. A tert-butyl group is an ortho/para director, but it is so big that the new group cannot squeeze in at the crowded ortho position. So substitution happens at the para position to avoid steric clash.
Step 5: Assemble the product. We now have two tert-butyl groups on the ring, sitting opposite each other at the 1 and 4 positions. That is 1,4-di-tert-butylbenzene.
Step 6: Reject the others. The isobutyl and sec-butyl products are wrong because the carbocation rearranges to tert-butyl. The 1,2 (ortho) product is wrong because it is too crowded. So only the para di-tert-butyl product remains.
